RV Solar Upgrade with Renogy and Go Power – QUICK & CHEAP!

We recently did an RV solar upgrade project that proved to be quick, easy and cheap. We spent just $480 to jump from 190 watts of power to 570 watts, more than enough for our boondocking off-the-grid RV lifestyle. PLUS it took less than three hours to install. What a great bang for the buck!

RV Solar Upgrade - CHEAP & EASY with Go Power + Renogy


Our Genesis Supreme 28CRT fifth wheel toy hauler came with a small factory-installed Go Power RV solar power system that included a single 190 watt solar panel, a 1500 watt pure sine wave inverter and a 30 amp PWM solar charge controller connected to four dealer-installed Group 24 12-volt wet cell batteries with a capacity of 280 amp-hours.

Factory-installed RV solar power systems like this one are now a common option on many new RVs, and Go Power (a subsidiary of Dometic) is often the brand that RV manufacturers use.

Although none of the components in the system are “best of breed,” the Go Power system worked fine for us as we boondocked every night for four months last summer. As the months wore on towards the Fall (and away from the summer solistice), however, the batteries struggled more and more each day to reach full charge. In the last few weeks in late August and September they never did.

Fortunately, the Go Power 30 amp solar charge controller that came with this system can handle up to 600 watts of solar panels, so an RV solar upgrade was possible without replacing the charge controller!

As we contemplated doing an RV solar upgrade all last summer, the debate was: do we ditch the whole factory installed system and replace it with top of the line components or do we simply add some more panels to the existing system?


How much solar power do you really need when you live in an RV?

Answering that question is really important because it’s incredibly easy to end up installing a far bigger and fancier system than you actually need after hearing people discussing their mammoth systems around the campfire.

Just because a friend has a huge system doesn’t mean it will make sense for you to break the bank to install one too!

How big an RV solar power system you need depends entirely on how much power you use in your day-to-day RV lifestyle and how often your boondock.

We boondock every night, but we don’t use much power. Also, since we are now seasonal travelers instead of the full-timers as we used to be, we travel primarily in the summertime when the sun is high in the sky at a good angle for the solar panels and the days are long, allowing the solar panels to work for a few extra hours.

Our primary power use is our two laptops (which we use a lot), the water pump, and the interior lights for an hour at night (we go to bed early). We don’t watch TV and we rarely use the microwave or hair dryer.

Running the air conditioning on battery power is not possible for any but the most massive RV solar power charging systems and battery banks, so it’s not part of the equation for most people. We rely on the generator for running our a/c.

With our traveling lifestyle of minimal power use, we happily lived on 480 watts and 555 watts in our trailer and sailboat respectivlely for 13 years. That was plenty of power for us except in the dead of winter when the sun was low in the sky (poor angle to the solar panels) and the days were short.

RV solar panel installation using Go Power and Renogy panels

Our toy hauler had one factory installed solar panel (center).
An easy RV solar upgrade with two more panels tripled our battery charging capacity!

When we did those installations in 2008 and 2010, they were considered to be sizable for a boat or an RV. Seeing a rig with 1,000 watts on the roof in those days made everyone’s head turn while they mouthed the word, “WOW!”

However, by today’s standards, we had small systems on both our RV and sailboat! The third owner of our boat Groovy upgraded the solar panels to 930 total watts instead of the original 555 watts.

Last year, we met a full-timing family who had 3,500 watts of solar power on the roof of their 44′ toy hauler. They also had two huge Victron solar charge controllers (the panels were wired in two separate arrays) and they had a massive bank of lithium-ion batteries in the basement.

They could run their air conditioning on battery power all day and they had a full-size residential refrigerator to boot. They liked to keep their TV on all day long and the kids spent hours watching videos on their iPads. The kids also did homework on their laptops and everyone in the family had had phones and laptops to charge. They also had several internet access devices that gave them a total of 500 GB of data each month. They used it all and sometimes fell a little short by month’s end!

So, the size of the system you need depends entirely on how you live your RV lifestyle.

We knew when we bought our toy hauler last year that 190 watts wouldn’t be enough for us long term, but we didn’t have time to fuss with and do an RV solar upgrade before starting our summer journey. We were also curious to see how it performed right from the factory.

The solar charge controller is a lower end PWM unit (Pulse Width Modulation) rather an MPPT (Maximum Power Point Tracker) type of controller that eeks out more power from the panels. We wondered if the system would work at all. We were pleasantly surprised that it worked quite well and did the job all summer long, although our batteries did get down to 11.9 or 12.0 volts on quite a few colder mornings at summer’s end, much lower than we’ve ever seen our house batteries before.

Go Power 30 amp PWM solar charge controller

Go Power 30 amp PWM solar charge controller mounted on a wall inside the rig.

Ultimately, we decided the simplest and most stress-free RV solar upgrade we could do would be to add more solar panels and leave all the other components alone.

RV Solar Upgrade – Adding New Solar Panels – Wired in Parallel or in Series?

The Go Power solar panel that came with the rig is a 12 volt 190 watt panel. Although the Go Power 30 amp solar charge controller can handle 600 watts of power coming from the panels, it is unable to operate on anything but 12 volts. Fancier charge controllers can work with the panels at 24, 36 or 48 volts and then step down the voltage to 12 volts to charge the batteries.

This limitation meant we didn’t have the option of using 24 volt panels which are generally cheaper per watt. Also, it meant that the new panels would have to be wired in parallel with the existing panel to keep them all at 12 volts rather than having the option of wiring them in series because it would put the solar array at 36 volts.

As a side note, even though we didn’t have a choice in this case, the decision whether to wire the solar panels in parallel versus in series is a matter of how much shade the panels might encounter and how long the cable runs will be versus the guage of the wire.

When solar panels are wired in series, if one panel gets shaded, all the panels reduce their power output dramatically. Also, the voltage of the panels is cumulative while the current stays the same. That is, three 12-volt panels will be at 36 volts but the current running in the wires will be the nominal current of a single panel, for instance, 10 amps.

When solar panels are wired in parallel, if one panel gets shaded, the others continue to produce power at their normal rate. So, in a three panel array, if one panel drops out you still get 2/3 of the power because the other two panels are still working. Also, the voltage of the panels remains the same but the current is cumulative. That is, three 12-volt panels will be at 12 volts but the current will be additive, or 30 amps.

The more current there is in a wire, the shorter that wire has to be before some of the current dissipates as heat, leaving you less current for charging the batteries. A heavier guage wire will retain more current over a longer distance, but it is harder to work with during the installation and it is more expensive.

For reference, we wired the panels on our old full-timing fifth in series, and that worked fine because we almost always parked in full sun and rarely had any kind of shade on the panels. However, we wired the panels on our sailboat in parallel because the mast and boom cast a huge moving shadow across the panels as the boat swung at anchor, so one or another of the panels was frequently knocked out of the system.

New Solar Panels – What Size?

Whether the panels were wired in series or in parallel, any new panels we added to our system would produce the same watts as the existing panel: 190 watts. Even if the new panels were bigger than 190 watts, they would match the lower wattage of the existing panel.

There weren’t many 190 watt 12 volt panels available, except the same model Go Power panel we already had on the roof, and their panel is very expensive.

Go Power 190 watt solar expansion kit

Instead, we got two Renogy 200 watt 12 volt panels, and these seem to be good quality. Because the new panels will drop down to 190 watts to match the existing panel in the system, this RV solar upgrade will give us 570 watts of total power (3 x 190).

570 watts is more than either our boat or our full-time trailer, so it should be more than enough!

Renogy 200 watt solar panel

As for the batteries, we don’t have room for more batteries, and the existing batteries haven’t died yet (to my surprise!). So, we’ve decided to hold off on swapping out the batteries until another season.


RV Solar Upgrade: Installation

The total cost of the solar power upgrade was about $480 which included:

The tools required to do this RV solar upgrade project were:

The installation was straight forward.

On the back of each panel — both the existing one on the roof and the two new ones — there is a junction box with two 10 AWG leads (positive and negative). They are about 18 inches long and have MC4 connectors on the ends.

Renogy solar panel junction box and MC4 connectors

Most solar panels have a junction box and short leads with MC4 connectors on the ends, one positive and one negative.

On the existing solar panel, the MC4 connectors at the ends of these cables were connected to two other cables that ran from the roof of the RV down to the solar charge controller inside the rig.

All of this cabling was invisible as you looked at the face of the solar panel on the roof because it was all underneath it. Also, beneath the solar panel, there were two holes in the roof where the cables went into the interior of the rig down to the solar charge controller.

Renogy solar panel MC4 wires and junction box

Most solar panels have a junction box and two leads with MC4 connectors on the ends.

Here is a rough diagram showing the solar panel with its junction box and two 10 AWG cables with their MC4 connectors. These connectors are attached to two MC4 connectors on the ends of a long length of 10 AWG cable that goes through a hole in the roof (the blue circle) down to the solar charge controller in the interior of the rig (not shown).

The holes in the roof are actually under the panel, but this drawing shows the holes being above the panel so the diagram isn’t too messy!

Diagram of single solar panel with MC4 connectors on an RV roof

Our factory installed solar panel had two leads, positive and negative, that attached to wires coming up through the roof from the charge controller inside the rig. The holes in the roof (blue circles) are actually located under the panel.

We purchased two 3-to-1 branch adapters that would make it super easy to wire the three panels in parallel. The adapters look like bird feet with three toes (one for each solar panel), and a leg that would attach to the cable that went through the roof into the rig.

One adapter would be connected to the positive side of the system and one would be connected to the negative side. That is, all three positive leads, one from each panel, would connect to the three toes on one bird foot (the “positive” 3-to-1 branch connector) and all three negative leads, one from each panel, would connect to the three toes on the other bird foot (the “negative” 3-to-1 branch connector).

We also bought two 6′ lengths of 10 AWG cable with MC4 connectors pre-installedat each end. These were essentially extension cables that would connect to the MC4 connectors on the cables coming up through the roof from the charge controller down in the rig.

They were color coded, so the red one would connect to the positive cable coming up through the roof and the black one would connect to the negative cable coming up from the charge controller.

Fortunately, Genesis Supreme had labeled the cables coming up from the charge controller so we could tell which one was positive and which was negative.

MC4 solar panel wire connectors for an RV installation

We got two 3-to-1 branch connector (“bird feet”) and one 6′ pair of 10 AWG cables with MC4 connectors pre-installed on the ends.

Here is a rough diagram showing the layout of the cables. As in the previous diagram, the two blue circles are the holes in the roof which are actually located beneath the original solar panel in the middle. However, for simplicity in showing how the cables connect, the “holes in the roof” are located above the panels in this diagram and the 6′ extension cables are really short!

Diagram of RV solar power upgrade from 1 panel to 3 panels in parallel

Our 2 new panels would be wired in parallel with the existing panel, connecting all the positives together on one 3-to-1 branch connector and all the negatives on the other. The extension cables would connect to the wires coming up through the holes in the roof (blue circles). Note that the holes in the roof are actually under the center panel and the 6′ extension cables are drawn super short.

Our mission was to :

  1. Lift the existing solar panel so we could access the cabling underneath
  2. Disconnect the MC4 connectors on the panel’s leads from the MC4 connectors on the cables that come up from the solar charge controller in the rig
  3. Reconnect the cables coming from the charge controller to the new 6′ “extension” cables
  4. Connect the “extension” cables to the legs of the 3-to-1 branch connectors which would designate one as “positive” and one as “negative”
  5. Connect each panel’s positive cable to the “positive” 3-to-1 branch connectors
  6. Connect each panel’s negative cable to the “negative” 3-to-1 branch connectors

All of this would be done by snapping the MC4 connectors together, simply inserting one end into the other and pressing it together. So easy!

There’s a special tool for disconnecting MC4 connectors, but you can also disconnect them with your fingers by keeping the tab on one side depressed as you pull the two pieces apart.

Connecting MC4 connectors in an RV solar panel installation

MC4 connectors snap together.

To get at the cables under the existing Go power solar panel, Mark removed the hardened sealant that was covering each of the mounting brackets. He used a screwdriver but a narrow and rigid putty knife would work too.

Removing a Go Power solar panel from an RV roof

First step was to lift up the existing panel which required removing the sealant on the mounting bracket screws and then unscrewing the screws.

Then he unscrewed each of the screws holding the mounting brackets in place.

We bought a wonderful cordless power screwdriver last year that we BOTH absolutely LOVE! It makes screwing and unscrewing things infinitely easier than doing it by hand, and it’s much less bulky than a cordless drill.

Ryobi cordless screwdriver
Removing a Go Power solar panel from an RV roof

Unscrewing the screws.
The cordless screwdriver is one of our favorite tools!

He unscrewed all four feet and then lifted up one side to get at the cables underneath.

Changing the wiring under a Go Power solar panel under an RV roof

Working under the existing solar panel.

A positive (red) and negative (black) cable came up through the roof from the interior of the rig where they were connected to the solar charge controller and were connected directly to the solar panel. Mark disconnected each cable from the solar panel and then reconnected them to the two 6′ extension cables we had purchased.

Changing the wiring under a solar panel on an RV roof

The positive and negative extension cables go between the 3-to-1 branch connectors and the cables coming up through the roof from the charge controller inside the rig.

Then he connected the extension cables to the “legs” of each of the two 3-to-1 MC4 branch connectors (bird feet) and connected the solar panel’s negative and positve leads to the “toes” of the 3-to-1 branch connectors.

RV solar panel MC4 connector wiring on an RV roof

The original panel (black leads going to the middle “toes”) and the solar charge controller (red and black extension cables going to the “legs”) are now wired into the 3-to-1 branch connectors. We ran into the rig to verify everything looked okay and we saw the float voltage of 13.5 volts on the charge controller display.

Next, we needed to get the two new Renogy solar panels onto the roof of the RV, place them on either side of the existing panel, and then connect their positive and negative leads to the positive and negative 3-to-1 branch connectors.

Before that, though, we needed to figure out how to get the panels up onto the roof which is 13.5 feet in the air! We opened the patio of the toy hauler and put a ladder on it. This was much more secure than carrying a heavy solar panel one handed up the ladder attached to the side of the rig!

Ladder roof access on a toy hauler RV patio

The most solid way to get the panels up to the roof was to put a ladder on the patio!

Lifting a solar panel onto an RV roof

Here comes the first one!

Once we got both panels up on the roof, we attached the MC4 connectors on the two new panels’ leads to the outer “toes” of the two 3-to-1 branch connectors, positive to positive and negative to negative.

Now all three panels were completely wired up in parallel.

Three solar panels wired with MC4 connectors on an RV roof


The next step was to mount the solar panels on the roof.

The roof is just wide enough (it’s an 8.5′ “widebody” trailer) that we could place the three panels side by side, leaving enough space between them so we could walk beyond them to the far end of the rig.

First Mark screwed the original Go Power panel’s mounting brackets back into the roof.

Then we used the Renogy mounting Z brackets to mount the new Renogy solar panels. The Renogy mounting brackets came with very handy hex head self-tapping screws.

Self-tapping screws for installing a Renogy solar panel on an RV roof

Self-tapping screws. So easy!

Then Mark used a scratch awl to make a starter hole for the self-tapping screws. Pounding a nail in a little ways would have worked too.

Mounting an RV solar panel on the roof

Mark made a starter hole for the screws with a scratch awl.

Then he used a cordless drill with a hex head bit to screw them in all the way.

RV solar panel installation- attaching the solar panels to the roof

The mounting Z brackets got screwed into the roof.

Solar panel mounting brackets screw directly into the RV roof


Last of all, he used Dicor Self-Leveling Lap Sealant in a caulk gun to cover all the screws and seal all the edges of the mounting brackets. This will ensure that no water can find its way through the roof!

Sealing the holes in an RV roof after mounting a solar panel

Dicor Self-Leveling Lap Sealant seals the whole mounting bracket so water can’t leak in.

Dicor Lap Sealant on a solar panel mounting bracket

After the Dicor Lap Sealant had leveled out, it completely surrounded and covered the mounting bracket

Ta da! The finished product looked great!

RV solar upgrade cheap and easy with Go Power and Renogy


I couldn’t believe how easy this project turned out to be. Of course, the hardest parts were already done for us: running the cables from the roof down into the interior of the rig, wiring up the solar charge controller and wiring up the inverter. All we had to do was add two more panels and wire them up with the handy MC4 connectors.

If you have purchased a rig that has a “starter” solar power system like the Go Power system on our toy hauler, it’s not difficult to upgrade it like we did so you have the maximum amount of solar panel wattage that the charge controller can accept.

One thing to consider before buying any solar gear, especially from an online retailer, is to buy each piece individually rather than in a big kit. The problem with a kit is that if one item in the kit doesn’t work and needs to be returned, online retailers, like Amazon, may require you to return the entire kit. If the failed element is a solar panel and you’ve already installed the other panels in the kit and they are working fine, it may be a hassle to get approval to return just the one broken panel. I’ve read of cases where the entire system had to be dismantled and reboxed and sent back. For that reason, we opted to buy each piece separately just in case.

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Is RV Solar Affordable? 3 Solar Power Solutions for RVs and Boats

Is RV solar power affordable? Or is installing a solar power system on a motorhome or trailer — or even on a sailboat — just too darn expensive to be cost effective? We never thought this question would be hard to answer until recently.

This article outlines three different RV solar power solutions and lists all the parts (and costs) of everything you need to buy:

1. A Small, Expandable Rooftop RV Solar Power Solution – For weekends and vacations
2. A Portable RV Solar Power Solution – To get you up and running effortlessly
3. A Big Rooftop RV Solar Power Solution – For full-time RVing

Solar panels on a fifth wheel trailer

Can a solar power installation on an RV or sailboat pay for itself?

Ever since we installed our first (very small) solar power system on our first full-time RV nearly ten years ago, we’ve been excitedly telling people it is a very affordable do-it-yourself project for anyone with some mechanical and electrical knowledge. And for those who can’t turn a wrench, it shouldn’t be that much more.

Our first 130 watt solar power system cost us about twice as much as the same system would today, but even at that high price, we felt it was dollar-for-dollar an equal value to buying a Yamaha or Honda 1000 generator. Best of all, once a little system like that was installed, it was a whole lot less noisy, expensive to operate and complicated to use than a generator would be.

At today’s super cheap solar prices, that little solar power system is even more valuable compared to one of those nice Japanese portable gas generators than it was 10 years ago!

Installing solar panels on a motorhome RV

Installing solar power can be a DIY project if you’re handy.

Recently, however, we’ve heard some crazy prices being quoted for installing solar power systems on RVs. We met one couple with a gorgeous brand new DRV Suites fifth wheel who were quoted $13,000 for a solar power installation. Not long after that, we read an article in a popular RV magazine describing a $12,000 solar power installation on a fifth wheel.

Yikes!! These are outrageous prices!!

We sure hope no one is finding they have to spend that kind of crazy money to get a solar power system installed on their trailer or motorhome or sailboat.

We’ve got oodles of articles on this website that go into the nitty gritty details of things to consider when designing and installing a solar power system on an RV or a boat (located HERE). However, all that theory aside, it’s not all that complicated.

Here are three solar power “packages” — with approximate prices — that will do the trick whether you’re a part-timer or full-time RVer.

Although it is possible to buy “pre-packaged RV solar power kits” online, we suggest hand selecting the components you want so that just in case any individual item has a problem it can be returned easily.

We’ve heard of cases where people bought a pre-packaged solar power kit online and then had problems returning a broken part because they had to return the entire kit — solar panels, charge controller, cables and all — just because the one item wasn’t working right.



Affordable solar panel with a popup tent trailer

For part-time RVers, installing solar on the roof isn’t a requirement.

The following is essentially what we put on our roof and what we camped with off the grid every night for a year when we started.

The brands are not exactly the same, but these components are highly rated and will do the trick for anyone that wants a roof-mounted solar power system on their motorhome or trailer.

This kit includes both a solar battery charging component and an 110 volt AC power component provided by an inverter. If you don’t understand the distinction, please see our post: RV Solar Power Made Simple.

The simplest inverter installation is to connect the inverter to the batteries using heavy duty cables and then to run an ordinary (but long) power strip (or two) from the inverter to somewhere convenient inside the rig.

Rather than using the wall outlets in the rig, just plug the AC appliances into the power strip as needed, taking care not to operate too many things at once and overload the inverter.

Prices always change, so check the links to see the current prices.

The nice thing about this kit is that it is easily expandable. If a second or third solar panel is eventually desired (to double or triple the size of the system to 300 or 450 watts, for another $200 or $400), those panels can be purchased at a later date. At that point the solar charge controller can also be replaced with a bigger and more sophisticated charge controller (for $600).



Portable folding solar panel suitcase for RV and motorhome use

A portable solar power kit that folds up and can be carried like a suitcase is an awesome solution for weekenders, vacationers and seasonal RVers.

A really nifty alternative for anyone that isn’t super skilled with tools or that’s a bit spooked by electrical things, is a portable solar power kit that folds into a suitcase. These come with two matching solar panels, battery cables with alligator clips, and a panel-mounted solar charge controller. The solar panels are hinged together and can be folded towards each other. A handle on the side of one of them makes the whole thing easy to carry and store like a suitcase.

These portable folding suitcase solar panel kits come in all sizes. A good size is anywhere from 120 to 200 watts:

The advantage of a portable suitcase solar kit like this is that it is self-contained. If you think you might upgrade to a different RV soon, then there’s no loss in investment when one RV is sold and another is purchased. Also, if you decide to install a roof-mounted system at a later date, the suitcase solar panel kit can be sold to another RVer.

As for the inverter, heavy duty cables and power strip, they are included here just to round out the package so you have AC power in the rig as well as the ability to charge the batteries just like the “small solar power kit” described above.


Affordable solar power on a motorhome

Installing solar panels on tilting brackets is popular, but only necessary in mid-winter. We’ve never done it.

With a big RV solar power installation, it is likely that the RV’s house battery bank will need to be upgraded or replaced too, so this package includes a “replacement” AGM battery bank.

The Magnum inverter is an inverter/charger that has a built in transfer switch, making it very straight forward to wire the inverter into the house AC wiring system so you can use the standard wall outlets in the rig rather than plugging things into a power strip.

We’ve been living exclusively on solar power since we started this crazy traveling lifestyle in 2007, and this system is larger than any system we’ve ever had on a boat or trailer. So it ought to work just fine for anyone who wants to RV full-time and do a lot of boondocking.



If you are not a DIY RVer, you’ll need to budget for the installation labor too. As a very rough estimate, I would allow for $500-$1,000 for a small system installation and $1,500-$2,500 for a big system installation. The variations in labor costs will depend on how difficult it is to work in your rig, how hard it is to mount the various components and run the wires from roof to basement, and whether or not you choose to have the batteries upgraded or replaced.



RV park and campground prices are all over the map, but assuming that the average cost is $25 per night for a site with hookups if you don’t take advantage of monthly discounts or $15 per night if you do, these systems can pay for themselves in anywhere from 18 camping days to 14 months, depending on what size system you buy, whether or not you do the installation yourself, and how you typically camp. Of course, this assumes the rig is equipped with a refrigerator that can run on propane and that if air conditioning is needed an alternative power source like a generator is used.

As with everything in the RVing world, starting small and cheap is the best way to go.



Solar panel arch with solar panels on sailboat transom

Installing solar power on a sailboat has its own set of challenges.

We have installed three different RV solar power systems and one solar power system on a sailboat.

We published an article in the February 2017 issue of Cruising World Magazine (one of the top magazines in the sailing industry) describing the solar power system we installed on our sailboat Groovy back in 2010. This system gave us all the power we needed to “anchor out” in bays and coves away from electrical hookups in marinas for 750 nights during our cruise of Mexico.

Cruising World has posted the article online here:

Sunny Disposition – Adding Solar Power – Cruising World Magazine, February, 2017

Installing solar power on a sailboat is very similar to installing it in an RV, but there is an added complexity because there isn’t a big flat roof to lay the panels on. Instead, we had to construct a stainless steel arch to support the panels. Fortunately, our boat, a 2008 Hunter 44DS, had a factory installed stainless steel arch over the cockpit already. So, we hired a brilliant Mexican metal fabricator named Alejandro Ulloa, to create our solar panel arch in Ensenada, Mexico.

Solar power installation on sailboat Hunter 44

We turned to Alejandro Ulloa of Ensenada, Mexico, for our solar panel arch
He can be contracted the=rough Baja Naval.

Solar panel arch installation on Hunter 44 sailboat

Alejandro is an artist. He wrapped the arch in plastic to prevent scratches until it was permanently mounted on our boat!

Solar panel arch on sailboat Hunter 44

The arch went back to Alejandro’s workshop for tweaking after this measuring session.

Solar panel arch on sailboat Hunter 44 installed by Alejandro Ulloa

Dimensions now perfect, Alejandro mounts the arch permanently.

Getting the 185 watt 24 volt solar panels up onto the arch was a challenge. Getting solar panels up onto an RV roof is tricky too!

Affordable marine Solar panel installation on sailboat Hunter 44

Getting the solar panels onto the roof of an RV or up onto this arch takes two people (at least!)

Installing solar panels on an arch on sailboat (Hunter 44) with Alejandro Ulloa Baja Naval Ensenada Mexico

The second of the three panels gets installed.

The solar panel arch was going to double as a “dinghy davit” system with telescoping rods that extended out over the transom. These davits supported a pulley system to hoist the dinghy up out of the water. So once the solar panels were mounted on the arch, we had to be sure it could handle the weight of the dinghy.

Our dinghy weighed a lot less than the combined weight of Mark and Alejandro!

Strong solar panel arch and dinghy davit extension

Alejandro and Mark test the arch to be sure it can support the dinghy (which weighed half what they do).

The solar panels were wired in parallel because they would be subjected to shade constantly shifting on and off the panels at certain times of the day as the boat swung at anchor.

Wiring solar panels on a sailboat (Hunter 44) marine solar power installation

Mark wires up the panels in parallel.

Affordable solar panel installation on a sailboat


Solar panel arch with dinghy davit extension supporting affordable solar power on sailboat

A beautiful, clean installation with wire loom covering the exposed cabling and the rest snaked down inside the tubes of the Hunter arch. The davit extensions for hoisting the dinghy are clearly visible under the panels.

Solar panels installed on arch on Hunter 44 sailboat


Down below the cockpit inside a huge locker in the transom, Mark mounted a combiner box that brought three cables in from the three panels and then sent out one cable to the solar charge controller.

Emily and Mark Fagan aboard sailboat Groovy

The transom locker in our Hunter 44DS sailboat was very large!

Combiner box for solar panel parallel wiring on a sailboat

A combiner box brings the wires from the three panels together before a single run goes to the solar charge controller (this is optional and not at all necessary).

The solar charge controller was installed in the cabin inside a hanging locker in the master stateroom.

Xantrex solar charge controller installed in sailboat locker

We have an Outback FlexMax charge controller on our trailer but chose a Xantrex controller for our boat because there were no moving parts. We compare the two HERE.

The solar charge controller was located about 8 feet from the near end of the battery bank which spanned a ~14 foot distance under the floorboards in the bilge.

Two 4D AGM batteries in bilge of sailboat

We had four 160 amp-hour 4D AGM batteries for the house bank and a Group 27 AGM start battery installed under the floorboards in the bilge.
One 4D house battery and the Group 27 start battery are seen here

This 555 watt solar power system, which charged a 640 amp-hour house bank of 4D AGM batteries, supplied all of our electrical needs, including powering our under-counter electric refrigerator.

Usually our engine alternator provided backup battery charging whenever we ran the engine. However, at one point our alternator died, and we were without it for 10 straight weeks while we waited for a replacement alternator.

Why such a long wait for a simple replacement part? Getting boat parts in Mexico requires either paying exorbitant shipping fees and import taxes or waiting for a friend to bring the part with them in their backpack when they fly from the US to Mexico.

During that long wait our solar power system supplied all our electricity without a backup while we were anchored in a beautiful bay. Diesel engines don’t require an alternator to run, so we moved the boat around and went sailing etc., and lived our normal lives during our wait.

Solar panel arch and dinghy davit extension with solar panels installed on sailboat

View from the water — cool!

The dinghy davit extensions on the solar panel arch made it easy to raise and lower the dinghy from the water and also to raise and lower the 6 horsepower outboard engine.

Solar panel arch and dinghy davit extension on sailboat

A pulley system on the davit extensions made hoisting the outboard and dinghy a cinch for either of us to do singlehandedly.

Solar panel arch and solar panels on sailboat transom

For 7 months we left our boat at the dock in Chiapas, unplugged from shorepower, and let the solar panels keep the batteries topped off. Everyday during that time they put 19 amp-hours into the batteries which was essentially the power required to operate the solar charge controller!

At anchor, sometimes the solar panels were in full sun all day long if the current and wind and the pattern of the sun crossing the sky allowed the boat to move around without the sun coming forward of the beam of the boat.

However, whenever the sun was forward of the beam, the shadow of the mast and the radome fell on the panels. We could watch the current production from the panels go from full on, to two-thirds, to one-third and back again as the shadow crossed one panel and then two at once, and then one and then none, etc, as the boat swung back and forth at anchor.

Mast and radome cast shade on solar panels on sailboat

RV solar installations have to avoid shade from air conditions and open vent hatches.
On boats the shade from the mast and radome is often unavoidable.

Mast and radome cast shade on pair of sailboat solar panels

When the shadow fell across two 185 watt panels at once, it knocked both of them out of the system so only one of the three solar panels was actually producing power.

The coolest and most unexpected benefit of having our solar panels mounted on an arch over the cockpit was the shade that they provided. The sun in Mexico is very intense, especially out on the water, and it was wonderful to have two huge forward facing jump seats at the back of the cockpit that fully shade as we sailed!

Under the shade of solar panels and a solar panel arch on a sailboat

Made in the shade — What a life that was!!

We have more solar power related articles at these links:




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RV Solar Power Made Simple

Thanks to solar power, we have lived completely off the grid in two trailers and a sailboat full-time since May 2007. Without doubt, our solar power installations have given us more independence and freedom as full-time RVers and sailors than anything else in these lifestyles. It has allowed us to go anywhere at anytime, and has revolutionized our lives.

On this page I describe the two systems we have had on our trailers. These were installed in 2007 and 2008 respectively. Prices for solar power equipment have dropped every year since then, however the prices listed throughout this page are from August 2014:

  • A Small (minimal) RV Solar installation for ~$700 that we used full-time for a year of boondocking in 2007
  • A Full-timer (all you need) RV Solar installation for ~$2,500 that we have used for full-time boondocking since 2008

I also offer a little theory and reveal some of the discoveries we have made along the way. For more info, please see our Solar Power Tutorial pages and our Sailboat Solar Power Installation page.

Links to all of our articles about solar power can be found on our Solar Power For RVs and Boats page.

You can navigate to different parts of this article by using these links:


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The biggest advantage of a solar power system in an RV is that the system works from dawn to dusk, silently, odor free and without requiring any fuel or maintenance, no matter where you are or what you are doing. Towing, parked at the grocery store, or camped, the batteries are being charged. They start getting charged before you finish breakfast, keep charging while you hike or go sightseeing, and continue all day, rain or shine. They don’t quit charging until nightfall. You never have to think about the batteries getting charged. It just happens. In our current rig, I feel like we have electrical hookups all the time — and we never get hookups any more!

Traveling full-time since 2007, we have connected to electrical hookups for a total of about 25 days, and that was during our first 18 months on the road. The last time we got electrical hookups was in October, 2008. Since we began our full-time travels in 2007, as of June 2019, we have boondocked in our RV nearly 3,200 nights. We also lived on solar power on our sailboat for over 900 nights during our sailing cruise of Mexico.

We do carry a Yamaha 2400i generator, but use it only a few days each year, either after a long period of winter storms to give the batteries a boost, or on hot summer days to run our 15,000 BTU air conditioner. We have used it a total of about 20 times since we purchased it in December 2007. We run it every six months or so to flush the gas through the lines. Little as we have used it, we have found the Yamaha to be a fabulous generator. It has always started on the first pull, even after it sat in storage for 20 months when we first moved onto our sailboat!

Our first solar power installation that we used for a year in 2007 was a “small” system that allowed us to use almost every appliance we owned, that is, laptop, TV, hair dryer, vacuum, two-way radio charger, power drill, etc. However, we had to be very conservative with our electrical use during the winter months. A similar “small” RV solar power kit can be found here.

Our second “full-timer” solar power system that we have been using since 2008 is like having full electrical hookups wherever we go. Very little conservation is necessary! On our biggest electrical use day to date, we watched our 26″ LCD TV with its huge surround-sound system and sub-woofer for 15 hours (it was the Olympics!) and ran two 13″ laptops for 7 hours, made popcorn in the microwave and ran several lights for 4 hours in the evening. It was July, and the next day was very sunny and the batteries were fully charged by mid-afternoon. A similar “full-time” RV solar power kit can be found here.

Here are some sample kits, smaller and bigger in size, and their prices. The only trouble with buying a component kit is that if one component fails the whole kit has to be returned. The third item, however, is a portable suitcase kit that does make a lot of sense for someone who doesn’t want to hassle with the installation just yet (you can always sell the portable kit later).


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The basic components of all solar power installations is the same, and is comprised of two major subsystems: BATTERY CHARGING to get the batteries charged up and AC (120v) POWER for appliances that can’t be run on DC (12v) power (i.e., TV, computer, vacuum, hair dryer, etc.).

The BATTERY CHARGING subsystem includes these components:

  • Batteries
  • Solar panel(s)
  • Charge controller to protect the batteries from getting overcharged

The AC POWER subsystem includes this component:

  • Inverter(s) to convert the batteries’ 12 volt DC power to 120v AC power


300 watt inverter for an RV solar panel installation

A 350 watt portable inverter
Plug it into a cigarette lighter

It is hard to “play with” the battery charging subsystem of a solar power installation to get a feel for how it works until you actually take the leap and buy a solar panel, charge controller and cables and hook it all up to the batteries. One great option if you don’t want to do any wiring but want some hands on experience is to get a portable solar panel kit. You can sell it later if you want to upgrade to a rooftop system.

You can get the hang of how the AC power subsystem works very easily. Simply run down to Walmart or any auto parts store and pick up a $15-$20 inverter that plugs into a cigarette lighter DC outlet. Plug it into the lighter in your car, turn it on, and then plug your laptop into it or your electric razor or any other small appliance. Now your 12 volt car battery is operating your 120 volt appliance.

Big inverters that can run the microwave, toaster, blender and vacuum cleaner work on exactly the same principal, the difference is just the amount of power the inverter can produce. Big inverters are also wired directly to the batteries rather than plugging into a cigarette lighter.


The difference between the “small” system we used for one year on our little Lynx travel trailer and our “full-timer” system we have now on our big Hitchhiker fifth wheel is simply the overall capacity of each of the components. That is, the capacity of the battery charging system (solar panels, batteries and charge controller) and of the AC power system (the inverter).

In functional terms this means that the difference between the “small” and “full-timer” systems is threefold:

1) the ability to run more appliances at once (i.e., have two laptops running while the TV and blender are going)
2) the ability to run larger appliances (i.e., using a VitaMix versus a small blender)
3) the ability to run more appliances for a longer time at night without discharging the batteries too much.

So, in a nutshell, the two subsystems — battery charging (batteries + panels + charge controller) and AC power (inverter(s)) — combine to do the same job as plugging a generator into the shore power connector on the side of the rig. The panels and charge controller charge the batteries. The inverter makes it possible to use AC appliances.

The cost of the parts for these installations is:

Small: $700 – Comparable to having a Yamaha 1000i generator
Full-timer: $2,500 – Comparable to having a built-in Cummins Onan 2.5KW generator

With solar power there is no noise, no fuel cost, no maintenance and no smell, unlike a generator. However, it is not possible to run the air conditioning in the summertime on solar power, unless you have a massive system with several hundred pounds of batteries and a roof absolutely loaded with panels. As mentioned before, we use our Yamaha 2400i generator to run our 15,000 BTU air conditioner.



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This setup is a fully functional, inexpensive solar power installation, and is what we used for a 350 nights in our first year in our Lynx travel trailer. It could power a 19″ LCD TV and DVD player, radio, laptop and vacuum as well as charge camera batteries, razor, toothbrush, cordless drill, cell phone, etc.

  • Two 6-volt batteries (wired in series) giving 220 amp-hours of capacity $250

    Ours were Energizers from Sam’s Club

  • 140 watts of solar power $175

    Ours was a Kyocera 130 watt DC panel. Today Kyocera sells the 140 watt panel instead.

  • A charge controller that can support at least 10 amps $90

    Ours was a Morningstar Sunsaver 10 amp charge controller (consider a Sunsaver 20)

  • A portable inverter that can supply 1000 watts of AC power $80

    Ours was a Pro One 800 watt inverter

  • Cables, connectors and mounting brackets $100

Here are the parts for the system (except for batteries and cables) — solar panel, solar charge controller and inverter:

This system is the smallest size system I would consider for an RV if you want to drycamp or boondock for more than a night or two and be comfortable. This setup worked great in the spring, summer and fall when the sun was high in the sky and the days were long. We never thought too much about our power use until the wintertime when the days got short and the nights got long and cold. Then we began to wish for a bigger system.

RV solar panel installation - wiring the panel's junction box

Mark installs our first solar panel on the roof.
He chose a nice spot by the ocean to do it!

On those long cold winter nights we had to conserve our use of lights and the TV to make sure our furnace (which used a lot of battery power) could still run. We used oil lamps a lot in the evenings. If we had stayed in that trailer longer, we would have installed a vent-free propane heater that did not use any battery power (we eventually did that in our bigger trailer the following winter: see our Vent-Free Propane Heater Installation page).

I think every RV should have this kind of a charging system installed as standard equipment, as it is useful even for the most short-term camping, like weekends and week-long vacations during the summer months.

When we installed this “small” system in our little Lynx trailer in June 2007, we were quoted $135-$350 for installation. Mark is very handy (although he is not a Master Electrician), and he found the installation was not difficult at all and completed it in one day.


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Here is some theory to explain why the above system is “sufficient” but is not great for “full-time” use. When it comes to a solar battery charging system, the concept of power charging and consumption is very simple. The amount of power you can use, or take out of the batteries, is essentially only as much as the amount you can put into the batteries. If you use (or take out) more power than you replace (or charge them with), sooner or later your batteries will be discharged and dead. The batteries are just a temporary storage place for electricity. They act as a flow-through area for the power you are going to use.

The most important part of any solar setup is the amount of charging going on (i.e., the total size, or capacity, of the solar panels), and you want that to be greater than the amount of electricity you use. More must go into the batteries than comes out. You can have an infinite number of batteries and eventually discharge them all completely if you repeatedly use more electricity than your solar panels put in.

We often find people want to add batteries to address their power shortages when what they really need to do is add more solar panels. As a rule of thumb, don’t use more than 1/3 to 1/2 of the total capacity of the batteries in one night. More important, though, is that the bigger the solar power panel array, the better. And lastly, Keep the size and age of all the batteries in the system fairly similar so the strong ones don’t waste their energy helping the weak ones keep up.


Appliances use amps to run. Another unit, the amp-hour (abbreviated as “Ah“), refers to the number of amps an appliance uses when it is run for an hour. For instance, an appliance that uses three amps to run will use up three amp-hours when it runs for an hour. These amp-hours will be drawn from the batteries, and the batteries, in turn, will look to the solar panels to recharge the amp-hours they have forked over to the appliance. It is for this reason that you need to know how many amp-hours you will use in a typical day. Ultimately those amp-hours must be replaced by the solar panels, so the size and number of panels you purchase will be determined by how many amp-hours you use in a day.

To estimate how many amp-hours you might use in a day, estimate how many hours each appliance will run and multiply that by the number of amps the appliance uses. We have measured some of the appliances in our trailer, and this is how many amps they use:

Single bulb DC light — 1.5 amps
Dual bulb DC light — 3.0 amps
Dual bulb fluorescent light — 1.5 amps
19″ LCD TV — 5.5 amps
DVD / CD Player — 0.5 amps
13″ MacBook laptop, on & running — 6-8 amps
13″ MacBook, off and charging — 1.6 amps
Sonicare toothbrush charging — 0.1 amps
FM Radio w/ surround-sound — 3.0 amps
12′ string of rope lights — 3.3 amps

We find that we typically use anywhere from 50 to 150 amp-hours per day, most commonly in the 70-90 range.


Since RV solar power systems are DC battery based, it is helpful to know how many amps (in DC) various appliances use. Multiplying that value by the number of hours the appliance is used each day then reveals how many amp-hours the appliance will require from the battery in the course of a day.

Most DC appliances list their amp usage in the user manual or spec sheet. In contrast, most AC appliances list their wattage in the user manual instead of amperage. So, for AC appliances that are run on an inverter you have to do some math to get their equivalent DC amperage rating.

You can get a rough estimate of the number of amps that an AC device will use on an inverter simply by dividing the wattage by 10.

Why is that?

Here’s one way to look at it: Technically, Watts = Volts x Amps. AC circuits run at ~120 volts. DC circuits run at 12 volts. An AC appliance will use the same number of watts whether running on a DC or AC. On a DC circuit (using an inverter so it can run), that AC appliance will use 10 times as many amps as it will on an AC circuit (that is, 120/12 = 10).

Here’s another way to look at it: Watts / Volts = Amps. So, to determine most precisely how many DC amps an AC appliance will use when running on an inverter, start by dividing the number of watts it uses by 12 volts to get its Amps DC. HOWEVER, keep in mind that inverters are not 100% efficient. Typically they are only about 85% efficient. That is, an inverter loses a bunch of watts to heat as it runs — about 15% of the watts it needs to run get dissipated into heat. So, it takes more watts to get the required amps out of the inverter, the exact figure being 1 / 85%. This means that after you divide the appliance’s Watts by its Volts (Watts / 12, as I mentioned above), then you have to divide that result by 0.85. This is messy.

Rather than dividing watts first by 12 and then again by 0.85, you can simply divide the watts by 10 and get a pretty close estimate. (That is, (1/12)/0.85 = 0.1)

Our AC 19″ LCD TV is rated at 65 watts. How many amps is that DC? 65/10 = 6.5 amps DC. We measured the TV at the volume we like to hear it and it was using 5.5 amps. If we cranked up the volume, the meter went up to 6.5 amps.

Likewise, our old white MacBook Pro laptop was rated for 65 watts. As we opened and closed files and started and stopped various programs, the meter zoomed all over the place between 3 amps and 8 amps. When we ran Adobe Lightroom, which is very disk and memory intensive, the readings hovered in the 7-8 amp range. So on average you could say it uses about 6.5 amps DC.

When we shut down the laptop and left it plugged in and charging, the meter dropped to 1.6 amps. This is important if you are trying to conserve electricity! Run your laptop on its own battery until the battery is depleted. Then turn it off and let it charge from the inverter while you do something else!


If you have nothing running in the rig (no computers running, no TV, no vacuum or toaster, etc.), you can measure the current a device is drawing from the batteries using a clamp-on meter around one of the battery cables. To measure the AC current of a small device, you can use a Kill-a-Watt meter. Simply plug it into an AC outlet and plug your device into it.


Battery storage capacity is measured in amp-hours (Ah), and more is better. As a starting point, most new RVs come equipped with one 12-volt Group 24 battery which will give you about 70-85 Ah of capacity. Assuming the sun has charged the batteries completely by nightfall, and sticking to the rule of using only 1/3 of your total battery capacity each night, you will have only 25 Ah available each evening. That isn’t very much!

What is the best upgrade strategy?

Upgrading to two 12-volt Group 24 batteries (wired in parallel) will give you 140-170 Ah of capacity.

However, a 6-volt golf cart style battery has the same footprint as a Group 24 12-volt battery (although it is about 3″ taller), and a pair of them wired in series will give you about 210-240 Ah of capacity.

So, rather than buying a second 12-volt Group 24 batteries and getting just 140-170 Ah of capacity out of the pair, why not sell the 12 volt battery and buy to two 6-volt golf cart style batteries for 210-240 Ah of capacity? That’s what we did on our first trailer. Just make sure that you have enough height in the battery compartment for the taller golf cart batteries.


So far I’ve been talking about wet cell batteries, and these kinds of batteries need to be maintained. Wet cell batteries are made with thick metal plates and liquid between them. Over time the liquid evaporates and needs to be replaced with distilled water. Also, over time, sulphite builds up on the plates and needs to be removed by “equalizing” the batteries.

Hydrometer Reading on Battery

Use a hydrometer to check each battery cell.

Before we upgraded to AGM batteries, Once a month Mark would check the liquid levels in each cell of each battery and pours in a little distilled water wherever needed. He also checked the condition of each battery cell using a hydrometer. This little device indicates whether a cell is functioning at full capacity. Then he equalizes the batteries by programming our charge controller to raise the voltage on them to one volt higher than their normal charging voltage for five hours. Last of all, he re-checks the liquid level in each battery cell and adds distilled water as needed and re-checks each cell with the hydrometer. Usually any cells that had a poor reading before equalizing now give a good reading.

This maintenance stuff can be avoided by buying AGM batteries which are maintenance free. However, AGM batteries are really expensive. One big advantage of AGM batteries for sailors and for people with tight battery compartments is that they operate fine in any position, that is, they can be installed on their sides and will operate when a sailboat is heeling. We had them on our sailboat.

On our trailers, we initially opted for wet cell batteries. We had Trojan 105 wet cell batteries for the first five years on our fifth wheel. Then we replaced them with cheaper Costco batteries from Interstate (Johnson Controls).

The Trojans worked very well, but replacing them with cheapo batteries was a mistake. The cheap batteries failed completely within 14 months.

We now have four Trojan T-105 Reliant AGM batteries which are truly awesome. They are a little more money than the T-105 wet cell batteries, but they are superior and, in our minds, worth the extra little bit of cash.

For price comparisons: Trojan Reliant AGM (single), VMaxTanks AGM (set of 4 & free shipping), Trojan T105 Wet Cell (single):

To learn more about our new batteries, why we chose them, and how we upgraded the power plant on our trailer in April 2015, visit:

Wet Cell vs. AGM Batteries – Why We Upgraded to AGM Batteries PLUS Wiring Tips!
RV Electrical System Overhaul

To learn more about batteries and what “single-stage” and “multi-stage” battery charging is all about, visit:

RV and Marine Battery Charging Basics


Battery capacity is only part of the story. The ultimate limiting factor is how many amp-hours the solar panels can put into the batteries during the day. If the solar panels are sized too small to charge the batteries sufficiently each day, you will eventually discharge the batteries over a series of days and they will be dead.

Solar panels are rated in terms of Watts. The relationship between the amp-hours that the panel can store in a battery and the panel’s watts rating is not straight forward. Suffice it to say that a 130 Watt panel produces 7.5 amps in maximum sunlight when the panel is exactly perpendicular to the sun, and both of those numbers are available in the specs for the panel. What isn’t stated, however, is how many amp-hours a panel will produce in a given day. That is because it varies by what latitude you are at, what angle the sun is to the panel (which changes all day long), how brightly the sun shines, how many clouds go by, etc.

We have found that each of our 120 watt and 130 watt panels typically produces between about 8 Ah and 40 Ah per day depending on the season, weather, latitude, battery demands, etc. Most commonly, they produce around 25-30 Ah per day each.

If you have the time and inclination (who’s got that stuff?), you can figure out how many amp-hours you use each night. Make sure that that number is less than 1/3 of your total battery capacity AND make sure your panels can provide that many amp-hours of charging each day.

But all that sounds very difficult.

Solar panels also come in a variety of flavors, including rigid or flexible and monocrystalline or polycrystalline as seen below:

To learn more about SOLAR PANELS, see our detailed review of the pros and cons of the different types of panels available today:

Solar Panel Selection – Flexible or Rigid? 12 volt or 24 volt? Monocrystalline or Polycrystalline?


As I have mentioned before, we changed how we lived when we had a small solar power installation and again when we got a big one. You can opt to live with very little electricity or not.

We met a couple living on their 27′ sailboat on its trailer in the desert in Quartzsite, Arizona (they were on their way to launch it in the Sea of Cortez). They were using just 6 amp-hours per day because they had a tiny solar panel. Lord knows, I never saw their lights on at night!

In our little Lynx travel trailer we used about 25-35 amp-hours per day. We relied on kerosene lamps for much of our lighting at night in the winter.

In our Hitchhiker fifth wheel we use an average of 60-120 amp-hours per day and we do not conserve electricity.

So as a rule of thumb, here is the number of amp-hours you might consume per day:

• 6 Ah = living ultra-conservatively
• 35 Ah = living very modestly
• 120 Ah = living much the way you do in your house

The amp-hour capacity of your battery bank should be three (to four) times your typical daily amp-hour usage.

A popular rule of thumb is to match (roughly) the amp-hour capacity of the batteries to the watts capacity of the solar panels. So, 140 Ah of battery capacity “goes with” 140 watts of solar power. 440 Ah of batteries “goes with” 440 watts of solar power.

However, having more solar capacity than that is not a problem, as it gives you much more flexibility in case you have cloudy days, the panels aren’t oriented well towards the sun, or you have periodic shading during the day from buildings or trees.

Side note: The average American house uses about 30 kilowatt-hours (kWh) of electricity per day (see here), whereas we use anywhere from 0.6 to 1.2 kWh per day in our RV. This is because houses are much bigger and more complex and have much larger appliances and systems that run on electricity (refrigerator(s), stove/oven, hot water heater, heat and air conditioning, etc.).

It is also interesting to note that the ~1 kWh of power that our fifth wheel requires to recharge its batteries every day is approximately the same amount of energy that is required to recharge the batteries of a Tesla Model S after it is driven three miles (see here). Charging a frequently driven Tesla’s batteries exclusively with solar power would require an immense solar panel array.



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Fifth wheel trailer solar power 681

In a nutshell, in order to run your RV with the same level of comfort as a house, using all of your appliances whenever you feel like it without thinking about conserving at all, you will need at least the following:

  • Four or more 6-volt batteries giving you at least 440 amp-hours of capacity

    We have four 6-volt batteries (2 pairs of batteries in series to make two 12-volt equivalent batteries, and then those 2 twelve volt equivalent batteries placed in parallel with each other). We had Trojan 105’s for the first five years, and after that we’ve had batteries from Costco, ~$480, which we soon replaced with Trojan T-105 AGM Reliant batteries, $1,200 (see note below).

  • 500 or more watts of solar power (preferably 600-800 watts)

    We have three 120-watt Mitsubishi panels and one 130-watt Kyocera panel, for a total of 490 watts of solar power, `$1,140

  • A charge controller that can support 40 amps or more (preferably 60 or 80 amps)

    We have an Outback FlexMax 60 60 amp charge controller (consider the FlexMax 80) $565
    For more info see our page: Solar Charge Controllers – Optimizing RV Battery Charging

  • A true sine wave inverter that can supply at least 1000 watts of AC power (preferably 2000 or 3000 watts)

    For 7 years we had an Exceltech XP 1100 watt true sine wave inverter $600.

PLEASE NOTE: In April, 2015, we upgraded to Trojan 105 Reliant AGM batteries ($1,200) and an Exeltech XP 2000 watt true sine wave inverter ($1,700). See our post RV Electrical Power System Overhaul to learn more.

This system will power everything except the air conditioner, regardless of weather or season. My notes indicating “preferably” larger sizes for everything reflects the fact that our installation is now quite old and component parts costs are half what they were when we were buying. More is definitely better.

I’ve never heard anyone say they wished they had less solar power!

Mark did the installation of this solar power system on our Hitchhiker fifth wheel. My rough guess is that the installation might have cost $700-$1,500 if done by an experienced installer. It took him three partial days, largely because we were boondocked in the woods about 15 miles from Home Depot, and I had to keep running back and forth to get little things for him!


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More and more solar power equipment manufacturers are selling complete kits for RVs, boats and cabins. Here is an example full-timer kit from Go Power, and a slightly smaller full-timer system from Renogy. Here is a small solar power kit from Go Power for “weekender/vacation” use and another small solar kit using Renogy panels.

Also, if you don’t want the hassle of doing an installation, here’s a nifty portable solar panel kit that folds into an easy-to-carry suitcase!!



If you are like me, the terms “inverter” and “converter” are confusing. They sound so similar it seems they must be one and the same thing. They are actually two very different components with very different missions in an RV.


A converter takes the AC power coming in from the shore power cord (via electrical hookups or a generator) and gives power to all the DC appliances in the rig so the batteries can take a break. It essentially does what the batteries do, but does it only when there is shore power.

The DC converter in an RV also charges the batteries while connected to shore power. Some converters have sophisticated multi-stage charging mechanisms, and others simply provide a trickle charge.

For more about single-stage versus multi-stage charging, click here.

The DC converter is not involved in the solar power system. In our “full-time” solar setup, the DC converter is actually unplugged because our inverter powers all the AC outlets in the rig. Because of our converter’s design, when it is plugged in it senses when there is AC power available and automatically turns on. This would impose a huge demand on our batteries whenever we turned on the inverter.

Once in a while, when the skies have been overcast or stormy for a few days, we fire up our trusty Yamaha 2400i generator to bring the batteries up to full charge. We plug our shore power cord into the generator, unplug the inverter and plug in the converter. Now the converter is charging the batteries.

The converter that came with our rig was a single-stage trickle charge Atwood 55 amp converter. This was very inefficient for use with the generator because it charges at such a slow rate that we had to run the generator for hours and hours to get the batteries charged up.

In April, 2015, we replaced that converter with a slick new Iota DLS-90 / IQ4 converter. This converter can put as much as 90 amps into the batteries and has a true multi-stage charging algorithm. To see our introductory post about our big electrical system upgrade, see this post: RV Electrical Power System Overhaul

For more about converters, visit: RV Converters, Inverter/Chargers & Engine Alternator Battery Charging Systems

Almost all trailers and many smaller motorhomes have a converter installed at the factory.


An inverter takes the DC power from the batteries and converts it to AC power so you can run things like TVs, computers, vacuum cleaners, hair dryers, toasters, etc., and also charge things like your phone and camera batteries. Turn on the inverter, plug an AC appliance like an electric razor or TV into it, and poof, the razor or TV works.

Inverters come in two flavors:

True Sine Wave (or Pure Sine Wave) which means the AC power signal coming out of the inverter is identical to the power signal of a wall outlet in a house (a smooth sine wave).

Modified Sine Wave which means the waveform is clipped at the top and bottom and is stair-stepped in between rather than being a smooth sine wave.

It is easier to convert DC power to a square-type wave than a smooth sine wave, so modified sine wave inverters are much cheaper. However, some sensitive AC appliances don’t work with a modified sine wave inverter.

We purchased a high-end true sine wave inverter for our “full-time” solar setup, because it matched the quality of the system and our particular unit was noted for its ruggedness (we run it 15 hours a day, sometimes 24). Our Exeltech true sine-wave inverter is designed to operate medical equipment, so it provides exceptionally clean and stable AC power.

See our story “How Much Inverter Is Enough?” to learn about what happened to us when we accidentally “blew up” our fancy Exeltech true sine wave inverter and had to live on a tiny cheapo 350 watt modified sine wave inverter while waiting for the parts to fix it!

Ironically, some RV parks have unstable AC power that can damage AC appliances in an RV. Our inverter power from our Exeltech is cleaner and more reliable (Exeltech inverters are designed to power sensitive medical equipment)! Desktop computers, laser printers, TV and stereo equipment and Sonicare toothbrushes are the most likely appliances to have trouble with modified sine wave inverters. However, when we used modified sine wave inverters exclusively with our small solar power setup on our Lynx travel trailer, we never had a problem with any of our appliances. Modified sine wave inverters often have loud fans, and Mark did have to put some WD40 on our Radio Shack inverter twice when the fan quit working unexpectedly.


To add to the confusion about inverters and converters, some inverters combine a little of the functionality of both an inverter and a converter. These are called inverter/chargers and have two independent functions: (1) convert the batteries’ DC power to AC (inverter), and (2) use the AC power from the shore power cord (connected to electrical hookups or generator) and charge the batteries.

These are pricey pieces of equipment and many higher end motorhomes come with them. Our sailboat came with both a 600 watt pure sine wave inverter (which we used for everything on the boat except the microwave) and a 2500 watt modified sine wave inverter/charger (which powered the microwave and charged the batteries when we plugged into shore power).


The distinction between inverters and converters is pretty easy, isn’t it? However, recently when I was in an auto parts store I noticed a box labeled “POWER CONVERTER” and the picture and description were very clearly that of an INVERTER! So, maybe the distinction is going to get all muddied up after all.

For more about inverter/chargers, visit: RV Converters, Inverter/Chargers & Engine Alternator Battery Charging Systems


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Because conventional propane RV refrigerators are inefficient and are (shockingly) expected to fail within ten years of service (see our blog post about that here), the current trend in full-time RVs is to manufacture them with residential AC refrigerators. These RVs are built with an inverter large enough to power the refrigerator while the RV is in transit. This is great for folks that are going to plug into electrical hookups 100% of the time. However, the electricity required to run a refrigerator, whether AC or DC, and no matter how Energy Star Efficient it is rated to be, is astronomical.

A typical 10 to 12 cubic foot Energy Star refrigerator will use over 300 kilowatts per year, or 822 watts per day. There is some energy lost when running on an inverter, so this will be roughly 822 Watts / 10 Volts = 82 amp-hours per day. To keep this fridge operating during the short days of winter when the sun is low in the sky, you will need 400+ watts of solar panels and 200+ amp-hours of battery capacity in addition to whatever you will need to run the rest of the household.

If you plan to boondock a lot, and you don’t want to run your generator 24/7, be prepared to outfit your rig with over 1,000 watts of solar panels and close to 1,000 amp-hours of battery capacity to power a residential refrigerator.

Non-Energy Star compliant DC electric refrigerators are even worse. Our sailboat had a 3.5 cubic foot DC refrigerator (“counter height” or “dorm size”) that was built for RV use. It did not have a freezer compartment. We had 710 amp-hours of AGM batteries and 555 watts of solar power. Granted, we were living in the tropics and the ambient cabin temperature was generally 85 degrees. The refrigerator compressor ran about 50% of the time and our solar power system was pushed to the max to keep the batteries topped off every day.

We had a separate standalone 2.5 cubic foot DC freezer on our sailboat. If we turned the freezer on, the solar panels could not keep the batteries charged without supplemental charging from the engine alternator every third or fourth day.

Residential refrigerators have vastly improved in recent years, running on a mere 25% of the electricity they used to use in 1986, and they are only getting better. For more information about refrigerator energy use and energy saving tips, see this resource: How Much Electricity Does My Refrigerator Use?

I have corresponded at length with a reader who has been boondocking 95% of the time for 6 months in a 40′ Tiffin Phaeton motorhome. He has a Whirlpool 22 cubic foot residential refrigerator, 1,140 watts of solar panels on his roof and 940 amp-hours of battery capacity in his basement. His fridge is powered with a dedicated Xantrex pure sine wave 2,000 watt inverter that is wired through a transfer switch to both his shorepower line and his generator, just in case the inverter fails (he had a 1,500 watt modified sine wave inverter that literally burnt up and started smoking).

So it can be done, but it will be easier in a motorhome that has a big payload capacity than in a fifth wheel or travel trailer that has a smaller payload capacity due to the weight of the batteries required. Even though we had to replace our RV refrigerator in its 8th year of service, we do not want double our battery bank and solar panel array just to power a residential fridge. I would rather put that extra 275 lbs into other things we need in our mobile lifestyle.


Still confused about the components and operation of an RV solar power system? See our four part RV SOLAR POWER TUTORIAL series where these concepts are re-introduced and discussed in greater detail:

Learn more about the different kinds of solar panels on the market:

Solar Panel Selection – Flexible vs. Rigid, 12 volt vs. 24 volt, Monocrystalline vs. Polycrystalline – PLUS Wiring Tips!!

Get the quick-and-dirty shopping list of things to buy for your solar power installation:

Three RV Solar Power Solutions: Small, Portable, and Big!

Want to learn more about BATTERIES and understand how battery charging works at a deeper level? Our Intro to Battery Types and our four-part tutorial series covers all the details involved in charging RV and marine batteries and takes a close look at a variety of specific charging systems, from converters to inverter/chargers to engine alternators to solar charge controllers. It also reveals how these systems work together:

Curious about the solar power installation we did on our sailboat? See our page: SAILBOAT SOLAR POWER INSTALLATION.

In April, 2015, we overhauled our electrical power plant on our trailer. See the introductory post about this upgrade here:

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Where do you buy solar panels, charge controllers, inverters and such? Surprisingly, Amazon offers solar power kits and more. Click the following links for a wider selection of:

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We offer all the information on our website free of charge in hopes of helping help our fellow RVers and cruisers. We have been alarmed and saddened to find portions of the copyrighted material on this page plagiarized in ebooks that are sold for profit, but so it goes. Imitation is the sincerest form of flattery. Good luck with your solar power installation — we hope our articles on this website are useful to you!

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RV Solar Panels – Flexible or Rigid? 12 or 24 volt? Mono or Poly? Yikes!

There are a lot of decisions to make when you install solar panels on an RV or boat. Some of the most basic are: what size solar panels to buy, whether to go with flexible solar panels or aluminum framed rigid panels, whether the solar cells should be monocrystalline or polycrystalline, and whether to install nominal 12 volt or 24 volt panels.

We have done several RV and marine solar panel installations, and we have used not only 12 volt and 24 volt panels of various sizes but we have also used both aluminum framed rigid solar panels and the newer semi-flexible solar panels. We have also worked with both monocrystalline and polycrystalline solar panels. This article outlines the pros and cons of the various types and sizes of solar panels and offers some things to think about when you are deciding which solar panels to buy for your RV or boat.

RV solar panel selection


Our article RV Solar Power Made Simple explains how to determine the overall wattage for an RV solar power installation. In general, a weekend / vacation RV can get by with 200 watts or less while a full-time solar power system is best with 500 watts or more.


Once you decide on overall capacity for your solar panel array, the next thing to think about is solar panel placement and wiring. The panels should be a matched set of identical or nearly identical panels. If you have a lot of real estate on the RV roof, then you can get a few big panels. If you have a truck camper or your RV roof is cluttered with a lot of things on it already (hatches, vents, antennas, etc.), then you may need to go with smaller panels that can be squeezed in and around everything else.

Solar panel installation on a ffith wheel RV

Our fifth wheel trailer is powered by four 120 & 130 watt 12 volt rigid polycrystalline solar panels wired in series


Solar panels are constructed internally with DC wiring, and they are sized to work on 12 or 24 volt circuits. So, they are commonly referred to as 12 or 24 volt solar panels. What’s confusing is that while the nominal voltage of a solar panel may be 12 or 24 volts, the open circuit voltage is higher. So, for a nominal 12 volt solar panel that is 100 watts, the open circuit voltage (“Voc“) will be 17 or 18 volts. Likewise, for a nominal 24 volt panel, the Voc will be 34 to 36 volts.

Also, smaller solar panels (both physically and in terms of watts) are typically nominal 12 volt panels while larger panels are typically 24 volts. Solar panels under about 150 watts in size are usually 12 volt panels. Solar panels over about 150 watts are usually 24 volt panels.

Solar panel installation on a sailboat

For nearly four years, we sailed our boat on Mexico’s coast relying on three 185 watt 24 volt
polycrystalline rigid solar panels, wired in parallel, for all our electrical needs.

Solar panels work best when they are a matched set. The electrical characteristics of all the solar panels in the array need to be very similar, preferably identical. When upgrading a solar power array this can make things complicated as you try to mix and match old small panels with new big ones.

One technique for upgrading is to wire two 12 volt solar panels in series to work on a 24 volt circuit. For instance, if you have two 100 watt 12 volt panels and you are buying a 200 watt 24 volt panel, you can wire the two 100 watt panels in series and then wire that pair in parallel with the new 200 watt solar panel.

This will work as long as the electrical characteristics of the pair of solar panels in series match the electrical characteristics of the single panel that is wired in parallel with them.


Shade is the biggest enemy of any solar power installation. Unbelievable as it seems, a tiny bit of shade will effectively shut down a solar panel. The impact is dramatic: a few square inches of shade can drop a solar panels current production down from 8 amps to 2 amps. A few more square inches of shade can drop the current production to 0.

Before deciding on the size of the panels, it is worthwhile to take some time to study the various things that might cast shade across them once they are in place. A closed hatch may cause little shade, but when it is open on a hot day, depending on where the sun is in the sky, it might cast a big shadow across a nearby solar panel. Satellite dishes, air conditioners and even holding tank vents can cast sizable shadows as well.

We put a book in one corner of a 120 watt 12 volt panel and discovered that even though it was a small percentage of the surface area of the panel, that 8.5″ x 11″ book was enough to knock down the current production of a 120 watt solar panel by 80%. Rather than producing 7 amps, it produced a measly 1.4 amps. Egads!

Shade on one corner of solar panel

Just 8.5″ x 11″ of shade from this book reduced current production by 80%!

Similarly, shade wreaked havoc on our three185 watt 24 volt panels on our sailboat. The shade from our mast traveled across the panels as the boat swung at anchor, and the current production dropped by 1/3 and then by 2/3 as the shade first crossed one of the three panels and then straddled two of them. It did this over and over, with the current rising and falling repeatedly, as the boat slowly swung back and forth at anchor.

Effect of shade on solar panels installed on sailboat

A line of shade from the mast on our sailboat reduced our solar panel array to 65% and then 35% of its capacity as it traveled across the panels and occasionally straddled two of them.

Shade is a huge concern in the solar power industry, and there are several white papers (here’e one) about the impact of shade on commercial solar panel installations. The gist is the importance of spacing the rows of commercial solar panel arrays in such a way that one row of panels doesn’t accidentally shade the bottom inch or so of the next row behind it when the sun is low in the sky.

If it does, the second row of panels shuts down. If there are rows and rows of solar panels spaced like this, none of the panels except the ones in the first row can function until the sun rises a little higher in the sky.

Solar panels are most sensitive to shade along the longest part of the panel, so in the case of our sailboat, when the sun was over our bow, the mast would shade the panels in a strip that had a maximum impact on current production (as you can see in the above photo)!

For RVers, besides rooftop obstructions, shade comes into play primarily if you park near a building or trees. Snowbirds boondocking in the southwest deserts of Arizona and California during the wintertime have little concern with shade from trees and buildings. But summertime RV travelers who boondock in wooded areas need to be cognizant of where the shade from the trees will fall during the course of the day.


One of the big decisions for a solar power installation on an RV or boat is whether to wire the solar panels in series or in parallel. There are several things to consider when making this decision.

When the solar panels are wired in series, then the developed voltage across all the panels is additive while the current remains constant from panel to panel. That is, if there were four 120 watt 12 volt panels producing 7 amps each, then the developed voltage across all the panels would be 48 volts (12 x 4) while the current would be just 7 amps.

In contrast, when the solar panels are wired in parallel, then the voltage of the panels remains constant through the circuit while the current is additive from panel to panel. For instance, for those same four panels, the developed voltage across them would be 12 volts but the current would be 28 amps (7 x 4).

The solar charge controller takes care of balancing everything out by ensuring the circuit between it and the batteries is 12 volts. In the case of the above solar panels wired in series, the solar charge controller steps down the voltage from 48 volts to 12 volts (if they are 12 volt batteries). The current then increases from 7 amps to 28 amps in the wire run going between the solar charge controller and the batteries.

In the case of the above solar panels wired in parallel, the voltage is already 12 volts, so the solar charge controller does not need to step it down for the batteries.


When solar panels are wired in series, if shade hits one panel and shuts it down (caused by that solar panel’s internal circuitry building up a massive amount of resistance), then the entire string of solar panels shuts down. For instance, if a tree shaded 1/3 of one solar panel in the string of four panels given above, wired in series, the current production of the entire array of four panels would be reduced to to 0 amps, even though the three other solar panels were in full sun.

In contrast, if the panels are wired in parallel, when shade knocks one panel out, the other panels are unaffected. So, even if 1/3 of one solar panel were shaded, reducing it to 0 amps of current production, the other three would be working just fine. The total current production would be 3/4 of what it could be if that one panel were in full sun (in this case, 21 amps), rather than 0 amps.

So, it would seem that the best way to wire solar panels is in parallel.

Unfortunately, it’s not that easy, and here’s why:


The more amps of current there are flowing in a circuit, the thicker the wire needs to be to ensure that no energy is lost to heat. Unfortunately, thicker, heavier gauge wire is a pain to work with. It’s stiff and doesn’t bend around corners easily. It is hard to tighten down in the solar charge controller connections and it’s hard to crimp ring terminals onto. It is also more expensive per foot.

So, when the solar panels are wired in series, a thinner gauge wire can be used for a given distance than when they are wired in parallel.

Of course, the thickness of the wire is also dependent on the length of the wire. The longer a wire is, the more energy is lost along its length. So, if you are installing the solar panels high on an arch off the aft end of a 50′ sailboat and the batteries are located at the bottom of the hull over the keel, the wire must be a lot heavier gauge than if you are installing the panels on an RV roof directly above the battery compartment.

What is the price difference in the cable? We like to use Ancor Marine Cable because it is tinned and it is very supple (the copper is fine stranded). Here are the price differences for 25′ of 2 gauge wire as compared to 25′ of 10 gauge wire.

Ultimately, there is a dilemma: Is it better to go for thinner, cheaper wire and an easier installation, and wire the panels in series, risking that the whole array will shut down whenever a corner of one panel is shaded by a nearby tree? Or is it better to pay the extra bucks for heavier gauge wire and endure a more challenging installation but have a system that will be more tolerant of partial shade?

What to do?


Luckily, there is another option: higher voltage solar panels can be wired with thinner gauge wire. Remember, Watts = Current x Voltage. So, for the same number of watts in a panel, a higher voltage panel will produce a smaller amount of current.

Rather than using four 120 watt 12 volt panels wired in parallel that would produce 28 amps at 12 volts, you can use two 240 watt 24 volt panels wired in parallel that produce 14 amps at 24 volts. The net effect on the battery bank will be the same, but the bigger panels can be wired with smaller gauge wire.

As mentioned above, the wiring that is most affected by these solar panel choices is the wiring that runs from the solar panels to the solar charge controller. The wiring from the solar charge controller to the batteries is the same in either configuration, as the same amount of current will be flowing in that wire regardless of how the solar panels are wired. In the case of solar panels wired in parallel, the voltage will be stepped down in the solar charge controller. So, in our example, the solar charge controller will step down the voltage from 48 volts to 12 volts, ensuring that the circuitry between the solar charge controller and the batteries is at operating at 12 volts.


The thickness of the wire, or wire gauge, depends entirely on how long the wire is going to be. That is, the wire gauge is determined by how far apart the solar panels and the solar charge controller and the batteries are.

Why is this? The more current that flows in a wire, the more the conductor in the wire will warm up. The more it warms up, the more energy is lost to heat. Eventually, this becomes measurable as a voltage loss between the two end points.

When wiring solar power circuits, you can choose how much voltage loss you are willing to have. Somewhere between 2.5% and 5% is typically considered okay. There are voltage loss tables that will help you decide on the proper wire gauge size for the distance you are spanning between the solar panels and the solar charge controller and between there and the batteries. Here’s a good one:

AWG Voltage Loss Table

An Example: 480 watts of solar power located 27′ from the batteries

  • Say we have four 120 watt 12 volt panels wired in series. If the distance is going to be 27′, then by looking at the third chart at the above link (the 12 volt chart) and going to the line for 8 amps flowing in the wire, it shows a wire run of up to 27′ can be done with 10 gauge wire.
  • Now, imagine putting those same panels in parallel. 32 amps will flow at 12 volts. For that same 27′ distance you’ll need 2 gauge wire.
  • Lastly, instead of using four 120 watt 12 volt panels, use two 240 watt 24 volt panels wired in parallel. For this you use the 2nd chart down (24 volt chart). There will be 16 amps flowing in the wire at 24 volts. You will be able use 8 gauge wire.

Of course, due to the nature of multi-stage battery charging and the changing position (and angle) of the sun in the sky, the solar panels will be operating at full tilt for a very short time each day. They may produce max current for 30 minutes near noon as they wrap up the Bulk Stage, however, as the Absorb stage takes over and continues in the afternoon, the solar charge controller will gradually hold the panels back so they produce far less than max current.

With less than peak current flowing in the wires, less energy will be lost to heat.

If this is confusing, see our articles:
RV and Marine Battery Charging Basics
How Solar Charge Controllers Work

So, although it may seem dire that you’re wiring is on the hairy edge size-wise, it is only that way for a little while each day. Depending on the overall size of the solar power array, the size of the battery bank, and the state of discharge when the batteries wake up in the morning, your system may not even hit the theoretical maximum current production or even come close.


Another method of keeping the wire size down is to install more than one big solar charge controller. For instance, you might install several smaller charge controllers for each pair of panels wired in series, or perhaps even one for each panel. Of course, this adds complexity and expense, and you will probably buy less sophisticated solar charge controllers that have fewer programming options than a single big one.

You must run more wires between the RV roof and the location in the coach where the solar charge controllers are installed (preferably next to the batteries), and so you must not only pay for additional solar charge controllers, but you must buy more wire and install it all. However, this design option does deserve mention and consideration.


Solar panels perform a whole lot better in the summer than in the winter. This is because the sun rides much higher in the sky and its rays hit the panels at a nearly perpendicular angle in the summertime. The days are also a whole lot longer. In the winter, the sun’s rays hit the panels at an angle and the sun is only out for a short while.

Solar panels on a fifth wheel RV roof

Tilting solar panels in winter can improve current production by 30%
Or…install more panels and save yourself from climbing up and down the RV ladder!

To get around this, rather than using ordinary Z-brackets to mount their solar panels on the roof, many RVers use tilting brackets. By tilting the panels towards the sun at about a 45 degree angle (technically, at the angle of your latitude), then the sun’s rays hit the panels at a nice 90 degree angle if they are oriented to face south. This can increase the overall power production by about 30% on a sunny winter day.

The only problem is that you have to climb up on the roof to tilt the panels each time you set up camp and then climb up again later to lay them flat when you are packing up before you drive away. We’ve seen many a winter snowbird driving their RV around with the solar panels still raised.

An alternative is simply to install more solar panels and to keep them lying flat all the time. This is easy for a big RV that has a huge roof but is not so easy for a little trailer with a small roof. We have not installed tilting brackets on our trailers.


Monocrystalline solar panel

solar panel

There are lots of different kinds of solar panels on the market today. There are two primary types of solar cells used in the manufacture of solar panels: monocrystalline and polycrystalline.

Monocrystalline solar panels are more efficient and more expensive, but they are also extremely intolerant of shade. Polycrystalline panels are slightly less efficient and less expensive, but they handle partial shade just a smidge better.

The way to tell if a solar panel is monocrystalline or polycrystalline is to look at the pattern of rectangles on the panel itself.

If the circuitry between the rectangles has large silver diamond shapes, it is monocrystalline. If the pattern of rectangles is just intersecting lines, it is polycrystalline.

Polycrystalline solar panel

solar panel

Examples of popular monocrystalline solar panels are here:

Examples of popular polycrystalline solar panels are here:


Flexible solar panel

Flexible solar panel

Solar panels can also be rigid or flexible.

Rigid panels are built with an aluminum frame surrounding tempered glass that covers the solar cells.

Flexible solar panels are built with the solar collecting material impregnated into a thin mylar film that is affixed to an aluminum substrate.

Flexible solar panels are not flimsy, they are simply bendable up to about 30 degrees.


There are a number of manufacturers selling flexible solar panels:


Flexible solar panels have several advantages over rigid panels. They are a little lighter than framed solar panels and you can glue them onto an RV roof using Dicor Lap Sealant, or something similar. This saves you from the complexity of drilling holes into a perfectly watertight roof and risking creating leaks. This is especially helpful with a fiberglass roof. It takes just a few minutes with a caulk gun to attach these panels to the RV roof.

Another nice feature is that on a rounded roof, like an Airstream travel trailer or Casita travel trailer, the panels can bend to follow the contour of the roof.

Installing solar panels on a motorhome roof

Mark uses Dicor Lap Sealant to affix flexible solar panels to a friend’s fiberglass roof.

One of the most important things for solar panels to work well is heat dissipation. Rigid aluminum framed solar panels stand up off the roof of the RV by about an inch, allowing air to flow underneath and for heat to dissipate. Air can’t flow underneath flexible solar panels. The aluminum substrate serves to dissipate the heat instead. This may or may not be as efficient a method of heat dissipation, and I have heard of a case where all the flexible solar panels on a sailboat had to be replaced after two years because they did not dissipate the heat sufficiently in the tropics and the panels self-destructed.


Our RVing friends Dick & Katie asked us to install six 100 watt 12 volt flexible solar panels made by Eco-Worthy on the roof of their motorhome, along with all the other projects involved in an RV solar power installation. Ironically, the hardest part of the entire installation was removing the plastic protection from the aluminum substrate of each panel (it kept ripping!). We all ended up working on this together.

Flexible solar panel installation on an RV

We had a tough time getting the plastic off the back of the Eco-Worthy flexible solar panels

Removing plastic from flexible solar panel

With all of us working together, we got the job done!

Once we got up on the roof, and got past a cute warning from Winnebego, the installation was straight forward.

Warning slippery roof on RV

Hmmm…I wonder what sage advice the manual suggests for this problem?

Solar panel installation on a motorhome RV

Flexible solar panels are lighter than their rigid counterparts

The plastic protection needed to be removed from the face of the panels as well. Interestingly, at one point Mark found himself picking at the corner of the mylar that has the solar collection circuitry embedded in it and almost began to peel that whole layer off the aluminum substrate! But once he got a hold of just the most superficial layer of plastic, it came off easily.

Removing plastic from flexible solar panel

Mark removes the plastic from the face of the flexible solar panels

He used Dicor Lap Sealant to tack down the corners of the panels and then ran a bead around each side of each panel.


Flexible solar panels are less efficient than rigid aluminum framed solar panels, which means you may want to get a few more total watts of solar panels than you would if you were buying framed panels. Bendable panels also can’t be installed on tilting brackets. So, again, buying more total watts may be the best solution.

Flexible solar panels are not as rugged as rigid aluminum framed solar panels built with tempered glass. Overhanging branches can scratch them. This is important for anyone that will be boondocking or dry camping a lot on public lands and in rustic public campgrounds, as it is often impossible to get in or out of a site without ducking under some low lying tree branches.

Some RVers have found that flexible solar panels installed on flat motorhome roofs tend to pool water when it rains. This can lead to debris building up and taking root and damaging the panels.

Perhaps for all these reasons, flexible solar panels are sold with a much shorter warranty than rigid solar panels. Whereas many solar panel manufacturers warranty their tempered glass aluminum framed rigid solar panels for 25 or 30 years, bendable solar panel manufacturers generally warranty their panels for 5 years or less.

This may or may not be relevant for RVers, as the fine print in almost every solar panel manufacturer’s warranty states that their solar panels are not warrantied for use on mobile vehicles.

Also, there has been a huge problem across the entire solar power industry with rigid solar panels failing prematurely in large numbers in big commercial installations (see a May 2013 NY Times article here). Apparently, just because those lovely rigid solar panels are warrantied for decades doesn’t mean they will last that long. We have already had a failure of one of our framed solar panels that was warrantied for 25 years, and we discovered the manufacturer’s warranty did not apply to RV installations.

However, as a general rule, when manufacturers warranty a product for 5 years versus 30 years, it says something about how they think their product will hold up over time.

Flexible solar panels installed on a motorhome RV roof

Nice job! (but don’t fall off that roof!)


There are many ways to go about installing solar power on an RV roof, and the solar panels that work best in one installation may not be the same as those that are best for another. Not only is every RV roof different, but every RVer’s needs are different.

If you have loads of space on a big motorhome roof or fifth wheel trailer roof, and you are setting it up for full-time use, you may be best off with three or four 200+ watt 24 volt rigid solar panels wired in parallel. If you have a little tear drop camper you use on weekends and store in the garage, a single flexible 100 watt 12 volt panel may be just the ticket for you.

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Related articles about Solar Power on RVs and Boats:

Related articles about Batteries and Battery Charging:

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RVers Jason and Nikki Wynn have written about the condition of their flexible solar panels after a year of use HERE

Solar Charge Controllers – Optimizing RV Battery Charging

The solar charge controller is the heart of any solar power installation on an RV or boat. It is the gatekeeper between the solar panels and the batteries, and it determines how much of the sun’s energy that is available to the solar panels will actually be converted into electrical current to charge the batteries.

Because solar power is a “set it and forget it” type of system, it is not “mission critical” to understand the inner workings of these complex pieces of gear. However, if you want to get the most out of your solar panels, you may want to fine tune your system to increase its battery charging capacity by programming the solar charge controller for optimal performance.

This page gives the low-down on how solar charge controllers work, presents ideas for how to size them, and explains what the typical input parameters are and how they affect performance. It then explores three specific charge controllers made by three different manufacturers, and compares the unique ways that each manufacturer has tackled the challenge of multi-stage charging via the sun.

Since we started traveling full-time in 2007, as of June 2019, we have used, worked with and lived with these particular units for over 4,000 nights of living off the grid in our RV and sailboat.

1200 Solar Charge Controllers and RV Battery Charging

An in depth look at solar charge controllers

This is the third part of our 4-part series on RV and marine battery charging systems.

So far in this series, we have reviewed the basic concepts involved in charging RV and marine batteries, including an in-depth review of multi-stage charging, and we also have looked at how “artificially powered” charging systems like converters, inverter/chargers and engine alternators go about the process of battery charging. The other parts in this series are:

This is a long post and you can read it in stages and navigate to the different sections by clicking on the links below:



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Solar charge controllers are a lot more complex than all of the charging systems described so far in this series (converters, inverter/chargers and engine alternators), and they offer a lot more flexibility for programming too, usually through a menu driven screen interface. What makes these systems so complicated?

— The sun not a consistent power source like the local power plant or an engine

“Artificially powered” chargers like converters, inverter/chargers and engine alternators have unlimited power backing them, either from electricity at a power plant or an engine. This allows them to perform optimally no matter what the circumstances are. In contrast, solar charge controllers are dealing with a very flaky power source.

The sun — flaky? Yes! The energy available from the sun varies all day long. At noon when the sun is high in the sky there’s a lot more energy available than in the morning and evening when it is low. The sun also gets covered by clouds now and then, and sometimes it goes away all together or never comes out all day.

Storm clouds swirl above our RV

The solar panels COULD be working, but…

In summertime, the days are long and the sun is out for many hours. In winter, the days are short and the sun is out very little (if at all — think Alaska). And every night all year long the sun vanishes for hours. Trees and buildings can cast shadows on solar panels, affecting their ability to generate current. For boats at anchor, sometimes the mast or boom will shade the solar panels every few minutes as the boat swings back and forth, making the current coming in from the panels rise and fall repeatedly.

— Solar panels can’t always do the job at hand

The batteries on an RV or boat are charged by the sun as long as it is light, regardless of what kinds of electrical appliances you are running inside. Sometimes there’s enough extra energy from the sun that the panels can do two jobs: charge the batteries AND support things like hair dryers and microwaves. But at certain times of the day, the solar panels may not be able to produce enough current to power those appliances AND charge the batteries at the same time by holding them at their target Absorb or Float voltage.

Solar power is difficult when cloudy

The solar charge controller keeps busy as the sun comes and goes

The net effect may be that the batteries are actually be being discharged while those loads are running, even though the solar panels are actively charging them. Sure, the sun mitigates the discharge rate, but overall the batteries are giving up more current than they are receiving from the solar panels. This temporary period of discharging means the solar charge controller will need to keep the batteries in the charging state a little longer to make up for the lost charging time.

— Solar charge controllers operate 24/7

Another difference between artificially powered and naturally powered charging systems is that solar charge controllers do not get turned on and off or plugged in and unplugged. Solar charge controllers operate 24/7, and they are busy communicating with the solar panels all the time to see how the sun is affecting them. At night, solar charge controllers stop talking to the panels quite so frequently since they know the sun won’t shine again for many hours. They “sleep” for a few hours, waking up periodically to see if the sun has risen yet.

Because there is no on/off switch, there isn’t necessarily an easy way for a solar charge controller to be forced into Bulk mode other than by virtue of the “wake-up” phase first thing in the morning. If, for instance, you want to force a solar charge controller into the Bulk stage at 2:00 in the afternoon, you may or may not be able to, depending on the unit.

— No two solar charge controllers are alike

Each solar charge controller manufacturer has a different way of dealing with the inconsistencies of solar power production. Some are easy to program and some are more difficult. Some have many adjustable input parameters and some have just a few. Some can be forced to start a Bulk charge at any time, and some can’t.




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Generally, a solar charge controller wakes up and immediately puts the batteries in the Bulk stage. Sounds great! However, the Bulk stage in low light may mean the batteries are getting just a trickle charge of an amp or two, because the solar panels can’t produce any more than that.

This means that frequently, for much of the morning, even though the solar charge controller is in Bulk and you’d expect the batteries to be getting blasted with current (which would be happening with an “artificially powered” charging system), the batteries are actually getting just a few anemic amps while the sun is slowly rising in the sky.

Depending on their state of charge at dawn and the size of the solar panel array, this trickle charge might actually be enough for the batteries to reach the Bulk voltage sometime before lunch. They will then switch out of the Bulk stage and into the Absorb stage before the sun has actually reached its peak in the sky where it can produce max energy.

Isn’t it ironic that by the time the solar panels are able to operate at full power, the batteries may not need it any more?!

However, having the batteries out of Bulk and into the Absorb stage during the hours that the sun is highest in the sky is actually optimal. The current delivered by the solar charge controller can slowly taper off as the sun falls lower during the afternoon. Once the Absorb stage is done, and the solar charge controller is operating in the Float stage, the low angle of the sun and the panels’ reduced ability to produce current is not a problem because the charge controller now wants to deliver less to the batteries anyway.

All this is great for sunny days… but not everyday is sunny!

On the other hand, it may be a cloudy morning until noon, or the RV may be in the shade of a mountain until noon, so by the time lunch rolls around, the batteries are still just as discharged as they were at breakfast — or even more discharged because you spent the morning playing on the computer or watching TV.

Lots of solar panels

Lots of solar panels

Now, when the sun comes out or the mountain’s shadow moves off the RV’s panels, the solar charge controller is still in Bulk mode. Suddenly the panels can run full blast and operate as close to their rated output current as possible (how close they can operate to their rated maximum depends on how close they are to being perfectly perpendicular to the sun’s rays).

In this case, having a bigger solar panel array is helpful because now it becomes a race with the clock to get the batteries through the Bulk stage and through the Absorb stage before the sun gets too low in the sky in the late afternoon.

And of course there are those cloudy days, or rainy days, and/or short winter days, when, try as they might, the solar panels just can’t produce the current needed to get the batteries through the Bulk and Absorb stages completely by the end of the day. On these days, you hope for more sun the next day or, if you get a bunch of these days in a row, eventually you turn to an artificially powered charging system like a converter or a inverter/charger ((via a portable gas generator or an onboard generator or shore power electricicity) or an engine alternator to finish the job.




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The rule of thumb for sizing solar charge controllers is not the same as for sizing artificially powered chargers. Remember, in Part 1 of this series, we mentioned there is a rule of thumb that says a battery charging system’s max output current should be roughly 25% of the capacity of the battery bank. This means that, in very approximate terms, a 440 amp-hour battery bank needs a 110 amp charging system.

However, solar charge controllers are generally sized to a solar panel array rather than to a battery bank. The sizing parameters for a solar charge controller are the maximum number of watts coming in from the solar panel array and the maximum current going out to the batteries. Add up the total watts in the solar panel array and the maximum amount of current the array can produce, and make sure those numbers are within the specs of the solar charge controller.

The traditional rule of thumb for sizing a solar panel array to a battery bank is that the total watts should be more or less equivalent to the amp-hour capacity of the battery bank.

Conventional Rule of Thumb:

Total solar panel array watts = Total battery amp-hours

However, this may end up under-sizing the solar panel array just a bit. As an alternative, you might start by sizing the solar charge controller to the battery bank using the 25% rule of thumb for sizing battery chargers to batteries:

1 – Solar charge controller output current = 25% Total battery amp-hours

THEN size the solar panel array so it maxes out the total watts and total open circuit voltage specified for the solar charge controller.

2 – Total solar panel array watts = Maximum input watts for Solar charge controller

Here’s an example using a 435 amp-hour battery bank of four Trojan T-105 Reliant AGM golf cart style batteries as a starting point. This is our battery bank and is the maximum amount of battery capacity our 36′ fifth wheel trailer can carry comfortably due to weight and space constraints.

Using the Conventional Rule of Thumb above, the total wattage of the solar panel array would be approximately 450 watts. This is sufficient in the summer months in North America and might be sufficient at the equator or in the Land of the Midnight Sun in the winter months, but in our experience, our 490 watts of solar panels on our RV roof is inadequate during winters in the southern US when the sun is low in the sky, the days are short and winter storms create overcast skies for days on end.

Using the Two Step sizing method above instead, you would choose a solar charge controller that has a maximum current output of 25% of 435 amps = ~108 amps. The Outback FlexMax 80 is an 80 amp solar charger (relatively close to the 108 we’re looking for). It can support up to 1,000 watts of 12 volt solar panels (and more watts for higher voltage panels). Note that to get 80 amps of current, you’d need to have the solar panels facing 90 degrees to the sun, and the solar charge controller would need to be operating in the Bulk stage.

Sizing the solar charge controller this way, we are now looking at 1,000 watts of solar panels instead of the 450 watts that the Conventional Rule of Thumb came up with — twice as much!

This sizing method is probably overkill. However, it might make sense to size the panels and controller both ways and choose something in between. As I’ve said, in our case, 600 to 800 watts lying flat on our RV roof without tilting would be nice in winter.

Boat solar power installation

Our sailboat had 555 watts of solar power.
Note the shade on the panels from the mast and spreaders.

For us, on our boat (710 amp-hour battery bank) we could have used a 750 watt to 1,000 watt solar panel array instead of the 555 watts we had to run the systems we had on board, despite having ample sunshine throughout our cruise.

All of this is given here as food for thought. Sizing panels and batteries and solar charge controllers is all very flexible. More of everything is better, but the reality is that there are roof space constraints for the panels, and there are both weight and space constraints for the batteries, and those limitations will ultimately dictate your particular options for panels and batteries.

A truck camper and a Class A diesel pusher (or a Catalina 27 sailboat and a Nordhavn 62 trawler) obviously have different constraints and needs.

In very general terms, anything from a 450 amp-hour / 500 watt system to a 900 amp-hour / 1,200 watt system is fine for both boats and RVs that are used to boondock or anchor out for months on end, depending on whether you run electric refrigeration and how much you stay up at night watching TV with the lights on and/or stay home during the day using computers, electric appliances and power tools.




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Now that we’ve seen the challenges that solar charge controllers face, let’s look at a specific example.

We installed an Outback MX60 MPPT solar charge controller in our fifth wheel trailer. It’s been in operation all day everyday that we’ve been in our trailer since we purchased it new in 2008. Since then, the Outback MX60 model has been discontinued and replaced by the new and improved Outback FlexMax 60 solar charge Controller.

The Outback FlexMax 60 MPPT Solar Charge Controller has the following algorithm:

BULK: Deliver maximum current until the Bulk voltage is reached.

ABSORB: Deliver as much current as necessary for the batteries to maintain the Absorb voltage. Transition to the Float stage when one of the following things happens:

  • The charger has been in the Absorb stage for as long as it took for the batteries to reach the Bulk voltage.
  • The current coming from the batteries has dropped below a certain level

If the sun fades and the controller can’t deliver enough current to keep the batteries at the Absorb voltage, extend how long the batteries stay in Absorb by the length of time the voltage fell below the Absorb voltage.

FLOAT: Deliver enough current to keep the batteries at the Float voltage.

EQUALIZE: Equalization voltage and time parameters are programmable, and equalizing can be done automatically or started manual. If Equalizing can’t be completed in one day, the batteries will resume equalizing the next day until the equalizing time has been completed.

Everything in the Outback MX60 charge controller (and the Outback FlexMax 60/80 Solar Charge Controllers) is programmable on a four-line LED menu driven display. You enter the battery type (Flooded, Gel, AGM) and that gives you default values for Bulk, Absorb and Float voltages. You can then override those values with values of your own if you wish.

So, how does this solar charge controller compare to a converter, inverter/charger or engine alternator?

If you compare the Outback MX60’s charging algorithm shown above to that of any of the artificially powered charge controllers described in the previous article, you can see just how very much more complicated this solar charge controller is. Here’s a little more detail:



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The key part of any multi-stage charging algorithm is when to switch from the Absorb stage to the Float stage. (If you are unclear about those stages, read more here: RV and Maring Battery Charging Basics). All charging systems use TIME as a basic criteria. The question is how long?? Should the batteries stay in Absorb for 2 hours or 4 hours? Should it always be the same amount of time?

To be most amenable to the batteries’ needs, the state of charge of the batteries when they first start charging must be taken into account. If the batteries are nearly fully charged when charging starts, why keep them in Absorb for three hours? That’s like forcing down extra helpings of pie after a big Thanksgiving dinner. Maybe just a small piece is enough on a full stomach.

On the other hand, if the batteries are deeply discharged when the charging begins, they should stay in Absorb longer to make sure they really get full. If you didn’t nibble on hors d’oeuvres before dinner and you skipped lunch and breakfast, then extras helpings of everything at the Thanksgiving table might taste and feel great.

Outback tackles this conundrum by looking at how long it takes the batteries to reach the Bulk voltage. If they are well charged already, they’ll zip to the Bulk voltage quickly. In that case, they don’t need to stay in the Absorb stage for very long. On the other hand, if they are deeply discharged, it will take a long time for them to reach the Bulk voltage. In that case, they should hang out in Absorb for a long time until they are really and truly fully charged.

The way the Outback charge controllers accomplish this flexibility in the length of time of the Absorb stage is that they make the Absorb stage last for the same length of time as the Bulk stage did. If Bulk took 2 hours, then Absorb will last for 2 hours. If Bulk took 3 hours, Absorb will be 3 hours. Clever!



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Unfortunately, the sun isn’t all that consistent for such a basic algorithm, and there is more to it than just a simple one-to-one relationship between Bulk and Absorb. What makes this business tricky is that the sun may not allow the charger to hold the batteries at the Absorb target voltage once they begin Absorbing. For instance, in the middle of the Absorb stage, the sky might cloud over. The charge controller will respond by instantly opening the floodgates for the batteries so it can get the necessary current from the panels to keep the batteries at the Absorb voltage. But if the panels can’t deliver, there’s nothing the solar charge controller can do, and the battery voltage will fall below the Absorb voltage.

Outback FlexMax 60 MPPT Solar Charge Controller

Outback FlexMax 60 MPPT Solar Charge Controller

In another scenario, someone in the RV or boat might turn on an electrical appliance that draws a lot of current — more than the panels can deliver — and this will temporarily lower the battery voltage below the target voltage. Running the vacuum or a hair dryer in addition to whatever else is running in the RV or boat might be just enough to draw more current from the batteries than the sun on the panels can produce.

In these cases, the solar charge controller will try to keep the batteries in the Absorb stage, but it’s failing. The thing is, if there isn’t enough current to keep the batteries at the Absorb voltage, are they really Absorbing? Not exactly. They’re getting as much current as possible, but the voltage has dropped below the Absorb stage threshold.

The Outback charge controllers view this as a kind of “timeout” period. So, for every minute of this “timeout,” they tack on a minute of extra time that the batteries must stay in Absorb before they switch to float.

For instance, if the batteries have been in Absorb for 53 minutes when the sky suddenly clouds over, the Outback charge controller will start counting how long the batteries stay below the Absorb voltage. If they stay below for 14 minutes, then once the sun comes back out and they get back to the Absorb voltage, they will need to stay in Absorb for an extra 14 minutes on top of the time period they were planning on (which is either the length of time that the Bulk stage took that day or a minimum amount of time programmed by the user). When they resume Absorbing, the Outback will resume counting from 53 minutes with a new target time that is 14 minutes longer than before.

This problem of the solar panels not being able to deliver enough current to keep the batteries at the target voltage exists in the Float stage as well as the Absorb stage. However, in the case of the Float stage there is no time consideration. Once they get into Float, the batteries will stay there (or attempt to stay there) until dark.

If you are confused, here is a real live example:

One day around noon our batteries had reached the Float stage (we’d gone to bed early the night before, so the batteries had charged up quickly). They were humming along getting about 4 to 10 amps or so to maintain a 13.5 Float Voltage with whatever stuff we had running in the RV (laptops, etc.).

I got out the vacuum, and when I turned it on, the charge controller jumped into high gear, demanding max output from the solar panels. The panels could deliver 25.6 amps, but that wasn’t enough to maintain the Float voltage of 13.6, and the battery voltage dropped to 13.1 until I finished vacuuming. Then everything went back to where it had been.

Lesson learned: use a broom not a vacuum!

You can see the display from the Outback charge controller here:

Outback MX60 Solar Charge Controller

Outback MX60 Charge Controller display at midday with vacuum & computers running.
Note the batteries have dropped to 13.1 volts (below Float) and the current coming from the panels to the batteries is a huge (for “Float”) 25.6 amps to support the load in the RV. “F-MPPT” means “I’m in the Float Stage but I need max power ’cause I can’t maintain the Float Voltage.”

Even if the sun is out all day long and the batteries reach the Float stage, at the end of the day when the sun begins to set, the charger will no longer be able to hold the Float voltage. As it gets darker and darker, the charger will try valiantly to hold the Float voltage, but the battery voltage will drop lower and lower while the charge controller delivers less and less current.

Eventually, when it gets completely dark outside, no current will be going to the batteries at all. If the batteries were in Float before the sun went down, they will settle out at 12.7 volts, fully charged. If they never reached the Float stage, however, you’ve gotta cross your fingers for good sunshine tomorrow!




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As I mentioned in the previous article in the description of the Xantrex Freedom 25 Inverter/Charger, a rule of thumb is to switch from Absorb to Float when the current that the batteries need to remain at the Absorb voltage drops below 2% of the amp-hour capacity of the battery bank.

For a 450 amp-hour battery bank, this would be 9 amps. For a 750 amp-hour battery bank, this would be 15 amps. So, for a 450 amp-hour battery bank, a reasonable time to switch from Absorb to Float is when the current drops below 9 amps. For a 750 amp-hour battery bank it is when the current drops below 15 amps.

The Outback FlexMax 60 (and 80) allow you to enter whatever number of amps seems right to you, whether it is 2% of your battery bank or some other number that you prefer.

Why is it important to switch from Absorb to Float when the amount of current the batteries need to remain at the Absorb voltage drops below a certain level?

The batteries may be nearly fully charged, but if the charging algorithm forces them to stay in Absorb for a set period of time — three hours for instance — they may need just 1 or 2 amps to maintain the Absorb voltage. It might be better for the batteries if they were allowed to slip back to the Float voltage at that point rather than forcing them to stay at 14.7 volts while accepting a minuscule amount of current until the 3 hours is up.

However, the reverse may also be true. There may be situations where you don’t want the batteries to be in the Float stage even though the charging current has dropped below 2% of the battery bank capacity. More on that further down.



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Because solar charge controllers operate 24/7, there are three more states that the Outbacks can be in:

  • SNOOZING: The voltage of the solar panels is greater than the voltage of the batteries but there is no current coming in from them
  • SLEEPING: The voltage of the solar panel array is less than the voltage of the batteries
  • ZZZZZ…: The solar charge controller has been in the SLEEPING state for 3 hours or more

The controller has an algorithm for waking up as well. As the sun rises, once the voltage of the solar panels is more than 2 volts higher than the voltage of the batteries (i.e., the panels are at 14.7 volts or more if the batteries are fully charged at 12.7 volts), it looks for current coming in from the panels. If the current is still near 0, it SNOOZES in 5 minute intervals while it waits for the current to reach about an amp. Then it goes into Bulk and starts its work for the day. This happens each morning as the sky becomes light and the solar panel voltage rises from 0.



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It doesn’t take much light to bring a 400+ watt 12 volt solar panel array up to 15 volts. A full moon with clear skies may raise the voltage on the panels to this level, and parking under a bright street light will definitely do it. This is not enough light for the solar panels to generate current, but it can sometimes be enough to fool the charge controller that the sun might be about to rise and give it a sleepless night.

We have seen our solar charge controller pull an all-nighter as it alternated between SNOOZING and WAKE-UP all night long because the solar panel array was steady at 15 volts from a street light overhead while the batteries were at 12.7 volts.

The charge controller couldn’t start the real SLEEPING phase because the panel voltage was higher than the battery voltage. But there wasn’t enough light to generate any current either. So, the controller would WAKE-UP, discover there was no current coming in from the panels, and then it would go back to bed and SNOOZE a little longer. It would repeat this unfortunate cycle all night long, never getting into the really good 3 hour long ZZZZ… sleep stage (poor thing!).

On the other hand, while staying in the Catskill Mountains about 120 miles from New York City, I crept out at 2:00 in the morning to see how the charge controller was doing. The city lights kept the sky from being very dark, and the panel voltage was elevated slightly to 9 volts rather than the usual 0 volts we see in more rural areas. However, the batteries were more than 2 volts higher than that at 12.7, so the charge controller was well into its REM sleep phase, dreaming of sunny days. (Mark crept out to photograph the fireflies…a much better reason to climb out of bed at 2 am!!)

Outback FlexMax solar charge controller sleeping near NYC

At 2:00 a.m. in the Catskills, the lights of NYC reflecting off low clouds raise the Panel Voltage to 9 volts. The previous day the batteries got 73 amp-hours of charge, so they are fully charged at 12.7 volts, more than 2 volts higher than the panels. The controller is sleeping soundly for 3 hours when it will check the panel voltage again.



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We installed a Xantrex XW MPPT 60-150 Solar Charge Controller on our sailboat. Xantrex is now Schneider Electric, and this unit has been replaced with the Schneider Electric XW MPPT 60-150 solar charge controller. I don’t know if this is just a name change on the unit or if the design of the unit has changed in any way.

This solar charge controller is about the same size as the Outback but has a two line LCD display instead of four, so you have to scroll through the menus a bit to get the same info you can see at a glance on the Outback.

The challenge for us on our boat was that we had a smaller solar panel array than we needed for our typical daily power consumption due to our electric (DC) fridge and standalone freezer. 555 watts of solar power was not enough. So, we needed the charge controller to get the solar panels to provide as much current as possible everyday.

Unfortunately, it took us a while to realize that the factory default settings on the Xantrex charge controller were preventing the solar panels from providing as much current as they could.

The Xantrex charge controller came with a factory default setting to switch from Absorb to Float when the current being delivered to the batteries dropped below 2% of the amp-hour capacity of the battery bank, or 14 amps.

Xantrex XW MPPT 60-150 Solar Charge Controller in a sailboat

Our Xantrex XW MPPT60-150 charge controller on our sailboat

The problem was that once the current going to the batteries dropped below 14 amps, the solar charge controller put them into the Float stage. In the Float stage they needed much less current to maintain the Float voltage, usually around 5 amps. That’s a lot less than the nearly 14 amps they had been getting in Absorb!

What this meant was that even if the sun was shining brightly, the batteries were being given less current than the panels were capable of delivering because the solar charge controller had put them in the Float stage. The gatekeeper had closed the gate most of the way!

We would watch the system go into the Float stage at 1:00 p.m. and waste the best sunshine of the day sitting in the Float stage all afternoon charging the batteries with a lot less current than it would have if the controller were still in Absorb.

So, because the Xantrex charge controller had the programming option available, we programmed it to switch into Float when the batteries needed only 5 amps to maintain the Absorb voltage instead of the 14 amps that was 2% of our battery bank size. This way we were able to charge the batteries up by an extra 25-30 amp-hours each day.

However, the Xantrex controller didn’t make this programming option obvious. Rather than having an input parameter for the current at which to switch from Absorb to Float like the Outback models have, you could enter only the size of the battery bank. The controller would then calculate what 2% of that value was and would use that value to switch from Absorb to Float.

So, we had to fool the controller by saying our battery bank was only 250 amp-hours rather than the 710 amp-hours that it actually was. Then it would switch from Absorb to Float when the current dropped to 5 amps (2% of 250) instead of at 14 amps (2% of 710).

This also could have been alleviated by throwing the system back into a Bulk charge, and in our first days of working with this system, there were many times when I wished there were a setting to force the charge controller to put the batteries back in the Bulk stage whenever I wanted. But unlike the Outback solar charge controllers, this Xantrex model did not have that option.

So, as you can see, the Xantrex XW MPPT 60-150 Solar Charge Controller takes a slightly different approach to the challenges of solar charging than the Outback models do. Here are the details:

The Xantrex XW MPPT 60-150 Solar Charge Controller charging algorithm is the following:

BULK: Deliver the maximum possible current to the batteries until they reach the Bulk voltage

ABSORB: Deliver as much current as necessary for the batteries to maintain the Absorb voltage. Transition to the Float stage when one of the following things happens:

  1. The current necessary to maintain the Abosrb voltage is 2% of the battery bank capacity
  2. The batteries have been in the Absorb stage for 2 hours (modifiable)
  3. The batteries have been at or above the Float voltage for 8 hours

FLOAT: Deliver enough current to the batteries to maintain the Float voltage.

EQUALIZE: The voltage and times for equalizing are user defined.

This charging algorithm is pretty straight forward, except for that odd 3rd way that the controller might switch from Absorb to Bulk. What’s going on there?

— What if the target voltages can’t be maintained — another technique!

That third trigger Xantrex uses for switching from Absorb to Float allows for the situation where the battery voltage has dropped below the Absorb voltage temporarily due to either clouds or shade or big loads in the RV or boat (vacuums or refrigerator compressors) drawing the voltage down for a while because the panels can’t deliver enough current. What it’s doing it that even if the batteries haven’t been at the Absorb voltage the whole time, as long as they have stayed above the Float voltage for at least 8 hours, they are considered ready to leave the Absorb stage and enter the Float stage.

Remember, the Outback solar charge controllers dealt with this same challenge of flaky sunshine by tracking how long the batteries fell below the Absorb voltage and then forcing the batteries to stay in Absorb for that same number of extra minutes to make up the lost time.

The Xantrex method is a little more simplistic than the Outback method, saying that as long as the battery voltage stayed above Float for 8 hours, they have been sufficiently charged and can switch to the Float Stage.

— Programming the charge controller for improved performance

As a recap, our goal was to keep the batteries in Absorb for as long as possible. So, I modified two of the Xantrex solar charge controller’s input parameters to allow this to happen:

  1. Pretend our battery bank was just 250 amp-hours instead of 710 so it would stay in Absorb down to 5 amps (modifying criteria #1)
  2. Increase the Absorb stage time limit from 2 hours to 8 hours (modifying criteria #2)

What these two programming changes ultimately did was they made the batteries stay in the Absorb stage for 8 hours, getting a healthy amount of current from the solar panels, unless the current happened to drop below 5 amps (2% of 250) before 8 hours was up.

This worked really well for 750 nights of anchoring out.



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We were extremely cautious with the AGM batteries in our boat and did not want to modify the solar charge controller’s default voltage settings for AGM batteries since AGM batteries are sealed and they can’t be charged at as high a voltage as flooded batteries (this is explained in more detail in Part 1 of this series).

The default charging voltages for AGM batteries on the Xantrex XW MPPT-60-150 Solar Charge Controller are:

  • Bulk: 14.3
  • Absorb: 14.3
  • Float: 13.4

* * * Lesson Learned * * *

Now that we have installed four Trojan T-105 Reliant AGM batteries in our fifth wheel and have been advised by the engineers at Trojan Battery to use Bulk and Absorb voltages of 14.7 volts on their AGM batteries instead of the 14.3 or 14.4 that most charging systems default to, I look back and realize I was probably too conservative with our boat’s AGM batteries.

If we had set the Bulk and Absorb voltage values to 14.7 instead of 14.4 (the setting I chose), then they would have charged faster (received more current from the charge controller) during those stages, and they would have won the daily race against the clock more easily. Obviously, more panels would have done the trick too, but finding unshaded deck space on a sailboat is tricky.

It only makes sense to program a battery charging system to the battery manufacturer’s specifications rather than assuming that the factory defaults on the charge controller are optimal. Afterall, charging system manufacturers — whether solar charge controllers, converters, inverter/chargers or engine alternators — will ALWAYS err far to the conservative side because they they are designing for a wide variety of battery brands and they don’t want to risk frying a customer’s batteries.

However, in the end, this might result in undercharging the batteries! Trojan Battery engineers have found that far more batteries die a slow death of chronic undercharging than a violent death of massive overcharging, so they prefer slightly higher charging voltages for their AGM batteries than are factory standard on many solar charge controllers, converters, inverter/chargers and engine alternators (with a caveat not to go to 14.8 volts or higher).



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We recently did a complete full-timer solar power installation on a friend’s motorhome. He specified the Morningstar TriStar TS-MPPT-60 Solar Charge Controller for his installation, so we had a chance to program it and work with it. This solar charge controller uses yet another methodology.

Morningstar TriStar MPPT 60 amp solar charge controller

Morningstar TriStar MPPT 60 amp solar charge controller

This solar charge controller is programmed via dip switches and the charging stages are indicated by LED lights rather than a digital readout. You can also purchase the additional TriStar Remote Digital Meter that has a two line LCD display similar to the 2-line and 4-line displays on the Xantrex and Outback models described above.

Separating the charge controller from the display is a great idea. It allows you to install the display inside the RV or in the boat’s cabin where you can read it easily and mess with its buttons whenever you wish. Yet you can still place the charge controller itself right next to the batteries where it needs to be (the cable going from the batteries to the charge controller must be as short as possible).

Our friend did not purchase the remote meter, but we found the system was easy enough to set up without it. The dip switches were a clunky interface, but that would be improved with the buttons and digital display of the remote meter. The lack of a digital readout made it difficult to know the details about the voltages and currents of the panels and batteries in the system. However, our friend did not plan on programming the solar charger any further, and he already had a battery monitor in his coach, so he had a way to monitor the battery voltage easily.

Here are the details on the charging algorithm:

The Morningstar TriStar TS-MPPT-60 Solar Charge Controller multi-stage charging algorithm is the following:

BULK: Deliver the maximum amount of current possible until the batteries reach the Bulk voltage.

ABSORB: deliver as much current as necessary to keep the batteries at the Absorb voltage until the following thing happens:

  • 2 to 2.5 hours has gone by (depending on battery type)

If the batteries fell below 12.5 volts during the previous night, then extend the Absorb stage by 30 minutes.

FLOAT: Deliver as much current as necessary to keep the batteries at the Float voltage. If the batteries are drawn down below the Float voltage for an hour or more due to big loads in the RV or boat (vacuum, power tools, microwave) or due to sudden cloud cover, the charge controller will switch back to Bulk mode and start the cycle over again. If the batteries fell below 12.3 volts during the previous night, then the solar charger will not enter the Float stage the following day.

EQUALIZE: The voltage and duration of the Equalization stage is determined by the battery type selected and is started manually.

This is yet another creative approach to the various problems caused by the unreliability of sunshine. The idea of setting up the charging parameters today based on the lowest voltage the batteries reached overnight is cool, since that is truly the biggest determining factor for how much charging the batteries need right now.

However, note that there is no criteria for switching from Absorb to Float based on the current falling below a minimum value as with the other charge controllers. There is also no provision for lengthening the Absorb stage if the Absorb voltage can’t be maintained, although there is if the Float voltage can’t be maintained.

The Absorb, Float and Equalize voltages are assigned in this controller when you select the battery type. AGM batteries are assigned:

  • Bulk/Absorb: 14.4
  • Float: 13.7

There seemed to be an option to override those values with custom values, however, it wasn’t clear how to enter the actual voltages using the dip switches. The TriStar Remote Digital Meter might provide more programming flexibility.

The Morningstar does come with PC based software, and it is possible to connect the solar charge controller to your in-house ethernet network via the controller’s ethernet port or to connect it using a wireless router. However, for me, that adds a level of complexity that isn’t really necessary.

There are just a few parameters to enter on any charge controller, and just a few values to monitor, and those only need to be monitored occasionally. Having a menu driven screen interface built into the charge controller rather than getting my computer involved in the action is worth a lot to me.



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As you can see, there is a significant variation in how the different manufacturers of solar charge controllers tackle the challenge of creating an algorithm to charge the batteries, given the vagaries of sunshine. All of the solar charge controllers described here get the job done, it’s just that the methodology varies and the ease of use and programmability of the units differ.

If you want to get the most out of your solar charge controller, the most important thing is to know what your battery manufacturer’s recommended charging voltages and time limits are, that is, what their preferrred Bulk, Absorb and Float voltages are and how long they want the batteries to remain Absorb. Then program the solar charge controller accordingly.

The reason I chose the Xantrex XW MPPT-60-150 Solar Charge Controller for our boat rather than purchasing another Outback charge controller like the one in our RV (the nice new FlexMax 60 was on the market by then) was that the Outback has a fan in it. I was concerned that in the hot tropical climates where we would be sailing, the fan would likely run a lot and might fail. I didn’t want any moving parts! I chose the Xantrex because it is cooled by large cooling fins instead of a fan.

In hindsight, the Outback charge controllers are rated to operate at up to 104 degrees, and the cabin of our boat never got that high. Probably an Outback charge controller would have held up just fine. The Morningstar with its Remote Digital Meter is a neat idea for separating the charge controller and the digital display. However it does require a few more installation steps to mount the remote meter and run the cable from the charge controller location to the remote meter location. It also has a simpler overall charging algorithm, which could be a pro or a con depending on your preference.


The next — and final — article in this series takes a look at what happens when two battery charging systems are running simultaneously. That is, what happens if you have solar power and you plug into shore power or turn on the boat engine?

To continue to the next article in this series, click here:

Solar and Shore Power or Engine Alternator Battery Charging Combined

4-Part Series on RV and Marine Battery Charging Systems:

Related posts about batteries, solar power and living off the grid in an RV or boat:

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RV Converter, Inverter/Charger, and Alternator Battery Charging Systems

This article discusses battery charging systems that are “artificially powered” by electricity or an engine (as opposed to sun or wind power) and the methods these systems use to chargeso RV and marine batteries. It is the second post in our four part series on RV and Marine Battery charging systems.

Converter Inverter-Charger Engine Alternator Battery Charging Systems

The first article in the series, RV and Marine Battery Charging Basics, explains how batteries are charged and describes the concepts of single stage and multi-stage charging. The third and fourth articles in this series are:

You can navigate to specific parts of this article with the links below:



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There are two basic types of multi-stage chargers for RVs and boats: those that are “artificially powered,” either by electricity, by an engine or by a generator, and those that are “naturally powered” by the sun (or wind). Note: Although this series doesn’t discuss wind charging systems, the same principles apply.

What is the difference?

Ability to Deliver the Maximum Rated Current

The biggest difference between these two types of charging systems is that artificially powered charging systems — converters, inverter/chargers and alternators — can all deliver the maximum amount of current they are rated for as soon as they are turned on. In contrast, “naturally powered” chargers may or may not be able to deliver their maximum rated current when called upon to do so.

Yamaha 2400i portable gas generator

Yamaha 2400i portable gas generator
As long as there’s gas, it’s good to go.

Solar charge controllers can deliver their maximum rated current only if they are connected to a large enough solar power array and that array is exactly perpendicular to full sunshine. Unfortunately, no matter how big the solar panel array is, these charging systems spend most of their time operating in sub-optimal conditions when the sun is low in the sky or filtered by clouds or totally absent because it is nighttime.

In addition, if a big appliance is turned on in the RV or boat while the batteries are being charged, the artificially powered charging systems can meet the challenge and provide the current that is needed (up to their rated current output and up to the limits of the power source) to keep the batteries at their target charging voltage.


The sun’s out — yay!
We can start charging!

However, solar charge controllers may or may not be able to meet the challenge, depending on the time of day and amount of cloud cover. In fact, if the current draw is big enough, not only will the solar charge controller fail to keep up with the sudden demand, but the net effect on the batteries may be that they are temporarily being discharged a little bit rather than charged.

Therefore, solar charge controllers have a lot of extra complexity built into their charging algorithms so they can handle the situations where, for whatever reason (lack of sun and/or too much demand from the appliances in the RV or boat) they aren’t actually charging the batteries but are just slowing down the discharge rate!

Ability to Restart the Charging Process with the Bulk Stage

Artificially powered charging systems can all be turned on or off with the flick of a switch. Most systems will test the battery voltage to see if they should jump into the Bulk stage as soon as they are turned on. This gives you a way to force the batteries into the Bulk stage and start the charging process from scratch.

Solar charge controllers operate 24/7, and they rely on an internal algorithm to determine when it is morning and time to start the Bulk charging stage. Not all solar charge controllers are designed to have an easy way for the user to put the batteries in a Bulk charging stage at any time of day other than dawn.




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Many artificially powered charging systems are programmable, but usually the choices are minimal. If they can be programmed at all, it is generally done with dip switches or simple buttons. In contrast, big solar charge controllers are complex enough and have so many programmable options that they often have a screen display and a menu driven interface.

Some charging systems have preset groups of voltage values, and all you can select is whether your batteries are Flooded, AGM or Gel. The charger then assigns voltage values for the charging stages based on battery type. In this case, the charging system manufacturer is guessing what voltages are appropriate for your batteries. The battery manufacturer may have different specs!

The most sophisticated (and expensive) charging systems allow you to enter any value you want for the individual charging voltages as well as the length of time to remain in the Absorb stage and other values as well.

Even if you don’t study the charging algorithm that is used by the charging systems on your RV or boat, it is worthwhile to find out what the default voltages are for the Bulk, Absorb and Float stages are on each device.



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There are rules of thumb for what the charging voltages should be for the various battery types, with flooded batteries requiring higher charging voltages than AGM and Gel batteries. The general consensus I found in my research was that flooded batteries preferred a Bulk/Absorb voltage in the range of 14.6 – 14.8 volts while AGM and Gel batteries prefer to be around 14.4 volts.

Because of this general consensus, I set up all the charging systems on our boat with Bulk and Absorb values around 14.4 volts so we wouldn’t fry our four Mastervolt 4D AGM batteries house batteries and our Group 27 start battery.

Needless to say, I was quite surprised when we installed our four new Trojan T-105 Reliant AGM 6 volt batteries in our trailer, that the engineers I spoke with at Trojan Battery recommended we set the Bulk and Absorb stages of our charging systems to 14.7 volts. They said the vast majority of battery failures are from chronically undercharged batteries, so they preferred that their AGM batteries be charged at this higher voltage.

I never spoke with anyone at Mastervolt back in our cruising days, and their documentation didn’t specify charging voltages. In hindsight, perhaps we should have been charging the batteries on our boat to higher Bulk and Absorb voltages. They would have charged faster, which would have been awesome, especially on solar, because our solar panel array was a little small (555 watts), and getting the batteries fully charged by day’s end was a challenge unless we turned off our DC freezer.

Lesson learned: If you can’t find your battery manufacturer’s recommended charging voltages in their documentation, give them a call!

The next sections take a look at a few artificially powered charging devices we have used and the algorithms they employ for battery charging.




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Most trailers are equipped with a converter to charge the batteries from shore power (via electric hookups or a portable gas generator). The shocking thing about these converters is that many of them are just single stage trickle chargers. (Note: if you are confused about what converters and inverters are, click here).

We had never thought much about our converter, because we use it very rarely. We don’t ever get electrical hookups, so our converter is used only when we fire up our gas generator, which happens just a few times a year. We had always assumed that the Atwood SRV 55 amp converter that came with our Hitchhiker II LS fifth wheel was a multi-stage charger. However, we discvoered a few months ago that this converter is actually a single stage trickle charger. It brings the batteries up to 13.4 volts and leaves them there indefinitely, as long as the converter has AC power supplied to it.

This is startling for two reasons.

First of all, since we boondock all the time, this means that whenever we turn on our generator to charge our batteries (after a few days of stormy weather), rather than giving the batteries a fast blast of Bulk charge followed by Absorb and Float, the batteries are immediately put into a Float stage and left there. Rather than getting a quickie does of lots of current and then trailing off to less and less current, the batteries get an anemic amount of current the whole time the generator is running.

What a waste of fuel! And who wants to listen to that noisy thing for that long! Rather than taking an hour or two to charge the batteries completely, it could take 8 hours or more. Ugh!

Secondly, single stage converters like this Atwood don’t exercise the batteries at all when they are left on shorepower via electrical hookups, and the batteries deteriorate more quickly. This is an important consideration for an RV that is plugged into shore power month after month. It is important for batteries to go through the Bulk and Absorb stages periodically.

We decided to replace our factory-installed single-stage Atwood 55 amp converter with an Iota DLS 90 converter / IQ4 smart charger a few months ago so that on the days that we use our generator we could use it for a very short time rather than running it all day.

Besides wanting a true multi-stage charger that could load the batteries up with a lot of current at the beginning of the charge cycle, we also realized our old factory installed converter was too small.

Remember that 25% rule for sizing batteries and chargers from the last post? Our converter had been sized for the two Group 24 12-volt batteries (total capacity 140 amp-hours) that had come with our RV, and we had upgraded to four Trojan T-105 Reliant AGM 6 volt batteries which gives us a total capacity of 435 amp-hours.

Our new Iota DLS-90 / IQ4 is a 90 amp converter which is much more appropriately sized to the new battery bank.

And what a world of difference there is between these two converters!

The Iota DLS 90 / IQ4 is far more sophisticated. It puts the batteries into a true Bulk charge state as soon as AC power is available (for us, that is when we turn on the generator with the shorepower cord plugged into it). Then, after cycling through Absorb to Float, it keeps the batteries in the Float stage for seven days (not applicable to us with our generator, but important for folks who get electric hookups), and then it cycles them through Bulk and Absorb again.

The multi-stage algorithm that the Iota DLS 90 / IQ4 uses is the following:

BULK: Whenver the batteries are below 12.8 volts (i.e., when first plugging into shore power or when a bunch of appliances are turned on in the RV or boat) deliver the maximum current possible (up to 90 amps DC) until the batteries reach a voltage of 14.6 volts, then switch to Absorb. If they don’t reach 14.8 volts within four hours, switch to Absorb anyways.

ABSORB: For eight hours, deliver enough current to hold the batteries at 14.2 volts.

FLOAT: For seven days, deliver enough current to hold the batteries at 13.6 volts. Then go through the Bulk and Absorb stages before resuming the Float stage.

The system is fully automatic and none of these values or times are programmable.

Note: For readers who have studied the spec sheets on the Iota DLS-90/IQ4, this outline differs slightly from what you read. I had a lengthy conversation with an engineer at Iota who explained the details of how this converter works. The documentation refers to the weekly return to Bulk and Absorb as an “Equalization” stage, but the voltages and times are actually those of the Bulk and Absorb stages. As noted in the first post in this series, equalization is generally done at 15 volts or more for less than 8 hours. In addition, the documentation describes the converter’s power supply ramping up to 14.8 volts during Bulk, but doesn’t explain that the actual trigger point that switches the batteries from Bulk to Absorb is 14.6 volts.

Using the Iota DLS 90 / IQ4 The First Time

A few weeks ago we endured several days of gray skies and rain while we were driving from Florida into southern Georgia. Our solar panels were producing very little current, and our new Trojan T-105 Reliant AGM batteries were becoming depleted. There was no sign of sun in sight.

We set up our Yamaha 2400i portable gas generator and plugged our shore power cord into it. We clamped the jaws of our trusty clamp-on ammeter around one of the battery cables and were truly astonished to see 67 amps going into the batteries. Yowza!! Within two hours the batteries had accepted roughly 100 amp-hours of charge and we turned the generator off. Our old converter would have taken about 8 hours or more to do the equivalent.




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Many motorhomes and cruising boats are equipped with an Inverter/Charger to charge the batteries when the RV or boat is plugged into shore power. Our Hunter 44DS sailboat was equipped with a Xantrex Freedom 25 inverter/charger which was factory installed in the boat. Xantrex has since become Schneider Electric, and a comparable model being sold today is the Schneider Electric 2500 watt inverter/charger. I haven’t found an online manual for it, so I don’t know if the charging algorithm or programmability of the unit has changed.

Schneider Electric 2500 watt inverter : charger

Schneider Electric 2500 watt inverter/charger
This is the updated model of our Xantrex Freedom 25 Inverter / Charger
(ours was buried under a settee and impossible to photograph!)

Unlike many converters, most inverter/chargers are multi-stage chargers. Our Xantrex Freedom 25 had minimal programming capabilities. You could enter the battery type (Flooded, Gel or AGM), and the voltages for the charging stages were automatically assigned according to the battery type you selected. You could not enter any other values. We had AGM batteries, and the Xantrex inverter/charger assigned them defaults of:

  • Abosrb: 14.3 volts
  • Float: 13.3 volts

If you wanted different voltages, you could select the Flooded or Gel values instead simply by indicating that your batteries were Flooded or Gel, even if they weren’t.

The multi-stage charging algorithm for the Xantrex Freedom 25 inverter/charger is the following:

BULK: Deliver the maximum current possible until the Absorb voltage is reached

ABSORB: For up to 3 hours, deliver as much current as needed to keep the batteries at the Absorb voltage. If the current necessary to keep the batteries at the Absorb voltage drops below 15 amps before the 3 hours is up, stop charging and let the battery voltage settle down to the Float voltage.

FLOAT: Deliver enough current to hold the batteries at the Float voltage., and keep the batteries at the Float voltage indefinitely.

EQUALIZE: Whenever you want to equalize the batteries, you can manually put them into an Equalize charging stage. The inverter/charger will deliver enough current to bring the batteries up to 16.3 volts and will keep them at that voltage for 8 hours.

Notice how different the Xantrex inverter/charger is than the Iota DLS 90 / IQ4 Converter!. Both the voltages and lengths of time are quite different.

Even more interesting, however, is where the heck did that 15 amp thing come from for switching from Absorb to Float?

As a rule of thumb, it is thought that when the batteries need less than 2% of the amp-hour capacity of the entire battery bank in order to maintain the Absorb voltage, then they are pretty close to full charge and can be put in the trickle charge Float stage.

This 15 amp switchover is an attempt at implementing this 2% rule. However, because the 15 amp value is not modifiable, the assumption is that the battery bank is 750 amp-hours (15 is 2% of 750). That’s quite an assumption! More sophisticated charge controllers allow you to program the current at which you want the system to switch from Absorb to Float.

Our boat’s battery bank was 710 amp-hours, so a more accurate number would have been 2% of 710, or 14 amps. 15 amps versus 14 amps — big deal, right? It’s true, for an inverter that is going to be running 24/7 when you are plugged into shore power, that slight difference is not significant.

But if you are using the inverter/charger with a generator (to supplement solar power during stormy days), you might want to stay in the Absorb stage for the full 3 hours rather than dropping into Float as soon as the current dips below 15 amps!

Also, as I’ll show in the next post in this series, 15 amps was still much too high a current — in our case — to switch from Absorb to Float when we charged our boat’s battery bank with our solar charge controller. We wanted the switch-over current from Absorb to Float to be only 5 amps.




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Cruising sailboats and motorhomes are equipped with an engine alternator that charges the batteries. Our sailboat had a 100 amp Balmar alternator with an ARS-4 Smart Charger which was a multi-stage voltage regulator.

Balmar 100 amp engine alternator

Balmar 100 amp diesel engine alternator

The multi-stage charging algorithm the ARS-4 Smart Charger uses is the following:

BULK: For 36 minutes deliver maximum current until the batteries reach the Bulk voltage. If the Bulk voltage is not attained in 36 minutes, then continue delivering that same current for 6 more minutes. If, again, the Bulk voltage has not been reached, continue for 6 more minutes and check again. Repeat this cycle until the Bulk voltage is reached.

ABSORB: For two hours, deliver enough current to keep the batteries at the Absorb voltage. If after two hours the batteries are not at the Absorb voltage (due to large current draws from systems on the boat or RV), check every six minutes until the Absorb voltage is achieved.

FLOAT: For six hours, deliver enough current to keep the batteries at the Float voltage. After six hours, increase the current being delivered to the batteries to bring them up to the Abosrb voltage and keep them at that voltage 36 minutes. Then return to Float for six more hours. Repeat this cycle indefinitely.

EQUALIZE: The equalizing stage is started manually and you can choose the voltage and time limit.

This charging system is quite programmable. The user can enter the length of time of each stage, and all the voltages can be programmed to any value as well. The factory default voltages are:

  • Bulk = 14.1 volts
  • Absorb = 13.9 volts
  • Float = 13.4 volts

Notice that with this particular engine alternator the batteries are not left in the Float stage indefinitely. Instead, they are put into Float for six hours and then in Absorb for 36 minutes, cycling between those two stages indefinitely.

How long is “indefinitely” when it comes to running a boat’s engine, anyway? Well, we had lots of 24 to 55 hour passages on our cruise where the engine ran nonstop. The alternator cycled between Absorb and Float quite a bit during those passages.




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One really important aspect of using an alternator to charge a large battery bank, especially if the engine will be running when huge loads are put on the batteries (like the anchor windlass or power winches), is the 25% rule of thumb I mentioned in the first post of this series: the rated output current of a charger should be roughly 25% of the capacity of the battery bank.

Most cruising boats have very large battery banks. Ours was 710 amp-hours, and we knew lots of cruisers with 600 amp-hour banks all they way up to 1,000 amp-hour banks. For us, 25% of our 710 amp-hour battery bank calculates to 177, so our alternator needed to be a 180 amp alternator to be sized correctly.

The problem is that most alternators over 100 amps require a double pulley system on the engine. That’s complicated, and very few cruisers choose to go that route. Instead, they tend to limp along with undersized alternators.

And what is the most common system failure we saw sailors experiencing on their cruising boats? Alternators!

Not only are most cruising boat alternators undersized, most alternators are called upon to power the anchor windlass, lifting a 60 or 70 lb. anchor along with 100 to 300 feet of stainless steel chain from a depth of 20 or 30 feet. Frequently, it does this in pre-dawn hours of the morning, after the sailors have spent an evening with lights and laptops running and maybe watching a movie. The boat’s batteries are depleted and the solar panels are still asleep and aren’t helping out. It’s like asking a weak and starving person to move furniture.



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The manufacturers of converters, inverter/chargers and diesel engine alternators each approach the methodology of multi-stage charging in unique ways, and the charging systems described on this page are just a few examples from our own personal experience.

If you have the time and the inclination, read the user manuals of the charging systems on your RV or boat, find out what your battery manufacturer gives for recommended settings, and set your charging systems up accordingly.

To continue to the next article in this series, click here:

Solar Charge Controllers – Optimizing Battery Charging from the Sun

4-Part Series on RV and Marine Battery Charging Systems:

Related posts about batteries, solar power and living off the grid in an RV or boat:

Never miss a post — it’s free!

Our most recent posts:

More of our Latest Posts are in the MENU.   New to this site? Visit RVers Start Here to find where we keep all the good stuff. Also check out our COOL NEW GEAR STORE!! *** CLICK HERE *** to see it!

RV and Marine Battery Charging Basics

RV and marine batteries can be charged using many different kinds of charging systems, and understanding the way these chargers work can make a huge difference in whether or not you get the most out of them.

Not only are there differences between single stage charging and multi-stage charging, but in our experience, no two multi-stage chargers use the same charging algorithm. Also, the ability to program the settings on each charging system varies a lot from unit to unit.

Furthermore, some chargers, like converters, inverter/chargers and engine alternators, are powered by a consistent power source that allows them to operate at their maximum ratings at any time of day or night. Others, like Solar Charge Controllers and wind chargers are powered instead by an energy source that comes and goes.

In our eleven years of living off the grid in an eleven years of living off the gridlesstraveled.us/hitchhiker-2/” title=”2007 NuWa Hitchhiker 34.5 RLTG fifth wheel trailer RV” target=”_blank”>RV and a sailboat, we have relied on a wide variety of systems to charge our batteries. At times, we have used a converter, inverter/charger or engine alternator in conjunction with our solar charging system, and we’ve learned a lot about these systems and how to make them work together harmoniously.

The four parts in this series cover the following:

1. Battery Charging Basics – (this article) – Explains single-stage charging and multi-stage charging and explores the ways that certain products implement a multi-stage charging algorithm (no two are alike).

2. Converters, Inverter/Chargers and Engine Alternators – Discusses the differences between converters, inverter/chargers and engine alternators, which I lump together as “artificially powered” charging systems

3. Optimizing Solar Charge Controllers – Examines these “naturally powered” solar charging systems whose power source is the sun, which is very unreliable.

4. Combining Solar Power with Shore Power or an Engine Alternator – Reveals some of the subtleties of solar charging and gives some ideas for how to get the most out of a solar charge controller when it is run alongside a converter, inverter/charger or engine alternator.

This first post in the series has many sections, and you can easily navigate directly to them by using the links below.


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Many people enjoy RVing and cruising without every relying on the house batteries for more than a few hours or an overnight. However, some of the joy of traveling with an RV or boat is being independent and free, and there is no better way to experience that freedom than to spend a few nights on your own, camped on public land or anchored in a quiet cove. Having well charged batteries makes a big difference in how comfortable you’ll be. Also, understanding the gear that charges your batteries can go a long way towards making sure your batteries perform optimally and are in the best condition possible.

In our household, Mark is the one who does the installation work while I (Emily) am the one whose head is in the clouds somewhere thinking about theory and design. When Mark asks me to hand him a box end wrench while he’s peering into some dark corner of our boat or RV, I go rummaging around in all our boxes and stare at all the wrenches and wonder what he wants.

When the installation is finished, however, Mark washes up and washes his hands of all concerns about it. If he flips the switch and it runs, then he’s off the hook. “The factory settings are fine!” He tells me. “Set it and forget it!” But that’s the time when my curiosity just begins to get going. I want to know how it works, what makes it tick, and how it’s designed.

I admire Mark’s carefree and trusting attitude, and truly:

Your batteries will probably be fine if you click off this page right now and go read something more amusing.

But for those folks out there who just can’t pry their minds away from these things, I hope this four-part series will give you some food for thought. I make no claims to be an expert and am simply passing on the things I’ve observed and learned.



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In order to have a consistent standard for rating how much power a battery can store, manufacturers indicate how many amps of current draw it takes to drain their battery to 80% discharge (down to 1.75 volts per cell, or 10.5 volts for a 12 volt pattery) over a given time period. For “deep cycle” batteries this time period is 20 hours, and it is called the 20 hour amp-hour rating.

Batteries are also manufactured in standard sizes, including Group 24, Group 27, Group 31, 4D and 8D, for 12-volt deep cycle batteries, and GC2 for 6-volt batteries that power golf carts. The ratings are given in the manufacturer’s specs for the batteries and is often shown on a sticker on the battery itself.

These Amp-Hour ratings can range from about 70 amp-hours for a single 12-volt Group 24 battery to 220 amp-hours for a pair of 6-volt GC2 batteries to 230 amp-hours for a single 12-volt 8D battery.

Wait, what was that about a PAIR of 6-volt batteries??

When batteries are wired in series, the current draw remains the same while the voltage of the pair of batteries doubles. For this reason, when a 6-volt golf cart battery is rated with a 220 Amp-Hour capacity, wiring it to a second 6-volt battery to create a virtual 12-volt pair does not double its Amp-Hour capacity. Those two 6-volt batteries wired in series have the same old 220 Amp-Hour capacity that the single battery did.

The physical size of these battery types varies too, with a Group 24 12-volt battery weighing as little as 47 lbs and an 8D 12-volt battery weighing as much as 160 lbs. 6-volt golf cart batteries are the same width and depth as 12-volt Group 24 batteries, however they are a little taller and heavier, and they offer a lot more storage capacity per pair than a single 12-volt Group 24 battery does.

RVs are typically sold with Group 24 or Group 27 size batteries, either a single battery or two.

To beef up an RV’s battery bank, the easiest and most effective upgrade is to replace the single 12-volt battery with two 6-volt golf cart batteries wired in series. This will typically increase the battery capacity from about 70 amp-hours to 220 amp-hours.

An alternative upgrade option, if there isn’t enough height in the battery compartment for 6-volt batteries, is to add a second 12-volt Group 24 battery (if the first battery is new) or to replace the single 12-volt battery with two 12-volt batteries for an overall capacity of around 140 amp-hours.



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In essence, discharged batteries are a lot like hungry people. If you’re super hungry, you’ll dive into a big dinner with gusto. If you eat too much too fast, you’ll get sick! If you eat at a normal pace, you’ll slow down as the meal progresses, and eventually you’ll be full and you won’t want any more food.

Batteries are very similar. The food they want is current (amps), but if you feed them too much they get damaged!

Discharged (hungry) batteries can accept a lot of charge (current) at first. However, as they become more and more charged, they accept less and less current. A fully charged battery is around 12.7 volts. A fully discharged battery that still has enough life in it to be able to be fully charged again is around 11.6 volts. RV and marine house batteries will last longest if they are always kept above 12.0 volts, preferably above 12.1 volts.

The way a battery is charged is that some external charging device temporarily forces the battery to a higher voltage than its “fully charged” voltage of 12.7 by feeding it lots of current.

The fastest way to charge a battery is to put as much current into it as possible. As long as the charger is delivering lots of current, the battery’s voltage will rise. The charger itself must be at a higher voltage than the batteries to do this. If the charger is around 13.5 volts, it can force a modest amount of current into the batteries. If it is around 14.5 volts, it can force in a lot more current.

During charging, the battery voltage will rise into the high 12 volt range, then it will move into the 13 volt range, then 14, and so on. It takes time for the battery’s voltage to rise as it is fed current. A more deeply discharged battery will take longer to reach a given voltage than a minimally discharged battery will.

If the charger is turned off so no current is going into the battery, the battery will gradually fall back to is own “internal” voltage. This may take 15 minutes or more. If it has been charged for a while, this voltage will be near or at the “fully charged” value of 12.7 volts. If it hasn’t been charged long enough, the battery’s internal voltage will be lower than that.

For instance, if a battery is partially discharged to 12.4 volts, the way to get it charged back to 12.7 volts is for a charging system to give it a bunch of current and temporarily force it up to some higher voltage in the 13 to 15 volt range. The charging system itself will need to be at a higher voltage than whatever voltage it is trying to get the battery to.

After a while, when the charging system is turned off and the battery is allowed to settle back down to its own internal voltage, it may drop back to 12.7 volts, in which case the battery is fully charged. However, the battery may settle back down a little lower — perhaps to 12.5 volts — which means it could use a little more charging to reach a fully charged state.


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The following chart shows the different voltages batteries have when they are charged or discharged. If you have nothing running in the rig (no computers running, no TV, no vacuum or toaster, etc.), you can measure the battery voltage using a hand-held voltmeter in DC volts mode by placing the two probes on the two battery terminals. This is what we do. You can also install a simple volt meter on the wall of your coach or install a fancier battery monitor.

Battery charge state chart

Data from Trojan Battery, rounded to tenths for easy memorizing.
Note that the values decrease by 0.1 volt for each 10% drop until 60%.

If the battery has just finished charged for a few hours, there will be a surface charge on the metal plates inside of it which will raise the voltage by a tenth of a volt or so. Running an appliance for a few minutes in the RV or boat will remove that surface charge so you can see the battery’s true internal voltage.

On the other hand, if a lot of appliances are running in the rig, current will be being drawn out of the battery and the battery’s voltage will be lower than its true internal voltage. Turning everything off and waiting a few minutes will bring the battery back to its true internal voltage.


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Batteries are filled with thin metal plates and battery acid (electrolyte). As a battery’s voltage is raised, the internal chemical reactions inside the battery make the electrolyte heat up. If the voltage is raised high enough for long enough, the acid begins to release gases (like hot water beginning to steam), and eventually the acid begins to boil.

Trojan Reliant 12 volt AGM battery metal plates inside

Looking down into the battery cells of four 12 volt Trojan flooded batteries
before the electrolyte is poured in.

Raising a 12 volt battery to a voltage in the high 14’s or more for a few hours is enough to make the batteries begin to start gassing. Reducing the voltage to the mid-13 volt range stops the gassing.

Some trickle chargers don’t allow the battery voltage to rise above the mid-13 volt range to avoid having the batteries begin gassing. However, the less a battery’s voltage is raised, the less current will go into it and the less the battery will be charged after a given number of hours. It is possible for the battery to become fully charged at a lower voltage, but it will take much longer.

The engineers at Trojan Battery have told us that almost all the dead batteries they have studied over the years have been chronically undercharged. Overcharging is a much less common problem.

When batteries are chronically undercharged, they develop lead sulfate crystals on the lead plates inside the battery. This is called sulfation. This material reduces the battery’s capacity, and it can even form a bridge from plate to plate, creating an internal short and rendering the battery useless.

With flooded (wet cell) batteries, raising the battery voltage very high (15 volts or more) for a few hours heats up the electrolyte until it gasses and boils and sloughs the sulfate material off the metal plates. The material then settles on the bottom of the battery underneath the plates where it doesn’t risk forming a bridge between the plates. This process is called Equalizing.

Equalizing is done only on wet cell (flooded) batteries. Gel and AGM batteries are sealed and cannot release gasses, so they can actually be damaged by charging them at a very high voltage in this manner.

There is no definitive moment when a battery is fully charged. It is similar to feeling full at the end of a meal. After a great dinner, you can usually find room for a yummy sliver of pie, or maybe just one bite of your spouse’s pie, but you can definitely leave the table feeling full without having any pie at all. RV and marine batteries are much the same in that they can usually accept another fraction of an amp of current from a charger even though they are essentially full charged.



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Batteries need to be used, and the worst thing that can happen to a battery is that it doesn’t go through regular discharging and charging cycles. Like a person who needs to exercise to to burn calories and give them a good appetite so they can consume some nutrition, batteries need to be used (discharged) and then charged up again to maintain peak health.

RVs and boats that are stored without being plugged in to shore power for long periods of time will slowly have their batteries discharge completely over a period of months. That’s not good! There’s nothing like coming back to the RV or boat to find dead batteries. However, if the RV or boat is left plugged into shore power to avoid this problem, even though the batteries will be fully charged at the end of a few months, they may still die a premature death due to not getting enough exercise and not being used.

For RVs and boats left on a charger for months at a time, whether or not the owners are living on board, a charger that periodically raises the battery voltage above a trickle charge will help prolong the battery life. Occasionally unplugging from shore power and running some appliances for a few hours will give them a good workout too.

The engineers at Trojan Battery have spent years studying car batteries that have died. The most common failure they find is what they call “Lot Rot” caused by cars that are used infrequently and drive only short distances.


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Battery chargers come in all sizes with maximum current output ratings that range from a few amps to hundreds of amps. One rule of thumb for sizing a battery charger to a battery bank is for its maximum current output rating to be roughly 25% of the amp-hour capacity of the battery bank.

RVers and sailors that plan to boondock or anchor out a lot tend to replace the factory installed battery banks with bigger ones. In this case, it is worthwhile to review the sizes of the factory installed charging systems to make sure they will be big enough to charge the new battery bank efficiently.

For instance, an RV or boat shipped with two Group 24 12 volt batteries that have a combined amp-hour capacity of 140 amps wil be fine with its factory installed 55 amp charging system. But if those batteries are upgraded to four 6 volt golf cart batteries with a combined capacity of 450 amp-hours, a larger charging system will perform better.


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A single stage charger will deliver enough charge to keep the batteries at a set charging voltage indefinitely. At first, the batteries will require a fair amount of current to be able to maintain that voltage. But as time goes on they will need less and less current to maintain that voltage. If the charging system is turned off, they will drop down to their own “internal” voltage. If that internal voltage is 12.7 volts, then they are fully charged. If not, they need to be put back on the charger!

This kind of single stage charging system works okay, but it is inefficient and risks undercharging or overcharging the batteries.

Automotive battery chargers generally charge the batteries at a high voltage (in the mid-14 volt range). This is fine for a while, but the batteries can’t be left on this kind of charger for very long or they will overcharge. An alternative is a single stage trickle charger that charges the batteries at a modest voltage (in the mid-13 volt range). This is how a lot of cheaper RV battery chargers (converters) work.

The problem with a single stage trickle charger is that it takes a very long time for the batteries to reach full charge. That’s okay if you are plugged into shore power for a few days, but if you are running from a generator, do you really want to run it for 12 hours just to get the batteries charged?

Also, a single stage charger never pushes the batteries up to a higher voltage, something that is considered helpful for prolonging battery life.



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A more efficient charging system is to give the batteries a lot of current at first, while they are most depleted, and then to back off, forcing less current into them once they are fairly well charged up. This is what multi-stage charging systems do.

Multi-stage chargers generally have three stages: Bulk, Absorb and Float.

Bulk Stage

In the Bulk stage, the battery is given as much current as the charging system can deliver. As the batteries accept this charging current, their voltage slowly rises. Eventually the batteries reach the “Bulk Voltage” which is something in the range of 14.3 to 14.8 volts, depending on the charger, the battery manufacturer’s recommendations and/or your own personal choice.

Absorb Stage

At this point the multi-stage charger switches tactics. Rather than giving the batteries as much current as the charger can deliver, the charger instead gives them only as much current as it takes to keep them at a particular voltage known as the “Absorb Voltage” (which is also usually between 14.3 and 14.8 volts). While the batteries are held at the Absorb voltage, they are in the Absorb stage (this is called the “Accept” stage by some manufacturers, but is more commonly known as the Absorb or Absorption stage).

The idea in the Absorb stage is that rather than force feeding the batteries all the current the charging device can deliver, the batteries are given just enough to keep them at the Absorb voltage. At first, this is pretty much the same amount of current they were getting in the Bulk stage. But after a while, the batteries don’t need as much current to be able to maintain the Absorb voltage. So, over time during the Absorb stage, the multi-stage charger delivers less and less current to the batteries, and the batteries just “hang out” at the Absorb voltage, getting force-fed a steadily decreasing amount of current.

Float Stage

At the end of the Absorb stage (and what defines “the end” of the Absorb stage is one of the areas where manufacturers and devices differ the most), the multi-stage charging system switches tactics again. Now, rather than holding the batteries at the relatively high Absorb voltage of 14.3 to 14.8 volts, the charger will hold the batteries at a much lower Float voltage in the range of 13.3 to 13.6 volts.

Of course, the batteries will require a lot less current to maintain this lower voltage, so the charger will now be delivering a much lower current. And again, as time progresses, the amount of current that the batteries need to maintain the Float voltage will diminish. At first, the batteries will need a fair bit of current to maintain the Float voltage, but as the hours go by they will require less and less. As with the Absorb stage, the batteries will just “hang out” at the Float voltage during the entire Float stage.

When the batteries reach the Float stage they are considered to be pretty nearly fully charged. If the charger is turned off at this point, the batteries will eventually settle down (after a few minutes) to their own internal voltage, and that voltage will be around 12.7 volts, indicating that they are fully charged.



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Of course, the multi-stage charger could be turned off at any time during the charging process, before the batteries are fully charged. Why? Well, during Bulk or Absorb or Float you might unplug the shore power cord so the RV or boat can go somewhere, or you might turn off the generator for quiet hours in the campground, or the sun might set, making the solar panels ineffective, or an engine with a built-in engine alternator might be turned off when the sails are raised on a sailboat or the motorhome is parked, etc.

These are all arbitrary events that could happen at any point in the multi-stage charging process.

When this happens, the batteries are more charged than they were, but they aren’t necessarily fully charged. In other words, if the multi-stage charger is turned off before the batteries are fully charged, the batteries will gradually settle down to their own internal voltage, whatever it is at that point. It might be 12.4 volts or 12.6 volts — who knows! Obviously, it should be a higher voltage than when the multi-stage charger first started charging the batteries.

For most mutli-stage chargers, when they resume charging the batteries, they begin the process all over again, first going through the Bulk stage, and then the Absorb stage, and then the Float stage. But again, different manufacturers and different products handle this scenario various ways.


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Most multi-stage chargers have a fourth charging stage which is intended to help wet cell (flooded) batteries last longer. This stage is not needed or used by Gel or AGM batteries. In the “equalize” stage, the charger raises the batteries to an even higher voltage than the Bulk or Absorb voltage for a few hours (generally in the mid-15 volt to low 16 volt range). During this time the battery acid (electrolyte) inside the battery will heat up and begin to boil, sloughing the sulfation off the metal plates in the battery and letting it drop down to the bottom of the battery underneath the plates.

Outback MX60 Solar Charge Controller in Equalization Stage

Here, our Outback solar charge controller has held the batteries at 15.8 volts for 47 minutes during an Equalize stage. At this moment it required 17.4 amps to keep the batteries at 15.8 volts.



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Converters and inverter/chargers on RVs and boats that are plugged into shore power all the time charge the batteries 24/7 and never stop. The way that multi-stage chargers manage their Float stage is one of the big differences between them.

Some chargers keep the batteries at a Float voltage all the time, forever, until they are turned off. Some periodically “reboot” automatically and go back through the Bulk and Absorb stages. A few provide you with a way to force the charger back into the Bulk stage to start the charging process over again manually if you need to.

Periodically leaving the Float stage and going into Bulk and Absorb will help prolong the battery ilfe.



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Generally, the Bulk voltage and the Absorb voltage are the same value, or very close, so the only difference between the Bulk stage and the Absorb stage is how much current the batteries are receiving.

In Bulk, the charger is delivering its maximum amount of current to the batteries to raise them up to the Bulk voltage. A small charger’s maximum current will be less than a large charger’s maximum current is, so a small charger will get the battery up to the Bulk voltage more slowly than a big one will. Either way, the chargers are working at their peak in the Bulk stage, pouring as much current into the batteries as possible.

In Absorb, the goal is to keep the batteries fixed at the Absorb voltage, so the batteries are given only enough current to keep them there. The amount of current they need to do this drops off over time.

So, in the first case the batteries are ramping up to the Bulk voltage due to receiving as much current as the charger can deliver, while in the second case the current going to the batteries is steadily decreasing because they are being given only enough current to keep them at the Absorb voltage.


These are the basic concepts involved in charging RV and marine battery banks. I’ve mentioned a few times how manufacturers and charging systems vary, and in the following posts I will be showing what those variations are.

To continue to the next article in this series, click here:

RV Converters, Inverters and Engine Alternators

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RV Electrical System Overhaul – New Batteries, Inverter & Converter!

April 2015 – For the past ten days we’ve been doing a total overhaul on our RV’s electrical power systems, and we’re really excited about the upgrades. Having installed several RV and boat solar and battery systems to date, both for ourselves and for friends, we’ve gone all out this time, researching, studying, and talking with the engineers at different companies to figure out which components will suit our needs best. Our upgrades include:

  • Trojan Reliant AGM batteries
  • Exeltech 2000 watt pure sine wave inverter
  • Iota 90 amp converter / multistage charger
Trojan Reliant AGM 6 volt battery

We’re getting new Trojan Reliant T105-AGM batteries!


Since we live on solar and battery power in our RV 100% of the time, having a robust power plant on board makes all the difference. Back in 2008 when we first got our fifth wheel trailer, we asked the RV dealership to install four Trojan T-105 6 volt wet cell batteries for us. These were terrific and served us very well for quite a few years.

However, because we had to leave the trailer in storage for stretches of 12 to 20 months at a time when we cruised our sailboat in Mexico, they deteriorated because no one was there to do the routine maintenance they require.

Wet cell batteries are inexpensive, which is why we chose them at the outset of our RVing life. However, once we started living with higher quality AGM batteries on our sailboat, we found AGM batteries have many advantages over wet cells (our boat had four Mastervolt 4D AGM house batteries and one Mastervolt Group 27 AGM start battery). So we decided to upgrade our RV battery bank to AGM.

Much to our surprise, we managed to time this upgrade really well, because Trojan Battery has revamped, redesigned and re-engineered their AGM battery line completely, and their new Reliant AGM batteries have just hit the market in the last month.

Trojan Reliant AGM 6 volt battery

Our new batteries go into the fifth wheel basement.

The batteries we are installing are their new 6 volt AGM battery called the Trojan Reliant T105-AGM.

Trojan Battery has been at the forefront of battery engineering and technology for decades, and this new AGM version of their ultra popular T-105 6 volt wet cell batteries is a true deep cycle AGM battery, designed to deliver steady power and withstand deep discharging of 50% of the battery’s capacity day after day after day (we plan to discharge them 25%-30% or less each day).

Most AGM batteries are actually dual purpose, designed not only to provide long-term power and deep discharging, but also to pack a high cranking power punch that can get an engine started without discharging the battery much at all. Our boat’s AGM batteries were all dual purpose marine batteries, despite their enormous size.

Obviously, a battery designed specifically for repeated deep discharging is going to be superior as a house battery to one that is designed to be both a deep cycle house battery and a start battery. So these new Reliant AGM batteries should work really well in an RV (or boat!).

The list of advantages of AGM batteries over wet cells is considerable:

  • Maintenance free – no equalizing and no adding distilled water (great if the RV gets stored for months on end)
  • Discharge just 3% per month when they aren’t being used (also important for longer term RV storage)
  • Charge more quickly than wet cell batteries
  • No gasses released during charging, so no special venting is needed in the RV battery compartment
  • Can be installed on their sides or ends since there is no liquid that can spill out

Mark has been very busy revamping our fifth wheel basement battery compartment, and he is taking this opportunity to rewire it entirely, applying all the things we’ve learned in 8 years of living off the grid!


At the same time as our battery upgrade, we also decided to upgrade our inverter. We have loved our Exeltech XP1100 Pure Sine Wave Inverter since we installed it in 2008.

Exeltech makes all the inverters used by NASA, and they supplied all the inverters to both the American and Russian sides of the International Space Station (the two sides run on different voltages and currents, so they need different inverters!).

Exeltech XP 1100 Inverter

Our old Exeltech XP 1100 pure sine wave inverter is getting replaced with the 2000 watt version

Exeltech XPX 2000 watt pure sine wave inverter

Our new Exeltech XPX 2000 watt pure sine wave inverter

The quality of the electrical signal produced by Exeltech inverters is so pure that they are used by field medical units to run sensitive medical equipment. One nice thing about living on inverter power exclusively is that we never have to contend with flakey RV park electricity, and we know our Exeltech inverter is giving us a great signal whenever we turn it on.

Our old Exeltech XP1100 inverter was too small, however. We have a 900 watt microwave, and 1100 watts of inverter power was shaving it just a little too close. A mishap last year made us realize we needed to go bigger. So we are installing an Exeltech XPX 2000 Pure Sine Wave Inverter that will give us 2000 watts of power.


Solar panels charge our batteries almost all the time, but once in a while we get stuck in overcast and stormy conditions for a while. After about 4 days of grey skies, we turn to our trusty Yamaha 2400i portable gas generator to bring our batteries back to full charge. When we run the generator, we plug the generator into our shore power input connector on the side of our trailer so the converter in the fifth wheel basement charges the batteries.

Our fifth wheel trailer came from the factory with an Atwood 32 amp converter which is a single stage battery charger. This is typical of converters installed in RVs. Rather than going through three stages of charging, these simple converters give the batteries a mere trickle charge at a low charging voltage.

RV manufacturers save on costs by installing basic single stage converters rather than robust multistage charging converters, and since most RVs are plugged into shore power all the time, it doesn’t matter if it takes 48 or 72 hours to charge the batteries completely.

Iota DLS-90 Converter

The Iota DLS-90 / IQ4 Converter does true multi-stage battery charging

Sperry Gardner Bender DSA 540A Clamp-On Volt - Amp Meter

Sperry stands behind their gear!

However, the only time our converter is charging our batteries is when we run our generator, and with that thing making noise and burning fuel, we want the batteries to be charged as quickly and efficiently as possible. We don’t want to trickle charge our batteries from the generator!

We are replacing our old converter with an Iota DLS-90 / IQ4 Converter which not only provides three stages of battery charging but will also put the batteries into a true bulk charge state when we first turn on the generator.

So, with all this wiring going on, we’ve been giving our trusty Sperry Clamp-on Amp/Volt Meter a good workout lately. And we’ve had a surprising experience with that little piece of gear.

We bought it back in 2010 when we were wiring up the solar power on our sailboat. But it died 10 days ago, right as we were starting our new RV power upgrade project. Of course, the warranty ran out a long time ago, but we called the company to see if there was anything they could do. We were shocked when they sent us out a replacement unit at no charge!

It is so rare these days for a company to stand behind its products like that, especially something small and inexpensive like a volt meter. Wow!

We’ll be posting much more detailed info about our electrical system upgrade once we’ve finished it all, so stay tuned!

For now, we’re extremely grateful to our good friend “Mr. G” who invited us to shoehorn our rig into his driveway in Sarasota, Florida, and make use of his workbench, tools and fabricating expertise as we tackle this exciting project.

Fifth wheel RV between houses in Sarasota Florida

A great spot to do a little upgrade work on our rolling home!

Want to learn more about all this, check out these informative posts:

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Sailboat Solar – Installing Solar Power & a Solar Panel Arch on a Boat

This page describes our solar panel installation on our sailboat, a Hunter 44DS.  This setup has allowed us to anchor out almost exclusively.

Groovy's solar panels.

Boat solar panel arch installation

Happy panels in full sun, Sea of Cortez.

Marine solar mount installation

Full sun & no shade (3 panels working):   22.5 amps

Boat solar mount

One panel partially shaded (2 panels working):  15 amps.

Boat Solar panel partial shade problems

Shade straddles two panels (only 1 panel working):  9.5 amps.

Boat solar arch for panels

Polished welds and drilled/tapped/screwed joints.

sv Groovy's solar panel arch welds. sv Groovy - sailboat solar panel arch.

Liquid Metal

boat solar power mount on a sailboat

Comparison: Factory weld on our Hunter arch.

Sailboat solar panel arch

The arch extension arrives for a fitting.

marine solar panel mount extension.

Alejandro tie-wraps it in place.

Sailboat solar panel installation

Mark helps hold it up.

Boat solar power Marine solar installation

The extension is in place -- without its legs yet.

Boat solar mount installed on a sailboat

Jose checks if it's level.

Sailboat solar arch extension

The arch extension returns -- now with support legs.

Marine solar panel arch support

It's maneuvered into place.

Boat Solar arch on a sailboat Telescoping davits on a solar panel arch

Telescoping davit arm (marine solar panel arch)

Boat solar installation with solar panel arch.

Held in place with tie-downs.

Boat solar power arch installation

Looking good!

Marine solar power

Alejandro drills and taps holes in the arch.

The boat's solar panels are ready!

The solar panels are ready!

Boat's solar panel arch extension removed while Alejandro taps and drills.

Arch extension removed from Groovy while Alejandro drills

and taps the arch on the boat.

Installing solar panels on a sailboat

Heave ho!!

Installation of marine solar power system Marine solar power system installation

The second panel is installed.

Boat solar installation with solar arch

Three panels - yay!

Sailboat Solar power setup

Alejandro and Mark test the strength of the arch extension.

Boat solar powert

Mark begins the big job of wiring it all up.

Marine solar power system diagram

Component layout:  3 panels, combiner

box, controller & 4 batteries

Sailboat solar power design

Combiner box (upper left) and controller (lower right).

Boat solar power design and installation

Wiring the panels.

Sailboat solar panel system design and installation

All done.

Solar panels on our Hunter 44DS Groovy.

In use 18 months later in Puerto Vallarta.

Marine solar power system and design

Sailing in Huatulco.

Sailboat Solar Power & Solar Panel Arch Installation

This page describes the solar power setup we installed on Groovy, our Hunter 44DS

sailboat.  This was our third solar installation.  Our two RV solar installations are described

here: RV Solar Installations, and we have a boatload of info here: Mobile Solar Power


We learned a lot from those installations, and have written lots of details about solar power on

this website, including a multi-part Solar Power Installation Tutorial for beginners. Going

into far more detail, we have a 4-part primer on battery charging which includes:

-- The basics of multi-stage charging

-- How converters, inverter/chargers and engine alternators REALLY work

-- How to optimize a solar charge controller

-- What happens when TWO systems (like solar/alternator) operate at once?.

The company Kyocera

Solar liked our solar

panel installation so

much, they featured

Groovy on their website.


For comparison, our solar power installations have consisted of

the following:

Lynx Travel Trailer

(1) 130 watt/12 volt Kyocera solar panel

(1) Morningstar 10 amp charge controller

Various 150 watt to 800 watt portable and

semi-portable modified sine wave inverters

(2) Energizer 6 volt batteries in series (220 amp-hours).

Hitchhiker Fifth Wheel

(1) 130 watt/12 volt Kyocera and (3) 120 watt/12 volt Misubishi solar panels (490 watts total), wired in series

(1) Outback 60 amp MPPT charge controller

(1) 2,000 watt pure sine wave inverter permanently mounted

(4) Trojan 105 6 volt batteries wired in series and in parallel (440 amp hours).

Hunter 44DS sailboat

(3) 185 watt/24 volt Kyocera solar panels (555 watts total), wired in parallel

(1) Combiner box (combines 3 panel wires into 1 going to the charge controller)

(1) Xantrex 60 amp MPPT charge controller

(1) 600 watt pure sine wave inverter

(1) Xantrex 2500 watt modified sine wave inverter/charger

(4) Mastervolt AGM 4D batteries, (1) Group 27 AGM battery (710 amp-hours)

Notes:  (1) Our odd collection of panels on the Hitchhiker was due to the Kyocera 130 panels not being available at the time of

our installation (we brought one over from the Lynx).  (2) Our switch from the Outback to the Xantrex charge controllers between

the Hitchhiker and the boat was due to the Xantrex being cooled by non-moving fins rather than a fan.  In hindsight I would

probably use the Outback charge controller in the future only because it displays more information on its screen rather than

having to scroll through multiple screens to get the voltage, amperage, watts and charging stage.  (3) Our Group 27 start battery

on the boat is isolated from the set of 4D house batteries only when the voltage of the bank drops too low.

The boat has a DC refrigerator and a DC freezer which together eat up some 100-130 amps or more every 24 hours, depending

on ambient temperature.  In addition we listen to music on the stereo with multiple speakers and a large subwoofer, we watch

DVD's many nights on a 22" TV, we use two laptops for several hours everyday.  We also have a water pump, electric flush

heads and VHF radio which we use at anchor.  Our cabin lighting is a combination of fluorescent and LED, and our anchor light is

LED.  So our typical daily amperage use at anchor is between 180 and 250 amps.

In December, around the winter solstice, on the southern mainland of Mexico (Zihuatanejo) our solar setup collected about 170

amp-hours per day.  In June, around the summer solstice, in the middle of the Sea of Cortez (San Carlos) our solar setup

collected about 250 amp-hours per day.  In hindsight, it would be nice to have at least 750 watts of solar power to meet our

power demands in winter.


The biggest problem with installing solar power on a sailboat is accidentally getting a little shade on the panels.  While swinging at

anchor, the mast, boom, radome and other things high up all conspire to throw pockets of shade on the solar panels and make

them quit working.  It is quite shocking to find out just how little shade is needed to reduce the panels to zero output.  We had

experimented a bit with partial shading issues on our fifth wheel solar installation (see bottom of Solar Setup), but we never park

near shading objects so it is not a problem on that moveable home.  A sailboat is a whole different story.

An interesting paper Shade Effects on Conventional PV (5th article down) from the Physics Department at the University of

Arizona describes how shading just half of one row of "squares" on a solar panel -- as often happens in the morning or afternoon

hours on a commercial installation if the rows of panels are placed too close together -- the panels shut down or reduce their

output significantly.  The opening sentence says it all:  A panel that is 8% shaded loses 94% of its productivity."  Deep down in the

meat of this paper the math lost me (sigh), but for a layman's explanation of just how devastating shade can be on solar panels,

this website delivers the skinny.

We placed our panels as high and as far back from the boom as we could.  We also pull the boom aside while at anchor, but the

panels still get shaded by the mast/forestay/radome when the sun is forward of the shrouds and they get shaded by the sails

when sailing.  As an experiment, we took some notes about how partial shade affects our panels.  This data was taken on

February 3rd at 10:00 a.m.  The shade was caused by the mast, forestay and radome (affixed to the front of the mast).  The

shade moved slowly back and forth across the panels as the boat swung at anchor.

Panels in full sun:

22.5 amps

One panel partly shaded:

15.5 amps

Two panels slightly shaded:

9.5 amps

As another experiment we sailed and noted the amperage

produced by the solar panels as we sailed on two different

tacks.  On one tack the mainsail shaded one entire end panel

and half of the middle panel.  On the other tack the boat was

heeled away from the sun but there was no shade on any of

the panels.  It was far better to be heeling away from the sun

than to have the panels shaded.  This data was taken at 11

a.m. on January 31.

1½ panels fully shaded by sails:

10 amps

No shade, tilted away from sun:

24.5 amps

So it seems to me that shade is the number one enemy of solar panel power production on a sailboat, and orientation towards

the sun is a lot less important.  If the solar panels are installed in such a way that a nearby radome or wind generator is always

partly shading one panel in the array, as too often happens in solar panel installations on sailboats, the result will be dramatically

reduced power production.


Our boat came with a fantastic arch that supports the traveler.  We used it as a base for an elegant stainless steel extension that

supports the three panels.  We hired Allejandro Ulloa of Ensenada, Mexico to create this arch extensions.  Alejandro is an artist

and a master craftsman.  And he is extremely professional.  We gave him a sketch of what we were looking for, he responded

with a written quote for half of what it would have cost in San Diego, and we were off and running.

Alejandro prides himself on the beauty of his work.  He polishes the welds and installs tubing that

seems to flow like liquid metal as it rounds corners and changes thicknesses.  In our opinion, his

arch extension dramatically increased the esthetics of our boat.  It also added functionality

besides just supporting the panels.  It makes a great spot for hanging on when you're sitting in

the rear jump seats, it has a

telescoping davit system,

and the panels provide

much needed shade.

If you need to have an arch

or any kind of stainless steel

structure fabricated for your

boat and you are heading to

Mexico from the US or

Canada, spend some time in

Ensenada and look up

Allejandro Ulloa (email:

alejandrossw [at] hotmail [dot] com,

Mexican phone: (646) 171-5207).  He can

be contacted through the excellent Baja

Naval boatyard as well.  There are other

stainless steel fabricators in Mexico but we

haven't seen anyone nearly as skilled or

as professional in their approach.

Alejandro built the extension in his workshop and then brought it

to the boat to size its supporting legs.  This was a thrilling process

for us, as we began to see it taking shape on the boat.  The entire

arch extension was wrapped in plastic for this phase to protect

the finish.

Mark helped wherever he could and I took endless photos.

Alejandro returned on another day with the finished arch extension.

Now it had tabs for the solar panels, and the supporting legs had

been cut and welded at the right length.

We wanted the arch extension to double as a davit system.

Alejandro designed clever telescoping tubes that snap into place in

an extended or contracted position, and he fabricated two beautiful

cleats.  We have found that we use the davits in the contracted

position most often because they hold the porta-bote tight to the

swim platform where it fits perfectly into the swim step cutout in the


We anticipated

mounting the solar

panels ourselves,

as the quote

Alejandro provided

was for building

and installing an

arch extension,

not for installing

panels.  We

weren't sure how

we'd get them mounted, but we knew

we'd figure it out.

Meticulously adhering to the

"measure twice cut once"

philosophy, Alejandro

dismantled the whole thing

for some adjustments and

then mounted it one last

time for the final installation,

tapping and drilling and

screwing each of the arch's

feet into place in a bed of


Then, to our amazement,

Alejandro and his assistant

began mounting each of

the panels.  Mark quickly

jumped in.  These are not

light panels, and it was

quite a stretch to get them

in position.  Alejandro was

concerned about possible

corrosion due to the

dissimilar metals of the

panels' aluminum frames

and the stainless steel arch

extension, so he placed a

plastic insulator in each

attachment point.

When it was all

finished, Alejandro

wanted us to be

confident that the arch

could support a dinghy

and engine.  He and

Mark swung from the

davits.  Both are

lightweights, but they

were still twice the

weight of our

dinghy and



Alejandro's work was done, but we still had a big project ahead.  We ran the wiring

inside the arch so it wouldn't show (it wasn't easy snaking it through!!), and we placed

the combiner box and charge controller in a transom locker.

The installation

looked beautiful

and it worked, but it did not work as efficiently as it

could have.  The whole system produced about

20% less power each day than it was capable of

doing.  We learned we'd made two vital mistakes.

One advantage of using 24 volt solar panels is that

we had half as much current in the wires as we

would have had if we'd used 12 volt panels.  Rather

than 36 amps (at 12 volts) at peak production we

had just 18 amps  (at 24 volts).  This allowed for a

smaller wire size, which is much easier to work with

as it is a lot more pliable, and it's cheaper to boot

(marine grade electrical wire is exorbitant).  Our

salesman at Northern Arizona Wind and Sun had recommended we use 10

gauge wire throughout the system.  This turned out to be inadequate

because the distance between the panels and the batteries is so long --

about 50'.  For wire gauge sizes, amps and

distances, see this chart.

Our second mistake was placing the charge

controller in an aft transom locker.  Our batteries

are next to the centerline of the boat at the lowest

point above the keel in the main salon.  The

charge controller needs to be close to the batteries

as possible.  The distance from the charge

controller in the transom locker to the batteries

was about 30' -- too far.    The combiner box was

fine back there, but the charge controller had to be


Although most of our circuit runs at 24 volts -- from

the panels to the combiner box to the charge

controller -- allowing for smaller wire, the portion

between the charge controller and the

batteries runs at 12 volts.  Therefore, the

cable between the charge

controller and the batteries

needs to be not only as short as

possible but very large as well.

We moved the charge controller

into the cabin in a hanging

locker about 10' from the

batteries and and switched to 8

guage wire to connect it, and we

saw a dramatic improvement.

When the distance between the

charge controller and the

batteries was 30' and we were

using just 10 gauge wire, the

resulting resistance in the wire created a large

voltage drop between the charge controller and the

batteries, artificially raising the voltage at which it

thought the batteries were operating.  The charge

controller would see the batteries at 14.4 volts whereas when we measured the batteries with a volt meter

they were actually at 13.2 volts.  This threw everything in the system way off, and ultimately resulted in a

daily loss of some 10-30 amp-hours that never made it from the panels to the batteries.  Once we moved the

charge controller to within 10' of the batteries and installed bigger wire, the resistance dropped.  The

controller saw the batteries within 0.2 volts of their actual voltage, and our daily power production increased.

Note: In three years of cruising Mexico, our boat was plugged into shore power for a total of 6 weeks

while it was in in-water storage in San Carlos. It was never plugged in while we lived aboard (even during

the 3 months we stayed at Paradise Village Marina in Puerto Vallarta).

















































































































































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In addition to living off the grid on solar power on our sailboat, we have also lived on solar power in our RV since 2007. As of February, 2016, we have now installed solar power on two trailers and a motorhome as well as our sailboat, as described here. We have a huge library of solar power and battery charging articles on this website that draw on all of our experience:




The solar power setup aboard Groovy has inspired stories and articles all over the internet.  Here are a few of the websites and online magazines that have featured stories about Groovy and our marine solar power installation:

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