RV / Marine Battery Charging – Solar & Shore Power Combined!

What happens when two RV or marine battery charging systems attempt to charge the batteries at the same time? The interactions between solar charge controllers, converters, inverter/chargers and engine alternators can be complex, and in our lives off the grid in a sailboat and RV, we have observed them working together in many different kinds of circumstances.

This page offers some insights into what goes on when two battery charging systems operate simultaneously, specifically: solar power and shore power, and solar power and an engine alternator. It is the fourth post in our series on RV and Marine Battery Charging Systems. The previous articles in this series are:

  1. RV and Marine Battery Charging Basics
  2. Converter, Inverter/Charger and Engine Alternator Battery Charging Systems
  3. Solar Charge Controllers – Optimizing Battery Charging from the Sun

This is a long post and you can navigate to the various sections using these links:

 

WHAT HAPPENS WHEN TWO CHARGING SYSTEMS OPERATE AT ONCE?

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When two battery charging systems are working side by side simultaneously, each follows its own internal algorithms to get the job done. However, when this happens, and the two charging systems measure the battery voltage to determine which charging stage they should each be in, they don’t see a “real” value. They see an artificially elevated battery voltage due to the presence of the other charging system. This can throw one or the other or both systems off of their normal Bulk-Absorb-Float cycle.

Because solar charging systems operate 24/7, the most common scenario in which two charging systems work simultaneously is solar charging and some form of artificially powered charging, either a converter or inverter/charger when the RV or boat is plugged into shore power or the generator is turned on, or an engine alternator when the boat or motorhome is under way.

The bottom line with two charging systems working simultaneously is that each will do a little work, but one will do more work than the other. Higher end solar charge controllers are designed to ensure that the batteries are never overcharged. As explained in the previous post about solar charge controllers, they are the gate keepers for the solar panels and will reduce the current coming in from the panels to 0 amps if need be.

There are many factors to consider when running an artificially powered charging system alongside a solar charging system. And in reality, just letting the two systems do their thing without worrying about how they get along is probably fine. But for those who want to ponder the relationships, here are some things we’ve learned.

SETTING COMMON BASELINE CHARGING STAGE VOLTAGES

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In order for all the charging systems on an RV or boat to work together truly harmoniously, it is helpful for the voltages at which the systems change charging stages to be the same across all the systems. For instance, each charging system should be set up with one common set of voltages similar to the following:

  • Bulk 14.7 volts
  • Absorb 14.7 volts
  • Float 13.5 volts

If these terms are confusing, have a peek at Battery Charging Basics.

Obviously, these voltages should be whatever values you have determined are optimal for your battery type. Unfortunately, some charging systems don’t allow you to enter specific voltages, so you may be stuck with whatever defaults the manufacturer chose or whatever “set” of voltages they provide that is closest to the values you want.

Flexible solar panels on a motorhome RV roof

Soaking up the sun:
600 watts of flexible solar panels we installed on a friend’s motorhome roof.

As you can see, if one system has an Absorb target voltage of 14.7 volts and another has an Absorb target voltage of 14.1 volts, there is going to be a conflict. What will happen is that the system that is aiming for the higher voltage will win out and raise the batteries to or towards the higher voltage. The reaction of the other system will depend on how it was designed to handle a situation where the battery voltage is higher than the stage it was in. This is true for all the target voltages (Bulk, Absorb and Float).

Similarly, all the charging systems on the RV or boat should be set up with the same algorithm for switching from one stage to the next. However, as shown in the posts about converters, inverter/chargers and engine Alternators and about solar charge controllers, this is not possible, because every product made by the many manufacturers who build these things has a unique charging algorithm.

There are some similarities, however. All multi-stage charging systems remain in the Bulk stage, pouring the maximum current they can into the batteries, until the Bulk voltage is reached. Then they switch to the Absorb stage. However, no two charging systems use the same criteria to exit the Absorb stage to go into Float. The Float stage is also handled differently by different chargers and manufacturers.

 

EFFECTS OF VOLTAGE LOSS IN THE WIRING

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Every RV and marine battery multi-stage charging system monitors the battery voltage to decide which stage to be in. How and where this voltage is measured and how each device is internally calibrated can make quite a difference.

For instance, the solar charge controller in a sailboat may be located as much as 20′ from the battery bank if the batteries are strung out from bow to stern in the bottom of the bilge and the charge controller is mounted in an aft compartment. Unless the charge controller is connected to the batteries with fairly beefy wires, there will be some voltage loss between the batteries and the charge controller, and the charge controller will get inaccurate readings of what the battery voltage actually is.

This can happen even if the distance is just 10′ but the wire used is too small for that distance. It can also happen if the engine alternator or the converter or the inverter/charger is a long distance from the batteries. Wire gauge sizes, distances and percentages of voltage lost are given in the following chart:

Wiring Gauge vs. Voltage Loss Chart

EFFECT OF DIFFERENCES IN CALIBRATION

Higher end solar charge controllers are complex pieces of electronic engineering that are likely to be calibrated pretty well coming out of the factory. However, a cheapie single stage converter, like the factory installed units that come with so many RVs, may not be calibrated as well, and may be off in its measurement of the battery voltage by a tenth of a volt or more. Likewise with a simplistic engine alternator.

It was a big surprise to me to read in the user manual for our boat’s engine alternator/regulator (a Balmar ARS-4 multi-stage regulator) that the voltages may be off by +/- 3%. That means that a target Bulk voltage of 14.4 volts could vary between 14.0 volts and 14.8 volts. Hmmm. Not a lot of precision there!

Solar panels on a sailboat

Our solar panels catch some tropical rays on the back of our sailboat during our cruise in Mexico.

If the two charging systems that are working simultaneously are detecting different voltages on the batteries — for instance, the solar charge controller is measuring the batteries to be 14.5 volts while the converter is measuring them to be 14.7 volts — they will each react according to their own internal charging algorthims.

For instance, say both the solar charge controller and converter are in Bulk mode, trying to attain a voltage of 14.7 volts before switching to Absorb. When the batteries reach 14.7 volts according to the converter, the converter will think they have achieved the Bulk voltage already and will switch to the Absorb stage, while the solar charge controller will remain in the Bulk stage because it sees only 14.5 volts, and it will continue aiming for 14.7 volts, according to its internal measurements and algorithm.

What does this mean? It simply means that the solar charge controller will continue to let as much current in from the solar panels as they can produce while the converter will already be backing off how much current it puts into the batteries to hold them steady at what it perceives to be 14.7 volts (and which the solar charge controller sees as 14.5 volts). Not a big deal. The solar charge controller will keep pushing while the converter keeps backing off, and the job will eventually get done.

 

LESSONS LEARNED FROM OUR ENGINE ALTERNATOR AND SOLAR CHARGE CONTROLLER

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The most challenging relationship we’ve had between charging systems was on our sailboat, and it was the one that forced me to investigate this whole business more deeply and to learn how to program a solar charge controller — and to discover, in the process, the value of programming one!

The two systems were our Balmar ARS-4 engine alternator/regulator and our Xantrex XW-MPPT-60-150 solar charge controller. The charging algorithms for these systems are described in detail here (for the alternator) and here (for the solar charger).

When I first observed them working together, I noticed two things right away.

1) Whenever we turned on the engine, the solar charge controller went into the Float stage soon afterwards.

2) Once the solar charge controller was in the Float stage, if we turned the engine off, it remained in the Float stage, even if the batteries hadn’t been fully charged by the engine alternator.

For instance, if the solar charge controller had been in the Absorb stage when we turned the engine on, and then we ran the engine for just 15 minutes and turned it off (not nearly long enough to charge the batteries), the solar charge controller would wind up in the Float stage and remain there for the rest of the day, depriving the batteries of a proper charge.

Engine Alternator Causes the Solar Charge Controller to Switch from Absorb to Float

The thing about batteries in a complex vehicle like a motorhome or a boat is that they are running many different systems that are continually turning on and off. In the case of our boat, when we were underway, any or all of our big systems might be in use at any one time: fridge and freezer compressors, radar, chartplotter, autopilot, anchor windlass, and even the microwave.

Marine diesel engine alternator Balmar ARS-4 100 amp

100 amp Balmar diesel engine alternator

Worst case, all of those things might be on at once for several minutes as we raised or lowered 200′ of stainless steel anchor chain with a 60 lb. anchor attached to the end of it (well, maybe not the microwave!).

Plus, there was no guarantee we’d run the engine long enough for the alternator to go through its Bulk and Absorb stages and charge the batteries completely.

We might run it for as little as a few minutes while moving from one anchoring spot to another, or for half an hour while we motored out of the bay to go daysailing.

We wouldn’t want to idle the engine at anchor just to charge the batteries, because the engine RPMs have to be fairly high for the alternator to generate a good charging current. These high RPMs happen naturally while driving the boat, but unfortunately, conventional wisdom says that revving the engine to high RPMs while not in gear (i.e., without a load on it) risks glazing the cylinder walls.

Besides it being random as to how long we might run the engine, it was also random as to what state the solar charge controller would be in when we started the engine up.

We might start the engine in the dark to raise the anchor, and in that case the solar charge controller would be asleep. Or we might do it early in the morning when the solar charge controller was in the Bulk stage and gamely trying to get whatever current it could from the wimpy sun on the horizon. Or we might do it later in the day when the solar charge controller was in the Absorb stage and cranking away.

We used a clamp-on ammeter to find out exactly what was going on at various points in the system. We put it around the alternator’s battery cable to see how much current the alternator was putting into the batteries. We also used it on the solar charge controller’s battery cable to verify that the current it displayed on its LCD screen was correct (it was).

Sperry Clamp-On Ammeter measures current from engine alternator

The alternator is pouring 77.9 amps into the batteries – WOW!!

Whenever we turned on the engine, regardless of what the solar charge controller was doing, the engine alternator would immediately go into the Bulk stage and dump as much current into the batteries as they needed to reach the alternator’s Bulk voltage.

If the solar charge controller had been in the Bulk stage already, its job would suddenly become much easier as it got a huge boost from the alternator.

If it had been putting 21 amps into the batteries and had been slowly raising the voltage towards 14.4 volts (the setting we had for the boat’s batteries), the engine alternator might contribute another 40 amps for a while, getting the batteries up to the Bulk voltage a whole lot faster than if the solar panels had continued working by themselves.

If the solar charge controller had been in the Absorb stage already, putting something like 18 amps into the batteries to hold the Absorb voltage of 14.4 volts, the engine alternator would begin its own Bulk stage regardless, and it would remain in the Bulk stage for 36 minutes as it followed its own internal algorithm.

The solar charge controller would react by backing off and delivering less current.

To make things more complicated, as these two systems worked through their charging stages, the loads on the batteries would be fluctuating widely as Mark and I went about our business of living on a boat.

If the fridge and freezer compressors were both running, and the autopilot was maintaining our course and the radar and chartplotter were on and we were making burritos in the microwave, the batteries would need a lot of current.

However, if neither compressor was on and someone was hand steering the boat, etc., then the batteries would need a whole lot less current. During those lulls in current demand, the solar charge controller would suddenly scale things way back and put just 8 or 9 amps from the panels into the batteries.

As soon as that happened, the solar charge controller would suddenly switch to the Float stage!

Huh?!

After some sleuthing, as described in the previous post, I realized that the charge controller was switching from Absorb to the Float stage because the current needed to maintain the Absorb voltage had dropped below 2% of the capacity of the battery bank.

2008 Hunter 44DS Sailboat Groovy in Tangolunda Bay Huatulco Mexico

In Tangolunda Bay (Huatulco, Mexico) we motored back and forth across the bay every few days to anchor out of the swell as it changed its flow.

Since I had entered the true value of the battery bank (710 amp-hours), the controller switched from Absorb to Float when the current dropped below 14 amps (2% of 710).

So, I lied to the controller and told it the battery bank was just 250 amp-hours. Then it would remain in Absorb down to 5 amps.

What I found (by trial and error) was that the solar charge controller pretty much always needed more than 5 amps when it was in Bulk or Absorb.

I don’t know why the alternator didn’t produce that last 5 or so amps on its own, but I suspect it was because the alternator’s Absorb voltage was set to 14.2 volts while the solar charge controller’s Absorb voltage was set to 14.4 volts (the alternator had “sets” of values for the three target voltages, and 14.2 volts for Absorb was in what I felt at the time was the most appropriate set).

The Solar Charge Controller Gets Stuck in the Float Stage

The second problem I encountered was that in the event that the solar charge controller went into the Float stage prematurely, then, after the engine was turned off it would remain there until the next morning.

Xantrex XW MPPT 60-150 Solar Charge Controller

Xantrex solar charge controller
(bottom plate removed)

Yet the batteries may not have been fully charged by the alternator, and they may have really needed to remain in Absorb with the solar panels charging them at a fast clip for another hour or two.

In this case, the solar charge controller needed either to resume the Absorb stage or cycle back through the Bulk stage as soon as the engine was turned off.

The only way the Xantrex XW MPPT 60-150 would cycle back through the Bulk stage is if the battery voltage dropped below a certain level.

I experimented with different voltages. The Float voltage was 13.4 volts, so if I set the “ReBulk” voltage to be 13.5 volts or higher, then the charge controller would never get into the Float stage at all, because it would keep cycling back to Bulk.

According to the user manual, this is actually a valid way to operate this solar charge controller, and they even provide a programming parameter that sets the charge controller up to be a “two stage” charger that has no Float stage and has just the Bulk and Absorb stages.

I wasn’t comfortable with not having a Float stage (although in hindsight that probably would have been just fine given the intermittent heavy loads that were on the batteries all day long). In the end, I settled on a ReBulk value of 12.9 volts.

So, if the solar charge controller was in the Float stage after the engine was turned off, and a big load came on some time afterwards that drew the battery voltage down from 13.4 volts to below 12.9 volts (microwave plus fridge and freezer, for instance), then the solar charge controller would cycle back through the Bulk stage and start the charging cycle all over again.

Programming For Storage

Periodically, we left the boat for a month or several months at a time when we traveled inland or went back to our RV for hurricane season. Since the fridge and freezer would be turned off, and there would be no loads on the batteries at all, I would undo these two programming changes. I would reprogram the solar charge controller with the true size of the battery bank so it would switch from Absorb to Float at 14 amps rather than 5, and I would change the “ReBulk” voltage back to 12.5, the factory default.

 

SOLAR CHARGING AND ELECTRIC HOOKUPS

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Sometimes There Are Good Reasons Not To Plug In!

Solar power is free, however, the electricity from shore power hookups may not be. If your shore power electricity is “free” (i.e., built into the overnight fee you are paying for your RV site or boat slip), then it doesn’t really matter which charging system is dominant.

If you have metered electricity (a common situation if you are renting your RV site or your boat slip on a monthly, seasonal or annual basis), and you are paying for your electricity, then you may want to ensure that your solar charger is running the show and doing the bulk of the work while your converter or inverter/charger is playing second fiddle.

One easy way to do this is just to flip off the electric switch on the shore power post. Flip it on only as needed when the batteries get low and need a boost.

We did this a lot when we lived on our sailboat. We lived at a slip in Paradise Village Marina in Puerto Vallarta, Mexico, as well as at slips at Hotel Coral and at Cruiseport Marina in Ensenada Mexico for months at a time without plugging in the shore power cord at all. During hurricane seasons, we also left our boat in a slip in Marina Chiapas for seven months without plugging it into shore power.

It was nice when we settled up the bills for these places at the end of each stay to have a big ol’ “$0” on the line item for electricity.

What Happens If You DO Plug In?

If your RV or boat is plugged into shore power, and the switch at the post is turned on, it is hard to get the solar power system to be dominant because its power source is flakey (as explained here).

We plugged our sailboat into shore power for several months while we lived aboard at Kona Kai Marina in San Diego at the end of our cruise.

Schneider Electric 2500 watt inverter charger Xantrex

Schneider Electric (Xantrex)
2500 watt Freedom inverter / charger

Our Xantrex inverter/charger went through the Bulk and Absorb stages the first time we plugged in, and then it remained in the Float stage forever after (except when we unplugged to go day sailing and plugged back in again upon returning)!

Each morning when our Xantrex solar charge controller woke up, it zipped through the charging stages and went into the Float stage after just a few minutes, because it saw the batteries were already fully charged.

In our RV, we plugged into shore power for 48 hours during rainy and stormy skies while we stayed at Narrows Too RV Resort in Maine. It was overcast when we plugged in. Our Outback solar charge controller was in the Bulk stage putting about 6 amps into the batteries at around 13.9 volts (it was aiming for 14.7 volts).

Ordinarily, since we live a solar power only lifestyle, our Outback solar charger is set up with Bulk and Absorb values of 14.7 volts, a minimum Absorb time of 2 hours and a maximum Absorb time of 4 hours. However, our Iota DLS-90 / IQ4 Converter has a fixed (non-modifiable) Bulk voltage of 14.6 volts and Absorb voltage of 14.2 volts and Absorb time of 8 hours.

I temporarily changed the solar charge controller to have Bulk and Absorb voltages that matched the converter, and minimum and maximum Absorb times of 0 hours so it would remain in Absorb only as long as it took to get to Bulk (the charging algorithm of the Outback solar charge controller is explained in detail here).

Iota DLS 90 IQ4 Converter and smart charger

Iota DLS 90 IQ4 Converter and smart charger ready for installation in our RV

As soon as we plugged in, the converter began dumping 49 amps into the batteries which zoomed the battery voltage up to the converter’s Bulk stage value of 14.6 volts. Then it backed way off to 30 amps, then 20, then 15 as it held the converter’s and solar charger’s Absorb voltage of 14.2 volts (our new Trojan Reliant AGM 6 volt batteries charge up extraordinarily quickly!).

The Outback solar charge controller responded by putting in a few amps at first, but then it displayed “Bat Full” and went to sleep!

From there, the Outback solar charge controller went through its usual Sleeping and ZZZZ stages as the Iota DLS-90 / IQ4 Converter quietly slipped from Absorb (14.2 volts) to Float (13.6 volts). When the Outback solar charge controller went through its wakeup sequence after being in the ZZZZ stage for 3 hours, it saw the batteries were fully charged, so it rolled over and went back to sleep in the ZZZZ mode.

Outback FlexMax 60 Solar Charge Controller

We catch our RV’s solar charge controller sleeping on the job at midday!
The solar panels are in full sun and are at 68 volts
The converter is in control and has elevated the batteries to 13.5v
But the controller sleeps soundly as 0 amps go from the panels to the batteries!

In fact, the whole rest of the time we were plugged into shore power, the Outback solar charger stayed in the ZZZZ mode, even in bright afternoon sunshine. Every 3 hours it would lazily open its eyes, yawn, look at the state of the batteries, see that they were fully charged and go right back to dreamland in the ZZZZ mode.

To summarize, these are two examples of how different solar charge controllers handled the presence of full-time shore power:

Do The Different Charging Voltages Have To Match?

No. On another occasion, while getting repairs done at an RV dealership, we plugged in our trailer for an afternoon while it was out on the lot next to the building on a cloudy day. The solar charge controller was putting in 6 amps at 13.8 volts in the Absorb stage (trying to keep the batteries at 14.7 volts) at mid-afternoon.

As soon as the shore power cord was plugged in, the converter began dumping 55 amps into the batteries and the battery voltage zoomed to 14.6 volts. The solar charge controller kept putting in around 6 amps.

For the next few minutes, the total current going into the batteries dropped from 61 amps to 33 amps and then settled there. If the solar charge controller could put in 8 amps, as the sky lightened, the converter put in 25 amps. If the solar charge controller could put in only 2 amps as the sky darkened, the converter put in 31 amps.

Suddenly, the converter switched to its Absorb stage where it holds the batteries at 14.2 volts, and the total current going into the batteries dropped to 20 amps. The solar charge controller was still in its own Absorb stage where it wanted to hold the batteries at 14.7 volts, so it kept putting in as much current as it could (5 to 8 amps and even as high as 12 when the sun came out for a few minutes) while the converter made up the difference, keeping the total at around 20 amps.

We didn’t stay plugged in long enough to see the solar charge controller switch to Float (the converter stays in Absorb for 8 hours), but at that point the converter would have held the batteries at 14.2 volts while the solar charge controller wanted them at 13.5 volts. It also would have been dark, so the converter would have been in complete control and the solar charge controller would have gone to sleep.

 

GETTING THE MOST OUT OF A BACKUP GENERATOR

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If you are using a generator to give the batteries a boost of charge because you’ve been in cloudy conditions or don’t have enough solar power to run everything on board indefinitely, then you’ll want the generator to charge the batteries as quickly as possible, saving you a few dollars in fuel (gas or diesel) and saving yourself from the loud noise and obnoxious fumes of the generator itself.

Yamaha 2400i Portable Gas Generator

Yamaha 2400i generator — our backup

In essence, the goal with a generator is to run it for as short a time as possible to get the batteries charged up.

With solar power, at the end of the day, before nightfall, the batteries are in their most charged state.

During the evening and into the darkest hours of the night, the batteries get depleted from running the lights, the TV, the computers, the microwave and whatever else your household uses until bedtime.

By dawn, the batteries are at their lowest state of charge. This is also a time when the sun is low in the sky and the solar panels are operating weakly and producing minimal current.

Early morning is the ideal time to turn on the generator!

An Example of Generator Use at Midday versus Dawn

The first time we fired up our generator to charge our batteries via the Iota DLS-90 / IQ4 converter, we’d had several overcast days in a row. It was mid-afternoon, and the batteries were fairly depleted from days of cloudiness. However, they had already gotten about 25 amp-hours of charge during the morning and noon hour, so they weren’t as depleted as they had been at dawn.

The solar panels were limping along in the Bulk stage with the batteries at about 13.5 volts. The solar charge controller was aiming at a Bulk voltage of 14.7 volts and the panels were valiantly trying to produce enough current to get there, but all they could muster was about 6 amps. It wasn’t likely the batteries would reach the Float stage before dark.

As soon as we turned on the generator, the the Iota converter went into the Bulk stage and began delivering about 60 amps to the batteries. It quickly got them up to 14.6 volts and switched to Absorb, dropping to about 20 amps. Great! But this converter is capable of putting 90 amps into the batteries, so why run it when Bulk mode delivers just 20 amps?

Solar panels on a fifth wheel RV roof

We let the solar panels do their job during the day.

We decided turn off the generator and let the solar panels do whatever they could for the rest of the day.

Early the next morning when the batteries were depleted from several days of inadequate charging plus a night of activity in the RV (they were down to about 12.3 volts), we fired it up again.

I did not modify the settings on the Outback solar charge controller to match those of the converter because we were just going to run the generator for a few hours and probably wouldn’t need it again for a few months.

This time the converter rolled up its sleeves and got to work, pumping 67 amps into the batteries as it aimed for its target Bulk voltage of 14.6 volts. The solar charge controller was in Bulk mode too and was busy putting in 1-2 amps of its own (it was early morning), and with the converter’s assistance, it briefly hit 14.7 volt Bulk target and switched to Absorb.

With both the converter and solar charge controller operating in the Absorb stage, the converter dropped the current to maintain the target Absorb voltage. The solar charge controller could still bring only 1-2 amps to the party due to the low light, so the converter was in control and doing virtually all the work.

We shut off the generator off after about two hours and let the solar charge controller take over. Now that the batteries were partially charged up, the solar charge controller was able to get the batteries up to its Absorb voltage target and finish the job, even in the overcast conditions, getting the batteries through its Absorb stage and going into the Float stage for the first time in a few days.

 

 

FINAL NOTES

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So, you can see, there are many ways to charge RV and boat batteries and many things to consider. Of course, it’s easy enough to leave the various charging systems at their factory settings after installing them, and there is nothing wrong with that!

But if you want to understand your system and get the most out of it — especially if you are using solar power and end up running a second charging system in conjunction with your solar power system — you may want to dig into the nitty gritty details buried in the user manuals and figure out what the charging algorithms are and how to program each system with the parameters that make the most sense for you.

All battery charging systems for mobile installations like RVs and boats have become increasingly more sophisticated over the years. A quick review of the older systems described in detail in the previous posts here and here show how the engineers designing these systems have become more and more knowledgeable about the real world applications of their products and what conditions they might encounter as they interact with other charging systems.

As the years go by from here forward, more and more solar charge controllers, inverter/chargers, converters and engine alternators will be designed with the understanding that they may not be the only charging system operating in the RV or boat.
 

This was the last article in our series on RV and Marine Battery Charging:

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

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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 May 2017, we have used, worked with and lived with these particular units for over 3,200 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:

 

SOLAR CHARGE CONTROLLER OVERVIEW

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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.

 

 

CHARGING FROM THE SUN AS IT RISES AND FALLS

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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.

 

 

SIZING A SOLAR CHARGE CONTROLLER

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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.

 

 

OUTBACK MX60 MPPT SOLAR CHARGE CONTROLLER

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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:

 

SWITCHING FROM ABSORB TO FLOAT BASED ON TIME

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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!

 

WHAT IF THE TARGET VOLTAGES CAN’T BE MAINTAINED?

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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!

 

 

SWITCHING FROM ABSORB TO FLOAT BASED ON CURRENT

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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.

 

WHAT HAPPENS AT NIGHT?

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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.

 

LOW LIGHT — FULL MOON and STREET LAMPS

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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.

 

XANTREX XW MPPT 60-150 SOLAR CHARGE CONTROLLER

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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.

 

PROGRAMMING THE CHARGE CONTROLLER TO THE BATTERY MANUFACTURER’S SPECS

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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).

 

MORNINGSTAR TRISTAR TS-MPPT-60 SOLAR CHARGE CONTROLLER

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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.

 

FINAL NOTES

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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.

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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:

 

“ARTIFICIALLY POWERED” versus “NATURALLY POWERED” CHARGING SYSTEMS

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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.

Sunshine

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.

 

 

PROGRAMMING A BATTERY CHARGING SYSTEM

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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.

 

WHAT VALUES DO YOU PROGRAM INTO A CHARGING SYSTEM?

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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.

 

 

CONVERTERS

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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.

 

 

INVERTER/CHARGERS

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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.

 

 

ENGINE ALTERNATOR

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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.

 

 

SIZING AN ENGINE ALTERNATOR TO A BATTERY BANK

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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.

 

FINAL NOTES

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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:

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New to this site? Visit RVers Start Here to find where we keep all the good stuff!!

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 nine years of living off the grid in an 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.

WHY IS BATTERY CHARGING IMPORTANT for RVers and CRUISERS?

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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.

 

HOW BATTERIES ARE RATED

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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.

 

HOW BATTERIES ARE CHARGED

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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.

 

BATTERY CHARGE STATES

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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.

 

UNDERCHARGING, OVERCHARGING and EQUALIZING

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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.

 

LEAVING A BATTERY DORMANT – “LOT ROT”

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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.

 

SIZING A CHARGER to a BATTERY BANK

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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.

 

SINGLE STAGE CHARGING

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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.

 

MULTI-STAGE CHARGING

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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.

 

PREMATURELY TURNING OFF A MULTI-STAGE CHARGER

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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.

 

EQUALIZING – A FOURTH CHARGING STAGE

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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.

 

LEAVING A MULTI-STAGE CHARGER ON INDEFINITELY – MANAGING THE FLOAT STAGE

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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.

 

WAIT – WHAT’S THE DIFFERENCE BETWEEN BULK and ABSORB AGAIN?

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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.

 

CONCLUSION

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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