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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6 thoughts on “RV and Marine Battery Charging Basics

  1. Excellent summary! Some stuff I knew, and some new info. Wish I’d had something like this when I was first trying to figure out the mysteries of RV electricity. Thanks!

  2. Batteries can be a complicated subject especially when you are using multiple sources (alternator, sun, wind) to charge them. We spend all summer anchored on our boat so power and power management becomes so important.
    I like the way you have started this off with some of the basics and it is so clearly written and explained.
    I am really looking forward to the rest of the series.
    Thank you for explaining something we really need to know.

    • I am so glad you like what I’ve got here, Mark. Coming from a seasoned sailor like yourself that means a lot! Sailors’ batteries are especially challenged by solar charging because there is so much shading from the mast and boom, and we also tend to use bigger battery banks than our engine alternators are designed to charge… I hope you enjoy the rest of the series!!

  3. Hi folks,
    When living in an RV full-time in a trailer park site, on shore power, do you think it would be beneficial to disconnect shore power now and again to run the batteries down a ways, and then reconnect to bring them back up and return to shore power for some length of time? In my case, the RV has been in this site for over a year; the batteries are two 6-volt batteries connected in series. I water them monthly. I should also mention that they build up a lot of corrosion on the terminals.

    • It’s definitely a great idea to exercise the batteries on a regular basis. Disconnecting from shore power and living on 12 volts for a while is an excellent way to do it! Some converters provide multi-stage charging which exercises the batteries to some degree (see this post). Also, an investment in AGM batteries will eliminate the need to water the batteries and will eliminate all corrosion as well (see this post).

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