Friday, March 24, 2023


Is it possible to overcharge my RV’s batteries?

Hi Dave, 
My Pinnacle is solar and generator prepped. I have three roof panels connected to a 60-amp Renogy solar controller, and I just installed an Onan 5500 generator. Do I need to disconnect my solar system from the battery bank while connected to shore power or running the generator?

Regarding the generator, if I start the generator while connected to shore power, will it switch over without causing a disaster? I am fairly confident the switch will save me, but I have no clue about the solar. Currently, I disconnect the solar when connected to shore and also when connected to my tow vehicle. I don’t know anything about that side of the system and worry I may damage something. Any advice you can give would be greatly appreciated. Thank you. —Jim, 2019 Jayco Pinnacle

Hi Jim,
This is a frequent question as more people are starting to boondock or dry camp. Solar panel technology has advanced greatly and the price is getting affordable. Keystone fifth wheels now come standard with 200-watt solar panels and lithium batteries with the system called SolarFlex. It can be upgraded to 800 watts with 2000-watt inverter!

Quick answer to your question

So, the quick answer to your question is no, you cannot overcharge your batteries and don’t have to shut off the solar charging system if your solar panel system has a charge controller. Let’s take a look at the typical battery system and converter/charger.

You did not indicate what type of battery or batteries you have. So let’s assume they are the standard flooded lead acid (FLA) that typically would have come with your rig when it was new. They would be either two 6-volt batteries connected in series (positive to negative), which gives you a 12-volt bank, or one-two 12-volt batteries.

To charge the batteries, you either have a converter or an inverter/charger. The basic converter used in RVs is part of the main power center called an all-in-one distribution center. It has circuit breakers for the 120-volt components, automotive-type fuses for the 12-volt functions, and the converter/charger on the side or bottom.

Some models have a standalone converter such as the WFCO version that was in the Thor Challenger we upgraded. This was located below the bed behind a false wall, as it runs a little hot. Getting it out of the kitchen area helps with temperatures in the summer.

Both converters provide a 13.6-volt charge when the batteries are drained 50%, then drop down to 13.2 volts when the battery hits 12.6 volts. A fully charged battery will not accept a charge from any source when it is 13.2 volt, so it’s called a float or maintenance charge. Therefore, when you are connected to shoreline power, the battery should always have 13.2 volts measured at the terminals provided by the converter.

Automatic transfer switch

When you installed your generator, you indicated it had an automatic transfer switch (ATS). This senses where the 120-volt power is coming from and switches inside the box to allow that power to pass through. It is typically defaulted to the generator, so any time the generator is running and providing power, the switch is closed on the generator side and open on the shoreline. The generator will provide 120-volt power just like your campground pedestal.

If you did not have the ATS, you would have what is called a “J” box and would have to manually plug the shoreline cord into the “J” box to get power from the generator and disconnect to plug into shoreline.


Some units come with an inverter that will take 12-volt DC power from the batteries and provide 120-volt AC power to outlets and certain appliances such as a residential refrigerator. The larger inverters are also a charger with multi-stage operation similar to the solar charge controller, and will charge the batteries. This only provides a charge if connected to 120-volt power from either the campground source or the generator, again through the ATS.

Now let’s look at the solar panel and charging system. You have three panels, which I would assume are 100 watts each, so 300 watts of solar panels, and a 60-amp Renogy MPPT Charge Controller. MPPT (Maximum Power Point Tracking) technology monitors the battery charge condition and intelligently adjusts voltage accordingly. According to the owner’s manual, when the battery is drained 50%, it will provide a boost charge for two hours that is a high voltage charge designed to break up sulfation on the plates of the battery. It will then go into an equalization and float charge, which is a constant charge of 13.2 volts, just like your converter.

This is not good for lithium

Keep in mind, this is all designed for FLA batteries and not good for lithium. It is programmable for lithium, which typically needs a constant 14.6 volts for about 1.5–2 hours. This is the 60-amp MPPT Charge Controller that we have used from Go Power! in the past.

So, when you are connected to shoreline power, or your generator is running, 120-volt power will be routed through the ATS and sent to the distribution center and then to the converter or inverter/charger, if applicable. If you are parked in the sun and plugged into shoreline power, the solar charge controller will manage the system and recognize the 13.2 volts available at the battery from the converter or inverter/charger. It will adjust accordingly so you will not overcharge your battery/batteries.

Therefore, you do not need to disconnect anything and should be able to see the voltage on the controller display. You can also take a multimeter and verify that at the battery compartment.

 You might also enjoy this from Dave 

Why aren’t my solar panels charging the house batteries?

Dear Dave,
I have solar panels to help charge the house batteries while boondocking, but after a couple of days the house batteries are dying. I turn on the engine to recharge the house batteries, but often it won’t charge them, even if I let the engine run for 15 minutes or more. Sometimes it charges as soon as I start the engine. What’s going on? —Brian, 2017 Leisure Travel Van (Unity)

Read Dave’s answer.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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10 days ago

What reverses sulfation is charging the batteries until there’s no lead sulfate left on the plates. Simply boosting the charging voltage for a few hours is not ensure that all lead sulfate has been converted to lead or lead oxide. Bogart Engineering has a good writeup on this topic.

Joe B
16 days ago

Good article Dave but I have a question regarding this passage:

‘According to the owner’s manual, when the battery is drained 50%, it will provide a boost charge for two hours that is a high voltage charge designed to break up sulfation on the plates of the battery. It will then go into an equalization and float charge, which is a constant charge of 13.2 volts’

I understand the equalization charge is what breaks up the sulfation. That charge can be as high a 16.4 volts and is set to a specific time limit. Then the charge controller will drop to an absorption charge of something like 14.4 volts and finally down to the float charge of 13.2 or so. Do I have it wrong?

16 days ago
Reply to  Joe B

I share your understanding .. It is confirmed in most explanations by other RV electricity experts.

Dave Solberg
16 days ago
Reply to  Joe B

You can adjust the bulk or boost charge to setting for your battery and yes it can go as high as 16.4 volts. Lithium batteries want only 14.6 volts for a specific amount of time which can also be set on the charge controller. The 14.4 volts would be for Flooded Lead Acid (FLA) and would be the equalization or absorption charge and the 13.2 volts would be the float or maintenance charge.

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