2 of 3 RV batteries overcharged—stunk. Replace all 3?

Dear Dave,
Two of our RV’s house batteries overcharged. They were hot and producing a nasty sulfur smell. We weren’t sure if it was the propane, so I called the fire department. The chief told me that the batteries had three different dates, and that I should replace all three at the same time. Is this true? Thank you. —Sybil, 2006 Winnebago Adventurer

Dear Sybil,
Quick answer is YES, you need to replace the batteries; however, maybe just two of them.

Your 2006 Winnebago Adventurer most likely came with two 12-volt NAPA deep cycle batteries connected parallel, which means positive to positive, negative to negative, for a 12-volt bank and twice the amp hours. According to the brochure on the Winnebago site (have I mentioned I love Winnebago questions as their documentation is outstanding?), your batteries were two Group 24 Flooded Lead Acid (FLA) batteries.

Like I said, Winnebago has been using NAPA batteries for years as they are some of the most dependable. This is a photo of the NAPA batteries in a 2023 Adventurer National Park Edition. That is designed for dry camping, so there are four batteries.

Since your rig is almost 20 years old, the batteries are most likely not new. Also, since there are three, at least one has been added. This is where specific information is required to identify what battery configuration you have. Yes, I believe you need to change the batteries, but maybe just two of them.

Battery dates

It is possible that the two original batteries are still there and could have different manufacturer dates. If you have two NAPA 12-volt deep cycle Group 24 batteries and the manufacturer date is within approximately one year, they could be original batteries. I have witnessed this many times where the original batteries went bad and the owners thought they could just add another battery to the bank.

If you don’t dry camp much, you can keep a set of weak batteries in the system as the converter will provide 12-volt power as long as the shoreline cord is plugged into power or using your onboard generator. The problem with this is the converter will provide 13.6 volts, thinking the batteries need to be charged all of the time and it could ruin the converter.

If the third battery is a different brand, it most likely has a newer manufacturer date and was added either to the original batteries or even to a replacement set since the unit is this old. What typically happens is owners do not dry camp much at first and everything seems to work fine, even for an overnight at a rest stop or Harvest Hosts. But eventually they need to spend two days and the batteries do not hold up. They think adding a third battery will lengthen the weekend—and it does for one or two trips. However, the two older batteries (or even one) will eventually drain the new battery and ruin it.

Overcharging the batteries and sulfur smells

Flooded Lead Acid (FLA) batteries are simply 12-volt energy storage boxes. They have lead plates submerged in acid. I won’t bore you with the chemistry; however, they are recharged by a converter or battery charging device which does emit a gas out the vent caps that smells like sulfur. That is why FLA batteries need to be in a vented, outside compartment and not inside the living area.

Most standard converters will provide a 13.6-volt charge at the initial charging profile when it senses the battery is low, and drop to 13.2 volts for a maintenance charge. Multi-stage chargers and solar panels with charge controllers will have a higher initial charge to break up sulfation, and then go into an equalizing and float charge—which your unit most likely does not have.

However, I do believe your situation is not normal, especially with three batteries and different dates. If the oldest battery was weak, it would drain the other two down. The converter would sense a very low charge and keep the high voltage charge stage continuously and boil all the batteries.

Battery could have a bad cell

Another issue could be one of the batteries has a bad cell. That would confuse the converter thinking there was not battery and provide an even higher charge.

We just replaced the distribution center in a 1996 Forest River Salem as the converter was bad and did not charge the battery. Since the distribution center did not sense a working battery in the system, the converter was putting out 18 volts to the 12-volt components, and we had issues. There needs to be a fairly good battery connected so the distribution center senses a closed loop, meaning voltage can pass through. Otherwise, it goes into high voltage orbit, and this could also be what was happening to your rig.

So back to the battery replacement. If one battery goes bad, you want to replace both as the older battery will drain the new one. I think if you look at the amp hours of the three batteries you currently have, you might see they are Group 24 and low amp hours. You would be better off getting two Group 27 FLA batteries and have as much or more. However, if you do not do much dry camping, two Group 24 FLA batteries will be fine.

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Understanding everything about 12-volt house batteries

This is Part 4 of Dave’s “Everything you need to know about RVing” series. In this post, Dave looks at 12-volt house batteries and tells you just about everything you need to know about them, whether this is your first or 101st RV trip.

Read it here.

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

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