Battery Boiling is Bad!

Here’s what could be going wrong…

This just posted on my RVelectricity by Mike Sokol group

I check water level on my RV quite regularly both while in storage and on the road. Early in June I had to add water (distilled) to every cell in both batteries and the top of the batteries were wet. Pulling the covers off I noticed the batteries were boiling but the water still covered the plates. After a number of trips, both hookup and boondocking the batteries have done well but require water and I still need to wipe the tops. This occurs both with shore power and parallel 2000iS generators. what might cause this? -Verne

Hey Verne,

Boiling a battery electrolyte is bad, so let’s take a look at what’s happening. First of all, you really need a multimeter to figure this out. I recommend a basic manual meter you can purchase for $20 to $30. Here’s the Southwire meter kit I use in my RVelectricity seminars, which is plenty accurate for this type of testing. And it only costs around $30, so it’s a great deal.

Basic Troubleshooting

The converter in your RV is also your battery charger, so it not only supplies power to your 12-volt electrical system, it also has to regulate the charging current to your batteries. And it does this by changing its output voltage. Here’s a simple chart on how this works.

Note that your converter/charger voltage should jump up to around 14.1 volts or so while in the absorption phase of charging. This helps equalize the individual cells, but then it should drop to around 13.5 to 13.6 volts in Float/Storage mode once the battery is fully charged.

First Possibility - A Faulty Converter

However, if your converter/charger is stuck in Absorption Mode and continuing to charge at 14.1 volts or more, then before long the battery electrolyte will begin boiling. So if you measure across your battery output (if you have two 6-volt batteries in series, then meter across both batteries) and you’re reading over 14 volts while the batteries are boiling, then your converter has likely gone bad and not reducing the charging voltage (and current). It’s probably time to repair or replace the converter.

Another possibility - A Shorted Battery Cell

It’s also possible that one of the cells in a battery has shorted out and that’s causing your converter to do crazy things. You can test for this by disconnecting your RV from shore power and letting your battery voltage stabilize for an hour or so. Then meter across the batteries and it should read around 12.7 volts. If it measures 2 volts low, (say 10.7 volts), then you have a shorted battery cell. Pull the batteries and take them to any battery store for a carbon pile current test. Once the batteries are boiled for any significant time, they’re probably beyond saving and need to be replaces.

Start Testing!

So get a meter and start measuring, and you’ll quickly figure out if the converter or batteries (or both) need to be replaced.

Let’s play safe out there - Mike