Monday, September 25, 2023


Why are my RV batteries drying out?

By Chris Dougherty

Chris Dougherty is a certified RV technician. Here is a letter he received from a reader while he was serving as’s technical editor.

Dear Chris,
boiled fishI have a 50-amp transfer switch. My question is if the power from the converter/charger goes through the transfer switch? I’m boiling batteries and trying to determine if the converter or transfer switch is bad, or none of the above? —James

Dear James,
I have good news and bad news. The good news is that the transfer switch is not involved. The transfer switch selects incoming AC power from the generator or shore power.

The bad news is this is a normal condition for basic AC/DC converters. Simply put, the purpose of a converter is to take AC power from an external source (electric utility or genset) and convert it down to DC. These will frequently apply 13-14 volts to the system continuously, which can lead to boiling the batteries.
The best fix would be to replace that converter with a smart converter system. There are several on the market, including the Xantrex TrueCharge2, the Iota IQ4,  and the Progressive Dynamics Intelli-power system. They range in price depending on the size and options of the unit, but installation is usually pretty straightforward.

In any case, until you resolve the issue, make sure you keep the batteries topped off with distilled water!

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Russ and Tiña De Maris
Russ and Tiña De Maris
Russ and Tiña went from childhood tent camping to RVing in the 1980s when the ground got too hard. They've been tutored in the ways of RVing (and RV repair) by a series of rigs, from truck campers, to a fifth-wheel, and several travel trailers. In addition to writing scores of articles on RVing topics, they've also taught college classes for folks new to RVing. They authored the book, RV Boondocking Basics.


  1. I have another question. I just installed 2 6v batteries and I had perfect power going to the trailer. 2 days ago I plugged into shore power to dewinterize my trailer and I guess I messed up and out of habit I turned the battery disconnect switch to on and it was charging the full batteries. Now there is 0 volts in both batteries, nothing. I guess I fried the batteries? They look totally new with no sign of stress. I guess I need new batteries, again?? I don’t have a charge controller to stop charging in my unit I am assuming.

  2. To James-
    If you’re not camping and you’re not a full timer, are you leaving the rig plugged in all the time? If the answer is yes than you’ll boil out the electrolyte and leave the cells dry in just a matter of time. Much depends on the type of charging your converter does and when. If you store your rig between trips you should be able to completely disconnect your batteries and have them fresh enough to work for you when you’re ready to go again. We leave our rig sitting and unplugged for 60 to 70 days between trips and the batteries are always ok. Our rv has a simple single stage charging circuit. Unless you leave your rig stored for months you should be fine with this way of doing things. If you feel compelled to keep your rig plugged in- I’d suggest using a device like the “Battery Minder”. It will keep the batteries safely charged and will also help de sulvate the plates if you get the correct model. In any case, my last resort would be to depend on the on board converter/charger.

  3. Follow up question on 50 Amp transfer switch. I leave my motorhome plugged into 115V shore power at my storage facility to keep the batteries charged. Recently I went to my storage facility and heard the transfer switch “thumping”. It turned out the transfer switch was bad and the thumping I heard was the switch, switching from generator mode to shore power mode. My question, could this problem have caused a fire had I not caught it in time?

    • Perhaps. What’s the make and model of the transfer switch? I’m getting a lot of emails about them failing.


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