Q. I noticed a faint whiff of sulfur somewhere near our 2007 motorhome, but I could not locate the source of the smell. Then, at 4 a.m., my wife and I awoke to a strong smell of sulfur throughout the motorhome. I soon found that our 12-volt, maintenance-free chassis battery was smoking and too hot to be touched without leather gloves.
I quickly disconnected the battery cables while it continued to smoke. I loosened the battery hold-down clamp and removed it from the motorhome (a task made more difficult by its location in front of the right front tire). After cooling down the battery by running water over it, the smoking stopped. I will never know how close we came to having the battery burst into flames.
Unlike a car battery, which is charged only when the motor is running, the motorhome battery is also being charged when plugged into park power. Apparently, all this charging can lead to uncovered plates in an older maintenance-free battery and the condition we experienced. Surprisingly, the battery had no problem starting the motorhome. It took only three days to go from a curious smell to a full-blown emergency. — Terry
A. I’m glad you avoided a fire or other damage; however, I am concerned about what caused your problem. The standard design for motorhomes does not tie the chassis (starting) battery to the converter for battery charging for the very reason that you inadvertently discovered. Starting batteries do not like to be constantly trickle charged!
Because of the battery design, the typical converter “float” voltage of 13.2–13.5 volts provided by the converter will cause them to constantly boil off electrolyte, leading to a dry battery.
You need to have that 12-volt system looked at. There should not be a charge path from your house batteries and converter to the engine starting battery. It could be a factory wiring error or a failure of some part of the 12-volt controls that has created a charge path that shouldn’t be there.
It could also have been a former owner who thought they were doing a good thing by running a wire from the house batteries to the starting battery. Either way, if it is not corrected, it is likely that the replacement battery will eventually suffer the same fate.
A quick test for the erroneous condition would be to check the voltage on the starting battery after the RV has been parked overnight and plugged into AC power. If the starting battery reads 13 volts or higher, that is an indication that the converter is attempting to charge it.
Normal resting voltage for a starting battery that is not under charge is around 12.6 to 12.8 volts. If the starting battery is indeed tied to the house-battery bank or converter, you need to identify the wiring mistake and correct it.