Reading the various posts on the subject makes it fairly obvious that something strange is happening when two 6 volts batteries are used in series as house batteries coupled with a single 12 volt chassis battery.
1. Electrolyte boils away in the 2 six volt batteries when being charged "underway" by engine alternator.
2. OP describes 2nd instance.
Guessing....
Temperature difference due to differing location? Warm/HOT chassis battery in the engine compartment vs house batteries at OAT, plus more cooling for house batteries.
When I rapid charge a 12 volt battery I have often noticed the 4 battery "interior" cell's electrolyte bubbling when the 2 battery end cells do not.
I have attributed this to the end cells receiving more cooling.
Two 6 volt batteries have a total of FOUR end cells being cooled by OAT...
It is well known that "flooded" lead-acid battery SOC, FULL SOC terminal voltage rises with declining ambient temperatures. Therefore ALL automotive VR's have an internal thermistor that is used to sense OAT and adjust the alternator charging voltage accordingly.
"WARM" VR/chassis battery, SOC (FULL) = 12.8. "COOL" house battery requires a higher charging source voltage (13.2?) in order to reach FULL SOC....
????
ell
