# and Type of house batteries

ron.dittmer wrote:
I should have made it clear in my reply that when one 12v battery slips in performance, my smart charger kept trying to get the pair to a full charge but could not. My smart charger would not give up and shut off.
The weak battery pulled down the pair, over charging the good one causing boil overs to both.

On my work bench with batteries separated, my smart charger completed the charge to the good battery and stopped. The ill battery would not accept a full charge, causing the charger to keep trying. So when they are back in the motor home together, it is an ill situation.

While on a trip, I would have been better off identifying the weak battery and take it off line and do the trip with just one battery.

The ill situation happened to two sets of 12v batteries, a repeatable condition that has me issuing my warning working with pairs of 12v batteries. If you replace your 12v batteries every other year, you avoid such problems.

Ron ... I think that I agree with BF ... you most likely had a bad cell in the battery that you call the weak battery. This is always possible with any lead acid batteriy - or with any type of batteries for that matter. I've had plenty of good old dry cell flashlight problems due to one battery being bad.

With two 6V RV batteries in series, if a cell goes bad in one battery you no longer have a battery power source for the RV. However as you say, with two 12V RV batteries in parallel, when a cell in one of them goes bad you can "maybe" determine which battery has a bad cell with a voltmeter and then continue camping using only the other good 12V battery. I say "maybe" with regards to a voltmeter determining which battery might be bad, because it is possible (I've had it happen more then once in various vehicles) for a vehicle battery to read on it's terminals a value of "12 volts" - but still not be able to deliver hardly any current at all. I call this a "high resistance current-path-breakdown" ... in which a voltmeter can still read a normal value (voltmeters require very, very little current passing through them read a voltage) but the current path has too much resistance (from corrosion?) to pass any current to speak of.

P.S. The conversation in this thread raised a question in my mind: I wonder if AGM RV batteries - either 6V or 12V - would have less tendency to develop a "bad cell" than liquid acid RV batteries? At least from the perspective of an RV's physical motion and jiggling causing a battery cell structure to fail, it seems that AGM batteries would be less inclined to have a cell go bad than liquid acid batteries.

FWIW, of the three sets of 12V parallel batteries I've had in our RV over the last 12 years: The 1st liquid acid set failed in only 6 months of ownership, the 2nd AGM set never failed but I replaced it after 8 years just in case, and the 3rd AGM set is still doing it's thing. I wish I'd checked the 1st set of liquid acid batteries at least with a voltmeter, but they were on the Winnebago warrantly so I just paid the difference to my RV dealer and had them replaced with the 2nd set which consisted of a pair of 12V 100AH AGM wheelchair batteries.