Bad Battery Problem

[sailingdog]

I would agree that it’s probably not the wiring causing this - Witzend having the opposite battery in a similar config have the water loss is interesting.

Why the advice not to mix different sizes of lead acid batteries in parallel - dont know. But once one battery starts to loose water (electrolyte), it "becomes" a different size battery - less plate surface in the electrolyte. Seems that this might be somewhat of a positive feedback effect. Once one battery looses a little more water than the other, the rate of imbalance increases causing even more loss.

Simple—different size batteries recharge at different rates and as such, if the two batteries are different in size, one will be charging at the wrong rate compared to the other. Also, a smaller battery would get to the "float" stage of charging sooner than its larger companion, and if the charger is still in absorption charge mode, the smaller battery will be overcharged and boil off electrolyte... likewise, if the smaller batter gets to float stage and the charger goes into float mode, the larger battery will be chronically undercharged...

 

[txtowman]

I have been using AGM batteries (12volt) for 4 years the only drawback is you have to have a charger that either has an AGM setting or the ability to adjust the chargers output to match the needs of the battery bank. I have been getting them from a recycler in houston for $35 they are 88 amp hours. I have also been testing one as a starting battery and its going strong after 4 years. the brand is johnson controls. they weigh 78 pounds and fit in a group 27 space.