After taking the boiled battery out of the circuit (at 135#, it remains in the battery locker), we recharged the remaining two batteries and then left the charger on while we were away for 76 hours. During that time they accumulated 21 amp-hours of overcharge — neither a good or a bad number, since we had no idea of their condition when we started charging them. I zeroed the meter and watched them for a while and they were floating to accumulate about an amp-hour a day. That’s a good number.
So, I turned off the charger and am monitoring a slow discharge to see what amp-hour consumption number I get when the batteries read 12.24 volts, the value for 50% discharge. This consumption x 2 will tell us what effective battery bank we actually have at nominal cruising usage.
It will be less that 2 x the labels, but it is looking pretty good (for a bad situation). So far we (I) have used 100AH, and we are at 12.42v which is about 33% discharge. So it’s looking like five years of cruising and the charger/sensor malfunction have left us with 2 batteries that can deliver ~300AH total.
Much better than having to buy three of the beasts to finish the season and then leave them floating over the winter. And with the Solar Panels in place the AH accumulation on the batteries will be less. Much less if the math is right.