Batteries Shot Or Over Reacting?

[Dave22q]

House bank is 3 AGM 105 group 31 hardwired to Prosport smart charger.
Had a problem several months ago, charger was off and batteries ran very low before I notice the problem. I recently installed a voltage monitor. Became concerned that batteries were not holding voltage. Tested by turning off all 12v devices and the charger for 5-6 hours. Voltage dropped from 13.3 which I think is the normal maintenance charge to 10.8. Of course when I turned the charger back on the reading jumped to a charging voltage of 14.7.

Is it clear that my 5 year old house bank is shot? Could just one of the batteries be bad? Could this be caused by a short? I'm no electrician but it sure seems that batteries should hold charge much better then this.

[roverhi]

If the voltage dropped to 10.8 with no load, you either have a short somewhere or your batteries or at least one of your batteries is/are toast. Resting battery voltage should be around 12.8 volts. If you have lead acid batteries, a cheap hydrometer will isolate a bad cell/battery. Best would be a load tester but you probably wouldn't want to spring for the cost of one of those. Pulling the batteries and taking them to a battery store would give you access to their test equipment. You'd have to do that anyway as it's not good form to drop the bad battery(ies) over the side.

[Maine Sail]

Originally Posted by Dave22q

Voltage dropped from 13.3 which I think is the normal maintenance charge to 10.8. Of course when I turned the charger back on the reading jumped to a charging voltage of 14.7.

Your batteries are toast. Anytime a battery jumps to absorption voltage that quickly, even from 12.1V, it is heavily sulfated and done for.

A typical 100Ah AGM should take approx 1:15 minutes to go from 12.1V to 14.4V with a charge rate of 20A or .2C... You went from less than dead to 14.7V in what sounds like seconds....