I live on the Gulf coast and am annually under threat of hurricanes, so I have a 12v 125 amp hour deep cycle battery I maintain so in the event of a power failure, I can use an inverter to run essential electronics during and after the storm. It was bought in 2012 brand new and ever since I've had it, it's been on a maintenence/float charger designed for this type of battery. It has rarely been used- maybe 3 hours total (we've been fortunate so far) and each time it was nowhere near being drained (load voltage never dropped below 12.5v) and was recharged immediately. Needless to say, I was shocked one day to find it was only putting out ~ 9 to 10 volts with no load (I have a habit of checking on it every month and testing the voltage without the charger on.) So I purchased a good hydrometer and all but one cell tested in a fully charged state. However, one cell was completely dead.
My question is, what causes one cell to go completely bad like this with very little use and being maintained properly? I had high hopes for this battery and now it is useless to me unless I can find a way to reverse whatever is causing this. I tried a change of H2SO4 in the "dead" cell and attempted a charge cycle which did nothing at all. Any ideas on what caused this just sitting there on a trickle charger and what, if anything, can be done? Thanks!