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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!

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The battery failure you can cause by abuse, so long periods resting with low voltage, or deep discharge, or over voltage, tend to affect all cells equally. So I suspect a mechanical failure in one cell.

Sometimes things just break, or have been built badly. The most likely causes of a single cell going down is some contamination in the cell causing a short between the plates. A badly built separator could fail early. The plates are heterogeneous, some of the active mass could have shed and be shorting the plates.

With 2012 vintage, you're probably out of warranty.

People who **need* backup power will often use a series connection of individual 2v cells. If one goes down, then 10v is often better than none. When designing the battery, make sure all your critical loads are tolerant of a lower supply voltage.

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