Check the specific gravity of the acid with a hydrometer. I'm going to guess you'll find one or more cells with a reading either much higher or lower than the rest. One big reason batteries fail is the loss of plate material off the grids that eventually builds up, touching the bottom of a cell group and shorting that cell out. A 12 volt battery becomes a 10 volt, and the shorted cell cannot recycle the acid during the recharging process, resulting in very low specific gravity in the shorted cell.
Jerry
