There are a few ways the battery can die. One is that a cell got dry and the plates bent, shorting out. Another is bits of metal growing from 1 plate to the other, eventually shorting out the cell. This is more common when you've got dirty water in it, as the ions need something solid to grow around. A friend bought a battery once and was presented with an empty one. The shop assistant asked if he wanted the cheap model or the expensive model. The difference was in which bottle of acid he poured into the battery...