The biggest culprit of car battery failure is due to sulphation. This is when lead sulphate crystals form on the lead plates eventually reducing the current within the electrolyte. As batteries sit without being charged, they start to sulphate. The longer a battery sits with increasing depth of discharge, the sulphation gets worse. In some cases it may not be recoverable. The way to reverse this is by either pulse charging the battery or by slow trickle charging the car battery. Sulphation may eventually cause the active material on the lead plates to break off and fall into the bottom of the cells which eventually will cause a short circuit as the material builds up in the bottom. There is no way to reverse this. Deeper cell basins is not a solution either since batteries rely on the balance of chemistry and deeper basins would mean larger casings, more electrolyte and thicker plates, etc.