Over time there is less conductive surface on the cell so the charge gets shorter. In extreme circumstances enough material can gather at the bottom and result in a short circuit. Another cause for the battery to wear out is something referred to as sulphation. This occurs when the car batteries are left at the zero charge state for too long. Nonconductive lead sulphate crystals start to grow around the cells. The more time the car batteries stay in the zero charge condition the more crystals grow and the more difficult it becomes to reverse. To get the most from your car battery make certain to look after it properly.