A car battery is typically a lead-acid type of energy storage device, consisting of six independent cells from the negative terminal side of the battery to the positive terminal side of the battery. The energy storage for each cell is around 2 volts each, meaning that a fully-charged battery with all of the cells working properly will show a stored voltage of around 12 volts. When, after a full overnight charge, a battery volt reading indicates a total voltage of 10.5 volts or less, this is a strong indicator that a cell has gone bad or that it has a short between itself and another on either side of it. To determine whether a battery has a bad or shorted cell, provide a proper charge and to then test the battery after the charging cycle is complete.
Disconnect the car battery, remove it from the automobile and set it next to an automotive battery charger.