How to revive dead batteries (at least temporary). Old batteries tend to suffer from shorted cells. If the battery voltage, after a charge cycle, is considerable less than 1.25 * (the number of cells), you probably have one or more shorted cells. To know for sure, you have to measure the cells individually. For this the battery has to be taken apart to some extend. If a cell measures (almost) zero volt during charging, it is shorted. For reviving you need a power supply capable of delivering 2 to 3 ampere. Set the voltage to 10 to 20 volt, connect the voltmeter and connect it briefly to the shorted cell only. This will draw some sparks, and the voltage will drop and rise again. Disconnect when it reaches 1 - 1.25 volt. Reconnect the battery to the charger and monitor the cell voltage. If it looks still shorted, repeat. Disclaimer: This could be dangerous. Do not attempt unless you know what you're doing.