How do you know when the battery is dead?

A visual check shows an excess of white buildup between plates. This is lead sulphate. If it is touching both plates in a cell, that cell is dead. This reduces power from the entire battery and may render it useless for applications that need full 12V. If a battery does not retain charge despite long hours of charging, it is dead. The lead oxide paste may have flaked off or the lead plates may have corroded leading to non-retention of charge and a dead battery is the result. If charging does not raise battery voltage or show adequate current according to its Ah rating, the battery is dead. If it is dried up and left unused without recharging for a long time, the battery dies.