What is a dead cell battery?

A battery that cannot charge higher than 10.5 volts has a dead cell or is considered a dead cell battery. A battery that does not charge at all past a reading of zero is likely short circuited.

If a battery's voltage is 12.4 or less but reads fully charged, then the battery is sulfated. Sulfations occurs through the discharge, and charging the battery changes the sulfation crystals back into electrolyte, which means it's ready to produce power. If a battery sits for a long period of time uncharged, the sulfation increases in size and hardens. This can keep the battery from fully charging and also can cause the battery to discharge at a faster rate.