A lead-acid cell produces 2.15 volts and a 12-volt battery therefore has six cells wired in series. Unless sealed, it has six caps on the top, one for each cell. The battery voltage is about 12.7 v at full charge (with no load) and drops to about 12.2 v at half charge and 10.5 v when empty. As the charge drops below half, it becomes ever more difficult to recharge. The last 20% of capacity is particularly difficult to replace (as is the first 10%). Discharging below 50% reduces the life of the battery.
Activation
cycleMost batteries cannot survive more than one or two discharges below 20%. They should never be left deeply discharged for very long. If a relatively new battery fails to hold a charge, however, it might be helpful to discharge it down to 20% of capacity and then recharge it fully. This kind of activation cycle can also be repeated up to five times when conditioning or breaking in a new battery. than simply providing 13 volts to the battery over time. An intelligent voltage regulator or AC battery charger will have settings for each type of battery, sensors to measure battery voltage and temperature, and a programmed schedule of voltages to apply at suitable intervals while recharging. The charging voltage should vary according to a precise schedule that depends on the elapsed time, the current level of charge, and the battery temperature. Standard wet cells, deep cycle batteries, gel cells and AGM batteries each require a different charging schedule. The charger should provide a quick bulk charge that takes the battery up to 80% of capacity, one or two stages of slower absorption charge that takes it almost to full capacity, and a float charge that maintains full capacity over time.
When an engine alternator is the source of charging current for deep cycle batteries, only the bulk charge is necessary and the batteries can cycle between 80% and 50% of capacity. They should be brought up to full charge occasionally, perhaps once a month, to prevent sulfation.
OverchargingDuring recharging, the battery voltage rises long before the battery is fully charged. Overcharging will soon destroy a battery by causing its plates to shed oxide, which precipitates to the bottom and cannot be recovered. Symptoms of overcharging include corrosion deposits on battery posts, loss of water exposing the plates, and voltage above 12.7 volts when the battery is not being charged. A brief, controlled overcharge, called an
equalization charge, can be used as a last resort to recondition some non-VRLA batteries and remedy sulfation.