The amount of time required to charge a 12-volt battery largely depends on how low the battery is in energy. A highly discharged battery may take up to 12 hours for it to be fully charged. Three-stage automatic 12-volt battery chargers are the most recommended for efficient charging.
Charging batteries slowly rather than quickly is recommended. Repeated quick charging of 12-volt batteries may lead to reduced battery life over time. Using 10 amps for recharging is considered slow while using 20 or more amps is considered a fast charge. Twelve-volt batteries have six cells with each providing 2.1 volts for a total of 12.6 volts when fully charged.