How much of a 12-volt battery has to be charged will significantly determine how long it takes. Up to 12 hours may pass before a highly drained battery is fully charged. The most suggested chargers for effective charging are three-stage automatic 12-volt battery chargers.
It is advised to charge batteries gradually rather than immediately. Battery life may be shortened over time if 12-volt batteries are frequently charged quickly. While charging at 10 amps is seen as slow, charging at 20 amps or higher is regarded as quick. Six cells make up a twelve-volt battery, and when completely charged, they produce a combined 12.6 volts from their individual 2.1 volts.