The time it takes to charge a 12-volt battery is mostly determined by how low the battery’s energy level is. It can take up to 12 hours to fully charge a highly drained battery. The most efficient charging is done with three-stage automatic 12-volt battery chargers.
It is preferable to charge batteries slowly rather than quickly. Quick charging of 12-volt batteries on a regular basis may degrade battery life over time. Recharging at a rate of 10 amps is regarded slow, whereas charging at a rate of 20 amps or higher is considered quick. When completely charged, twelve-volt batteries have six cells that each provide 2.1 volts for a total of 12.6 volts.