A car battery charger restores energy in a car battery by converting the AC power to 12 volts DC stored in the battery.
It charges the battery at a much lower voltage than hooked up to a cable or hooked up to the starter. This allows the battery to last longer and is safer for the user. Pushing too much power into a battery can cause hydrogen gas to be released, potentially causing the battery to burst under extreme conditions.
It typically charges at 2 amps, which means will take a long time to fully charged—up to 24 hours if the battery is completely flat. While it’s a long time, it’s the best way to extend battery life.