To choose a battery charger, determine how many Amp-hours your car battery stores. You can find this in the owner's manual. And then determine whether you want to charge your battery slowly or more quickly. Keep in mind that a fast charge can buckle the battery plates. Once you know how many Amp-hours your battery stores, you can estimate how long it will take to charge and then decide which battery charger will work best. For example, a typical full-size auto battery is about 48 Amp-hours. A basic charger usually charges at around 2 amps which means it will need 24 hours to deliver the 48 amps needed to fully charge a flat, 48 amp hour battery.