How much electricity does it take to charge a car battery?

Answer 1

No matter if it is a 6 volt or a 12 volt battery, the amount of power it takes to charge it depends on its "charge capacity", measured in ampere-hours. A 60 A-h battery needs half the amount of power to charge it as a 120 A-h battery.

The amount of power you have to supply from a battery charger depends on how old the battery is and also on how old the battery charger is. The older they are, the more power they will waste during the battery charging period.

Very roughly, a nominal 12 volt 60 A-h battery in good condition but "flat" requires about 5 amps of current supplied at between about 15 to 17 volts (which the battery charger unit should take care of automatically) for about 13 to 15 hours.

When the battery has been fully charged and disconnected from the charger, its voltage should be around 13.6 volts when it is not supplying any load current. The electrical power needed to do that would be roughly equal to 17 volts x 5 amps x 15 hours = 1275 watts.