Construction

• A 12-volt battery charger is roughly the size of a car battery. It has a thick cable ending in two heavy-duty copper alligator clips, a black clip for the battery's negative terminal and a red one for positive. The charger has an on-off switch and a meter to indicate charging power.
  
  

Action

• The 12-volt battery charger uses household current to put an electric charge into a discharged battery. The charger's circuits convert 120-volt household alternating current (AC) to the 12-volt direct current (DC) that the battery needs.
  
  

Types

• Trickle chargers, also called battery maintainers, put a small amount of current into the battery to keep it charged while in storage. A heavier-duty charger can recharge a battery in about a day. The professional-grade charger your mechanic uses delivers more current and can charge the battery in an hour or two.