A car battery is charged by the alternator on modern cars or by a dynamo on earlier ones. The charging system consists of the alternator, battery, wiring, and electronic control unit (ECU) . The alternator creates electrical power to run accessories and recharge the battery. It turns mechanical energy from the cars crankshaft into electrical energy. The computer (or regulator in older cars) controls the alternators output and senses when the battery needs charging or when the demand for electricity changes. The battery is a reservoir of electrical power that cranks the engine to start the car and supplies power to the accessories when the engine isnt running.

If something is not right with the charging system, a warning light labeled "Gen," "Bat," or "Alt" will appear. If this warning light lights up while the engine is running, it means that there is a problem in the charging system, usually that the battery has gone bad or the alternator has stopped working.

It is a common myth that driving a car will fully charge the battery. While the alternator does charge the battery, it only does so when the car is being driven at highway speeds. Otherwise, the alternator is busy powering all the onboard electronics, from the AC to the little lights on the windows, and the computers managing the transmission and sensors. Actual battery chargers take 10-24 hours to charge a car battery, and trickle chargers can take three or more days.

In summary, a car battery is charged by the alternator or dynamo, which creates electrical power to run accessories and recharge the battery. The battery is a reservoir of electrical power that cranks the engine to start the car and supplies power to the accessories when the engine isnt running. While driving a car can charge the battery, it only does so when the car is being driven at highway speeds, and actual battery chargers take 10-24 hours to charge a car battery.