A battery charging circuit in a car is the assembly that connects the alternator to the battery and sends electricity to the battery. The motion of the wheels is transferred to the alternator, which acts as a simple generator. Some energy, however, is always lost to friction and heat.
The alternator produces alternating current, which has too much voltage for the car battery. It passes through diodes that convert AC to 12 volts DC. The peak voltage that passes into the battery is about 14.8 volts, after which the voltage regulator opens the circuit and prevents further charging. Electricity then flows back to the alternator.