The electrical system of a car is a closed circuit with an independent power source the battery. It operates on a small fraction of the power of a household circuit.Current flows along a single cable from the battery to the component being powered, and back to the battery through the car’s metal body. The body is connected to the earth terminal of the battery by a thick cable.
This type of circuit is called an earth-return system any part of it connected to the car body is said to be earthed.The strength of the current is measured in amperes (amps); the pressure that drives it round the circuit is called voltage (volts). Modern cars have a 12 volt battery. Its capacity is measured in amp/hours. A 56 amp/hour battery should be able to deliver a current of 1 amp for 56 hours, or 2 amps for 28 hours.