Automotive Electricity

The electrical system of a car supplies the energy needed to start the engine, generate a spark for combustion, and run the many electrical devices on board, from lights to fuel injectors. It centres on two energy sources working in turn: a battery that provides power before and during starting, and an alternator that takes over once the engine is running.

The lead-acid battery stores energy chemically and delivers the heavy current that the starter motor needs to turn the engine over. Once the engine runs, the alternator, driven by a belt from the engine, generates electricity. Within it a rotating electromagnet, the rotor, spins inside fixed coils called the stator, inducing alternating current. Because the car's circuits use direct current, a rectifier stack of diodes converts the alternator's alternating output to direct current. A voltage regulator adjusts the rotor's field to hold the output steady regardless of engine speed, and a charge warning lamp signals the driver if the charging system fails.

The ignition system, in petrol engines, produces the high-voltage spark that ignites the fuel-air mixture. An ignition coil acts as a transformer, stepping the low battery voltage up to the thousands of volts required to jump the spark plug gap. Timing circuits ensure the spark occurs at the correct moment in each cylinder's cycle. All of these subsystems share the same supply, so a weak battery or failing alternator can disturb the entire electrical operation of the vehicle.

Frequently asked questions

What does an ignition coil do?

An ignition coil is a transformer that raises the car's low battery voltage to the very high voltage needed to create a spark across the spark plug gap and ignite the fuel mixture.

How does the rotor and stator in an alternator generate electricity?

The rotor is a spinning electromagnet that moves inside the stator's fixed coils; this changing magnetic field induces an alternating current in the stator windings.

Why does a car need both a battery and an alternator?

The battery provides the strong current needed to start the engine, and once running the alternator generates power for the electrical loads and recharges the battery.