Thursday, 17 November 2011

What is an Alternator & What does it do?

Quite simply, without an alternator your car, or whichever vehicle you have, would not start. The alternator is part of a very important electrical charging system in your vehicle and is integral to starting the engine.

So, What is an Alternator?

To prevent battery failure, most vehicles are fitted with an Alternator, whose purpose is to provide continual charge to the battery whilst engine is running. This supply of power prevents the battery from going flat.

But why? Every vehicle is fitted with a battery, which has a dual purpose:

1. Providing the power to turn the starter motor
2. Running the vehicles electrical systems e.g. car stereo and headlights

A problem common to all batteries is that their charge (power) is used up over repeated usage, eventually resulting in failure. Not a problem in your TV remote, just replace the batteries every 6 months or so. But a car battery costs significantly more and is not something you want to be replacing every few weeks!

How does an Alternator work?

The alternator’s role is to charge the battery and it does so by converting kinetic energy from the crankshaft, into electrical power. The alternator is connected to and powered by the crankshaft, by a drive belt. Most modern vehicles are fitted with a serpentine drive belt, which drives multiple rotating components. When the engine is running, the drive belt rotates the alternator, which converts the kinetic energy into AC electrical current (alternating current), via a process known as electromagnetism.

However one problem remains, the fact that the battery requires DC power (direct current). Before power can be sent to the battery, the vehicle’s electrical system first needs to convert the AC power to DC power and it does so by passing the AC current through the diode pack, where the rectifier converts it into DC current.

After successfully navigating the rectifier, the DC electrical current leaves the alternator along the thick B+ terminal, providing the required charge to ensure the longevity of your battery.


  1. Does an alternator consume any electric current when the vehicle as well as all equipments and lights are off. My car is found to consume some 2.5 amp electric current when off but when alternator is disconnected it becomes zero.

    Happy to have an explanation

  2. Thanks for your question. Under normal circumstances an alternator should not consume any current at all.