If you’ve been in a car recently (and unless you’re part of a small minority, the chances are that you will have), then you might have noticed that there are lots of components which run on electricity.The radio, the headlights, the windscreen wipers, the air conditioning, the heated seats – each of them is drawing electrical power from somewhere.But you don’t need to charge your car (unless it’s an electric car).Which suggests an obvious question: from where is the vehicle drawing its power?

The answer, perhaps unsurprisingly, is that the energy is coming from your petrol tank – or, at least, indirectly.Let’s take a look at the closed circuit inside your car, and see exactly how it sustains itself.

Car battery

A car’s battery is where its electricity is stored.Each component in your car is wired to the positive terminal, and to the negative terminal via the body of your car.This type of circuit is known as an ‘earth return’ circuit, and it causes every part of your car to be earthed.

Most modern cars have a twelve-volt battery, which is roughly equivalent to the sort of power that a desktop computer receives.Voltage measures the force which drives the current around the circuit.The current itself, however, is measured using amperes.A simple way of appreciating the difference is that devices draw current, while power sources provides voltage.

The capacity of a car battery tends to be measured according to the amount of current that can be drawn in a specific time frame, typically amp/hours.So, a 48 amp/hour battery will be able to deliver one amp for forty-eight hours, or two amps for twenty-four.

Starter motor

Without doubt the most important electrical component in your car will be its starter motor.Without this little piece of kit, the engine won’t be able to turn and your car won’t switch on.This electromagnetic motor is activated through a solenoid electromagnet, which uses a low-current circuit to complete a high-current circuit.This helps to prevent dangerous arcing, as the magnet is capable of actuating the switch far faster than a human being could manually.


Obviously, you don’t need to replace a car’s battery every time it runs out of power.Instead, it’s being constantly recharged through a device called a generator.These devices fall into two categories.In newer cars, it’s an alternator, while older ones might use a dynamo.

Within an alternator you’ll find a set of coiled wires, which are wrapped around an electromagnet which rotates along with the movement of the engine.This causes electromagnetic induction to generate a current in the surrounding wires.This current, however, is alternating – it changes direction with each turn of the rotor.In order to fix this, a special circuit called a rectifier must be used.This device lops off the negative part of the signal and then uses a series of capacitors to smooth is into a nice, even signal.

A dynamo works somewhat differently.It provides direct current, but it’s far heavier than an alternator, and thus no longer widely used.Since rectifiers are inexpensive and easy to implement, there’s no reason to.A dynamo reverses the arrangement of an alternator, so that the electromagnets are stationary.The current is generated by another set of coils inside the shaft known as the armature.

How do I know when my battery is running low?

When your battery is not being properly charged, you’ll get a warning light on the dashboard.This might occur when your car is left with the engine turned off but the headlights on for a sufficient length of time.A modern car, for this reason, will emit a helpful reminder beep whenever you open the door while the lights are on.If your vehicle is especially sophisticated, it might even come with a readout on the dashboard to show you the current charge in your battery, along with how much electricity is being generated at any given time.

What if I need a replacement

While modern automobile components are reassuringly long-lasting, they’re still vulnerable to wear and tear and so will occasionally benefit from a replacement.You’ll be able to secure the necessary replacements online.You can buy starter motors for a variety of makes and models are easy to find, and the same can be said of generators.You’ll find Audi, Ford, Vauxhall and BMW alternators available from online outlets which specialise in car parts.