What You Need to Know About Your Car's Alternator

Jack Dreyer | Monday 9th January 2023 8:00am

Alternator in a clean engine bay – likely being inspected for performance.

If you’ve owned a car for any length of time – especially an older car – you’ll likely have heard something or other about its alternator. They’re not the part most prone to wear, but they can be really pesky when they start to go because they tend to become less effective rather than suddenly stop working. If your exhaust breaks, you tend to know about it pretty quickly – if your alternator needs replacing then you might get signs you wouldn’t necessarily expect!

So what are alternators exactly, what do they do, and what do you need to know in order to keep them happy for as long as possible?

What is an alternator?

The alternator is a component in combustion engine vehicles that acts like an electricity generator to recharge your car’s 12v battery and power anything that needs electricity while driving. So this means that without your alternator you’ll be stuck without lights, heating, cooling, power steering, or even the beloved radio.

Perhaps more surprisingly, if your alternator is on the way out then you may find you can’t even start your car!

How does an alternator work?

Clean, new-looking alternator being driven by a drive belt for inspection.

The car’s combustion engine, when running, provides a great deal of rotational power. This power is used to actually move the car but a certain amount of it is transferred to the alternator using a belt system (one of a few belt-driven systems). A simplified version of how this then works is that the belt turns a magnetic core within a coil housing in much the same way that the motor of an electric car works – the magnetic field of the core then creates an electric current in the wires, which is sent to the car’s components as well as to the battery.

Why do you need an alternator?

Originally, engines had to be hand-cranked in order to start up, but this was actually quite dangerous if an engine misfired and you were in the way. So starter motors were developed to provide the starting power that the engine needs.

The problem is that, by virtue of being necessarily quite small, starter motors need to output a lot of force very quickly to get the engine going – so the battery exists primarily to provide this power, and the alternator exists to recharge the battery so that you’re not left stranded.

How do you know if your alternator needs replacing?

A technician inspects a disassembled alternator for cleaning & fault-finding

A simple test is to use a multimeter on your car battery terminals. When turned off, you should see a reading of between 12v-13v – when you start the car, this reading should jump to somewhere around 14.5v. If you’re getting a reading in the low 13v area then there’s a good chance that the alternator isn’t providing enough power or that there’s an issue with the connections.

Most usually, people realise that their alternator is failing because their battery keeps going dead. The two main things that happen are that the alternator fails to charge the battery enough to start the car, in which case you realise something’s wrong as soon as you can’t start your car. Or, the alternator will dip out its power output while the car’s running – so you’ll notice the lights dim and brighten, or other odd behaviours.

The first assumption is usually that the battery needs replacing – because this is usually the correct assumption. 12v car batteries get put through their paces so tend to need to be replaced every 2-4 years (though this varies greatly depending on the brand and quality you have installed). But an alternator can usually be expected to last for around 10 years.

Concerned about your car’s alternator?

If you’re unsure whether your battery needs to be replaced or whether your problems lie with the alternator, book your car for a free battery health check at your local Kwik Fit centre – the experts there are always happy to advise!

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