“A bad alternator will cause your headlights to dim, other car electrics will start to malfunction and you’ll have to jump-start your car to get it started”
Is the starter motor turning over slowly? Do you have to keep charging the battery, even though it was replaced recently or you know it's in good condition? Your alternator may be faulty, which means your battery isn’t being charged (you should see a red warning light or other message on your dashboard) and it’ll go flat quickly.
Other clues for when an alternator goes bad include the electric windows slowing down or the headlights being dimmer than normal. Basically, if you’re having electrical issues, but a new battery hasn’t fixed it, your alternator could well be to blame.
An alternator’s bearings cause most failures (it starts to get noisy when the engine is running), but the drivebelt that powers the alternator can also snap or become loose. If it's the latter, you should be able to get away with tightening or replacing the belt, but if the alternator is faulty it should be replaced without delay.
How long does an alternator last?
An alternator is designed to last the lifetime of the vehicle, which should be 20+ years if it's well maintained. However, nothing's guaranteed and parts can fail.
In the first instance, check the drive belt (as suggested above), because that is most likely to fail before the alternator itself.
Can an alternator be repaired?
An alternator is usually unserviceable and will need to be replaced. Without a functioning alternator, a car battery isn't much use. It'll last about 30 miles in daytime driving conditions, even less at night when you need the headlights.
The battery in a car is not there to provide endless power like the batteries in a torch. Instead, it’s there to receive power, hold onto it for a bit, and then send it out into the car. But it can only do that if the alternator is in good health.
How an alternator works
An alternator comprises a voltage regulator, a rotor, a stator and diode, with the electricity being produced by the rotor spinning within the stator. The diode converts the AC current to DC and the voltage regulator ensures the current is manageable for the battery.
The DC electricity is used by the car for various things, and it’s sent to the battery to keep it topped up for extra duties the alternator can’t help with, such as starting the car, or keeping things operating when the engine is turned off.