What does an alternator do?
Tuesday 27th October 2026
An alternator generates electricity while your car engine is running It keeps your battery charged and powers the electrical systems you rely on every journey.
Most drivers never think about the alternator until a warning light appears. In our guide, we’ll explain how it works, what an alternator belt does and the signs that point to a problem.
What does a car’s alternator do?
The alternator converts the engine’s rotational energy into electrical energy. It has two main functions, to charge the battery and power the electrical systems, and it does this simultaneously while the engine is running.
Starting the engine takes a huge burst of energy from the battery. The alternator recharges the battery as you drive so that the battery has enough charge for the next start. Once the engine is running, the alternator supplies electricity directly to the car. This includes:
- Headlights and dashboard lights
- Windscreen wipers
- Heating, ventilation and air conditioning
- Electric windows
- Heated seats
- Radio and infotainment
- Power steering
- Engine management and fuel systems
If the alternator fails, extra pressure is put on the battery to power everything else, so it runs flat much faster.
How does an alternator work?
The engine turns the alternator through a belt. Inside the alternator a spinning magnetic rotor moves within a set of fixed copper coils called the stator. This movement generates an alternating current (AC).
Your car’s electrical system runs on direct current (DC), so the alternator contains a rectifier that converts AC to DC. A voltage regulator then keeps the output steady, typically between 13.7 V and 14.7 V when the engine is running. That voltage sits above a fully charged battery’s resting voltage, which is what allows the battery to recharge.
What does an alternator belt do?
The alternator belt transfers power from the engine to the alternator. It loops around a pulley on the engine’s crankshaft and a pulley on the alternator, so the alternator spins whenever the engine runs.
On many cars, a single belt (the drive belt) also turns other components, such as the power steering pump, air conditioning compressor and water pump.
Why does a car need both a battery and an alternator?
The battery and alternator work together to share the load. The battery stores the energy needed to get the engine running, then the alternator takes over to recharge the battery and maintain power.
This is why short, frequent journeys can leave a battery low on charge. Each start draws heavily from the battery, and a short drive doesn’t give the alternator enough time to replace the energy used.
Where is the alternator located?
The alternator sits at the front of the engine, bolted to the engine block and driven by the belt. It has a round vented metal housing, usually about the size of a grapefruit. On most cars you can spot it by following the belt from the crankshaft pulley.
What are the signs of a failing alternator?
Alternators usually lose output gradually before they fail completely. Watch for these warning signs:
- Battery warning light: The battery symbol on your dashboard can indicate an issue anywhere in the charging system. Some cars have a separate alternator light that illuminates when the alternator starts to fail.
- Dim or flickering lights: Headlights and interior lights may dim when you switch on several electrical items at once.
- The battery doesn’t hold a charge: Even after charging or replacing the battery, it keeps going flat. Electrical faults: Electric windows, radio or air conditioning may work slowly, intermittently or stop working altogether.
- Difficulty starting the car: A battery that the alternator has failed to recharge may lack the power to turn the engine over.
- Whining, grinding or squealing noises: Worn bearings inside the alternator or a slipping belt can produce these sounds.
- Burning smell: A burning rubber or electrical smell can indicate a slipping belt or overheating electrical wiring. Have the car inspected straight away.
How often should your alternator belt be checked?
A visual inspection during each service can spot cracks, fraying or loose tension before the belt fails. Replacing a worn belt costs far less than the recovery and repair costs caused by a failed alternator.
Can you drive with a faulty alternator?
You can, but not for long. With the alternator out of action, the car runs on stored battery power until it runs flat. Switching off any non-essential electrics like heated seats and radio will buy you a little more time.
Jump-starting a car with a faulty alternator will get the engine going again, but the battery will quickly drain again as there is nothing recharging it.
Concerned about your alternator?
If you’re concerned about your alternator or battery performance, book an appointment at your nearest Formula One Autocentres branch for a diagnostic test.
