If the battery light comes on while you're driving
Pull over somewhere safe and switch off. The RACV says the car may keep going for a while, but the battery can lose power and stop it restarting, so don't count on reaching home. If the steering suddenly goes heavy or the temperature gauge climbs, stop straight away: the drive belt may have broken, and on many engines that belt also runs the water pump. Call roadside assistance if you have it. If you're stuck near traffic, get everyone somewhere safe, and if anyone is hurt or in danger, call 000.
What does an alternator do?
The alternator is a small generator, turned by a belt from the engine. While the engine's running, it powers the car's electrics and charges the battery. The battery's main job is starting the car, and the alternator takes over from there.
When the alternator weakens, the car starts living off the battery. Everything can seem normal until the charge runs out, which is why the car often dies a while after the first warning.
Signs of a bad alternator
- The battery light on the dash. It may flicker at first, then stay on.
- Dim or overly bright lights. AAA, a US motoring club, says a failing alternator can send inconsistent voltage to them.
- Slow electrics. Windows, wipers or the radio that lag or play up.
- A battery that keeps going flat. Above all if a new one follows the old one.
- A whining or groaning noise from the engine bay.
- A smell of burning rubber or wire. Treat this one as urgent.
- Repeated stalling. The engine needs electrical power to keep running.
Alternator or battery?
| Points to the alternator | Points to the battery | |
|---|---|---|
| When it happens | While the car's running or driving | Mostly when starting, above all after the car has sat |
| After a jump start | Runs for a while, then fades or stalls | Starts and keeps running, then may not restart next time |
| The lights | Change with engine speed | Dim mainly when the engine's off or cranking |
| Noise or smell | A whine, a groan or a burning smell | Usually none. A swollen case or corrosion on the terminals is the giveaway |
| The battery's age | Any age, even nearly new | Often past three years |
Why you can't tell by the symptoms alone
RAC WA's Car Doctor says a slower start, a slight clicking or fading dash lights can come from a failing battery, and the same symptoms may also indicate a faulty alternator. NRMA agrees that a flat battery might be caused by a faulty charging system.
AAA's advice is not to assume a dead battery is the battery's fault, and to have a mechanic check the alternator too. It works in both directions, because a worn battery can make a good alternator look guilty. A test settles it in far less time than swapping parts.
What else can it be?
The RACV lists four common causes for that light: a failing alternator, a damaged battery, loose wiring or a broken drive belt. So a mechanic may find the alternator's fine and the fault is a loose connection or a slipping belt.
The belt matters more than people think. Kia's schedule for one of its Australian models describes a single alternator, water pump and air conditioner drive belt. On an engine like that, one worn belt can affect all three.
What to do if you think it's the alternator
Cut the load
If you have to move the car a short way to get off the road, turn off the air conditioning, heated seats and stereo, so the battery lasts longer. Then stop somewhere safe.
Don't plan a long drive
A car with a failing alternator can stop anywhere, and on a modern car that can mean losing electric power steering and more.
Don't just buy a new battery
If the charging system is the fault, the new battery will go flat as well. NRMA also warns that repeated jump starts can damage a battery.
Have both tested
A mechanic tests the battery and measures what the alternator is putting out. Mention when the problem started and what you noticed.
Get it towed if you need to
If the battery light's on and the car's lost power, roadside assistance or a mobile mechanic is safer than trying to limp across town.
What the mechanic should check, and what the estimate should say
- A battery test, with the result.
- The alternator's charging output, with the engine running.
- The drive belt's condition and tension.
- The wiring and connections between the alternator and battery.
- Whether the alternator is being replaced or reconditioned, and the warranty on the part.
- Whether the belt should be replaced while it's off.
What you can do, and what to leave
You can
- Note when the battery light comes on, and whether it stays on.
- Watch how the headlights behave when you rev the engine gently while parked.
- Check when the battery was last replaced.
- Look at the belt with the engine off and cool, if you can see it, for cracks or fraying.
Leave to the mechanic
- Testing the charging system and the battery.
- Removing and replacing the alternator.
- Fitting and tensioning the drive belt.
- Tracing wiring faults.
If it were our car
We wouldn't guess, and we wouldn't start with a new battery. If the light's on or the car's been dying after a jump start, we'd stop driving it around and have the battery and charging system tested first. If the car still runs, a mobile mechanic or a workshop close by makes more sense than a long trip across town. The RACV's word for it is immediately, and we agree.
Who should look at it?
A good general mechanic can test and replace most alternators. For an electrical fault that comes and goes, an auto electrician is usually the better choice, because that's their whole trade. Hybrids and EVs handle charging differently, so tell whoever you're talking to exactly what you drive.
Tell us the job and the car. A local mechanic who specialises in your car.