Battery light on while you're driving?
Find somewhere safe to pull over soon. RACV says the battery light can mean a failing alternator or a broken drive belt, and that needs checking immediately. The car runs on what's left in the battery, and it may not restart once you stop, so stop somewhere you can wait safely. If the temperature gauge is climbing too, switch off straight away: on many cars the same belt drives the water pump. If anyone is hurt or in danger, call 000.
Alternator, starter or battery?
| Alternator | Starter motor | Battery | |
|---|---|---|---|
| When it shows up | While you're driving | When you try to start | When you try to start, often after sitting |
| What you notice | Battery light on, lights dimming, electrics playing up | A click or a clunk, or nothing, with bright lights | Slow crank, dim lights, a click |
| Will a jump start help? | It'll start, then it may die again | Usually no | Usually yes, for now |
| Other clues | A whine or a burning rubber smell from the belt area | Starts sometimes and not others | It's three years old or more |
How does a mechanic find the fault?
They test before they replace. RAC WA says a multimeter test can tell the battery from the alternator, because the symptoms overlap. A good mechanic checks battery condition, charge rate with the engine running, the belt and tensioner, and the wiring and earths.
Those small things catch people out. RACV lists loose wiring and a broken drive belt alongside the alternator as causes of a battery light. A corroded terminal can look a lot like a dead starter, and it costs far less to fix.
Newer cars add a wrinkle. RACQ says the charging system is deactivated under certain conditions, so a single reading doesn't tell the whole story. That's a reason to use someone who knows how your make charges.
What's the job itself?
An alternator sits at the front of the engine, driven by a belt. A starter sits low down, where the engine meets the gearbox. Most are unbolted and replaced whole, though some can be repaired or swapped for a reconditioned unit.
How big the job is depends on where it sits. On some cars it's near the top and quick to reach. On others, the starter is buried under the inlet manifold, or the alternator comes out from underneath.
These jobs are mechanical and electrical at once. Most mechanics can do them. A tricky charging fault or a wiring problem suits an auto electrician, which NSW treats as its own trade, covering electrical equipment, systems and circuits.
Why a failing alternator kills batteries
RAA says both under and overcharging reduce battery life. A weak alternator leaves the battery flat and working too hard. A faulty regulator can overcharge it. Either way, a replacement car battery won't last if the cause stays.
RAA suggests having the charge rate checked at every major service. It's a quick check, and it's the sort of thing that picks up a problem before you're stranded.
What a good written estimate shows
- The test results: battery, charge rate and starter draw.
- Which part has failed, and how that was confirmed.
- New, reconditioned or repaired, and the warranty on each.
- The belt and tensioner, if they're worn.
- Any wiring or earth repairs, listed separately.
- A note if the battery has been damaged too.
What you can do, and what to leave
You can
- Note exactly what happens when you turn the key: click, clunk, slow crank or nothing.
- With the engine off, check the battery terminals look clean and tight, as RACV suggests.
- See if the headlights stay bright when you try to start.
- Turn off the air con and accessories if the battery light's on and you need to reach a safe stop.
Leave to the mechanic
- Testing the charging system with the engine running.
- Anything near a moving belt. Keep your hands clear.
- Removing and fitting the alternator or starter.
- Wiring, fuses and the battery cables.
Where we'd start
We'd get a charging system test before buying anything. That one step separates a battery from an alternator from a loose wire, and it's the difference between a small bill and a wasted part. If the car won't start where it's parked, a mobile mechanic can often test it there. Tell us what it does when you turn the key, and the mechanic will have a good idea before they arrive.