Battery light came on while you were driving?
The alternator has probably stopped charging, and the car is running on what's left in the battery. Turn off what you can, like the fan, the rear demister and the stereo, and head for the nearest safe place to stop rather than the long way home. If the temperature gauge starts rising as well, pull over and switch off. Stopped somewhere busy? Everyone out on the side away from traffic, and 111 if anyone is in danger.
Alternator, starter or battery?
| Alternator | Starter motor | Battery | |
|---|---|---|---|
| Battery light while driving | The classic sign | No | Rarely |
| One click, or nothing, at the key | Not directly | Possible | Often |
| Slow cranking on cold mornings | Only if it's been undercharging | Sometimes | Often, on a battery over three years old |
| Lights dimming or flickering as you drive | Often | No | Sometimes |
| Grinding or a whirr with no crank | No | Often | No |
| Flat again soon after a new battery | Often | No | Only if something's draining it |
How the mechanic tells them apart
Three quick checks do most of the work. A load test shows whether the battery can hold up under strain. A charging check with the engine running shows whether the alternator is putting out enough. A test while cranking shows whether the starter is drawing too much or the cables are losing power on the way.
They'll also look at the drive belt that turns the alternator, and at the connections and earth straps. A corroded or loose cable can mimic a failed part, and it's a far smaller fix. AA New Zealand recommends a diagnostic check after any flat battery for exactly this reason: to find out why it happened.
Why a weak alternator keeps killing batteries
AA New Zealand says the alternator is designed to keep a battery topped up and run the car, not to bring a dead battery back to full. So a jump start followed by a drive doesn't really recharge it.
A failing alternator makes that worse: it undercharges, the battery sits part-flat, and AA New Zealand names poor charge as the biggest single thing that shortens a battery's life. Fit a new battery without fixing the alternator and you'll likely be buying another one soon.
Rebuild, reconditioned or new?
Alternators and starter motors can often be rebuilt with new brushes, bearings, a regulator or a solenoid, or swapped for a reconditioned or new unit. Ask which you're getting and what warranty comes with it. How big the job is mostly comes down to access: some sit right at the front of the engine, others are buried under the intake.
Hybrids are their own case. Toyota New Zealand says a hybrid's 12-volt battery is there for the auxiliary electrics, so a hybrid that won't power up is a different diagnosis from a petrol car that won't crank. Say in the note if yours is a hybrid.
What the written quote should show
- The test results for the battery, the charging output and the starter.
- Whether the replacement is new, reconditioned or rebuilt, and its warranty.
- The condition of the drive belt and tensioner, if the alternator is coming off.
- Whether the battery needs replacing too, backed by its test result.
- Cleaning or replacing corroded cables and earth straps, if found.
Charging, starting and the WoF
Neither part is on the WoF list. The lights are, though, and a car that isn't charging properly can show it in dim headlights, so a charging fault is worth sorting before inspection day.
What you can do, and what to leave
You can
- Listen at the key: one click, rapid clicking, a whirr with no crank and silence each point somewhere different.
- Watch whether the dash lights dim or stay bright as you try to start.
- If you jump start, follow the order in the owner's manual. AA New Zealand warns that connecting the wrong way round can damage the electronics.
- Note whether the battery light came on before the car stopped.
Leave to the mechanic
- Battery, charging and starting tests.
- Removing and fitting the alternator or starter.
- Tracing a drain that flattens the battery while the car is parked.
What we'd do in your shoes
We'd test before buying anything. A battery, charging and starting check stops you replacing a good battery because of a bad alternator, or the other way round. If the alternator is the culprit, ask for the drive belt to be checked while it's off.