Dead on the road, or the battery looks damaged?
If the car has died in traffic, get everyone somewhere safe, away from the traffic. Use the hazard lights if they still work, and call roadside assistance if you have it. Don't attempt a jump start beside moving cars. Don't jump or charge a battery that's cracked, leaking or bloated: NRMA says a misshapen case is often a sign of overheating and internal damage. If anyone is hurt or in danger, call 000.
What to do right now
Make sure it's the battery
Dim or dead dash lights with a click or a slow crank point to the battery. A strong crank that won't fire is a different problem.
Look at the terminals
With everything off, look for a loose clamp or crusty build-up on the battery posts. Don't touch anything that looks wet or leaking.
Jump start only if you know how
Read the owner's manual first. RACV warns that an incorrect jump can cause issues with electrical systems, and that the last black clamp goes on bare metal on the flat car, not on its battery. Not sure? Leave it to roadside assistance or a mobile mechanic.
Once it starts, drive it
RACT says running the engine without driving won't help. RACV suggests 10 to 20 minutes on the road after a jump, and don't switch off anywhere you can't afford to get stuck.
Why did it go flat?
| The clue | What it needs | |
|---|---|---|
| Something left on | Headlights, an interior light or a boot light found on, and it's the first time | A proper charge, and usually nothing more |
| Short trips | Mostly drives under 20 minutes, which RACV says won't allow much recharging | A longer weekly run or a smart charger |
| Parked for weeks | NRMA says a car still drains the battery even when it's locked | A charger or maintainer while it sits |
| Old battery | More than three years old, with slower starts on cold mornings | A test, then probably a new one |
| Charging fault | Goes flat soon after a charge, or the battery light came on | An alternator and belt check |
| Something draining it | Flat overnight with a healthy battery. RACT names dash cams, reverse cameras and trailer brake controllers | A drain test by a mechanic |
Will a drive recharge a flat battery?
Not fully. RAC says it generally takes up to eight hours of continuous driving for a battery to become fully charged. RACT says modern charging systems won't fully recharge a battery once it has drained.
So a jump gets you going, but as RAC puts it, jump starting isn't the same as recharging. To bring it back properly, use a smart charger, which RACT says can take up to three or four days depending on the battery and the charger. Standard batteries aren't designed to be run nearly flat and charged up again, so each deep flat takes something out of them.
How do you know the battery's finished?
RACV and NRMA both put a standard car battery at about three to five years, and RACV says AGM batteries can reach five to seven, sometimes less with stop-start. Heat and cold both wear them down, and RACV says batteries are more likely to go flat in winter.
A multimeter gives a quick guide after the car has sat overnight. RACV says 12.6 volts is full and anything under 12.4 volts is considered discharged. A load test by a mechanic tells you more, and NRMA recommends regular testing once a battery is past three years.
Don't count on a warning in a newer car. An NRMA patrol describes batteries that seem fine one minute and are gone the next.
What will the mechanic do?
First they load test the battery, then test the charging system with the engine running, because a good battery on a bad alternator just repeats the problem. If both pass, they measure the drain with the car asleep to find what's pulling power.
The right battery matters more than it used to. RACT says modern cars need the correct type, whether that's calcium, EFB, AGM or lithium, and some European cars have a second, well-hidden battery. RACV warns that the wrong one can severely damage the electrical system, so ask what's going in and why.
A mobile mechanic can do all of this in your driveway or a car park, which suits a car that won't move. Ask for the test result, not just a new battery.
What you can do, and what's the mechanic's job
You can
- Check for loose or crusty terminals with the engine off.
- Keep the key fob at least three or four metres from a parked car, as NRMA suggests.
- Use a smart charger if the car sits for weeks at a time.
Leave to the mechanic
- Load testing the battery and the charging system.
- Tracking down a drain from an accessory or a fault.
- Fitting the right battery type for the car.
- Helping with hybrids, EVs and cars with hidden batteries, where a jump can go wrong.
Where we'd start
First flat, and you know why: we'd jump it or charge it, then have the battery tested within the week. Second flat in a month, or a battery past three years: we'd stop jumping it and ask a mobile mechanic to test the battery and alternator where the car sits. RAC's rule holds up well. If it keeps losing charge and nothing else is wrong, it's probably time for a new one. Annoying, but rarely a big problem.