Flat battery on the side of the road?
Get the car as far off the road as you can and put the hazard lights on, if they still work. Everyone waits outside the car, on the side away from traffic. Don't try a jump start beside fast-moving traffic: ring your roadside cover instead. If anyone is hurt or in danger, call 111.
Signs the battery is on its way out
- The engine cranks slowly, worst on cold mornings.
- A rapid clicking when you turn the key or press start, and no crank.
- Headlights or the dash dim noticeably while the engine is starting.
- The battery light comes on while you're driving (that one often points to charging, not the battery).
- White or green crust building up around the terminals.
- The car has needed a jump start more than once lately.
Is it the battery, or something else?
A battery can only hold what it's given. If the alternator (the part that charges the battery while the engine runs) is failing, a new battery goes flat just like the old one. The same happens when something keeps drawing power after the car is locked, like a boot light that stays on or a faulty accessory.
That's why a proper test comes first. A load test shows how the battery copes under strain, and a charging check shows whether the alternator is keeping up. If it's the alternator or the starter motor, the job becomes an alternator or starter repair instead.
Jump starting without hurting the car or yourself
Read the owner's manual first
Some cars have dedicated jump points under the bonnet, away from the battery itself, and the manual shows how the maker wants it done. Hybrids have their own instructions.
Look at the battery
If it's cracked, leaking or swollen, don't jump it. Batteries can give off a flammable gas, so no smoking or flames nearby.
Connect in the order the manual gives
AA New Zealand warns that getting the connections the wrong way round can damage sensitive electronics. If you're unsure, stop and ring someone who does this every day.
Don't trust the drive home to recharge it
The alternator is built to keep a battery topped up, not to bring a dead one back. A jump gets you going; it doesn't fix the cause.
Get it tested
AA New Zealand recommends a check after a flat battery to find out why it happened. A mobile mechanic can often test and replace it where the car is.
Why the battery type matters on newer cars
Batteries aren't all the same inside. AA New Zealand says cars with start-stop (the engine switches off at the lights) need an AGM battery, and that fitting another type hurts the fuel-saving system.
Some newer cars, many European models among them, also need a fresh battery registered with a scan tool so the charging system knows it's new. And a hybrid like an Aqua or a Prius still has a small 12-volt battery that wakes the system up. When that one is flat, the car won't start, even with a fully charged hybrid battery.
What a good quote for a new battery says
- The test result for the old battery and the charging system.
- The battery type and size, matched to what the maker specifies.
- Registering or coding the new battery, if the car needs it.
- Whether it's fitted at your place or at the garage.
- Taking the old battery away for recycling.
- The battery's own warranty, in writing.
What you can do, and what to leave
You can
- If the car sits for weeks at a time, use a smart maintenance charger. AA New Zealand says not keeping the battery fully charged is the biggest thing that shortens its life.
- On short-trip days, let the engine run for a bit before switching on the heater and other extras.
- Look at the terminals for crust, without touching them, and mention it to the mechanic.
- Keep the old battery out of the household rubbish. A battery in landfill eventually corrodes and leaks chemicals into the soil.
Leave to the mechanic
- Load testing the battery and checking the alternator.
- Choosing the right type and fitting it, plus registering it on cars that need it.
- Tracking down a drain that flattens the battery overnight.
What we'd do
We'd ask for a battery and charging test before buying anything, especially after a jump start. If the battery is the problem and the car is at home or work, a mobile mechanic can often swap it on the spot. Mention the make and model so the right type comes with them.