Flat in a risky spot, or the battery looks damaged?
If you're stopped on a busy road, hazards on and get clear of the traffic before anything else, and ring roadside help rather than jump starting beside moving cars. If the battery is swollen, cracked, leaking or smells of rotten eggs, don't connect anything to it. If anyone is hurt or in danger, call 111.
What to do right now
Rule out the simple things
Gear in Park, key fob working, lights and accessories off.
Look at the battery, hands off
Crust on the terminals or a loose clamp can stop a car just as well as a flat battery.
Jump it the way the manual says
The owner's manual shows where the leads go and in what order. AA New Zealand warns that a jump pack connected the wrong way round can damage the car's electronics.
Then give it a real charge
Normal driving won't bring a flat battery back to a reliable charge, AA New Zealand says. Its advice is a multistage charger, allowing up to 48 hours.
Get it tested
A test shows whether the battery is worn out or something else flattened it.
Why did it go flat?
| Clues | What sorts it | |
|---|---|---|
| Something left on | A light or the boot left open; first flat in ages | A full charge, then a test to be sure |
| A worn battery | Slow cranking on cold mornings for a while; battery more than three years old | A new battery of the right type |
| A charging fault | Battery light on while driving; flat again soon after a jump | Alternator or belt repair |
| Something draining it | Flat after a few days parked, no obvious reason | A drain test to find the circuit |
| Short trips | Mostly short runs, rarely a long drive | An occasional longer drive or a smart charger |
| Computers left on standby | A modern car often parked without locking it by remote | Lock it with the remote |
Why short trips and parked cars go flat
Starting the engine takes a big gulp of charge, and a short drive doesn't put it all back. The MTA says frequent short trips shorten a battery's life, and suggests a longer drive about once a month. AA New Zealand adds that every battery slowly loses charge when the car isn't used.
Modern cars also keep their computers awake. The MTA notes that a car not locked with its remote can stay on standby and drain the battery over time. For a car that sits for weeks, a smart charger is the simple answer.
Hybrids, EVs and the small battery
An Aqua, Prius or Leaf can show a full main battery and still refuse to wake up. The culprit is the 12-volt battery, which the MTA says hybrids and EVs still carry for things like lights and door locks. It goes flat the same way, especially when the car sits. Follow the manual's jump-start steps exactly on these cars.
What will the mechanic do?
Load-test the battery, check the alternator's charging with the engine running, and if both pass, measure the current draw with the car locked to find what's draining it.
If it needs a new battery, ask for the type the car was built with. AA New Zealand says start-stop cars need an AGM battery, and another type impairs the fuel-saving system. A mobile mechanic can usually test and fit one at your place.
What you can do, and what to leave
You can
- Make sure lights, the boot and the doors are shut and off.
- Use a smart charger if the car sits for long spells.
- Keep the old battery out of the household rubbish: the MTA says that's illegal.
Leave to the mechanic
- Battery load tests and charging checks.
- Tracking down a drain that flattens the battery while parked.
- Choosing and fitting the right battery type, and registering it where the car needs it.
Where we'd start
We'd get going with a careful jump or a charge, then have the battery and charging tested before buying anything. If it's the second flat in a month, we'd go straight to a mechanic, because something is either worn out or draining it.