Swollen, leaking, or stuck on the roadside?
A battery that's swollen, cracked, leaking or smells of rotten eggs shouldn't be jump started or put on charge: leave it to a mechanic. If the car dies on a busy road, hazard lights on, get as far left as you can and clear of the traffic before you think about the battery. Call 111 if anyone is hurt or in danger.
What decides how long yours lasts?
AA New Zealand names the things that matter: the right battery, fitted correctly, the climate the car lives in, how regularly it's driven, and how well the battery and the car are looked after. Of those, keeping it fully charged makes the biggest difference.
That's why two identical batteries can have very different lives. One in a car that does a daily motorway run stays topped up. One in a car that only goes to the shops, or sits in the driveway for weeks, spends its life partly flat and wears out sooner.
What shortens a battery's life, and what helps
| Why it hurts | What helps | |
|---|---|---|
| Short trips | Starting takes more charge than a short run puts back | A longer drive about once a month, the MTA suggests |
| Long spells parked | Every battery slowly loses charge when the car isn't used | A smart maintenance charger |
| Cold mornings | A battery has less starting power in the cold | A test before winter |
| The wrong type | Start-stop cars need an AGM battery | Fitting the type the maker specifies |
| Being left flat | Sulphate builds up on the plates | Recharging fully as soon as you can |
| Electronics on standby | A modern car not locked by remote can stay awake | Locking it with the remote |
Which battery does your car have?
Most older petrol cars use a standard lead-acid battery. Cars with start-stop (the engine cuts out at the lights) need an AGM battery, and AA New Zealand says fitting a different type impairs the fuel-saving system. Some European cars also need a new battery registered with a scan tool before the charging system treats it properly.
Hybrids and EVs carry a small 12-volt battery as well as the big one. Toyota New Zealand says the hybrid's 12-volt battery supplies power for auxiliary functions, and AA New Zealand says an EV with a flat 12-volt battery may not start. An Aqua or a Leaf that has sat unused for a while is worth testing before you assume anything bigger.
How to tell if your battery is on the way out
Listen to the start
A slow, laboured crank, worst on cold mornings, is the classic early sign. AA New Zealand says a battery more than three years old that cranks slowly may be failing.
Watch the lights
Headlights or the dash dipping hard while the engine turns over.
Look at the terminals
White or green crust around them, which the MTA lists as a warning sign.
Find its age
Many batteries carry a date on the label or case. The MTA treats an unknown age as a warning sign in itself.
Have it tested
A load test shows how the battery copes under strain, which a simple voltage reading can't.
How often should you replace a car battery?
There's no fixed date. Replace it when a test says it's failing, not on a calendar. A battery that tests well in its fourth year can stay; one that tests weak in its second should go.
The MTA makes the point that waiting until you break down to replace a battery is avoidable. A test at each service from the third year on is the easy way to stay ahead of it.
What a good battery replacement includes
- A test of the old battery and the charging system first.
- The type and size the maker specifies, AGM for start-stop cars.
- Registering the new battery on cars that need it.
- Clean, tight terminals.
- The old battery taken away for recycling: the MTA says putting one in household rubbish is illegal.
What you can do, and what to leave
You can
- Take the car for a longer run now and then if it mostly does short trips.
- Use a smart charger if it'll be parked for weeks.
- Lock it with the remote so the electronics go to sleep.
- Note the battery's date, and mention slow starts at the next service.
Leave to the mechanic
- Load testing and charging checks.
- Fitting and registering a new battery.
- Finding a drain that flattens it while parked.
If it were our car
From the third year, we'd ask for a battery test at every service and again before winter. We'd replace it when the test says so, with the type the maker specifies, rather than on age alone or after it strands us.
Who should look at it?
Most garages and mobile mechanics test and fit batteries. If it's a start-stop car, a European car that needs the battery registered, or a hybrid, give the make and model in the note so the mechanic who rings arrives with the right battery and tools.