Flat car battery

Signs your battery is flat or failing

• The engine turns over slowly, or you hear clicking when you try to start.
• Dashboard and interior lights are dim, or flicker when you turn the key.
• Central locking or the key fob works slowly or not at all.
• The battery warning light comes on while driving (this points to the charging system).
• Electrical items such as the clock or radio settings have reset.

Why car batteries go flat

• Something left on: lights, an interior light or a phone charger in some cars.
• Short journeys: starting uses a lot of charge, and short trips don’t give the alternator time to put it back.
• Cold weather: a battery gives less power in the cold, just when the engine needs more.
• Standing unused: modern cars draw a little power even when parked, so a car left for a few weeks can go flat.
• Age: batteries wear out. Many last around four to six years, less with lots of short trips.
• A charging fault: if the alternator isn’t charging, the battery will keep going flat even when it’s new.

How to jump start a car safely

Check your handbook first: some cars, especially hybrids and electric cars, have specific jump-start points or shouldn’t be used to jump other cars.

1. Park the working car close by without the cars touching. Turn both engines and all electrics off.
2. Connect one red lead to the + terminal of the flat battery.
3. Connect the other red lead to the + terminal of the good battery.
4. Connect one black lead to the − terminal of the good battery.
5. Connect the other black lead to bare, unpainted metal on the engine or body of the car with the flat battery, away from the battery itself.
6. Start the working car and let it run for a few minutes, then try to start the car with the flat battery.
7. Once it starts, remove the leads in reverse order, without letting the clamps touch.
8. Drive for at least 20 to 30 minutes, ideally not in stop-start traffic, to put some charge back.

Don’t jump start a battery that is cracked, swollen, leaking or frozen, and keep sparks and flames away from the battery.

Recharge or replace?

If the battery went flat because something was left on, a jump start and a good drive, or a few hours on a battery charger, is often enough. If it keeps going flat, struggles on cold mornings or is more than four or five years old, it probably needs replacing.

Cars with stop-start need the right type of battery (usually AGM or EFB), and some need the new battery registering to the car’s computer. A garage or mobile mechanic can test the battery and the charging system and fit the right one.

When it isn’t the battery

If a new or freshly charged battery goes flat again, or the battery warning light comes on while driving, the alternator or a fault that drains power while the car is parked may be to blame. That needs testing rather than another battery. If the car won’t start even on a jump, see car won’t start for the other causes.

Frequently asked questions

Can a mobile mechanic replace my battery at home?

Yes. Many mobile mechanics test and fit batteries at your home, workplace or the roadside.

How long do car batteries last?

Many last around four to six years. Lots of short journeys, cold winters and long periods parked all shorten that.

How long should I drive after a jump start?

At least 20 to 30 minutes, at steady speeds if you can. Even then a deeply flattened battery may not fully recover, so get it tested.

Why does my battery keep going flat?

Usually short trips, an old battery, a charging fault, or something drawing power while the car is parked. A battery and charging test will show which.