How to check and replace a car battery

Safety: only do jobs you are confident with. Never work under a car supported only by a jack, and if a repair affects brakes, steering or suspension and you are unsure, have a garage do it or check your work.

A flat battery is the most common reason for a breakdown call-out, especially in the first cold snap of winter. Most batteries last four to six years. A battery doesn’t affect the MOT directly, but one that is insecurely mounted, or a dashboard warning light caused by low voltage, can.

Signs your battery is failing

• The engine turns over slowly, especially on cold mornings
• Dashboard lights flicker or dim when starting
• Stop-start stops working (the car protects a weak battery by disabling it)
• A battery warning light while driving, which can also mean the alternator isn’t charging
• White or green crusty deposits on the terminals

Testing the battery

A cheap multimeter (about £10) tells you a lot. With the car off for a few hours, set it to DC volts and touch the red probe to the positive (+) terminal and the black to negative (−):

• 12.6V or more: fully charged
• About 12.4V: around 75% charged
• 12.2V or less: discharged; charge it and retest
• Below 12V after a full charge: the battery is likely failing

With the engine running, you should see roughly 13.8–14.7V. Much less means the alternator may not be charging properly. Many motor factors and tyre centres will test a battery free.

Jump starting safely

1. Park the cars close but not touching, both engines off.
2. Connect the red lead to the flat battery’s positive (+), then to the good battery’s positive.
3. Connect the black lead to the good battery’s negative (−), then to bare metal on the dead car’s engine (an earth point or unpainted bracket), away from the battery.
4. Start the good car and let it run for a few minutes, then start the dead one.
5. Remove the leads in reverse order: black from the dead car first.

Check your handbook first. Many modern cars have dedicated jump-start points under the bonnet even if the battery is in the boot or under a seat.

Replacing the battery

1. Get the right battery. Match the size, terminal layout and type. Cars with stop-start need an EFB or AGM battery; fitting a standard one will cause problems. Parts shops look it up by registration.
2. Make sure you have the radio code if your car needs one, and note that some cars lose window and clock settings.
3. With everything switched off, disconnect the negative (−) terminal first, then the positive (+). Disconnecting negative first means a spanner touching the bodywork can’t cause a short.
4. Undo the clamp or bracket holding the battery down, and lift it out. Batteries are heavy (15–25kg), so lift with your legs.
5. Clean the terminals and tray, fit the new battery and its clamp securely.
6. Connect the positive (+) first, then the negative (−).

Some cars, mostly BMW, Mercedes, Audi and other cars with stop-start, need the new battery “registered” with a diagnostic tool so the charging system knows it is new. Without it, the new battery can be overcharged and fail early. If your handbook mentions this, a garage or auto-electrician can do it in minutes.

Safety

• Wear eye protection. Batteries contain acid and can give off explosive hydrogen gas.
• No smoking or naked flames near the battery.
• Take the old battery back to the shop or a recycling centre. It is illegal to put it in household waste, and most shops take it free.