Flat car battery? Red cable first, then black to bare metal

A jump start from another car revives a flat battery in minutes, but done in the wrong order it can damage both cars’ electronics or hurt the person doing it. These are the steps published by the AA, the British motoring organisation. If your car’s handbook says something different, follow the handbook.

Before you touch the cables

• You need a pair of undamaged jump leads, rubber gloves and a helper car with a fully charged battery of the same voltage, usually 12 volts. Don’t use a hybrid or electric car as the helper; it could be damaged.
• Never try to jump a battery that looks damaged or is leaking. Batteries give off flammable gas, so no smoking and no naked flames nearby.
• Take off rings, watches and chains, and keep a ghutra, scarf or anything loose away from the engine. Any metal object touching the battery can cause a spark.
• If the leads get hot, stop.

Connecting, step by step

1. Park the cars so the leads reach both batteries without the cars touching. Handbrakes on, ignitions off.
2. Red lead: from the charged battery’s positive (+) terminal to the flat battery’s positive (+) terminal. Never let the red and black ends touch.
3. Black lead: from the charged battery’s negative (−) terminal to a point of unpainted metal on the engine block or chassis of your car, well away from the flat battery and the fuel system. Not onto the flat battery’s negative terminal.
4. Wait 3 minutes with both engines off, then start the helper car and let it run for 1 minute.
5. Start your car.
6. Leave both cars idling at a fast idle for about 10 minutes.

Disconnecting, in reverse

Switch both engines off first; pulling leads off a running car can damage its electronics. Then remove them in the reverse order: black from the bare metal on your car, black from the charged battery, red from the charged battery, and finally red from your battery. Keep the leads from touching each other or either car.

Then try starting your car. If it won’t start, the problem is probably bigger than the battery and needs a mechanic. If it does, drive for at least 30 minutes on open road, not in stop-start traffic, to recharge the battery.

With a booster pack

A charged portable booster pack does the job without a second car. Put it somewhere stable, not on the engine. Connect red to the positive terminal and black to unpainted metal, switch the pack on and start the car. Let the engine run for 5 minutes, switch the pack off and let the engine run another 5 to 10 minutes, then turn the engine off and disconnect black first, then red.

The AA puts the average battery life at 5 to 7 years. If yours is past 5 and struggling to start the car, have it tested before it strands you.

Photo: red and black jump lead clamps, an illustrative image. By Karl Gruber, CC BY-SA 4.0, via Wikimedia Commons.

Source: The AA, “How to jump start a car”, theaa.com, read 29 October 2026.