How to jump-start a car safely, and when not to try

A jump-start lends a discharged 12-volt battery enough power to start the engine. Done in the right order it is routine; done wrongly it can damage control units or injure you. Always follow the procedure in your owner's manual, and if you are unsure, the battery is damaged, or the car is a hybrid, electric or high-end model, call a professional instead.

Do not attempt a jump-start if

• The battery case is cracked, swollen or leaking, or you smell rotten eggs
• The terminals are badly corroded or the cables are damaged
• You cannot identify the positive and negative terminals with certainty
• The owner's manual prohibits it, or names special jump points you cannot find
• The car is a hybrid or electric vehicle and you do not know its 12-volt procedure
• The car is in a live traffic lane or another unsafe position

The general sequence with jump leads

This is the commonly published sequence for a 12-volt car. Your owner's manual takes priority wherever it differs.

1. Park the donor car close without the vehicles touching. Switch both off, apply the parking brakes and turn off lights, air conditioning and audio.
2. Connect a red clamp to the positive (+) terminal of the flat battery.
3. Connect the other red clamp to the positive (+) terminal of the donor battery.
4. Connect a black clamp to the negative (−) terminal of the donor battery.
5. Connect the last black clamp to the earth point named in the manual, or to solid unpainted metal on the engine or body of the car with the flat battery, away from the battery itself.
6. Start the donor car and let it idle for a few minutes. Do not rev it.
7. Start the car with the flat battery. If it does not start within a few seconds, stop, wait and try once or twice more at most.
8. Once it runs, remove the leads in exactly the reverse order, keeping the clamps apart.

Why modern cars need more care

Current cars carry dozens of electronic control units and often a battery sensor on the negative terminal. Reversed leads, clamps touching, or a voltage spike when leads are pulled off can damage them, and the repair costs far more than a battery. Many manufacturers provide dedicated jump posts under the bonnet for this reason.

A portable jump pack with reverse-polarity protection, used by someone who knows the car's procedure, is generally the gentler method. That is what a mobile technician brings.

After the engine starts

A jump-start does not repair anything. A short drive may not restore the charge, and a battery that went flat without an obvious reason, such as lights left on, is likely to do it again. Have the battery and charging system tested before you rely on the car, particularly if it has needed more than one jump.

Last reviewed: 5 October 2026. General guidance only. Follow your owner's manual and ask a technician about your specific car.