How to jump-start a car safely

A jump-start borrows power from a good battery to start a car with a flat one. It takes 10 to 15 minutes. Done in the right order it is safe; done in the wrong order it can cause sparks or damage the electrics.

Before you start: when not to jump

• The battery is cracked, leaking, bulging or smells of rotten eggs. Do not jump it. It needs replacing.
• The voltages do not match. Almost all cars are 12 volts. Do not connect a car to a 24-volt truck system.
• You are in traffic or visibility is poor (darkness, fog). Get to a safe spot first or call a breakdown service.
• The donor is a hybrid or electric car, unless its manufacturer specifically allows it. Their electronics can be damaged.
• Your owner's manual says otherwise. On some cars the battery sits in the trunk or under a seat and the manual tells you to use separate jump-start points under the hood (bonnet) instead. Always follow the manual.

What you need

• Jumper cables (jump leads) in good condition. AAA recommends heavy-duty cables, 6-gauge and 16 feet (about 5 m) long.
• A donor car with a healthy battery of similar or larger size, or a charged portable jump starter.
• Safety glasses and gloves if you have them.

Step by step

1. Park the cars close but not touching. Both in Park or Neutral, parking brakes on, ignitions off. Switch off lights, radio and other electrics.
2. Remove rings, watches and other metal jewelry. Do not smoke, and do not lean over either battery.
3. Find the terminals. Positive is marked + (often red cover), negative is marked -. Clean off heavy corrosion if you can.
4. Red clamp to the flat battery's positive (+) terminal.
5. Other red clamp to the good battery's positive (+) terminal.
6. Black clamp to the good battery's negative (-) terminal.
7. Last black clamp to bare, unpainted metal on the dead car, such as a solid bolt on the engine block, well away from the battery. Not on the dead battery itself.
8. Check the cables are clear of fans and belts.
9. Start the donor car and let it run for 3 to 5 minutes (AAA) to put some charge into the flat battery.
10. Start the dead car. Crank for no more than about 5 seconds at a time. If it does not start, wait a few minutes and try again. After three failed tries, stop: the battery has probably failed.
11. Once it starts, leave both cars running for a few minutes.
12. Remove the cables in reverse order: black from the earth point on the jumped car, black from the good battery, red from the good battery, red from the jumped battery. Keep clamps from touching each other.
13. Drive for 20 to 30 minutes with the air conditioning and rear demister off, so the alternator can recharge the battery.

Using a portable jump starter

Charge the pack fully first. Most packs follow the same idea: red clamp to positive (+), black clamp to bare metal or negative as the pack's instructions say, switch the pack on, then start the car. Read the pack's manual, as some have their own order and safety checks. Our jump starter roundup compares popular models.

After the jump

A jump-start fixes the symptom, not the cause. If the car was flat because you left the lights on, a good drive or a charger will sort it. If it happens again, or the battery is more than three years old, have the battery and charging system tested. See how long car batteries last and, if the battery warning light comes on while you drive, read our battery warning light guide.