A flat battery is one of the most common reasons a car will not start. Jump-starting is straightforward, but the order of the leads matters — get it wrong and you can damage expensive electronics.
Before you start
Check the battery is not cracked, leaking or frozen. If it is damaged, do not jump it — call for assistance. Make sure both cars are switched off, in park or neutral, with handbrakes on, and that they are close enough for the leads to reach without the cars touching.
Important: many hybrids and electric cars must not be used to jump-start another vehicle, and some have specific jump points rather than a normal battery. Check the manual before using an EV or hybrid as the donor.
The correct order
1. Red clamp to the positive (+) terminal of the flat battery.
2. Red clamp at the other end to the positive (+) terminal of the good battery.
3. Black clamp to the negative (−) terminal of the good battery.
4. Final black clamp to a bare, unpainted metal point on the engine or chassis of the flat car — not to its negative terminal. This keeps any spark away from the battery, which can give off flammable gas.
Starting up
Start the working car and let it run for a few minutes. Then try the flat car. If it does not fire after about 10 seconds of cranking, stop, wait a minute and try again. Repeated long cranking overheats the starter motor.
Once it starts, remove the leads in the reverse order, keeping the clamps from touching each other or any metal.
Afterwards
Drive for at least 20 to 30 minutes, ideally with minimal use of headlights, aircon and stereo, so the alternator can put charge back in. Idling in the driveway does very little.
If it keeps happening
A battery that goes flat repeatedly is telling you something: it is at the end of its life, the alternator is not charging properly, or something is drawing power while the car is parked. Most auto shops will test a battery for free. Do not keep jump-starting it and hoping.
A portable lithium jump pack is a good investment — it fits in the glovebox and means you do not need a second car at all.