How to Jump Start a Car Safely

• Check the flat battery. If the case is bulging, cracked or wet, or you smell rotten eggs, stop. Do not connect anything.
• Check both cars are 12-volt systems, and that the leads are undamaged with clean clamps.
• Park the cars close but not touching, handbrakes on, in Park or neutral, ignitions off.
• Switch off lights, AC and accessories in both cars.
• Keep flames, cigarettes and sparks away. A charging battery gives off hydrogen, which is flammable.
• Take off rings and metal watch straps, and wear eye protection if you have it.

1. 1

   Red to the flat battery's positive (+)

   Clamp one end of the red lead to the positive terminal of the flat battery, or to the positive jump post if the car has one.

2. 2

   Red to the donor battery's positive (+)

   Clamp the other end of the red lead to the positive terminal of the working battery.

3. 3

   Black to the donor battery's negative (−)

   Clamp one end of the black lead to the negative terminal of the working battery.

4. 4

   Black to bare metal on the flat car

   Clamp the last end to an unpainted metal part of the engine or body of the car with the flat battery, well away from the battery, or to the earth point the manual specifies. This keeps the final spark away from battery gas.

5. 5

   Start the donor car, then the flat one

   Let the donor run for a few minutes, then try to start the flat car. If it does not start after a couple of short attempts, stop — more cranking will not help.

6. 6

   Remove the leads in reverse order

   Black from the bare metal, black from the donor, red from the donor, red from the car that was flat. Do not let the clamps touch each other or any metal while removing them.

• The battery is swollen, cracked, leaking or has heavily corroded terminals.
• There is a smell of rotten eggs (sulphur) around the battery.
• You cannot identify the positive and negative terminals with certainty.
• The car is a hybrid or electric vehicle and you have not read its manual. These still use a 12-volt battery, but the jump points and rules differ, and many manufacturers say not to use them as the donor car.
• The car has already been jumped recently and gone flat again. The battery or charging system needs testing, not another boost.

In any of these cases, a professional jump start is the safer route. We use the manufacturer's jump points, check the battery's condition first, and test the battery and alternator afterwards.

Keep the engine running and drive for at least 20 to 30 minutes so the alternator can put some charge back. Avoid switching the engine off during that drive. Then find out why it went flat: if you left the lights on, the battery may be fine; if it went flat on its own, have it tested before you rely on it.

Older cars tolerated a clumsy jump start. Newer ones carry dozens of control units, a battery sensor on the negative terminal, and often the battery itself in the boot or under a seat. A reversed clamp or a voltage spike can damage electronics that cost far more than a battery. Clamping the black lead directly to the negative post on a car with a battery sensor can also bypass the sensor, which is one reason manufacturers specify a separate earth point.