How to Jump Start a Car Safely

Jump starting is simple, and it is unforgiving of mistakes. Done in the right order it takes five minutes. Done in the wrong order it can blow fuses, damage control units or injure the person holding the leads.

What follows is the general method used for most cars. Your owner's handbook has the final word, because some models have their own jump points and a few should not be used as a donor at all. If anything looks wrong with the battery, or you are not sure, leave it and call for help.

Before you connect anything

A minute spent checking saves most of the trouble:

• Switch both cars off, put them in Park or neutral and apply the handbrakes
• Look at the flat battery. If the case is cracked, leaking or swollen, stop there
• Keep flames and cigarettes away, and keep rings, watches and tools clear of the terminals
• Find the jump points. Cars with the battery in the boot usually have marked posts under the bonnet
• Make sure the clamps on the leads never touch each other once one end is connected

Connecting jump leads, step by step

Positive first, and the last connection away from the battery.

1. 1Park the donor car close enough for the leads to reach, without the two cars touching, and switch both off.
2. 2Clamp one red lead to the positive (+) terminal of the flat battery, or to the car's positive jump point.
3. 3Clamp the other end of the red lead to the positive (+) terminal of the donor battery.
4. 4Clamp one black lead to the negative (−) terminal of the donor battery.
5. 5Clamp the last black end to unpainted metal on the stalled car's engine or body, or to its marked earth point, away from the battery.
6. 6Start the donor car and let it run for a few minutes.
7. 7Start the stalled car. If it does not catch, wait a few more minutes and try once more.
8. 8Remove the leads in reverse order: black from the earth point, black from the donor, red from the donor, red from the car that was flat.
9. 9Keep the revived car running and drive it for at least half an hour.

Using a booster pack instead

A portable booster pack does the same job without a second car, which makes it the practical choice in a basement bay. With the pack switched off, connect red to the positive terminal and black to the earth point or negative terminal, as the pack's own instructions say. Switch it on, start the car, switch the pack off and remove the clamps.

Packs differ, so read the instructions that came with yours and keep it charged. A booster pack that has sat in a hot boot for a year may be as flat as the battery it is meant to rescue.

Mistakes that cause damage

Nearly all jump start damage comes from one of these:

• Connecting positive to negative. Check the + and − marks twice before each clamp goes on
• Letting the two clamps at one end touch while the other end is connected
• Making the last connection on the flat battery itself, where a spark is closest to any gas
• Using a hybrid or electric car as the donor when its handbook says not to
• Cranking for long stretches. Try for a few seconds, then rest the starter
• Pulling a battery cable off a running engine to test the charging

When not to jump start at all

Leave the leads in the boot and call for help if:

• The battery case is cracked, leaking or bulging, or there is a smell of rotten eggs
• The terminals are so corroded that a clamp cannot get a clean grip
• The car is a hybrid or electric model and you cannot find the maker's jump points
• You are stopped in a live lane or anywhere it is unsafe to stand beside the car

After it starts

A jump start that works tells you the battery was short of charge. It does not tell you why. If you left the lights on, a long drive may be all it needs. If there was no obvious reason, expect it to happen again.

Have the battery and the alternator tested within a day or two. In this climate a battery that has gone flat once without a clear cause is usually near the end of its life.