8 steps · 2 common mistakes · two 12-volt cars, a pair of jumper cables
How to jump-start a car: interactive 3D guide
A car that will not start because its battery is flat can be started from another car’s battery with a pair of jumper cables. Every step, in the order that keeps the spark away from the battery, in 3D. Press play to watch a step, drag to look around, or switch to Drawing for a manual’s black lines.
What you need
A set of jumper cables, a car with a good 12-volt battery, and safety glasses. Both cars must have 12-volt batteries. Check the flat battery first: do not jump it if it is cracked, leaking or frozen.
- Read the jump-starting page in both cars’ handbooks: some cars have their own jump points under the bonnet.
- Take off rings and watches: a metal ring across a battery’s terminal can burn badly.
All 8 steps
- 1Park the cars and switch everything off
Park the booster car close enough for the cables to reach, nose to nose or side by side, without the two cars touching. Put both in park or neutral with the parking brake firmly on, turn both ignitions off and open both bonnets.
- 2Red clamp on the flat battery’s positive
Take one red clamp and connect it firmly to the positive (+) post of the flat battery: the post marked + or with a red cover.
- 3Red clamp on the good battery’s positive common mistake
Connect the other red clamp to the positive (+) post of the booster car’s battery. Common mistake: Red goes only to positive (+) posts and black to negative (−) and the ground point. Clamped crossed, the last clamp joins the two batteries positive to negative: a short circuit. It sparks hard, the cables heat up, and it can damage both cars’ electrics. Check the + and − on each battery before every clamp goes on.
- 4Black clamp on the good battery’s negative
Connect one black clamp to the negative (−) post of the booster car’s battery.
- 5Last black clamp on bare metal common mistake
Connect the last black clamp to a clean, unpainted, solid metal point on the flat car’s engine, away from its battery: the engine’s lifting bracket is a good one. The small spark as it goes on stays away from the battery. Common mistake: The last clamp to go on makes a small spark. A battery gives off hydrogen gas, and a spark next to it can make it explode, which is why the last black clamp goes on bare metal on the engine, well away from the flat battery. Kia’s handbook also warns against joining the booster’s negative straight to the flat battery’s negative.
- 6Start the booster car
Check that no cable lies near a fan or a belt, start the booster car’s engine, and let it run for three to five minutes to charge the flat battery a little.
- 7Start the flat car
Now start the car with the flat battery. If it does not start, let the booster run five minutes more and try once more; if it still will not start, the battery probably needs replacing.
- 8Take the cables off in reverse order
With both engines running, take the clamps off in the reverse order: the black one from the ground point first, then the black from the booster, the red from the booster, and last the red from the car you started. Keep the clamps apart as they come off. Then drive for 20 to 30 minutes so the alternator can charge the battery.
What you need
- Jumper cables · heavy-gauge, long enough to reach
- A booster car · with a good 12-volt battery
- Safety glasses · and gloves
- Both cars’ handbooks · for any jump points of their own
Questions
What order do jumper cables go on?
Red on the flat battery’s positive, red on the good battery’s positive, black on the good battery’s negative, and last, black on bare unpainted metal on the flat car’s engine, away from its battery. They come off in the reverse order.
Why not put the last black clamp on the flat battery’s negative?
The last connection sparks, and a battery gives off hydrogen gas that a spark can set off. Bare metal away from the battery, such as the engine’s lifting bracket, keeps the spark away from it.
How long should the booster car run first?
AAA says three to five minutes before you try to start the flat car, and five more minutes if it does not start the first time.
How long should I drive after a jump-start?
AAA suggests a continuous drive of 20 to 30 minutes so the alternator can recharge the battery. If it goes flat again, have the battery and the charging system tested.
What happens if the cables are crossed?
Red on a negative post joins the batteries the wrong way round: a short circuit that sparks hard, heats the cables and can damage the cars’ electrics. Check the + and − on each battery before every clamp goes on.
Safety
Car batteries give off hydrogen, which can explode, and contain sulphuric acid: wear safety glasses, keep sparks and flames away, and never lean over a battery while connecting cables. Do not jump a battery that is cracked, leaking or frozen, and never let the clamps touch each other. If in doubt, call a breakdown service.
About this guide
This guide shows two small cars with ordinary 12-volt batteries, parked nose to nose. The order of the clamps, the bare-metal ground point and the reverse order for taking them off follow AAA’s guide and Kia’s owner’s manual, which agree on them. Some cars have jump-start points of their own under the bonnet: if yours does, its handbook comes first.
The voltages and currents on the readings are illustrative: a flat battery reads well under 12 volts, a charged one about 12.6 at rest, and a running engine’s alternator charges at about 14.
Sources: AAA: How to jump a battery, Kia owner’s manual: Jump starting.
The 3D models, drawings and text are our own.