Step 5 of 8
Jump-start a car step 5 of 8: Last black clamp on bare metal
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.
Tools: none, just your hands
Common mistake: the last clamp on the flat battery’s negative post. 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. The switch in the player shows what goes wrong.
All 8 steps
- 1Park the cars and switch everything off
- 2Red clamp on the flat battery’s positive
- 3Red clamp on the good battery’s positive common mistake
- 4Black clamp on the good battery’s negative
- 5Last black clamp on bare metal common mistake
- 6Start the booster car
- 7Start the flat car
- 8Take the cables off in reverse order
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.