Measurements in
8 steps · 2 traps to avoid · two 12-volt cars, a pair of jumper cables, the bay door open
How to jump-start a car: interactive 3D repair 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, in a garage bay with its door wide open. 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, safety glasses and gloves. Both cars must have 12-volt batteries. Check the flat battery first: do not jump it if it is cracked, bulging, leaking or frozen. Open the bay door wide before either engine runs, and keep flames, sparks and cigarettes away from both batteries.
Tools: Jumper cables
- Both engines will run for several minutes, and their exhaust carries carbon monoxide: work only with the bay door wide open. In a garage that is part of a house, never run an engine even with the door open: move the flat car outside and jump it there.
- 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.
Before you open it: the risk map
Under the closed bonnets: no sparks by the flat battery, whose hydrogen can ignite, hands and leads clear of each car’s fan (it can start by itself) and belt, the booster’s exhaust hot, and the four clamp points in order, the last on bare metal away from the battery.
Where not to spark or reach
- No sparks Hydrogen from the flat battery
- Moving parts Flat car’s fan: self-starts
- Moving parts Flat car’s drive belt
- Moving parts Booster’s fan: self-starts
- Moving parts Booster’s drive belt
- Hot Booster’s exhaust manifold
- Never touch Red and black clamps together
Where the clamps go, in order
- 1 · Flat battery + · red, first
- 2 · Good battery + · red
- 3 · Good battery − · black
- 4 · Earth point · black, last
Press Risk map (K) on the player to see them in 3D, through the closed car.
All 8 steps
- 1Park the cars and switch everything off
Bring the booster car close enough for the cables to reach, nose to nose or side by side, without the two cars touching, and leave the bay door wide open. Put both in park or neutral with the parking brake firmly on, turn both ignitions and all lights 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 trap
Connect the other red clamp to the positive (+) post of the booster car’s battery. Avoid this trap: 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 trap
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 and the gas it gives off. Avoid this trap: 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 (an electric fan can start by itself), start the booster car’s engine, and let it run for three to five minutes to charge the flat battery a little. The bay door stays wide open while it runs.
- 7Start the flat car
Now start the car with the flat battery. Once it runs, leave both cars running, still connected, for three to five minutes more. 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. Close both bonnets, switch the booster off, and drive the started car out of the bay 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
- Fresh air · the bay door wide open; outdoors if the garage is part of a house
- 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.
Can I jump-start a car inside a garage?
Only with the garage door wide open, and only in a garage that stands apart from the house: both engines run for several minutes, and their exhaust carries carbon monoxide. The CDC advises never running a car in a garage attached to a house, even with the door open, so move the flat car outside first. Keep the door open for the whole job: the battery’s hydrogen rises and gathers under the roof.
How long should the booster car run first?
AAA says three to five minutes before you try to start the flat car, five more if it does not start the first time, and, once it starts, three to five more with both cars running and still connected.
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, more while they take a charge, which a spark or a flame can explode, and contain sulphuric acid: wear safety glasses, keep sparks, flames, cigarettes and phones away, and never lean over a battery while connecting cables. Run the engines only with the bay door wide open, and never in a garage attached to a house: exhaust carries carbon monoxide. Do not jump a battery that is cracked, leaking or frozen, keep hands and cables clear of the fans and belts, and never let the clamps touch each other. If in doubt, call a breakdown service.
- Jacks and axle stands: never get under a car, or put a hand or a foot under it, while a jack alone holds it up. Lift it only at its jacking point, on firm, level ground with the wheels chocked, and set it on axle stands rated for its weight for any work under it.
- Car batteries: a lead-acid battery gives off explosive hydrogen. No sparks, flames or smoking near it, eye protection on, and the last jump lead clamped to bare metal away from the battery.
- Hot parts and fumes: brakes, exhausts and engines stay hot long after a drive, and a running engine fills a closed garage with carbon monoxide: let parts cool, and keep the door open.
About this guide
3D Spatial Reconstruction: models are engineered from factual measurements to show the parts, the fixings and the order of the job before you start. Always check the handbook of your own car.
This guide shows two small cars with ordinary 12-volt batteries, nose to nose in a workshop bay with its door wide open. 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; the three to five minutes of charging before the start, and the three to five minutes with both cars running after it, are AAA’s. Some cars have jump-start points of their own under the bonnet: if yours does, its handbook comes first.
Why the door stays open: the booster’s engine runs for several minutes, then the started car’s too, and exhaust carries carbon monoxide. The CDC’s advice is to open the door of a detached garage whenever a car runs inside, and never to run one in a garage attached to a house, even with the door open: there, move the flat car outside first. A battery taking a charge also gives off hydrogen, faster as it nears full charge, and the gas rises and gathers under a roof, as the UK’s Health and Safety Executive explains: no flames, sparks, smoking or phones near the batteries, and the last clamp on bare metal away from them.
Under each bonnet the risk map marks the radiator’s electric fan, which can start by itself even with the car switched off (Kia’s handbooks warn of it), the drive belt, which runs whenever the engine does, and the exhaust manifold behind its heat shield, hot once the engine has run. Where they sit differs from car to car: ours is a generic layout.
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, Kia owner’s manual: the cooling fan, CDC: Carbon monoxide poisoning basics, HSE: Using electric storage batteries safely (INDG139).
The 3D models, drawings and text are our own. The car shown is a generic one of our own design, not any one maker’s model.