Do it yourself guide
How to Jump Start a Car Safely, Step by Step
About 6 minutes to read
A jump start is simple electrically and easy to get wrong physically. The battery you are connecting to can deliver several hundred amps into a spanner or a badly placed clamp, and the car around it is full of control modules that do not enjoy voltage spikes. Done in the right order, with the right lead sizes, it is a two minute job. Done in the wrong order, it can cost an alternator, a fuse box or a bodywork burn.
This guide gives the lead order in the sequence a technician actually uses, explains why the final connection goes to bare engine metal rather than the flat battery's negative post, and covers the extra care needed on cars with stop start, absorbent glass mat batteries or a 12 volt lithium pack. If you have no leads, no second vehicle or no confidence, skip to the bottom and ring us instead.
The correct jump start sequence
1Park and isolate both vehicles
Bring the donor car close enough for the leads to reach without stretching, but do not let the two bodies touch. Handbrake on, both in neutral or park, both ignitions off, lights and heated screens off on the flat car so the first surge goes into cranking rather than accessories.
2Check the flat battery before you connect anything
Look for a cracked case, a bulged side, a strong sulphur smell or ice inside a translucent case. Any of those means stop. A frozen or physically damaged battery can rupture under charge, and no jump start is worth that.
3Red lead, flat battery positive first
Clamp the red lead to the positive post of the flat battery, then to the positive post of the donor. Doing the dead side first means that if the free clamp touches metal while you work, the lead is not yet live from the good battery.
4Black lead, donor negative next
Clamp the black lead to the donor battery's negative post. Keep the last clamp clear of everything and do not let it swing near the positive posts.
5Final black clamp to bare engine metal, not the battery
Attach the last black clamp to an unpainted engine lifting eye, a bracket or a manufacturer marked earth point on the flat car. Any spark then happens away from the battery, where hydrogen collects. On many modern cars the negative post carries a battery monitoring sensor, and clamping onto it hides the jump from the charging system, which then undercharges afterwards.
6Run the donor, then crank
Start the donor and hold about 2000 rpm for two to three minutes to put surface charge into the flat battery. Then try the flat car. Crank in bursts of five to ten seconds with a rest between so the starter does not overheat.
7Disconnect in exact reverse order
With the recovered engine running, remove the earth clamp from the engine, then donor negative, then donor positive, then the flat battery positive. Keep clamps apart as you go.
8Drive, do not idle
Idling puts very little back. Drive for twenty five to forty minutes, ideally with some steady road speed and the heavy consumers off, or the car will very likely be dead again next morning.
What happens when you get the order wrong
The failure modes below are the ones we are called out to after a home attempt has gone wrong.
| What you see or hear | What it means | What to do |
|---|---|---|
| Clamps reversed, red to negative | Reverse polarity across the whole electrical system | Do not crank. Disconnect and have the main fuses, alternator diodes and modules checked |
| Last clamp put on the flat battery negative post | Spark next to venting hydrogen, and the battery sensor is bypassed | Move the earth to bare engine metal and expect the charging system to relearn |
| Thin domestic style leads | Voltage drop, hot insulation, not enough current to turn a diesel | Use leads rated well above the engine's cranking need, or a proper jump pack |
| Cranking for thirty seconds at a time | Starter motor and solenoid overheating | Five to ten second bursts with a pause between attempts |
Things not to do
- Do not jump a battery that is bulged, cracked, leaking or frozen.
- Do not lean over either battery while cranking.
- Do not connect to a hybrid or electric car's traction system. Only the 12 volt auxiliary battery is ever involved, and on many models the jump point is a dedicated terminal under a cover, not the battery itself.
- Do not leave the ignition on with the leads connected and both engines off, which just drains the donor.
- Do not assume a successful start means a healthy battery. A battery that needed help once will usually need it again.
No leads, no second car, or it still will not turn over
That is exactly the call we take all day and all night. We arrive with professional boosters that will turn a large diesel, test the battery and charging system on the spot, and can supply and fit a replacement battery there and then if that is what it needs.
Would rather someone came out and did it?
24 hour mobile jumpstart across the West Midlands, with battery testing and replacement on the spot.
Questions
Q.Which lead comes off first when you finish?
Reverse the order you fitted them. Earth clamp off the engine first, then the donor negative, then donor positive, then the flat battery positive, keeping the clamps apart as you work.
Q.Can I jump start from a smaller car?
Sometimes, but a small petrol engine will struggle to feed a large diesel. Give it longer at raised revs before cranking, and stop if the leads get warm. A dedicated jump pack is a far better match.
Q.Why does the car die again the next morning?
A jump start only puts surface charge in. If the battery is at the end of its life, or something is drawing current with the car locked, it will be flat again by morning regardless of how well the jump went.
In short
Red to flat positive, red to donor positive, black to donor negative, black to bare engine metal on the flat car. Run the donor, crank in short bursts, disconnect in reverse, then drive properly. If anything looks damaged or nothing turns over, stop and call us on 07886 626640.