How to jump-start a car safely
By James Snow
Owner and mechanic, SV Autos
Updated 25 July 2026
A flat battery is one of the most common reasons a car won't start, and a jump from another car will usually get you going again in a few minutes. It's a simple job, but the order you connect the leads matters, and getting it wrong can give you a nasty spark or damage the electrics. Here's how to do it safely, and how to tell when a jump won't be enough.
When a jump-start is the right fix
A jump-start is for a flat battery, nothing more. If you left an interior light or the headlights on overnight, or the car has sat unused for weeks in cold weather, the battery can drain flat and a jump will bring it straight back to life.
The tell-tale signs of a flat battery are the dashboard lights being dim or dead, the engine turning over slowly or just clicking, and everything electrical being sluggish. If the car cranks over strongly but still won't fire, the battery probably isn't your problem, and a jump won't help.
Safety first, before you touch anything
Park the good car close enough for the leads to reach, but not touching the flat one. Switch both engines off, put both handbrakes on, and take the keys out of the ignition. Make sure both cars are in neutral, or park if they're automatic.
Keep anything loose well clear of the engine and the battery terminals: dangling scarves, ties, jewellery, long hair, and any metal tools. A battery can put out a big spark, so no smoking and no naked flames nearby.
- Both engines off, keys out, handbrakes on
- Nothing loose or metal near the terminals
- Never jump a battery that's cracked, leaking or visibly swollen
- Check both batteries are the same voltage (nearly all cars are 12 volt)
The correct order to connect the leads
This is the part people get wrong. Connect the leads in this exact order, and it's worth reading it through once before you start. The red lead is positive (marked +) and the black lead is negative (marked -).
For that last black clip, use a solid piece of unpainted metal on the flat car's engine, a bolt or a bracket, well away from the battery. This is called an earth point. Doing it this way keeps any spark away from the battery itself, which is where the flammable gas sits.
- Red clip onto the positive (+) terminal of the flat battery
- Other red clip onto the positive (+) terminal of the good battery
- Black clip onto the negative (-) terminal of the good battery
- Other black clip onto a solid unpainted metal earth point on the flat car, not its negative terminal
- Start the good car and let it run for a minute or two
- Start the flat car, then let both run for a few minutes
Removing the leads, and what not to do
Once the flat car is running, take the leads off in the reverse order: the earth point first, then the black clip on the good battery, then the red clip on the good battery, then the red clip on the car that was flat. Don't let the clips touch each other or any bare metal while any of them are still connected.
Never connect the two black clips to both battery negatives, never let the red and black clips touch while attached, and don't rev the engine hard to force it. If the leads get hot or you smell anything burning, stop and disconnect. Cheap, thin leads can struggle, so decent jump leads make the job safer and quicker.
After the jump, and when it won't hold
Once you're running, drive for a good twenty to thirty minutes, or leave the engine running that long, to put some charge back into the battery. Switch off unnecessary electrics like the heated rear window and the stereo while it charges. If the car starts fine next time, a one-off flatten is all it was.
If the car goes flat again within a day or two, jumps but dies as soon as you stop, or the battery is more than a few years old, the battery is likely on its way out or something is draining it. A jump is a get-you-going fix, not a repair, so get the battery and charging system tested rather than relying on jumps.