To jump-start a car, connect a working battery to the flat one with jumper cables in a strict order: positive to positive, then negative on the donor battery, and the last clamp on bare metal of the stalled car. Done in this order, it takes about ten minutes and avoids sparks near the battery.
This guide explains what you need, the exact order of the steps, the mistakes that damage cars, and what to do when the engine still won’t start.
What you need
- A set of jumper cables, with thick, undamaged insulation and clean clamps. Thicker cables carry current better.
- A second car with a healthy 12-volt battery, or a portable jump starter.
- Gloves and eye protection, if you have them.
- Your owner’s manual. Some cars have special jump-start terminals under the bonnet, away from the battery.
Before you start: safety checks
- Look at the dead battery. If the case is cracked, swollen, leaking, or frozen, do not jump-start it. Call roadside assistance instead.
- Switch everything off in both cars: engines, lights, radio, air conditioning. Take the keys out of the ignition on the stalled car.
- Park the cars close, but not touching. Apply the handbrake on both. Keep the cables away from moving parts such as fans and belts.
- No flames or smoking. Batteries can release flammable gas.
- Check the voltage. Both cars must have 12-volt batteries. This is the case for nearly all passenger cars.
The 6 steps, in the correct order
- Red clamp 1: dead battery, positive (+). Positive terminals are marked with a plus sign and often have a red cover.
- Red clamp 2: good battery, positive (+).
- Black clamp 1: good battery, negative (−).
- Black clamp 2: bare metal on the stalled car. Use an unpainted metal part of the engine or the chassis, away from the battery. Do not clamp it to the dead battery’s negative terminal. This last connection can spark, and keeping it away from the battery reduces the risk.
- Start the working car and let it run for a few minutes.
- Start the stalled car. If it doesn’t start after a few seconds, wait a minute and try again. Do not crank for long periods.
Once the engine runs, remove the cables in the reverse order: black from the bare metal, black from the good battery, red from the good battery, red from the dead battery. Never let the clamps touch each other or any metal while they are connected.
After the engine starts
Drive for at least 20 to 30 minutes, ideally at a steady speed, so the alternator can recharge the battery. Switching the engine off right away often leaves you stranded again. If you turn off the engine before the battery has recharged, it may not start next time.
Common mistakes
| Mistake | Why it is a problem |
|---|---|
| Connecting the clamps in the wrong order | Can cause sparks, and can damage the electronics. |
| Crossing positive and negative | Can short-circuit both cars and blow fuses. |
| Leaning over the battery | Batteries can vent gas or, rarely, burst. |
| Using damaged or very thin cables | They overheat and may not deliver enough current. |
| Jump-starting a frozen or leaking battery | It can burst. The battery needs replacing. |
Hybrid, electric and start-stop cars
Hybrid and electric cars have a small 12-volt battery that runs the electronics, separate from the large traction battery. Many manufacturers specify exact jump points, and some do not allow these cars to be used as the donor. The same applies to cars with start-stop systems and AGM batteries. Check the owner’s manual first, and call roadside assistance if you cannot confirm the procedure.
What if it still won’t start?
- Clicking, then nothing: the battery may be too weak or the terminals corroded. Clean the terminals and check the clamps have a firm grip.
- Engine cranks but doesn’t fire: the battery is probably not the problem. Fuel or ignition faults are more likely.
- Starts, then dies again soon: the alternator may not be charging, or the battery may be at the end of its life. Many car batteries last roughly 3 to 5 years, depending on climate and use.
- The battery flat every few days: something may be draining it while the car is parked. A garage can test the battery, the alternator and the electrical drain.
If the battery warning light is on while driving, read our guide on what the battery light means. If you need a replacement, see our comparison of the best car batteries.
FAQ
Can I jump-start a car in the rain? Yes, with care. Keep the clamp connections as dry as you can and never stand in water. If there is a lot of water around the battery, wait or call for help.
How long should I leave the cars connected? Let the working car run for a few minutes before you try to start the other one. A very flat battery may need a little longer.
Is a portable jump starter better than cables? It is more convenient because you do not need a second car. Follow the instructions of your model, since the order of connection can differ.
Will jump-starting damage my car? Not when done correctly. The risks come from wrong connections and from damaged batteries.
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