Car batteries, 3 minute read

How to Jump-Start a Car Safely, Step by Step

By The Roadworthy Editorial Team, updated 2026-09-29

A flat battery is one of the most common reasons a car will not start. If you have a second car and a set of jumper cables, you can usually get going again in a few minutes. The order in which you connect the cables matters, because a wrong connection can cause sparks or damage the electronics.

Before you start

• Read your owner's manual. Some cars have special jump-start terminals under the bonnet, and some modern cars have specific instructions.
• Check the flat battery. If the case is cracked, leaking or swollen, or if it looks frozen, do not jump-start it.
• Turn off the engines, lights, air conditioning and radio in both cars.
• Wear eye protection if you have it, and keep flames and cigarettes well away.
• Make sure the cables are the correct size and in good condition, with no exposed wire.

Step by step

1. Park the cars close, nose to nose or side by side, so the cables reach without stretching. Do not let the cars touch. Put both in park or neutral with the handbrakes on.
2. Identify the terminals. Positive is marked with a plus sign and often has a red cover. Negative is marked with a minus sign.
3. Red clamp 1: attach one red clamp to the positive terminal of the flat battery.
4. Red clamp 2: attach the other red clamp to the positive terminal of the good battery.
5. Black clamp 1: attach one black clamp to the negative terminal of the good battery.
6. Black clamp 2: attach the last black clamp to an unpainted metal part of the dead car's engine block or chassis, away from the battery and moving parts. This is called grounding, and it keeps any spark away from the battery.
7. Start the good car and let it run for a few minutes at slightly raised revs.
8. Try to start the dead car. If it does not start, wait a couple of minutes and try again. Do not crank for more than about ten seconds at a time.

Remove the cables in reverse order

1. Black clamp from the metal on the car that was flat
2. Black clamp from the good battery
3. Red clamp from the good battery
4. Red clamp from the formerly flat battery

Keep the clamps from touching each other while any of them are connected.

After the car starts

Keep the engine running and drive for at least 20 to 30 minutes so the alternator can recharge the battery. Avoid switching the engine off before then. If the car will not restart the next day, the battery is probably worn out. See the signs of a failing battery and how to test it.

If it still does not start

• Check that the clamps are gripping clean metal and not corroded terminals.
• If you hear clicking or nothing, the battery may be beyond recovery, or the problem may be the starter, alternator or a fuse.
• If the car starts but dies soon after, the alternator may not be charging.

When not to jump-start

If you are unsure, call a mechanic or roadside assistance. Never jump-start a battery that is damaged, and never connect the cables to the wrong terminals. Reversed connections can destroy fuses, control units and the battery itself.

Once you are moving again, it is a good idea to choose a replacement battery before it lets you down again.