How to Jump Start a Car Battery

A flat battery is the most common reason a car won't start. ADAC's 2026 breakdown statistics put the 12 V battery behind 45.4 % of the 3,691,813 breakdowns it attended in 2025. A jump start gets you moving again, and done in the right order it is safe for you and for the car's electronics.

This guide is the jump itself, as the owner's manuals describe it. For why batteries die, how to test one and what the voltage on your multimeter means, read the car battery and charging system guide. If the battery needs replacing, how to replace a car battery covers the job.

Your own owner's manual wins over everything here. Makers differ in the details, and many cars have dedicated jump-start points.

Before you connect anything

Don't jump a battery that is frozen, damaged or leaking. HELLA says no starting aid should be used on one.

Both cars must be 12 V. Kia and Mazda allow only a 12-volt booster system and warn that a 24-volt supply, such as two 12 V batteries in series, can permanently damage the starter, the ignition system and other electrical parts. HELLA adds that the donor battery should have the same rated voltage and the same or a greater capacity than the flat one.

Not every car can be a donor. Hyundai says not to jump-start another vehicle with its hybrid, because it damages the hybrid's lithium 12 V battery. Renault says an E-Tech full hybrid must not restart another vehicle. Volvo says never to use the 48 V battery to jump-start another vehicle, and Tesla says a Model 3 cannot be used to jump-start another vehicle.

Use proper leads. Volkswagen specifies a cross-section of at least 25 mm² for petrol engines and at least 35 mm² for diesels. The clamps must be fully insulated, the two cars must not touch each other, and Volkswagen says not to use a mobile phone while the leads are going on or coming off.

Look for the jump-start points. HELLA says cars with a battery management system must be jump-started via the jump-start connection points in the engine compartment, because using the battery poles directly can damage the electrical system or the battery sensor. On a BMW, for example, a starting-aid terminal in the engine compartment serves as the positive, and the body ground or a special nut acts as the negative.

The clamp order

Volvo's order:

1. Red clamp to the donor's positive terminal.
2. Red clamp to the flat car's positive terminal or its positive jump-start point.
3. Black clamp to the donor's negative terminal.
4. Black clamp last, to the flat car's negative ground point, away from the battery.

The black clamp must not touch either positive terminal or the red clamps. VARTA's generic procedure ends the same way: the final negative connection goes to the engine block of the stalled car, away from the battery.

Why the last clamp doesn't go on the flat battery

Kia, Mazda and Volkswagen all put the last clamp on a ground point away from the battery, and they give three different reasons:

• Kia: connecting to the discharged battery's negative terminal can overheat and crack the battery and release acid.
• Mazda: a spark there could ignite gas around the battery.
• Volkswagen: the car's electronics can then make an incorrect evaluation of the battery's condition.

Starting the car

Which engine runs when differs by maker, so follow your manual. Volvo runs the donor at about 1500 rpm for a few minutes before the start attempt, and says not to touch the cable connections during it. Kia's Niro manual runs the donor at about 2,000 rpm for several minutes. BMW runs the donor for several minutes at increased idle.

If the car doesn't start, don't keep cranking. Volkswagen says to switch off the starter after about 10 seconds and try again after about a minute; BMW says to wait a few minutes before retrying.

Taking the leads off

In reverse order, black clamp first. Keep the black clamp away from both positive terminals and the red clamps while it comes off.

Afterwards: the battery is still flat

A jump start puts very little back into the battery. Kia says to keep the car running for at least about 30 minutes after a successful start. Volvo says a flat battery needs at least 1 hour of charging at about +15 °C and 3 to 4 hours at lower temperatures, and recommends an external charger instead. A short drive home won't do it: Midtronics says trips under 20 minutes prevent the alternator from fully recharging the battery.

A smart charger is the better fix. CTEK's in-car connection is red clamp to battery positive, black clamp to the chassis away from the fuel pipe and the battery, mains plugged in last. The battery guide covers chargers in full.

Then find out why it went flat. ADAC says a battery lasts about 5 years, and to have it checked before autumn if it was last replaced more than three years ago.

Using a jump pack instead

A jump pack follows its own manual. The NOCO GB40, for example, is for 12-volt lead-acid batteries only: positive (red) clamp to the positive terminal first, then the negative (black) clamp to the negative terminal or the chassis, and the negative clamp comes off first. NOCO says to wait 20 to 30 seconds between attempts and make no more than five consecutive jump starts within 15 minutes, then rest the unit for 15 minutes. It won't boost a battery below 2 volts; its manual override forces it on, but disables the spark-proof and reverse-polarity protection.

Safety

CTEK's manual says batteries generate explosive gases during normal operation. VARTA says to wear eye, face and hand protection and never to lean over a battery while boosting, testing or charging.

Log it

Log the jump in GarageHub with the date. One jump is bad luck; two in a few weeks is a battery or a charging problem, and Rev, the AI mechanic, can tell you what to check next.