How to Jump-Start a Dead Car Battery

Before you connect anything

A jump start borrows electricity from another vehicle’s battery (or a portable jump pack) to turn over a dead car’s engine. It is a common fix, but batteries can release explosive gas and cables can spark, so the order of connections matters.

First check the basics:

• Read the owner’s manual for both cars. Some vehicles have special jump-start terminals under the hood, and some, especially hybrid and electric models, have specific instructions. Do not skip that step.
• Look at the dead battery. If the case is cracked, swollen, leaking or frozen, do not try to jump it. Call for roadside assistance instead.
• Make sure the two batteries have the same voltage, which is 12 volts for ordinary cars.
• Switch off both engines, headlights, the radio and the climate system, and apply the parking brakes. Keep the cars from touching.
• Keep everyone back, remove loose jewelry, and avoid smoking or open flames. Eye protection is sensible.

If you are not sure what is wrong, the problem might not be the battery at all. A basic checklist for an engine not starting helps separate a dead battery, which often shows as dim lights and a rapid clicking sound or nothing at all, from a starter or fuel problem.

Cable order

Jumper cables have a red (positive) pair of clamps and a black (negative) pair. Use cables in good condition, with no frayed insulation, and make sure the clamps never touch each other once a cable is connected to a battery. A widely taught sequence is:

1. Clamp one red clamp to the positive (+) terminal of the dead battery.
2. Clamp the other red clamp to the positive (+) terminal of the good battery.
3. Clamp one black clamp to the negative (-) terminal of the good battery.
4. Clamp the last black clamp to a bare, unpainted metal part of the dead car, away from the battery, such as a sturdy engine bracket or a metal part the manual identifies.

The last connection goes to metal rather than the dead battery’s negative terminal because a small spark can occur there, and it is better away from any gas the battery may give off. If your manual shows a different method, follow the manual.

Starting the car

1. Start the engine of the working car and let it run for a few minutes. Some people hold the engine at a slightly raised speed; follow the guidance in the manual.
2. Try to start the dead car. If it does not start after a short try, wait a minute or two and try again. Do not crank for long, since this can overheat the starter.
3. If the engine starts, leave both cars running.

Removing the cables

Reverse the order exactly, and avoid letting clamps touch:

1. Remove the black clamp from the dead car’s bare metal.
2. Remove the black clamp from the good battery.
3. Remove the red clamp from the good battery.
4. Remove the red clamp from the formerly dead battery.

After the car starts

Keep the engine running and drive for a while so the alternator can recharge the battery. A drive of around half an hour is often suggested, although the amount actually needed varies with the battery’s condition and how drained it was. Short trips may not be enough. Avoid switching off the engine until you are somewhere you can get help, since the battery may not start the car again.

If the battery went flat because you left lights on, it may recover. But a battery that is several years old, or one that has been fully drained more than once, may have lost capacity. A shop can test it, and if it fails, battery replacement is the next step. Dead batteries are more common in winter, and cold starts put extra strain on them.

If the car still will not start

Consider the following:

• Check the connections. Wiggle the clamps to be sure they bite into clean metal. Corroded terminals can block the current, so cleaning may help.
• Wait and retry. Letting the donor car run a few more minutes can transfer more charge.
• Listen for clues. A single click or no sound may point to a starter or connection problem. A spinning engine that does not fire suggests fuel or ignition trouble rather than the battery.
• Look at the dashboard. A lit battery or charging symbol while driving suggests the charging system is not working. The warning light articles explain the charging warning.
• Try once more, then stop. Repeated attempts can damage components.

If it still does not start, call a roadside assistance provider or a tow, and consider towing if the car cannot be driven safely. Do not keep trying in a dangerous place; if you are on a busy road, the guidance for a highway breakdown matters more than the jump itself.

A portable jump pack

A compact jump pack avoids the need for a second car and can be kept in the trunk with an emergency kit. Follow its instructions, which differ between products, and keep it charged. If you jump-start often, the cause deserves attention, not just the cables.