How to jump start a car with a dead battery

If your car will not start because the battery is discharged, there are several possible solutions. The safest method depends on the vehicle, where it has broken down and what equipment you have available.

Before doing anything, check the owner's manual.

Option 1: Use a portable jump starter

For many drivers this is the most convenient solution because another vehicle is not required.

With a compatible booster pack:

1. Turn the vehicle and electrical accessories off.
2. Identify the manufacturer's correct jump points.
3. Connect the positive/red clamp to the positive jump point.
4. Connect the negative/black clamp as instructed by the vehicle manufacturer, commonly to the designated earth point.
5. Activate the booster.
6. Attempt to start the vehicle.
7. Disconnect according to the booster and vehicle instructions.

The RAC advises placing the booster securely rather than on the engine and using the appropriate positive terminal and earth point.

Option 2: Use jump leads and another suitable vehicle

First confirm that the vehicles and manufacturer instructions allow it.

The RAC specifically advises against using hybrid or electric vehicles as donor vehicles unless their manufacturer permits it.

A commonly recommended sequence for conventional 12V systems is:

1. Red lead to the positive terminal of the flat vehicle.
2. Other red lead to the donor battery positive terminal.
3. Black lead to the donor battery negative terminal.
4. Other black lead to the designated earth point on the disabled vehicle.

Follow your manufacturer's procedure where it differs.

Never improvise around unfamiliar terminals.

Why use an earth point?

Connecting the final negative clamp at the manufacturer-approved earth point keeps the final connection away from the battery itself.

The AA advises attaching the final black clamp to an earthing point such as suitable unpainted metal on the engine block or chassis, away from the flat battery and fuel system.

What happens after the engine starts?

Do not assume the job is finished.

A jump start supplies enough external electrical power to get the vehicle operating. The battery may still be heavily discharged.

The AA advises normal driving rather than stop-start traffic for at least around 30 minutes after a successful jump start, while a proper battery charger can provide a more complete recharge.

What if it still will not start?

Stop repeatedly cranking the engine.

Possible causes include:

• a battery too badly deteriorated to recover
• inadequate booster capacity
• poor jump-lead connection
• starter-motor failure
• charging-system problems
• immobiliser problems
• another mechanical or electrical fault

At that point, diagnosis is more useful than another ten starting attempts.

When should you not attempt it?

Do not jump start if:

• the battery is visibly leaking or damaged
• you cannot positively identify the correct connections
• the manufacturer prohibits the procedure
• the vehicle is in an unsafe road position
• you suspect high-voltage system damage
• you are uncertain whether the vehicle uses an unusual battery system

A discharged battery is inconvenient. Incorrectly connecting jump leads can turn a small problem into damaged electronics, personal injury or a much more expensive repair.