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How to jump start a car

Last updated November 30, 2021

Jump starting a car is the most common way to start a car in the event of a flat battery. However, it can be very dangerous to jump start a vehicle if you misuse the jump leads.

Our simple step-by-step guide explains how to jump start your car correctly and includes a dedicated section on jump starting your car with a battery booster pack.

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What you'll need

  • Jump leads

    You’ll need a set of working jump leads that have no faults, to link the flat car battery to the working battery of another car.

  • A donor car

    You’ll need a donor car with a working battery to transfer electricity to the flat battery on your car.

  • Safety goggles

    Sometimes there are dangerous sparks created when jump starting a car. To protect your eyes, you should always wear safety goggles.

How to remain safe when using jump leads

When jump starting a car, you mustn’t take any unnecessary risks. Stay safe by following the guidelines below:

  • Check for any damage to either car batteries or the jump leads. If there is any leaking or damage, you shouldn’t attempt a jump start. If the jump leads become hot at any point, stop using them immediately.
  • Make sure you wear suitable clothing and remove any jewellery.
  • The engine should be off when you remove the jump leads. Removing them while either engine is running could damage the electronics.
  • Never put naked flames close to the battery - this includes smoking.

How to jump start a car: step-by-step

  • Take safety precautions

    Before you jump start your car, remove any loose clothing that could get tangled in the engine. Next, remove your jewellery and put on your protective glasses.

  • Position the two cars

    You’ll need to park the two cars nose-to-nose. Ensure enough space is left between each car to open both bonnets and connect the jump leads.

  • Attach the red jump lead

    With both ignitions turned off and the parking brakes engaged on both cars, attach the red lead to the donor car's positive (+) terminal and the other end to the flat battery’s positive (+) terminal.

  • Attach the black jump lead

    Once the red jump lead is attached, connect the black jump lead to the negative (-) terminal on the donor car. The other side of the jump lead should be attached to an unpainted metal surface, well away from the fuel system and the battery on the car with a flat battery as an earthing point.

  • Start the donor vehicle

    Once the jump leads are securely connected, start the donor vehicle’s engine and allow it to run for 3-5 minutes. It should remain in park or neutral with the handbrake locked.

  • Start the car with a flat battery

    After the donor vehicle has been running for 3-5 minutes, start the car with the flat battery.

  • Allow both cars to run

    Leave both cars in idle and allow them to run for 10 minutes with the jump leads still attached.

  • Turn off both cars and disconnect the cables

    After you’ve left both cars to run for ten minutes, turn off both cars. In reverse order to how you applied them, disconnect the cables without letting them touch each other.

  • Run the car with a flat battery

    Once you’ve safely removed all jump leads, turn on the car with a newly charged battery and leave it running for 15-20 minutes. Next, drive the car for 20-30 minutes on a route where stop-start driving is unlikely to further charge the battery.

How to jump start a car using a battery booster pack

  1. As detailed above, turn off all electrical devices in the car and remove your ignition key. The battery boost pack should also remain off at this stage.
  2. Connect the red positive lead to the positive terminal on the battery, then connect the black negative lead to an uncovered (earthing) metallic part of the engine.
  3. Switch the booster pack on to charge the flat battery.

How to jump start a 24-volt battery

A 24-volt battery is simply two 12-volt batteries combined in one unit, and you can use the same boosting process detailed above to jump start them.

Before changing the cables from one battery to the other, turn off the assisting vehicle’s engine or booster pack before removing and reconnecting the cables. Conveniently, you can charge both batteries simultaneously using a working vehicle and a booster pack.

  1. Make you’ve switched the booster pack off before connecting it to the battery unit. Connect the red positive cable to the positive terminal and the black negative cable to a bare-metal connection, no less than 45cm from the battery.
  2. For the second battery, attach the jump leads as per the instructions above.
  3. Activate the booster pack and switch on the assisting vehicle’s engine, allowing it to run for 5-10 minutes.
  4. Switch on the ignition of the flat battery vehicle and cut the booster vehicle’s engine. Once the flat battery has received the all-important boost, let it run for a few minutes to charge.
  5. Finally, switch off the car with the newly charged battery before switching off and removing the booster pack.

Get expert help

Not everybody will be comfortable jump starting a car, even with the detailed step-by-step process we’ve provided. It might also be the case that you need to replace your cars battery if it still isn’t working after you’ve followed our instructions, which will require an expert to diagnose and fit.

Perhaps your battery needs replacing? Every battery has a shelf life, and car batteries can last around 3-5 years.