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How to jump start a car

October 10, 2022 by

Battery left you feeling flat? Our guide will talk you through how to jump start a car

Most drivers will at some point have the unwelcome experience of returning to their car to find it has a flat battery and won’t start. If this is you, we’ll talk you through the steps needed for jump starting a car.

We’ll also highlight the three different methods of jump starting a car, detailing how to jump start a car with jump leads, how to jump start a car with a power pack, and how to bump start a car.

How to jump start a car with jump leads

Jump leads come in pairs: the red lead is for the positive battery terminal, and the black lead for the negative terminal. They have large ‘crocodile’ clips that are designed to clamp onto battery terminals, and are designed to be hooked up to the car with the flat battery, and another vehicle that’s running healthily.

It’s important to follow steps in a specific order so as not to damage either vehicle, or risk an electric shock.

Preparing for a jump start

While it’s a relatively simple procedure, jump starting a car is still a mechanical task, which brings with it risks. If you are in any doubt about how to jump start a car, or don’t feel confident, call a breakdown firm to help.

If you have long hair, tie it back, while also making sure you don’t have loose clothing or jewellery that could get tangled in moving parts of the engine, or potentially cause an electric shock. Don’t allow anything metal like a spanner or watch strap to touch battery terminals.

Work out where the batteries are on both cars – they’re usually under the bonnet, but can be in the boot or even under the back seat in rare instances.

Jump leads are often not very long, and you’ll need to position both cars so their batteries are close to each other. Often this will involve placing the cars nose-to-nose, or potentially lining them up side-by-side if both cars’ batteries are on the same side of the engine bay and the leads won’t stretch diagonally.

Remove any covers that may be over the batteries, but don’t disconnect any wires or leads from them. Check the jump leads are in decent condition and don’t have any missing insulation on their wires, and check the cars’ batteries aren’t damaged or leaking.

How to connect cars for jump starting

This is where getting things in the right order is important. Follow these steps:

  1. Make sure both cars are switched off and have their handbrakes on.
  2. Connect the red (+) lead to the positive terminal of the working car’s battery.
  3. Connect the other end of the red (+) lead to the positive terminal of the flat car’s battery.
  4. Connect the black (–) lead to the negative terminal of the working car’s battery.
  5. Connect the other end of the black (–) lead to an earthing point on the car with the dead battery. Check your car’s handbook to find this – it will generally be unpainted metal on the engine or chassis that’s not close to the battery or the fuel system.
  6. Leave both cars connected with their engines off for three minutes or so.
  7. Start the working car’s engine and leave it running for a couple of minutes.
  8. Start the previously dead car’s engine. If it doesn’t start, leave the healthy car running for another five minutes.
  9. Leave the cars running, connected, for about 10 minutes.
  10. Remove the jump leads in the following order: black (–) lead from the previously flat car; black (–) lead from the healthy car; red (+) lead from the previously flat car; red (+) lead from the healthy car.

How to jump start a car with a power pack

Portable power packs for jump starting cars are essentially heavy-duty powerbanks, similar to those people use to keep their phones topped up when on the go. They are charged at home (often via USB cable) and can hold their charge for many months. Power packs can be a good addition to the ad-hoc toolkit some people keep in the boots of their cars.

Different power packs work in different ways, so consult the instructions of any one you have (and keep the manual with the pack in the car). They feature crocodile clips that connect the power pack to the car’s battery. You’ll need to make sure the power pack you buy is compatible with the type of battery your car features (EG lead-acid).

In general terms, follow this process, bearing in mind the safety points set out in the previous section.

  1. Connect the red (+) crocodile clip to the positive terminal of the car battery.
  2. Connect the black (–) crocodile clip to the negative terminal of the battery, or an earthing point on the car as detailed above.
  3. Press the power button on the power pack to turn it on, then try starting the car. If it doesn’t start straight away, leave the power pack connected for 30 seconds before trying again.
  4. Once the car is started, remove the black (–) crocodile clip first, then the red (+).

Some power packs feature a ‘boost’ or override function, intended for use if the car’s battery is really low on charge. Follow the instructions and use this mode with caution, as it may disable the safety features some power packs have, such as the ability to know if the leads have been connected to the correct battery terminals; this function can also involve a higher current being sent to the battery to get it going.

How to bump start a car

The two methods described above involve giving your car’s battery the power it needs to start the engine, but bump starting (also known as push starting) uses motion from the wheels to turn and start the engine.

Bump starting is only possible in petrol and diesel cars with a manual gearbox, and also requires a clear section of road (or a layby or long driveway) with no traffic. It should only be attempted if you are 100% sure you know what you are doing, and can do so safely. You will also need someone with you to help, and they should know how to push a car safely without a) injuring their back, and b) putting a dent in the car’s bodywork.

The steps are as follows:

  1. Turn the ignition to the ‘on’ position. This is typically the second ‘step’ out of three on the ignition (1 being accessories on, 2 being ignition on, and 3 being start).
  2. Depress the clutch and keep it depressed.
  3. Select second gear.
  4. Get the other person who is helping to push the car from behind.
  5. Once the car is moving at about 5mph, release the clutch quickly. The engine should restart at this point.
  6. Immediately depress the clutch again and put the car into neutral so it stops moving, then let it run for 15 minutes or so to allow the battery to get some charge. You should then go for a decent-length drive fully charge it.

What to do after jump starting a car

As well as leaving a car that’s been jump started running for 15 minutes or so, you’ll also want to take it for a long-ish drive to get the battery property charged. A 30-minute run should do it, but bear in mind driving at 60mph will charge the battery faster than driving at 30mph, so you may need to go for longer if you can’t get to a road with a high speed limit.

Be prepared for the fact that modern cars tend not to like having their batteries go totally flat; you may find that some systems – like electronic stability control (ESC) – don’t work initially, or there may be warning lights for the power-steering, or other systems. These should disappear either as you drive the car, or once it has been turned off and restarted again. Drive extra carefully if you have any of these symptoms (a lack of ESC could present increased skid risk, for example).

Why did my car battery go flat in the first place?

Healthy car batteries tend not to go flat unless the car has been parked up for some considerable time (maybe three weeks or so). If your car goes flat after sitting just for a few days, you could have one of the following problems:

  • A battery that needs replacing (this is often the case if the battery is more than five years old).
  • A defective alternator that isn’t charging the battery properly – this requires a mechanic’s trained hand.
  • A ‘parasitic drain’ – IE an electrical component that is drawing excess current from the battery when the car is switched off. This might be an improperly installed aftermarket accessory (like a hardwired dashcam) or it could be a faulty vehicle component (EG the glovebox light doesn’t turn off when the glovebox is closed and the car is off). A competent auto electrician will be your friend here.

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