How to jump-start a car

If your car won’t start when you switch on the ignition, it might have a flat battery. With a friend, one working car and a set of jump leads, you should be able to get it going again.

Written by

Julie Daniels

Motor insurance expert

Posted

15 FEBRUARY 2021

4 min read

Should I jump-start my car?

Jump-starting a car means borrowing a bit of power from another car’s battery to get yours working again. It’s not complicated to do, but it involves handling electrically-charged components. For safety’s sake, ask a qualified mechanic if you don’t feel confident doing it yourself.

This guide gives you a general idea of how to jump-start a car. Things can vary between vehicles though, so check your car’s manual before you get started.

How does jump-starting work?

In petrol or diesel vehicles, the battery charges up when the engine’s running. If the battery’s gone down, for example because you’ve left the internal lights on overnight or it’s been exposed to too much cold weather, you can use jump leads to get the engine started again. You can then drive the car around and about to charge your battery up fully.

Jump-starting is for conventional engines (petrol or diesel). It’s not suitable if you, or the person who wants to help you, has an electric or alternative fuel vehicle.

What you need to jump start a car

A fully-working petrol or diesel car that has the same voltage as yours does (usually 12 volts).
A set of jump leads in good condition.
Somewhere you can safely park both cars nose-to-nose.
Another person to hold the other ends of the leads while you attach them.
If it’s dark, a torch.

Safety musts

Never try to jump-start a battery that is leaking or visibly damaged.
Never allow the clamps on the ends of the leads to touch each other. Ask the other person to hold the ends until all the connections are made.
If the jump leads get hot, stop using them.
The black (negative) lead should only be attached to the other car’s battery. Don’t attach it to your battery at all.

Jump-starting a car step by step

Stage 1: Prepare

1. Park both cars nose-to-nose (but not touching) in a safe place.

2. Apply the parking brakes and switch off the engines. Take keys out of ignitions.

3. Open the bonnets and identify where the battery terminals are.

4. Remove anything metal you’re wearing and anything metal in your pockets, including rings, keys, watches and belt buckle (basically, remove anything that could conduct electricity). Tie back long hair and tuck away loose clothes.

Stage 2: Connect the jump leads

Jump leads need to be connected in a specific order. With the other person’s help:

5. Connect one end of the red (positive) lead to the positive terminal of the other car’s battery.

6. Connect the other end of the red (positive) lead to the positive terminal of your battery.

7. Connect one end of the black (negative) lead to the negative terminal on the other car’s (fully functioning) battery.

8. Connect (clip) the other end of the black (negative) lead to an unpainted metal surface on your car - the side of the engine bay for example - to earth it. Keep it away from your battery.

Tip: If the battery terminals look corroded, give the connected clamps a twist to make sure there’s a good metal-to-metal connection.

Stage 3: Turn on the power

9. After waiting for three to five minutes, start the engine of the other car (the car with the fully charged battery).

10. Wait a few minutes for power to trickle into your battery.

11. Start your car’s engine.

12. Let both engines idle for around 10 minutes.

13. Switch off both engines and disconnect the clamps, reversing the order you connected them in (that means first removing the black negative lead from your car). Take care not to let the clamps touch until everything’s disconnected.

14. Start your engine again and take your car for a drive for about 30 minutes to fully charge up the battery.

What if jump-starting doesn’t work?

If the battery isn’t holding a charge after your jump-start, you might have a different problem, like a faulty alternator, or your battery may be at the end of its life and need to be replaced.