How to jump start a car safely

A flat battery is one of the most common reasons a car refuses to start, and a jump start will often get you moving again in a few minutes. Done in the wrong order, though, jump starting can damage sensitive electronics or, in the worst case, cause a battery to vent gas near a spark. The method below is the safe, widely recommended sequence. Take it slowly and it is straightforward.

Before you begin, make sure the flat battery really is the problem. If the car is completely dead with no lights and no click, a flat battery is likely. If you are not sure what is happening, our guide on why your car will not start can help you narrow it down.

What you need and a few safety checks

You need a set of jump leads in good condition or a charged jump pack, and if you are using leads a second running car with a battery of the same voltage. Nearly all cars are 12 volts, but do not attempt this if either battery looks damaged, cracked, leaking or frozen. Remove any loose scarves or ties, take off metal jewellery, and if you have safety glasses to hand, wear them.

Position the working car close enough that the leads reach comfortably, but make sure the two cars are not touching. Put both in park or neutral, apply the handbrakes, switch off the ignition on both, and turn off lights, heaters and other electrics.

The correct connection order

This is the part that matters most. Connect the four clamps in this exact order.

1. Red lead to the flat battery positive. Clip one end of the red, positive lead to the positive terminal of the flat battery. The positive terminal is usually marked with a plus symbol and often has a red cover.

2. Red lead to the good battery positive. Clip the other end of the red lead to the positive terminal of the working car battery.

3. Black lead to the good battery negative. Clip one end of the black, negative lead to the negative terminal of the working car battery, marked with a minus symbol.

4. Black lead to an earth point on the flat car. Clip the final black clamp to a solid piece of bare, unpainted metal on the engine block or chassis of the car with the flat battery. Do not attach it to the flat battery negative terminal. A bolt or bracket on the engine, well away from the battery, fuel lines and any moving parts, is ideal.

That last step catches a lot of people out, so it is worth understanding why. A flat battery can give off a small amount of flammable gas. Making the final connection away from the battery means that if there is a tiny spark as the clamp touches down, it is nowhere near that gas. It is a simple habit that keeps the whole job safer.

Starting the engines

With all four clamps on and secure, start the engine of the working car and leave it running for a few minutes. This lets it feed some charge across to the flat battery. After that, try to start the car with the flat battery. If it fires up, leave both engines running together for around five minutes before you touch the leads.

If the flat car will not start after several attempts, stop and check your connections are clean and tight. If it still will not go, the battery may be too far gone or the fault may be something else entirely, and it is time to call for help rather than keep cranking.

Disconnecting in reverse

Once the previously flat car is running, remove the leads in the reverse of the order you fitted them, and take care that the clamps do not touch each other or any metal while the other end is still attached. Remove the black clamp from the earth point on the previously flat car first, then the black clamp from the working car battery, then the red clamp from the working car, and finally the red clamp from the car you have just started.

Run the engine afterwards

A jump start only gives the battery enough to fire the engine. It does not recharge it. To put charge back you need to drive the car for a decent run, around half an hour on faster roads is a good guide, so the alternator can top the battery back up. Avoid stopping and restarting straight away, and try not to switch off before that charging run is done or you may find the car flat again.

When a jump means the battery needs replacing

An occasional flat battery can be down to a one off, such as leaving the lights on overnight or a very short journey pattern. But if your car needs jump starting again and again, or struggles to start every cold morning, the battery is likely near the end of its life or something is draining it. Most car batteries last roughly five to seven years, so age is often the answer.

It is worth learning the car battery warning signs so you can plan a replacement rather than get caught out. Repeated jump starts are hard on the car and inconvenient, and a battery that keeps dying should be tested by a garage. They can check the battery, the alternator that charges it, and whether anything is drawing power when the car is off, then fit a new battery if that is what is needed.