How to jump start a car safely at home

Before you open the bonnet

A flat battery is one of the most common reasons a car refuses to start, and it almost always happens on the coldest morning of the year when you are already late. Jump starting at home is straightforward, but it does involve a large amount of electrical energy, so a few minutes of preparation are worth it.

You will need a set of jump leads in good condition. For a petrol engine up to about 2 litres, look for cables with a copper core of roughly 25mm²; for larger petrol engines and most diesels, go for 35mm² or more. Thin, cheap leads get hot and simply will not pass enough current to turn a cold engine over. Check the clamps for corrosion and make sure the insulation along the length of the cables is not cracked or cut.

Then prepare both cars properly:

• Park them close enough for the leads to reach comfortably, but make sure the bodies are not touching.
• Switch off both engines, remove the keys and put the cars in neutral or Park with the handbrake firmly on.
• Turn off lights, heaters, radio and anything else that draws power.
• Put on gloves and eye protection. Never smoke or use a naked flame nearby.

Find the battery, which may be under the bonnet, under a seat or in the boot. Some cars have jump start terminals under the bonnet instead, with the battery hidden elsewhere, so check your handbook. Look closely at the terminals: positive is marked + and usually has a red cover; negative is marked – and is often black. If the battery is cracked, leaking, hissing, frozen or smells of rotten eggs, do not jump start it. That battery needs to be replaced.

The correct order for connecting the leads

Order matters, and so does where you put the final clamp. Work through these steps slowly and never rush them.

• Connect one red clamp to the positive terminal of the flat battery.
• Connect the other red clamp to the positive terminal of the good battery.
• Connect one black clamp to the negative terminal of the good battery.
• Connect the final black clamp to a bare metal earth point on the car with the flat battery — an unpainted engine bracket, lifting eye or sturdy bolt head well away from the battery.

That last step surprises people, but there is a good reason for it. A lead-acid battery gives off hydrogen gas, and the final connection is the one that usually produces a small spark. Making that spark away from the battery keeps any gas well out of reach. Never let the clamps touch each other or any other metal while the leads are live. A brief touch can weld the jaws together, damage wiring or cause a burn.

Take a moment to check every clamp is biting clean metal, not paint or plastic, and that the leads are clear of moving parts such as belts and fans.

Starting both cars

Start the engine of the car with the good battery and let it idle for two or three minutes. This puts a bit of charge into the flat battery and steadies the voltage. Then try to start the car with the flat battery. If it turns over slowly, give it a little longer before trying again.

Once it fires, leave both engines running for a few minutes. Do not switch the donor car off immediately, as the newly started car may stall without that support.

Disconnecting safely in reverse

Switch off both engines before you remove anything, then work backwards through the connection order:

• Remove the black clamp from the earth point on the previously flat car.
• Remove the black clamp from the negative terminal of the good battery.
• Remove the red clamp from the positive terminal of the good battery.
• Remove the red clamp from the positive terminal of the flat battery.

Keep the clamps apart as you go and coil the leads up neatly afterwards. Do not let them drop onto the battery terminals.

If the car still will not start

Check the obvious things first. Are the clamps sitting on clean, bright metal? Wiggle each one slightly and try again. Give the donor car a few more minutes of idling, and hold the revs at around 1,500rpm while your helper turns the key. Do not crank the engine for more than about ten seconds at a time, and leave a minute between attempts so the starter motor can cool.

If the engine turns over briskly but will not fire, the battery may not be your problem. Fuel, ignition or an immobiliser fault can all produce the same symptom. If there is no sound at all, or just a rapid clicking, the connection may still be poor or the starter motor may be at fault. At that point it is worth calling a professional rather than persisting.

Helping the battery recover

A jump start gets you moving, but it does not fully recharge the battery. Drive for at least twenty to thirty minutes, ideally on faster roads rather than crawling through traffic, and keep electrical loads such as heated seats and rear demisters switched off where you can.

If your battery is more than four or five years old, or if it has gone flat several times recently, plan to replace it. Short, stop-start journeys in winter are hard on batteries and rarely put back what starting takes out. A cheap multimeter will tell you the state of charge — around 12.6 volts at rest is healthy, and anything below 12.2 volts means it needs attention. Keeping the terminals clean and lightly greased, and using a maintenance charger if the car sits unused for weeks, will save you a lot of cold, frustrating mornings.