Dummies' Guide - How to Jump-Start a Car (and What to Do If That Doesn't Work)

A flat battery is one of the most common reasons a car won't start, and it usually happens at the worst time: a cold morning, a car park, or a remote trailhead. Jump-starting is simple once you know the order, and the order matters because it keeps sparks away from the battery.

For certain cars, you can jumpstart from the boot as well. Please search your make/model/year up for more details, there are plenty of video tutorials around.

Before You Start

• Check your owner's manual. Some cars have special jump-start terminals under the bonnet, and some hybrids and modern cars have specific instructions or keep the 12-volt battery somewhere unexpected, like the boot.
• You need: a second car with a healthy battery (or a portable jump pack), and a set of decent jumper leads with clean, tight clamps. Alternatively, get yourself a spare 12-volt car battery.
• Park the cars close enough for the leads to reach, but not touching. Turn both engines off, apply the handbrakes, and switch off lights, radio and the air-con.
• No smoking, no naked flames, and keep your face and hands away from the battery.

The Order That Matters

1. Red clamp to the positive (+) terminal of the flat battery.
2. The other red clamp to the positive (+) terminal of the good battery.
3. Black clamp to the negative (-) terminal of the good battery.
4. The last black clamp to bare, unpainted metal on the car with the flat battery, such as a strong bolt on the engine or the engine block, away from the battery and away from moving parts.

Start the good car and let it run for a few minutes. Then try to start the flat car. If it starts, take the leads off in the reverse order: the black clamp from the bare metal first, then the other clamps.

Why the Last Clamp Goes on Bare Metal

It's tempting to clip the last black lead onto the flat battery's negative terminal, because it looks like the matching pair. But batteries can vent explosive hydrogen gas, and connecting the final lead makes a small spark. Making that last connection on metal away from the battery keeps the spark well clear of the gas. Mechanics widely recommend this, and it's the method taught for good reason.

Once It Starts

• Leave it running, and don't switch it off straight away. Driving for a decent stretch, generally 20 to 30 minutes, gives the alternator time to recharge the battery.
• Don't leave the leads clipped on while you rev the engine.
• If the car stalls or won't restart after you turn it off, the battery may be failing.

What If It Won't Jump-Start?

• Poor connection or a bad ground. This is a very common cause. Wiggle the clamps to bite through corrosion, clean the terminals, and try a different bare metal spot for the last clamp.
• Give it a few minutes. Let the good car run for longer before you try again, as a very flat battery can take a while to take a charge.
• Clicking, or nothing at all? A rapid click usually means not enough power reaching the starter. Silence can mean a completely dead battery, a loose terminal, or something else.
• It starts, then dies. The alternator may not be charging the battery.
• The battery is old or damaged. Batteries wear out. A battery that keeps going flat needs testing or replacing, and a workshop can test it for you.
• It's not the battery. The starter motor, fuses, or a fault elsewhere can also stop a car from starting.

Quick tip: A rested, healthy 12-volt car battery reads about 12.6 volts on a multimeter. If yours is well below 12 volts, it's flat.

Keep the Battery Alive

• Don't leave lights, the stereo or a charger on with the engine off.
• Short trips don't recharge the battery properly, so take a longer drive now and then.
• Cold weather makes weak batteries fail. See our winter checklist.
• Consider a portable jump pack for the boot so that you don't need another car.

This guide is general advice. For your car, follow the owner's manual, especially for hybrids and newer models. If you're not comfortable doing it, call roadside assistance.

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