How to Jump-Start a Car Safely, Step by Step

What you need

• Jumper cables (also called jump leads), ideally at least 3.5 metres (12 feet) long and reasonably thick.
• A second car with a healthy battery of the same voltage (nearly all cars use 12 volts), or a charged portable jump starter.
• Gloves and eye protection, if you have them.
• Your car's handbook, which may show dedicated jump-start terminals.

Before you start: safety

• Check the battery. Do not jump-start a battery that is cracked, leaking, swollen or frozen. A frozen battery can explode. Call for roadside help instead.
• No smoking or flames nearby. Batteries can give off flammable hydrogen gas.
• Park the cars close but not touching, facing each other or side by side.
• Turn both cars off, put them in park or neutral, and apply the handbrakes.
• Switch off lights, the radio, heater fans and other electrics in both cars.
• Keep the clamps apart once any of them is connected. Never let red and black touch.

Connecting the cables

Identify the terminals: positive (+) is usually red, or has a red cover; negative (−) is usually black. Clean off heavy corrosion with a wire brush if needed.

1. Red clamp to the dead battery's positive (+) terminal.
2. Other red clamp to the good battery's positive (+) terminal.
3. Black clamp to the good battery's negative (−) terminal.
4. Last black clamp to bare, unpainted metal on the dead car, such as an engine bolt or a dedicated earth point, away from the battery.

Why the last clamp goes on metal: the final connection can make a small spark. Making it away from the battery keeps any spark away from the gas a battery can release.

Starting the car

1. Start the working car and let it run for 3 to 5 minutes.
2. Try to start the dead car. If it starts, go to the next section.
3. If it does not start, wait another few minutes and try again. Do not crank the starter for more than about 10 seconds at a time.
4. If it still will not start after a few tries, check the clamps are firmly attached. If they are, the battery may be completely dead, or the problem may be elsewhere, such as the starter motor or fuel.

Removing the cables

Remove them in the reverse order, keeping the clamps apart:

1. Black clamp from the bare metal on the jumped car.
2. Black clamp from the good battery.
3. Red clamp from the good battery.
4. Red clamp from the jumped battery.

After the jump

• Keep the engine running and drive for at least 20 to 30 minutes, ideally on faster roads rather than in town, so the alternator can recharge the battery.
• Do not switch the engine off straight away, or it may not restart.
• Get the battery tested at a garage or parts shop. Many test batteries for free.

Using a portable jump starter

A jump pack is a compact battery that does not need a second car, and is worth keeping in the car in winter.

1. Make sure it is charged, and the pack is switched off.
2. Connect red to the battery's positive (+) terminal.
3. Connect black to bare metal on the engine, or the negative terminal if the instructions say so.
4. Switch on the jump starter, and start the car.
5. Switch off the pack, then remove black first, then red.

Recharge the jump pack after use, and every few months when stored, since it loses charge, especially in cold weather.

Why batteries die, especially in winter

• Cold weather slows the chemical reaction inside the battery and makes the engine harder to turn.
• Short trips do not give the alternator time to recharge.
• Lights or accessories left on.
• Age. Most car batteries last about 3 to 5 years.

Signs a battery is failing: a slow, laboured start; dim headlights at idle; a battery warning light; or needing jumps more than once.

Planning a winter journey? Check the forecast for winter storm warnings, and keep jumper cables or a jump pack, a torch, warm layers, water and snacks in the car.

Electric and hybrid cars

EVs and hybrids have a small 12-volt battery for their electronics that can go flat. Many can be jumped through dedicated terminals under the bonnet, but most manufacturers advise against using an EV or hybrid to jump-start another car. Check the handbook. Our electric vehicle buying guide covers EV ownership more broadly.

Key takeaways

• Connect: red to dead +, red to good +, black to good −, black to bare metal on the dead car.
• Start the good car, wait a few minutes, then start the dead one.
• Remove the cables in reverse order, and never let the clamps touch.
• Drive for 20 to 30 minutes afterwards, and get the battery tested.