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Maintenance and tyres , On the road , Safety and breakdowns · Updated 2026-10-18

How to jump-start a car safely

A flat battery is one of the most common reasons a car will not start. You turn the key and hear clicking, or the lights are dim and nothing happens. A jump-start can get you going, but a car battery can release explosive gas and carries enough current to cause burns and damage electronics. Done carelessly, it is risky. This guide gives the general idea; always follow your owner's manual, which may have specific instructions or warnings for your car.

First: is it really the battery?

Signs of a weak battery are slow cranking, clicking, dim interior lights, and a dashboard that flickers or goes dark when you try to start. A battery problem is more likely in cold weather, after the car has stood for days, or after lights were left on. If the engine cranks strongly but does not start, the problem is probably fuel or ignition, and a jump will not help. If you smell fuel or see smoke, do not attempt anything and call for help. See 12-volt battery care for what makes batteries last.

When not to jump-start

• The battery is cracked, leaking, swollen or frozen.
• You smell rotten eggs (sulphur) or see corroded or damaged terminals and cables.
• The manual says not to, or the car has a special start system you do not understand.
• You are unsure about any step. A tow or assistance service is cheaper than damaged electronics.

Modern cars have sensitive electronics, and some hybrid and electric cars have rules for jumping or being jumped. Their high-voltage system is dangerous and must not be touched. Even the small 12-volt battery in these cars may have special connection points. Check the manual first and call assistance if in doubt. More in electric car basics and hybrid types explained.

What you need

• A second car with a healthy battery of the same voltage (usually 12 volts for passenger cars), or a portable jump starter that you know how to use.
• Jump leads in good condition, with thick wire and clean clamps, long enough to reach.
• Gloves and eye protection, if you have them.

The general order of steps

These are the usual steps. Your manual comes first.

1. Park the cars close enough for the leads to reach, but not touching. Switch off both engines, apply the handbrakes and switch off lights, radio and other electrics.
2. Open the bonnets and find the battery terminals, marked plus (+, often red) and minus (-, often black). Some cars have remote jump points instead.
3. Connect one red clamp to the plus terminal of the flat battery.
4. Connect the other red clamp to the plus terminal of the good battery.
5. Connect one black clamp to the minus terminal of the good battery.
6. Connect the last black clamp to a bare metal part of the engine block or the body of the car with the flat battery, away from the battery and from moving parts. This is not to the minus terminal. It reduces the risk of sparks near the battery.
7. Make sure the leads are clear of fans and belts, and that the clamps do not touch each other.
8. Start the engine of the good car and let it run for a few minutes.
9. Try to start the car with the flat battery. If it does not start, wait a bit and try again. Do not crank for long.
10. When the engine runs, disconnect the clamps in reverse order: the black on the metal point, the black on the good battery, the red on the good battery, then the red on the formerly flat battery.

Keep the leads away from moving parts and never let the clamps touch each other while connected. Do not lean over the battery while connecting, and no smoking or open flame nearby.

After the car starts

Do not switch the engine off straight away. Drive for a while so the alternator can recharge the battery. A short trip may not be enough, and the battery might fail again the next day. If the battery was flat because you left the lights on, it may recover, but a battery that has gone flat several times, or is several years old, often needs replacement. A workshop can test it and check the charging system. In the meantime, consider a proper battery charger if you leave the car unused for long periods.

Common mistakes

• Connecting plus to minus, which can damage electronics and the battery. Check the markings twice.
• Letting the two cars touch.
• Trying to jump a frozen battery.
• Using thin or damaged cables that overheat.
• Revving the good car hard instead of a steady idle.

Other ways to get going

A portable jump starter is a good alternative, and avoids the need for a second car. Follow the instructions of the device. Roadside assistance can also start the car or replace the battery on site. If you are on a busy road, stay safe first; see what to do in a breakdown. Keep the leads in your emergency kit. In winter the battery works harder, so see winter driving tips.

What to remember

• Read the manual first; some cars have special jump points or rules.
• Never jump a damaged, leaking or frozen battery.
• Plus to plus, minus to a bare metal point on the engine, in the right order.
• Remove the leads in reverse order.
• Drive afterwards, then have the battery tested.
• When in doubt, call assistance, especially with hybrid and electric cars.

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