How to Jump Start a Car Safely, and When You Shouldn't

Quick answer

Before you start: safety checks

• Read the owner's manual. Many modern cars have dedicated jump-start terminals under the bonnet, especially when the battery sits in the boot or under a seat. Mercedes-Benz, for example, tells owners to use only the designated jump-start connection point.
• Check the battery. Do not jump start a battery that is cracked, leaking, bulging or smells of rotten eggs.
• Match the voltage. Both cars must have 12-volt systems.
• Use proper leads. Varta recommends cables with a cross-section of at least 16 mm², or 25 mm² for larger engines. Thin, cheap leads can overheat.
• Keep the cars apart. The two vehicles must not touch.
• No sparks, no smoking. Batteries give off hydrogen gas, which is flammable and explosive. Don't lean over the battery while connecting.
• Mind the engine bay. Keep leads, hands and clothing clear of fans and belts.

Step by step with jump leads

1. Park the donor car close enough for the leads to reach, without the cars touching. Switch off both engines, lights and accessories.
2. Connect one end of the red lead to the positive (+) terminal of the dead battery.
3. Connect the other end of the red lead to the positive (+) terminal of the donor battery.
4. Connect the black lead to the negative (−) terminal of the donor battery.
5. Connect the other end of the black lead to an unpainted metal point on the dead car's engine or body, or its marked earth point, away from the battery. Never connect it to the dead battery's negative terminal: the final connection can spark, and a spark near a battery is what you want to avoid.
6. Start the donor car and let it run for a few minutes.
7. Start the dead car. If it doesn't start, wait a few minutes before trying again rather than cranking continuously.
8. Once it runs, remove the leads in exactly the reverse order: black from the car body, black from the donor, red from the donor, red from the car that was dead.

Advice differs on protecting electronics while you disconnect. Varta suggests switching on a load such as the headlights or rear demister in the car that was dead to absorb voltage spikes; The AA suggests switching engines off first. Follow your owner's manual.

Using a jump starter pack

A portable booster pack avoids the need for a second car, which is handy in basement car parks. Connect it in the same way: red to positive, black to an earth point, pack switched off while connecting. The AA suggests that once the engine starts you leave it running for around five minutes, switch the pack off, then keep the engine running for another five to ten minutes before disconnecting. Keep the pack charged; a flat booster is no help.

When you should not jump start

• The battery is damaged, leaking or swollen. Leave it alone and call for help.
• You would use a hybrid or electric car as the donor. The AA advises against it, and the RAC says not to unless the manufacturer specifically permits it. Their 12-volt systems are not designed to start another car.
• The dead car is a hybrid and you don't know where its jump points are. Only the 12-volt auxiliary battery can be boosted; Toyota states that the high-voltage battery pack cannot be jump started. The 12V battery is often in the boot, with a remote terminal under the bonnet.
• Your car has a second battery. Some vehicles with start-stop have a supplementary battery. Jeep, for example, warns that the Wrangler must only be jumped from the main battery.
• You are stopped on a highway. Your safety comes first. Dubai Police advise heading for the nearest exit if the car can still move, and calling 999 if it can't. Switch on the hazard lights, get out on the side away from traffic and wait behind the barrier if there is one.

Why a jump start sometimes doesn't work

• The battery is so deeply discharged, or so badly damaged, that it can't take enough charge.
• The clamps aren't gripping bare metal, or the leads are too thin.
• The problem is not the battery at all, for example the starter or the fuel system. See battery, alternator or starter?

After the jump: get the battery tested

A jump start gets you moving; it doesn't fix anything. If the battery went flat because the lights were left on, a good drive may be all it needs. If it went flat for no obvious reason, it is probably failing, or something is draining it. A battery and charging test tells you which before it lets you down again.

Prefer not to do it yourself?

Our technicians come to your car anywhere in Dubai, including basements and roadsides, with portable equipment. We test first, jump start the car if the battery is worth saving, and replace it on the spot if it isn't.

Sources

• The AA: How to use jump leads
• Varta: Jump start a car step by step
• RAC: How to jump start a car
• Toyota: Prius emergency response guide (PDF)
• Khaleej Times: Dubai Police advice on stopping on roads