A flat 12V battery can turn a normal morning into a nuisance, but a jump-start is not a job to rush. Connect the leads in the wrong order, use a damaged battery, or treat a hybrid like an older petrol car and the problem can become bigger and more expensive.

For a conventional 12V petrol or diesel car, the safe sequence is simple: red to the flat battery positive terminal, red to the donor battery positive terminal, black to the donor battery negative terminal, then black to a solid earth point on the disabled car. The final black connection goes away from the flat battery, not to its negative terminal, unless the vehicle handbook specifically directs otherwise.

Before you connect anything

First, check the handbook for both vehicles. Modern cars can have designated jump-start terminals, batteries hidden under trim, battery-monitoring sensors or manufacturer-specific instructions. Follow those instructions over a generic guide.

Only proceed when both vehicles have compatible 12V systems. Do not use a hybrid or electric vehicle as the donor car unless its manufacturer expressly says that you can. The RAC makes the same distinction in its jump-start guidance.

  • Park the cars close enough for the leads to reach, but do not let the vehicles touch.
  • Apply both parking brakes. Put manuals in neutral and automatics in Park.
  • Switch off ignition, lights, radio, heated screens and other electrical loads in both cars.
  • Keep the leads clear of fans, belts and other moving parts.
  • Take off rings, watches and other metal jewellery before working near the terminals.

Stop before starting if either 12V battery is cracked, leaking, visibly swollen, badly corroded or appears frozen. Do not try to jump-start a car with damaged leads either. A battery can produce flammable gas, while a damaged case or poor connection raises the risk of sparks and acid exposure. Call breakdown assistance instead.

The correct jump-lead order

Use good-quality, insulated jump leads with clean clamps. Keep the red and black clamps from touching each other at any point.

  1. Red to flat positive: connect one red clamp to the positive (+) terminal or designated positive jump point on the car with the flat battery.
  2. Red to donor positive: connect the other red clamp to the positive (+) terminal or jump point on the donor vehicle.
  3. Black to donor negative: connect one black clamp to the negative (-) terminal on the donor vehicle.
  4. Black to earth: connect the final black clamp to a clean, unpainted metal earth point on the disabled vehicle, well away from its battery and fuel system. A substantial engine or chassis earth point is often specified in the handbook.

That last connection is the one people most often get wrong. Connecting it to an earth point rather than directly to the discharged battery helps keep any spark away from the battery. If you cannot identify a suitable earth point, stop and consult the handbook or get help rather than improvising.

Starting the car without abusing the starter

Start the donor vehicle and let it idle briefly. Then try the disabled car. Avoid long, repeated cranking: the RAC advises no more than around five seconds at a time. If it does not start, wait a couple of minutes before one more short attempt.

Do not rev the donor engine hard to force a result. If the car still will not start after a sensible attempt, there may be a poor connection, a battery that has failed, a starter fault or another electrical issue. More attempts can overheat the starter or create a new problem. This is the point to call a breakdown service or a mobile mechanic.

Disconnect the leads in reverse order

Once the previously flat car is running, leave it running and remove the leads carefully in the reverse order:

  1. Remove the black clamp from the earth point on the revived car.
  2. Remove the black clamp from the donor battery negative terminal.
  3. Remove the red clamp from the donor battery positive terminal.
  4. Remove the red clamp from the revived car positive terminal.

Keep the clamps apart as you remove them. Refit any terminal covers. If a warning light remains on, or the car immediately struggles again, do not assume a short drive has fixed the underlying fault.

What to do after a successful jump-start

A jump-start gets the engine running. It does not diagnose why the battery went flat or guarantee that the battery is now healthy. Give the car a proper run, ideally a continuous drive rather than a few minutes at idle, then deal with the cause.

Common causes include an ageing battery, a light or accessory left on, lots of short journeys, loose or corroded terminals, or a charging-system fault. If the problem repeats, read our guide to why a car battery keeps going flat before spending money on a replacement. A smart charger can be useful when a healthy battery has simply been run down, but it is not a cure for a defective alternator or a battery at the end of its life. Our home battery charger guide explains what to look for.

Hybrids and electric cars: do not assume the rules are identical

Hybrids and EVs still have a 12V battery, and it is that battery, not the high-voltage traction battery, that is relevant to a no-start or no-ready situation. But the location of terminals and permitted procedure vary considerably by model. Some vehicles provide clearly marked under-bonnet jump points; others have specific instructions about an external jump pack.

Never touch orange high-voltage cables, traction-battery components or damaged wiring. The DVSA MOT inspection manual warns that damaged high-voltage components can pose a fatal electric-shock risk. If there is any visible damage, leakage, smoke, an impact near the battery area, or uncertainty about the procedure, do not attempt a jump-start. Contact the manufacturer, a recovery provider or qualified assistance.

Can you use a portable jump starter?

Yes, provided it is suitable for your car and you follow both the pack and vehicle instructions. A lithium jump pack removes the need for a donor vehicle, but it does not remove the need for care. Confirm that it is a 12V unit, check its clamps and charge level, and use the same connection logic: positive to the vehicle positive point, negative to the approved earth point where the handbook says to use one.

Do not buy on peak-amp claims alone. Check that the pack is intended for your engine size and fuel type, has reverse-polarity protection, and can be stored and maintained as its maker requires. A compact pack is useful contingency equipment, not a substitute for investigating a recurring flat battery.

When not to jump-start a car

  • The battery or cables are damaged, leaking, swollen, heavily corroded or frozen.
  • You do not know whether the vehicles’ 12V systems are compatible.
  • The donor vehicle is a hybrid or EV and its handbook does not explicitly allow donor use.
  • The disabled car is a hybrid or EV and you cannot confirm its approved 12V jump points and method.
  • The car has been in a collision, has damaged wiring, smells of fuel, or you see smoke.
  • The engine repeatedly refuses to start after correct, short attempts.

Quick checklist

Red flat positive, red donor positive, black donor negative, black disabled-car earth. Start gently, disconnect in reverse, then investigate why the battery went flat. If the handbook disagrees with any part of that sequence, the handbook wins.

A cautious 10-minute wait for professional help is cheaper than damaged electrics, a ruined battery or an injury. For a battery warning light while driving rather than a simple flat battery, use our separate guide on what to do when the battery warning light comes on.