How to jump-start a car

Flat batteries always occur at the worst possible time. You're off to pick up the kids from school. You're running late for a business meeting. You need to catch a plane. But you've turned the key or pressed the 'start' button and… nothing. Your car battery is dead and you're going nowhere fast.

Don't panic. Depending on whether you've planned for such an incident, a few options may be available to you. You could use a set of jump leads and a donor vehicle to start your car. Or you may have what's knows as a jump starter (basically a power pack you use instead of a donor vehicle). Or, if your car has a manual gearbox and you have a flat-ish stretch of road ahead of you (and a spare person or two), the car can be push-started. Here, though, we're concentrating on using jump leads from either a donor car or a power pack.

11 things to watch out for when jump-starting a battery

  1. First, read your car's manual to see if there are specific instructions for your model. For example, some modern cars have two batteries – one that is used for starting and another to maintain the car's electronics. So you'll need to know which is which.
  2. Be wary when jump-starting cars with engine management systems – careless jump-starting, particularly if the leads are allowed to spark, can cause damage to the car’s electronic components. Some cars, such as certain BMWs, are fitted with special connectors to allow jump-starting, and on these models the jump leads should only be connected to the special terminals provided. Again, you'll find details in your manual.
  3. Jump-starting cables often allow only limited current flow, and some are designed to be used with engines of a particular capacity. Depending on how much the battery’s discharged, jumper connections may or may not start a vehicle with a weak battery.
  4. A 1.0-litre city car isn’t going to jump-start a Range Rover. You need cars of comparable size.
  5. Locate the flat battery and the donor vehicle's battery. Move the latter vehicle as close as you can to the flat battery so that the jumper cables will reach easily, but make sure the vehicles aren't touching (this is very important).
  6. Make sure the cables are not in the way of the fan, drive belts or any other moving parts for when the vehicles are started.
  7. Make sure the booster battery is the same voltage as the flat one - nearly all modern car batteries are 12 volts.
  8. Make sure the ignition switch is in the off position, and the transmission is in Neutral (manual) with the parking brake set, or Park (Automatic).
  9. Turn off the lights and other electrical loads on the car with the dead battery, but turn on the heater blower motor which will minimise the damage an excessive electrical surge can cause.
  10. Wear safety goggles – there's always a small chance of an explosion and batteries are full of acid.
  11. A jump pack may be more useful for future 'events'. These pocket-sized lithium-ion battery packs cost around 50 quid and are an absolute godsend when you can't find your jump leads or another vehicle.

Want to know how to jump-start a car? Let’s go…

03 Identify the dead car’s battery and its positive terminal

1 Which one’s the positive terminal?

Usually, your battery will be located in the engine bay, but it may be in the boot. There are two terminals (short stubs) on the top. The positive terminal is the one we want first and it'll likely be marked by a '+' (plus) symbol, it may have a red cable attached to it, or it could be covered by a protective flap, as shown above.

It is imperative that you're sure which terminal is which, because connecting jumper cables incorrectly can, at best, blow fuses and, at worst, destroy your car's computers, costing thousands to replace.

04 Connect one end of the red jumper cable to the positive terminal on the dead car's battery

2 Connect the red jumper cable…

…to the positive terminal on the dead car's battery. Make sure the clamp has a secure metal-to-metal connection, and check that the other end of the cable isn't about to contact another metal object, such as the car.

The second (black) jumper cable should also be well out of the way at this point to avoid possible short circuits.

05 Connect the other end of the red jumper cable to the positive terminal on the working car's battery

3 Connect the other end of the red jumper cable…

…to the positive terminal on the donor car's battery.

06 Connect one end of the black jumper cable to the negative terminal on the working car’s battery

4 Connect one end of the black jumper cable…

…to the negative terminal on the donor car’s battery, marked with a '-' (minus) symbol. Again, ensure the other end of the cable isn't contacting anything metal.

07 Nope, wrong!

5 Now leave the flat battery alone

The other end of the black jumper cable should NOT be connected to the dead car's battery. That would create sparks, and since a dead battery can leak flammable hydrogen gas, that's not advisable.

Instead, the fourth and final connection is made to a metal grounding point. Look for some solid, unpainted and un-chromed metal in the engine bay away from any moving parts such as fans or belts. A spot on your car's engine block is an ideal place to clamp on to.

6 Start the donor car

Allow it to run (at idle) for five minutes. This gives it time to supply some charge to the dead car's battery.

7 Try starting your car

If it fires up, leave the engine running and unclamp the cables in the reverse order to how they were attached, being careful to avoid them touching each other as you unclamp them.

8 Time to hit the road

It takes time for your resuscitated battery to be fully recharged by your engine's alternator, and the quickest way to generate juice is to go for a drive of at least 20 minutes. Just make sure you don't stall your car because you may not be able to restart it.

Unless there's an obvious explanation for this battery blip – such as leaving your headlights on with the engine off – your battery could just be ripe for replacement. Car batteries have a lifespan of around four years, with extreme winters and sweltering summers accelerating their deterioration.

When jump leads don’t work 

Connected everything correctly, but your engine won't start? Try letting the donor car idle for longer, or hold its engine at around 2000rpm so it can supply more charge to your car's dead battery.

Still nothing? Find an alternative grounding point in your car's engine bay for the black jumper cable.

And if that fails, it could be because your battery is refusing to accept a charge. If you've got a multimeter handy, you can check how much voltage the battery is outputting (12.6 volts or higher is optimal, anything less than 12V is bad news).

Which jump leads are best?

Not all cables are created equal. Decent jumper cables should have both length and girth. A thicker cable can transfer more electrical current, giving your dead battery a better boost. This is especially important if your car has a large engine or is equipped with engine start-stop technology, because both require a heavy-duty battery. The packaging will say what size engine the leads are suitable for.

As for length, 3 metres/10 feet is fine if both cars are parked close to each other, but 5-metre cables or longer could be useful if you’re stranded in a tight spot. However, the longer the cable, the thicker it’ll need to be in order to transfer the same current.

What order do you put jump leads on?

Place your jumper cable connections in EXACTLY this order:

  1. Connect the red jumper cable to the positive terminal of the dead battery (or jump start terminal on a car with a battery located elsewhere).
  2. Connect the opposite end to the positive terminal of the booster battery or jump start terminal.
  3. Connect the black jumper cable to the negative terminal of the booster battery or jump start terminal.
  4. Connect the other end of the black cable to a well grounded bolt or bracket on the engine block of the vehicle being jumped, not the battery itself. This prevents sparks near the battery which may cause an explosion.
  5. Start the engine of the vehicle with the good battery and let it run at a moderate speed to charge both batteries.
  6. Start the engine of the vehicle with the discharged battery.
  7. Reduce the engine speed to idle on both vehicles.
  8. Remove the jump leads in the reverse order they were attached, making sure to never touch the red and black terminals to each other.

If the charging system and the battery of the car which had to be jumped is in good condition, 30 minutes of driving should bring it back up to a full charge.