Jump leads to jump start a car

Jump starting a car is something that almost all of us will have to do one day, and there are several important rules to follow when jump starting a car using jump leads. In this walkthrough from Haynes experts, we focus on how to jump start a car with leads connected to another car's battery rather than with a portable battery pack. Let's start with some key things to know about this method of getting charge back in your car's battery.

Jump start a car: important things to know

  1. First, read your car's manual to see if there are specific jump starting instructions for your model. For example, some 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. 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 is discharged, jump leads 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 – 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 – batteries are full of acid.
  11. A jump pack may be more useful for future 'events'. These pocket-sized lithium-ion battery packs cost from around 50 quid and are an absolute godsend when you can't find your jump leads or another vehicle.

What jump leads do I need?

Not all leads are created equal for the purpose of jump starting a car. Decent jump leads 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.

Locating a car battery's positive terminal

Jump start a car in 8 easy steps:

1. How do I find the positive terminal for my jump lead?

With the cars parked next to each other and the engines turned off (see above), we first need to connect one end of the red jump lead to the flat battery. Your car's battery will usually be in the engine bay, but it may be in the boot. There are two terminals (short stubs) on the top of it. 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 to know which terminal is which, because connecting jumper cables incorrectly will result in damage to your car's electrical systems.

A red jump lead being connected to the positive terminal on a car battery

2. How do I connect the red lead to the positive terminal?

All jump lead 'jaws' are spring-loaded. You squeeze the handles to open the jaws releasing the handles clamps the jaws tightly around whatever they're in contact with – in this case the positive terminal. Make sure the jaws have 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) jump lead should also be well out of the way at this point to avoid possible short circuits.

Connecting the other end of the red jump lead to the positive terminal on the donor car battery

3. Where does the other end of the red jump lead go?

Connect the other end of the red positive lead to the donor car's battery. Hopefully by this stage you've already established whether the car with the healthy battery has its power source under the bonnet or in the boot. Don't forget to make sure the cars aren't touching.

Connecting the black jump lead to the negative terminal on the donor car’s battery

4. Which car should the black jump lead connect to first?

The first black lead connection should be made to the negative terminal on the donor car’s battery. You should have already checked which terminal was which before connecting the red lead, but the negative terminal is marked with a '-' (minus) symbol. Again, ensure the other end of the cable isn't contacting anything metal.

Connecting the black jump lead to a car's grounding point

5. Why shouldn’t the other end of the black jump lead connect to the dead battery?

The other end of the black jump lead 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. Which car should be started first?

With all of the leads connected, wait a few minutes before starting the donor car's engine. Allow it to run (at idle) for five minutes. This gives it time to supply some charge to the dead car's battery.

7. Can I jump start my car now?

Yes. 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. Is that it? Can I switch off now?

No! It takes time for your car's flat 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. If you've got a multimeter handy, check how much voltage the battery is outputting (12.6 volts or higher is optimal, anything less than 12V is bad news).

Quick guide for how to jump-start a car:

Connect your jump leads in this order:

  1. Connect the red jump lead 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 jump lead 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 are in good condition, 20-30 minutes of driving should bring it back up to a full charge.