How to jump-start a car

The humble car battery really doesn't get the love it deserves. Without it we'd be thrown back to a time when the world was black and white, you started your car with a cranking handle and plenty of elbow grease, and though autonomous driving did exist, it was powered by some brekky, not leccy.

But just as the battery in your laptop or smartphone eventually loses its mojo, so does your car battery. And unless it's fighting fit, it won't be able to supply enough juice for your starter motor.

It's at this point you'll then need to borrow a boost from a healthy car via the lifeline of jumper cables.

These are simply a pair of heavy duty cables with a chunky crocodile clip at each end. One cable carries the positive current, and the other negative. Once connected, you're then able to use the battery in the working car to recharge your dead battery just enough for it to start your ride.

Here's how to use jumper cables safely, and hopefully successfully. But first, let's review some of the precautions...

Precautions when jump-starting a car

  • Position the vehicles close enough so that the jumper cables will reach easily,  but, MAKE SURE THE VEHICLES DON’T TOUCH EACH OTHER!
  • Make sure the cables are not in the way of the fan, drive belts or any other moving parts when the vehicles are started.
  • Make sure the booster battery is the same voltage as the dead one in the vehicle - Nearly all modern car and truck batteries are 12 volts.
  • 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).
  • Turn off the lights and other electrical loads on the car with the dead battery, though if it is a modern car with a computer, it’s a good idea to turn on the heater blower motor which will minimize the damage an excessive electrical surge can cause.
  • It’s a good idea to wear safety goggles, as there is always a chance of an explosion and batteries are full of acid.

It’s all fairly straightforward stuff, but before you crack on, there are just a few more common-sense things to be aware of...

  • A tiny 1.0 hatchback isn’t going to jump start a Range Rover. You need cars of comparable size.
  • If it’s cold, it might take longer. Batteries perform slower when the temperature is down, so you might need to leave the cars running longer to get enough juice into your flat battery.
  • A jump start should be a rarity, not an everyday occurrence. If you’re having to jump your car three times a week, it might be time to think about getting a new battery fitted.
  • Jump packs are your friend. Modern times bring modern tech, and that means you can buy pocket-sized lithium-ion battery packs that will jump your car. They’re about fifty quid and are an absolute godsend.

Now we're ready to begin.

01 Does your car actually need a jump start?

A jump start isn't a magic cure that'll restore life to all motionless motors. If your car tries to start (you can hear the starter motor running normally, and it doesn't sound like its dying) but the engine won't fire up, your battery probably isn't to blame.

What we're looking for is no starter motor action when you turn they key – just the sound of silence, and possibly a Christmas tree of warning lights appearing on the dashboard.

02 Park the working car next to the dead car

Get both engine bays close to each other, but don't let the cars touch. Turn off both cars.

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

03 Identify the positive terminal

Usually your battery will be located in the engine bay and should resemble a fairly large, usually black plastic-cased box about the size of a normal toaster. On the top will be two terminals.

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.

It is absolutely, 100%, vitally and imperatively important you're sure which terminal is which, as connecting jumper cables incorrectly can blow fuses, fry your car's brain, and potentially even your own.

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

04 Connect one end of 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.

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

05 Connect the other end of the red jumper cable…

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

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

06 Connect one end of the black jumper cable…

… to the negative terminal on the working car’s battery. This is usually marked with a – (minus) symbol, but it should be obvious as it’ll be the only terminal left. Again, ensure the other end of the cable isn't contacting anything metal.

07 Nope, wrong!

07 Don't connect to the dead car's battery!

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, we don't want sparks.

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

08 Start the working car

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

09 Try starting your car

If it successfully fires up, leave it turned on and unclamp the cables in the reverse order to how they were attached. And though you're now likely to be pumped that you no longer need to get the bus to work, remember not to let those cable clamps touch each other as you unclamp them.

10 Got your motor running? Head out on the highway

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. A garage could be a good destination.

Unless there's an obvious explanation for this battery blip – like leaving your headlights on with the engine off – your battery could just be ripe for replacement. Car batteries only have a lifespan of around 4 years, with extreme winters and sweltering summers accelerating their deterioration.

Troubleshooting

Connected everything correctly, but your engine won't start? Try letting the working car idle for longer, or hold its engine at around 2000 rpm 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 has gone completely kaput and 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 12 is bad news).

Buying the right jumper cables

So it’s a good idea to keep a pair of jumper cables in your car, but not all cables are created equal. Decent jumper cables should have both length and girth.

The latter is preferable as a thicker cable can transfer more electrical current, thereby giving your dead battery a better boost. This is especially important if your car has a large engine or one equipped with start/stop technology, as both require a heavy duty battery.

Cable thickness can be graded by how much current (amperage) a cable is capable of transferring, or sometimes by the cross-sectional (mm2) area of the cable itself.

We’d recommend steering clear of cables with anything less than a 400-amp rating, but when you’re trying to start your car, you don’t want to worry about whether your jumper cables could be a weak link.

Consequently 500-amp cables should give total peace of mind, while anything beefier is overkill unless you plan on jump starting a Jumbo Jet.

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 literally in tight spot.

One last thing to remember though. The longer the cable, the thicker it’ll need to be in order to transfer the same current.

What order do you put jumper cables 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 remote mounted battery).
  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 and leave all switches off to prevent damage to the vehicle electrical system.
  8. Remove the jumper cables 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 being it back up to a full charge.

Remember, sometime in extreme cold conditions a perfectly functional car may need a boost to get started because batteries put out less energy the lower the temperature, while at the same time need more power to turn in the cold.