How to use jumper leads correctly

The humble car battery really doesn't get the love it deserves. It sits there in high temperatures and sometimes cold, and we expect it to power the starter motor time after time without fail.

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

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.

How to use jumper leads?

1. 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.
2. 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.
3. Identify the dead car’s battery and its 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 very important you're sure which terminal is which, as connecting jumper leads incorrectly can blow fuses, fry your car's brain, and potentially even your own.
4. Connect one end of the red jumper lead 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 lead should also be well out of the way at this point to avoid possible short circuits.
5. Connect the other end of the red jumper lead to the positive terminal on the working car's battery
6. Connect one end of the black jumper lead 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.
7. Nope, wrong. The other end of the black jumper 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, 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 ideal place to clamp on to.
8. 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.
9. Try starting your car. If it successfully fires up, leave it turned on and unclamp the leads 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. You've got your motor running, now 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 four years, with extreme winters and sweltering summers accelerating their deterioration.

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Why won’t my car start?

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 lead.

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).

Which jumper leads are right for my car?

So it’s a good idea to keep a pair of jumper leads in your car, but not all cables are created equal. Good jumper leads 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 leads 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 jumper lead length, three metres is fine if both cars are parked close to each other, but five-metre leads 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.