How to jumpstart your car

Carlton Boyce / 10 September 2019

Your car battery's gone flat, so you need to jumpstart it. All you need is some jump leads and some knowhow - and we've got the knowhow!



How to jumpstart a car - made simple

• Select jump leads (also known as booster cables) of the correct size.

• Connect the positive terminal on the battery of one car to the positive terminal of the other car using the red lead.

• Connect the negative terminal of the car with the good battery to an earthing point on the car that won’t start using the black lead.

• Start the car with the flat battery.

• Disconnect the red positive lead.

• Disconnect the black negative lead.

We’ve all done it; you forget to turn your car’s lights off when you park it, which means you come back to a car with a flat battery. Or perhaps you ignored the battery getting slower and slower over the previous few days and weeks, only to find that the first cold morning killed it completely. In either case, it generally happens at the most inconvenient time (but then, is there ever a convenient time to have to jumpstart your car?).

Of course, if you have breakdown cover then help is only a phone call away - but the first cold morning of the year will see a surge in callouts for the same thing, which means you’ll be kept waiting for longer than normal until help can arrive.

And anyway, wouldn’t you rather be completely self-sufficient and take care of the problem yourself? Of course you would, and here’s how you can do it…

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Buying a set of jump leads

Jump leads, or booster cables as they are sometimes known, are thick copper cables with a big crocodile clip on each end. They are used to connect the dead battery on your car to that of a car with a good battery. So far, so good.

The only snag is that jump leads need to be thick enough to be able to carry the necessary current, and a diesel engine or a car with a big petrol engine will need a lot of power to start it, so the sort of thin jump leads you see advertised for a fiver in your local petrol station or supermarket probably won’t be up to the job. At best, they won’t be thick enough to carry enough power from one battery to the next - and at worst they could melt and catch fire.

But please don’t worry, because finding the right size is easy. Simply go into somewhere like Halfords and ask them what set they would advise.

Free-thinkers, or those buying online, can look at the packaging for themselves and read what it says. At the time of writing, Halfords offers five different sizes, listed by engine size and they range in price from £10 a set all the way to £35. If your budget can stand it, I’d recommend buying the heaviest, thickest ones you can because they’ll last forever and can be used if/when you buy a bigger vehicle in the future.

Heavy duty booster leads also tend to be longer than cheap, thin ones, which makes the job much easier than it might otherwise be.

And don’t forget to keep them in your boot; they’ll be no use to you if they’re stored safely in your garage!

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Using your jump leads

Jump starting a car using your jump leads or booster cables is easy.

• Park the car with the good battery next to the one with the dud, or flat, battery. Most, but not all, cars have their batteries under the bonnet but it might be in the boot. Check first, and then park the cars next to each other so that the distance from one battery to the other is as short as possible.

• Make sure that the engine and ignition is turned off on both cars. Also, make sure that any electrical items like the radio are turned off too, and remove the keys from the ignition too to be on the safe side.

• Take a quick look at the duff battery. If it is damaged in any way, or leaking battery acid, then you must not try and start it. Call for help instead.

• The most important thing to remember is to connect the positive terminals of the dead car first. The positive battery terminal is often marked with a ‘+’ sign or might even be obscured with a red plastic cover. If there is a plastic cover, lift it away carefully and clip the crocodile clip on one end of the red lead to the positive battery terminal of the car with the flat battery.

• Then connect the other crocodile clip of the red lead to the positive terminal of the car with the good battery.

• Now attach one of the crocodile clips at the end of the black lead to the negative terminal of the battery with the car with the good battery.

• Next, clip the other black crocodile clip to a good earthing point on the car with the faulty or flat battery. A solid piece on the engine is usually your best bet.

• Wait for a few minutes (3-5 is plenty) and then start the car that is acting as the power source and then start the car with the flat battery.

• If it doesn’t start immediately, try a couple more times. If it still won’t start then there is probably another fault with your engine or electrical system and you will need to seek professional help.

• If it starts, then leave both cars running for five minutes or so. Then turn off both vehicles and take the keys out of the ignition.

• The next step is to disconnect the earth (black) lead from the car that wouldn’t start first, followed by the black lead from the jumper vehicle.

• Then disconnect the positive (red) lead from the car with the good battery and then remove it from the car with the (previously) flat battery.

• Start the car that had the problem. If you can, put the battery on a battery charger until it is fully charged. If you can’t do this immediately, then you should take it for a decent drive to put some more power into the battery from the car’s alternator.

• Finally, unless you have left your lights on then your battery will have failed for a reason. Batteries usually last for between five and seven years, and places like Halfords or your local garage will test it for you to see if it was a one-off or whether it is starting to deteriorate. Cars will often warn you that they’re having problems before they leave you stranded, and while none of us likes spending money unnecessarily, you’ll end up paying it at some point, so why not save yourself the hassle and inconvenience of another breakdown?

Mechanics translated

Alternatives to jumpstarting your car

If you want to be totally self-reliant then you might like to consider a small, portable powerpack. No bigger than a paperback book, these little marvels of modern technology pack a powerful punch and will start all but the largest diesel engines.

They will also charge phones, laptops and cameras, and I think they’re a great all-round investment; I’ve got a WorkshopPlus SmartBox and can’t recommend it highly enough.

Expect to pay around £60 upwards.



The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.