How do I jump start my car safely?

We received an e-mail from Mary who went on holiday for a month and her battery was flat. She wanted to know how to jump start her car correctly.
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Q: Hello. I bought a BMW X1 at the start of this year and after going on holiday for a little over a month, my battery is flat. Firstly, how do I jump start my car properly? And, can I just buy any jump starting cable to do this?

A: Jump starting a car isn't as easy or safe as some people assume it is. There is a very real potential to incorrectly connect two vehicles using jumper cables and cause degradation of electrical components or injury.

Firstly, let's have a look at first principles: An electric charge is required to operate a starter motor, which basically moves engine components to a point where a fuel can combust to create sustainable internal combustion. A battery's primary use is to provide this charge to the starter motor and generally operate headlights and auxiliaries.

When a battery is going flat, it can sometimes operate auxiliaries, but doesn't have enough charge to operate the starter motor. Then, when the battery is totally flat, it won't operate auxiliaries or the starter motor. You can tell that your battery is flat by using a simple voltmeter across its terminals, or by observing a clicking sound when you try and start the car.


So, the idea behind jump starting a car is to connect a charged battery from a donor car in parallel with the flat battery using jumper cables. This process may seem straight forward and easy, but simple mistakes can cause injury, or can fatigue or destroy electrical components.

When purchasing a set of jumper cables, it's important to make sure they come with a built in surge protector. It's also important to ensure they are suitably rated for jump starting your particular vehicle. As a rule of thumb, 600A jumper cables will be capable of jump starting most passenger vehicles with eight cylinders or less. You can check this with your local mechanic or automotive store if you are unsure.


It's importing to follow the steps below in this exact order:

- Purchase suitable jump starting cables. These will look like electrical cords and come with alligator clips at both ends.

- Park the donor vehicle close enough to the receiving vehicle so that you can stretch the jumper cables between the two cars.

- Turn all lights, radios and accessories off on both vehicles.

- Find and identify the positive (+) and negative (-) terminals on both batteries.

- Connect your first jumper cable to the positive (+) terminal of the receiving car battery (the vehicle with the dead battery) and then connect the other end of the first jumper cable to the positive (+) terminal of the donor vehicle (the vehicle with the working battery).

- Connect your second jumper cable to the negative (-) terminal of the donor car battery and then connect the other end of the second jumper cable to a bolt on the engine, or a clear metallic part of the engine bay.

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- Before you go any further, double and triple check that the jumper cables are clear of any moving parts in both engine bays, this is absolutely critical.

- Start the donor vehicle and allow it to run for around 3 minutes. This process allows the dead battery to receive some initial charge.

- While the donor vehicle is still running, attempt to start the vehicle with the dead battery. The engine may crank for slightly longer, or it may sound somewhat laboured when this happens — this is normal.

- Once the car is started, it's a good idea to turn on the accessories and headlights of both vehicles. This process will reduce the chance of voltage spikes once cables are removed.


- Remove the jumper cables in reverse order. So, disconnect the second jumper cable hooked up to the engine or metallic component from the car with the recipient car and then disconnect the other end of the second jumper cable from the negative (-) terminal of the donor car. Then, disconnect the positive (+) terminal of the first jumper cable from the donor car and finally disconnect the other end of the first jumper cable from the recipient car.

- It's incredibly important to not switch off the recipient car now. If you do, the car will not start again. You need to drive the vehicle for at least 30 minutes, preferably at a constant speed, to allow the alternator to charge the battery so that the next time you stop, the levels are topped up again.

Hopefully that answers your question, Mary!