How to safely jump-start your vehicle

The 1-2-3 of jump-starting your car

car jump start
dead car battery
flat car battery

Why do car batteries fail in winter? That is a question frequently asked of motoring journalists, and the answer is found not so much in cold weather being the root cause, but rather in how the battery copes with warm temperatures experienced during the preceding summer.


What happens?

Heat quickens corrosion of internal plates and causes electrolytes to vaporize faster. Damage done during the summer months does not manifest until the battery begins to work harder as winter sets in. A battery operating in cold conditions has reduced cranking power, and a drop in temperature also thickens motor oil, making it harder for the unit to turn the engine.


During balmy weather modern electrical and fuel systems can help to mask a weak battery by starting an engine with a minimum of cranking but, when winter arrives, the compromised power source no longer finds itself able to supply enough charge to overcome the effects of the drop in temperature.


If, on a frosty morning, you find that your car will not start because the battery is dead, it is most likely suffering the consequences of the stresses it suffered during the long, hot days of summer.

And that brings us to another problem – how to safely jump-start your vehicle, since it can be a dangerous exercise if step-by-step procedures and precautions are not followed.


Important tips

First thing to know is that the jump-start guidelines in this article relate to vehicles that have electrical systems that are negatively earthed – which is by far the majority of today’s cars. However, be aware that some older vehicles were built with positive earths, in which case these instructions are superfluous. 


Also, take into account that not all modern cars can be jump-started without risking the vehicle’s warranty. If either vehicle involved in the process has an electronic ignition system, for instance, use of jumper cables may cause damage to components and may not be covered by the manufacturer.


Another thing – you might not even know where to find the battery in your car since a number of automakers no longer position it under the bonnet. So, before hauling out the cables, read your owner’s manual carefully – it will tell you where the battery is situated and if specific procedures need to be followed when connecting cables, or whether jump-starts are permitted at all.


Tip: When working, wear protective clothing, eyewear and gloves – vehicle batteries contain sulphuric acid and other harmful chemicals. And there is always the danger of electrical sparks which could cause a fire.  


Follow these guidelines to ensure a safe jump-start:

  • You will need to bring the vehicle that is supplying the donor charge close to the one that is being jump-started. Make sure the vehicles are not touching but are parked close enough together for the cables to reach each of the batteries. Check that the vehicles’ transmissions are in neutral and that their parking brakes are on.


  • Switch off all electronic systems in each car – the ignition, radio, air-conditioning, interior and exterior lights, windscreen wipers and any other accessories that could drain the battery. Then, in order to hook up the cables in the right sequence, begin by identifying the terminals on the dead battery.


  • The positive terminal will be marked with a plus sign (+) and will usually have a red cable attached to it. The negative terminal will be marked with a minus sign (-) and will usually have a black cable attached to it.  


  • While checking the terminals, inspect the dead battery for cracks and leaks. If you discover any damage, do not attempt to jump-start the car. However, if the battery casing looks to be sound, get out your jumper cables and unfurl them. Good quality jumper cables are usually colour coded like battery terminals and feature heavy-duty clamps at each end.


  • Connect the clamps one by one in this sequence: Attach a red clamp (+) to the positive (+) terminal of the dead battery. Make sure the connection is properly secured to enable good contact, and that no part of the clamp touches the car.


  • Connect the other end of the red cable to the live battery’s positive (+) post. Again, make sure the clamp is properly and safely secured. Similarly, connect the end of the black cable to the live battery’s negative (-) post, then clamp the remaining black end to the bolt on the chassis or engine where the black cable from the negative terminal is secured. You may see a small spark when you connect the clamp.

Tip: Be warned - If you connect the black cable to the negative (-) post of the dead battery, any spark generated runs the risk of igniting any gases which may be venting from the casing.


  • Make sure the cables do not dangle into the engine compartments of either car, where they could be exposed to moving parts, then start the donor car’s motor.


  • Let it idle for a few minutes, before revving it slightly for about a minute. The procedure is aimed at putting some charge into the dead battery, since most of the current the starter motor will draw to crank the engine – in excess of 100 amps – will come from that source, not from the cables.


  • Try to start the disabled vehicle. If it doesn’t crank, turn off the ignition, then turn off the ignition in the donor car. Disconnect the last cable you attached – the black one to the securing bolt on the dead car’s chassis – then slightly twist or wiggle each of the other clamps to help ensure good electrical connections. Reconnect the cable you removed and restart the donor car once more.


  • Allow another five minutes for charging before attempting to start the disabled vehicle. If the engine refuses to start after a few tries, you will need to call a service provider to have the battery replaced – it’s probably reached the end of its life.


  • However, if the car starts, the jumper cables need to be removed in reverse order. Switch off the engine of the donor car, but leave the engine of the disabled car running. Disconnect the black clamp from the grounded metal on the dead car’s chassis, then disconnect the black clamp from the negative (-) terminal of the good battery.


  • During this process, do not let any of the clamps touch each other, or dangle into the engine compartments. Disconnect the red clamp from the positive (+) terminal of the good battery and, finally, disconnect the last red clamp from the positive (+) terminal of the dead battery.


  • Keep the engine of the disabled car running slightly above idle – at about 2 000 rpm, if the vehicle has a rev counter – for about five minutes, then revert to idle for at least 20 minutes before turning it off. Alternatively, drive the car immediately for at least a similar period.


  • In both cases you will be attempting to give the battery enough charge for a successful restart. If it fails to respond, you will probably need to have it replaced and have the vehicle’s alternator or generator checked for faults.


Read "Tips to avoid getting stranded this winter with a flat car battery" here.