Car Advice

How to jumpstart a car…and should you bother?

The idea behind jumpstarting a car is to use a donor car’s battery to re-charge a flat battery via the use of jump leads…but if you own a post-2000 car you risk damaging your vehicle.

JUMPSTARTING A CAR might seem like a very simple thing to do but you need to pay attention to what you’re doing and, if you own a car built after 2000 then jump-starting it off your neighbour’s car could cause a whole mess of trouble. This guide is intended as an explanation only, if in doubt seek professional advice.

There’s nothing worse than coming out to start your car and hearing that dreaded click when you turn the key in the ignition or press the starter button. It means your car’s battery is flat. See, your battery is there just to start your car, once the starter motor has fired and the car is running then the alternator takes over it’s a generator converting mechanical energy into electricity) and it runs your vehicle’s electrics and provides a top-up charge for the battery, but the alternator can’t recharge a flat battery. And, so, despite what some people will tell you, going for a drive with a battery that’s on the way out won’t recharge it, it will just surface charge it and cause more damage to the battery. Moving on.

What is jumpstarting?

So, your battery is dead. While most people will likely reach for their phone and call for roadside assistance you can always knock on your neighbour’s door and ask if they can help get your car started; see, the idea behind jumpstarting a car is to use a donor battery to kick-start the flat battery.

Listen for warning signs

If your car battery is labouring when you turn the key in the ignition or press the starter button and is taking longer than normal to start then take this as a warning that the battery is struggling and may need to be replaced. If you’ve had your battery recharged via a battery charger but it continues to fail even after a short period of inactivity then there could be an issue with your car’s electrical system that will need to be diagnosed by a mechanic. And in a vehicle built after 2000 with all their processors then the chance of an electrical gremlin creeping in is even higher, and so is the risk of damage if you try and jumpstart the thing, but we’ll cover this shortly.

How to jumpstart a car (pre-2000)

Let’s consider your car is pre-2000, because jumpstarting an older car is a much safer prospect than one built after 2000 (and we’ll explain this in more detail shortly). To jumpstart your car you’ll need a willing helper with a car and battery in good working order (we’ll assume their car is a pre-2000 car too) and a set of jumper leads.

Jumper leads are two cables, red and black, with large alligator-type clips at either end with handles covered in plastic. The cables are usually very thick to handle the high current draw from the battery. Only the toothed clip should be exposed metal…if there are any cracks in your jumper leads then throw them out and buy a new set.

One jumper cable, the red one, will be marked with a ‘+’ and this should only ever be clipped to the positive terminals of the batteries. The black lead is marked with a ‘-‘ and is connected to the negative terminal on the battery or the vehicle’s earthing point.

First thing is to locate the batteries in both vehicles and position them so the leads will reach from one to the other. Once you’ve done that, remove any cover from the battery or the terminal covers, but don’t remove the leads from the battery terminals.

Starting with the donor car, attach the red cable (+) to the positive terminal on the working battery then connect the other end of the cable to the positive terminal on the flat battery. Then connect the black cable (-) to the negative terminal on the working battery and then the other end needs to be connected to a suitable earthing point on the vehicle with the flat battery. This can usually be a bare metal bolt or another type of solid metal component under the bonnet; look in the owner’s manual as it might explain where to find a suitable earth on your vehicle.

Once done, start the car with the good battery and let it run for a few minutes before cranking the car with the flat battery. Once you’ve started the car with the flat battery let them both run for a few minutes.

After the cars have been running for a five or so minutes you can start the disconnection process. This involves removing the cables in the reverse order you connected them, so: remove the negative cable from the earthing point and then the negative clip connected to the donor battery; then remove the red lead from the ‘flat’ battery and then from the donor battery. It’s vital when you’re removing the leads that the clips don’t touch one another.

What about jumpstarting a post-2000 car

Many car makers and most roadside assistance services will recommend not jumpstarting a car built after 2000. See, newer cars have more complex electrical systems with many more on-board computers than older cars do.

Everything from electric seats and powered tailgates to sat-nav, Bluetooth and Wi-Fi capable infotainment systems, as well as active safety systems like autonomous emergency braking have added a level of complexity to vehicles that just didn’t exist a decade ago or, in some cases, several years ago. Indeed, most modern cars these days have between 25-50 computers to control the various electrical systems and jumpstarting a battery can fry these computers.

Most auto accessories stores sell jumper leads or portable power packs with surge protection built-in, this surge protector acts like a fuse in your home’s fuse box and stops too large a current from reaching the flat battery and overwhelming the stricken vehicle’s electrical systems. But, even with surge protection, if in doubt, don’t go ahead with the jumpstarting because the potential risk to the car’s electrical system is huge and, in some cases, could be catastrophic.

Anything to watch out for?

Beyond knowing the age of your vehicle and having surge protected jumper leads or portable power pack, you should always sit down with your owner’s manual and check to see what it says about jumpstarting your car. Quite often the manual will warn against it.

Once your battery has been ‘jumped’ and your car is running check the dashboard for any warning lights as these will be evidence of damage to the car’s electrical system. And it’s worth checking in your car with your mechanic after a jumpstart to get them to check it over and potentially replace the battery.

Notify of
1 Comment
Newest Most Voted
Inline Feedbacks
View all comments
3 years ago

and more, make sure that the jumper cables are safe and straighten them out nicely. Just in case that the cables might be affected by the weather, so my advice is that they should be stored in a dry place, preferably in a toolbox to avoid corrosion (hahaha that’s why it is produced in a toolbox, right?) Before doing anything with the cables or particularly starting to jump start your car, remember that we are dealing with direct electrical contacts, so you don’t want your jumper cables frayed or your metal parts rusted for safety.

Isaac Bober

Isaac Bober