How to jump-start a car

How to get your vehicle moving when the battery is as flat as a pancake.

There’s nothing more frustrating than when you come out to your car on a cold, frosty morning, put the key in the ignition, turn it and… nothing, barring a dull-sounding ‘click’. Obviously, turning a key in the ignition is becoming a thing of the past, but even keyless-entry-and-go vehicles can be susceptible to a flat battery. Here’s how to get your car working again if the battery has no life left in it…

Jump-starting with jump leads and a second car

This is the main way to get your car working in the event of a flat battery. The most crucial things you will require are a set of jump leads, which are one red lead and one black lead (the colours don’t actually matter, but they make it less likely that you’ll make a mistake) with a pair of strong crocodile clips at each end of each cable (so four clips in total), and then a fully working car belonging to a friend, family member or a Good Samaritan of the general public.

With jump leads, you’ll usually find that they come as a ‘set’ with the red and black cables joined together for much of their length, so there’s no way you can lose just one of them, but there are also jump leads that are completely separate cables as well. You should find that a decent set of jump leads is no more than €20, so everyone should really buy a set and have them on hand (i.e., in the boot of their car) at all times.

Jump-starting can be a hazardous process, so follow these steps very carefully. You are dealing with a high-voltage battery here, so you can injure yourself if you do things wrong or. One final addendum: this process is assuming both cars involved (your car with the flat battery and the ‘donor-power’ vehicle) are combustion-engined vehicles, not electric cars.

  1. Make sure both cars are in neutral with their handbrakes on (if a manual gearbox) or in Park with their handbrakes on (if an automatic gearbox). They must also have their ignitions switched off completely.
  2. Open the bonnets on both cars and safely prop them open. Almost all combustion-engined cars in production have their batteries mounted in the engine bay, which is at the front of the car, although some high-performance vehicles will place the batteries elsewhere for weight distribution. However, the terminals for the battery are almost always in the engine bay or at the front of the car, so look there first.
  3. With the power still turned off on both cars, attach one end of the red ‘positive’ cable to the positive terminal (it is marked with a ‘+’ sign and it should have red colouring) on the flat battery of your car. Then attach the other end to the positive terminal on the working car’s battery. Whatever you do, DO NOT attach the positive battery terminal to the negative battery terminal from one car to the other.
  4. Select a piece of bare metal, somewhere away from the battery itself, in the engine bay of your flat-battery car and attach one end of the black ‘negative’ cable to it. This is the ‘earth’ for the car being jump-started; you do not need to attach the crocodile clip of the lead to the negative battery terminal (marked with a ‘–’ symbol and black colouring).
  5. Attach the other end of the black negative lead to the negative terminal of the battery on the car that is working.
  6. In all of the above, DO NOT cut corners and attempt to attach both the positive and negative cables at the same time. Follow the process above, step-by-step, otherwise you could create an electrical short that may do damage in one or other of the cars’ electrical systems.
  7. Once the leads are connected safely and correctly, get the friend, family member or helper who owns the car with the powered battery to start their car. If the flat battery is particularly flat, leave the car running for a few minutes to let it build up some ‘cranking’ charge in the flat battery of your car that needs jump-starting.
  8. Try to start your vehicle with the previously flat battery. As soon as you turn the ignition key in the barrel or press the start-stop button, if there’s power flooding through the system then instead of getting the customary ‘click’ and dead displays of a car with a flat battery, all of the dashboard should light up. Try and turn the engine over; if it still fails to start, wait a few more minutes with the other car still running, then try again. At most, you should try two or three times to start the car with a flat battery. If your battery refuses to accept charge after three attempts to juice it up, you may have to call your relevant rescue service to come and sort the problem professionally or replace the battery itself.
  9. If the car with the flat battery starts up, leave both engines running for a few more minutes  with the jump leads still connected. For safety, you should not try to disconnect the jump leads with either car’s engine running or its ignition on.
  10. Once a few minutes have passed, turn both cars completely off. Disconnect the jump leads in the reverse order you attached them: so take the negative crocodile clip off the car with the good battery first, then the negative clip off the bare metal in the engine bay of your car that has been successfully jump-started, then the positive clip off the car with the good battery and, finally, the positive clip off your car which has been jump-started. DO NOT allow the clips to come into contact with each other at any point during this process.
  11. Once all the clips are disconnected, leave the bonnet on your car open and try starting it again. If it fires up, close the bonnet, put your jump leads away and let the other person in their car get on their way. If it doesn’t fire up, repeat all of the process above again to connect the vehicles back up, allowing even longer charging time between the two cars (i.e., keep the jump leads connected and both engines running for longer this time around).
  12. Once your car with the flat battery fires up on its own safely, then unless it is already in either location, drive it to a place (home or a garage) where it will not be a problem if it fails to start again. Ideally, you should take the car to a garage and purchase a new battery for it, as a flat battery could be a sign that the battery in your vehicle is nearing the end of its life and is due for replacement. Alternatively, it could signal the fact that your alternator is not working properly; the alternator is the component in your car that charges the battery once the engine is running. Of course, you might just have had a flat battery because you left the lights on in your car all day (or night), in which case your battery should be OK to use for a longer period once it has been jump-started.
  13. However, a deep discharge on any battery is not good and will affect its life-span – so once you’ve got the vehicle running again, make sure you take it on a longer journey to allow the alternator to get some juice back into the battery pack, so that the car will start easily again once you come back to it the next time you need to use it. If, for any reason, you get the car jump-started and you begin driving, and it runs out of battery power again while driving and the engine dies, then this is a sure sign of alternator failure, rather than the battery being tired due to age, or it being discharged through simple forgetfulness of you leaving an electrical drain (lights, radio etc) switched on while the car was parked.

The jump-pack method

This is exactly the same as above, except – standing in for a friend or family member’s donor car with power – you have your own jump-pack. These are heavy-duty, portable and rechargeable battery packs with two jump leads (one red, one black) with crocodile clips attached. Instead of needing another car, you simply connect the jump-pack’s positive clip to your positive battery terminal and the negative clip to a section of bare metal away from the battery; yes, it’s the same process here, just without a second car involved.

These battery packs will have different ratings, according to the size of engine you are trying to jump start – bigger cars with larger, more powerful engines require more cranking power and so require more potent jump-packs. Check the specifications of the jump-pack you are purchasing carefully against your own car and its battery specs to ensure it will work. Also, and this may sound obvious but, make sure the jump-pack itself has been fully charged on a mains terminal before you need to use it – if it has no or little charge itself, it is not going to jump-start your defunct car. These jump-packs have clear indicator gauges on them, showing how much charge they have in them, so you will know before you hook it up to your vehicle if it is going to work or not.

Incidentally, the reason you don’t attach the negative clip to the negative terminal of a car being jump-started is that hydrogen gas can build up above the battery itself, and hydrogen is explosive. By attaching the negative to a bare-metal ‘earthing’ patch away from the battery, you vastly reduce the risk of sparks and a possible explosion in your engine bay.

The bump-start

This is not recommended unless it’s a particularly dire situation or you need to be somewhere in an emergency. If you don’t have jump leads and you don’t have a jump-pack, and there are no other cars around with jump leads and a working battery, then you can try a bump-start. First things first, this will only ever work on manual cars – so if you have an automatic gearbox, forget about this method.

For it to work, you ideally need yourself and (at least) a second person to push the car, or (at the absolute outside chance) if you’re on a downhill gradient then you can get this to work on your own. Put the car into second gear, then turn the ignition to ‘ON’, even if there’s absolutely no life in the battery. If you’re on a downhill, release the handbrake and stay off the brake pedal (although cover it with your right foot), and – keeping your left foot on the clutch pedal – let the car roll until it is travelling at a speed of approximately 20km/h or so; if someone is pushing, get them to push and then, once the car is moving, to run as fast as they can while still pushing the back of the car; this is why, if you have a particularly big or heavy car, you might need more than one assistant because, otherwise, they will not have the strength to push the vehicle at a fast enough speed that will allow for bump-starting.

Once you attain a suitable speed and it is safe to do so (i.e., you have enough road to complete this manoeuvre), release the clutch pedal abruptly. If you have enough speed built up, the car should jolt into life (hence the ‘bump’ of bump-start) and the engine will be running. Provided you have enough road or the person/people pushing the vehicle have enough strength and the wherewithal to do it, if it does not work the first time then you can repeat the bump-start process to try and get the car to fire up. If the engine does start, do not let the car stall immediately, so once the engine is running then you must quickly depress the clutch pedal again to keep it active. Put the car into neutral, apply the handbrake and leave the engine running, and – if someone pushed you – let them climb, exhausted, back into the passenger seat(s). Once again, head to the nearest garage and get the car checked over, to see if there’s a problem with the battery, its electrical systems or the alternator, accordingly.