Flat battery? You're in good company - it's the most common cause of roadside breakdowns.

Using a set of jump leads could get you back on the road quickly. However, you need to ensure you understand how to use them safely, since any mistake could be dangerous, or may damage your vehicle's electronic components.

Read your car's handbook first

Disclaimer: This article provides a basic overview of how to jump start a car. Neither startrescue.co.uk nor Call Assist Ltd. will be held responsible for any damage that occurs to your vehicle as a result of reading this article. It is your responsibility to read the applicable section in your vehicle's handbook or, if you do not feel comfortable carrying out the jump start yourself, to enlist the help of a professional mechanic.

Batteries release flammable gases, so it’s crucial to remember:

  • If the battery appears to be damaged or leaking, do not jump-start the car.
  • If your jump leads are damaged, do not use them - find another set.
  • Ensure nobody smokes nearby while using the jump leads.
  • Ensure there are no naked flames nearby.
  • Ensure no metal objects touch the battery. Remove Jewellery, rings, watches etc. - indeed anything made of or containing metal.  If you touch either battery with a metal object it could cause a spark - or the battery may even explode.
  • Remove any loose clothing so it does not get caught in any moving parts.

How to connect the jump leads:

In order to jump-start your car you need a working car with a similar engine size to yours. Ensure the voltage of both is the same; most cars feature 12V batteries. You will also need a set of jump leads in good working condition. You should also wear a pair of safety goggles, in case of any sparks/explosions.

Park both cars near each other  

Open and secure the bonnets of each car, then ensure all electronics are switched off. The best way to guarantee this is by removing the keys from the respective ignitions. Ensure both handbrakes are on.

Next, attach one end of the red lead (positive +) to the positive terminal of the working car.

Now attach the other end of the red lead (positive +) to the positive terminal of the car with the flat battery.

Now attach one end of the black lead (negative - ) to the negative terminal of the working car.

Finally, you need to attach - or earth - the other end of the black lead (negative - ) to a piece of metal on the car with the flat battery. This might be a bolt or bracket on the engine. Some cars have a special earthing rod for this very purpose - check your manual/handbook. This earthing point must be away from the battery and fuel systems.

Ensure all leads are firmly attached and wait a few minutes for the voltages to equalise.

Next, start the engine of the working car and let it idle for a few minutes, return to the car with flat battery and start its engine too.

If it does not start, wait a little longer with the healthy car's engine running, before trying again.


How long to leave jump leads on

If the engine does start, leave the engines of both cars running for ten minutes.

Important: If the jump leads become hot, turn the engines off for a few minutes to let them cool off.

Once you have waited ten minutes, turn off both engines.

Next, you need to disconnect the jump leads in reverse order. Do this very carefully and ensure the clip teeth do not touch any part of the body work, or each other - because a loud bang is likely to result if this happens (a spark or even battery explosion).


Carefully disconnect the jump leads in the following order:

  • Black lead from earthing point
  • Black lead from the negative terminal
  • Red lead from healthy car on positive terminal
  • Red lead from newly-charged car's positive terminal


What to do next?

If your battery has run flat there could be a fault with the battery itself, the alternator, or some other component. Take your car to a reputable garage and they will be able to assist.


Replacing old batteries

To be on the safe side, it's a good idea to replace your battery once every three years. Batteries aged four or five years - or older - tend to be unreliable.