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How to use jump leads

AA Rescue safety advice

Flat batteries are the single most common breakdown fault attended to by AA Patrols, accounting for 30% of breakdowns.  Most of the battery-related breakdowns attended by AA breakdown services are due to faulty or old car batteries in need of replacement.  Other driver error like a vehicle’s head lights left on, faulty car door switches, using electronics with the car’s engine off and cars being left idle also lead to flat batteries.

While it’s a common misconception that jump starting your car is easy, drivers should remember that delving under the car bonnet to correct vehicle faults can be dangerous. We advise motorists to seek assistance from a professional breakdown service like AA Rescue or bring your car to a reliable garage.

Here AA Patrol, Paddy Finn, shares important dos and don’ts of jump starting your car.

Always check the vehicle handbook. All vehicle handbooks include general advice on using jump leads however some include model specific procedures. If this is the case drivers must follow the car manufacturer's procedure rather than the below guidelines.

Jump starting a vehicle when done correctly is a safe procedure however it can be dangerous if performed incorrectly.  To avoid damaging your car or putting yourself at risk, motorists should follow the below guidelines.

Jump starting your car – safety rules

  • Drivers should keep metal objects away from the top of the car battery – rings, watch straps, hand tools etc. just brushing a battery post can cause a massive spark, possibly exploding the car’s battery and releasing the acid.
  • Drivers should never attempt to jump-start a car battery that is leaking or looks damaged – an explosion could result.
  • Drivers should avoid smoking or naked flames – car batteries give off flammable gases and an explosion could result.
  • Drivers should keep their hands well away and avoid loose fitting clothing – with the vehicle’s engine running it's easy to get caught-up and seriously injured on moving parts.
  • Jump leads must be in good condition – damaged conductors or clamps can result in overheating and even fire.

Step by step

Before connecting leads to your car make sure the vehicles are the same voltage and parked with their handbrakes on and ignition off. The vehicles must not touch as this can cause sparks or an explosion.

  • Use the red jump lead to connect the positive terminal of the donor vehicle's good battery to the positive terminal of the flat battery.
  • Then use the black lead to connect the negative terminal of the good car battery to a suitable earthing point on the engine or chassis of the other vehicle. This earthing point must be away from the battery and vehicle’s fuel system.
  • With both leads connected wait three minutes for the voltages to equalize before starting either car’s engine.
  • Start the engine of the donor car and allow it to run for a minute, then while still running, start the engine of the other car and leave both running at a fast idle for ten minutes. Don’t remove the jump leads while the cars’ engines are running as this can cause serious damage to the electronics on either car. 
    If the jump leads get hot, avoid a possible fire by switching off both vehicles’ engines and allow the leads to cool.
  • Turn off the ignition on both cars and then disconnect the leads carefully in the reverse order to the way they were connected to the cars. Be careful not to touch the clips against each other or the car bodywork.
  • Start the car that had the dead battery using its own battery power. If it won't start there could be a more serious problem that’ll need investigating by a professional.

If the car battery can’t be jump started

Motorists who discover that their car battery can’t be jump started or who have accidentally done damage by using the incorrect jump start method can avail of AA Battery Assist where AA mobile battery replacement technicians, who have an extensive range of car batteries, will come to you and fit your car with a brand new battery.

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