How to jump start a dead battery

Jump start

Flat batteries are the most common cause of breakdowns. Jump starting is the easiest technique to start a car with a dead battery. This is where you use the power of another car to charge a second vehicle’s dead battery.

Always check your car’s handbook before doing this: some manufacturers recommend their cars aren’t jump started because it can damage the engine management system.

Safety first

If you’re confident jump starting won’t harm either car, follow these basic safety rules:

  • Manoeuvre the ‘working’ car so that it’s close to but not touching the ‘dead’ car. Ensure both cars are parked safely with parking brake on and ignition switched off
  • Don’t attempt to jump start a battery that is damaged or leaking
  • Don’t use anything other than proper jump leads and make sure these are in good condition
  • Keep all metal objects including watch straps and belt buckles away from exposed battery posts
  • Ensure you’re not wearing anything that might catch an engine’s moving parts
  • Ensure the cars are the same voltage (Most are 12-volt)
  • No smoking!

Switch off anything that draws power from the battery on both cars. That’s lights, ventilation, radio, windscreen wipers or phone chargers and portable sat nav systems. This will stop the battery draining further and prevent damage if there’s a power surge.

Find the battery: if you don’t know where it is, check the vehicle handbook for its location; the battery of some cars can be in the boot. You can then join the cars. Simon Smith from Shield Batteries explains: “You should connect the batteries in the following sequence to avoid a possible power surge through the car’s alternator.”

  1. Red lead connected to the positive (+) terminal of the dead battery
  2. Red lead connected to the positive terminal of the working car
  3. Black lead connected to the negative (-) terminal of the healthy car
  4. Black lead attached to a bolt or other piece of bare metal on the disabled car’s chassis or engine, away from the battery and any obvious parts of the fuel system

When two become one

Smith adds: “You are essentially combining the voltage of the two batteries so leave them connected for a few minutes before firing up the healthy car so the working battery can charge the flat battery.” When you start the working car, run it for at least three minutes to give the disabled battery a further charge.

Now try the dead car. The battery should be sufficiently charged for it to start. Leave the two cars connected and running at a little more than idling speed for about 10 minutes. This will hopefully allow the dead car’s battery to charge fully.

Switch off the working car and remove the leads as follows:

  1. Black lead from the previously dead car
  2. Black lead from the healthy car
  3. Red lead from the working car
  4. Red lead from the dead car

Leave the previously disabled car running for at least 20 minutes, or better still, go for a short drive, then see if it’ll start on its own. If it won’t, check with a professional: the car may have alternator problems preventing the battery from charging correctly.

If you’re unsure of any of the above process, seek professional advice.

Knowing how to jump start your car is a great skill to have. To learn about more car maintenance tips, you can check out our blog here.

3 comments on “How to jump start a dead battery

  1. Michael davidson 27/02/2016 7:05 AM

    Can I disconnect a flat battery and use jump leads from a working car battery to recharge flat battery

  2. allan 28/02/2017 3:01 PM


  3. Saleh 24/07/2023 6:06 AM

    This article has undoubtedly empowered me with the knowledge and confidence to handle battery-related emergencies with ease. Thank you, GreenFlag, for delivering yet another outstanding piece that sets a benchmark for automotive guides!

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