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How to jump start your car safely

By Jade Harding

Jump starting a car with a flat battery using jump leads is really simple. Just follow our 10-step guide and you'll be back on the road quickly – and safely

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A flat battery is normally to blame when your car won’t start. Instead of calling a breakdown firm, you can fix it yourself. Grab a set of jump leads and look out for a helpful driver with an engine at least the same size, or larger than yours. 

Follow our short guide on jump starting your car – or watch the video above – to get you and your car back on the road swiftly.

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Not the first – or second – time you’ve jump started your car this year? Maybe it’s time for an upgrade. Here's our round-up of the best cars.

Jump start a car using jump leads  

  1. Grab your jump leads and find a vehicle with the same engine size or larger than yours
  2. Park so the batteries are as close as possible and ensure both cars' engines, lights and radios are switched off
  3. Attach the red lead to the positive terminals of each battery – live car first
  4. Next, attach the black lead to the negative terminal on the live car
  5. Then to the negative terminal or earthing point on the dead car
  6. Start the working car and let it run for a couple of minutes
  7. Try to start the dead car. If you can’t, leave it for a few more minutes
  8. When you start the dead car, leave both to idle for 10 minutes with the leads attached
  9. Turn off both cars and remove the leads in the exact reverse order, starting with the negative lead from the previously dead car
  10. Now restart your car.

Doing a short journey and turning the car off again may mean you’re unable to restart, as the alternator hasn’t had enough time to sufficiently charge the battery.

 

Jump start a flat battery safely 

Jump starting your car can be hazardous if you don’t follow these basic safety precautions:

  • Always check your owner’s manual before doing anything
  • No smoking. Sparks near a battery can cause an explosion
  • Make sure both engines are off to begin with, and always set the parking brakes
  • Don’t jump a battery (or leads) that appear damaged or leaking. Call a professional
  • Remove jewellery such as rings and bracelets. Metal objects coming in contact with the battery terminals could cause a spark
  • Avoid wearing loose-fitting clothing that may get caught in moving engine parts
  • Never jump start a battery if fluids are frozen. This could lead to an explosion.

Don’t cut corners when it comes to safety. If you don’t feel confident jump starting your car, call a mechanic. 

Use Which? Trusted Traders to find a reliable mechanic you can trust. 

Only mechanics that meet our standards get our Which? Trusted Traders endorsement. All Which? Trusted Traders go through an assessment process that includes credit and customer reference checks, and a visit from a qualified assessor. Once Trusted Trader status is awarded, businesses are reassessed on an ongoing basis.

Do you have a dead battery? 

Here are some quick signs that your car battery is flat:

  • The starter motor won’t turn over on ignition
  • Interior electronics and exterior lights won’t work or are very dim
  • The car has problems starting on cold mornings
  • You’ve already had to jump-start it to get going.

A flat battery's one of the most likely culprits when your car won’t start. But there are other issues, such as a broken alternator or a defective fuel gauge, that could stop it from firing up. 

If the car still won’t start after you’ve tried to jump start it, call a professional to assess the problem.

Want to reduce your chances of calling out a breakdown service at all? We reveal the most reliable cars.

Useful tips about jump starting a car 

The car you're using to jump start the dead car must have the same battery voltage as the immobile vehicle (normally 12 volts). As a general rule, jump start from a car that has an engine at least the same size (in litres) as your own.

Certain car manufacturers (such as BMW) locate their batteries in the boot rather than under the bonnet. So make sure you know where yours is.

Keep any auxillary electrics, such as lights, air conditioning and radio, turned off on the dead car while you're waiting for the battery to charge. 

Once the dead car's ready to drive again, don't just take it on a short journey. You may be unable to restart it again because the alternator hasn’t had enough time to sufficiently charge the battery. 

If a jump start doesn’t work, your battery may need replacing, or there may be a more serious issue with your ignition system.

Can you jump start an automatic car? 

Yes. Being an automatic or manual car won’t make a difference when it comes to jump starting using jump leads. You can follow the same process as above.

You can’t, however, push or jump start an automatic car.

Stuck on the side of the road? We've surveyed breakdown firm customers so we can reveal the best car breakdown providers.

Can you jump start an electric car? 

In most cases, you can jump start an electric car. But it’s always worth checking your manual beforehand.

Hyundai told us that its Ioniq Electric and Kona Electric both feature conventional 12v auxiliary batteries. These can be jump started as per any conventional internal combustion engine using the instructions in the owner’s manual.

Tesla also told us its electric vehicles can be jump started, but advised owners to call the Tesla roadside assistance first - one of the team will talk you through the steps.

How do you jump start an electric car? 

Most electric vehicles have two batteries: 

  • Lithium-ion battery that supplies the electricity for the motors 
  • 12-volt lead-acid battery, the same type that is used in petrol and diesel cars. This powers the lights and wipers, as well as ensuring the lithium-ion battery can be charged. It’s the latter that is used when jump starting.

If the lithium-ion battery has completely depleted, you will need to find the 12v battery – typically stored in the boot or passenger side. Jump start that using a petrol or diesel car or a portable jump start device as the booster.

You can’t use another EV as a booster because it won’t have enough power.  

Jump starting your car can be dangerous if it’s not done correctly. Always check the manual beforehand.

Always check the manual beforehand. If you don’t feel comfortable, call out a recovery service.

Jump straight to our pick of the best electric cars

Can a car battery be too flat to jump start? 

Jump starting your car will only work when the problem is a flat battery – and as long as the battery is still in a decent enough condition to actually respond to the charge.

If the battery is too old and worn or has completely depleted, then a jump start might not be enough. Instead, you need to get a professional to fully replace the battery.

While jump starting a car that has a problem with the alternator or engine won’t fix the problem, it will help rule out the battery as the cause.

Need to replace your battery? Use our top tips on how to find a trustworthy garage that’ll charge you a fair price.

How to avoid a flat battery  

Is a car battery charger worth it? 

Yes, if you leave your car parked up or in storage for long periods of time.

A battery will deplete if it isn’t used enough. By regularly charging your battery when you’re not using it should help keep it in good condition, and lower the risk of it dying.

Some smart chargers can be used for charging and maintenance. They can determine if your battery needs a full charge or just a top up (trickle charge) and will act accordingly. 

This will avoid over-charging a battery, which can cause damage, and can even completely kill a battery. 

Only cars that are reliable, economical, comfortable, safe and good to drive can become a Which? Best Buy - these are the best new and used cars you can buy today.

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