How to jump start a car

A flat battery can often be to blame if your car won’t start. Tell-tale signs include a starter motor that won't turn over and interior lights that dim significantly when you start the ignition.

Instead of calling a breakdown firm, you can fix the problem 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 and you'll have your car back on the road in no time.

Having battery issues and don't know where to start? Read our guide to finding a good garage near you. 

How to jump-start a car using jump leads

1. Grab your jump leads and some rubber gloves and find a second vehicle to help jump-start your car
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 – attach the live car first
4. Next, attach the black lead to the negative terminal (-) on the live car
5. Then attach the black lead to an earthing point on the dead car; any unpainted metal on the engine block or chassis will work
6. Leave the cars off for three to five minutes, then 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.

How to jump-start a flat battery safely

Jump-starting your car doesn't have to be hazardous. Just follow these basic safety precautions:

• Always check your owner’s manual before doing anything
• Wear rubber gloves if possible
• Make sure both engines are switched off to begin with, and the cars are parked in neutral (or 'P') with the handbrakes engaged
• Don’t jump a battery or use leads that appear damaged or leaking. Call a professional
• Remove jewellery such as rings and bracelets. Metal objects coming into 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 damage.

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.

How to use a jump-starter pack

Portable battery packs with jump leads are widely available online and in high-street stores, so you can jump-start your car without needing a second vehicle to help you. Before you buy, check that the pack is rated for your car; the listings pages will typically mention the maximum engine size in litres and the battery voltages they work with. 

Is your car battery dying?

Here are some quick signs that your car battery is flat, or failing to hold charge:

• 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 it going.

A flat battery is one of the most likely culprits when your car won’t start, although there are other issues, such as a broken alternator or a defective fuel pump, 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.

Top tips for 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).

Certain cars (such as BMW) have their battery located in the boot rather than under the bonnet. Make sure you know where yours is.

Keep any ancillary electrics, such as lights, air conditioning and radio, turned off on the dead car while you're waiting for the battery to charge, as they will sap any remaining power. 

Once the dead car is ready to drive again, don't just take it on a short journey as you may be unable to restart it again because the alternator hasn’t had enough time to sufficiently charge the battery. Drive around for at least half an hour to ensure the battery has time to charge.

Can you jump-start an automatic car?

Yes. Whether it's 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.

However, you can’t push or 'bump-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 or hybrid car?

In most cases, you can jump-start an electric car's 12V battery (which is separate from its main battery pack), but it’s always worth checking your manual beforehand.

For example, Hyundai told us that its Ioniq Electric and Kona Electric both feature conventional 12V auxiliary batteries. These can be jump-started in the same way as any conventional internal combustion engine, using the instructions in the owner’s manual.

Tesla also told us that its electric vehicles can be jump-started; however, it advises owners to call Tesla roadside assistance first and one of the team will talk you through the steps.

One thing you definitely shouldn’t do is jump-start another car using an EV, as this can void your warranty and damage the electrical systems in your car.

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'll need to 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 

Don't: 

• Leave your car inactive for weeks at a time 
• Run the radio or lights when the engine is off 
• Ignore warning signs (for example, a clicking sound when you turn the ignition, dim lights, slow retraction of electric wing mirrors).

Do: 

• Turn off all the lights when you leave your car
• Regularly service your vehicle
• Ensure battery terminals don't become corroded
• Use a battery trickle charger if you only tend to make short trips in your car.

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 overcharging a battery, which can cause damage.

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