Flat batteries are an all too common problem, especially during winter. This means that most drivers will have to jump start a car at one time or another - so how do you do it safely?

By the end of this article, you will know:

  • What safety precautions to take before jump starting a car
  • How to attach and disconnect the leads correctly
  • How to jump start a car
  • Some common reasons why a battery goes flat
  • How to prevent your car battery going flat



Before you try to jump start your car, you must check that it is safe to do so. This means checking the manufacturer's recommendations and that you're using the right equipment.

You should never try and jump start a battery using faulty leads or by attaching them to a leaking battery. Once you have connected them in the right order, you must follow a set procedure. You must then disconnect the jump leads properly.

A flat battery could be caused by a lack of use, regular short journeys, a charging problem, faulty electrics, electrics left on overnight or an old or severely undercharged battery.

If you notice regular problems with your battery, you should seek professional help at a garage near you.


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Jump Starting Safety Tips

First of all, you should check your manufacturer's guidance about jump starting. The sudden surge of power can damage your car's delicate engine management systems, so some advise against it as a result.

Check in your vehicle handbook for your manufacturer's guidance.

If you get the all-clear on that, your next task is to ensure you don't hurt yourself during the process.

Before jump starting your car, check the following things:

  • That the jump leads are in good condition. You should stop using them if they get hot.
  • That the battery is not damaged or leaking.
  • Both cars have a battery with the same voltage. This should be a 12-volt battery.
  • If you're wearing any metal (jewellery, a watch strap or belt buckle) that could touch the battery. Remove these if you are.
  • If you're wearing loose clothing, either tuck it in or change. You don't want it to get caught in the engine's moving parts.
  • All cigarettes or naked flames are extinguished. You should never allow these near a car battery.

Once you're satisfied that you can jump start your car safely, you need to attach the leads in the right order.


How to Attach Jump Leads in the Correct Sequence

If you don't connect the jump leads in the right order, you can damage the car's electrics by surging power through the alternator. You can also hurt yourself.

The correct order to attach jump leads is:

  1. Attach the red lead to the positive terminal (+) of the dead car’s battery.
  2. Attach the red lead to the positive terminal of the working car’s battery.
  3. Attach the black lead to the negative terminal (-) of the working car’s battery.
  4. Attach the black lead to a piece of bare metal on the disabled car’s chassis or engine. You should attach this away from the battery and the fuel system.


BookMyGarage infographic detailing how to jump start a car battery


To disconnect the jump leads, follow the process in reverse. This means you should disconnect jump leads in the following order:

  1. Remove the black lead from the ‘dead’ car.
  2. Remove the black lead from the working car.
  3. Remove the red lead from the working car.
  4. Remove the red lead from the ‘dead’ car.


How to Jump Start a Car

  1. Find the battery in both cars. It should be obvious, but you can also check the user manual for help.
  2. Move the working car close to the dead car but without the two touching. Both cars should be in neutral (Park for an automatic) and have their parking brakes engaged. Switch off everything that draws power from the battery on both cars. That means the lights, wipers, ventilation, sound system, touch screen and anything you might be charging.
  3. Connect the jump leads in the correct order, as shown above.
  4. Switch on the engine in the car with a working battery and run it for at least 3 minutes. This should pump some charge into the 'dead' battery. Then, try to start the other car. The battery should have enough charge to turn the engine over. If it doesn't, check that you have connected the jump leads correctly and try again. If it still doesn't work, there is a more serious problem and you should call a tow truck to get you to a local garage.
  5. Once the 'dead' car starts, keep them both connected and run their engines for about 10 minutes. You should keep both revving at about 1200RPM during this process (just over idling speed). This will charge the dead car's battery.
  6. After 10 minutes, switch both engines off and remove the jump leads in the correct order as shown above. Then, try and restart the 'dead' car's engine. If it refuses to turn over, there is a more serious problem as the battery cannot hold any charge at all.
  7. If the engine restarts, whoever owns the ‘dead’ car should take it for at least a 20-minute drive. This will fully charge the battery and the car should start on its own again.

If this process fails to restart your car's battery, you need to contact a local garage to find why it isn’t holding its charge.


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Why Has My Car Battery Gone Flat?

Once you have your car working again, you must work out why you needed to jump start it in the first place. There are many reasons for this. You can avoid some by changing your driving habits but others are the result of more serious mechanical faults.

If you can't work out why your battery has gone flat, or you have to jump start your car regularly, you should book an appointment at a local garage to have a professional inspect the issue.

Here are 5 reasons why your car battery may have gone flat.


Car Hasn't Been Used

As many drivers found out during the Coronavirus lockdowns, prolonged periods of inactivity are no good for car batteries. The AA reported that 55% of all breakdowns during the first lockdown in April and May 2020 were battery-related.

Try and drive your car for at least 20-30 minutes a week to keep the battery well-charged.


Car Has Been Used For Short Journeys

If you only use your car for one 10 minute journey a week, you use more electrical charge than you regain. The same goes for stop-start journeys.

Try to walk, cycle or use public transport if you know the journey is only going to be a short one.


Lights Were Left On Overnight

If you forget to switch an interior light or your headlights off with the engine, you'll probably need to jump start your car when you return. Car lights drain the battery very quickly and can cause a fully-charged battery to go flat overnight.

Make sure you check that all your internal and external lights switch off when you walk away from the car. If your headlights switch off on an automatic timer, make sure this process still works.

If not, you will need to book an appointment with a local garage as something is faulty.


A Charging Problem

This is a more serious issue. The alternator charges the battery while you drive along and your battery won't last long on its own charge if there is any problem with this.

You must book an appointment at a local garage to find and fix the issue.


Old or Severely Undercharged Battery

CTEK reported that 1 in 2 drivers had an undercharged or dead battery in the winter of 2020. Any battery struggles to perform during cold weather, but it's even worse for a poor quality battery. In fact, even a fully-charged battery produces 35% less current in winter!

If you can't remember the last time you changed your battery, it may not survive the winter.


If you're concerned about your battery, why not book a Winter Health Check on our comparison site at a participating UK garage? This 17-point check covers your battery's condition as well as your windscreen wipers, tyres, brakes and other vital car parts.

Plus, over 700 garages offer it free of charge, and over 1,000 offer it for less than £25! That makes it much more affordable than a service - just what you need to save money in the run-up to Christmas.

Speaking of Christmas, you don't want to have to cancel your plans because your car won't make the journey, do you? Book a FREE Winter Health Check today and get the peace of mind you deserve. You'll also catch any problems early, allowing you to sort them out on your own terms. Otherwise, you'll have to fork out for an expensive repair bill at the wrong time.

Enter your vehicle reg and postcode to find participating garages near you and book online today.


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How Can I Stop My Battery Going Flat When I'm Not Using It?

If you can't drive your car regularly to help the battery recharge, you should invest in a charger. This could be a trickle charger, intelligent charger or solar charger.

An intelligent charger offers protection against overcharging and an auto cut-off function as well as monitoring of the battery's voltage, temperature and charge time. They differ from trickle chargers which deliver a very low voltage to slowly charge a battery over a long period of time.

The same is true for a solar charger. It can't recharge a flat battery but it will use sunlight to keep an unused battery in good condition.

Whichever you choose, make sure you follow the instructions to set them up and use them correctly.

Don't overcharge the battery either as this can shorten its lifespan, which makes maintenance more expensive in the long run.