How to change a car battery and choose the right battery for your car

Flat car battery? Our guide to replacing it, installing a replacement and using a battery charger should help

Winter driving - battery check

If you Google ‘How to change a car battery’, the results will bring up an extensive range of convoluted and sometimes utterly useless articles on how to replace a dud set of cells. It goes without saying that care should be taken whenever you decide to work on electrical components, especially the high current circuits which are found in car electronics. That being said, all the information required for you to install a new car battery safely is fairly basic and easy to follow.

How long do car batteries last?

Sadly, the life of a car battery is a finite thing, even though modern variants last much longer than they did back in the black and white times. As the amount of technology we all use in the average car increases, so too does the demand on batteries, hence the need for capacity to increase.

Your modern day battery not only has to crank the engine and create a spark (unless you drive a diesel) but there are a host of complicated on-board computers and ECUs to power, too. Add to this features like high-power speakers, electric windows and heated seats and the drain on your battery is huge, and that’s before you put your phone on charge!

Modern automated stop-start systems might be good for fuel economy and the environment, but they help to give your battery a proper pasting too. You won’t be surprised to hear then, that flat or dead batteries are one of the most popular callouts for breakdown services.


You don’t have to wait for the inevitable emergency though, because if you keep an eye on your battery’s condition it’s possible to spot the early signs of a potential failure. Look out for easy to spot clues like dim interior lights or an engine that’s reluctant to start. If these signs are showing then it may be a good idea to get your battery tested.

Catching the problem early means that you can find the best replacement car battery deal at your leisure, instead of being forced into an expensive emergency purchase. A professional battery check should also confirm that it is indeed the battery at fault, and not another of the car’s electrical systems. So, either way, you could avoid a very expensive bill later on.

Car battery types and sizes

First off, you need to find the right battery for your car. Sadly, this isn’t a case of ‘one size fits all’ but finding the correct battery needn’t be intimidating. Car batteries come in various sizes and types. Each battery has a three- or four-digit code such as 075 or 096T. This code indicates the size, power rating and terminal positions of the battery, amongst other things, and helps you find the correct replacement battery for your car.   

Though the traditional lead-acid battery is still commonplace, different cars require different battery types depending on their on-board systems etc.  If your car has stop/start, the battery will need to be tuned with the on-board computer, which is why fitting an ECM or AGM battery is probably a job for a professional. Also, If you fit the wrong type or size of battery to any vehicle, it could cause a severe lapse in performance. As a general rule, always fit the same type and size as the manufacturer’s original. 

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Although this may all look confusing, A quick look at the owner’s manual should have all the information you need. Additionally, while the internet is a valuable resource, many suppliers’ websites will allow you to enter your registration number to find the correct specification for your vehicle. 

Plus, if you shop around online, you’ll have a good chance of finding a big discount, along with the added bonus of having your heavy new battery delivered to your door.

Before you disconnect your failing old battery, remember that it powers various features in the car, which are likely to be reset when their power supply is cut. Do you have the code for the stereo, for example? You may need this to use it again when you connect the new battery, otherwise you could face another bill, or worse, no music.

Disconnecting your old battery

Most batteries are found under the bonnet, however some are under the footwell or in the boot. If you are unsure then check your owner’s manual. Wherever it is, make a careful note of which terminal is which. Once you are ready to remove the old battery, firstly disconnect the negative terminal ( – ). Then disconnect the positive (+) terminal. Be sure to do it in this specific order, otherwise you could cause an electrical short or sparks that can cause damage and injury. Once your terminals are safely removed, keep the connectors away from each other and remove any clips holding the battery in place and lift or slide it out of its plastic cage. There may be additional brackets or bolts holding it in place so do not force it. 

To fit your new battery, simply place it into the cage and reverse the removal process, connecting the positive terminal first, then the negative. It is likely that you will now need to reset any electricals inside the car – such as the clock or infotainment system.

Care for your car battery

Now that you’ve splashed out on a new battery, you want to protect your investment. The latest chargers have sophisticated programmes to maintain the level of charge and are especially useful if you only use your car sparingly – classic car owners swear by these products.

These smart chargers vary the current that they pump into the battery in order to prolong its life. They can even be left attached to a car for long periods of time, and just top the battery up gently. Hence they are often referred to as trickle chargers. Plus, if your battery is showing signs of being on its last legs, one of these products might be worth a try first; a smart charger can bring a dying battery back to life.

Emergency jump starting

Another handy investment is a jump start pack. These work like the traditional set of jump leads you run from your car to another vehicle’s battery to get you started if you’ve left your lights on all night by accident, for example. The difference is, you don’t need another car.

Just charge up the jump start pack, attach the leads to the correct terminals on your battery, and they’ll provide a boost to get your car started. The best systems can jump petrol cars with engines up to 3.0 litres in capacity, and promise multiple starts before they need recharging. They’re definitely handy gadgets to keep in the boot.

If you do get stuck with a flat battery, here's our guide to jump-starting your car...

Current affairs and features editor

Chris covers all aspects of motoring life for Auto Express. Over a long career he has contributed news and car reviews to brands such as Autocar, WhatCar?, PistonHeads, Goodwood and The Motor Trader.

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