A guide to car batteries

How does a car battery work? How long will it last? What can you do to extend the life of your car battery?

Here’s our guide to the ins and outs of car batteries and what to do if yours dies.

How does a car battery work? How long will it last? What can you do to extend the life of your car battery?

Here’s our guide to the ins and outs of car batteries and what to do if yours dies.

Written by

Rory Reid

Car and technology expert

Last Updated

10 AUGUST 2023

6 min read

What is a car battery?

A car battery is an essential rechargeable power source needed to start your car. The battery supplies electricity to your car’s starter motor to turn the engine over when you turn the key. It then powers the spark plugs to ignite the fuel and air mix in the combustion cylinders, which is what gets the engine going.

When your car is parked but running, the battery also powers important systems like the radio, lights, air conditioning, electric windows and on-board computer.

Dead car batteries are one of the major causes of car breakdowns. And one of most common reasons for battery problems is people leaving the lights on while the engine’s not running. This is easily done, especially if you’re a new driver.

How do car batteries work?

Without getting too deep into the science, car batteries store chemical energy that can be converted into electricity to power up the car’s engine.

Most car batteries are lead-acid, meaning they contain lead plates immersed in a solution of sulphuric acid and water. When you turn the key in the ignition, it triggers a chemical reaction between the lead and the acid which creates an electrical current strong enough to turn the engine over and get it firing.

While you drive, your car battery is recharged by the alternator belt from the engine. As the belt goes round it creates an electrical current which is used to power various electrical systems in the car, like the lights and air conditioning.

The alternator also sends some electrical current back to the battery, where it is converted into chemical energy by the same reaction in reverse and stored in the battery.

How much are car batteries in the UK?

How much a car battery costs depends on what kind you need, the quality and level of warranty you want, and where you buy it.

UK car battery prices vary enormously – you could pay anything between £45 and £330 for a replacement, with stop/start batteries among some of the most expensive.

On top of the price of the car battery, you’ll also need to factor in the cost of fitting it, unless you’re handy with a spanner. Some high street names offer relatively inexpensive battery installation, from as little as £20, but you could end up paying more if you go to a dealership.

Can I change a car battery myself?

It’s often not a good idea to change your own car battery. For a start, knowing which car battery to fit takes expertise.

If you make a mistake, it could not only damage your car’s electrical system but could well be dangerous, as well as expensive to put right. Even if you do choose the right replacement battery, only a trained mechanic will be able to tell if the fault is with the battery or the car itself.

How long do car batteries last?

Assuming you buy the right battery for your vehicle make and model and maintain it correctly, your car battery could last between three to five years, or potentially longer.

Long drives help car batteries stay in optimum condition as your battery will have more time to charge as the engine keeps topping it up. If you make lots of short trips, you may find your battery wears out faster.

Batteries also discharge over time, so if you leave your car parked up for a long time, it could shorten the life of your battery.

How do I know if I need to replace my car battery?

Here are a few tell-tale signs that your car battery needs replacing:

The car is slow to start – as a battery ages, internal parts of it may become corroded, making the battery less effective.
The electrics don’t work – as a battery nears the end of its lifespan, it will have less power output available to run the car’s electrical systems.
The dashboard warning symbol is illuminated – you’ll need to get your car checked and your battery tested as soon as you can if either the battery warning light or the engine warning light come on.
There’s a bad smell – if you notice a rotten egg type smell when you open the bonnet, it might indicate that your battery is damaged. Get it tested right away.

How do I avoid running down my car battery?

To maximise your car battery’s lifespan:

Don’t use the radio or other electronics when the car’s engine is off.
Take a look at the battery. Are the cables loose? Is there gunk on the cables? If you’re experienced, you may be able to fix these problems by yourself, but it’s normally best to seek help from a professional.
Avoid making lots of short journeys. If that’s unavoidable, try to take the occasional longer journey to help improve the health of your battery.
Drive regularly to make sure that the alternator can do its job and recharge the car battery.

Use a trickle charger to stop your battery going flat if you don’t use your car regularly.
When you leave the car, check the internal lights and headlights are turned off. Also switch off any electronics, such as dash cams, which might drain the battery.
Keep your car in a garage overnight if you can, or park it in a shady spot. Extreme temperatures can affect your battery’s performance and lifespan.

How to fix a dead car battery

Provided it’s not completely dead, you can try to charge your battery using a car battery charger that plugs into the mains.

If the battery is completely flat, you can try jump-starting the car. For this, you’ll need some jump leads and a driver willing to attach them to their own car. Once you’ve started the engine, go for a drive for at least half an hour to fully recharge the battery.

If neither of these methods work, it’s time to get a mechanic to diagnose what the issue is and fix it – replacing the battery if necessary. If you have breakdown cover, you may be able to call your provider for help.

Why has my car battery died?

There are a number of reasons for car batteries to stop working, such as:

You haven’t driven your car in a while
Cars need to be driven regularly. If you go away for an extended period, you may come home to find your car’s battery is flat.
You’ve left the radio or lights on without the engine running
Forgotten to turn your car’s lights or heater off? Shut down the infotainment system? Or left your indicator stalk in the left or right position? Again, you may return to find your battery’s dead.
There’s a problem with the alternator
If the alternator isn’t working properly, it won’t be recharging your battery while you’re driving. In this case your battery will keep draining and eventually fail. Watch out for flickering lights and strange noises – this could be a sign of a faulty alternator.
Your battery is in poor condition
How old is the battery? If it’s been with you more than five years, it may be time to get a new one.

Remember, the problem might not be with the battery.

If you can’t start your car, the first thing you should do is check the headlights. If they work, the problem may be with your car’s electrical components rather than the battery. If so, you’ll need to call the garage.

How to dispose of car batteries

Car batteries shouldn’t just be thrown in the rubbish. They contain harmful chemicals, like lead dioxide and sulphuric acid, that can be extremely harmful to the environment. That means it needs to be recycled by professionals.

If you can’t get your mechanic or battery replacement specialist to take yours for you, take it to your local recycling centre.

Frequently asked questions

What is the right size battery for my car?

To find out what type of battery is in your car, check your owner’s manual and look for the section on the battery or battery replacement. It could also be detailed under technical details or specifications.

The battery size is shown as a two or three-digit code, which can contain both letters and numbers. Getting the right size of car battery means firstly, that the battery will fit snugly into its compartment, and secondly, that the terminals will be in the right place.

If you don’t have the manual for your car, the size should be displayed on the battery itself. Alternatively, some shops that sell batteries will allow you to search by your car’s make and model, or registration number, to find the correct battery size.

How long does it take to charge a car battery?

How long it takes to charge your battery will vary, depending on your car’s make and model and how powerful your charger is. If you’re in a hurry, you could probably do it in two to four hours. But ideally you should opt for a slower, overnight charge.

If you’ve jump-started your car, then driving it for half an hour or so should do the trick.

How to start a car with a dead battery

If your car won’t start and you’ve identified that the problem is a dead battery, the easiest solution is to jump-start it.

To jump-start a car, you’ll need a pair of jump leads and access to another car with a working battery. For step-by-step instructions and important safety precautions, read our guide on how to jump-start a car.

Can a car run without a battery?

Although in theory a car could run without a battery, using the alternator to power the engine, you wouldn’t be able to start it without one.

What does it mean if my car’s dashboard battery light is on?

If your car’s dashboard battery light is on, it doesn’t necessarily mean your battery is on the blink. It could be an electrical fault, or a problem with the charging system. Either way, you should call a mechanic and get it checked out.

How does the weather affect my car battery?

When the weather is extremely cold, your battery can lose power as it has to work harder to keep you and the car warm. You often see this with electric cars, which see their batteries die far more quickly in winter than summer.

Meanwhile, in the summer, intense heat can cause some of the vital liquids inside the battery to evaporate, which can lead to corrosion and battery failure.