A guide to car batteries

What is a car battery?

A car battery is an essential rechargeable power source needed to start your petrol, diesel or hybrid car. It also powers lights, heating, air con, electric windows and on-board electrics and computer.

Batteries in electric cars (EVs) are much bigger than standard batteries. They can be metres long and typically weigh around 450kg. Made up of hundreds of individual cells packed together in modules, the EV battery typically lives hidden inside the car’s chassis.

EVs also have a second 12-volt battery, which looks much like the battery found in a petrol or diesel car. Also known as the auxiliary battery, this powers non-driving related systems like the central locking, in-car entertainment and interior lights. As with petrol/diesel batteries, these can drain and cause breakdowns.

In fact, dead car batteries in all vehicles are one of the major causes of car breakdowns. And one of the 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?

Petrol/diesel cars: 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.

EVs: Unlike petrol or diesel cars, electric vehicles get their power to move from a lithium-ion battery pack made up of cells that can store and release energy, much like the battery in a mobile phone or laptop. Starting the car triggers an electrochemical process which produces a current to power an electric motor that drives the wheels.

As with a mobile battery, energy goes both ways – out when you’re driving, but back into storage for future use when the car is put on charge.

How much are car batteries in the UK?

Petrol/diesel cars: 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 £50 and £300 for a new car battery. Stop/start batteries (which turn off the engine when it’s not moving, at a red light for example) are among 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 confident to do it yourself. Some high street names offer relatively inexpensive battery installation, but typically you could pay between £162 and £460 for battery plus professional installation.

EVs: If you need to replace the small battery that powers the auxiliary systems this is similar to replacing a battery in a petrol or diesel car. But replacing the EV battery that powers the car is very costly, on average over £7,000 according to one estimate. This shouldn’t alarm EV drivers unduly, however, as EV batteries typically last a very long time and you may never need to replace one.

Can I change a car battery myself?

It’s not a good idea to change your own petrol, diesel or EV auxiliary car battery. Knowing which car battery to fit takes expertise. And if you make a mistake, it could not only damage your car’s electrical system but could well be both dangerous and 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. If your car won’t start because of a battery issue, breakdown insurance could come in handy to help here.

The primary EV battery is unlikely to need replacement and if it did, the job would need to be done by a specialist.

How long do car batteries last?

Assuming you buy the right battery for your petrol/diesel vehicle make and model and maintain it correctly, your car battery could last between three to five years, or potentially longer. The same goes for the auxiliary battery in an electric vehicle, which powers the car’s non-driving systems.

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.

EV battery warranties for 100,000 miles or eight years are typical, and in practice EV batteries often last much longer than that, as much as 20 years.

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

Here are a few tell-tale signs that your petrol/diesel 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?

Petrol/diesel cars: To maximise your petrol/diesel 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.

• Check the internal lights and headlights are turned off when you leave the car. 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.

EVs: Lithium-ion batteries gradually degrade over time, but there are ways to optimise the lifespan of your EV battery:

• Park in the shade: Avoiding extremes of heat (and cold) will reduce strain on the battery’s thermal management system.

• Don’t run the battery right down. Try not to discharge below 20%.

• Avoid over-charging. Charging to 100% too often can reduce capacity. (Many EV models now have built-in buffering to stop you over- or under-charging.)

• Use rapid charging sparingly. Standard charging gives an estimated 10% longer battery life over time.

• Allow the battery to cool before putting it on charge.

• Drive slower. Minimising acceleration and high speeds will slow the drain on the battery, which impacts positively on lifespan.

How to fix a dead car battery

Provided it’s not completely dead, you can try to charge your petrol/diesel car 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 and fix the issue – 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 petrol/diesel 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 in the car for more than five years, it may be time to get a new one.

Remember also that 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.

What if my EV won’t start?

There are a number of possible reasons why an electric car might not start:

• The car simply needs charging. If you’re not near a charger, however, you may need to call for roadside assistance. They may have a portable car charger or can arrange to tow you to a charging point. It’s not recommended to try to jump start an EV, as this could damage the car and invalidate your warranty.

• The car needs a software update. You may need to add some charge to complete the update. You can adjust settings later so updates don’t happen without your permission.

• Heat or cold may be an issue. In bitter cold, the charge port switch could freeze, which can be carefully thawed with a hairdryer. In hot weather or after extensive driving, it’s possible your converter unit may overheat. This may need to be looked at by an EV specialist.

• Corrosion in the motor. Excessive salt or water can damage an EV motor and prevent it starting. A garage will need to take a look.

• Dead key fob. If your car is keyless, it might be that your key fob battery needs replacing. Your manufacturer can do this for you.

• Dead auxiliary 12-volt battery. Check with your garage to see if it needs replacing.

• The auxiliary 12-volt battery has a loose connection or has become sulphated. With the car completely switched off, check everything is properly screwed in. Check the terminals haven’t become sulphated, which is when moisture gets into the battery and builds a green-and-white residue between terminal and connector. This can be very gently cleaned with water and baking soda, again with everything switched off. If in doubt, contact your garage.

Do I own my EV battery?

Battery lease arrangements are increasingly rare. At one time, some EV models, such as the Renault ZOE and Nissan Leaf, effectively rented the battery to the car owner for a monthly fee. This brought the cost of the EV down, and usually came with other perks such as a battery performance guarantee, with replacement as needed, and breakdown cover.

Leases have become less popular as battery lifespans have increased. These arrangements could also make selling on the car more complicated as the new buyer has to take on the lease and its ongoing monthly payments. Manufacturers now typically sell EVs with battery included.

If you’re buying a used EV, check that you understand the terms and costs involved. Manufacturers now often offer buy-out options so you can take ownership of the battery, which is well worth considering.

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 they need 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. All of this goes for the 12-volt auxiliary batteries in EVs too.

When it comes to disposing of the main battery in an electric car, your local brand dealer has the option to send the battery to landfill, give it a second life, or recycle parts.

The industry is taking steps to reduce landfill disposal (which has negative environmental impacts) and increase reuse and recycling.

EV batteries can be used as storage units to capture surplus solar energy, for example, or broken down and used in the making of new EV batteries. Many car companies have now introduced recycling schemes.