How to check a car battery: a guide to charging and jump starting

A healthy car battery is key to ensuring your car starts first time, every time. We show you how to check yours...

Stuart Milne

Published20 December 2024

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A car battery is essential for your car to function properly. It will power the starter motor when you fire up your engine, and will power every one of your car’s electrical components. They perform a similar role in an electric car, too.

Ways to check your car battery

Start the car
If you turn the key or push the start button and the car won’t fire up, it takes a little longer than expected, or the whole process feels laboured, it’s likely the battery’s charge is low.

Are your headlights working as expected?
With the ignition on, and the engine off, turn on your headlights. If they’re dimmer than you’d expect, your battery may be struggling to power them. Note that this won’t necessarily be the case for cars fitted with LED headlights.

Check the battery status indicator
Many car batteries have a status indicator. This is usually a small window on top of the battery which shows whether the battery is in good health or not.

If it’s green, then all should be well. Red indicates low voltage, and if there’s nothing there at all, it’s likely the battery is completely flat.

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Checking a car battery with a multimeter
A multimeter – sometimes called a test meter – is a handy tool for measuring all kinds of electrical properties, and can cost less than £10.

When checking a car battery, you’ll need to select its voltage modes, so twist the dial to the setting which displays a V beneath a solid and dashed line. Some older multimeters will require you to set the range to 20, which will ensure it measures between 0-20-volts; more sophisticated ‘auto-ranging’ multimeters will do this for you.

You’ll then need to touch the red probe to the positive terminal, marked with a ‘+’ and the black probe to the negative terminal, marked ‘—’. The display should read no less than 12.6. If it doesn’t you’ll need to charge it.

How to charge a car battery

If you’ve noticed your car is taking a little longer than usual to start, don’t wait to get stranded. If the battery is old, your local garage, fast-fit centre or main dealer will be able to swap it for you. It’s a job you can often do yourself with basic tools, but call in the pros if you’re not sure.

There are also plenty of things you can do yourself to keep your battery in tip-top condition.

A battery charger will help keep your battery conditioned if it’s parked up for longer periods of time. The NOCO Genius5 UK was named our favourite when we tested the best car battery chargers on sale.

Each car battery charger will work slightly differently, so you should always refer to the instruction manual. But in broad terms:

Locate the battery in the car. It will usually be under the bonnet, often hidden by plastic panels, but you may also find it in the boot or under the rear seat. In these cases, the charging terminals will usually be located under the bonnet for better access.

Remove the covers on the battery terminals, if fitted. Connect the red clamp to the positive battery terminal (marked with a + sign), then connect the black clamp to the negative terminal (marked with a — sign).

Set the appropriate mode on the battery charger for your needs. Consult the manual on the best mode for your needs.

Then it’s time to plug the charger into the mains and check it is charging correctly. Most modern ‘smart’ battery chargers will vary the charging rate to best revive the battery, and they’ll provide some indication of the length of time it’ll take to recharge. This can take several hours or more.

Once the battery is charged, switch the charger off from the mains and disconnect it from the battery terminals. Replace the terminal covers and any protective panel.

If the charger has done its job, the car should start as normal. If not, you should speak to your local garage, mechanic or car spares supplier. Replacing a battery is usually quite straightforward, but it isn’t recommended for the novice.

How to use a jump starter

Jump starters, also called car jump packs, have largely replaced traditional jump leads. They’re convenient and easy to use and store, and many are inexpensive.

But more importantly, they don’t require a friendly neighbour or a helpful automotive samaritan to offer up their car.

A jump starter is an invaluable piece of kit, then. You can think of them as a powerful battery in a small case – simply attach the clamps to the battery terminals and start the car as normal. Once the car has started, disconnect the jump starter, pop it back in the boot, and don’t forget to recharge it when you get home ready for its next use.

Some jump starters also feature torches and hazard warning lights, and even tyre inflators. The Clarke JSM 1200 is currently our best car jump starter.

How to avoid a flat battery

While car batteries can – and do – fail, there are some things you can do to preserve the longevity of yours.

The easiest is to make sure you use your car regularly, avoiding letting it stand for long periods of time. When you drive, the car’s alternator will charge the battery, keeping it topped up.

But the starter motor is one of the most power-hungry features your car will have, so you’ll need to make sure you’re taking longer journeys, rather than lots of short ones.

If you do need to leave your car for longer periods, consider using a trickle charger, which will provide just enough to keep the battery conditioned.

Why does a car battery fail in winter?

If a car battery is likely to fail, it’ll probably do so when the temperature drops. That’s partly because the nature of a car battery’s chemical makeup means it’s harder to deliver full power in the cold – quite simply, they don’t really like the cold.

Starter motors in petrol or diesel-powered cars require a great deal of power to start an engine, and this coupled with the increased likelihood of drivers making more short journeys, and the increased drain of headlights and air-conditioning systems means that any cracks in a car battery’s performance will start to show.