How to test a car battery

Release time:2017-10-17

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A flat battery is a common cause of a car failing to start. Following our guide will help you check your car battery.

Many will be familiar with that stomach-churning feeling when your car fails to start – or even unlock – on a dark, cold morning. A flat battery – one with no electrical charge – is often to blame and, fortunately, this is something that can be avoided.

Our advice guide tells you how you can check your car’s battery to help prevent it failing at the worst possible moment.

When to check your car battery

It’s an unfortunate fact that many drivers barely ever look under their bonnets, let alone check their battery. There’s no specific rule as to how often you should check the condition of your car’s battery, but it’s advisable that you give yourself the peace of mind that your battery is in good condition by performing regular checks.

If you ever notice that your car isn’t starting as quickly or easily as it once did, this may be a pointer that the battery isn’t performing as it should be. At this point, it’s vital that you check its condition, before it’s too late and your car fails to start altogether.

Several outside factors influence the performance of a car battery, one of which is the time of year. Any battery will perform at its best in warm weather – cold winter months can see them struggling to provide as much power as they can in the summer.

Batteries like to be exercised, too. If your car is likely to sit unused or only be driven occasionally for a prolonged period, a regular battery check is essential.

What does my car battery do?

The most important task your car battery performs is starting its engine. Once the engine is started it then plays a vital role in ensuring that all the electrical functions in your car will operate reliably.

The battery sits at the very centre of your car’s electrical system. Though modern car batteries are usually reliable and virtually maintenance-free, regular checks are a good idea to prevent finding it to be flat when you least expect it.

Testing your car battery voltage

The simplest and best way to perform a car battery check is to use a car battery tester. These are available from any good car accessory retailer and are very simple to use.

They range from basic types that simply give a green light if the battery is charged, to models with a display that shows the exact voltage of the battery.

A car battery tester should come with straightforward instructions. The red lead will need to be attached to the positive charging terminal and the black lead will need to be attached or held against the negative terminal as indicated in your car’s owner’s manual.

If the battery tester has a voltage read-out, you’re looking for a figure of more than 12.4v with the engine switched off. Around 12.7v is a good indicator of a fully charged battery.

Below 12.4 volts, the battery will need to be charged. If the engine starts, it may be worth going for a short drive and checking the battery voltage again on your return.

When the engine is running, the battery is constantly being charged by the car’s alternator, so if the car has been sitting for a while, it may just be a case of putting the car back into regular use to maintain proper battery voltage.

This test can also be conducted with a multimeter or voltmeter. If you’re going to use one of these, make sure that the ‘0-15v DC’ range is selected.

It should be noted that cheap battery testers will only display how much charge the battery has. If a battery is faulty, it may not be able to deliver sufficient current even when fully charged.

How to charge a car battery

There are a wide range of battery chargers on the market, available from all good car accessory stores. Your local stockist will be able to advise on which charger is suitable for your car and its battery – some cars have larger batteries that require a more powerful charger.

We recommend you use an automatic battery charger. These have built-in systems that stop delivering power once the battery has reached optimum voltage. They then switch to a ‘maintenance’ or ‘trickle’ mode that keeps the battery topped up. Some chargers also have a built-in battery tester.

Your battery charger will come with instructions from its supplier and you should follow those as well as the instructions in your car’s owner’s manual.

Always ensure the car’s battery leads are attached to the battery throughout the charging process.

Unless the instructions for the charger specifically say otherwise, do not switch the charger on until the clamps are correctly secured to the charging terminals. These are the same charging terminals that you’d earlier attached the battery tester to.

With the battery tester removed, connect the black clamp to the negative terminal and the red clamp to the positive terminal.

Check the clamps on the charging leads are securely attached to the terminals – and only the terminals. They must not contact any other part of the car, or a short circuit could occur.

Check the cables from the charger have a straight run and aren’t coiled or tangled together, as this could cause them to overheat.

When you’re happy the charger is connected according to the instructions provided, as well as the instructions in your car’s owner’s manual, you can switch the charger on.

The charger will go through a charging process, which will take as long as the manufacturer specifies, depending on the battery’s voltage level at the beginning of the process.

When the charger indicates the charging process is complete, switch it off. Disconnect the positive lead from its charge terminal first, then the negative, and store them as detailed in the instruction manual for the charger.

You can then perform a battery test again if necessary, or attempt to start the car. If the battery level during cranking still drops below 9.6v or the car fails to start, this indicates the battery is defective and will need to be replaced.

Car battery replacement is recommended only to be performed by qualified vehicle technicians.


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