Tips and Advice

How to test a car battery

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 sinking feeling that occurs 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.

What do I do if my car doesn’t unlock?

If your car fails to unlock using the remote-control key fob, it may just be that the battery in the fob itself requires replacement. However, if your spare fob doesn’t work either, it could indicate a flat or very low car battery.

If remote unlocking doesn’t work, you should still be able to open the car by turning the key in the lock of the driver’s door. Many keys, including those from Mercedes and BMW, have a metal key blade hidden inside the key fob, which can be folded out by sliding a button on the fob.

Turning the key blade in the driver’s door lock may operate the central locking and unlock all the doors, or it may only open the driver’s door. The door handle may also need one or two sharp tugs after turning the key before it’ll open.

Sometimes, though, the battery is too run down to unlock the doors. It may still have sufficient power to operate the alarm and this may sound once the door has been opened. It should silence when you turn the key in the car’s ignition or press the unlock button on your key fob.

If the car still can’t be opened, it might be time to make contact with your recovery or breakdown service. If the car is still within its warranty period, you may be entitled to assistance under the manufacturer’s recovery service. The phone number should be in your owner’s documentation pack along with the instruction manual.

If you’ve been able to get into the car and found that the engine won’t start, it’s now time to check the battery.

How do I check my car battery?

If you don’t have any other equipment to hand, the only thing you’ll be able to check is that your battery is properly connected. The owner’s manual should be your first point of reference.

Somewhere in the manual you should find an entry titled ‘charging the battery’ or ‘checking the battery’. This should show you the location of the battery charging terminals on your car. These are also what you use to check your battery, although they may not be located on the battery itself.

With the bonnet open, if the battery and its terminals are readily visible, you may see it has two terminals. The black one marked with a ‘-’ is the negative terminal, while the red one marked with a ‘+’ is the positive terminal. One or both of these may be located under a hinged or sliding cover.

The batteries on some cars are hidden from view. The owner’s manual for your car will tell you where the battery is located. It may be referred to as the ‘main battery’ or ‘starter battery’. Note that some cars also have an ‘auxiliary’ battery, which is best left alone.

I can’t see my car’s battery charging terminals

Before going any further, make sure your owner’s manual is to hand. For battery checking purposes, you’ll need to know where your car’s charging terminals are. Your manual will contain this information.

In many cases these days, the battery is concealed and the terminals are not easily accessible, with special safety connectors designed to stay permanently attached. These should only be removed by a qualified service technician.

In cases where the battery cables are permanently attached to the battery, the charging terminals are usually separately located and clearly marked under the bonnet.

The positive terminal will often be marked boldly in red and may have a metal post, sometimes under a sliding cover. The negative terminal may be attached directly to the metal body of the car. It’ll be made from unpainted metal and may be shaped to accept a charging lead.

Once you’ve located these terminals, you can proceed with testing or reconnecting the battery. If you still can’t find them, we’d advise that you contact your breakdown or recovery service.

Reconnecting your car battery

If either of the terminals are visibly bare, then one of the battery leads may have become detached. If the positive (red) or negative (black) battery cables have parted company from their terminals but are visible and can be reattached, you can do so carefully.

You should only attempt this if the retaining nuts that tighten the battery leads to the terminals are still in place – if they’re missing the battery lead will soon become detached again. You’ll need a suitable spanner to tighten the nut effectively.

Also, only attempt reconnection if you’re wearing suitable rubber gloves to avoid risk of electrocution. Also note that when reconnecting the battery, the power surge can damage delicate components in the car’s electrical system. If you have any doubts about performing this operation you should contact your breakdown or recovery service.

If both the positive and negative lead have come adrift, reattach the red positive battery terminal first. The lead should fit over the battery terminal post or stud – push it so that the whole stud is covered and then tighten the retaining nut until the lead is securely in place.

When reconnecting the positive terminal cable, you may experience electrical sparking. These sparks can be minimised by connecting the terminal lead quickly and accurately. Prolonged sparking will cause the electrical system to receive power surges and can cause damage.

As you did with the red positive terminal lead, connect the black negative lead firmly and tighten to the terminal post or stud securely.

When both terminals are attached, the car alarm may begin to sound as it detects that the bonnet is open. You should be able to silence it by turning the key in the ignition or pressing the ‘unlock’ button on the remote key fob.

If the battery is in good health now it’s reconnected, the car should start as normal. If the car doesn’t start, it may be necessary to test the battery.

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. It's worth carrying a portable jump start pack in your car for emergencies – our test of the best can help you choose.

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.

While changing a battery is usually a relatively straightforward process, it can be more complex on certain cars – particularly newer models. We would always recommend getting a professional to do the job if you're in any way unsure about how to do it properly.

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