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How to test a car battery

How to test a car battery

Every car has one, and without it you'd be going no where! We check out car batteries, and how you can test them with a simple multimeter...

How many volts should a car battery have?

A good healthy car battery should register no less than 12.6 Volts. Due to the way batteries discharge it's important that you test the battery after it's been sitting for a period of time to get what's called the 'resting voltage'.

Leave the car overnight (preferably longer) and test before you start the car in the morning to get an accurate gauge of the health of your battery.

If you've recently been for a drive, the battery is likely to give a higher reading than the resting voltage, and so could be misleading. 

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How to check car battery with multimeter

How to check car battery with multimeter

Testing a battery with a multimeter is a simple process. The first thing to do is make sure you can access the battery terminals (the metal bits on the top of the battery).

Batteries are commonly located in the engine bay to one side of the engine, and often have a plastic cover which will unclip, lift off, or occasionally require the removal of a couple of bolts. You may also have a cover over the positive (+) terminal that will lift off too.

Once the battery is exposed be careful that nothing metal touches the terminals, so don't leave any spanners or other tools on top of the battery.

With your multimeter you want to measure DC voltage (this is indicated with a solid line and a dashed line above a letter V). Set the dial to 20. This will allow you to accurately measure between 0-20 Volts.

Hold the red probe to the positive terminal, and the black probe to the negative terminal. The terminals will be marked + and -. If you're getting a reading with a minus in front of it (-12.6 rather than 12.6) you've got the probes the wrong way round!

As we've discussed the resting voltage should ideally be no lower than 12.6V. Bear in mind that when a battery goes down to 12.2V it's actually only 50% charged, and below 12V it's classed as discharged!

One thing to bear in mind is that some cars can experience 'parasitic loss' where something electrical is faulty and drains the battery even with the engine turned off. If you suspect this is the case you can either remove battery leads from the battery, or remove the battery from the car entirely.

You should then fully charge the battery with a battery charger then test no less than 12 hours later. If the battery holds a charge when it's not connected to the car – something is at fault other than the battery or alternator.

How to check the alternator with multimeter

The alternator's job is to produce electricity and charge the battery. As you drive along the belt driven alternator takes over the role of delivering power to the car's electrical systems and also changes the battery.

So with the engine running (and being mindful about moving parts) conduct the same test as above with your multimeter. A healthy battery alternator combo should give a reading of between 13.8V and 14.4V.

Anywhere outside that range and your car's either under, or over charging – both of which will shorten the battery life and require further investigation.

How to test car battery for dead cells

If you suspect your battery isn't charging properly it's likely either to be the battery, or the alternator that's at fault. The battery light may come on the dashboard to alert you.

Many new car batteries are of the sealed 'maintenance' free variety. But some older batteries allow you to access the individual cells. The cells will either have individual caps, or a plastic cover that clips over all, or some of the cells.

The cells contain a mix of water and sulphuric acid, so you don't really want to be messing around inside them too much.

You can buy a hydrometer-style battery tester that measures the specific gravity of the battery acid, and can tell you whether any cells are dead or not.

However knowing a cell is 'dead' is of no more use to you than knowing that the battery won't hold a charge, so a test of the resting voltage is just as effective a diagnosis.

TOP TIP: Before you disconnect your battery make sure you know the code for your stereo – You may find it won't work when you reconnect it otherwise!