Test a car battery – what should the voltage be?
A good, healthy car battery should have no fewer than 12.6 volts. Because of the way lead-acid 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 battery's health.
How do I use a multimeter to test a car battery?
Testing a car battery with a multimeter is a simple process. The first thing to do is make sure you can access the two terminals on the top of the car battery.
Car 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. There may also be a cover over the positive (+) terminal, which usually lifts off.
Once the car battery is exposed be careful that nothing metal touches the terminals, so don't rest any spanners or other tools on it.
Using 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 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.
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.
What do I do if the car battery is repeatedly going flat?
If the battery is continually going flat, it could be because of a 'parasitic loss'. This is where something electrical - a light or a motor for example - is draining the battery, even with the engine turned off. Apart from doing the obvious (setting your phone to record video before you place it in the boot or glovebox to see if the bulb is staying on), you can either remove the leads from the battery, or remove it from the car entirely.
Then, fully charge the battery, leave it for 12 hours and test it. If the battery holds a charge when it's not connected to the car, it's not faulty.
TOP TIP: Before you disconnect your battery make sure you know the code for your stereo – otherwise you may find it won't work when you reconnect it!
How do I test a car battery for dead cells?
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.