What’s the right voltage for a car battery?
A healthy car battery should have at least 12.6 volts, but you should 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. This is because of the way lead-acid batteries discharge. This way you'll be able to measure the 'resting voltage'.
Testing a battery with a multimeter
First, make sure you can access the two terminals on the top of the battery.
Batteries are commonly located under the bonnet 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.
If the battery isn't in the engine bay it may be under the boot floor. Your Haynes Manual will show you where it is.
With the terminals exposed, makes sure nothing metal touches the terminals – don't rest any spanners or other tools on the battery.
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 12.6 volts. Bear in mind that when a battery goes down to 12.2 volts it's actually only 50% charged, and below 12 volts it's classed as discharged.
But the battery is going flat all the time!
This could be because of a 'parasitic loss', which is where 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 and closing it 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 check for dead cells?
Most new car batteries are 'maintenance free'. 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.