Knowing how to perform a simple test of a car battery with a multimeter can save a lot of time and hassle over the winter. Car batteries lose about 20% of their capacity at 0°C compared to 25°C, so it’s vital to give them a periodic check to stop you getting stranded.
How to Test Your Vehicles 12v Battery
- First the battery needs to be in a resting state, so leave the car overnight. This allows the alternators given charge to the battery from driving around to drop to it’s ‘resting’ voltage, which is the measurement you want to take. The resting charge is what starts the engine on freezing mornings don’t forget!
- On some vehicles you’ll need to know your cd/radio players unlock code, as when the battery is disconnected and reconnected, it will reset.
- Open the bonnet
- Remove the positive red terminal safety cover from the battery
- Set your multimeter to 20 DC volts. The DC Volts symbol looks like a V with 3 dots above it with a horizontal line on top. See below:
- Don’t use the symbol with a V and a squiggly line next to it, as that’s AC volts.
- Before you begin testing, make sure the ignition and all of your vehicle’s lights are off.
- Connect the red or positive multimeter test lead to your battery’s positive terminal.
- Then, connect the black or negative multimeter test lead to your battery’s negative terminal.
- Jot down the reading given by the multimeter
Any reading below 12.2v is very bad, bear in mind that this reading means the battery is only charged by about 50% which can be surprising for many people. A reading of 12.4v may just need an overnight recharge if it’s an old battery. However note that a 100% charged battery should read a healthy 12.6v+. If your battery won’t charge overnight to anywhere near these figures, then it’s completely bust.
Use the guide below to distinguish your batteries charge state:
- 12.66 volts = 100% charged
- 12.45 volts = 75% charged
- 12.24 volts = 50% charged
- 12.06 volts = 25% charged
- 11.89 volts = 0% charged
Cold Cranking Amps (CCA) is the battery specification you want to look for when buying a new battery, as the higher the number means it’s better equipped to deal with colder weather.