If you’ve been having problems getting your car started or it has been starting slowly your battery may be getting weak. Knowing how to test a car battery could prevent you from wasting money on a new battery when you don’t need one. It’s also a good idea to test car battery at the start of winter to help make sure that your battery is up to the task of starting your car on a cold morning.
Before testing a car battery you will need to make sure that it is fully charged. A fully charged battery should measure over 12.5V across the terminals. If you cannot get the battery up to this voltage it may have a dead cell. If your battery is completely discharged it’s best to use a battery charger to top it up as alternators are not meant for this task and do a poor job.
To test a battery you need to apply a load. Most shops use a specialized battery tester that can put a large load on the battery, however these are not necessary (though they are convenient). A quick and easy test is to turn the headlights on for 5 minutes or so and then read the voltage across the battery terminals, which should remain above 12V. Another test is to read the voltage across the terminals while a helper starts the car. The battery should read more than 10V when cranking, and shouldn’t drop to below 9V even after 10-15 seconds of cranking on a cold day.