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.
To determine if your alternator or charging system is causing the problem measure the voltage across the battery terminals when the car is running. The voltage should rise higher than it was before the engine was started, normally somewhere over 13.5V. Turn on as many accessories as you can and monitor the voltage. With a helper giving the car some gas to get the revs up to normal cruising speed the voltage should stay above 13.5V. If it drops below this value there may be a problem with the charging system.
If you plan on leaving your car for a couple of weeks without starting it, consider hooking it up to a battery maintainer/float charger to keep it topped up, which should help the battery last longer.