How To Test a Car Battery With a Multimeter
You get in the car and turn the ignition but don't hear the comforting hum of your engine. It won't start and the lack of power indicates it's probably the battery. Sometimes the reason is obvious, such as realizing you left your headlights on all night. But other than searching "how to tell if car battery is dead," how do you know if you can jump it or just need to replace it? A cheap tool and a simple test can let you know if your battery is holding a charge. Learn the easy solutions to potential problems that can come with a busted battery.
Testing Your Battery
Using a multimeter, you can determine a couple things about your battery – whether it is at sufficient charge, and how well it handles several demands. This is generally a first step before going through the removal process, or jumping the battery and bringing it into AutoZone for further testing. Any car battery can be tested in this method with a multimeter. Batteries that are not “maintenance free” have caps across the top of the cells, sometimes individually, and sometimes 2 caps. You can use a hydrometer-style battery tester to test the individual cells by testing the specific gravity of the acid in each cell, but even once you find a bad cell, you’re arriving at the same place you would by simply using your multimeter to do some simple voltage tests.
Testing Batteries with a Multimeter
Before you learn how to test a car battery, there are a couple of things to keep in mind. A multimeter itself is going to do one thing – measure voltage, and in this case, DC voltage. Where a battery test at AutoZone has an advantage is it’s going to simulate a starting load on a battery. Many bad batteries will appear to have full voltage, but when it comes to holding that voltage under load, the battery reveals it is bad. There are several things you can do at home to simulate a load, which we will walk through, but none is as good as a thorough load-test that AutoZone can perform. Next, you should only test a battery after it's been sitting unused for at least an hour. This is known as “resting voltage”. If you test it immediately after use, you can receive a false reading from a unit charged by your car's electrical system. Once you're sure you can use your multimeter, you're ready to get started.
Test Your Battery
How to Test a Car Battery With a Multimeter
Prepare Your Multimeter
Set your multimeter to voltage and ensure it's adjusted to 20 DC volts, or if your voltmeter does not have incremental settings (2,20,200,2000) then simply set it to DC volts.
Touch the Probes to Your Battery Terminals
Press each probe to the correct terminal, touching negative to negative and positive to positive. Your multimeter and battery are probably both color coded, so the negative terminal and probe will be black while the positive terminal and probe are red.
Check the reading
Turn the vehicle's key to the run position and check the multimeter. It's a good idea to write down the reading. Then, compare it to the voltage figures below.
The temperature outside affects the voltage of the battery. At about 80 degrees, a fully charged battery will be around 12.5-12.6 volts. To be more specific: a fully charged battery ideally measures at 12.66 volts and above, but 12.6 volts at 80 degrees, 12.588 degrees at 30 degrees, and 12.516 volts at 0 degrees are acceptable readings. A 75% charged battery will measure closer to 12.45 volts while anything below 12 volts indicates the battery is effectively discharged.
If you get a reading between 12.3 and 12.5 volts and have the ability to charge the battery, try charging the battery up to full, which shouldn't take long. Next, turn on the headlights, and the heater blower motor and check the voltage. The voltage should drop by a few tenths, but shouldn't be drastic. At this point, if the car still is not starting – meaning you are getting a click or buzz when attempting to start, do a multi-meter test on the battery while someone attempts to crank the car. Observe the reading as this happens. Most good batteries when a serious load like a starter is put on them will drop 1 to 1.5 volts during the load, then quickly return to full charge once over. If you notice the battery drop from 12.5 down to say, 8 or 9 volts or lower, this is an indicator that the battery may indeed be bad.
Bring the Battery (or Vehicle) in to AutoZone
It's important to note, that jump-starting and driving on a potentially bad battery is not a good practice. A bad battery that cannot keep a charge puts an incredible strain on an alternator, which is not designed to put a high-amp charge on a battery, unlike generators of old, which could do this. This is one of the prime reasons why many vehicle owners change out their battery, only to come back where they purchased the battery 3-4 days later, angry that the battery has died again, only to then find out the worse news – the alternator is also bad. If you suspect the battery is bad based on your multimeter testing, remove the battery and bring it into AutoZone for further testing. If the vehicle will start on its own, bring it into AutoZone where it can be checked. If you do end up replacing the battery, you can learn how to test your charging system here, or bring the vehicle back up to AutoZone for a charging system test. Don't assume once the battery is changed that everything has returned back to normal.