How To Test a Car Battery With a Multimeter
Having a no-start situation on your car is frustrating, and stressful. In most cases, you’re left wondering what’s actually going on with the vehicle, and culprit #1 tends to be the battery. 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? How do you know if the battery is actually charged properly?
A cheap tool and a simple test with a multi-meter can let you know if your battery is properly charged. There are options with multi-meters from $20 to several hundred dollars, and they’re extremely helpful for any electrical work you need to do. Learn the easy solutions to potential problems that can come with a bad battery with a multimeter, and here’s how you use it.
Testing Your Battery
It’s important to note that a multi-meter is great for checking if your battery is fully charged, and can be used to test a small amount of load on the battery, but should never be used as an alternative to an actual load-test like AutoZone can provide. The multi-meter test 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 two 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. If one of the battery’s cells is low when you remove the caps, you will need to fill that cell with a small amount of distilled water. Make sure that any time you remove the battery’s caps, you always have proper nitrile or rubber gloves, and eye protection!
How to Use a Multimeter
Essentially, a multimeter is a device to test electrical circuits for a variety of industries. It can be used for measuring current, voltage, resistance, continuity, and a gamut of other things. There are analog multimeters still around, but the majority you’ll see available today are digital multimeters, with a small screen that displays your readings.
It has one black lead and one red lead that need to be connected or touched off to a point. You will be connecting the red lead to the positive terminal of the battery, and the black lead to the negative or ground terminal of the battery. You will want to read your multi-meter manual to determine that the test leads are in the proper connecting spots on the meter for measuring voltage and continuity, and that the meter is set to DC voltage.
If your multimeter has increments, set it for the next number higher than the voltage you’re testing. For example, if your digital multimeter has settings for 2, 20, 200, and 2,000 volts, choose 20 since it’s the next highest number to the expected 12+ volts in a car battery.
Then, it’s a matter of attaching or touching the black lead to a ground source and probing your circuit with the red lead. Watch the display for your voltage reading.
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 test one thing on the battery, and that’s to measure voltage. A battery test at AutoZone has an advantage in that 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, also called a surface charge. Once you’re sure you can use your multimeter, you’re ready to get started.
How to Test a Car Battery with a Multimeter
1. Prepare Your Multimeter
Set your multimeter to voltage and ensure its adjusted to 20 DC volts, or if your voltmeter does not have incremental settings (2, 20, 200, 2,000) then simply set it to DC volts.
2. Touch the Probes to Your Battery Terminals
Press each probe to the correct terminal to test your battery, 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.
3. Check the Reading
Take a look at the initial reading with the vehicle off. If the battery is below 12 volts to start with, the battery is immediately suspect. Starting voltage on any battery is 12.4 volts or more. At this point, you’ll want to remove the battery and have it tested at AutoZone. If the battery is extremely low (8 volts or below) its generally not a good idea to jump-start the battery as this can cause a massive strain on the alternator. If the battery voltage is above 12 volts, 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. See if you have a large drop in voltage once the vehicle is switched to run.
The temperature outside affects the voltage of the battery. At about 80 degrees, a fully charged battery will be around 12.5 to -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.
4. 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-amperage 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 a few 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.
FAQ for Testing Your Battery with a Multimeter
Touch the red lead to the positive battery post and the black lead to the negative post. The result will indicate whether the battery has a sufficient charge or needs to be recharged or replaced.
To test a car battery, the multimeter should be set on DC Volts. If your tester has increments, choose the next highest increment to 12 volts
If your voltage measured is at 12.5V to 12.6V when it’s tested during the summer, it has a sufficient charge. A fully charged battery will measure nearly 12.7 volts.
Set a multimeter to DC Volts to check a battery. Choose the 20V option if your tester has increments.
Knowing how to use a multimeter can help detect if a battery is discharged or has a significant parasitic load. However, you should have your battery load tested to determine if it’s, in fact, faulty. You can also learn the <a href=”https://www.autozone.com/diy/battery/is-your-car-battery-dead”>signs of a bad battery</a>.
