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How to Tell If Car Battery Is Bad With Multimeter

Fernando the electronics guy is an electronics engineer. He earned his Bachelor of Science in Electrical Engineering from UC Riverside.

how-to-tell-if-car-battery-is-bad-with-multimeter

Your car battery has 6 cells. Each cell is 2.1 V at full charge. Let's illustrate this below.

6 Cells

6 Cells

Each cell in series adds up to a larger amount. Shown below is what voltage readings look like when you measure between each cell.

how-to-tell-if-car-battery-is-bad-with-multimeter

Notice the lines are getting higher and higher. Having multiple cells one after another means added voltages.

The highest line above, in gray, is the battery voltage total of 12.6 V at full charge.

Now, let's walk away from the specifics for a second. Let's look at the battery from positive to negative.

how-to-tell-if-car-battery-is-bad-with-multimeter

At large, your car battery's voltage is 12.6 V. All 6 cells are inside your battery's exterior housing. You know, the one below.

how-to-tell-if-car-battery-is-bad-with-multimeter

Perhaps your car battery won't look exactly the same. However, they're cousins. They're similar.

So, how do you test the damn thing?

How to Tell If Car Battery Is Bad With Multimeter

how-to-tell-if-car-battery-is-bad-with-multimeter

See the image above. You're going to put red to red, and black to black. Red goes with positive. Black with negative.

Let's pretend you don't have a multimeter. Let me show you mine. See the image below.

how-to-tell-if-car-battery-is-bad-with-multimeter

This is a Fluke 27 multimeter. It's not the best but it works.

Now, make sure you're in DC voltmeter mode and not in AC voltmeter mode. Two completely different modes.

Scroll to Continue

Note: AC has a wiggle line. DC has a straight line above a dashed line. See below.


how-to-tell-if-car-battery-is-bad-with-multimeter

Now, connect your testing probes. Positive to positive, negative to negative.

Or, by color, red to red, black to black.

Note: avoid the "A" section. Connect in the "V" section as shown below.

how-to-tell-if-car-battery-is-bad-with-multimeter

Here are what the test probes look like with a corresponding unit below.

Note: model shown below is a different Fluke model. For the bottom model, negative is universally the black spot. When wishing to test DC voltage, such as a car battery, always connect the red probe to a red spot with the name: "V", or "VDC", or "DC", etc. Avoid "A".

how-to-tell-if-car-battery-is-bad-with-multimeter

The non-pointy parts of the probes are called banana plug terminators. You'll need to push with extra force to get them in.

Test Car Battery With Multimeter

Now, you have your unit on DC voltmeter mode and your probes are in. It's time to finally test your car battery's voltage.

Apply the red positive probe to the + terminal of your car battery.

Next, apply the black negative probe to the - terminal of your car battery.

Note: If you do this in reverse, it's perfectly safe. It only makes the reading negative.


Diagnosing Your Car Battery

Now, there are a couple of scenarios to look out for.

  • Battery is above 12 V. Battery is looking great.
  • Battery is above 12 V but a few hours later falls below 12 V. This means your car battery is unable to hold a charge. The cells may be leaky and thus unable to hold a charge. Consult with a technician for advice. Or, get it tested at a local auto parts store or with your local mechanic.
  • Battery is above 13 V. This only happens when you're checking your battery voltage while the car is on, silly. Turn that thing off. This is great for checking the alternator. However, this tells us nothing more about the battery.
  • Battery is above 12 V last night. This morning it fell under. Same thing as the last time. Only this time you have an overnight discharger. This is common with bad alternators. However, it's also possible you have a leaky system such as an extra external lighting of some sorts. Or your battery sucks.
  • I'm not able to get a reading. Check your multimeter. Make sure it has batteries. Ensure your probes are connected properly. Lastly, ensure you're in DC Voltmeter mode and not AC mode. AC mode has the wiggly line. DC mode does not.
  • Battery is below 12 V. The only information this tells us is that your battery is not fully charged. It makes sense if your battery is old. However, this is not ideal for a 12 volt system. Get the battery charged. Ensure that it is fully charged and above 12 V. If it persists below 12 V you need a new battery.

Lastly, ensure your voltage stays night and day. This is the best test for a bad battery. Easy enough with a voltmeter.

That's all there is to it. Use your best judgment if you're going to check this yourself.

Let me know your diagnosing stories. What happened and what worked for you?

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 electronicsguy

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