A Definitive Guide To Test Your Car Battery With A Digital Multimeter

May 14, 2019December 1, 2020 The Mechanic Doctor battery, multimeter

Multimeters are every handyman’s best friend. They can be used to perform numerous important tasks and they are absolutely essential in troubleshooting faulty electrical components, especially car batteries.

As it turns out, checking the voltage of your car battery is not at all difficult but yet again, make sure you get correct readings and end up with a correct diagnosis, you need to know how a multimeter works. To avoid any errors and possible damages, it would be wise to have this done by a professional but yet again, this would not be as convenient as doing it yourself.

Go get yourself the best volt-ohm meter possible and follow the step by step guide below to learn all you need to know to quickly and accurately check the health of your car battery

Testing a car battery using your digital multimeter

Testing the voltage

On some car models, the voltage of the battery can be checked at the dashboard but that won’t help you know how good is your battery since the voltage reading in the dashboard actually shows the power coming from the alternator and not the battery. Here’s how you should check the battery voltage readings.

Step 1: Set the multimeter

First off, make sure the red probe is inserted in the voltage jack and the black in the COM jack.

The dial setting on an auto-ranging and a manual-ranging digital multimeter work differently. If you don’t have a multimeter yet, I’d recommend that you go ahead and get one with the auto-ranging feature. They are simply easier to use since the multimeter will automatically select the ideal range of measurement for you.

Step 2: Battery polarity

Obviously, you’ll want to connect the red probe to the positive post of the battery and the black probe to the negative side. To save out the trouble, the positive terminals of most car batteries are normally red.

When measuring voltage, the polarity doesn’t matter as much as if you were measuring amps or jump-starting the vehicle. The only difference is that you’ll get a reading of -12.6v instead of 12.6v if the polarity is reversed. Simply switch the probes and you’ll be good to go!

Step 3: Provide a light load and check the readings

First, turn the ignition key to the ON position and turn the car headlights on for 30 seconds to eliminate any chance of parasitic charges that could give you false readings.

A car battery’s voltage is dependent on the outside temperature so assuming it’s 27 degrees Celsius, here’s what you should make of the readings you see on the multimeter display:

For readings that are over 12.5 volts, it means that your battery is fully charged and it is in good condition.
If the readings range between 12 and 12.3 volts, your battery is operating at ¾ of its capacity and it should take you sometime before you’ll need to replace it.
Lastly, if your battery is operating in critical condition and is at around 11.8 volts, this means that it is operating at around ¼ or 25% of its full capacity.

It’s worth mentioning here that your only measuring the voltage of the battery and not the cold-cranking amps. A fully charged battery could still die right away when the ignition key is turned as being full charge doesn’t been that it can still deliver enough power to make the starter turn in extreme cold weather conditions. You’ll need to perform a battery load test for that. Load testers are professional tools and regular people don’t usually have one at home so you might need to stop by your local auto repair shop to have your battery checked

What to do in case of a low reading

If you get a reading below 12.6v, the first thing you’ll want to do is to recharge your battery using a smart battery charger. The voltage of a battery will gradually lower as time goes by so if the car hasn’t been started for a couple of days, it’s maybe totally normal. A quick charge will fix that right up. Once it’s charged, perform the same test again and see if the readings are better. If they aren’t, you’ll probably want to have your car battery replaced in the near future.

You could also just start the car and let it idle for about 10 minutes. You can also use that time to take a reading of the battery while it’s running, and see if your alternator is still doing his job. A faulty alternator could prevent the battery from recharging correctly while you drive so that maybe your problem.

When the car is at idle, the voltage reading of the battery should range between 13v and 14.6v. Anything below that will let you know that your alternator needs to be replaced. Ignoring it will only make it worse and a faulty alternator will slowly damage the battery. Simply replace it and you’ll end up saving money in the end.

Checking the terminals of your car battery using a digital multimeter

In some cases, the problem may not be with your car battery charge at all but your battery terminals instead. When the battery terminals are dirty, corroded or loose, the car may be a little difficult to start or may not even start at all.

While a loose battery terminal may be pretty obvious, other bad connection problems may be quite difficult to figure out with a simple visual inspection. Once again, your multimeter will be your best ally here.

You’ll need an assistant for this one.

Here’s what you should do:

The first thing you’ll want to do is to disable the ignition or the fuel injection system. To do that, either disconnect the ignition coils to disable the ignition system or remove the fuel pump relay or fuel pump fuse to disable the fuel system. Take a look inside your vehicle’s repair manual if you need help locating them.

Disabling the ignition system basically prevents the engine from being started. In this case, I strongly recommend you disable the fuel system as it will prevent the engine from being flooded while you crank.
The next thing you should do is have the red probe of your multimeter attached to the positive post of the battery and the black one to the positive cable terminal.
Turn the dial of the multimeter to the 2V measurement station and then ask an assistant to crank the engine up.
On the multimeter display, you should register a reading of 0.5 volts or below. If otherwise, you’ll have to either check the physical condition of the post and respective terminal and clean it up.
Repeat the same process for the negative terminal.

Taking a leak test on the battery cover using a DMM

The long-term result of poor maintenance of your car battery is that you’ll end up with a build-up of deposits on the cover. In most cases, it happens when electrical charge can leak from the battery because of dirt and acid deposits on the battery posts.

A visual inspection is not enough to determine whether the battery has reached the leaking point and using a digital multimeter will help you confirm it in a jiffy.

First, have the dial of your multimeter set at the lowest voltage reading available and once again confirm that the red probe is in the Voltage port and the black one is in the COM port.
Turn on your multimeter and touch the black probe on the negative terminal of the car battery and the red probe at the battery cover.

In the likely event that you register even the tiniest voltage reading from the battery on your multimeter, the build-up of deposits has resulted in the charge leaking from the battery. This would then you should clean it up a dedicated car battery cleaner or simply using warm water and baking soda which neutralizes the acid hence allowing you to lift the dirt from the battery cover.

If you suspect that another electrical component is draining the battery when the key is in off position, commonly called a parasitic drain, you’ll have to perform a parasitic drain test to locate the faulty components and prevent it from draining the battery every time the engine is turned off.

Final verdict

If you follow these steps carefully, it should now be a walk in the park for you to troubleshoot the health state of your car battery using a multimeter. Since visual inspection cannot be entirely relied on, stepping things up and using a digital multimeter is the next best alternative to a full charging system test performed by a certified auto mechanic. When in doubt though, it is still highly recommended that you give your mechanic a call if you suspect something may be wrong with your car’s ability to start in the morning. Professional advice from a trustworthy mechanic will often save you time and money in the end.

It’s all about finding the right one.