How to Use a Multimeter on a Car Battery

Using a multimeter to check your car battery is a simple DIY procedure that you can perform at home. It enables you to know if the battery has enough charge to start your car. You surely wouldn’t want to hear that click, click, click sound when on a road trip or when you stuck in the middle of nowhere. That clicking sound usually means the battery can’t output enough Cold Cranking Amps (CCA) to start your car.

Checking your car’s battery health is a pre-departure preparation that every driver should know. It requires you to use a multimeter that measures the resistance, voltage, and electric current present in electronic devices. A digital multimeter is preferred since it gives a more accurate reading than an analog multimeter or category I (CAT-I) multimeter. Below is a step by step guide on how to use a multimeter on a car battery.

Basic Equipment &Tools 

Apart from a digital multimeter, to test your car battery safely, efficiently, and accurately you also require

• Protective gloves
• Safety glasses
• Battery-terminal cleaner
• Phillips head screwdriver – for tightening any

#1 Familiarize Yourself with the Multimeter

If it’s your first time dealing with a digital multimeter, it is advisable to familiarize yourself with it first. You will notice it comes with two electrical connectors that have metal probes at the end.

Note: The wires are colored red and black; the red-colored wire is positive while the black colored wire is negative. It is crucial to note this as each should be connected to the correct terminal of the battery of your car.

Testing your car battery involves measuring the DC voltage of the battery; you should select the DC volts position on your multimeter. The DC voltage is marked as a V; it’s followed by a short line and a dotted line below it. Do not select the wavy line (sine wave) after the V, that’s for measuring AC (alternating current) voltage.

#2 Turn off Your Car’s Engine

Testing your car’s battery requires that the engine, lights, and radio be turned off. However, ensure that you turn the headlights on for about 2 minutes; it will help to get rid of any surface charge which can influence the readings.

Also, the battery should be tested after sitting for more than an hour if you’ve been driving to get the correct voltage reading that’s called ‘resting voltage.’ Failure to do this may give inaccurate readings.

Locate Your Car’s Battery

The initial step of checking a car battery is first locating it. Most modern cars have a maintenance-free battery i.e., those batteries which don’t have removable caps. In most car models the battery is mostly housed in one the following places

• The engine bay – open the bay and look for the battery near or around the engine. Most likely, it will be under the hood near the fender or to one side of the engine.
• The trunk – this is mostly for two-seater cars and small ones.
• Under the back seat cushion or any passenger seat.

Some carmakers may hide the battery in obscure places. If you are unable to find the battery consult the owner’s manual or do a quick internet search on your car model to find out where the battery is placed.

At this point, you will have to wear your protective glasses and gloves as you will start handling the battery. The battery will most likely have a plastic cover which can be unclipped or hinged up. Other batteries require removing the plastic by untightening a few bolts or screws; that’s why we recommended you have a Philips head screwdriver. The positive (+) terminal may have a red cover that can be lifted off or snapped open.

Note: If you remove the plastic cover, the battery is exposed, ensure that no metal touches the terminals as it might cause a short.

Inspect And Clean The Battery Terminals If They Are Corroded.

Closely inspect the terminals and cables of your car’s battery for any corrosion. If there is corrosion, it mostly resembles a greenish-yellow crust buildup. Corroded terminals can cause car starting problems, charging system problems as well as interfere with your multimeter readings.

Use a battery-terminal cleaner and a terminal brush to clean and remove any corrosion. Furthermore, make sure you tighten any loose cables using the Phillips head screwdriver.

Set Up Your Multimeter

If the multimeter leads aren’t connected, insert them to their correct connectors on the multimeter. The black lead into the black-colored connector (labeled COM), and the red lead into the red connector (labeled ohms, volts, milliamps).

Turn the multimeter function dial to 20V. As of now, the multimeter display should read 0.00.

Connect the Negative and Positive Probes to The Battery Terminals

Connect the red cable probe of your multimeter to the positive terminal on your car battery. Connect the black multimeter probe to the negative terminal on your car battery.

If your car’s battery is placed or mounted in a difficult to reach the area, you can use the jump-starting terminals to connect your multimeter.

Read & Interpret the Test Results

Read the voltage reading on your multimeter screen.

When the car’s engine is turned off, the battery voltage readings are as follows for a battery at 80 degrees Fahrenheit.

• 12.66 volts or more = 100% charged – The battery is good
• 12.45+ volts = 75% charged – The battery is good
• 12.24 volts = 50% charged – Battery should get charged
• 12.06 volts = 25% charged- Battery should get charged
• 11.89 volts = 0% Discharged battery

Note: Battery voltage will change 0.01 volts for each 10-degree temperature change.

Load Testing A Battery Using A Multimeter

A multimeter state of charge reading isn’t an indication of a good or bad battery. A load test gets used to check if a battery should be replaced.

To load test the battery, follow the steps below;

• Note battery voltage while the engine off.
• Get someone to crank up the car as you watch the voltage drop on the multimeter

A voltage drop of fewer than 2volts e.g., from 12.4 volts to 10.5 volts, indicates the battery is good. A voltage drop of 2V and above or a reading of under 10V indicates the battery needs replacement.