How To Test A Car Battery With A Multimeter | Shopping Guides

See All Shopping GuidesHow to Test a Car Battery with a MultimeterJessica Shea Choksey | Jul 02, 2024

When your car doesn’t start, often a low or dead battery is the culprit. Unfortunately, most vehicle owners do not check their battery until it fails. As preventative maintenance, the suggested best practice is to conduct a car battery voltage test regularly – at least twice per year – using a multimeter.

What Is a Multimeter?

A multimeter is an electronic measuring instrument that is used to gauge volts, amps, and resistance from an electrical source. The most common automotive application for a multimeter is to test the strength of a car battery. When used properly, it will provide voltage information to a high degree of accuracy on a digital readout. Understanding the data provided is vital in determining whether the battery is strong and healthy or should be replaced.

Multimeter Cost

Multimeters are versatile mechanisms that can be leveraged beyond automotive applications. Even basic multimeters can measure voltage and current in household items such as switches, outlets, and other batteries. Depending on accessories and capabilities, the price for multimeters varies greatly. For testing a car battery, you can find multimeters for as little as $13. 

Prepare the Battery

The first step is to locate the vehicle battery (consult the owner’s manual) and determine if there is any dirt or corrosion at the positive and negative terminals. The positive terminal usually wears a red cover and a “plus” sign, while the negative terminal has a black cover and a “minus” sign. Since corrosive buildup can keep the multimeter from taking accurate voltage readings, it should be scrubbed off using fine-grit sandpaper. Gloves should be worn to prevent skin exposure to harmful chemicals and battery acid. Once the terminals are clean, they will serve as the connection points for the multimeter’s probes.

Multimeter Setting

The multimeter may look complicated due to its various measurement settings, but the general operation is fairly simple. For testing the electrical output from a vehicle battery, the multimeter dial should be turned to the “20 volts” setting. But before the multimeter can be used, all surface charge from the battery must be removed to allow for an accurate reading. To do this, the headlights should be turned on for about two minutes and then turned off.

What Voltage Should a Car Battery Be?

There are three different voltage tests to run for your battery. The first is to test your battery when the vehicle is at rest. Here, the multimeter should measure at least 12.2 volts. The second test is to measure battery voltage upon starting the ignition at which point the battery should not drop below 10 volts. Finally, once the vehicle is running, the multimeter should measure at least 14 volts.

How to Test a Car Battery?

To measure battery load, conduct three voltage tests with the multimeter. Start the first test by making sure the vehicle is turned off. Multimeters have two probes: red and black. The red probe is for contact with the positive terminal and the black probe is for contact with the negative terminal. Connect the multimeter’s red probe with the positive battery terminal and the black probe with the negative terminal. The multimeter display should show a reading of 12.2 to 12.6 volts indicating a full charge. This voltage range means the battery is in good condition for starting the vehicle. If the measured reading is less than 12.2 volts, the battery’s resting voltage is weak, which means it most likely needs to be charged or replaced.

Once the resting voltage has been determined, it is time to get a reading on the crank cycle. This is the moment that the vehicle is turned on and the battery is under the most draw because of the higher amount of energy needed to drive the starter motor. To get this reading, a second person will be needed for the purpose of starting the ignition. As soon as the car is turned on, the voltage reading will drop for a quick moment, but should not fall below 10 volts. If it falls below 10 volts, it means the battery does not have sufficient turnover strength and is prone to failure. Again, in this case, recharge or replacement of the battery may be necessary.

Immediately following the crank cycle, the vehicle will begin to idle and maintain a steady draw from the battery. With the motor running, the multimeter rating should stay in the 14 to 14.5 volt range. Dropping below 14 means either the battery is weak and unreliable for sustained vehicle operation or the alternator is failing. The alternator’s job is to generate energy to feed the electrical system and charge the battery while the vehicle is running.

Alternator Test

To test the alternator, start the vehicle and turn on all of the vehicle’s electrical equipment – headlights, interior illumination, climate controls, and stereo. This will maximize the voltage load. To measure the voltage, attach the multimeter’s red probe to the positive battery terminal and the black probe to the negative terminal. If the multimeter displays a reading below 13.5 volts, then the alternator is struggling to properly charge the battery and may require replacement. This result confirms it is time to consult with a licensed professional for a second opinion.

Summary:

Multimeters provide car owners with valuable information about their vehicle’s battery and electrical system. Biannual testing can predict and prevent imminent failures that often occur without warning. The sooner owners are made aware of a possible issue, the sooner they can react by shopping for a battery or alternator replacement and budget accordingly. Similarly, it also gives owners peace of mind to hopefully avoid being stranded due to an electrical issue with their vehicle. 

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