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.
How to test a car battery with a multimeter
A multimeter is an electrical 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.
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.
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, then turned off.
Measure and Analyze
For measuring battery load, the multimeter has 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.
When the probes touch the terminals while the car is off and the battery is resting, the multimeter display should show a reading of 12.2 to 12.6 volts (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 the 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.
To test the alternator, turn on all of the vehicle’s electrical equipment – headlights, interior illumination, climate controls, stereo. This will maximize the voltage load. If the multimeter reading drops below 13.5 volts, the alternator is struggling to properly charge the battery and may need replacement. It is time to consult with a licensed professional for a second opinion.
Utilizing a multimeter can provide a car owner with valuable information about their vehicle’s battery and electrical system. Periodic testing can help predict and prevent imminent failures that often occur without warning.