How to Test a Car Battery With a Multimeter
Checking your car’s battery with a multimeter is a simple, easy process. Don’t let yourself be scared away by any complex-looking dials and probes – Test your car’s battery safely and easily with our guide.
What is a Multimeter?
Multimeters measures several different properties related to electricity; typically voltage, current and resistance. Most home multimeters have a digital display, a large dial, and are no larger than the average cell phone.
To use a multimeter, two “probe” wires are inserted into the base unit, then attached to something with an electrical charge (like your battery!); the dial usually tells the multimeter what sort of measurements to take. The dial may have a multitude of settings but for testing your car battery we’ll only be looking for one: DC (or “direct current”), set to 20 volts. The DC side of the multimeter dial will be designated by a “V–” symbol (there will be three dots underneath the straight line as well); “V~~” (with tildes) is AC (alternating current) and is the wrong setting for this test.
Your probe wires should be colored red and black, with corresponding red & black ports on the multimeter base for them to fit into. If multiple ports exist your black probe should go into the slot labeled “COM”, while your red connector should go into a port labeled “volts/homs/milliamps”. If a port labeled “10A” is present do not use it for testing your car battery; this could damage your multimeter.
Locating & Preparing Your Vehicle’s Battery
Before you begin make sure your vehicle has been at rest for at least one hour before testing your battery – This gives you the “resting voltage” of the battery, rather than what could be a false reading after the battery has been in recent use. Testing your battery first thing in the morning, before using your vehicle for the day, is ideal.
Most car batteries are located in the hood of the car, near the engine block, though some models of vehicle (and many European cars) have the battery located in the trunk, underneath the floorboard. Once the battery has been located it may have an additional plastic hood on the top that can be removed to access the battery underneath.\
There should be two terminals on top of the battery, one labeled “positive” (colored red) and one labeled “negative” (colored black). There may be a pair of small plastic sheathes covering the terminals – These will need to be removed prior to testing.
Once the terminals have been exposed take caution, as the battery is very much “live” and can cause shock and injury if touched. Of particular note: Never let an object bridge the two terminals, as this can lead to a dead battery at best and a literal explosion at worst. Work with care and attentiveness, and make sure to keep metal tools and jewelry away from the battery terminals.
If the terminals are heavily corroded the battery may need to be removed and cleaned before it can be properly tested – Corrosion can give off false or otherwise imprecise readings. Remove the battery according to manufacturer’s instructions and clean the corroded bits with terminal cleaner and a terminal-cleaning brush. If it comes to this point, taking it into a shop for this step might be your safest option.
Using the Multimeter
Again, we want our multimeter set to the “DC 20v” setting on the dial with our probes in the approperiate ports. With the multimeter on, and making sure your car engine is off, take your red probe and touch it to the positive (red/”+” sign) terminal, doing the same with your black probe and the negative (black/”-” sign) terminal.
Your reading should appear instantly. A healthy, fully-charged battery will have a multimeter reading of 12.6 volts; 12.4 is considered 50% charged and 12.2 is considered 25% charged. Anything reading below 12 volts on the multimeter is a sign of a dead battery.
If your car hasn’t been starting you can also perform a crank test: Have a friend or family member crank the car for you while the terminals are attached. If your reading dips below 10.0 volts (or more than 2 volts less than your initial reading) this is a sign that your battery may not be producing enough current to handle the starter motor.
No matter what the end result we hope our guide to using a multimeter has helped you feel confident about testing your car battery from home.
No Power? Need to find the Best Car Battery?
If your battery has gone flat and can’t be recharged, it might be time for a new one. Check out our guide to the best car battery for more info on what brands and models might be suitable for your car.