DIY

How to Test a Car Battery with a Multimeter

Reading Time: 5 minutes

Your car’s battery is responsible for providing the jolt of electricity needed to start your car. The battery also supplies electrical energy to the rest of the vehicle whenever the engine is not running.

You can easily check your car battery’s state of charge using a multimeter. Although checking your car battery’s state of charge is not necessarily reflective of your battery’s overall condition, it’s a good place to start. 

Testing Your Car Battery’s State Of Charge With a Multimeter 

A good car battery should be able to accept and hold a charge. If you want to check your battery’s overall health, you may ask a  professional to do a load test using a hand-held digital analyzer. However, if you simply want to check the state of charge, you can do so with a multimeter.

To put it simply, a multimeter refers to a device used to measure voltage, amperage, continuity, and resistance in electrical components and circuits. 

Closeup of digital multimeter
A digital multimeter comes with two electric connectors with metal probes⁠—the red wire stands for positive and the black one represents negative.

If you’re the type who loves to DIY, then you probably own one. 

A digital multimeter comes with two electric connectors with metal probes⁠—the red wire stands for positive and the black one represents negative. 

Before researching how to test a car battery, it is important to familiarize yourself with the device. If you’re used to doing a DIY car test, then you may proceed with doing the following steps on how to test a car battery with a multimeter.

How to Remove Surface Charge

If you’re going to test your car battery with a multimeter, make sure to remove the surface charge to avoid an inaccurate voltage reading. 

Surface charge refers to a higher-than-normal charge that is just on the surface of the battery plates. If the battery has just been charged or if the car has just been driven, it will most likely have a surface charge. 

Since a surface charge is quickly removed once a car battery is loaded, not removing it before testing may cause inaccurate results. 

  1. Turn the high-beam headlights on for one minute. 
  2. Turn the headlights off and wait for two minutes. 
  3. Make sure that your engine is off. All electrical accessories must also be turned off. Keep the door shut to make sure that the interior lights are turned off. 

Once you’re done removing your car battery’s surface charge, you may proceed with connecting the voltmeter or multimeter. 

How to Check Your Car Battery with a Multimeter

  1. Set the device to read DC (direct current) voltage. 
  2. After doing this, set the dial to 20 volts so you will be able to measure 0-20 volts. A fully charged battery should give out at least 12.6 volts, so setting the dial to 20 volts is enough. If you’re using an auto-ranging meter, you will not need to perform this step. 
  3. It’s time to connect the leads. Make sure to remember which one is positive and which one is negative. 
  4. Get the black metal probe (negative) and then hold or touch the tip to the negative battery terminal post.
  5. The red metal probe (positive) should also be touching the tip of the positive battery terminal post. 
  6. As a guide, the terminals are marked “+” or “-.”  After connecting the probe, make sure that you are not getting a reading of -12.6 or any reading with a “-” symbol in front. That would indicate a negative voltage and it means that the wires are wrongly connected. 

Note: The resting voltage should not be lower than 12.6 volts. Take note that if a battery goes below 12.2V, it means it is 50% charged. If it goes below 11.9V, it is considered “discharged”. 

  1. If you find that the car reading is less than the ideal charge, you may want to remove your battery and charge it.
  2. After charging, leave the battery to rest overnight and go back to test it again the following morning without connecting it to your vehicle.
  3. If the battery continues to lose its charge, you should have it tested by a professional. Professionals can determine the health of a battery using advanced equipment, such as a load tester and hand-held digital battery analyzer. 

How Many Volts Should a Fully Charged Battery Have?

What should car battery voltage be? Fully charged automotive batteries should have a voltage of approximately 12.6 volts with the engine off. 

Meanwhile, a battery voltage of less than 12.45 volts (less than 75 percent) is considered low. In this case, you may want to charge it until it reaches the normal car battery voltage. You may do this by driving your car around for 15 to 20 minutes at a speed of 40 mph or faster, or by connecting a portable charger to your battery. 

Closeup of fully charged car battery
For optimum performance, you need to keep automotive lead-acid batteries at least 75 percent charged.

For optimum performance, you need to keep automotive lead-acid batteries at least 75 percent charged. If you allow the battery to run down and you do not bring it up to 75 percent or more after a few days, you may permanently damage it. 

What Causes a Car Battery to Drain? 

There are several reasons why your battery can’t hold a charge. A parasitic drain may be caused by electrical problems such as a short circuit somewhere or an accessory (e.g., the headlights) that are being left on when the engine is off.

Leaving your headlights on, for example, may cause your battery to drain continuously overnight. To avoid this, make sure your automotive lights are turned off before leaving your vehicle for the night. 

Also, a faulty alternator could be the problem.  

If you’re unsure of what is causing your battery to drain, it is best to have your mechanic do the necessary tests to accurately diagnose any car battery issues. 

Symptoms of Low Car Battery Levels

The most obvious sign of a low battery level is slow cranking when attempting to start your vehicle. In worse cases, there may be no cranking at all. If your power window malfunctions or if your heated seats or electronic defrosters are not functioning as they should be, it may also be a sign of a low car battery. 

With a low battery level, you may also have problems opening your car door and trunk if your vehicle is equipped with remote keyless entry. 

Keep in mind that a low or dead battery does not automatically mean that it needs replacement. Several factors may affect your car battery’s state of charge, such as not driving your car for long periods of time or not driving it long enough. 

Also, you may want to check your vehicle’s alternator and the charging system for issues. Aside from a faulty battery, there may be something wrong with your vehicle’s electrical system, causing your battery to drain.

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In the Garage with CarParts.com is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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