How to Test a Car Battery

How to Test a Car Battery with a Multimeter

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Whether you’re feeling ill or maybe it’s your usual health checkup, your doctor will have to perform a few tests to get information about the health of your various systems. When it comes to automobiles, the same applies where your mechanic will run a few tests on your vehicle to ensure it’s fine. One of the tests your mechanic will perform is on the battery. Since the battery is not a complex system, learning how to test a car battery on your own is really important for personal safety.

Why personal safety? You see, the battery is one of the most important components in your vehicle. It powers your car’s onboard computer, the car’s electronics, and lastly, it provides power to start the engine. So, if the battery is unable to work, then it means your vehicle won’t start which is risky if you’re not in a safe location.

Therefore, to avoid getting stuck in risky situations, you need to perform regular tests on your battery. In this insightful guide, we’re going to discuss how exactly you’re supposed to test your battery using a multimeter and a hydrometer. We’ll also discuss a few other essentials you need to be aware of when testing your car’s battery. So, if you’re ready, then let’s begin.

When Should I Test My Car Battery?

 

Now, most motorists have been asking when the best time to test their batteries is. The most unfortunate fact about most of these car owners is that they barely spare any time to check the bonnet, let alone the battery.

Now listen. There’s no specific time as to when you should test or check the battery. Battery check should be performed regularly to ensure it’s in good shape. Just like paying regular visits to a doctor, your battery requires regular tests to guarantee preventative maintenance.

But since most motorists are reactive rather than being proactive, here are several signs that can point to a faulty battery.

  • Slow Starting the Engine: The first major sign that points to a faulty battery is the slow-starting of the engine. Here, you’ll notice that the engine is taking quite long to start when you turn the ignition on. Sometimes the engine will crank then make a sluggish whirring sound before starting.
  • Cranking Sound: A cranking or clicking sound might be heard when you turn on the ignition. If this is what you’re hearing, then it’s evident the battery has a low charge. What happens here is that the battery has insufficient current to power the starter solenoid. Since the starter is the one responsible for igniting the engine, the car will not start at all.
  • Dim Headlights: If you’re driving and you suddenly notice that your car’s electrical components are struggling to function, then there’s a possibility the battery is failing. Some of the signs you’re likely to notice include dimming or flickering of the headlights or dashboard lights whenever you start or idle the engine.
  • Check Engine Light: Another sign that indicates it’s time to test your battery is when the “check engine” light turns on. In most cases, this light signifies the battery is running out of juice. If this is what you’re seeing, then it’s about time that you check your battery and hopefully charge it.
  • Frequent Jump Starts: If you notice that you’re jump-starting the battery frequently, then it means it needs further inspection. A car’s battery is very similar to a laptop or a Smartphone battery. The battery can get depleted due to old age or overusing your device.

Your car’s battery too can get depleted due to old age, premature wear (caused by poor driving habits), and lastly, parasitic drain where some car electronics draw power even when the car is turned off. So, if you’re used to jump-starting your battery, then you need to take it for inspection immediately.

 

How Do I Test My Car Battery Using a Multimeter?

 

The digital multimeter is the first tool we’re going to use to test the amount of charge available in the battery. Since most motorists are unaware of how to use this tool, we’re going to discuss a few steps that will help you perform this test in the comfort of your garage.

Step One: Understand the Amount of Charge a Fully Charged Battery Should Have

The first thing you need to know is the amount of charge a fully charged battery should have. This is a very important parameter that should guide you during the testing process. Otherwise, it would be useless to test a battery if you’re unaware of what you’re testing.

A fully charged 12V battery should have a voltage of around 12.6V to 12.7V when the vehicle is at rest. Supposing the engine is running, the voltage should be slightly higher at around 13.7V to 14.7V.

Step Two: Turn the Headlights On

Once you’re aware of the voltage you’ll be comparing to, the next step is to turn on the headlights and let them sit for about 2 minutes. The reason for this is to get rid of any surface charge that might be on the battery.

In case you’re testing the battery after being in a long drive, then you have to allow the vehicle to rest for about an hour. If you happen to omit this step, then you might end up getting a false multimeter reading as the battery is still holding some charge from the alternator.

Step Three: Inspect the Battery

In this step, you’ll be doing two things. One, you’ll be locating the battery, and two, you’ll be inspecting the battery for any signs of wear. To locate the battery, simply refer to your car’s manual for the exact location.

Once you’ve found it, inspect it closely for any signs of corrosion build-up on the positive and negative terminals. In case you don’t know how corrosion looks like, it’s generally a whitish or yellowish crust forming on the terminals. The reason why this is important is because corrosion can interfere with the multimeter reading.

Step Four: Set Your Multimeter

Now that you’ve removed any surface charge from the battery, turn off the headlights and ready your multimeter. If you’re using a multimeter for the first time, you might find it a bit complex due to the various measurement settings that are available.

In our case, for instance, we’ll be using the DC since we’ll be measuring the voltage of a battery. So, don’t switch to AC whatsoever. Lastly, you’ll need to adjust your multimeter to 20 volts for you to get the correct reading.

Step Five: Test and Analyze the Results

The testing process is pretty much self-explanatory. Here, you’ll only need to touch the positive probe of the multimeter to the positive battery terminal and the negative probe to the negative battery terminal. The best thing about battery terminals and multimeter probes is that they’re color-coded to make things easier. So, the positive terminal/probe is red while the negative terminal/probe is black.

Once you’ve tested the battery’s voltmeter readings, the next step is to analyze the results. How do you analyze these readings? In the beginning, we mentioned that a fully charged 12V battery should have a reading of 12.6V to 12.7V when your car is at rest. When the engine is idling, the reading should rise slightly to 13.7V or 14.7V.

In our case, these readings indicate the battery is sufficiently charged. However, if you record a reading of around 12.3V, then it means the battery is 75% charged. Anything lower than 11.8V means that the battery is less than 25% charged. In such a case, the battery is termed as insufficiently charged. For easy reference, here’s a table you can refer to.

Recorded Battery Voltage (V) Depletion Level (%)
12.5V 90%
12.42V 80%
12.32V 70%
12.2V 60%
12.06V 50%
11.9V 40%
11.79V 30%
11.58V 20%
11.31V 10%
10.5V 0%

 

Lastly, when testing the multimeter reading, you need to ensure room temperature is around 80°F or 26°C. Note that, as temperature rises, the multimeter readings are likely to drop and vice versa.

 

Testing a Car Battery Using a Hydrometer

 

This alternative test is also known as an electrolyte gravity test and it’s used to confirm the status of the battery to see whether it’s charged or discharged. Now, this test is usually performed on traditional batteries that have removable cell caps to access the interior cells.

So, if your battery is a sealed or maintenance-free type of battery, then you can’t perform this test. Lastly, an electrolyte gravity test is performed using a special self-adjusting hydrometer that has an in-built thermometer.

Step One: Wear Protective Gear

Since you’ll be working with corrosive acid, you need to start by wearing protective gear to stay safe. Here, you’ll need to put on your gloves and goggles to protect your hands and eyes from any accidental acid spillage.

Next, run the engine or charge the battery to ensure it’s sufficiently charged before proceeding with the test.  When you’re done, allow the battery to settle for about 2 to 3 hours.

Step Two: Disconnect the Terminals

After the battery has settled, disconnect the negative terminal to eliminate any load. Next, open the caps for all the six cells to expose them.

Step Three: Take the Hydrometer Readings

Start by squeezing the bulb of the hydrometer then insert the tip inside the first cell. Release the bulb to allow the electrolyte to draw inside the hydrometer chamber. Allow the electrolyte to settle then record the gravity readings on a piece of paper. Perform the test on the rest of the cells and record the readings.

Step Four: Analyzing the Tests

Now, if your hydrometer readings are in the range of 1.265 and 1.299, then it means your battery is sufficiently charged. However, if the readings fall below 1.265, then it means the battery needs to be recharged.

Lastly, if you notice a difference of 0.025 to 0.050 in one or more cells, then it means the battery is sulfated and hence needs to be replaced.

 

What if the Problem is the Alternator?

 

In some cases, the battery might be working perfectly well. In case you’re still experiencing power issues in such a scenario, then there’s a possibility the alternator is faulty. Since the alternator is responsible for charging the battery and also powering the car’s electronics, power loss can be evidenced if this component is faulty. So, how do you test the alternator?

So, to test the alternator, simply turn on all the car’s electronics such as the headlights, interior lighting, the car radio, and the heater among others. The reason for doing this is to maximize the voltage load. Next, turn on the engine and leave it idle.

With the battery and the rest of the instruments working, use a multimeter to test the battery readings. If the readings are below 13.5V, then it means the alternator is struggling to charge the battery.

On the other hand, if the readings are above 14.4V, then it means the alternator is overcharging the battery. In either case, the alternator will demand further inspection by a licensed professional to repair or replace it.

 

Conclusion

As you can see, testing the condition of your battery using a multimeter or a hydrometer is not as hard as it sounds. The process is pretty simple provided you follow the right procedure. Since there’s a lot of misguiding information on the internet, this guide has discussed vital steps you’ll need to follow to make the procedure as accurate as possible.

As you can see, testing your car’s battery is really important when trying to pinpoint the exact cause of an electrical failure. In case the battery seems to be working normally, then the next possible culprit is the alternator.

Thankfully, we’ve also discussed how to test the alternator. So, all you need is to follow the instructions we’ve given. In case the alternator is at fault, then you’ll have to hire a licensed mechanic to inspect it and determine the next cause of action.

If the problem is the battery, then you can either recharge it or replace it depending on the severity of the condition. With that said, we wish you all the best in your testing procedure and as always, don’t forget to share your experience in the comments section below.

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