How to Test an Alternator by Disconnecting the Battery?

The alternator is considered the brain of a vehicle as it keeps your vehicle charged and converts mechanical energy to electrical energy. And, the converted energy helps you to operate different electrical components in your car.

But what if the alternator doesn’t work? How to test an alternator by disconnecting the battery?

The process is quite lengthy because there is mechanical knowledge included in the steps. For example, you need to disconnect the negative and positive battery terminals with enough care. Let’s get to the article to know the details.

Before we head into the detailed steps of disconnecting the battery, know that it’s not the safest option for the job. There’s another option that can cause less damage to the parts of your vehicle and we’ve covered it too in this article.

Let’s dive right in!

Symptoms of a Faulty Alternator

An important point before starting the test – you have to look for different types of confirmation indicators because this problem is always predictable in different places.

However, how do you know if the problem is related to your car’s alternator? Of course, there are some symptoms, the most common of which are:

Engine Shutdown

If you notice your engine stalls now and then or it just shuts down all of a sudden, there could be an alternator issue. The fuel injection of your car requires enough amount of electrical power.

Also, you may experience a hard-to-start engine that cranks slower, which also means that the alternator fails to make enough current while driving. 

Service Engine Light Lits

Check engine light or battery warning light can be a symptom of an undercharged or overcharged alternator. Also, the reason can be different (e.g malfunctioned sensors).

If your car is modern and it’s equipped with screens and microchips for almost everything in your car, it might cause some different symptoms.

Overly Bright or Dim Light

This is probably one of the most common symptoms because a failed alternator is more likely to provide inconsistent voltage to the electronic accessories of your car. The result can either be over- or over-performing equipment.

So, you either can experience too dim or extremely bright light. Also, the symptom can be lights that gradually go from dim to bright (and vice-versa) or flickering lights.

However, these symptoms might confuse you with any other issues apart from the alternator.

Here’s a more specific sign- see whether the lights illuminate at the same time when the RPM increases. If yes, then it’s a clear and safer sign. 

Weird Noise

Let’s admit it, car owners hear a lot of weird sounds; some are harmless while others can indicate serious issues. If you continuously hear a squealing noise from under the hood, that’s not normal and you should get your vehicle checked by a professional in no time.

The main reason for the noise is that the alternator is driven by a serpentine belt which can be stretched over time. If it’s stretched, the belt cannot spin the alternator pulley effectively anymore.

And, if you notice a grinding or growling noise, chances are the internal bearings have been worn-out.

Battery Dies

If the shelf-life of your battery has come to an end after using it for a few years, then it’s a natural case and there might not be any connection with a malfunctioning alternator.

However, in other cases, a dead battery can indicate a malfunctioning alternator. If the alternator malfunctions, it will not efficiently charge the battery when your engine is running.

And, it causes the charge to drain unusually faster.

Since troubleshooting or recognizing the problem is quite tricky, consult a mechanic and let him test whether the issue is related to the battery or the alternator. 

Electrical Problems

An alternator is the most important electronic component of your car as it keeps your car battery charged and allows you to use the radio, headlights, or other electronic accessories.

Sometimes you may notice that the electrical equipment in your vehicle does not operate properly. If there’s not enough power, the alternator doesn’t work and the pieces of equipment can shut off.

How Much Does It Cost to Repair The Alternator?

Repairing an alternator depends on the degree of damage it already has. If you don’t get it checked soon enough and the issue becomes more serious, it will lead you to more expensive repairs and higher bills of course. 

However, if you’d like to replace the alternator, the average price range can be $100-$350 (it varies according to different models).

However, this price is for replacing only the alternator and not changing any other parts. If you’re planning to get a serpentine belt as well, it might cost around $20-$50 to get one.

How to Test an Alternator by Disconnecting the Battery?

So here we are about to learn how to test your car alternator easily and quickly by disconnecting the battery. Pay attention and follow the steps.


The first step is to park your car in a safe and open place, such as a driveway. Lift the hood of the car. Make sure the car is in stable condition. And now, go to the driving seat of the car.


Start the engine and notice if you face any issues in starting the car. It’s because a faulty alternator can cause you to have issues with the starts.


Let the engine in a running position and detach the negative battery terminal in the meantime. Now, see whether the car runs as usual. Notice if there is any dirt or corrosion around the battery. If so, clean it up.


Disconnect the positive connection of the battery while the vehicle is on.


Now notice if the engine of the car is running as before.

There is nothing to worry about if the car continues to run even if you disconnect the positive connection of the battery at the time the car is switched on. The alternator is OK. In this case, the problem is probably in another part. 

On the contrary, if the engine stutters, stumbles, or stalls out, then the problem is with the alternator. Since the engine isn’t getting enough electric power from the alternator, this is what causing the problem.

Is It Safe to Disconnect The Battery to Test The Alternator?

Unfortunately, it’s not safe to disconnect the battery to test the alternator. Chances are your engine performance will decrease eventually. 

The biggest damage it can cause is to create a voltage spike. Detaching the negative or positive terminal battery now and then can cause damage and a voltage spike is enough to ruin the electrical system of your car. 

So, yes, you may have to repair the damage in exchange for thousands of dollars which you never want.

Moreover, there are safety hazards because the batteries produce flammable hydrogen gas. It’s a bit of too much risk to take the battery terminals off and experience a spark.

So, it’s best to avoid the method if you have another option. Let’s talk about the other option now.

Alternative Methods That Are Comparatively Safe

Here’s the good news- there are alternative methods that are safer than disconnecting the battery. You need to use a multimeter for this method.

Step 1

Switch on the engine and turn off all other electrical components and take a multimeter. After that, get your multimeter prepared. See whether there’s any increment setting in the multimeter. If there is any, adjust the voltage to 20 DC volts. But if there is not any, just set it to DC volts.

Step 3

This time, connect the positive to positive and negative to negative terminals of the battery. You can simply do it by touching and pressing each multimeter probe to the correct terminals. It shouldn’t be that though.

Step 4

Now, take the voltage reading of the battery. The reading range should be 12.4-12.6 volts this time. If the reading is below the minimum level, chances are you have a bad battery.

Step 5

And finally, start your car and check the reading again. When pressing the probes to the battery terminals, the multimeter reading range should be 14-14.5 volts.

If you notice that the voltage is lower compared to the initial reading, maybe the charger is malfunctioning. If this is the case, just head to an automotive repair shop and get the alternator checked.

How to Maintain an Alternator From Going Bad?

You know the importance of an alternator for a car very well. So it is your responsibility to take proper care of this part. The question here is how you can take care of the alternator. No worries, they are given below.

Check the Electrical Connection

Sometimes all the devices are fine but the connections get loose causing various problems. So you should check all the connections associated with the alternator. If there is a connection that is loose or missed, fix it.

Check for Any Damage

If there is a fault in the alternator, it will not be able to charge the car battery. So check if there is any such problem with the alternator. See if the connections are tight enough. If the problem is serious, replace it as soon as possible.

Clean the Outside of the Alternator

Over time, lots of dust accumulates in the alternator. Excess dirt makes it overheat. To do this, wash the outside of the alternator with water and dry it well.

Symptoms of a Faulty Alternator Infographic


Now consider what you have learned from this article. You just learned how to test a car’s alternator by detaching the battery. With this article, you know what the symptoms of a faulty alternator can be. 

You learned how reliable this method is and whether there are any exceptions. However, there is only one goal in adding so much information to this article so that you can accomplish this task properly. 

After reading the article, you should not have any hesitation about it. However, before you resort to any method related to automotive, be sure to ensure safety and do not delay. 

In the case of an alternator, any delay will not be acceptable. If you get stuck somewhere, our advice is to take the advice of a pro. Hope you got all that you need to know. 

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William Baldwin

This Is William Baldwin, The Founder & Managing Editor of this website. Me and my team share automotive tips, tricks, and news

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Talha Atta

Talha Atta, a Mechanical Engineer and experienced technical content writer and editor at with a passion for the automotive industry.