How To Test Alternator By Disconnecting Battery?

Sometimes, a vehicle malfunction can happen without any notice. This can leave you stranded if you cannot figure out the problem and fix it immediately. For instance, if a car doesn’t start, the battery and alternator are some of the first suspects to examine.

Although battery problems are common, the alternator is next in line if you’ve verified that your battery is intact. This is why knowing how to test alternator by disconnecting battery is crucial to determine the actual problem.

Therefore, disconnecting the wrong battery terminal can result in a severe disaster. So, let’s find out the ideal process of testing your alternator by disconnecting the battery.

how to test alternator with battery

How to Test Alternator by Disconnecting Battery

The alternator test is one of the easiest operations to perform in a car. Although there are a couple of options to implement, disconnecting the battery is the simplest way to check whether your alternator is good or bad. 

Another option is knowing how to test if alternator is bad with a multimeter. However, if you prefer the first approach, you can’t just start your car, open the hood, and pull out the cable from any of the battery terminals (positive or negative).

Performing the test incorrectly can trigger a massive spark or fire outbreak and damage your vehicle. Here you go with a step-by-step procedure on how to check if alternator is bad by disconnecting the battery.

Step 1: Start the car’s engine.

First, start your car’s engine to commence the alternator test process. If the car was already running, you might want to proceed from there.

However, it would be best if you’re just starting the car because of the injury a hot engine can cause if you’re not careful enough. So, you may want to park, turn off the engine, and allow the hot engine to cool down before you restart the engine and commence the check.

Also, ensure that you’re performing this test in a safe place, especially if you couldn’t get to your home garage, before delving into the process. Use a safety material like rubber hand gloves if you have a pair. If not, it’s okay!

Step 2: Open the hood and locate the battery

Once the engine starts running, pull the hood lever beneath the dashboard on the driver’s side to open the hood.

Then, get out of the car to open the hood properly and hook it in place, depending on the kind of car you drive. While some car hoods can stay unassisted, others may require a tiny metal hood-lift support to hold the hood in place while it is lifted up.

Step 3: Identify the negative battery terminal

Now, it’s time to identify the correct battery terminal to pull in order to get your desired outcome. So, the negative terminal is the ideal point to use in testing the alternator. The terminal is marked with a minus sign (–) on the battery.

Kindly note that removing the positive terminal will trigger a spark that can easily damage your battery and other components in the car. So, you must avoid this costly mistake. On the other hand, removing the cable on the negative terminal helps prevent a battery dead short from occurring.

Step 4: Remove the negative battery terminal

Once you identify the negative terminal, simply pull the cable connected to the terminal while the car’s engine is still running. You can use a wrench or pull it directly with your hand if a wrench is not available.

After that, carefully observe the outcome. If the car’s engine performance drops, stalls, or dies down entirely, you have a bad alternator. Kindly replace the component immediately to protect the battery from further damage.

Meanwhile, if the engine keeps running without any interruption, it means the alternator is okay. For those asking how to test alternators without a multimeter, you may want to check out this video to guide you through the process properly.


Q: Which battery terminal do you disconnect to test the alternator?

First and foremost, you carefully disconnect the negative battery terminal with the minus sign (–) to test the alternator. Once you do this after starting the car, the outcome will determine whether your alternator is good or not.

For instance, if the engine stalls or automatically dies down, you most likely have a bad alternator. On the other hand, if it keeps running, you can be sure that the alternator is in good condition. However, be careful when performing this test.

Q: What happens if I disconnect the battery while the car is running?

Disconnecting your car’s battery while the engine is running has a tendency to endanger some components in a computer-equipped vehicle. For example, equipment like the electronic control unit, cruise control, speed-sensitive steering, car stereo, etc., may be affected in the process.

Even though disconnecting your car’s negative battery terminal can help you test your alternator, ensure to use any other better strategies to test the component instead of risking a costly damage you’re not expecting.

Q: What happens if you remove the positive terminal first?

Removing the positive terminal when disconnecting your car’s battery to test the alternator can trigger a serious spark that can damage the battery and other components. This is why you should remove the negative terminal first.

When you remove the positive terminal, there’s a tendency for the wrench you’re using to touch the car’s body or engine, thereby grounding the battery. If you’re contemplating how to tell if alternator is bad by disconnecting the battery, carefully follow the procedure above in this post.

Q: Can an alternator run without a battery?

Of course! An alternator can run without a battery in a car. The component has the capacity to run a vehicle when the battery is flat. However, you will have to use another car’s battery to jump-start the car with the dead battery first.

After that, the alternator can take over from there and power the vehicle. However, this is not the best. If you have a flat or bad battery, ensure to recharge or replace the component as soon as possible. At least, it will help you overcome the challenge of constantly jump-starting your car.

Q: What happens if your alternator goes out while driving?

If your car’s alternator suddenly goes out while driving, the car will gradually lose power until the battery dies completely, and the car won’t start. A bad alternator will affect virtually every electrical component, including the headlights, radio, etc.

Driving with a bad alternator is risky. If you find yourself in such a situation, ensure to contact an expert auto mechanic immediately for help. Some technicians offer free alternator testing. However, before that, you may want to call for the assistance of a tow truck to get you to the nearest auto repair shop.

alternator testing near me

Final Words

Knowing how to test alternator by disconnecting battery is arguably one of the simplest ways to check the state of the component. However, “what is worth doing is worth doing well.” In other words, a simple mistake can result in severe damage to your car.

But you need not worry now if you suspect a faulty alternator in your car. To your rescue, this step-by-step guide above will help you test correctly whether the component is okay or needs a new replacement. 

If you have questions like, “Can I test my alternator by disconnecting the battery?” Never remove the positive terminal cable, as it can trigger a spark and cause damage. Take off the negative terminal cable instead. Good luck with the alternator test!

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

2 thoughts on “How To Test Alternator By Disconnecting Battery?

Leave a Reply

Your email address will not be published. Required fields are marked *

Recent Posts