Battery Dead After Replacing Alternator [Why+ How To Fix]

If you find a dead battery in your car after recently replacing the alternator, you are sure to be frustrated.

But why is the battery dead after replacing alternator?

The battery may have died because of the incorrect installation of the alternator. There may be one or multiple loose connections. Or, it’s the battery that has a problem. In that case, a replacement is in order.

battery dead after replacing alternator

If you haven’t replaced the serpentine belt, that may be at fault.  In the worst case, your new alternator may have a defect and is not charging the battery.

In the next parts of the article, we will troubleshoot how to solve a dead battery with a new alternator.

Battery Dead After Replacing Alternator: What Should You Do Now

So, you have just replaced your alternator but for some reason, it did not go very well and now your battery is dead. No worries. here’s what you should do-

Note: Read our ultimate guide on bad Alternator diode draining battery.

Step 1. Check For Loose Connection

The first question you need to ask yourself is, “Did I properly replace the alternator?” Even if you think that you did, you may have missed something crucial. Trace back all the steps and check if there are any loose connections.

check the battery loose connection

In this case, especially check the battery terminals. If they are not tightened properly, the alternator will not be able to charge up the battery.

If you do find any loose connection on the battery or the alternator, reconnect it properly.

Step 2. Check For Corrosion

While you are checking the battery terminals for loose connections, also check if they are corroded. Corroded battery terminals will prevent the flow of current to the battery, causing the battery to die. 

check for battery corrosion

If you find the battery terminals corroded, you can clean them up with a corrosion cleaner. Here is how you can do that-

  • Remove the battery terminals. Always remove the negative terminal first and then the positive terminal.
  • Make sure you coat the entire affected area with the cleaning product.
  • Using a wire brush, scrub the corrosion away wearing rubber gloves. Clean up the battery case while you are at it. 

Or, you can try using baking soda. Mix 6 tablespoons of baking soda with 4 cups of water. It will create a paste that you need to spread over the entire corroded area. Wait for 30 minutes, this will give the paste enough time to neutralize the corrosion. Then simply, wipe it all away with a clean piece of cloth.

Step 3. Check The Battery

Did you check your battery or the new alternator before installing it? If not, now is the time. 

check the car battery 

To test these, you will have to charge your battery first. For this, you can use a Jumpstarter, battery charger, or another car. If you did not already know how to jumpstart your car, watch the below video.

Now that you have jumpstarted, you can test the battery and the alternator. First, let’s test the battery. To do this, you will need a load tester. 

Here’s how you can test your battery with a load tester-

  • Connect the load tester to the battery terminals. Connect positive to positive and negative to negative. 
  • Now, turn on the load tester for at least 10 seconds, doing so should send a load to the battery.
  • Notice the battery voltage in the load tester and see if the voltage drops below 9v. If it does, then your battery is ruined and needs to be replaced.

Another sign of a faulty battery could be the car running rough after replacing the alternator.

Step 4. Check The Alternator

Now, if you still have no power after replacing the car battery, you should check if the alternator is doing its job. To test that, you will need a multimeter.

check the alternator
  • Connect the multimeter to your battery terminal and follow the same rule as the load tester. Connect positive to positive and negative to negative.
  • Then, start your car.
  •  With the car turned on, the alternator should charge the battery and the battery voltage should be around 13-14.6 volts. If it’s lower than that, that means the alternator is not charging the battery.

If the alternator is not charging the battery, it could be one of three things-

  • A damaged serpentine belt,
  • A blown alternator fuse, or
  • A defective alternator voltage regulator  

The best way to check a serpentine belt is to take it out and check for wear and tear. 

check a serpentine belt

This is how to remove a serpentine belt

If your serpentine belt is worn out, you will have to get a new one. Now, in the case of a good serpentine belt, let’s check the alternator fuse.

To check the alternator fuse, you must first locate it. For that, it’s best to refer to your vehicle’s manual. 

Once you have located the alternator fuse, take it out and look for signs of burning and melting. If there is none, you need to test its continuity with a multimeter.

 located the alternator fuse

Set the multimeter to continuity setting and touch the probes on both sides of the fuse. If there is continuity, you will hear a beep sound. This indicates that the fuse is fine.

If the fuse is also fine, it is most likely a bad alternator diode draining the battery. Since you have replaced the alternator and the battery still not charging, that surely is the alternator’s fault. 

Step 5. Call A Mechanic

If none of what we described for you works, then we suggest that you consult with an expert. 

call a car mechanic

There may be some problems with the car’s electrical system that are causing it. So, it would be best to let a professional handle this.

How To Prevent A Dead Battery After Alternator Replacement

To prevent such problems after replacing the alternator, you can take some small steps that would go a long way. Here are some tips you can follow-

prevent a dead battery after alternator replacement
  • It’s a good idea to replace the serpentine belt when you replace the alternator. Serpentine belts are not that expensive and can be a good investment.
  • “Do you have to replace the battery when replacing the alternator?” is a very common question that we face. But no, you do not have to replace the battery while replacing the alternator. But check your battery health regularly and replace it if it’s older than three years.
  • Before buying the alternator, spin the alternator’s pulley by hand. It should rotate smoothly and quietly. If you notice any noise or roughness, it may indicate worn bearings.
  • You should also perform an alternator diode check with a multimeter. 
  • If you are replacing the alternator yourself, follow the alternator replacement process word by word. Or have someone else do the job for you.

By using these tips as a safety measure you can prevent a dead battery in your car.


Can a dead alternator drain the battery?

Yes, a dead alternator can drain the battery. The alternator is responsible for charging the battery while the car is running. If the alternator is not working properly, it will not be able to keep up with the demands of the electrical system and the battery will eventually run down.

Can a new alternator drain a battery?

Yes, a new alternator can drain a battery if it is not installed correctly or if there is a problem with the alternator itself. If you suspect that your new alternator is draining your battery, you should have it checked by a qualified mechanic.

What to do after replacing the alternator?

After replacing the alternator, check if the battery is fully charged. Next, verify that all connections are tight and secure. Finally, examine the fan and pulley to ensure the fan spins freely and the pulley is undamaged. 


If the battery is dead after replacing the alternator, it could mean a range of problems. That’s why it’s important that your approach the situation with proper caution.

While pinpointing the cause of the problem, make sure you wear safety goggles and gloves to avoid any unwanted situations. If the troubleshooting seems difficult or confusing to you, do not hesitate to contact a professional.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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