Do You Have To Replace Battery When Replacing Alternator?

Hi, I'm Luis Johnson, an automobile technician, and power equipment professional. By profession, I'm a businessman and operate a car workshop. I have created this...Read more

If you are thinking about replacing the alternator, your car must be causing major issues. Normally, that’s when people decide to get a new alternator. Indeed, that’s a good initiative. But, what about the battery? Do you have to replace battery when replacing alternator?

No, you don’t have to replace the battery with your car’s alternator. If you detect a bad alternator, replacing it should be enough. Make sure to disconnect the battery first during the replacement process to avoid sparks or short circuits.

Through this article, we will explain this question in more detail. Plus, we will answer everything regarding how the alternator works and how you can tell if it has gone bad.  So, without further ado, let’s start.

What Is an Alternator and What Does It Do in A Car Engine?

Batteries take all the credit for running your vehicle. However, it’s the alternator that works silently and contributes the most. It transports electricity to the whole car. So that you can listen to the radio, watch Google Maps, or turn on the interior lights in the car.

What Is an Alternator and What Does It Do in A Car Engine

Each of these elements draws a good amount of electricity. As you already know, the car battery is nothing but a chemical mixture and stays in a liquid form. As for the engine, it only produces kinetic energy through moving crankshafts.

So, where does so much electricity come from? Yes, the alternator. Let’s find out how it works.

How Does An Alternator Work?

You can locate the alternator just beside the car engine. Both units are joined by a “pulley” or “belt.” Once the engine starts running, the movement causes the “belt” to mimic it.

As a result, the rotor starts to circulate along with the coil installed around it. Now, the moving coil has an active electromagnetic field around it.

We all know AC is nothing but a by-product of electromagnetic energy. That’s what happens here as well. Using the crankshaft’s motion, the alternator produces usable electricity for the car.

How Car Alternators Charge the Battery?

When the engine is turned off, the alternator can’t produce electricity. It’s simple. That’s why you need batteries to jump-start a car. And, it loses most of its power during that. It needs the alternator to charge it again when the car is running.

Now, we have seen how the alternator produces AC. But car batteries can only work on the DC type. If you don’t know, DC means the electricity can move only in a singular direction. So, how does the alternator charge your car battery?

Well, alternators come with a part called a “rectifier.” Its job is to restrict the AC and make it flow only in a direction. After passing through the rectifier, the electricity is ready to be used in the battery.

Can A Bad Alternator Damage A Car Battery?

The answer is both no and yes. No, because batteries are separate units and won’t get affected due to the condition of the former. However, there are cases when a bad alternator causes troubles that end up damaging the battery for good. Here are some instances that can happen:


Alternators make sure the battery as well as the other gadgets are getting the prescribed amount of electricity. Battery cells need 12V of current to keep the ions in an ideal state.

Overcharging or undercharging is a big “NO.” However, a faulty alternator might not put a cap on the supply. That’s how it messes up the ionic state inside the battery cell.

Refusing To Redirect

The alternator might altogether refuse to transport electricity to the battery. In that case, after jump-starting the car, the battery doesn’t have much power to survive. It will be troublesome when your car needs a battery for extra support. Slowly, the battery will die.

How To Know If Your Alternator Is Bad And Needs To Be Replaced?

To replace the bad alternator and save your battery, you have to first recognize it. Luckily, it’s not rocket science. There are plenty of signs that let you know. Take a look.

Frequent Electrical Problems

The alternator is responsible for providing electricity for your car accessories. So, in case you notice this equipment is not working properly, check the alternator.

Sometimes, one of these gadgets can become obsolete and only need a replacement. But, if you see all the settings like interior lights, headlights, windscreen wiper, or seat heater have stopped responding, it’s a faulty alternator

“Low Battery” Light Is On

Most cars have a LED light that flickers when there is something wrong with the battery. For example, the battery is down and needs immediate charging.

If you see this happen, it’s because of a misbehaving alternator. Because feeding electricity into the car battery and keeping it charged is the primary job of an alternator.

Car Not Starting

It echoes what we have said previously. A faulty alternator will stop charging the battery. And, you need the battery to start the engine of your car and drive. So, if the car refuses to start, you can blame the alternator.

Bad Odor

It’s optional. Not every bad alternator produces bad smells. However, some of them might. So, if you find a smell like “burnt rubber”, it might be the “belt” of your car’s alternator. Those belts can lose shape over the years and get damaged. Plus, the continuous rubbing puts a toll on its durability.

Do You Have to Replace Battery when Replacing Alternator?

If your battery is in good shape, there’s no need to replace it while changing an alternator. Yes, the battery gets affected due to a bad alternator. But, that’s due to a lack of electricity supply only.

Do You Have To Replace Battery When Replacing Alternator in 2022

A new alternator shouldn’t have this supply issue at all. Once the battery recharges itself fully, there shouldn’t be any further issues.

However, if the car faces issues even after replacing the bad alternator, it’s time to reconsider. The car battery might have gone obsolete too. in that case, you can go ahead and change the battery too.

How Do You Know Whether the Battery Is Working or Not?

You can use a multimeter or voltmeter to test a car battery. Just, insert the red node into the “positive side” of the battery. Do the same thing with the black node, now, read the output on the screen. if it’s 12.00 to 12.6V, you are good to go.

Now, run the engine and let the alternator charge the battery. Repeat the process again. This time the reading should be above 12.6V. If you don’t see such results, your battery has a problem.

What Are the Consequences of Not Replacing a Bad Alternator?

If you choose to go ahead with the bad alternator, it’s not probably a good idea. Soon, your car will become useless. And, here’s how:

Major Electrical Shut Down

First of all, a bad alternator will lead to a major electrical shutdown in your car. For example, you are driving at night. Suddenly the headlights will start flickering. The windscreen wiper might also stop responding to the rain.

It will surely cause you to stop driving since you can’t see what’s ahead of you. All these car accessories are powered by an alternator. So, if you don’t replace a bad one, you might have to sacrifice all these features as well.

Dead Battery

If the alternator doesn’t recharge the battery, it will go dead pretty soon. And, once a battery is out, you can’t start the car whatsoever. It’s only a matter of 20-30 minutes before the car comes to a halt.

Alternators only work due to a running engine. And, the battery is in charge of supplying a surge of electricity to turn on the engine. That means you will end up with an obsolete car.

Car Stalling

If the alternator is not fully damaged, it might provide intermittent electricity. That means the battery receives a supply of electricity from time to time. But, it should have been continuous. Anyway, with a little amount of power, you won’t be able to drive the car at its optimum speed.

You May Also Read: How Long To Charge Car Battery With Jumper Cables?


You are at the bottom of this article. We have already explained our answer above. Now, let’s take a look at some of your most asked questions about this topic.

How Long Should I Let My Car Run After Changing The Alternator?

Mostly 20-40 minutes is a good range to drive your car. If it’s a hot start, you can get away with only 10 minutes of driving. And, if you are crawling through heavy traffic, the time will increase significantly. So, it all depends on your engine’s rpm. You can use an Amp-meter to check whether the battery is fully charged or not.

How Can I Tell If My Alternator Is Draining My Battery?

When the car accessories refuse to respond or the “low battery” light flickers, your alternator might be the culprit. But, the battery won’t die instantly. Another trick is to use a voltmeter and test the battery. If the reading is below 12V, the battery is draining. Or, you can ask for a mechanics’ help.

Why Does My Car Keep Dying With A New Battery And Alternator?

This could simply indicate a parasitic draw. It means something is drawing charge from the battery when the car is not in use. You can try to disconnect the negative cable before leaving the car. It would break the path and help you store the charge for a long time.

Should I Replace The Alternator Or Battery First?

If you are having severe power issues in your car, we would suggest replacing the alternator first. The alternator is the most crucial power source. It also charges the battery after the car starts. It means that if your alternator is bad, having a new battery won’t improve the situation. You would still face all the issues. So, go with the alternator first.

Should I Disconnect The Battery When Replacing The Alternator?

It’s not advisable to have the battery connected while replacing an alternator. Not just that, whenever you are working on any electrical part of your car, try to disengage the battery first. Otherwise, facing a short circuit is pretty common. Especially if you touch the wrong cable first. Removing the battery would eliminate this risk.


Alternators are the most prominent source of electricity for your vehicle. Batteries are only there to support it when it’s not producing enough electricity.

As we have already seen in this article, a bad alternator can literally shut the car down. If you want to avoid it, we would suggest replacing it after 7 to 8 years.

That’s the average lifespan of an alternator. As for the batteries, they also expire after 5 to 6 years due to the cell drying up. It’s advisable to dispose of the damaged units and protect your car. Happy driving!

Hi, I'm Luis Johnson, an automobile technician, and power equipment professional. By profession, I'm a businessman and operate a car workshop. I have created this site Batteryquery to help people purchase the right battery for their vehicles. The reason behind creating this site is to help my customers purchase the right car battery who often purchase the wrong one due to their lack of knowledge.

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