Can a Bad Alternator Ruin a Battery? (Find the Truth)

Published on: May 16, 2022
Written by Liam Jaxon / Fact-checked by David Rowan

One of the most common problems with vehicles is the alternator. If it’s not working properly, the battery will not charge, and this can cause serious problems. If you don’t want to repair your car battery, the only way to fix this problem is to replace the alternator. But how can you tell if an alternator is bad? The only way to tell is if the voltage output of the alternator is less than 12V.

Do you know” can a bad alternator ruin a battery?” The most common reason for a bad alternator is not charging the battery up. When the alternator charges the battery, it only charges about 90 percent, leaving some battery capacity in reserve. 

can a bad alternator ruin a battery

This reserve is called “dead,” or “shorted” battery capacity and a weak or defective alternator most often causes it. If the alternator only charges about 90 percent, the battery will lose its ability to hold a charge. It is called “discharging” the battery.

How Long Does a Battery With a Bad Alternator Last?

An alternator is a piece of internal car equipment that converts direct current (DC) from the engine to alternating current (AC). It is the primary source of electricity for the car.

When a battery has a bad alternator, it can cause battery corrosion, which may not hold a charge as long. Plenty of things can cause a battery to go bad, including a bad alternator. That is why it is essential to check your battery to see if it’s in good condition or not.

What Is the Sign of a Bad Alternator?

A bad alternator can be affected in many ways. So, ensure that your alternator is working or not. There are few signs of a bad alternator.

Interior Dim Light:

Check the lights in your vehicle by turning them on. If they are flickering, dying, not working at all, or steadily dimming, there is a problem with the alternator. It is likely that your car’s alternator is failing, which can ruin your car’s battery, especially if the alternator is failing at the same time your car’s battery is failing as well.

Headlight Is Over Bright or Dim:

An alternator is a device that converts electrical energy from a car’s battery into DC power for accessories such as lights, gauges, and radios. If the alternator is not working properly, headlights will turn bright or dim, and you will lose power to accessories. If the alternator is not working correctly, it may cause a low voltage and engine electrical system to die.

Noise Growing:

Bad alternators are responsible for the noise growing that you hear when you first start your car. But that’s not the only thing that can go wrong. When your battery is weak, your car may run rough, not start at all, or not start for long periods. It can also take longer to crank, but this isn’t always the case. 

Electric Failures Reasons:  

We all know that alternators are vital to our cars, so it can be a real inconvenience when they stop working. The main reasons for failure are poor or bad electrical connections, which you can easily remedy by replacing the part. 

Alternator Overheating:  

Alternators often fail before their time. The harsh weather conditions and pollution of the car’s environment can wear down the rotor. Causing voltage fluctuations can create stresses on the magnetic fields and cause the alternator to overheat. If your car is prone to an alternator failure, you should consider other problems such as faulty voltage regulator, faulty alternator wiring, and bad connections.

How Can a Bad Alternator Ruin Your New Battery?

Bad alternators can ruin your batteries. If you purchased a car new from a dealer, it is highly unlikely to have a bad alternator. After a few years, a new alternator can go bad, but a new battery is a different story.

A new battery often gets a bad alternator because the battery is very new and not fully charged. When a new battery is put in a vehicle, it is charged to a certain voltage. If the charge is too high, the alternator will over-work and over-heat. Eventually, it will die and need to be replaced.

Let’s know some ways where it can ruin a new battery.

Not Charging Battery Completely:

The alternator is an important part of a car’s electrical system, and a battery’s ability to hold a charge depends on it. So, when a car battery is charging very slowly or not at all, what can cause this? A corroded or damaged alternator is a common cause.

Battery Over Charging:

If your car’s battery is overcharged, the battery will eventually die. In some cases, this can be caused by a faulty alternator. Alternators are responsible for converting the DC from the battery to AC, then converted back to DC by your car’s electronics. If you have a bad alternator, the battery may be damaged and will cause your battery to overcharge.

Charging Intermittent:

The Battery Charging Intermittent reason for a bad alternator is that the alternator belt is not turning fast enough. The battery will keep charging but still not be able to maintain a steady, constant charge. If the alternator is not functioning properly, it can’t charge the battery properly. When this happens, the battery charges well but won’t maintain a steady charge.

Voltage Ripple:

If you have a bad alternator, you should also have a bad battery. The alternator regulates the voltage that gets sent to the car battery. If the alternator fails, it can greatly harm your car battery. When the alternator turns off, the voltage is supposed to go down, which causes the battery voltage to ripple.

Is It Compulsory to Change the Battery and Alternator Together?

It is not compulsory, and the reason is that the alternator and battery can independently work without interference. Most people will likely never experience problems, and if they do, they can wait until their car is serviced for an alternator replacement. Then the car’s battery will be checked and replaced simultaneously.

How Much Cost to Fixing an Alternator?

If you have a new car or a new set of batteries, you must replace both simultaneously. It is a matter of convention and not a requirement. Many car manufacturers recommend changing both of them together. 

However, if a battery stops working, it is [amazon link=”B01L2RIU4M” title=”good to replace it” /] first since a bad alternator can damage a battery. Changing the battery and alternator can save time, money and is easier on the car.

Could a Faulty Alternator Cause the Car Battery Light to Stay On?

Yes, a faulty alternator can cause the car battery light to stay on, even with a new battery and alternator problem. The alternator is responsible for charging the battery while the car is running, so if it’s not functioning properly, the battery light may remain illuminated.

How Is Long Driving With a Bad Alternator?

Sadly, a bad alternator is the root cause of a car’s electrical issues most of the time. A bad alternator can also signify a potential electrical fire, and a bad alternator will certainly cause a weak battery.

But a car can make a couple of hundred miles with a bad alternator. If you fall into the situation, you have to avoid using those tools operated by electrics. Please don’t go with a bad alternator for a longer time. Change it as soon as possible.


The question is: can a bad alternator ruin a battery? Well, that depends on your battery, of course. Bad alternators will only damage the battery if the current is greater than what the alternator is intended to provide. Also, if the battery is already worn out, a bad alternator will not produce a sufficient charge to power the vehicle.

In some cases, a bad alternator will result in a dead battery. In other cases, the issues are more complicated. If the alternator is the culprit, you can make some repairs to get it up and running, but a new battery is needed in some cases.

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About This Writer

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Hi, I am responsible for the 'Homeowners Power Solutions' category. My name is Liam Jaxon and a licensed technician with 7 years of experience in vehicle batteries, electrical gadgets, and home appliances. My working experience in different residential & light commercial electrical sectors and the automobile industry helped to acquire vast knowledge in this industry.

Hi, I am responsible for the 'Homeowners Power Solutions' category. My name is Liam Jaxon and a licensed technician with 7 years of experience in vehicle batteries, electrical gadgets, and home appliances. My working experience in different residential & light commercial electrical sectors and the automobile industry helped to acquire vast knowledge in this industry.

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