Bad Alternator Vs Bad Battery: How To Tell The Difference?

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So your car’s probably not starting or your having issue with your car running properly. 

The two most likely culprist are the alternator and the battery. But how do you tell which one is the source of the issues. 

Don’t worry. I’m going to make you life much easier. 

In this post, I am going to help you tell wheather your alternator or battery is bad. 

Let’s get started. 

What Does The Alternator Do?

The alternator is the power source for your car when driving or when your car is idle. It doe this by converting mechanical energy into electrical energy. To create electrical energy, the alternator uses a pulley system that rotates the rotor shaft. As the rotor shaft rotates, it creates alternating current using a set of magnets.

But the alternating current is not electrical energy and can’t power your vehicle. So finally, the alternating current is converted to electrical energy. To do this, the alternating current is directed through the rectifier bridge, where it is converted to direct current and can not power your electrical system.

In addition to powering your vehicle as you drive, the alternator also recharges your battery. Car batteries can only last between 30 minutes to two hours of driving if there are not being recharged. This is why the alternator is such as crucial part of your vehicle.

What Does the Car Battery Do?

Your car battery’s main function is to start the car. Alternators are powered by a pulley system that is connected to your engine. Unfortunately, this means that your alternator won’t generate any energy if your car is off.

This is where car batteries come in. The car battery will supply the initial energy required to get the car started. While the alternator will supply the energy required to drive the car and recharge the battery to ensure it can start the next time it’s turned off.

The battery also stabilizes the energy or electricity that your car needs to drive. This ensures that each component receives the proper amount of electricity.

How To Check Your Battery Or Alternator?

When your battery or alternator is going bad, they can result in similar symptoms.

One of the best things you can do is test your battery and alternator. Fortunately, you can test your battery and alternator with a voltmeter or multimeter.

This is an easy, quck, and effective way that you can determin what is causing the issue on your vehicle. 

How to Test Your Alternator?

There are a few things that you can do to test the starter yourself. The easiest thing to do is to test the starter solenoid. The starter solenoid acts as an electromagnet relay that works to engage the starter. The starter solenoid is attached directly on the starter.

To check the starter solenoid, you can do a visual inspection of the wires. If all the wires are not damaged and connected correctly, you will need to test the voltage.

To test the voltage, you can use a simple multimeter or a circuit tester. If the multimeter or the circuit tester doesn’t show any voltage or light up, this is likely your starter’s issue. This means that the starter is not receiving the electricity it needs to start.

If the solenoid passes the test, then the issue is likely with the starter itself. Checking and testing the starter is not an easy process. I recommend you take your car to a professional to inspect your starter unless you are an experienced car enthusiast.

When testing your alternator with a multimeter, it should read between 12.6 and 14.2 volts.

Typically 12.6 volts is considered approaching low, but your alternator can still work properly at this time.

If you receive a lower reading, then you’re alternator is likely having a voltage issue.

If you are getting a reading higher than 14 volts, your alternator is likely an issue.

How To Test Your Battery?

There are effective and inexpensive battery testers that you can purchase online. You can also use a simple multimeter or voltmeter. 

To test you would adjust your multimeter to 20DC. Then place your red probe on the postive battery terminal and the black probe on the negative battery terminal. Once both probes are both on the terminals you will get your reading. 

If your battery is not giving you a proper reading after both situations, this is a sign that your battery is dying, and you should test further.

A good battery should read between 12.2 and 13 volts when the car is off.

If the car is running, your battery should read between 13 volts and 14.8 volts.

If you experience anything outside of these ranges, this is a sign your car battery is draining and not being replenished.

It’s important to note that if your engine is reading higher than the range for a good battery, this is a sign of a faulty battery or issues with the alternator.

Signs of a Failing Battery

Battery issues are much more common than dead batteries. This is because the lifespan of a battery is much shorter than an alternator.

Batteries need to be replaced every 3-5 years, while alternators need to be replaced every 7 to 10 years.

There are some common signs that you can look for that will help you identify a faulty battery.

1. Trouble Starting Your Car

One of the main issues associated with a bad battery is when your car has trouble starting.

In most cases, this is due to issues with the battery. This is because your car relies on your battery to start your car and not the alternator.

In most cases, your battery may not be able to keep a charge, or there may be connection issues.

In some cases, the issues with your car starting may originate with your alternator’s inability to recharge your battery. In this case, your battery can be working completely fine, but the alternator is not recharging your battery.

2. Corroded or Damaged Battery Terminals

If your car doesn’t start, there is a high chance that it has something to do with your battery.

This doesn’t mean that your battery is dead; this can mean you have issues with your battery terminals.

The battery terminals are what transfer electricity from the battery to the rest of the car.

So even if your battery is in perfect condition, if your battery terminals are corroded, damaged, or not on properly, your car won’t get the electricity it needs.

This will cause a very similar effect that a dead battery would. Your car will not make any noise when it starts because there is no electricity to start the alternator, starter, or spark plugs.

If your car doesn’t start, the first thing you should do is inspect your battery.

The battery terminals should be corrosion-free and should not have any damage on the lines.

A corroded terminal is one of the most common issues. Corrosion can begin to form between the battery terminal and the electrode. This will prevent any electricity from reaching the rest of your car.

If there is corrosion, you want to clean it off. If your lines are damaged, you want to replace these to ensure a safe and secure transfer of electricity.

3. Your Car Cranks Slow

Another sign of a dead battery is if your car is cranking slow. 

You will experience this as soon as you turn your ignition. If your car is turning over or starting slow this is a sign of a bad battery. 

This is a sign that your starter is barley getting enough electricity to start your car. This is the reason for the slow turn-over of your vehicle. 

This is something that takes experience to identify. You will typically notice that you car is taking longer to start. 

You can also identify this if car sounds weak or slow when its starting. 

4. Dim Headlights or Interior Lights

Another common sign of a bad battery is if your headlights are dim. If your battery is starting to fail, your lights may become dim or dull due to the limited electricity available.

If you feel that your headlights look dimmer than usual, then you should have your battery checked.

You might also experience the other lights in your car looking dim or dull—for example, your dashboard, radio, or other lights indoors.

You will likely experience this just after you start your car or before you try and turn the ignition.

In many cases, when your car is on, the lights might be dim or dull.

5. Bad Smell Coming From Under The Hood

when your battery starts going bad, you may also smell a foul or rotten egg smell coming from under your hood.

This smell is often compared to the smell of sulfur.

This smell is a sign that your battery is leaking. Leaking typically happens when there is damage to your battery or due to natural wear and tear.

You should inspect your battery for any liquid coming from the sides or the top of the battery. You should also inspect the body of the battery of any cracks, bloating, or other damage. 

Signs of a Bad Alternator

Your alternator is the powerhouse of your car. As a result, you can drive very long with a bad alternator. 

Unfortunatly, a bad alternator is often confused with a bad battery. So how do you tell the difference. 

There are some key signs you can look for to determine the difference between a failing alternator and battery. 

1. Dead Battery

A dead battery is a tell-tell sign of a bad alternator. One of the main functions of the altnerator is to recharge your car battery. 

Your battery is not designed to power your car for extended periods. Without the alternator your car battery will only last around 30 min to 2 hours. 

If your battery is dead or continues to die even after your drive then this is a sign your alternator is not working properly. 

To check if your battery is dead you can test your battery usign a battery tester or multimeter. 

Anything lower than 12.9 volts is a sign that your battery is not holding a charge properly. 

2. Bright Lights That Begin To Dim

With a failing alternator what you will experience is that your dashboard lights will start off brigh and begin to dim after some time. 

This happens becuase when you start your car your lights are being powered by your car battery. When you start driving, the alternator is not able to provide the neccessary power to keep your lights bright. 

You might experience these issues with your headlights and other lights inside your vehicle as well such as your radio or dashboard. 

You can test this by driving you car at night. After starting your car, drive for 10 to 3o minutes to determine if you experience any diming. 

3. Dashboard Warning Light

Another key sign that your alternator is failing is when the warning light comes up on your dashboard. The alternator warning light is a small battery. 

This warning light will turn on to inform you that your alternator is not recharging your battery. 

If this light does turn on then you should try to aviod using the other electrical components of your car. 

Also you test your alternator and battery after you the light turn on. If you continue to drive with this light your car battery and alterntor will eventually stop working. 

4. Car stops running After Disconnecting Positive terminal

A very simple way to test if your alternator is bad is to first start your car. 

After starting your car, you need to disconnect your positive terminal from your battery. 

If your car turns off after disconnecting then your alternator is bad. 

This is a sign that your alernator is not generating enough power to support your car’s requirments. 

5. Engine Stalling or Slow Cranking

Another sign of a failing alternator is if your car starts stalling. This typically happens because your cars power is fluctuating. 

As a result, when your power is too low then your car will begin to stall. 

Another sign is if your car is cranking very slow. 

This will happen becuase your alternator is not working properly. Your alterntor will crank very slow or not crank fully. As a result, your car won’t run and also your car won’t be able to generate enough power. 

6. Grinding or Squeling Noise

Another sign of a failing alterntor is grinding or loud sounds coming from your alternator. 

Grinding or strange noises coming from your alternator is a sign that it can’t rotate properly. When your alternator can’t turn properly it will not generate the proper power your car needs. 

When you hear grinding or other strange noises its typically a sign of friction or broken internal parts of the alternator. In most cases, you will need to replace your alternator. 

This is the case becasue these noises are typically a sign that your alternator has damage on the inside which is likely too expensive to repair. 

Frequently Asked Questions (FAQ)

Can you jumpstart a car with a bad alternator?

Yes, you can jumpstart a car with a bad alternator, but you won’t be able to drive it very long.

The alternator supplies the car with the power it needs when your drive. As a result, you will only be able to drive it a short distance.

If you do jumpstart a car with a bad alternator, you should only drive it to a repair shop or to a location where it will be repaired.

You will need to get your alternator replaced before you can begin driving your car normally. Also, you will need to jumpstart your car each time you try to start it until your alternator is repaired or replaced.

Will a Dead Battery Damage My Alternator?

No, a dead battery won’t necessarily damage your alternator.

One of the alternator’s main functions is to recharge the battery. As a result, if you have a dead battery, you can fully charge your battery if you drive for between 30 min to two hours.

If your battery does not hold a charge, this can cause unnecessary wear on your alternator.

How Long Does an Alternator Last?

Most alternators are meant to last around 150,000 miles. For most people this is between 7 to 10 years.  As a result, when you have alternator problems there is typically something else causing the issue. 

If you drive through rough conditions such as dirt, sand, rain, and snow frequently this can cause your alternator to fail sooner. The dirt and grime from these conditions can cause your alternator to fail ealry. In other cases, direct contact or rough driving conditions can damage the internal parts of your alternator. 

Although, alternators are meant to lifespan of your car, this doesn’t mean that this is not the issue in your car. Be sure to inspect your car throughly and keep and eye our for the common signs before you rule it out. 

How Long Do batteries last?

Batteries have a much shorter lifespan than alternators. The lifespan of a battery also varries drastically depending on the quality and type of battery you purchase. 

The stock battery on most new cars is expected to last between 2 and 4 years. 

When you replace your battery the lifespan will depend on the quality of battery. 

High quality batteries can last between 5 and 7 years whereas less expensive batteries typically last between 2 and 4 years. 

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