Signs of a Bad Car Battery vs Alternator – Learn the Difference

When you get in your car on a cold winter morning and your car won’t start the first thing your mind goes to is the battery. And while that certainly can be the case, it could be something like the alternator too.

Tons of people end up replacing the wrong part first and then they need to head out and spend even more money to get their vehicle running again. It’s easy to mix the two problems up with each other since they have so many similar symptoms.

We’ve highlighted signs that you need both a new battery or a new alternator here, and we’ve highlighted how you can test each component to figure out if that’s the problem.

The Car Battery vs the Alternator

The car battery and the alternator are two very different components in your vehicle, even if they work hand-in-hand to keep your car running with electricity. But to understand how each one can affect your car you need to know what each one does in the first place.

The car battery is just like any other battery in that it stores power and then uses that power to perform various tasks. The radio, headlights, and just about every other electrical component use power from the battery to do their job.

However, just like any battery, if you don’t take the time to charge it eventually it will run out of voltage. That’s where the alternator comes in. Just like you use a charging cord and an electrical outlet to charge your cell phone, your car uses the alternator to charge the battery.

The alternator only works when the car is running, and it pumps electricity into the battery to keep it at a full charge while it provides power to all the different components in your vehicle. Even better, once the battery has a full charge the alternator pumps all that extra voltage straight to a ground so it doesn’t overcharge and damage the battery.

What Are Signs of a Bad Car Battery?

Now that you know a little more about both the car battery and alternator, it’s time to learn about a few classic signs of a faulty car battery. Just keep in mind that many signs could mean there’s a problem with either the battery or the alternator.

1. Car Won’t Start

It’s the first thing most people think of when their car won’t start and there’s a reason for that. Car batteries typically only last between three and five years, so it’s a common part to replace.

If your car won’t start because of a faulty car battery, you should hear a clicking sound when turning over the ignition, and sometimes the engine will turn over slowly. It’s also more likely to happen on cold days or mornings.

Also read: 5 Dead Car Battery Tricks To Get You Back On the Road

2. Corroded Connections

If you look at your car battery and see a bunch of corrosion around the terminals, that’s a clear-cut sign that there’s something wrong with the battery. We recommend cleaning the terminals, charging the battery, and see if that helps with your problem. Just keep in mind that even if it helps you’ll likely need a new car battery before too long.

3. Warped Battery Case

If you look at your car battery and see it has a warped casing you need a new car battery right away. This is from excessive heat inside the car battery and there’s no fixing the problem.

Replace the battery immediately and then double-check that the alternator isn’t pushing out too much voltage. If it is you’ll need to replace that too, or you’ll just end up warping the new battery.

4. Dimming Lights

Your vehicle’s battery only has so much power to go around. So, if it’s starting to fail, then it’s certainly possible that the headlights or even dome lights in your vehicle will begin to dim. Just know that this isn’t a battery-specific condition, so you can’t narrow it down to just the battery if this is happening.

5. Electrical Problems

The battery is the component in your vehicle that provides power to everything else. So, if there’s not enough voltage in the battery you might start to notice electrical problems throughout your vehicle.

The radio, headlights, and even the key fob might not work quite as well as they should, and they might not work at all! But once again, this isn’t a battery-specific symptom.

6. Battery Warning Light

Did you know that most vehicles have a warning light specifically for the battery? This warning light tells you the voltage isn’t where it should be and there’s a problem somewhere in the charging system. But surprisingly the battery warning light isn’t for just the battery.

The problem could still be the alternator, even if the battery warning light is on.

What Are Signs of a Bad Alternator?

While most people think about the battery when their car has an electrical problem, it could be the alternator too. Below we’ve highlighted five common signs that it might be time for a new alternator.

Related: How Much Is an Alternator? (With Repair & Replacement Cost)

1. Car Won’t Start

Even if the battery is in perfect condition it’ll still run out of power if the alternator isn’t charging it back up. The more you use the car’s electrical components the more the battery will die, and it won’t matter how long you leave it running since the alternator won’t charge it again.

2. Dim Lights

If you’re driving around and notice that your headlights are slowly starting to dim the more you drive, that’s a clear-cut sign that there’s something wrong with the alternator. The battery is running out of power as all the different components use it, and the alternator isn’t filling it back up with more power.

But while this is the most likely reason for this slow dim, it is possible that there’s a problem with the battery and it just won’t accept the charge.

3. Accessories Don’t Work Correctly

As your battery runs out of power since the alternator won’t charge it, various accessories on your vehicle will start to stop working properly. Once again, it’s more likely that the problem is the alternator than the battery if the components slowly lose functionality as you drive, but you can’t completely rule out the battery.

4. Engine Dies While Driving

If your battery runs out of voltage completely while you’re driving then it won’t be able to power all the necessary components to keep it running. No matter what you do, your vehicle needs electricity to stay running, and if the alternator isn’t providing electricity to the battery, it will eventually run out.

5. Battery Warning Light

Even though the battery warning light has “battery” in the name, it really could be a problem with the alternator. That’s because the warning light really only looks at the voltage in the battery, and if the alternator isn’t charging the battery properly then of course it won’t be at the right amount!

Testing the Battery

One of the most common mistakes people make when they’re testing a car battery is that they only think about the voltage, and sometimes they don’t even think about the right voltage. When you test a typical “12-volt” car battery it should have 12.6 volts.

If you test the battery and it only has 12 volts, that means it only has a 75 percent charge. Not only that, but for a battery to start a car it needs enough cranking amps, and the voltage doesn’t tell you anything about that.

To accurately test a battery you need a dedicated battery tester, and you should only test the battery after it has a full charge.

Testing the Alternator

While you need a dedicated battery tester to test a battery accurately, all you need to test an alternator is a voltmeter or multimeter. Simply hook up the negative lead to a good ground, then put the positive lead on the back output terminal of the alternator while the engine is running.

Turn on all the accessories like the headlights, radio, and air conditioning to get the best test results. You’re looking for an output voltage between 13.5 and 14.5 volts. Anything below that means the alternator isn’t working the way it should and it’s time to replace it.

Keep in mind that this still doesn’t rule out a faulty battery too, but it does mean there’s something wrong with the alternator.

Troubleshooting Your Vehicle’s Electrical Problem

If you notice any of the symptoms we’ve highlighted for either the car battery or the alternator, we highly recommend testing the entire charging and starting system to rule out any other potential problems.

Even better, auto part stores like AutoZone or Advanced Auto Parts will test all of these components for you for free. Just keep in mind the battery needs a full charge for accurate results.

But you can even take your vehicle’s battery to the store for them to test when it’s not in your vehicle and they’ll test it for you. From there, you can test the alternator wherever your vehicle is as long as you get it running long enough!

Final Thoughts

After reading this article you now learned that many of the symptoms are similar when it comes the alternator and battery. The best option is to test the entire charging system to avoid replacing the wrong part.

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Rickard Cefalk

Rickard is the owner of and a dedicated and avid do-it-yourselfer who has always enjoyed working on his own vehicles since childhood. He now devotes his time to sharing his expert knowledge of car maintenance and other car-related information through his website.

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