Signs of a Bad Car Battery vs. Alternator – Know the Difference

a man checking on his car alternator and car battery

You may think your battery is the sole supplier of power to your car. But the truth is, it’s the alternator that produces the majority of your car’s electricity. It also charges your battery while the engine is running. When your car doesn’t start, it’s probably due to a bad battery or a faulty alternator. 

Either way, any issues with these two components contribute to your car’s startup failure. To identify the problem sooner, we’ll go through the difference between the signs of a bad car battery vs alternator so you could fix the problem before it gets worse. 

Bad Car Battery vs Faulty Alternator 

Your battery provides power to your car when the engine isn’t running. The alternator then charges it as soon as you start your car. Since both make up your car’s starting system, identifying the cause of the problem can be quite challenging to identify. That said, here are the signs of a bad car battery versus a faulty alternator:

Bad Car BatteryFaulty Alternator
Battery ageThe engine stalls
The smell of rotten eggsBurning smell
The engine barely turns overUnusual sounds
Can be fixed by jump-startingElectrical issues
Dimming lightsDim or overly bright lights
Battery corrosionDashboard warning light is on
Distorted battery casingDead battery (New)

7 Signs of a Bad Car Battery

1. Battery age

A typical car battery lasts around 3-5 years. Over time, your battery’s ability to hold a charge decreases significantly. When this happens, you’ll start to experience car starting problems. So, the next time your car fails to start, check if your battery is already older than 5 years. It might be due for a replacement. 

2. The smell of rotten eggs

One of the more obvious signs of a bad car battery is when you notice an odd smell, particularly the smell of rotten eggs. The smell comes from the sulfuric gases released from a leaking lead-acid battery. Battery leakage may be caused by several factors including:

  • Damage to its body
  • Overcharging it
  • Cold weather

Important: Never use a leaking battery and replace it as soon as possible. 

3. The engine barely turns over

Another sign of a bad battery is when your engine barely turns over. This may mean that your battery doesn’t have enough voltage to start your car. A quick solution is to charge or jump-start your battery. 

4. Can be fixed by jump-starting

How to tell if the alternator or battery is bad? Well, if you successfully jump-started your weak battery and your engine starts running again, then your battery is definitely the problem. Otherwise, even jump-starting your battery wouldn’t be enough to start your car.

5. Dimming lights

Having a bad battery can result in dimming lights because your battery doesn’t have enough voltage to power all things electrical in your car. A dim light indicates a weak battery. If it doesn’t light at all, it means your battery is dead. 

6. Battery corrosion

If you’re having car  start-up problems, check battery terminals for corrosion. A corroded battery terminal prevents the battery from receiving a proper charge, which eventually drains the battery. 

7. Distorted battery casing

Another telltale sign of a bad battery is when it looks bloated or distorted in any way. A swollen battery may be caused by overcharging or extremely cold weather. If you notice that your battery is swelling out of its case, call for roadside assistance as soon as possible. 

Note: DO NOT drive your car or try to remove the battery as it can cause serious damage to you and your vehicle if it ends up exploding. 

7 Signs of a Faulty Alternator

1. The engine stalls

A dead alternator often leads to a car that won’t start. A faulty alternator may only work sporadically and won’t be able to consistently charge your battery. As a result, you may find your engine stalling from time to time because your battery doesn’t have enough electricity for the fuel injectors to do their job. 

2. Burning smell

A burning smell may indicate that the alternator belt is starting to wear out due to constant tension and friction. It may also mean that your alternator is overworked or some of its wires are damaged. 

3. Unusual sounds

Faulty alternators often produce loud growling or squealing sounds. The sounds may come from a misaligned or excessively worn-out alternator belt. If this belt wears out, it may not turn the alternator fast enough to generate adequate power.

4. Electrical issues

Since your alternator provides the bulk of the electricity, a dying or damaged alternator may lead to a variety of electrical issues. That includes slow or malfunctioning power windows, power locks, stereo, dashboard lights, and air conditioning. 

5. Dim or overly bright lights

How to tell if it’s the battery or alternator that’s causing electrical issues? A faulty alternator causes fluctuation of lights from dim to bright due to irregularities of an electrical surge. A bad car battery, however, doesn’t have enough power to produce a bright light. 

6. Dashboard warning light is on

If you notice that a battery symbol on your dashboard is turned on, this could mean there’s something wrong with your charging system. If your lights and other accessories function correctly, then you can be sure it’s an alternator issue. 

7. Dead battery (new)

It may seem confusing to include a dead battery as a sign of a bad alternator. But if your newly bought battery dies all of a sudden, it could mean that your alternator does not charge the battery even when the engine is running. 

Frequently Asked Questions

What does an alternator do?

An alternator provides power to your car by turning mechanical energy into electricity. It also charges your battery while the engine is running.

Can a bad alternator affect the car battery?

Yes, it does. A bad alternator can lead to a dead battery as it won’t be able to consistently charge your battery.

Can you drive a car with a bad alternator?

Technically, you can. However, it’s best not to since the engine can stall at any time, leaving you stranded somewhere.

How do I check the alternator or battery output?

Ans: The easiest way to check for output is to use a voltmeter. Your alternator or battery is working fine if the voltage reads anywhere between 12.5 – 12.8 volts.


You need both your car battery and alternator to function properly if you want to have a smooth driving experience. If any of the two have issues, you’ll experience several car starting problems and electrical issues. 

So, if you notice any of the signs above, have your battery or alternator checked as soon as possible. Should you need to replace your battery, you can always count on Neighborhood Roadside Assistance for the fast and reliable service that you deserve.