Car Battery versus Car Alternator
Car Battery | Roadway Questions

Bad Car Battery Vs. Alternator-4 Signs to Tell the Difference

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Click click click….click click click…Oh no, the click of doom! You’re in a hurry but your car isn’t cooperating.  But, ‘what the heck?’, you just replaced this battery; why isn’t your car starting?

This is always fun, especially when you’re in a hurry and need to get to work or drop the kids off.  However, maybe your battery isn’t to blame.  Your car’s electrical system is a complicated thing with many components.  

Today, we will focus on a bad alternator versus a bad car battery. It matters because you obviously will take different steps to resolve the issue.  FYI, a bad alternator can actually damage a new battery, but more on that later. 

So, let’s look at the signs of a bad alternator versus a bad battery…in table format

Bad AlternatorBad Battery
Car Won’t StartBattery Age (main culprit)
Car Struggles to StartThe smell of Rotten Eggs
The car starts but stallsPower windows slow to move
Dimming LightsBattery light
Issues with Sound SystemEngine slow to turn over
Other electrical issues*Radio loses presets
*If you are unsure, please consult a professional mechanic for a proper diagnoses.

As you can see it can be difficult to discern the difference between a bad battery and a bad alternator. Let’s get into a little more detail about the signs of a bad alternator and how it affects your car.

Is the Alternator at Fault?

Does your car show any of the following problems?

  • Trouble starting
  • No starting
  • Dimming lights
  • Problem with a sound system output
  • Your vehicle starts but stalls

All these are signs of your vehicle’s battery not getting recharged and a faulty alternator can be one of the key reasons for this. Your vehicle giving out a squealing sound that becomes louder when you turn on the sound system or the heater could be an indicator that the bearings of the alternator are under stress.

Here are some signs that indicate that your vehicle’s alternator is failing. 

Indicator Light: Your vehicle may have an indicator light to tell that something is not right about the alternator. If you can locate this indicator and if it is turned on, you should not ignore the message and immediately take your vehicle to an automotive workshop for inspection.

Dimming or Flickering Headlights: The alternator supplies power to the headlights in a vehicle. If the headlights are dimming or flickering, it’s a strong signal that the alternator is not working alright.

Other Electrical Problems: When the alternator is struggling to perform, some other systems linked to the alternator’s performance start showing problems. For example, power windows, air conditioning, dashboard lights, power locks may face difficulty in their smooth operation. There could be some other reasons but even the alternator could have some problems.

For Those Who Have an Adventurous Automotive Side

For those of you who like to get your hands dirty. I found this video, that explains how to test your alternator and what to look for.

Is the Battery at Fault?

If your car is having problems getting started, you need to get to the bottom of the problem. If your car’s engine revs up on jumpstarting, and everything seems to work fine, your car’s battery is likely old and perhaps needs to be replaced.

But before you actually do that, you need to check other things such as the connection at the battery’s terminals. The cable may have got corroded or loosened up. You can change the cable and clean the terminals to see if it helps. If the battery is depleted faster, it could also be because of additional draw from lights, fuses, alarms, or sound systems.

There are many symptoms for a battery going bad, for a complete checklist read my article 10 Symptoms of a dying Battery.

What Does the Alternator Do?

The alternator is an important component of your vehicle’s electrical system. The alternator converts the energy created in the crankshaft to electricity that can be used. It supplies the electricity to the car’s battery to recharge it. The battery powers the vehicle’s starting system and all the accessories that run on electricity.

If there are problems with the alternator, it will not convert the energy in the crankshaft into electricity. As a result, the battery will not get the supply of energy even when the vehicle is running and the crankshaft is producing energy. This, in turn, would mean the battery is not getting recharged. 

Once the charging of the battery depletes, it will not be able to provide the necessary electricity for the vehicle to start or run any of the electronics. Before the battery is fully depleted, your vehicle may start giving you strange problems. These may include flickering headlights and/or strange noises while driving. You may mistake these as signs of failing battery, which may be the case.

How Long will a Battery Last with a Bad Alternator?

A bad alternator will not supply the charge to the battery while the battery may have to supply power to the headlights and all other accessories and systems in the car. This may drain the battery quickly. Depending on how much charge was there in the battery and how old the battery is, the battery may run flat within minutes to a few hours. 

The old diesel engine vehicles have simpler engineering and there are fewer items drawing power from the battery. So, these vehicles with a bad alternator may have their battery supporting the vehicle for longer. Modern cars that run on gas have many systems drawing power from the battery and this may lead them to run out of battery faster if the alternator is bad.

Can You Drive a Car with a Bad Alternator?

Yes, you can drive your car with a bad alternator for a few minutes or in case of an emergency. The point to remember is that your car’s battery may not be getting the usual amount of charge and it may completely run out of power suddenly.

However, this is unadvisable. Your car will quit quickly and without warning and you will be stranded.

Will a Bad Alternator Kill a New Battery?

Yes, it can. So, instead of replacing the battery, get the alternator checked first. Otherwise, a bad alternator can damage a new battery in no time.  

Final Thoughts

A bad alternator is bad news. Assuming you are not a mechanic, this problem is difficult to diagnose. However, if you have had your battery tested and are fairly sure that it is not a battery issue, then take your car to a mechanic to have it correctly diagnosed.

To be safe, if you suspect an issue have your car inspected. It may cost you up front a little, but in the long run, it will save you.

Safe travels…

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