Can't tell if there's an alternator or battery issue


Asked by Thomas Feb 12, 2020 at 01:49 PM about the 2012 Honda Civic EX

Question type: Maintenance & Repair

I went to the store the other day and when it was time to leave, my car
wouldn't start. I called a buddy to come to jump my car. He jumped it and the
car started up and I made my way back to my apartment just fine. When I
tried to start my car the next day for work, it wouldn't turn on.

I thought it would be a simple battery issue but the battery I have now is not
even 2 years old (Duralast battery). It's still under warranty at AutoZone. So I
head to AutoZone and the manager tells me that he's going to charge and
test it and to come back later. So, that's what we do. A couple of hours later I
get a phone call that the battery charged to ~90% capacity just fine and (his
words not mine) that there isn't an issue with the battery...So I put the
charged battery in my car, drive around a bit, come back to see that the
capacity of the battery dropped to 66% meaning that the alternator may not
be charging the battery correctly OR that the battery may not be holding a
charge properly.

I was convinced that it was an alternator issue UNTIL a friend and I turned
the car on, disconnected the negative terminal of the battery (while the car is
on), and proceeded to turn on the radio, AC, and high beams. We ran the car
like this for about 20 minutes and it ran fine. So now this leads me to believe
it's a battery issue since the alternator could support all of that for upwards of
20 minutes.

For further information, I tested the voltage of my battery while idle (not
running) and it came out around 12.2 volts. Tested it again (while turned on)
after running the car for about a minute and it came out to about 12.95 volts.
12.95 volts may be a little low while the car is running but I may have not let
it run long enough. I can test that again if anyone thinks it'll help.

Does anyone have any other thoughts? Maybe the AutoZone guy is trying to
prevent me from cashing in on my warranty?

Thanks in advance.

2 Answers


12.95 is low with the engine running, it should be 14V. I suspect your alternator is faulty. Your auto zone tech can check the alternator for proper function like he did with the battery.


Thanks for your input TDolby. I went ahead and convinced AutoZone to give me a new battery based on a few things. 1) Since the battery level was higher after turning the car on, the alternator is indeed able to provide power to the battery 2) I tested the battery again later that day and found that the idle (car not running) voltage was ~12.06 volts, insinuating that the battery can no longer hold a charge 3) After turning on features that would pull a bigger load from the alternator (headlights, AC, etc.) while the car was on, the voltage of the battery shot to a perfect 14.40 volts I even tested for continuity between the alternator and battery just to be sure as well as a couple of fuses. It's been 2 full days since I've changed the battery and everything seems fine. I just checked the voltages of the new battery and they're currently at 12.27 volts idle, 12.48 volts running, and 14.41 volts running with the headlights on. The voltage, while the car is turned on (not with extra things turned on), seems like it should be higher, but who knows.

1 people found this helpful.

Your Answer:


Looking for a Used Civic in your area?

CarGurus has 33,589 nationwide Civic listings starting at $1,495.


Own this car?

Share your experience with others.

2012 Honda Civic

Review another car

CarGurus Experts

  • #1
  • #2
  • #3
View All

Know more, shop wisely


Related Models For Sale

Used Honda Accord
1,265 Great Deals out of 29,395 listings starting at $700
Used Toyota Camry
1,855 Great Deals out of 41,769 listings starting at $1,100
Used Toyota Corolla
1,571 Great Deals out of 25,032 listings starting at $1,900
Used Honda Civic Coupe
152 Great Deals out of 1,940 listings starting at $900
Used Honda CR-V
1,710 Great Deals out of 38,686 listings starting at $1,450
Used Mazda MAZDA3
492 Great Deals out of 12,334 listings starting at $1,100
Used Hyundai Elantra
862 Great Deals out of 23,590 listings starting at $1,350
Used Toyota RAV4
1,857 Great Deals out of 31,522 listings starting at $1,950
Used Lexus IS
283 Great Deals out of 5,867 listings starting at $3,499
Used Nissan Altima
1,184 Great Deals out of 34,311 listings starting at $1,400
Used Toyota Tacoma
1,415 Great Deals out of 22,388 listings starting at $3,700
Used Ford Mustang
855 Great Deals out of 24,642 listings starting at $1,900
Used Dodge Charger
709 Great Deals out of 38,388 listings starting at $1,600

Used Cars for Sale

2024 Honda Civic For Sale
12 Great Deals out of 18,107 listings starting at $22,849
2023 Honda Civic For Sale
48 Great Deals out of 692 listings starting at $19,856
2022 Honda Civic For Sale
110 Great Deals out of 1,617 listings starting at $16,995
2021 Honda Civic For Sale
106 Great Deals out of 1,920 listings starting at $14,549
2020 Honda Civic For Sale
161 Great Deals out of 2,320 listings starting at $12,945

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use. Content will be removed if CarGurus becomes aware that it violates our policies.