Battery or Alternator?
 

Hello,

Can someone confirm if the following is a battery or alternator that is dying?

I used a multimeter.

At rest, my battery reads 12.51V.
When the ignition is on, I opened max beamlights, fans, AC, etc. an my battery read 13.51V.

I then drove my car and the red battery icon started flashing. I turned off my car and grabbed my multimeter.

At rest, my battery started to read 11V.
Then I tried to turn on my car but it wouldn't turn on.
I then re-read the battery and it was at 9.80V.

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Does anyone know if this is a sign of a dying battery or alternator?

Thank you in advance!
Eric

Re: Battery or Alternator?
 

anytime your battery light comes on with the engine running that mostly likely means a faulty alternator

from what you have described i would say your alternator is most certainly fried, however i would highly recommend replacing them both at the same time, running the engine with a faulty alternator will kill a battery very quickly

Re: Battery or Alternator?
 

This can be determined with a few tests and some inspection. Grab a shop manual.

Do you mean to say that the engine is running for this check? It must be, because that is the ONLY way you can achieve that number.

The alternator IS doing something at this point.
So the wiring for the alternator can be assumed to be ok for the moment.



1) Is the alternator belt loose? A slipping belt will let it stop charging, while you are driving it.

2) Are the alternator bolts tight? Loose hardware can fry the PCM.


Battery ran dead while driving. Alternator wasn't charging enough. See #1 above, or the alt could be bad. Check that belt first. It has to be pretty damned tight.
Charge up the battery. Does it pass a load test?

If these are all ok, then pull the alternator and get it checked.
Does the alternator howl, whine, growl, or make any other unusual noises? Those are all signs.

Does the car have a quarter million miles on it? Could just be worn out.

Re: Battery or Alternator?
 

You have not stated what car you have. Yes, people with older Civics come in here asking questions.
Do you have a stereo system with an amplifier and larger than stock speakers?

Nathan

Re: Battery or Alternator?
 

something wrong with the charging system, not battery. maybe loose cables to the battery, maybe loose wiring at the alternator (check the bolt under the boot), or its not making as many amps as it should be (maybe 30 amps instead of 60 amps at load). and looks like whatever the reason, the battery is already ruined as a result. so once you get the alt situation fixed, get a new battery.