Alternator problems?
 

While waiting for my wife, I had the engine running with the a/c and radio on. When she got in the car, the radio started flashing off and on. After trying to adjust it, I decided to shut off the engine then restart it. The battery was dead. The voltmeter I installed showed only 10 volts. I boosted the car and took the battery out and put it on a tester at Pep Boys. They fully charged the battery and it tested to be fine. Now I see that the voltmeter is only showing 12 volts when I am driving. I know it should be over 13, but it is a cheap gauge and I have no engine codes showing. Think the alternator could be on it's way out? Tomorrow I want to bring it to my friends shop, have the codes read, and put a good voltmeter across the battery.

You should be running 14-14.5 volts.

I know, but the gauge is a cheapo and I don't recall it reading much higher than 12. But wouldn't I have gotten some warning light if the alternator was not working right?

I don't think you had a alternator problem. You just drained the battery by sitting and using the radio and a/c, probably on a higher blower setting.....and remember there are other electronic/electrical powered functions in the "invisible" background.........for engine operation, dash gages, etc.

My daughter did this every summer waiting for kids at school, swim club, track team, etc.

I'm told, by a GM engineer, that occasional idling is acceptable with SOME power accessories running. But an owner should run the car at higher RP M's if sitting more than a couple of minutes......conversely, a run on an Interstate is the best resolutions.

So, I suggest taking a long run on the Interstate to see a friend (remember to turn the engine off while talking with him/her.

Oh, and if I remember correctly....when you drained the battery that low, it may not return to full and reliable charge.

Warning lights or CEL's are not always accurate. Do not depend on them to be your only diagnosis.
They're nice when they give you a warning but don't blame the car if something breaks and you don't get a "light".

Sounds like you have a bad alternator OR a bad wire/power connection to the charging system.

Have it tested. Most, if not all, auto parts stores will test it for free. Then you will know.

This morning when I started the car to drive my daughter to camp, the voltmeter read 14 volts for a few seconds then dropped to 12. Stayed that way as I drove, dropping to about 11 when stopped at a light and engine RPM low. I went by my friends shop and told them I was going to bring the car in to test/change the alternator (and belt while at it). I went home to get my other car keys and tell my wife to follow me there, then drove back to the shop. Now the voltmeter was reading 10 at idle. Once I got to the shop, I tried to restart the car and the battery was dead. Feel pretty sure it is the alternator.

Just got my car back and it turned out to be very lucky for me. The alternator was bad, but so was the serpentine belt. It had cracks and was ready to break.

And now my voltmeter is back at 14!