It now being noonish o'clock in Noo Yawk, you've probably done some serious screwdriver twisting and wrench bending on that puppy.
Let's sit and analyze what we know. The airplane, so far as you know, was working just fine on Friday. Saturday you departed home and flew for an hour with the alternator side of the master off. Battery volts dropped to near 12 volts or slightly below. In normal charge mode, that particular alternator regulator is set for 14.2 +/- 0.2 volts. Taking the battery out and charging it got you back up to 12.5 volts. You fired the airplane back up and put a load on the battery and it immediately dropped to 11.9 volts.
So far as I can tell, this is what we KNOW.
Good analysis.
Questions to help us along a little:
1. When you noticed the master switch half off, did you turn the master completely on at cruise RPM? If so, did the voltmeter and/or the ammeter tickle in the least?
At cruise (2300-2400). The voltmeter rose some and the ammeter rose from negative to zero, then no further action.
2. When you re-placed the battery after charging, did you do the load test at higher than idle RPM or down in the 500-600 rpm idle range?
I was at 1000 rpm.
3. Did you "cycle" the alternator master a few times? That rocker switch was truly supplied by the lowest bidder.
Yes, we cycled one extra time. Since then, it has been off and on several times. We haven't tested the switch itself yet.
4. That battery should accept a charge to at least 13.5 volts ten minutes after a good healthy charge. Why you only got it up to 12.5 volts is a good question that I don't have an answer to.
It was on the charger more like 45 minutes and got to 12.5 volts.
Jim