I pulled the EMS logs for yesterday's flight. It appears that about four minutes after startup (about a minute before takeoff) the voltage climbed above the 14.1 v set point on the alternator.
At the same time, the log shows the value from the amperage shunt - which measures amps into and out of the battery - went to zero.
From that point the voltage goes up to / bounces around at 14.6 - 14.8 for the next 16 minutes. Amperage reads zero all this time.
At 2:02:50 PM, about 20 minutes after startup, the voltage suddenly jumps to 15 and then bounces between 15.1v and 16.9v. At 2:03:28 I pulled the power to the avionics bus, ending the log.
If I recall correctly - and that's a bit of an if, as I was focused on safely operating the airplane - I popped the alt field breaker to try to switch to battery-only before I shut down the avionics. If so, I don't see any indication that doing so made any difference in the situation: voltage remained high, battery amperage remained zero.
If anyone cares to look at the log, I have it posted here:
First 13 characters are the timestamp. Voltage is "vbat". Amperage is "ishunt" (multiply by 1000 to get amps).