The day after I returned from SteamCon in Seattle I got into the car to go to the day job and discovered that my battery light was on in my 2007 Toyota Yaris.  Being an old Volkswagen man I resisted the urge to get out and hammer on the the alternator with a monkey wrench.  Instead I turned everything off and then listened carefully to the engine RPM as I turned on the rear window defroster.  When I pressed the defrost button the engines revs did indeed drop about 100 and the engine lugged for a moment until the ECU gave it a bit more gas to smooth out the idle.  Good. This exercise told me that the alternator was charging the battery and that the problem was with the warning light itself. I could safely drive the car for the rest of the week and figure things out at my leisure.

This morning I started working the problem . . .