Back to rapid battery discharge.... I may have found some useful evidence. I drove the car on the 9th and pugged it in to charge when I returned home. That evening it finished charging no doubt but it remained plugged in, and the next morning (the 10th) I unplugged it. It sat for several hours and then I checked the battery voltage. It was 12.8 volts.
The car sat all day and in the evening of the 10th it was 12.65 volts. This morning (the 11th) it was still sitting there, unplugged and unused for over 24 hours now, and at 7:30 AM the voltage was 12.30 volts. I went back out a while ago (11:30 AM) and checked it again. The voltage had gone back up to 12.50 volts.
The only way I can see that the voltage could go up while the car sits, unplugged, after more than 24, is if the computer is detecting the battery voltage and at some point when it determines it is needed, it begins a charging routine taking power from the HVB to do it. How else could what I described happen?
And if I'm correct, then the problem of unexpected, inexplicable battery discharge must be due to a computer bug that occasionally fails to correctly identify the need to charge and then charge it.
By the way, during the entire time referenced here no car door was ever opened and no accessory was run. IOW there was no drain nor input on the 12 volt battery that I had control of. All voltage testing was done with a multimeter under the open hood.
Edited by daudette, 11 September 2016 - 12:58 PM.