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This problem is still unsolved, although the dealer wants me to bring the car in again soon to take another look at it.
One suggestion which has been brought up both here and by the dealer is the idea that this may be a measurement or display problem as opposed to an actual battery discharge. I rigged up a camera mount which allows me to drive the car while making a video recording of the dashboard, and I have used the rig to capture videos of me starting and driving the car on two different days on which the HVB drain problem happened. The behavior of the car and dashboard indicator during these drives is, I think, consistent with the theory that the battery has actually been run down, and inconsistent with the measurement/display glitch theory. I post them here for your amusement and in case anything shown in them gives any of you any ideas.
When the car is turned off the HVB is isolated when the contactors open. Unless it is self discharging there is no way for it to discharge. Have them check the contactors to see if the contacts have welded in the closed position.
Everything looks normal except the low HVB at start. The temperature bulb Icon switches back to white from green in both videos but that might be normal in cold temperatures. I left the car in accessory mode overnight once and discharged the LVB to dead and the HVB to an even lower point than yours. That was the emergency HVB shutoff point. I had to jump the LVB but then the car started normally from the low HVB and charged normally. I think you'll have to leave it at the dealer. Maybe you'll get a new HVB under warranty.
Update to my post #2 of this thread, my 2010 FFH hasn't experienced another similar HVB drain. Only problem on this car that happened subsequent, was the 12V battery went dead and was replaced. That was about 6 weeks after the HVB drain occurance.
If the HVB contacts are closed and it is powering the car, the DC to DC converter keeps charging the LVB and system. When the HVB gets to it's emergency low shutoff point, the contacts open. The LVB powers the 12 vdc system until it dies.
I have had the low HVB three times in the last two years. The car will now not go as fast in EV and the display of available EV power is not as high as it once was. I believe the events are a cell permanently shorting. This doesn't change the total energy capacity much but it would lower the voltage and maximum power output capability. I usually have had a reconditioning event every 8000 miles but I haven't seen one since this started. Maybe once a cell shorts it is inhibited. I don't know what this bodes for the future. HVBs can tolerate shorted cells but not open ones as all the cells are in series. There are 204 cells in series so if 3 are shorted that's about a 1.5% voltage loss. Available power is related to the square of the voltage so peak power loss would be 6%. EV operation needs higher voltage the faster you go because of "back EMF" (voltage) of the motor so the maximum speed may be inhibited even more than the 6%.