Odd HV Battery SoC behavior

I noticed some, never before witnessed and what I will call odd, HV battery state of charge behavior today.

Car had not been run for ~36 hours and was parked outside in the hot Texas Sun. Upon backing out of the parking spot, the HV pack SoC indicated 4 bars, typical for this car is 4-6 when parked. It reversed electrically and the AC was in automatic mode, therefore on FULL BLAST, after all it was 140*F+ inside the car. Upon changing directions to Forward, the engine started, this is not unusual in any way. I drove about one block to the intersection of the main road. By this time, I was somewhat alarmed to find the SoC to be in the orange - 1 bar! Now remember, the engine was running the entire distance from our home to the intersection.

As I attained highway speeds, the SoC began to slowly increase and after about 5 miles was about 3 bars. Upon return from my errand for diesel, it had attained 6 bars and undergone mild regeneration a few times bringing it to 7 bars briefly. This car normally operates between 4 and 6 bars, under ideal conditions it will attain 7 and has only shown "full" twice in about a year while I was at the wheel.

Much has been discussed about HV pack life in the hotter climates. This car has been in Texas since new - that is 12 years and 185k miles, give or take and the HV battery seems to function normally. I believe it is not where/under what conditions the vehicle is DRIVEN that is the major factor in high temperature HV pack loss, but under what conditions it is PARKED that are most important.

A few reasons why I believe this is an important consideration:
- Interior vehicle temperatures while parked can quickly rise to 40*F++ over ambient, in my case that was almost 150*F!
- Battery temperature is controlled ONLY during vehicle operation
- While running, HV battery temperature is essentially only modestly greater than that of the interior, so under "normal" conditions, well below 90*F
- Solar radiation is the primary mode of temperature increase INSIDE of a parked vehicle
- Parking a Hybrid/EV in a garage or at least under cover, essentially eliminates the effects of solar radiation on interior temperatures
- Our vehicle was garaged for the first 10 years of life when not being used
- As the interior cools and therefore the HV battery, it can more easily accept charge or discharge

This behavior could help explain why two vehicles, operated in very similar high ambient conditions, can have such different HV pack lives, if one was shielded and the other unshielded for extended periods.

 

We do not typically lose even 1 bar during the block drive to the main road. Again, this is not my daily driver, but I have never seen it drop that far in that short of a time. My instincts are telling me that it is not measuring the HV pack voltage, but rather the useable capacity. At high temperatures, that drops FAST.