Regen disabled and engine runs constantly in warm weather.

My 2010 Prius has developed a worrisome problem since the end of last summer. On any day above about 85 degrees, light to moderate use of the battery such as regenerating down a hill or driving under 40 mph using battery causes the car to go into a failsafe mode which does not use the battery at all and does not throw any errors. When this is happening, it does not use regeneration at all and runs the engine 100% of the time in motion at any speed or load. I have a gauge on my dash which shows charging, as well as Torque app, so this problem is quite apparent and is not "normal". It is definitely heat related and only happens on warm days.

I initially suspected it was based on the temperature of the traction battery. Oddly the HV battery fan does not automatically speed up at all during this, though the fan does work fine if I command it to a higher speed, such as fan speed 6 via torque.

Today I ran a stress test with the AC on high and HV fan on high on rolling hills where I was able to repeatedly charge and discharge the battery to see if the problem could be triggered while keeping the HV battery cool. Under this extreme stress, the battery never got above 35 degrees, and everything worked great, which I consider to be a very interesting data point suggesting battery temperature is the trigger for the failsafe mode. Furthermore, since the battery ran cool under stress with the fan at max in a cool cabin, I suspect that the problem is that the HV fan is not automatically speeding up when it needs to.

Questions I'm hoping the community can help me with:
1. At what HV battery temperature should the HV fan usually start to noticeably speed up on a normal gen III Prius? I monitor this with Torque app.
2. Has anyone else experienced this?
3. Is there any way with Torque App to persistently force the HV fan to always run or be more aggressive, without the app running and being connected at the time.
4. What other data should I collect, or what other tests should I run? (I'm reading all the inverter/boost temperatures as well, but don't know what normal ranges are.)

As an aside, I did verify that the inverter coolant is circulating and the radiators are clear.

Thanks for your help!
Michael

 

Following up on my own post, my current theory is that when the car sits in the hot sun, the battery heats up to near the high operating limit. Then with some driving (using the battery before the catalytic converter warms up, regenerating, or operating in stealth mode) the battery gets just enough warmer to hit the limit and puts the car in failsafe. The main question is why does my HV fan not automatically speed up in this condition. The Torque monitored set of three battery temperature sensors are sending good data, and the fan responds to manual speed settings. The next warm day I'll repeat a battery stress test without manually setting the HV fan speed, and monitor temperatures and fan speed. I'll also determine if there is a consistent temperature where failsafe gets triggered.