Auto A/C Periodically Vents

I am curious about an operation in the HVAC system of my 2008 package 5. I am the second-owner and have personally logged almost 78,000 miles in a little over two years. With all this time behind the wheel, I have become very well versed in Prius systems operation. And of course what I haven't figured out on my own through the owner’s manual or trial and error, I have found excellent descriptions of in this wonderful forum. I am also an avid DIY and do all of my own routine maintenance, on time and properly. Here is my question. When operating the vehicle in Auto A/C, I have noticed that periodically the re-circulate setting will change by itself to "outside air". I have not noticed yet if it will set itself back to re-circulate after some period of time. I wonder why it does this. Could it be normal operation, as I recall several other posts about this behavior so other folks notice it too. It isn’t a problem really, but the times I take notice that it has switched are when I’m following behind a semi truck or a car that’s burning oil and the smell pervades the cabin. Not very pleasant. I was thinking the other day about the HV battery cooling system. The exhaust ductwork as everyone knows extends to the right-rear corner of the vehicle, passing over the auxiliary 12V battery to the body vent. In my snooping around I have determined that when the trunk cover is in place, there appears to be a restriction to the normal airflow that this system would need. Even with the HV battery vent fan running, there is no return path to the cabin. That is until the air handler is taken out of re-circulate mode and air can flow into the vehicle, into the HV battery cooling intake on the rear bench seat, and out through the body vent. The vehicle appears to be rather air tight when everything is buttoned up, so I am wondering if maybe a high temperature prompt from the HV battery is changing the re-circulate mode to vent. At 144K total miles, I am beginning to worry a little about my HV battery health. Your thoughts.

UPDATE: I have been watching the conditions under which the vent automatically cycles to outside air and noticed that it happens every time the preset interior temperature in AUTO mode is reached and the fan speed is dropped down either one or two speeds. I don't think it has anything to do with any other system in the vehicle. I seem to recall one or two times when it was set to outside vent and reset itself to recirculate, so it probably works in both directions according to some strange programming. You would think that once the selected interior temperature was reached that the fan speed would decrease to maintain that temperature and external venting would be optional depending on the preset condition. Select recirculate and it will remain recirculate. Something else to wonder about during my long commute.

 

Here in the desert heat, the converse of that process works best for me. A/C on and draw outside air with one window down a crack. The cooled air drawn into the cabin helps to force out the oven-like air inside. At some point then switch back to re-circulate and close the window. A person could also roll down all windows, let the heat out, then roll back up with A/C on re-circulate.

Maybe the automatic re-circulate switching and HV battery state of health are completely unrelated. I hope so. I monitor my SOC and I don't see any other indications of problems, large swings in SOC, etc. I can discharge it down to two purple bars by sitting in a parking lot in Ready with A/C on and get a full green bar screen display when arriving home from a lengthy descent in altitude. Most times at highway speed the meter stays right in the middle.

 

I've been thinking about having some more diagnostic information available, since the instrument panel is practically devoid of this useful data. Not even something as simple as a coolant temperature gauge. Really ... Looking around at the various devices, I am wondering about this new Actron U-Scan. Seems simple enough. The website isn't all that informative on just what types of data are displayed or if it will even be useful in something as sophisticated electronically as the Prius. I think I'll wait until someone else gives it a try.