You can bet there's plenty of reserve power in the big battery, and you won't get stranded. Without it, the car could not operate in hybrid mode, as the small motor controls the sun gear in the planetary gear system, using the big battery. The small motor/sun gear is what starts the engine, using the big battery. The small motor/sun gear is what gives you an infinitely variable ratio transmission, using the big battery. Without plenty of power left in the battery, even when it says less than 1%, neither of these features would function and the car would not be driveable as a hybrid on long trips.
Big battery conditioning, and the message, only happens when the car is plugged in. If you leave it plugged in over night, and it's not charging, it will periodically wake up to condition the battery, depending on the ambient temperature. You can start the car if you get the message. The system will heat/cool the big battery as necessary. What you must remember is that the small motor/sun gear combination acts as a generator when the engine is running and the car is stopped. Same thing as being plugged in.
It is very cold here in Northern Michigan (-8 at night, -4 all day today. Actual temps, not windchill) I am very pleased with how my PacHy is performing, now that we've had it a month and the system has learned our local driving style. Most of our trips are 5 miles or less. I crank it up on the remote start (after opening the garage door) and the engine always starts under these conditions, as I leave the climate control on auto with all 3 zones set to 70 degrees. The 5 minute remote start period is enough to get everything warmed up and comfortable. (Love that heated steering wheel.) I have a digital tach that plugs into the OBD II port, and engine speed at startup is 1400 rpm. After it warms up a little, idle speed is 1100 rpm.
The engine will usually run most of the way to our destination, then turn off. The warmer it is outside, the shorter the run time. Engine rpm will get up to about 2400 if I hit the big grade at 75mph on the freeway south of where we live. In warmer temps, the engine doesn't come on until the battery is supplying precisely 60kW. It's fun to watch this happen as the battery power drops to 5kW or less, and engine zips up to 60.
If we've been in at our destination for around 45 minutes or more, the engine will start up again as we depart, but usually only runs a short time, then it's EV all the way home, 75mph freeway and all. I've been monitoring the engine coolant and oil temps, and the engine going off line corresponds with oil temps of 150-160 degrees, and coolant temps somewhat higher. The radiator shutters and the electric heating elements in the various cooling circuits do a marvelous job of keeping the engine temps where they belong. (I retired from the company that first applied radiator shutters to vehicles in the 1930s, so I'm kind of proud about that. Where they were first applied is tonite's quiz question.)
Many owners have gotten their knickers in a twist over the "fuel/oil refresh mode", where the vehicle enters hybrid only mode. They feel they have to run right out and put gas in the tank to reset it. Nonsense. I've had fuel/oil refresh come on four times in the last month, and it only lasted long enough to get the oil temp up to around 180 degrees, then it went away. This is an engine service feature. It does this so the moisture can be cooked out of the engine oil, thus extending its life. Without this, on a car where the engine does not operate 100% of the time, oil life would be very short. This kind of service used to destroy conventional engines in a very short time if the oil was not changed every 90 days. Ask any Ford "Y" block owner.
I have noticed only two software bugs, and I'm not really concerned. 1.) Every now and then, if I turn the vehicle off, engine running or not, the tachometer, the instrument panel and the center stack displays stay on, then cycle on and off while the car is parked. This could quickly kill the small 12V battery, so cannot be tolerated. Cure is simple: without touching the brake, turn it on then turn it off. (The Direct TV cure for anything.) This might be related to my tach, but I doubt it. 2.) I originally scheduled charges to take place from 11pm until full. That worked for awhile then it began turning off the charge in the middle of the night. I just reset it for 11pm to 10am, and it does fine now.
Really great info here! Thanks for taking the time to share all the great insight!