warming the engine may warm the battery by charging, but then you've already started the engine, negating the premise of the o/p.
li-on batteries don't like it too cold. chemistries may differ, but toyota's limit discharge rate depending upon several factors. if the stars align properly, the engine comes on so the battery cannot be fully used.
Owner manual (at least mine) states that the engine will start if the traction battery temp is 32F of below. Since the o/p is charging outside and does not use delayed charging by timer, it is possible that the battery is cooling down to below 32 deg. during the night colder hours after charging (morning was 32 deg. he said). Toyota states: "use the timer whenever possible" and there is a good reason for doing this.
Lithium chemistry sees a significant increase in electrical resistance when its temperature is below freezing. Waiting until it has warmed up some before using it exclusively is a better use of the electricity within.
^^ This is why modern designed PHEV and BEV's have a Thermal Management System dedicated to the Li-ion battery pack. The TMS heats or cools the pack as required when plugged in or when the car is powered up and 'READY'. If you want a PHEV that you can just drive and not have to 'consider' when and where to use it as such, you'll need a better design than the PIP.
actually, you can drive the pip without considering anything. i have been doing so for over 3 1/2 years.
It's 23°F outside. I just went out into the garage, unplugged the Prius, and drove to the coffee-shop. Top speed was 55 mph. Distance was 4.4 miles. Efficiency was 999 MPG. What part of that needed consideration? Spin of the past attempting to downplay the abilities of PIP fall on deaf ears now. We have proof those claims made were misleading. We're tired of that blatant greenwashing. Plugging in significantly boosts overall efficiency. It's that simple. Our driving results clearly confirm that.
First I'd like to say that I don't know anything about PIPs, but this may be some info I am may be able to contribute: When I went to test drive a 2012 PIP a few days ago, the salesman drove the car from the lot to the front of the dealership and parked it. Distance was probably a hundred yards and he drove slow, maybe 10 mph. While I was inspecting the car's paint from the outside while it was parked, the engine turned on by itself for approx. 1 minute and then shut off. The temp. Was approx 55 to 60 degrees. Here is the pic of the remaining battery charge (dashboard) at the time and another pic of a little history of the car sitting so long with recent limited driving. I don't know how to read it (since I don't have a PiP). So I don't know if the battery charge was low and that is why the engine turned on or that the car had very limited driving for approx 3 months was a contributing factor. I hope some of this data is helpful. iPhone ?
actually, it's not. the battery charge indicator looks like a flashlight battery, and the remaining miles estimate is below it. i think it's over to the left in your first pic.
That last photo shows bars in the battery, rather than a solid mass. That indicates EV capacity was not available, that electricity had been used up.
It only shows that because when I signed up for this forum from a phone, there were only Prius cars to can choose from, so I chose a PIP No I didn't buy that car, however, the car I test drove a few days ago, a Blizzard Pearl 2012 Prius Pip w/ 37k miles will be mine, hopefully soon. iPhone ?