Have you tried jump starting the Insight in cold weather, or tried replacing the existing 12V with a fully charged up new one?
I can't tell you at exactly what temperature the 12V starter is supposed to be activated. However, we had a couple days recently, one was about 1ºF and the other about 5ºF in the morning when I started the car.
Both mornings, the car started quickly using the 12V battery even though the hybrid pack was only one bar down from max charge.
I had just replaced my 12V battery with a new one, of the spec'd Group 51 size. For whatever it's worth, mine is an AGM type and the rated CCA is a bit lower than what's shown on the oem Honda battery that's been kicking around on my workbench for a couple years. I replaced it because I had been using an undersized AGM in the car - which clearly was used up by now. I bought the small, lightweight AGM used, then used it myself for several years. It never was able to start the car on a cold day but now it also couldn't even keep the car's computer running below about 25ºF; in the morning I'd find the dash display had shifted from Trip A over to the Lifetime Mileage display.
Last edited by brucepick; 01-24-2019 at 11:55 AM.