It’s the cold IPhone will die when the temperature is below about forty. I have a Mophie charging case for mine that will trickle charge and keep it at 100% all day. But, if cold temperatures crash it too fast the Mophie won’t bring it back.
I’m out in Kansas this week and today the wind chill hovered around 0. Even in my front pants pocket my phone went from 100 to dead in about 10 mins