The battery level isn't actually close to dying, in my case it just rapidly depletes from up to 40% when it's very cold. But that 40% remains intact seemingly with the screen off. Could be my phone also, but it has happened often enough that I wanted to solve it and thought this was an interesting approach.
For me, this was just a matter of convenience and not optimizing for most resilient or reliable solution. I do have backup plans for home entry.
Batteries are generally less able to source current when very cold. At low temperatures, the battery was sufficient to power your phone in a low-draw state (minimal network activity, no screen, low power CPU state), but the voltage dropped when current draw increased.
I experienced this. I ran to work at -10F. When I arrived, I took out my phone to take a photo of my ice beard and although it almost immediately died.
For me, this was just a matter of convenience and not optimizing for most resilient or reliable solution. I do have backup plans for home entry.