I think the initial problem was clearly-enough stated, and the GP's point is valid - why bother accounting for the case where your phone is so close to dying that you can't unlock the screen, when just a bit less battery life would mean that your phone dies regardless (and you're stuck outside anyways)?
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.
