> Yesterday I was skiing at a resort and my phone was dying at an incredible rate, like 25% per hour. I don't know for certain but I suspect some app or website was retrying a download of something while in a dodgy service area.

Whenever you have poor service (but not none) that's when phones waste the most energy trying to crank up RF transmit power and doing retry loops. I doubt it was actually trying to download much.

You can try this by putting your phone in a homemade Faraday cage with tin foil in a Tupperware or something.