How do you keep a phone battery alive when out in the cold for a whole day?

When ski touring I'll usually keep my iPhone in my right ski pants pocket (close to your skin). On really cold days I'll throw in a hand warmer, to keep the phone warm. I'll also try not to take the phone out of my pocket too much.

My older iPhone 5 use to shut off much more in the cold, but I've never had a problem with my newer iPhone 7 shutting off due to the cold. So it my have something to do with your phone and/or crappy battery if your phone shuts off a lot in cold weather.

Always keep your phone next to your body, either in a chest pocket or pants pocket. Put your phone in a plastic bag along with a footwarmer to keep it warm. I always bring a phone external battery as a backup, so if my phone battery does completely die, I have an extra battery to power the phone.

If don't want to put your phone in your pocket, then put inside a sock along with a chemical hand warmer and stick it in the middle of your backpack to stay warm. You can also use a waterproof pouch to place the phone in and hang it around your neck, in between layers to keep warm.