I have a Moto G and I just turn it off when not using it. I charge it from the bike while setting up camp or doing maintenance. I also have a £9.99 dumb phone in my bag as a standby; it gets a better signal and the battery lasts for ages.

Generally, even a cheap digital camera will take better pickies than a phone an a decent head-torch is more useful than a flashlight app. Get one that has the option of a red light if poss.