I work in very rural areas on a road crew. We are constantly going down into deep ravines and staying there for hours.
Cell sites disappear or are very weak.
I use MacroDroid to turn off Data, Wifi when the phone's screen is locked.
When the screen is unlocked, I have it automatically turn on Data and Wifi.
* MacroDroid - Device Automation - Android Apps on Google Play
Taking over the phone's access to the internet stopped my phone from wasting battery power needlessly.
The phone will now last me 3 to 5 days or more easily. It is currently at 64% after 3-1/2 days off charge.
I do a lot of newspaper reading and all of their videos, emails, and a lot of texting.
I also have the screen set at 10 minutes to make reading easier.
Just tap the Power Button and the screen is locked instantly (settings for that in Lockscreen)