Making that phone last

maximum power saving can make a lot of changes to the interface on some phones and turn a lot of functionality off you may need, so you need to test it out and see what it does to the phone in general. you should be able to go through the apps and choose which ones get put to sleep, uninstall apps you don't need there's also settings functions to stop apps you really dont want, some apps may not uninstall because they came packaged with the phone OS. power saving apps can do the opposite because they can constantly trawl your phone for ways to save power, using up power in the process. the fanciest phones can be the worst for power usage because of all the functionality in them. safest option is have a battery pack to recharge the phone with. i find power saving modes can stop gps tracking apps from tracking, some tracking apps may recommend no power saving. bright screens are a big battery drain.

You got a lot already I would say. Definitely enable power save mode. I'm assuming you only need your GPS to know where you are, and are not doing continuous tracking in which case you need to experiment if power save mode works. Here some more: Reboot the phone once you are about to turn it into flight mode. That will stop any power sucking app, and you go to a clean slate. Turn off sync or turn on data saver. That means that as soon as you go off-flight mode your phone doesn't immediately download the latest Facebook update. Turn down screen brightness. Turn off phone at night. I would say with your phone you should be able to get more than 3 days, unless you take it off airplane mode a lot, or do a lot of videoing.

This wont work for you but I can get a full week out of my Huawi 6 year old phone. gps off 4g data off and not a lot of calls. Gps hogs the battery but not as much as turning on the 4g and hotspotting it

put it in airplane mode as soon as you can before the trip if you arent able to charge it in a vehicle while travelling. using mobile connectivity will drain more power faster, the further away it is from cell phone towers. if you're in the city normally you'll be used to the phone draining at a certain rate, which will be low with cell towers close by everywhre, the phone will use the minimim power necessary to maintain a connection. but get out into the countryside even before you go tramping and the phone will drain faster than normal with the cell towers being further away.

Keep the phone warm - nothing drains the battery faster than the cold. Some sleeping bags have internal pockets for electronics.

Does coldness drain the battery or does it just make it less effective whilst cold? I've not paid close attention but in the past I'd though my cold devices have sometimes started working again when warmed up. Also, +1 for power saving modes on phones. About a year ago I messed up my camera batteries around Taranaki, so reverted to using my phone for photos... not ideal because it still doesn't take as-good photos for me, but better than no camera. On a typical day of sitting around doing nothing it'll often be down to 30% or 40% by the end of a day. I've never used it this way before, but I put it in flight mode and maximum power saving mode, and didn't significantly use it for anything else. Despite having it out frequently for photos it was probably still at about 70% after 3 days.

other way around batteries in hot environment lose charge faster cold temps just temporarily reduce the speed the battery can discharge making it look like its lost its charge https://www.thoughtco.com/why-batteries-discharge-quickly-cold-weather-607889