I see mention of apps for phones, use this battery over that one, etc. Realistically this is ok, as if a battery dies you go home and get a new one.

What if - you are out in the tickie weeds and your battery powered whosis fails. I mean way out in the weeds. And your batteries are all dead and dying. What now?

Yes, I carry some spare batteries for devices. But mainly I refuse to rely on battery powered stuff. My emergency radio had a crank. My small emergency flashlight can recharge by solar. My watch is solar powered for that matter (I hate having a watch die in the field.) If it's dark, yes I have a flashlight, but I can also usually do most things without one (years of training as a grunt, sans NVGs).

So what's your plan if the batteries go out?

not so much for out in the bush but for hurricane preps.... have been procuring adapters for a Goal Zero 7w solar panel... just received a Nimh charger controller with USB, 12V, and 120v connectors for AA... 8 pack of AA Sanyo Eneloop cells, probably later C and D adapters...12v male plug to battery clips for a auto battery..

my older Grundig radio is crank to Nimh cells, and AA powered

digital portable TV is 12v adaptable

12v ventilation fan with female adapter off auto battery

Dietz Air Pilot and Little Wizard kerosine lamps

toying with a BYU funnel solar oven next week (making ice cream using black body radiation would be pretty cool...pun intended)

my 4x32 ACOG has an exit pupil diameter of 8mm, and really sucks in light

1) Carry spare batteries for, and with, the critical stuff. You're not having fun until you're 100 yards into the trees and the batteries in your flashlight decide to quit.
2) Don't rely exclusively on them. I've seen too many of the latest and greatest go belly up at the worst possible moment. Map and compass is much more reliable then a smart phone GPS.

My phone and my flashlight are my two important battery-powered devices. I carry spare batteries, at least one spare flashlight, and two ways to charge my phone. I also use a pulse-load battery tester and replace batteries as appropriate before going into the field.

It depend on what device we're talking about, and what situation. Nowadays I try to have as much as possible run off standard AAs, so I can take batteries out of non-essential gear and use them in essential gear. One AA will last 50 hours in my torch, so it'd have to be an extreme situation to be out of light. My phone uses a specialist battery, so for that I keep track of its charge state and don't let it run flat. I have equipment to recharge from sun or a car battery.

My watch is solar powered too. If I know I'll be navigating I'll have a compass and map.

I'm a simple girl JBMat. The only things I really EDC which use batteries are my flashlights, headlamp, watch and radio. Once my extra batteries are pooched, I also carry a wind-up watch, a crank flashlight, candles, snaplights, and the radio has a crank too.

As others have said, it depends on the situation.

Out in the bush, genuine wilderness, it often doesn't take much light to do simple tasks. The moonglow mode in most flashlights will last a very long time if used sparingly. High-end lights can run for many days straight in moonglow mode. Travel in rough terrain would be a daylight affair, due to the risk of injury and encountering night-travelling wildlife.

A secondary light source is my Bic lighter (usually several are at hand). I don't waste butane; rather, I simply flick the flint wheel twice and rely on persistence of vision to let me find something or move from one location to another without breaking my neck or poking an eye out. This is highly effective when it's very dark, indoors or out. These lighters were designed for lighting cigarettes, so they have a lot more flint than I need for lighting fires. I've even used Bics that were discarded on the roadside (out of fuel) just to prove the point. (Not for checking gas tanks though.)

In other situations, with more of our modern technology at hand, there's always a way to kludge something together. I like to take stuff apart, find how it works, repurpose it, find out its boundaries and limits. There's always a way.