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.