I got this idea soon after some friends visited and forgot their charger cords for their phones. Since I had my own to use overnight, we had to fish around for other solutions. One day, in my shop I was reflecting on this dilemma when I remembered there were these new USB wall sockets that would be perfect to turn into a portable DIY charging station whenever I need it.
This is real MacGyver territory. I've been stuck in a handful of urban survival situations and the inevitably dying phone battery is by far one of the more pressing issues. Most important will of course be immediate safety, shelter, water and food, etc., but it's likely that the use of a phone will help you secure those things. And when the power is knocked out it can be a real challenge.