We've all been in this situation before: You haven't plugged in your iPhone in a couple of days and now you have just a sliver of battery life left. Your phone has been warning you that it's near death, but you have nowhere to charge it. At this point, you don't care if your phone is the fastest or most powerful; you're waiting for an important call and you just want it to live as long as possible before it faints to a black screen.