Garmin 800 or better can be charged while riding. Not sure about the 520 or newer. Also, the Wahoo GPSes can be charged while riding.
There was a route I rode for the first time and forgot to print a cue sheet. So I d/l the cue sheet onto my phone and left it open. I think I might have gotten a little off route on that course once, but I knew where I was going so it never became obvious that I was off course.
I only got lost because of my gps once, it was in a town with a lot of turns. I also didn't realize the route was going to turn back on itself. I feel pretty uncomfortable without a cue sheet. My GPS has a number of deficits as far as guiding me. It's very slow for one thing.