I use an app that allows downloadable map sections and GPS routing while the phone is in airplane mode. Have navigated for two days straight on tour without needing to charge the phone. There are several out there. Build routes in Strava or Mapmyride, then email it to self, open on phone with Maps.me, download appropriate maps if I don't already have them and I'm good to go (assuming no cell phone failure of course) I usually carry a paper map of the area just in case.