I use a mophie helium juice pack and slide it into a Timbuk2 tube bag. It is a tight fit but I can see the phone through the clear plastic top. I use it in connection with the new Garmin 810 which does the navigation but feeds a live track to the phone if you want people to follow where you are. Also, the 810 auto uploads your ride to the iphone. With that use. I can get more than an eight hour ride. If you were using the iphone for the navigation, not sure what the combined iphone plus helium juice pack battery life would be.