Have an iPhone mounted using a Ram Mount
http://www.ram-mount.com/. They make several combinations depending on the type of case you use. MotionX works excellent. Have use it for a year and never go done the raid without it. You do have to turn the screen off as it runs in the background. Other wise it would indeed drain the battery. The nice thing about MotionX is that it does all types of sports when you are not on the bike. Recent updates for the app make is a bargain at $2.99. Of course first you need the iPhone