In our experience, 8 miles is about maximum for the battery with slight hills and good wind (20+mph). Your comments about leaving the battery on the charger after it's green is interesting, but we probably will continue to leave it on until the green light remains steady which is normally about 3 1/2 to 4 hours of recharging. We still have good voltage on the batteries, but your experience might help someone search for the absolute maximum distance from a battery pack. For a 15 mile commute, I, personally, would have TWO battery packs since I don't want to discharge the batteries over 75% which will shorten life considerably, and I try to be frugal with my money so I can help others and still carry on my work in other areas, e.g. on-board solar designs. From my experience with Currie's e-Zip, I'm thinking of buying one for myself this time. Although the e-Zip has its problems, it's a solid value and you're using it wisely, i.e. making money without spending much, getting exercise, and mainly enjoying the refreshing commute.