I bought Enercell 2400. Got it at Radio Shack and I can run about 8 hours on Strava with it. That way I don't have to close down any of my other locator programs when on Strava. If I had to do it again I might get a bigger one for multi day rides. But they come in about three sizes I believe. Here are some good ones as well. Though the Stichwell got some bad reviews. I have used my enercell for about two months and it works well.

http://www.iphoneness.com/iphone-acc...ery-extenders/