There are a multitude of "battery extenders" in use by leveraging the charging capabilities using the USB port. These extenders (such as the Power2Go) can give up to 24 hours of run time on the Edge 305. There are even plans to make your own but given the approximate cost of a purchased battery extender at $15, there's little motivation to build your own.
If you need ideas and are so inclined, feel free to contact me and I will find you a copy of the build it yourself plans.
I did a century (2 actually) last year with my Edge...more than 8 hours of operation on the unit and still a bar on the battery gauge (not sure what was left, but it was still showing a bar).
There are a bunch of small battery extenders out there. I invested in a AAA version (planning a 200 km / 125 mile trip this summer) as my mp3 player takes the same and it is easy enough to have a couple extra batteries around for it. I'll let you know how it does.