Thanks, I will try that.
I found more details on http://talk.maemo.org/showthread.php?t=38230 as well as reports on different cache batteries.

What happens when you charge your Long March battery pack with the dynamo while it's connected to the N900? If you cycle fast enough, do both devices charge, or the long march charges X minutes then delivers current to the N900 x minutes, or ... ? I don't want to loose any "cycling energy", so the current must go somewhere, either in the N900 either in a cache battery, ideally without changing cable connections.