Some batteries don't like to be charged and used at the same time. It could be done I guess, but for anything but a lead-acid battery, you'd probably need some kind of charge controller to regulate what goes in and what goes out. For the extra weight of the dynamo, the resistance it introduces, and the weight of the extra controller, you may as well add a bigger battery. It's a cool idea through, if it were implemented from the design phase-up.