I mean... regenerative braking is a thing.

My hybrid Prius C had this, and yes, you actually can build up some useful amount of charge from just rolling down a decently large or long hill, and you can also run the car on pure EV mode, though you're probably not gonna top 20 mph on a flat road.

Obviously this does not create an over unity situation or perpetual motion machine, but, if this guy can figure out a way to put a regenerative braking type device onto an E-Bike, or maybe motorcycle/moped, in a way that isn't stupid expensive...

That could increase overall range, and I think it would be neat.

Though I... don't really know why you wouldn't just use one battery for the whole system, just have a modulation/regulation system for it.