My guess is that the generator can't handle the stresses (both physical and electrical) of someone cranking on the battery with all their strength. The spring

My guess is that the generator can't handle the stresses (both physical and electrical) of someone cranking on the battery with all their strength. The spring would allow some slippage once it's completely wound, than then it would be able to feed a steady low-voltage charge into the battery. That said, it doesn't seem likely that this would provide a significant charge. It would be nice, but when things like this seem too good to be true, then they're most likely a pipe dream.