This concept from over at Yanko Design would charge batteries using a traditional jump rope action. Also, your self-respect. Start practicing!

The charging mechanism would be placed in the handles, with the charging itself taken care of by the spinning rope. Just a few hundred hops, or 20 minutes worth, will charge the batteries 100%. Whether you'll have the energy required to do whatever it is you were trying to use after your marathon charging session is a question best left unasked of this concept. [Yanko Design via Technabob]