[slash-dot]

Since the lock will probably be in motion when not not in use, couldn't you just use some kind of kinetic charging mechanism to charge the battery? Is kinetic charging not enough powerful? Or is the circuitry too complex?

[bentcorner]

Maybe store some mechanical energy in a spring to help reduce the amount of "stuff" that needs to be moved just by the battery?

[mehrdad]

that will complicate everything! and also increase the cost. besides you can't harvest much energy from that kind of mechanical movement...

[chid]

cheaper and simpler to have a replaceable battery.