using magnets
I wonder if they could use magnets instead of a battery. Put strong enough magnets a set distance apart and they exert considerable force to come together. This force could drive a flywheel as they get closer together. To recharge it spin the flywheel in the opposite direction forcing the magnets apart. It would probably never wear out unless the magnets got too hot. It might even be possible to use this in a car instead of a battery. Some of the latest magnets are very powerful for their weight.