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no comments for a couple years on this idea--still, maybe these will be useful.
1) for safety, longevity and size have you considered modifying a pacemaker battery? pacemakers can also be programmed externally.
2) the ratcheted mechanism used in self-winding watches would bypass the chemical component entirely while taking advantage (arguably) of every movement made by your body before and during sex.
though storage devices for accrued energy of a [reasonably] portable size i cannot comment on.