I am curious about a resonant LC circuit. Assuming a perfect inductor and capacitor, would it be possible to tap off the energy in the oscillations to use it as a battery? Literally, it would be an AC battery. With a bridge rectifier and filter cap would this act as a DC battery?
(This is just theoretical stuff – I doubt you could get useful energy from an LC circuit as it would dissipate too quickly due to losses in the inductor and capacitor, plus to store any reasonable amount of energy would probably require a massive inductor and capacitor.)