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Scientists Develop New Dirt-Powered Fuel Cell That Runs Forever
(scitechdaily.com)
This is a most excellent place for technology news and articles.
I'd say a battery is at least something that should be "chargeable", either one time or rechargeable. I dont think you can use solar cells to store energy back into the sun.
Not saying that my definition does work for the dirt fuel cell, talked about in the article, though.