How about a combination of solar and an ultra thin lithium ion battery? Finding a small, thin solar cell cheap enough is the trick I think... http://www.phdene

How about a combination of solar and an ultra thin lithium ion battery? Finding a small, thin solar cell cheap enough is the trick I think... http://www.phdenergy.com/products/rechargeable-batteries/ultrathin-battery/ http://www.phdenergy.com/products/rechargeable-batteries/ult... https://www.digikey.com/product-detail/en/powerfilm-inc/MP3-25/1996-1023-ND/9559473 https://www.digikey.com/product-detail/en/powerfilm-inc/MP3-... I haven't done any calculations to see if these would provide enough power or not...