I saw a review of something called Banna Pro. It is a raspberry pi formfactor, but has 2 gb of ram instead of 1(Raspberry Pi) A better processor, and other faster features. Combined with a cheap lithium-ion protection circuit, a boost converter, and a few lithium-ion batteries, it could run for a very long time without power. Plus you charge it from USB.
Protection Circuit:https://www.amazon.com/gp/product/B071RG4YWM/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
Boost Converter:https://www.amazon.com/eBoot-Converter-Voltage-Adjustable-Step-up/dp/B06XWSV89D/ref=sr_1_1_sspa?s=electronics&ie=UTF8&qid=1528678633&sr=1-1-spons&keywords=boost+converter&psc=1&smid=APHB2NWNKDSBL