Wishful Thinking Dept :)
A 4 HP Honda Engine
A 70 ampere rated newer alternator such as an ND (Denso) from a Toyota
Belt driven with an idler pulley to maximize belt wrap
In a booondock situation a Honda would give commendable fuel to kWh efficiency. The four phase Denso alternators are small and relatively lightweight. The spring loaded idler pulley keeps optimum belt tension.
The alternator would be sucking air almost 175 degrees cooler than underhood air so cooling for continuous max charging would not be an issue.
Northertool.com is a good place to find a Honda consumer engine.
It's nice to daydream...