How about solar charging
Here's someone doing a solar generator/charger for his vruzend battery https://www.youtube.com/watch?v=ucK1HF9g7zc&t=10s
Be sure to buy Micah’s new book about DYI solar projects....
DIY Solar Power: How To Power Everything From The Sun https://www.amazon.com/dp/098990671X/ref=cm_sw_r_cp_api_6UqIAbEWRJQJ6
I have not finished reading it but it is a great resource. As far as using solar to charge home made battery packs, check out the picture below. I am new to the DYI battery pack and solar charger world but it was not too difficult for me.