Any modern filmmaker who uses the latest video follow-focus systems or cameras (for example, the Sony A7Rii) will need a portable USB battery and a way to charger it on the go.
This new USB portable solar charger from Choetech is the best out there that I’ve found. It retails right now for just $54.99.
You can get it here
Choetech solar charger on 
Also, here is my favorite portable USB battery. It’s shock and water proof and holds 7500mAh. Plus, it costs only $17.99.
You can get it here
Coocheer 7500mAh Portable Battery on 