https://arstechnica.com/cars/2017/09/this-cement-quarry-dump-truck-will-be-the-worlds-biggest-electric-vehicle/

 

Generating a net of 10KWh per trip, not bad...

 

-G-

Interesting. Unfortunately it is relatively rare that mining trucks move downhill when laden, mostly they are going uphill. The larger trucks are usually diesel-electric, and may have a direct electrical pickup for the main haul roads out of a pit.