What cells do you have that allow charging to 4VDC? I wouldn't advise fast charging anything past the nominal voltage marker if you try. What do you mean by "Large DC generator"? Sounds like you want to use a mobile generator that has DC outputs on it. Generators will have frequency fluctuations when under load.....especially if that load is over 80% of it's load max. Supplying a fluctuating DC voltage to anything is never a good idea. Most, not all, but most devices have basic circuitry to buffer incoming power supply fluctuations, but not to this magnitude.Instead of charging out of a chademo or factory dc fast charger, it would be directly out of a large DC generator for the 80% to 4.0vdc.
Basically, can the diy dc fast charge just be linear power out of power supply and then the topping up saturation charge on the exponential curve be done by a Brusa nlg513 or similar charger?
Packs would be manually checked for balance between events after six charge cycles. Looks like 400 cells per pack would be incredibly difficult and expensive to monitor via bms.