The stock JG 416 battery is a Sub-C pack. An 8.4V sub-C pack fits perfectly. I'm sure a 9.6V would as well.
Currently, I have my 416 stock on my C8 and I run a 9.6V 2200 mAh crane stock battery made from 4/5A cells. The battery is loose in the cavities, but works great. A couple of pieces of foam prevent the batteries from rattling around.