Just received my Nikon MB-D18 battery grip today and I have been looking forward to finally testing for myself which batteries provide the uplift from 7 fps to 9 fps. I have read countless reviews, blogs and followed the forums and there are many versions out there on this topic. Nikon lists that the Nikon EN-EL18a or Nikon EN-EL18b are required to get 9 fps. Another forum I read states only the EN-EL18b will work. None of the blogs, forums or Nikon’s sites mention that the original Nikon EN-EL18 that shipped with the Nikon D4 would work. The EN-EL18a is a 10.8v 2500 mAh battery in contrast to the original EN-EL18 which is also 10.8v but only packs 2000 mAh. Thus it made sense to me that the original EN-EL18 might not have enough juice to power the Nikon D850 to the 9 fps.
I tested it this evening for myself and I can confirm that both the Nikon EN-EL18 and the Nikon EN-EL18a provide the 9 fps so if you have a few of the original 18’s there does not appear to be a need to purchase new 18a or 18b batteries to boost the frame rate.