The MB-D18 is great but quite expensive. It comes with 2 battery packs - one for an EL15a and the other takes 8 AA batteries (you would really not use this except in emergency) - these give you longer battery life for the camera but DO NOT ENABLE 9fps on a D850. To get 9fps you need an EL18 battery, a BL-5 battery holder for the EL-18 and a charger for the EL18 (different to EL15). All this comes to quite a lot if you get Nikon - I got these new from 3rd party suppliers and it all works fine. 9fps at last!