The Nikon Z9 can use any EN-EL18 battery (Nikon has made five variations, the original EN-EL18, plus EN-EL18A, EN-EL18B, EN-EL18C, and now EN-EL18D versions0. Only the EN-EL18B, EN-EL18C, and EN-EL18D batteries can be charged by the MH-33 charger that comes with the Z9 camera. Older battery versions will need to use Nikon's original MH-26 battery charger, or one of the many third party EN-EL18 chargers that have appeared. I've received some reports that older EN-EL18 (no letter) batteries may not properly power some Z9 bodies, probably because of a combination of lower overall power coupled with battery aging.
Third party batteries and battery chargers exist for the EN-EL18 line and can be found on Amazon. Be careful about these. So far, I've found that a few batteries seem to work in the camera, but don't necessarily work on the charger. Likewise, the third-party chargers may or may not fully charge the EN-EL18D battery supplied with the camera. The original Kastar EN-EL18 (3200mAh) battery does not work on the MH-33 charger, nor does it work in the camera. Ditto the Wasabi BTR-ENEL18-JWP. Wasabi has since come up with the BTR-ENEL18D, which does work in the camera and charger.
The other third-party battery that I've been able to verify powers the camera, recharges via USB-C in the camera, and charges in the MH-33 charger supplied with the Z9 is the 3800mAh PowerCore2000 ACD-800 (available from B&H).
Others report that the ProMaster EN-EL18D 3300mAh battery works, as do the Wasabi BTR-EN-EL18D 3350mAh battery (note the D) as well as the newer Kastar BA-2B-ENEL18D (again note the D).
As I learn more, I update this page.
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