All variations of the Nikon EN-EL15 battery will work inside the Nikon Z6 and Z7. That is the EN-EL15, EN-EL15a and EN-EL15b. The B version comes standard with the Z6 and Z7. However, if you would like to charge the battery inside the Z6 and Z7 via the USB connection, only the EN-EL15b will work that way.