The first thing to know is that there are two possible battery configurations for these cameras:  1) Rolleiflex or Sinar style batteries, and 2) the Leaf Style batteries.   The cameras come standard with the Rolleiflex type contacts in the battery compartment in the grip arm, however they can be ordered with the Leaf style battery contacts in the grip arm on request. 

Click on the link to read the whole story and find out which aftermarket batteries are compatible - and why you might want to stick to the OEM.