The cylindrical Shimano Di2 battery designs (BT-DN11, BT-DN300) were developed to be mounted internally inside a seatpost. Seatposts can have various internal diameters and shapes, so Ritchey offers these two-piece injection molded rubber internal seatpost mounts for their posts, and these actually do work with many other brands as well. The system is easy to use, and the two sizes offered here fit most round seatposts commonly sold at this time.