Made by PRO for Shimano, the Di2 Seatpost Battery Holder is a simple two-piece set-up that works with Shimano's BT-DN11 and BT-DN300 batteries and most size associated round seatposts. These cylindrical batteries for the Di2 electronic shifting system are designed to mount internally within a seatpost for ease and convenience. The resin pieces feature a flexible fin-like design that allows them to fit round seatposts with marginal variable internal diameter. Depending on shape and size, these make work with various aero seatposts as well, but those cannot be specified here.