Small parts have a way of disappearing, and the magnetic charging port cover on your Dura-Ace rear derailleur is exactly the kind of piece that can go missing during a cleaning session or get knocked off on a ride. The Shimano RD-R9250 Di2 Rear Derailleur Charger Cover is the genuine replacement for that plastic cap, identical to what came installed on your derailleur from the factory.

The R9200-series Di2 system moved the charging interface from a separate junction box to the rear derailleur body itself. You charge by connecting the EW-EC300 cable magnetically to the port on the derailleur, which means that port is exposed to road spray, dust, and whatever else gets kicked up while riding. The cover keeps debris from accumulating on the magnetic contact surface and provides a layer of protection against the elements, even though the port itself is designed to handle exposure.

Losing this cover won't stop your Di2 system from functioning, and it won't prevent you from charging the battery. The magnetic interface works fine uncovered, and water intrusion isn't a significant concern with the port design. But letting grime build up on the contact surface can eventually affect charging reliability, and the cover keeps things clean with minimal effort. It's a cheap insurance policy against having to clean out a packed-with-dirt charging port on the side of the road when your battery is low.... Read More