Older Di2 setups running the SM-BTR1 external battery can gain wireless E-Tube connectivity and Synchronized Shift functionality without replacing the whole system, and this is the piece that makes that upgrade happen. The Shimano BM-DN100 Short Battery Mount isn't just a bracket—it's the master unit that adds Bluetooth communication to a Di2 drivetrain that predates the wireless era. The short version is the one to grab when the downtube real estate is tight or a longer mount would crowd the bottle cage or bottom bracket junction.

The IBMDN100S1 variant ships with M4 x 10mm bolts, which is the standard length for mounting directly to a frame's water bottle cage bosses. If your build needs longer hardware to stack a cage over the battery, that's the S2 variant with M4 x 16mm bolts—worth double-checking before you order if you're planning to run both mount and cage on the same set of bosses. The mount itself supports both internal and external wire routing, so it works whether the Di2 wires stay hidden inside the frame or run externally along the downtube.

Functionally, the electronics inside this mount are what enable pairing with the E-Tube Project app through a compatible wireless unit like the EW-WU101 or EW-WU111. Once connected, you can adjust shift points, customize button assignments, and enable Synchro Shift or Semi-Synchro Shift from a phone or head unit rather than plugging into a computer. For a rider still running an SM-BTR1 battery who wants the newer generation's tuning flexibility, this is the bridge component.