First, it is important to know that this assembly is not juts a mount, but also an integrating connection point for a BTR1 External Battery. The external style batteries with integrated mounts are rarely used in road bike Di2 set-ups at this point, but are more commonly an option for externally wired Shimano STEPS commute and utility oriented builds. The mounting section itself offsets the battery position from water bottle bosses, and is generally run on the downtube under a bottle cage- though the frame and routing may offer cleaner wiring from a different mounting position, but you must leave a gap of 108mm or more at the end of the battery mount.

An e-Tube compatible battery can be monitored and get firmware updates from the e-Tube Project site. The Di2 BM-DN100 mount is considered a Master unit for allowing a BTR1 to connect with and power a wired system, as well as get the e-Tube access for the external battery. The end of the mount features one e-Tube Port.