I’m just about finished with my 105 Di2 build and thought I’d share a little workaround I came up with for the battery install in case it helps anyone else.

The stock clip that’s supposed to hold the battery under the seatpost never seated properly for me—the tabs just wouldn’t click into the holes on the post. The “proper” fix would probably be trimming a couple millimeters off the seatpost, but I wasn’t too keen on that.

Instead, I went with a simpler solution: I added two small zip ties to the battery so I can pull it out easily after sliding it into the seattube. One zip tie holds the second in place, which acts as a little grab handle. The battery sits in the seatpost with the connectors facing up, and the extra cable tucks neatly inside.

With a bit of wiggling (but no force) the battery slides all the way into the narrower section of the seattube and is still very easy to remove. For reference, it sits about 20 cm down from the seattube opening to the top of the battery. The seatpost itself goes in just as far as it would without the battery/wiring (sticky layer fully inserted), and I was also able to use the hole behind the front derailleur mount without any trouble

I hope this is useful for others facing the same issue.