The transparent CAD image below shows where the Di2 battery (red) fits inside the hidden pocket storage compartment which is built into the P5’s frame. The original Shimano Di2 battery is neatly stored inside the P5’s seat tube in the area above the BBrightTM bottom bracket: this is the “hidden pocket.” This is an aero benefit, since a typical external Di2 battery can add up to 11 grams of aero drag. Unlike a custom battery, using Shimano’s original battery and connectors keeps Shimano’s Di2 warranty intact.
To access the hidden pocket, simply remove the rear wheel and unscrew the cover. The battery slides out easily as the electrical cables are also inside the frame and stay neatly connected to the battery holder.
If your P5 is assembled without Di2, then the hidden pocket is available to store other things, such as a spare inner tube, etc.
The Aduro aero bar is also Di2 ready, as the shift wires route through the extensions and stem, and directly into the frame through the ICS passage in the top tube.
The CAD image above shows mechanical routing but Di2 cables follow a similar path.
The junction box fits inside the stem near the front of the base bar under the bolted cover. Alternatively, if you want to have access to the junction box at all times, it can be attached to the side or top of the stem with double sided tape. The Aduro aero bar has a Di2 cable exit port at the trailing edge of the stem to let the cables out. Another more visible option would be directly on top of the stem. Of these two options, the best aero position may be on the side of the stem, in the wake of the base bar.