To answer your question walfisch, the Di2 battery location is precut/predrilled into some black polystyrene at the bottom of the rear seatpost mount hole. It fits the BT-DN110 perfectly. I used a 550mm wire to Junction B routing under the bottom bracket spindle sleeve and it's long enough to be able to pull it out and have it clear the seatpost hole for servicing. One note, it's hard to get out without some pliers shaped to get a secure hold.

For context and/or to anticipate any other Di2 questions, I just purchased a clearance 2017 TM02 3 which I stripped bare and transferred Di2 from my road bike with clip-ons.

Here is the breakdown of the Di2 components with their weights from a calibrated lab balance:
Shimano Dura-Ace ST-R9160 Dual Control Di2 TT Levers 127.7g
Shimano Dura-Ace SW-R9160 Bar-End Di2 TT Shifters 36.8g
Shimano SM-EW90-B Di2 Junction A 14.6g
Shimano SM-JC41 Di2 Internal Junction B 4.1g
Shimano EW-WU111 Di2 D-Fly Wireless Unit 3.0g
Shimano EW-SD50 Di2 400mm E-Tube Left TT Brake Wire 4.6g
Shimano EW-SD50 Di2 400mm E-Tube Right TT Brake Wire 4.6g
Shimano EW-SD50 Di2 600mm E-Tube Left TT Shifter Wire 5.9g
Shimano EW-SD50 Di2 600mm E-Tube Right TT Shifter Wire 5.9g
Shimano EW-SD50 Di2 600mm E-Tube Down Tube Wire 6.6g
Shimano EW-SD50 Di2 550mm E-Tube Battery Wire 6.0g
Shimano EW-SD50 Di2 350mm E-Tube Front Derailleur Wire 3.7g
Shimano EW-SD50 Di2 600mm E-Tube Rear Derailleur Wire 6.6g
Shimano EW-SD50 Di2 300mm E-Tube D-Fly Wire 3.2g
Shimano BT-DN110 Di2 Internal Battery 51.4g
Shimano EW-SD50I Di2 E-Tube Wire Holders (x4) 1.1g
Shimano SM-GM01 Di2 Wire Grommets (x2) 0.5g

Have fun!