I've just did it, here's the information you need :
For internal battery, you need to order this part with your LBS :
For di2 wire length, here is the schema sent from specialized :
For all other ultegra di2 components, I choose "old" ultegra 6871 with last battery version :
- Shimano Di2 BCR2 Charger for BTR2 (battery charger)
- shimano E-Tube SEIS di2 (battery)
- Shimano SD50 Di2 Electric Wire (see schema for length)
- Shimano ST-6871 (STI Gear levers for TT/tri bar without wire) + 2x 350mm di2 wire length
- Shimano SW-R671 (triathlon shifters, comes with wire)
- shimano SM-EW90-B (junction A - 5 ports)
- shimano SM-JC41 (junction B)
- shimano RD-6870 ultegra Di2 (rear derailleurs)
- shimano FD-6870 ultegra Di2 (front derailleurs)