Originally Posted by
mtseymour
With a new bike, the Di2 battery is usually mounted inside the seatpost with the wires threaded through the frame. For retrofit to an older bike, the battery is attached the water bottle cage near the bottom bracket. So I don't understand how she can "forget" the battery if it was installed properly. The battery last a long time, possibly a whole year on one charge.
11spd Di2 batteries are rated for 500-600 miles operating duration. Unless your tandem is a garage queen, a year is unlikely on one charge.
External batteries IME recharge on the charger in 15-30 minutes. Dunnoh about internals with USB connection. Internals assume a particular
seatpost and wiring capacity in the frame, ie the frame has to be relatively open at tube joins so you can snake the wires around.
To the OP, Santana has found that 11spd Di2 road brifters can operate XT Di2 RD, so you can use whatever cassette the XT can handle. Whether
this requires fiddling with the firmware, you need to talk to Calfee or Santana about that. The XT Di2 FD will also run off road Di2 controls but you
may want to stick to road Di2 FD and a suitable crank, there being some question as to the max size of XT FD chainwheel capability and the FD
mounting variants.