Is the XTR Di2 battery compatible with Ultegra Di2?
According to BikeRadar, "The battery unit as well as the wiring for XTR Di2 are identical components to the ones used in Shimano's electronic road groups. The battery can be mounted on a bottle cage, in a seat tube, frame and can even be contained within the steerer unit of certain forks (via the use of PRO's new Tharsis components)."