Shimano EW-WU101 Di2 D-Fly E-Tube ANT+ Bluetooth Wireless Unit

This little component opens up the insides of your Di2 system. Feeding data from your derailleurs to your phone or tablet so you can monitor drivetrain metrics, gearing and battery life. This is the most effective way to make sure you don't ever run out of charge on your way home.
The Di2 D-Fly Wireless Transmitter Unit is compatible with Shimano's E-tube Di2 systems and uses the DN110 battery. It transmits using either ANT+ or Bluetooth to connect with compatible head units or your smartphone.