Shimano BT-DN300 Built-In DI2 Battery - For Dura-Ace 9250 and Ultegra R8150 Systems, Compatible with SD300 Wires

Shimano Di2 Batteries. Must purchase battery, charger, and power cable to complete the system.
All Di2 components on the bicycle require the same firmware version. Using Di2 components with different firmware versions may cause malfunctions
Use Di2 system checker to update component firmware from http://e-tubeproject.shimano.com/