SHIMANO SM-BTR1 DI2 External Built-In Type Battery 

Di2 External Battery, battery charger, and power cable are universal between Dura-Ace 9070, 7970, Ultegra 6870, 6770 and Alfine Di2 systems
Weighs only 71g
Lasts around 1,000 miles on average

Type: Lithium Ion
Nominal Capacity: 530 mAh
Weight: 71g
Operating Temperature Range: -20 - 60°C (when discharging), 0 - 45°C (when charging)
Storage Temperature Range: -20 - 60°C
Nominal Voltage: 7.4V

Original Shimano parts guarantee that all components work perfectly for optimum performance and maximum safety and longevity.