Shimano Di2 External Battery SM-BTR1
Shimano's Di2 SM-BTR1 Battery is their external power source. It works with all versions of Di2, provided you have a SM-BMR2 Battery Mount to attach the battery to the system. Battery life depends on many factors. It's not only the number of times you shift, but the climatic conditions, and how often you use the front derailleur, as it takes more energy per shift.
*Battery Mount Not Included