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. The general guideline is 1,000-2,000km riding per charge, and with approximately 300 charge cycles before battery performance starts deteriorating and needs replacement.
If you're worried about running out power on a ride, know that you'll first get a warning light on the stem-mounted junction box. If you don't see that, you'll lose the ability to shift the front derailleur first, and then you'll have 100-200 shifts left before it goes dead. Charging a battery from empty, and you'll need to pick up the SM-BCR1 Battery Charger, and the SM-BCR1 Power Cable separately, will take 90 minutes. Power up your Di2 System with the SM-BTR1 Battery.
MODEL NO: SM-BTR1
SERIES DURA-ACE 9070 Series
weight: 70g