Shimano SM-BTR2 Di2 Internal Mount Battery For Dura-Ace 9070

This is the power house of Shimano Di2 is this long-life 7.4v li-ion battery that is both compact and reliable, taking shifting effort away from you. Excellent sealing and reliability have been proven through repeated field tests in various riding conditions, including extreme temperatures, rain, mud, and cobble stone roads.

If you're worried about running out of juice 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.  The battery takes 90 minutes to charge from empty.