The Shimano SM-BCR2 is required to charge the SM-BTR2 internal seatpost battery used for Dura-Ace 9070, Ultegra 6770, and Alfine electronic systems.
The SM-BCR2 Charger includes a USB (SM-USB1) power adapter.
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Mfg PartNum: ISMBCR2
misplaced the charger for my Roubaix dI2 and eventually ran out of juice (no shifting). BTD carefully explains which components you need for a recharge (it can be slightly confusing) and several days later 90 minutes-back on the road. Fortunately there were other bikes to use in the mean time.