Short external battery mount.
etube
Compatible with EW-SD50 etube wires
SHIMANO JUNCTION-A, SM-EW90-A, 3 PORT
PRO DI2 BATTERY HOLDER
SHIMANO BATTERY, BT-DN110-1, BUILT-IN TYPE
SHIMANO BOTTLE CAGE MOUNT ADAPTER, SM-BA01
SHIMANO JUNCTION-A, SM-EW90-B