From the manufacturer

Sitting at the center of the Shimano DEORE XT fully wireless technology, this battery powers fast and smooth shifting. It fits snugly into the DEORE XT rear derailleur for protection from impacts.

Features

• Tucked safely inside an enclosed space, the battery won't budge in rough terrain while remaining completely removable for charging
• This 310 MaH battery provides up to 211 miles (340 km) of riding on a single charge, thanks to its power-saving management system