Shimano BT-DN320A Battery - On Component Type 1st Group. Full wireless technology means an all-new Battery. The BT-DN320 fits snuggly and protected within the new Wireless XTR Rear Derailleur and powers the fast and smooth shifting XTR is known for. Protected Battery is tucked safely inside an enclosed space so the battery won't budge even in rough terrain while remaining completely removable for charging 310 MaH provides for more riding on one charge Charger sold separately