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. FEATURES: Protected Battery: Tucked safely inside an enclosed space, the battery won’t budge even in rough terrain, while remaining completely removable for charging. Longevity: 305 mAh provides for more riding on one charge.