The MB-D15 Multi Battery Power Pack offers both enhanced handling capabilities as well as extended battery life while shooting. It accepts one additional EN-EL15 lithium-ion battery pack to increase the camera's battery life or, alternatively, six AA batteries can be used in conjunction with the included MS-D14 AA Battery Holder. The design of the grip improves shooting in the vertical orientation and also provides an overall more ergonomic structure for holding the camera. 

Nikon MB-D15 Specs