The EN-EL14 is a rechargeable Li-ion battery that powers select Nikon D-SLR cameras, providing 7.4v and 1030 mAh of power.
Used together with your Nikon D-SLR, the camera's fuel gauge accurately displays the remaining charge and number of shots since the last charging.
The battery uses the MH-24 charger.
The EN-EL4a even gives notice when calibration is necessary.