Nikon EN-EL 4a Compact Lithium-ion power supply for Nikon D2 series D-SLR cameras. Up to 3800 images can be shot on one charge. The EN-EL4a is free from "memory effect", so it can be recharged at any time or any level. A microchip built into the battery measures discharge capacity and compares it to the full (maximum) capacity recorded when the battery was last calibrated. To charge the EN-EL4a use the MH-21.