7.2 V 2000mAh power supply for the D1, D1H and D1X Digital SLR cameras. The EN-4 is charged using one of the following: Quick charger MH-15 (1 EN-4 at a time) or Quick charger MH-16 (2 EN-4 successively) and Quick charger MH-17 (1 EN-4 12V cigar lighter). Charging takes about 90 minutes with an MH-15.