YONGNUO New External Battery Pack SF-18 for Nikon SB900 with 8AA Protective Case: Protective soft case can be worn on belt or put in a bag; light and easy to carry. The one-way connector plug is designed to avoid mis-connections which can lead to flash unit damage. 1/4'' Thread Screw: Standard thread for tripod mount. Performance: The Pack reduces recycling time to less than half of a normal cycle and can give up to double the amount of flashes as the original ( a ). It is suitable for continuous flash use ( b ). Convenience: The Pack uses 8 AA batteries (same batteries as flash) and with different connecting plugs, supports most Nikon flashes to greatly increase flash capacity. Its ideal for the professional.