The JJC FB-2(II) Compact Battery Pack is used to greatly increase the number of flash firings of the NIKON SB-900Speedlight, and also shortens the recycling time. It has a total capacity to hold 8 AA batteries. It can be used with any type AA battery. Total minimum flash recycling time can be as fast as 1.5 seconds when used with AA Ni-MH batteries, and the capacity per round of batteries is about doubled. Batteries must be in place in the flash in order to operate.