The Nikon SD-9 Battery Pack is used to greatly increase the number of flash firings of the SB-900 and SB-910 Speedlights, and also shortens the recycling time. It has a total capacity to hold 8 AA batteries, but can also be used with just 4 batteries. It can be used with any type AA battery, but provides the fastest recycling time when used with 4 quality AA Ni-MH rechargeable batteries. Total minimum flash recycling time can be as fast as 1.0 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.