Nikon SD-9 Battery Pack for SB-900
The Nikon SD-9 Battery Pack is used to greatly increase the number of flash firings of the SB-900, SB-910 and SB-5000 Speedlights, and also shortens the recycling time. It has a total capacity to hold eight AA batteries, but can also be used with just four batteries. It can be used with any type of AA battery, but provides the fastest recycling time when used with four 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.