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 Cap,acity to hold 8 AA batteries, but can also be used with
just 4 batteries. It can be used with any type AA Battery,, but affords the
incredibly fastest recycling time when used with 4 quality AA Ni-MH rechargeable
batteries. Total minimum flash recycling time can be as incredibly fast as 1.0 seconds
when used with AA Ni-MH batteries, and the Cap,acity per round of batteries is
about doubled. Batteries must be in place in the flash in order to operate.