I have previously written a review for the SB-900 flash. I have several using them for location work with studio modifiers, often using two or three together. I use PocketWizard TT5's to control the flashes remotely. My tendency is to run my speed lights at 1/2 power or less. This gives very short recycle times, extends the life of flash tubes, and reduces melting/fading of gels. When I need full power the recycle time drops to about 3 seconds with Eneloop NiMh batteries. Adding the SD-9 with 8 Eneloops drops the time to about 1 second. Further the concern about an overheat shutdown of the flash is substantially reduced. I have never experienced such a shutdown. The SB-900 has 3 internal heat sensors. All of them are close to the internal battery compartment. Adding the SD-9 removes the battery heat source.