The SD-9 is a must, period! This is the external battery pack for the SB-900 and when shooting at 8fps, this is the only way you’re going to keep up with the action. If you look at my Whimberly, you’ll see a gray velcro strap, that’s to hold the SD-9 onto the Whimberly Head and out of the way. This also keeps the cord policed so it’s not dangling around to scare wildlife.
The case for the SD-9 is designed to attach easily to a lighting stand, your belt loop and the one thing most folks miss, to keep the power cord policed when not in use. This keeps the cords from getting twisted and knotted or tangled with other gear in your camera bag. The SD-9 holds 6 AA batteries and their power go directly to the capacitor in the SB-900. The batteries in the SB-900 power its computer. In both devices I use PowerEx 2700mHn batteries that I get from Thomas Disturbing. I really don’t know how many I have, a few pounds worth but as you can see, I mark the year I bought the battery right on it with a gold pen.