The SD-9 is an external battery pack for the Nikon SB-900 / 910 / SB-5000 Speedlights.
It can hold 8 AA batteries, but you can also use it with just 4 batteries. It’s compatible with any AA battery, but provides the fastest recycling time when used with 4 quality AA Ni-MH rechargeable batteries.
This battery pack doubles the number of flashes possible and reduces recycle time to as little as 1.0 second when used with NiMH batteries (other types of batteries work, but a bit more slowly).
Note: This is a powerful tool. You can run amuck and ruin other flashes if you plug it into them. If you shoot as long and fast as you can, you could possibly even burn out an SB-910. Use it carefully.