Powered by AA Batteries. It can hold eight AA batteries, but you can also use it with just four batteries. It’s compatible with any AA battery, but provides the fastest recycling time when used with four high-quality AA Ni-MH rechargeable batteries.

More Flashes, Less Recycle Time. 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.