The Vivitar Battery Grip is specially designed for Nikon D7100 DSLR Camera.
Reviewed by 1 customers
It actually comes with a battery tray for AA and the Nikon battery... I have two Nikon batteries in my D7100 now. It feels a little flimsy, I'm sure the Nikon version is more sturdy, buttons feel a bit cheap, but it works fine and is a good alternative for $60. You don't need to remove it to attach to a tripod.