Man, I thought I was going crazy? I never buy aftermarket batteries for my cameras, but I got the extra battery as a package deal when I ordered my D600. I too am having issues with it. I'll put the fully charged into my camera, power it up, and I'm good, right? Wrong! When I turn the power to the camera on the battery icon flashes showing it's a dead battery. Only way I can get it to work is by... completely removing the battery, than power up really quick. Once I turn the camera off, and back on, it'll do it again, and I'm forced to repeat the process to get it to work. Blows!