I purchased a second EN-EL15 battery as a spare for my D800 and have had it for approx. two (2) months. It was never used in the camera as I didn’t need the spare until just the other day. That is when I discovered that the battery will not work at all. When I charge my EN-EL15 battery in the supplied Nikon charger, the charger indicates the battery is fully charged (Light stops flashing). When I place the battery in the D800 camera, the camera will not turn on and the only display is a battery indicator bar on the top LCD that only shows one “bar”. The camera will not turn on at all with the question battery so I can’t do the in camera check to see if the camera will provide any more information. The battery charger will charge another Nikon EN-EL15 without any problems, so it appears that the charger is not the issue. Will Nikon replace the battery?
I think this problem is well known now and I had two batteries showing 2 and 3, both were under 2 years old. I've also read of folks like you with "1" and Nikon supplying new ones. Nikon swapped them for new ones without any hesitation even paying postage in both directions. You'll have to have the original receipt.
Give Nikon a ring and I'm sure they'll sort it out