Nikon D780 Reference Manual page 864

3
Unplug the charging AC adapter when charging is
complete.
Disconnect the charging AC adapter from the camera.
D
The Charging AC Adapter
If the battery cannot be charged using the charging AC adapter, for
example because the battery is not compatible or the temperature of the
camera is elevated, the charge lamp will flash rapidly for about 30
seconds and then turn off. If the charge lamp is off and you did not
observe the battery charging, turn the camera on and check the battery
level.
818 Technical Notes