Nikon D6 Reference Manual page 1166

Calibrating Batteries
The MH-26a battery charger can detect whether batteries
require calibration and calibrate them as necessary to ensure
the accuracy of the battery level display. Calibration status and
progress are shown by lamps on the battery charger:
If the calibration lamp for the current battery chamber flashes
when a battery is inserted, the battery needs to be calibrated.
To begin calibration, press the calibration button for the current
chamber for about a second. The chamber lamp for the current
battery chamber flashes while calibration is in progress.
The display can be read as follows:
1114 Specifications
Chamber lamps (green)
1
Charge lamps (green)
2
Calibration lamps (yellow)
3
Calibration buttons
4