Nikon D3 - Manual (Page 460)

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Battery Warning
If the chamber and calibration lamps flash on and off in sequence when
no battery is inserted, there is a problem with the charger. If the chamber
and calibration lamps flash on and off in sequence when a battery is
inserted, a problem has occurred with the battery or charger during
charging. Remove the battery, unplug the charger, and take the battery
and charger to a Nikon-authorized service representative for inspection.
A
Charging and Calibrating Two Batteries
The MH-22 charges only one battery at a time. If batteries are inserted in
both chambers, they will be charged in the order inserted. If the
calibration button for the first battery is pressed, the second battery can
not be calibrated or charged until calibration and charging of the first
battery are complete.