Nikon D4 - Manual (Page 467)

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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.
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Charging and Calibrating Two Batteries
The MH-26 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.