Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 56)

Inserting/Removing Batteries
Insertion
Removal
Insert a fully charged Battery Pack LP-E19 into the camera.
Viewfinder brightness increases when a battery is inserted and decreases after
removal.
Insertion
1.
Remove the battery compartment cap.
2.
Insert the battery.
Insert the battery firmly, all the way in, then turn the battery release
handle as shown by the arrow.
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