I claim the ornamental design for a battery holder cover, as shown and described.
FIG. 1 is a perspective view of a battery holder cover in accordance with my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation view thereof, the rear elevation view being identical;
FIG. 4 is a right side elevation view thereof, the left side elevation view being identical;
FIG. 5 is a bottom plan view thereof, the broken lines showing an indeterminate thickness and are for illustrative purposes only and form no part of the claimed design; and,
FIG. 6 is a perspective view of the battery holder cover of FIG. 1, showing the battery holder cover removed off a battery held in a battery holder assembly with an optional second battery holder cover removed off a second battery held in the battery holder assembly with the battery holder assembly shown in broken lines for illustrative purposes only and forming no part of the claimed design.