We claim the ornamental design for a battery pack, as shown and described.
FIG. 1 is a perspective view of a battery pack.
FIG. 2 is a right side elevation view thereof.
FIG. 3 is a left side elevation view thereof.
FIG. 4 is a front elevation view thereof.
FIG. 5 is a rear elevation view thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dashed broken lines in the drawings are for the purpose of illustrating portions of the battery pack that form no part of the claimed design. The dash-dot broken lines in the drawings are for the purpose of illustrating the boundaries of the claimed design, and form no part thereof.