I claim the ornamental design for a battery pack, as shown and described.
FIG. 1 is a perspective view of a battery pack of my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines are for the purpose of illustrating portions of the battery pack that form no part of the claimed design.