The ornamental design for a batterypack, as shown and described.
FIG. 1 is a front view of a battery pack showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines showing the remainder of the battery pack are included for the purpose of illustrating environmental structure and form no part of the claimed design.