FIG. 1 is a rear elevational view of a battery pack, showing my new design;

FIG. 2 is a right side elevational view thereof, the left side being a mirror image thereof;

FIG. 3 is a top, rear, right side perspective view thereof; and,

FIG. 4 is a bottom plan view thereof.

The broken lines illustration of an outline of a portable telephone is for illustration purposes only and forms no part of the claimed design.