The ornamental design for battery charger, as shown and described.
FIG. 1 is a front perspective view of a battery charger of our present invention.
FIG. 2 is a top plan view of FIG. 1.
FIG. 3 is a bottom plan view of FIG. 1.
FIG. 4 is a left-side view of FIG. 1.
FIG. 5 is a right-side view of FIG. 1.
FIG. 6 is a front view of FIG. 1; and,
FIG. 7 is a rear view of FIG. 1.
The dash-dot-dash lines in FIGS. 1-3 are for the purpose of illustrating portions of the battery charger that form no part of the claimed design. The surfaces are lightly lined for contour.