What is claimed is the ornamental design for a battery charger, as shown and described.
FIG. 1 is a top plan view of a battery charger showing my new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a rear side elevational view thereof;
FIG. 6 is a front side elevational view thereof; and,
FIG. 7 is a perspective view thereof.
Any elements shown in dotted and/or dashed lines are provided for illustrative purposes only and form no part of the design claimed. Any surface shading is used solely to show the character or contour of the surfaces represented.