The design consists of the features of shape, configuration, pattern and ornamentation of the BATTERY CHARGER as shown in solid lines in the drawings. The stippled line portions do not form part of the design. One variant of the design for the BATTERY CHARGER is depicted in FIGURES 1 to 9. A second variant of the design for the BATTERY CHARGER is depicted in FIGURES 10 to 18. A third variant of the design for the BATTERY CHARGER is depicted in FIGURES 19 to 27. FIG. 1 is a front perspective view of a BATTERY CHARGER bearing a first variant of the design; FIG. 2 is another front perspective view of the BATTERY CHARGER shown in FIG. 1; FIG. 3 is a rear perspective view of the BATTERY CHARGER shown in FIG. 1; FIG. 4 is a front elevational view of the BATTERY CHARGER shown in FIG. 1; FIG. 5 is a rear elevational view of the BATTERY CHARGER shown in FIG. 1; FIG. 6 is a left-side elevational view of the BATTERY CHARGER shown in FIG. 1; FIG. 7 is a right-side elevational view of the BATTERY CHARGER shown in FIG. 1; FIG. 8 is a top plan view of the BATTERY CHARGER shown in FIG. 1; FIG. 9 is a bottom plan view of the BATTERY CHARGER shown in FIG. 1. FIG. 10 is a front perspective view of a BATTERY CHARGER bearing a second variant of the design; FIG. 11 is another front perspective view of the BATTERY CHARGER shown in FIG. 10; FIG. 12 is a rear perspective view of the BATTERY CHARGER shown in FIG. 10; FIG. 13 is a front elevational view of the BATTERY CHARGER shown in FIG. 10; FIG. 14 is a rear elevational view of the BATTERY CHARGER shown in FIG. 10; FIG. 15 is a left-side elevational view of the BATTERY CHARGER shown in FIG. 10; FIG. 16 is a right-side elevational view of the BATTERY CHARGER shown in FIG. 10; FIG. 17 is a top plan view of the BATTERY CHARGER shown in FIG. 10; FIG. 18 is a bottom plan view of the BATTERY CHARGER shown in FIG. 10; FIG. 19 is a front perspective view of a BATTERY CHARGER bearing a third variant of the design; FIG. 20 is a rear perspective view of the BATTERY CHARGER shown in FIG. 19; FIG. 21 is another front perspective view of the BATTERY CHARGER shown in FIG. 19; FIG. 22 is a front elevational view of the BATTERY CHARGER shown in FIG. 19; FIG. 23 is a rear elevational view of the BATTERY CHARGER shown in FIG. 19; FIG. 24 is a left-side elevational view of the BATTERY CHARGER shown in FIG. 19; FIG. 25 is a right-side elevational view of the BATTERY CHARGER shown in FIG. 19; FIG. 26 is a top plan view of the BATTERY CHARGER shown in FIG. 19; and FIG. 27 is a bottom plan view of the BATTERY CHARGER shown in FIG. 19.