The ornamental design for an accessory for a portablebattery, as shown and described.
FIG. 1 is a perspective view of an accessory for a portable battery, showing my new design;
FIG. 2 is a front elevational view thereof, the rear elevational view being identical thereto;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a right-side elevational view thereof, the left-side elevational view being identical thereto.
The broken lines shown in the drawings illustrate portions of the accessory for a portable battery that are environmental structures and form no part of the claimed design. Grayscale shading illustrates contour, not any particular color.