The ornamental design for a mobile powerbank, as shown and described.
FIG. 1 is a perspective view of a mobile power bank showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view showing the mobile power bank in an alternate position.
The broken line shown in the drawings of the mobile power bank are included for the purpose of illustrating environmental structure, and form no part of the claimed design.