We claim the ornamental design for an charging unit for mobile devices, substantially shown as described.
FIG. 1. is a front side view of a charging unit for mobile devices in accordance with the present design;
FIG. 2. is top view thereof;
FIG. 3. is a bottom view thereof;
FIG. 4. is a top perspective view thereof; and,
FIG. 5. is a bottom perspective view thereof.
The broken lines shown in FIGS. 1, 3, 4 and 5 are included for the purpose of illustrating an environment for an embodiment of my new design and form no part of the claimed design.