Abstract: The present disclosure describes systems and techniques relating to wireless charging of mobile devices, such as smartphones and wearable devices. According to an aspect of the described systems and techniques, a mobile data processing device includes: a display; a processor; a memory system; wireless communication hardware; a battery; a coil; and wireless charging circuitry coupled between the coil and the battery; wherein the wireless charging circuitry is configured to charge the battery using power received wirelessly through the coil, and wherein the wireless charging circuitry is further configured to draw power from the battery and operate the coil to wirelessly transmit the power from the battery to another mobile data processing device.