An auxiliary electronic device attachable to a wearable electronic device. The auxiliary device includes a housing, electronic circuitry within the housing, and an attachment mechanism configured to attach the auxiliary electronic device to the wearable device while the device is being worn by a user. In some embodiments the electronic circuitry includes a power transmitting unit that can wirelessly transmit power to charge a rechargeable battery within the wearable electronic device. In some embodiments the attachment mechanism includes a pair of lugs that extend, from opposite ends of the housing, above the housing towards a center of the auxiliary device and are adapted to fit within corresponding recesses of the wearable electronic device.