Power is delivered from a power source in a head-mounted device to a separate accessory that is coupled to the head-mounted device. Power and signals are delivered on a common conductor that couples the head-mounted device to an accessory. One or both of the head-mounted device and the accessory are configured based on the signals. Power may be received at an accessory from the head-mounted device through a dedicated power conductor. Power may be delivered from a power source in a portable accessory to a head-mounted device that is coupled to the portable accessory and uses the power for circuitry in the head-mounted device that delivers audio to a user.