Accessory device authentication techniques are described. In one or more embodiments, connection of an accessory device to a host computing device is detected. Responsive to the detection, an authentication sequence may occur to verify an identity and/or capabilities of the accessory device. Upon successful authentication of the accessory device, the host device may authorize the accessory device for power exchange interactions with the host device. The host device may then draw supplemental power from a power source associated with the authorized accessory device, such as a battery or power adapter. The host device may also enable the accessory device to obtain and use power supplied by the host device in some scenarios. Power exchange between a host device and an authorized accessory may be managed in accordance with capabilities of the accessory device that are identified during authentication.