1. A method implemented by a host computing device comprising: maintaining a data structure configured to associate authorized accessory devices with power contract settings for each authorized accessory device, the power contract settings including at least a power exchange direction and current limits; detecting connection of an accessory device to the host computing device via an accessory interface; determining whether the accessory device is an authorized accessory device by comparing identity data of the accessory device to known data that indicates respective identities of the authorized accessory devices; when the accessory device is determined as an authorized accessory device, setting an active power contract for power exchange between the host computing device and the authorized accessory device, including: obtaining, from the data structure, the power contract settings associated with the authorized accessory device; and setting active power contract settings for the authorized accessory device based on the power contract settings obtained from the data structure; monitoring power exchange conditions between the host computing device and the authorized accessory device; detecting a change in the power exchange conditions prompting modification of the active power contract settings including modifying the data structure to reflect the modified active power contract settings; and communicating an update message that includes the modified active power contract settings to the authorized accessory device.