15. A host computing device as described in claim 12 , wherein the current limits include a maximum for power delivery from the accessory device to the host computing device.
12. A host computing device comprising: one or more microcontrollers; one or more computer-readable storage media storing instructions that when executed via the one or more microcontrollers cause the host computing device to perform operations including: managing power exchange between the host computing device and an accessory device connected to the host computing device in accordance with settings for at least a power exchange direction and current limits defined by an active power management contract; maintaining a data structure configured to associate the accessory device with the settings for the active power management contract including at least the power exchange direction and the current limits defined by the active power management contract; monitoring to detect a change in conditions of the power exchange between the host computing device and the accessory device; monitoring to detect receipt of a power contract update message from the accessory device indicating a change in the conditions of the power exchange between the host computing device and the accessory device; determining whether the change to the conditions of the power exchange prompts modification of the active power exchange contract; and when the change to the conditions of the power exchange prompts the modification of the active power exchange contract: updating the settings of the active power management contract for at least the power exchange direction and the current limits in accordance with the conditions; and modifying the data structure to reflect the updated settings of the active power management contract.