This disclosure relates to methods and devices for transmitting acknowledgment messages on a preferred cell in a carrier aggregation scenario. A user equipment device (UE) establishes a connection with a primary cell and one or more secondary cells, and determines a preferred cell from among the primary cell and the one or more secondary cells based at least in part on one or more quality metrics. The UE may determine the preferred cell at least in part in response to determining that transmission power of the UE has been limited due to a specific absorption rate (SAR) backoff feature of the UE. The UE transmits a notification to a network indicating that the UE intends to transmit an acknowledgement message to the preferred cell, and transmits the acknowledgment message to the preferred cell.