A wearable electronic device, comprising: first, second, and third connectors (480, 485, 490); a flexible printed circuit; a first battery pack operatively coupled to the flexible printed circuit with the first connector; a second battery pack operatively coupled to the flexible printed circuit with the second connector; a main logic board operatively coupled to the flexible printed circuit with the third connector; a first protection circuit module disposed in the first battery pack and configured to selectively enable and disable power output from the first battery pack; a second protection circuit module disposed in the second battery pack and configured to selectively enable and disable power output from the second battery pack; and a sensing circuit disposed along the flexible printed circuit and main logic board and including one or more sensing signal paths (705, 805) coupling the first and second protection circuit modules (435, 440), through each of the first, second, and third connectors (480, 485, 490), wherein the first battery pack provides a first sensing signal over the sensing circuit and the second battery pack is configured to provide a second sensing signal over the sensing circuit, and the first protection circuit module enables power output from the first battery pack when it detects the second sensing signal over the sensing circuit and disables power output from the first battery pack when it fails to detect the second sensing signal over the sensing circuit, and the second protection circuit module enables power output from the second battery pack when it detects the first sensing signal over the sensing circuit and disables power output from the second battery pack when it fails to detect the first sensing signal over the sensing circuit. first, second, and third connectors (480, 485, 490); a flexible printed circuit; a first battery pack operatively coupled to the flexible printed circuit with the first connector; a second battery pack operatively coupled to the flexible printed circuit with the second connector; a main logic board operatively coupled to the flexible printed circuit with the third connector; a first protection circuit module disposed in the first battery pack and configured to selectively enable and disable power output from the first battery pack; a second protection circuit module disposed in the second battery pack and configured to selectively enable and disable power output from the second battery pack; and a sensing circuit disposed along the flexible printed circuit and main logic board and including one or more sensing signal paths (705, 805) coupling the first and second protection circuit modules (435, 440), through each of the first, second, and third connectors (480, 485, 490), wherein the first battery pack provides a first sensing signal over the sensing circuit and the second battery pack is configured to provide a second sensing signal over the sensing circuit, and the first protection circuit module enables power output from the first battery pack when it detects the second sensing signal over the sensing circuit and disables power output from the first battery pack when it fails to detect the second sensing signal over the sensing circuit, and the second protection circuit module enables power output from the second battery pack when it detects the first sensing signal over the sensing circuit and disables power output from the second battery pack when it fails to detect the first sensing signal over the sensing circuit. wherein the first battery pack provides a first sensing signal over the sensing circuit and the second battery pack is configured to provide a second sensing signal over the sensing circuit, and the first protection circuit module enables power output from the first battery pack when it detects the second sensing signal over the sensing circuit and disables power output from the first battery pack when it fails to detect the second sensing signal over the sensing circuit, and the second protection circuit module enables power output from the second battery pack when it detects the first sensing signal over the sensing circuit and disables power output from the second battery pack when it fails to detect the first sensing signal over the sensing circuit.