Aircraft battery systems

Aircraft with battery thermal regulation systems are described. One such aircraft 10 comprises: an aircraft air inlet 16 and an aircraft air outlet 17; a battery pack 20 comprising battery cells 200; and a battery thermal regulation system comprising a first air channel 21, a surface 211 of which is in thermal communication with the battery cells 200 so that air flowing through the first air channel 21 exchanges heat with the cells 200 through the surface 211; and an airflow device 40 for driving an airflow. The battery thermal regulation system has a first mode of operation and a second mode operation. In the first mode of operation, the first air channel 21 is fluidly connected with the aircraft air inlet 16 and the aircraft air outlet 17 whereby, during flight of the aircraft 10, air can enter the aircraft through the air inlet 16, flow through the first air channel 21 and exit the aircraft 10 through the air outlet 17. In the second mode of operation, the airflow device 40 drives an airflow through the first air channel 21.