Aircraft battery systems

An aircraft 10 comprising a battery pack 20 having a plurality of battery cells 200 is provided. Each of the plurality of battery cells 200 is arranged to vent into a venting region 230, 235a, 235b of the battery pack 20. A path 21, 210, 213, 232, 235a, 235b is defined between the venting region 230, 235a, 235b and an exterior of the aircraft 10, whereby thermal products vented by the cells 200 can exit the aircraft 10 via the path 21, 210, 213, 232, 235a, 235b. In some embodiments, the venting region 230, 235a, 235b is or is in fluid communication with an air channel 21, 210, 213, 235a, 235b through which air flows from an aircraft air inlet 16 to an aircraft air outlet 17. In other embodiments, the path joins the venting region 230 and an opening in an external surface of the aircraft 10.