Thermal runaway only happens on nicads, it is most likely to happen when a battery is discharged to much then somebody starts the aircraft using external power and the generators are then putting to much current through the batteries whilst charging them. Large amounts of heat generated and the interesting thing that then happens is that one of the cells which are joined in serial to each other to make up the battery reverses polarity and feeds current to the adjacent cells without any current ie circuit external to the battery required hence the term runaway. The only remedy is to isolate the battery from the Bus and let it run it's course ,do not move it cool it or interfere with it in anyway. It can explode. Once it has cooled down it is now unserviceable and needs sending to a battery shop where all the cells will be discharged to zero by resistors then shorting to ensure they are all equal then recharged in stages using a specialist charger.