Airsafes:
Heat and smoke are the first signs of a thermal runaway, once one cell starts to overheat, if the battery is not cooled by quenching the other cells will quickly follow suit. The bigger the battery the more cells.
Remember, to avoid confusion and unnecessarily scaring the public, when discussing situations like yours, we must qualify the words "thermal runaway" with either "cell" or "battery" because Boeing has chosen to use "thermal runaway" ONLY to refer to "aircraft thermal runaway" in which the end result is destroying the plane.
When they say that thermal runaway can only result from overcharging, they are using their definition of thermal runaway.
So the two accidents involving 747 cargo craft were probably what Boeing would call thermal runaway, (although as I understand it that has not been proven.) The two 787 incidents were not.
