Dead batteries are not dead. They often just cant deliver the voltage they are supposed to.
Usually a cell or two, out of many, fail, and the battery cant deliver enough voltage to power a device. But the unaffected cells usually still have a lot of energy in them, ajd if there is corrosion inside the battery container, shorts can occur and thats where you get the fire risks. Because they will release their energy real quick then.
Thanks to both of you! I learned something.