What is a battery fire like in an ammo box? I imagine flames shooting up, catching the ceiling on fire, and the bottom getting so hot that it catches the shelf on fire.
I made a lithium battery pack for amateur radio once. I was scared of it for a while and charged it in my fireplace. If it catches on fire, well, that's the place, right? (They were A123 cells though. Not as firey as other chemistries.)