I have discussed it at length with the USAA insurance people, I expect there are policy documents on their web site. The exact discussion was, "If the source of the fire is tracked back to a leaking gas can or a propane tank we won't cover the loss, we recommend you store flammables in a shed away from your house."

My impression from them was they were very dubious about "off book" uses of things as well, which leads to my expectation that they would push back on covering a loss due to a "home made" battery wall. Could I sue them and get them to cover it? I don't know, perhaps. But, as with my propane tanks, I would keep this stuff out away from the house to limit my losses and avoid having to go there.