[Dylan16807]

A single lead-acid battery is putting out such a tiny amount of hydrogen. Put it in any room. It's a big fat nothing in a fire too: https://www.youtube.com/watch?v=ndfe0c00gwo Something could short but shorts are possible and dangerous even if no batteries are involved.

Or get a LiFePo battery and put it in a metal tub.

[Macha]

That looks like enough fire that spreading to my wooden floors or desks, carpets or skirting boards seems more likely than not?

[Dylan16807]

It burns about as well as any random piece of electronics in a plastic shell. Any of those could spread too. If you want a battery that's less flammable everything else in the room I guess you're out of luck, but I don't think that's a reasonable standard.