Comment by Ourgon

Pull the thing out of the house and let it burn outside. It is a relatively small battery so outside it won't do much harm, this in contrast to what it seems to do in that room. Tossing it in the pool is also possible but given the small size that'd be a waste of pool water which would need to be replaced. Just let it burn outside. Use a rake, broom or whatever other long thing at hand to pull the thing outside.