I have a friend that his dad worked at a place that recycled car batteries. We talked about melting down a few to make counter balance weights for a race car. He over heard us. He came up and said in exact words. "Do not even think about trying that anywhere near me, or this property. It is not worth it."
He explained about the sulfur and other chemicals in the batteries made highly toxic gas. The resulting gas could also turn to acid when in contact with any water like the tears in ones eyes reverting back into acid. Also within the lungs. He said doing so would be similar to the pour and run method of making meth. It has blinded, and killed more than few people.
So I urge anyone that is thinking about melting batteries to consider the fact that they are running the risk or blindness, and/or death. It is not worth it.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.