Well, it looks like car batteries are straight out.
Here is an excellent article as to why it is dangerous to try and melt down car batteries. Long and short of it: melting them releases arsine and stibine, which can kill at 50 parts per million. 10 parts per million is said to be used to fumigate for mice.
http://castboolits.gunloads.com/showthread.php?t=40769
Steve