Having worked as a rep for a battery manufacturer, I would strongly recommend not going there. The alloys used in car batteries are very specific, and most of them have large proportions of calcium in them, which will mess with the casting process. The ones that don't will be sulfated and flaky, add in the fact that there will be a good amount of hydrogen sulphide infused into the alloy from use (a major carcinogen) which could be released by melting, and you have an ugly, toxic proposition.