I have made my own sinkers for years now and have never used Car battery lead. Sounds like too much hard work.
I prefer to use left over flashing from building sites and even scabbed some weights from the local tyre fitting business's.
I make sinkers about every 2 years and somethimes longer.
Maybe you could place the batteries in a large ( 60ltr ) old oil drum and
burn them, scraping off the plastic slag to leave the lead in the bottom. Even that sound no good for the environment.
I'm sure someone here has done this before and can offer better advice.
???