Which is the best site to ask a question about the batteries used in electric cars including environmental stuff?
If you are willing to wait for an answer and support a new site try Area 51's Electric Vehicles proposal. If it's a great question you can earn a few points on a site where rep is fairly rare.
If you need an answer quickly then Motor Vehicle Maintenance and Repair has a tag for electric vehicles. There is also Engineering.SE for technical questions about electric vehicle batteries. For even more technical answers try Physics.SE which has tags for renewable-energy and batteries.