There is a plastic thing on the battery that is difficult to take out even though I 've taken out the the right-end cable and unscrewed the left oneHow to change the battery in Audi A3 (2001)?
That plastic thing is most likely the protective cover for the positive terminal. It has 2 small pins to locate it on the battery as well as double sided tape to keep it there. If this is the item you're describing, it will just pry up with a flat blade screwdriver or even a putty knife. If that's not the item, then re-post a question here.
