I have a TI-30 STAT calculator I bought at a yard sale and it does not work. The guy said it just needed batteries, but he did not say what kind. I don't know if the batteries I put in are proper or not, as there is nothing on the calculator that says what type of battery it takes. I don't understand how any battery can work if there is a piece of plastic between the batteries and the terminals on the cover. Can you help me?\ Thanks Pete Mills Mt. Gilead, Ohio