What is the third terminal on the batteries for?? The one in middle between the + & - . I've taken apart a dead battery apart and it apppears to be connected to the - terminal but is insulated from it!! There does not seem (not on my drills) any corresponding terminal in the drill. The reason behind this question is to try and get a 18V mains transformer to fit inside the empty battery case to see if I could construct an emergency mains connection. Just a thought.