That is the charge sense line that tells the radio that a rechargeable battery is attached. ?It contacts the edge of the bottom battery. ?The AA battery pack is insulated from this so it won't try to charge alkaline battery's. ?By the was, if you do want use rechargeable batts in the AA pack you can very carefully cut the paper cover on the bottom cell so the contact will touch the battery. Then you can recharge nimh in the alkaline pack. ?There is an instruction on this somewhere on the internet. ? Rick/N0HKE