A battery includes a case housing an electrode assembly, the case having an opening for receiving the electrode assembly, a cap plate combined with the opening so as to close the case, a first insulator on an outer surface of the cap plate, an electrode terminal electrically connected to the electrode assembly, the electrode terminal extending through a first terminal hole in the cap plate and a second terminal hole in the first insulator, a first connection portion that mates the cap plate to the first insulator so as to oppose rotation of the first insulator relative to the cap plate, the first connection portion being eccentric with respect to a central line of the first terminal hole, and a second connection portion that mates the first insulator to the electrode terminal so as to oppose rotation of the electrode terminal relative to the first insulator.