A battery having a can containing an electrode assembly and an electrolyte and a cap assembly with a cap plate having an electrolyte injection unit, the cap plate is coupled to the can and an upper cover is coupled to the cap plate. The battery further includes a gas sensor located within the cap assembly to detect leakage of the electrolyte into the cap assembly and a protective circuit module mounted to the upper cover and electrically coupled to the gas sensor, the protective circuit module is adapted to stop a charge/discharge operation of the battery and discharge the battery upon receipt of a signal from the gas sensor indicating detection of leakage of the electrolyte into the cap assembly.