Rechargeable battery and rechargeable battery module

A secondary battery module comprising a cell stack comprising a plurality of battery cells located along one direction, the plurality of battery cells comprising one or more secondary batteries, each comprising:

• an electrode assembly comprising:
• a first electrode;
• a second electrode;
• and
• a separator between the first electrode and the second electrode;
• a case having an opening at one or more sides to accommodate the electrode assembly in the case through the opening;
• one or more cap plates sealing the opening at the one or more sides of the case, respectively;
• and
• a reference electrode located in a void volume inside the case.