NOVELTY - The sensor device (150) has a reference electrode (160), which is arranged within a housing (140) of a battery cell (100) in ionic contact with electrolytes, where a detection unit (170) is provided for detection of sensor data. The detection unit has an anode terminal (110), which is electrically connected with a battery terminal (115) within the housing, a cathode terminal (120), which is connected with another battery terminal within the housing and a reference terminal, which is electrically connected with the reference electrode within the housing.    USE - Sensor device for a battery cell of an electric energy storage device (Claimed) in battery management systems, particularly for electric vehicles and hybrid electric vehicles.    ADVANTAGE - The detection unit has an anode terminal, which is electrically connected with a battery terminal within the housing, a cathode terminal, which is connected with another battery terminal within the housing and a reference terminal, which is electrically connected with the reference electrode within the housing, and thus reliable and accurate determination of a state of charge is ensured through accurate and reliable evaluation of the overall condition of the battery cell, as well as secure transmission of sensor data without affecting the sealing and protection function of the battery cell housing, and cost-effective manufacturing of the battery cell is ensured.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a battery cell for an electrical energy storage device;    (2) a method for producing a battery cell of an electrical energy storage device; and    (3) a method for monitoring a battery cell of an electrical energy storage device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a battery cell with a sensor device.    Battery cell (100)    Anode terminal (110)    Battery terminal (115)    Cathode terminal (120)    Housing (140)    Sensor device (150)    Reference electrode (160)    Detection unit (170)