NOVELTY - A fuel cell assembly comprises power generating fuel cell and sensor cell (1010). The sensor cell comprises anode gas diffusion layer (GDL) (1020), cathode GDL (1030), membrane electrode assembly (MEA) (1040) interposed between the GDLs. The sensor cell is located at hydrogen gas outlet to receive fuel from residual fuel in the gas outlet from the other cells. The anode and cathode are connected with each other via a resistor.    USE - A fuel cell assembly for electrical device or for use as power source for electronic apparatus (claimed).    ADVANTAGE - The fuel cell assembly includes a sensor cell which is located at the outlet of the in-plane fuel cell assembly and which is driven by fuel that has passed through the assembly without being consumed, thus generating an electric signal, providing feedback to the fuel cell, controlling the fuel feed or alternatively the power draw of the fuel cell, and consequently optimizing the fuel consumption. The sensor cell has small size, thus allowing the gas concentration to be uniform and providing fuel cell assembly having design with longer service life; can provide information on whether there is too much or too little gas supplied to the assembly; can ascertain that all gas is consumed provided that the sensor cell is big enough; and is cheap to produce.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a sensor fuel cell;    (2) an apparatus comprising unit operating by electric power and chargeable battery; and    (3) a method for operating a fuel cell assembly, comprising supplying hydrogen gas to fuel cells, monitoring the voltage of the sensor cell, during operation, continuously monitoring the voltage of the sensor cell, and performing a control function to adjust the performance of the assembly when the voltage of the sensor cell deviates from the predefined value.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-section of four cell assembly incorporating a sensor cell.    Sensor cell (1010)    Anode GDL (1020)    Cathode GDL (1030)    MEA (1040)    Current collector (1050)