test under this fixed output current, and go into the next workingcurrent test. The optimal working temperature data under everytest current are shown as Tab.1.The least squares fitting is done to the data in Tab.1, and get therelational expression of the load current Iout and the stack workingtemperature Topt as shown in equation (1), and the fitting curve asshown in Fig. 4, Tmopt is the actualtest point, Topt is the fitting curve.