A semiempirical equation was used to represent the performance characteristics of a 20-cell proton exchange membrane electrolyzer stack. The coefficients of the equation are the exchange current densities and membrane conductivity. These coefficients were determined using experimental data and a nonlinear curve fitting method. The anode exchange current density was found to be , the cathode exchange current density , and the membrane conductivity . External programmable power supplies were used to obtain the characteristic curve of a commercial proton exchange membrane electrolyzer. Stack current, voltage, and system temperature were monitored while current steps were applied to the electrolyzer stack.