Voltage Reduction Technique for Use With Electrochemical Impedance Spectroscopy in High-Voltage Fuel Cell and Battery Systems

An attractive application of electrochemical impedance spectroscopy (EIS) is for diagnostic of a fuel cell (FC) or a battery system during operation. The use of EIS, however, is mostly limited to low-voltage (LV) FC systems and laboratory environments. Hence, the application of EIS in advanced diagnostics of a high-power system certainly lacks due to the voltage limitation and/or cost of the equipment. In this paper, a precision, low-cost electronic interface is proposed which enables the use of existing LV ac diagnostic tools with a production-size FC or battery stacks without the need for postprocessing of data. The interface, a dc level reducer (DLR), reduces only the dc component of the stack voltage to a safe voltage of <60 without altering the ac diagnostic components. This paper explains in detail the development of the DLR circuitry. The scalability and real-world capability of the interface are demonstrated by developing it for two voltage ratings. A set of circuits rated for 30 V is tested with a nine-cell proton exchange membrane FC (PEMFC) stack, and circuits rated for 200 V are tested on 90- and 110-cell commercial PEMFC stacks. The stack voltage is reduced by 60% on the nine-cell stack, and 60%-90% on the 90- and 110-cell stacks. The accuracy is measured using EIS data for 74 frequency points in the range of 0.1-20 kHz, with and without the DLR. The maximum relative error for point-versus-point comparison of the impedance is measured at 0.8% and 1.4% for 30- and 200-V rated circuits, respectively. These errors are well within the error of the industrial measurement equipment used, proving the fidelity of the ac signal output from the DLR.