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Impedanzmessungen bei hohen DC Strömen
Dr. Bernd Evers, FuelCon
Electrochemical impedance spectroscopy (EIS) is one of the most common tools to gain a closer look inside an electrochemical energy storage device such as fuel cells or batteries. Most equipment is limited by the maximum DC current while conducting an EIS measurement. Thus, for the measurement of an EIS the battery charge/discharge cycling has to be interrupted to measure the EIS at zero amperes (OCV). In most setups a frequency response analyzer is connected to a 2-quadrant electronic load or a potentiostat/galvanostat outputting a control signal according to the desired frequency. Thus, the bandwidth of the electronic load is limiting the maximum possible DC and AC current.
In the FuelCon TrueData-EIS impedance setup the impedance analyzer is located between | the test item and the charger/discharger. Latter only outputting the DC current (up to 1000A) while the EIS system outputs the AC modulation current. The advantage is that the bandwidth of the electronic load does not impact the measurement. With this configuration EIS Measurements under very high currents, as achieved during real operation conditions, are possible.
To examine the influence of the measurement method different Lithium batteries were tested with high charge and discharge currents while measuring impedance spectra at different state of charge (SOC) levels, a) during charging and discharging under load condition and b) in an open cell voltage (OCV) period.
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