We have conducted comparison measurements with impedance spectrometers from different manufacturers. These measurements revealed that in the low impedance range of high-energy battery cells, which is difficult to access, the devices exhibited deviations of up to 4 % in the relevant frequency range. By definition, the end of life of battery cells for electric drives is reached when their initial capacity has decreased by 20 %. Measuring the remaining capacity by means of impedance spectroscopy is thwarted by the fact that the measurement errors induced by the impedance spectrometers are therefore much too high, especially in the face of further measurement-induced uncertainty components, such as inaccuracies in determining the temperature or inductive effects, for example.
To be able to determine the measurement error and to correct it, where appropriate, it is necessary to have suitable standards traceable to the International System of Units (SI). By developing various reference impedances for the calibration of impedance spectrometers, PTB has taken a first important step. The reference impedances are to cover different impedance ranges between one ohm and 100 microhm in the frequency range from µHz up to one kHz. To date they can, however, only be used as stable references for comparing different impedance spectrometers, as no reliable traceability to the standards of the SI system is available yet. For this purpose, PTB is currently developing a reference spectrometer which is optimized for the use in this frequency range and for the impedances of high-energy Li ion battery cells.