2.3.1. Selection of Test Amplitude

Depending on the battery cell application, the amplitude of the test frequency should be selected taking into account the trade-off between measurement accuracy and investment costs for battery management. In order for the appropriate test amplitude to be selected for impedance measurements, it must be selected to be small enough not to violate the linear criteria of the electrochemical battery system but large enough to obtain a suitable signal-to-noise ratio for robust measurements. The amplitude of the test signal must be small enough to satisfy the pseudo-linearity of the cell response voltage. This is crucial because the current–voltage curve of a Li-ion cell shows a nonlinear relationship that follows the Butler–Volmer kinetics. An excitation amplitude of 5 to 10 mV is generally recommended [51], e.g., if it is used for CC measurement (galvanostatic), the excitation amplitude of the current must be set so that the maximum absolute value of the voltage amplitude does not exceed 10 mV. Especially when a multi-signal signal is used, the maximum amplitude of the applied signal should be considered. Because the amplitudes at each frequency increase the total amplitude of the test signal, pseudo-linearity can be violated. For more test frequencies to be used, the amplitude of each frequency must be lowered, i.e., if a multi-sine signal of N frequencies is used, the output voltage amplitude at each frequency must be reduced to a maximum 10/N·mV. Amplitudes that are too small can be a problem as well. This is due to the finite resolution of the hardware that acquires and generates the test signal. Moreover, impedance measurements are not possible when the output voltage amplitude at individual frequencies becomes less than the system noise.
