Temperature measurement of large format pouch cells with impedance spectroscopy

Temperature influences significantly the behaviour of a battery cell. For online state estimation, knowledge of the battery cell temperature is therefore indispensable. However, temperature sensors and the wiring are expensive and increase the cost of the battery management system. The temperature dependency of the cell impedance can be used as a temperature gauge to estimate the inner cell temperature. Several concepts are possible, the change of the complex impedance at a certain measurement frequency can be measured or the frequency change for a certain value of the complex impedance can be used. The selection of the impedance value, i.e. phase value, absolute value, real part or imaginary part constitutes an additional design parameter. The dependency on other factors as State of Charge and DC-offset currents determine whether the approach is useful for temperature measurements. The applicability of the different approaches has been evaluated for a 60 Ah large format pouch cell. After characterisation of the cell, all possible combinations have been evaluated. The result showed that the real part and absolute value of the impedance have a strong SoC and DC-offset current dependency and are not advisable for temperature measurements. The use of the imaginary part and the phase value of the impedance proofed to be much more beneficial. Also tracking a fixed phase value with changing frequency is less noisy than measuring the phase value at a fixed frequency.