State Estimation of a Lithium-Ion Battery Pack By Using Lamb Waves Generated from Piezoelctric Transducers

Currently the state of Lithium-Ion batteries in an electric vehicle (EV) is determined with a battery management system (BMS) that monitors the voltage, current and temperature of the cells. With this information the batteries are operated within their safe operating limits and important parameters like the state of charge (SOC) and state of health (SOH) are calculated. In this work a novel and realistic approach to determine these characteristics is presented by using piezoelectric sensors. During operation, the mechanical properties of LIBs change which affects the speed of the propagating waves, that are generated by the piezoelectric transducers. These changes primarily occur during cycling due to the intercalation and deintercalation of lithium ions in the active materials. Over the lifetime additional mechanical changes based on the degradation of the battery are taking place, for example the growth of the solid electrolyte interface (SEI), particle cracking, or lithium plating. It is very challenging to measure the above-mentioned degradation processes solely based on the electrical behaviour of the LIB. By utilizing piezoelectric transducers not only the SOC can be determined more precisely but also these degradation processes that in some cases can lead to safety critical events can be tracked. This subsequently results in an accurate estimation of the SOH of the LIB. With the knowledge that the propagating waves inside a LIB are so-called Lamb waves it is not only possible to track the state of the battery but also to simulate the propagation of these waves and get more precise information 1. By applying Lamb waves that use excitation signals in the lower frequency range of around 10kHz, this approach - compared to works that use higher frequencies, permits the usage of less expensive electrical equipment. This makes the presented method more cost-effective and suitable for an integration in a BMS for an EV. This has been shown by the authors recent work, where an ultrasonic BMS (UBMS) was developed for a single pouch cell. An SOC and temperature estimator based on the ultrasonic measurements were proposed where the average error of the SOC estimator at room temperature was 3.48% and the maximum error of the temperature estimator at SOC 50 was 1.6°C 2. In this work, an experimental setup was created where a battery pack consisting of four 60Ah pouch cell was built. This setup includes the developed UBMS, a balancer and piezoelectric transducers mounted on two different pouch cells. Two load cells measure the changes in pressure that are caused by the mechanical property changes which occur during cycling. The whole setup is placed in a climatic chamber and can be seen in Figure 1 a). The shown battery pack was cycled at different temperatures and various current rates (C-rate) to reproduce a realistic use case. The UBMS, which excites the piezoelectric sender and measures the signal generated by the piezoelectric transceiver is processing continuously the gathered data to calculate the so-called group velocity of the propagating wave. Additionally, the UBMS tracks the standard values as cell voltage, current and temperature of the LIBs. The interplay between the cell voltage, the applied current, the ultrasonic measurements, the temperature, and the changes in pressure can be seen in Figure 1 b). Measurements with different C-rates and at different temperatures were conducted and the changes in the measured group velocity caused by the mechanical property changes will be discussed. With the help of ultrasonic measurements safety critical events like a rise in pressure or temperature or certain chemical events e.g., lithium plating, can be, compared to a regular BMS, immediately detected and necessary steps to avoid safety risks can be implemented. An outlook on these topics based on the measurement results for future research and possible industrial implementation will be given. 1 M. Koller, G. Glanz, A. Bergmann, und H. Popp, „Determination of Lamb Wave Modes on Lithium-Ion Batteries Using Piezoelectric Transducers“, Sensors , Bd. 22, Nr. 13, Art. Nr. 13, Jan. 2022, doi: 10.3390/s22134748. 2 M. Koller, G. Glanz, R. Jaber, und A. Bergmann, „Ultrasonic Battery Management System for Lamb wave mode tracking on Lithium-ion pouch cells“, J. Energy Storage , Bd. 74, S. 109347, Dez. 2023, doi: 10.1016/j.est.2023.109347. Figure: a) Demonstrator of a Lithium-Ion Battery pack consisting of four 60Ah pouch cells connected in series inside a climatic chamber. Equipped with the developed ultrasonic battery management system (UBMS), balancer, piezoelectric transducers, temperature sensors and pressure sensors. b) Measurement results at room temperature, cycled with 0.5C. Figure 1