Publications

Enhancing the performance, safety and reliability of battery management systems is crucial for advancing the state of the art in battery electric vehicles. Current research explores the potential of ultrasound to monitor state of charge (SoC) changes in individual cells. Understanding spatial variations in SoC is essential, as non-uniformities could lead to sub-optimal performance, premature ageing, and possible safety risks.

This study uses ultrasound immersion C-scans to map wave speed and attenuation at different SoC levels during battery cycling. Results indicate non-uniform wave speed and attenuation suggestive of SoC spatial variations within single cells, emphasising the importance of addressing this issue. Acoustic measurements under various C-rates and relaxation periods are discussed, providing insights into lithium-ion rearrangement in graphite particles. Potential causes of structure and manufacturing variations of the cell are discussed, highlighting the need to address these issues to prevent overcharging or overdischarging in specific battery areas.

Traditional direct status measurement methods for assessing lithium-ion batteries, such as Coulomb counting and open-circuit voltage (OCV), provide limited information into internal mechanical changes in operando. Ultrasound nondestructive evaluation (NDE) offers a complementary approach by probing the mechanical response of cells.

This study investigates ultrasound monitoring of state of charge (SoC) and degradation in lithium-polymer (LiPo) pouch cells under controlled cycling and temperature variations. Time of flight (ToF) and signal amplitude (SA) were extracted from through-transmission waveforms via cross-correlation and electrochemical data during CCCV protocols. ToF showed closed-loop behaviour and stronger correlations with SoC than SA. Repeated 0.25C cycling induced capacity fade and ultrasonic shifts, though successive temperature calibrations partially masked these changes. Tests at 15°C, 25°C, and 35°C revealed ToF shifts of up to ∼0.7 µs relative to the 25°C baseline. Therefore, a temperature correction factor normalised ToF to this reference, unifying the ToF–SoC trend at 95% state of health (SoH) and clarifying degradation signatures.

These results show the importance of accounting for temperature effects in the ultrasonic signatures in support of accurately monitoring degradation.

Accurate state-of-charge estimation is critical to the safe and reliable operation of Li-ion batteries. The accuracy of established state-of-charge estimation methods is limited due to measurement drift over many cycles or models which are challenging to reliably parametrise. Ultrasound sensing techniques have the potential to overcome these issues by providing a direct link between the state-of-charge and physical properties of the cell. Ultrasonic time-of-flight is the predominant measurement used to predict state-of-charge, but this measurement is complicated by changes in cell thickness during cycling, the effect of which has gone underappreciated until now.

Unlike time-of-flight, ultrasonic velocity is unaffected by cell thickness changes, and this paper demonstrates for the first time the advantages of using velocity to predict state-of-charge. Velocity is found to be more consistent than time-of-flight at a range of different C-rates, temperatures, and across multiple cycles. This culminates in a large improvement in the prediction of state-of-charge during a US06 drive cycle, where the mean difference between predicted and actual state-of-charge is improved from 12.3 % to 3.8 % when velocity is used instead of time-of-flight. These results are a significant step towards the realisation of state-of-charge prediction using ultrasound and hence safer and more reliable Li-ion battery operation.