Ultrasonic velocity as an improved ultrasound state-of-charge prediction method that compensates for cell thickness change

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.