Ultrasonic time of flight technique for state of charge estimation in lithium-ion batteries

Accurate State of Charge (SoC) estimation is essential for the safe and reliable operation of lithium-ion batteries. Conventional electrical methods often require complex models and are sensitive to temperature variations and hysteresis effects. This work proposes a non-destructive ultrasonic Time-of-Flight (ToF) technique for SoC monitoring. The method exploits the sensitivity of longitudinal ultrasonic wave velocity to internal mechanical changes occurring during charge and discharge. Experiments were conducted on a lithium-ion pouch cell immersed in oil to ensure stable acoustic coupling. Ultrasonic transducers were positioned on opposite sides of the battery to measure transmitted signals and track ToF variations throughout cycling. Results reveal a clear correlation between SoC and ToF. A reduction in ToF during charging corresponds to an increase in acoustic velocity, attributed to changes in density, stiffness, and lithium concentration within the electrodes. These finding demonstrate the potential of ultrasonic sensing for real-time, in situ SoC estimation and future integration into advanced battery management systems.