Preliminary Analysis of Eddy Current Resonant Sensing for State of Charge Estimation in Lithium-Polymer Cells

Accurate State of Charge (SoC) estimation is essential for the safe and reliable operation of lithium-based energy storage systems, particularly in applications such as electric mobility, stationary storage, and second-life battery repurposing. Current battery management systems predominantly rely on galvanically connected electrical measurements, which limit observability at the cell level. This work presents a preliminary analysis of a resonant eddy current (EC) sensing technique aimed at indirectly estimating the SoC of lithium-polymer (Li-Po) pouch cells. The proposed technique exploits the swelling phenomenon of batteries to estimate SoC by detecting variations in the sensorto-cell lift-off distance. A dual-channel resonant EC sensing system was implemented and experimentally validated through repeated charge-discharge cycles under controlled operating conditions. The resonant frequency variation exhibited a clear and monotonic dependence on the SoC, with good repeatability across cycles and across two independent sensors. A polynomial regression model was employed to derive an analytical mapping between resonant frequency and SoC, yielding an R2 of 0.84 during charging and 0.97 during discharging. A quantitative comparison against a Coulomb-counting reference SoC was also performed, reporting the mean relative estimation error and the associated uncertainty at selected SoC points for both charge and discharge conditions. These preliminary results confirm the feasibility of resonant EC sensing as a fully contactless, lowcost, and integrable method for cell-resolved monitoring, with potential relevance to durability certification and early fault detection.