Novel State-of-Charge Determination in Lithium Ion Batteries

Lithium ion batteries are an important component in many green energy technologies that demand the storage of energy at high energy density. Accurate knowledge of the state of charge (SOC), i.e. the amount of available energy remaining for use, is critical for efficient and safe operation. Current technology uses voltage measurement and current integration, where there are significant limitations in accuracy, robustness and computational demand. In order to address this need, three novel SOC measurement methods were developed. Because the intercalation of lithium ions in the interstitial sites of the cathode and anode changes the volume of these materials, the cathode and anode in lithium ion batteries expand/contract during the charging/discharging of the battery. Changes in the physical properties of the anode/cathode material resulting from the intercalation process were probed in three ways: vibro-acoustics, pressure sensing and mechanical strain. First, the battery response to excitation from a piezoelectric stack actuator was recorded with an accelerometer, where the resonant vibration frequencies are sensitive to changes in SOC. Specifically, the resonant frequency of the most significant peak moved to higher frequencies as the battery was charged. Second, a pouch cell battery and 2D thin film pressure sensor were constrained between metal plates, thereby significantly constraining the macro-scale expansion. The spatial average of the 2D pressure distribution correlated directly to SOC and changes in the spatial non-uniformities in the pressure field provided information on local damage and manufacturing characteristics. Third, using strain gauges, the deformation that results due to intercalation during charging/discharging was measured via (i) the bending of aluminum plate constraining a pouch cell, (ii) the stretching of stainless steel bands constraining a battery back and (iii) the radial expansion of a cylindrical cell. The strain gauge SOC sensors showed a piecewise linear correlation of the strain to the SOC. The SOC measurements from both thin film pressure transducer and strain methods were insensitive to discharge rate and temperature effects are readily compensated, which is an important improvement over traditional SOC estimation and modeling techniques that rely on noisy current and voltage signals. In addition to constant current cycling, both the strain gauge and pressure sensor SOC sensors were evaluated for a complex charge/discharge cycle similar to that occurring in an electric vehicle, where again the two new SOC sensor provided an accurate and robust determination of the SOC