Electromotive force characterization of secondary battery cells using estimated electrolyte molality

Existing methods for electromotive force (EMF) characterization that make use of readily available data, such as terminal voltage and applied current, require a significant time commitment and a constant temperature environment. Both linear interpolation and extrapolation rely on state-of-charge (SOC) calculation by Coulomb-counting which only serves to reduce the accuracy of the resulting EMF-SOC curve(s). A battery management system requires accurate EMF-SOC data for periodic recalibration otherwise it cannot apply proper charge control. This paper presents an alternative concentration-based method for EMF characterization and the key lies in the fact that molality is not influenced by temperature. A modified version of the Nernst equation and temperature-compensated open-circuit voltage measurements are used to estimate the molality when the cell is at rest. During operation, the energy into or out of the cell is mapped to changes in the estimated molality to calculate the EMF as the cell charges or discharges. The concentration-based method is verified using experimental data from valve-regulated lead-acid cells and its EMF curves are compared to those of linear interpolation and extrapolation. The proposed method has less stringent practical requirements and validation results indicate a significant improvement in accuracy and applicability over the existing methods.