State of Charge Estimation for Advanced Batteries: Reduced Order Electrochemical Modeling with Error Compensation
Type of Degreethesis
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Advanced battery management systems are essential for efficient electrification of vehicles. Correct state of charge (SOC) estimates will maintain a battery within its charge limits and extend its usable life. Existing SOC algorithms for high-power Li-polymer batteries are not capable of accuracies greater than 95% in realistic operating conditions. This thesis proposes the use of a reduced order electrochemical model with error correction to achieve that accuracy target in the current range of 1-5C with various load profiles and ambient temperature (Tamb) range of 0-40°C. Hybrid electric vehicle (HEV) and electric vehicle (EV) drivers deserve to precisely know their remaining driving range and should not have to worry about premature battery pack exhaustion or failure. Coulomb counting, equivalent circuit model, and electrochemical model approaches are evaluated based on their deviation from a standard open circuit voltage curve. The equivalent circuit is a 2nd order Randle circuit and the electrochemical model is a single particle model. Order reduction was accomplished by assuming that reaction current is not influenced by the electrolyte concentration, the Butler-Volmer equation may be linearized, and ion conductivity is constant. Two methods are proposed; one based on empirical data and one based on a closed-loop feedback system. In the first method, error rates are calculated depending on operating current rate and ambient temperature and are then applied to the reduced order electrochemical model SOC estimates during constant current and pulse loading. In the second method, model SOC estimates are updated continuously by comparing the model terminal voltage estimate to battery terminal voltage measurements with a variable gain. Performance during charge-depleting and charge-sustaining modes is investigated. This thesis is the first to document SOC estimation performance of an equivalent circuit model and electrochemical model at ambient temperatures other than 25°C or room temperature. Also pioneered is the exploration of high current rates, charging efficiency computations as a function of temperature, and use of an open circuit voltage curve as a comparison tool rather than coulomb counting. The model error characterization and minimization techniques employed are novel as is the ROM feedback error compensation technique.