Iterative capacity estimation of LiFePO₄ cell over the lifecycle based on SoC estimation correction

Real time state of charge (SoC) and state of health (SoH) monitoring plays an essential role in electric vehicles, hybrid electric vehicles and generally in battery powered applications. Between these two state estimations, only the SoC has been studied rigorously until the current dates, while SoH or capacity estimation are much less referenced on the literature. Additionally, the SoC and the SoH estimation are strongly correlated by widely used coulomb counting equation and consequently wrong capacity estimation would lead to a SoC estimation error, which in turn, will lead to a further capacity estimation error. In this sense, the first job was to develop the equivalent electric model, design SoC estimator and to verify both of them experimentally. Then, different alternative techniques for estimating the capacity are analyzed, selecting the best choice considering the observability degree of the object of estimation: capacity. Then, in this paper we propose a new method for estimating the capacity, called iterative transferred charge, which adapts the current capacity estimation value based on the SoC correction made by the corresponding estimator. Finally, the developed algorithm is evaluated by comparing the capacity estimation with the reference over the life of the cell, by extensive experimental tests.