Dynamic electric behavior and open-circuit-voltage modeling of LiFePO4-based lithium ion secondary batteries

Accurate battery modeling is one of the key factors in battery system design process and operation as well. Therefore, the knowledge of the distinct electric characteristics of the battery cells is mandatory. This work gives insight to the electric characteristics of lithium ion batteries (Li-ion) comprising LiFePO4-based cathode active materials with emphasis on their specific open-circuit-voltage (OCV) characteristics including hysteresis and special OCV recovery effects, which last for several minutes or even hours after a current load is interrupted. These effects are elucidated incorporating OCV measurement data of high power cells. Simple empiric models are derived and used in a model-based state estimation algorithm. The complete battery model includes an impedance model, a hysteresis model and an OCV recovery model part. The introduced model enables the assessment of the cells’ state-of-charge (SOC) precisely using model-based state estimation approaches.