Modeling of Voltage Hysteresis and Relaxation for HEV NiMH Battery

SOC (State of Charge) estimation based battery management is essential for HEV (Hybrid Electric Vehicle) applications. SOC can be estimated by the relationship between OCV (Open Circuit Voltage) and SOC, which is based on Nernst equation, in combination with coulomb counting. However voltage hysteresis and relaxation make the measurement of OCV difficult. Measured OCV after charge (discharge) is higher (lower) than estimated OCV by Nernst equation, and is slowly relaxed with time constant from few minutes to few hours. In this paper, to express voltage hysteresis and relaxation, three layers model concerning Nickel active materials is proposed. A voltage gap between surface and inside of Nickel active materials is introduced, and voltage hysteresis is modeled as surface partial battery decides OCV of the whole battery. Voltage relaxation is also expressed as equalization between surface layer and relaxation layer with higher internal resistance. The statical and dynamical behavior of proposed model is confirmed through the experiments of 7.2V 6.5Ah NiMH battery module.