State of charge Estimation for NiMH Battery based on electromotive force method

State of charge (SOC) estimation of NiMH battery is a key technology in electric vehicle/hybrid electric vehicle (EV/HEV). In this study, a relationship between SOC and electromotive force (EMF) is obtained based on the study of discharge experiments at different SOC. In addition, equivalent circuit model is used to estimate the SOC of NiMH battery. The analysis of experiment data shows a relation between EMF and SOC. Then, according to the analysis of the equivalent circuit, an equation between EMF and terminal voltage is built. Therefore, SOC can be estimated by terminal voltage. Two different tests that include constant discharge and variable current discharge are undertaken to verify precision of the SOC estimation method. The result of two tests show the method is suitable for NiMH battery SOC forecast in static or steady state.