Battery state of charge estimation solution based on optimized Ah counting and online calibration strategy for electric vehicle

Abstract

Abstract State of charge (SOC) estimation is critical for electric vehicles (EVs); the typical solution is the ampere-hour (Ah) counting strategy + open circuit voltage (OCV) strategy as they are straightforward and easy to implement. However, this solution makes a significant SOC estimation error if the driver needs to drive long distances or in winter. This article aims to optimize the Ah counting strategy and propose an online SOC calibration strategy. For the former, we evaluate the effects of temperature, initial SOC, and current on the Coulomb efficiency and the impact of temperature and discharge current on the battery capacity and take them into account when estimating the battery SOC; for the latter, we conduct theoretical analysis and argue that after a while of small-current fluctuations in the battery, the OCV of the battery can be obtained based on the battery voltage, current, and direct current (DC) resistance, and can be calibrated online. We designed experiments to validate the proposed strategy. The experimental results show that the optimized Ah counting strategy does not pull away from the standard Ah counting strategy at room temperature or high temperature, which is because the effects of the Coulombic efficiency and the battery capacity can be canceled out, but the optimized Ah counting strategy has a better performance at low temperature, and vice versa for standard Ah counting performs poorly; for SOC online calibration, the OCV estimated online by the proposed strategy differs from the reference OCV by only 2 mV, and its performance is excellent. The solution proposed in this article can be applied to EVs to obtain better performance.