I. Introduction
With the rapid innovation of battery technologies, lithium-ion (Li-ion) batteries have exhibited outstanding performance, in comparison with other types of batteries such as lead–acid and nickel–metal hydride batteries. Li-ion batteries offer high energy and power density, fast charge capability, no memory effect, longer cycle life, and low self-discharge rate. These outstanding features facilitate Li-ion batteries as the most promising power sources for the eco-friendly electric vehicles (EVs) [1]– [3]. Among existing EV technologies, a reliable battery management system (BMS) is a remaining design challenge, and the pivotal function of this BMS is the indication of battery state of charge (SOC). The SOC reflects the ratio of the utilizable capacity to its nominal capacity [4]. It is substantial for managing battery energy utilization efficiently, preventing the battery from being overcharged and overdischarged [5], [6]. Thus, a well-developed method is essential for accurate SOC estimation.