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The State Of Charge (SOC) is the most important index in a Battery ManagementSystem (BMS) to regulate charge/discharge decisions and to ensure the battery’s safety,efficiency, and longevity. There are many methods to estimate SOC of a battery and the model based-methods exhibit higher accuracy compared to other methods. Among them the EquivalentCircuit Model (ECM)-based methods are employed in power system applications due to theirflexible nature. These models consist of a voltage source to represent Open Circuit Voltage(OCV) which depends on the SOC of the battery. The accuracy of the SOC estimation highlydepends on the adopted Equivalent Circuit Model. To accomplish accurate battery model,battery SOC should be precisely estimated. This paper investigates various types of SOCestimation methods for lithium-ion batteries in-depth in view point of Battery Energy StorageSystems (BESS). Different SOC estimation methods are compared and evaluated to assess theirsuitability under both static response and dynamic conditions.