Battery State of Charge Management by Voltage Feedback Modification Niloofar Ghanbari, and Subhashish Bhattacharya Department of Electrical and Computer Engineering North Carolina State University, Raleigh, NC, USA E-mail: nghanba@ncsu.edu Abstract—Due to increase in utilization of DC loads and renewable resources which mostly produce DC powers, there has been an increasing popularity in DC microgrids. In the previous research studies, the focus is more on power balancing among parallel voltage sources of the system while balancing the State of Charge (SoC) of batteries has been overlooked. In this paper, by modifying droop control method as the basis of power sharing in the DC microgrid, the objective of SoC equalization can be achieved. DC bus voltage scheduling is the core of modification to keep the batteries SoCs balanced. First, the paper discusses about an existing droop control based SoC balancing method with voltage scheduling. Then, it proposes a new DC bus voltage scheduling to overcome the drawback of the mentioned method. The methods have been simulated in MATLAB/Simulink and a comparison study has been performed. Index Terms—DC Microgrid, Droop Control Method, State of the Charge (SoC) Balancing, DC Bus Voltage Scheduling I. I NTRODUCTION Microgrids are categorized as AC microgrids and DC mi- crogrids, according to type of sources connected. Mostly, generation stage is comprised of renewable resources, espe- cially Photovoltaic (PV) arrays and Battery Energy Storage Systems (BESSs). BESSs help renewable resources in power production, specially in their lack of generation period due to their stochastic nature. As PV arrays and BESSs produce DC powers, DC microgrids are gaining attention. [1]. In the DC microgrids utilizing renewable resources, there are lots of stochastic behavior and uncertainties. Hence, BESSs, consisting of battery units and their controllers, are utilized in the generation stage of DC microgrids to ensure power balance. Moreover, to make the system redundant, more than one BESS is required. Thus, controlling more than one battery unit should be done in an integrated form. In a DC microgrid with several generation units, there should also be predictive control to handle the transient response issues [2]. Previous research items emphasized on equal load sharing and power balance using droop control method as a well- known method in paralleling voltage sources [3]. Droop con- trol can obtain scheduled power sharing without any commu- nications. Therefore, droop control method is commonly used in DC microgrids consisting of parallel sources to schedule an arbitrary power sharing among converters. For instance, to achieve power sharing among BESSs with a reasonable accuracy, a droop control method has been proposed in [4]. However, when more than one BESS is utilized in a DC microgrid, there is possibilities that some batteries exposed to Fig. 1: Schematic diagram of the studied DC microgrid. deep-discharging or overcharging without any control on their power sharing and droop control algorithm [5]. Therefore, it is necessary to manipulate droop controller based on BESSs State of Charges (SoCs) and dynamically change the power shared between them to keep their SoCs balanced. Thus, droop control method needs to be modified to have more focus on balancing BESSs SoCs besides power sharing. For the purpose of balancing the SoC of BESS, it is demanded that the BESS with higher SoC provides more power in the discharging phase, and less power in the charging phase [6]. To address this point, some of recent research studies manipulate droop coefficients by taking BESSs SoCs into consideration [7]. Beside these methods of SoC balancing that change droop coefficients, another control method based on voltage schedul- ing has been proposed in [8] - [9]. These methods keep the SoCs at the same level, by modifying voltage reference based on BESSs SoCs. In this paper, the method of voltage scheduling presented in [9], has been discussed. Then, a novel method of SoC balancing based on DC bus voltage scheduling is proposed to overcome its drawback. The paper is organized as following. In Section II, a back- ground of droop control method and BESS SoC is presented. Section III, describes modification of droop functionality by voltage scheduling: one of the existing droop based control