A Battery Energy Storage System (BESS) mainly consists of four parts: the Battery System (BS), the Power Conversion System (PCS), the Battery Management System (BMS), and the monitoring system. In practical applications, for ease of design, management, and control, the BS, PCS, and BMS are often reassembled into a modular BESS, while the monitoring system is primarily used to monitor, manage, and control one or more modular BESS units.

1. Battery System

The battery system is the primary component for storing and releasing electrical energy in a Battery Energy Storage System (BESS). Its capacity and operational status directly influence the energy conversion capability and safety reliability of the BESS. The capacity of the battery system can be expanded through the series/parallel connection of battery cells, forming a Large Capacity Battery System (LCBS). Due to the limitations of individual battery cell voltage, specific energy and power, and charge/discharge rates, an LCBS typically consists of thousands of battery cells connected in series and parallel. A common configuration in practical development involves first forming battery modules (BM) from multiple battery cells connected in series and parallel, then connecting these modules in series to form battery strings, and finally paralleling multiple battery strings to form an LCBS. The number of series/parallel connections in the battery modules is designed for ease of management and replacement, while the number of modules in series within a string is determined by the voltage requirements, and the number of strings in parallel is based on capacity, redundancy, and operational mode requirements.

2. Power Conversion System
The PCS is a device composed of power electronic converters that connects the battery system to the AC grid, facilitating energy exchange between the BESS and external systems. As the core part of the Battery Energy Storage System (BESS), the PCS performs several key functions: charging and discharging the battery system in both grid-connected and islanded modes, switching between these modes, enabling four-quadrant operation to provide bidirectional controllable active and reactive power for system balance, and supporting advanced applications such as black start, peak shaving, power smoothing, and low voltage ride-through. Depending on the PCS topology (e.g., single-stage AC/DC, two-stage AC/DC+DC/DC, single-stage parallel, two-stage parallel, cascaded multilevel), control strategies are implemented to manage battery voltage and state of charge (SOC). The control strategy and experimental platform for the PCS are crucial aspects of this study.

3.Battery Management System

The BMS is an electronic circuit system that monitors various states of the battery system (voltage, current, temperature, SOC, health status), manages safe charging and discharging (preventing overcharge and overdischarge), handles alarms and emergency protection for faults, and optimizes the battery system's operation to ensure safe, reliable, and stable performance. The BMS is essential for effective and reliable Battery Energy Storage System (BESS) operation. Accurately estimating the SOC of the battery system and its components is a critical function of the BMS.

Currently, BESS research and development are still in the early stages, with no fully standardized system structure. The structure and capacity expansion methods of a Battery Energy Storage System (BESS) depend on the system configuration. There are two main capacity expansion approaches: increasing the capacity of a single PCS through high-voltage, high-current converters or cascaded multilevel technology, and running multiple modular BESS units in parallel. Although the first method is simpler and more suitable for high-voltage, large-capacity systems, it is limited by the development of power electronics, investment costs, and control technologies. Therefore, large-scale BESS applications often use the second method.

For the second method, based on engineering applications in power systems, BESS can be categorized into low-voltage small-capacity BESS, medium-voltage large-capacity BESS, and high-voltage ultra-large-capacity BESS. Low-voltage small-capacity BESS, connected directly to a 400V AC grid, typically have a rated power of 500kW or less and discharge durations of 1-4 hours, suitable for microgrids, residential or building energy storage, and small renewable energy grid integration. Medium-voltage large-capacity BESS, formed by paralleling multiple modular BESS units and connected to a 10kV or 35kV grid via step-up equipment, have rated power up to 10MW and discharge durations of 1-4 hours, suitable for power quality management, peak shaving, backup power, and renewable energy integration. High-voltage ultra-large-capacity BESS, formed by paralleling multiple medium-voltage BESS units and connecting them to a 35kV or 110kV grid via high-voltage step-up equipment, have rated power above 10MW and discharge durations of 15 minutes to 6 hours, suitable for peak shaving, grid balancing, backup power, and large-scale renewable energy integration.

Comparing these BESS structures shows that capacity expansion fundamentally involves paralleling multiple modular BESS units, offering linear scalability, plug-and-play capability, and strong maintainability. This flexibility allows for the creation of large-scale distributed energy storage systems (D-BESS), enhancing system reliability. Thus, paralleling modular BESS units is an effective way to achieve large-capacity BESS.