BMS, EMS and PCS: the three control systems in battery storage

Awareness of climate change and the harmful effects of conventional energy sources such as coal and oil continues to grow. At the same time, falling costs and improving efficiency make renewable energy more attractive to investors and consumers, supporting steadily rising global demand. In the context described by this April 2024 edition, renewables account for approximately 28% of global electricity generation, including hydro, wind, solar and biomass. China is expected to contribute nearly 60% of worldwide additions to renewable generating capacity through 2028.

One challenge in expanding renewable energy is continuity of generation. Solar and wind output depend on weather and can fluctuate substantially. Storage therefore plays an important role: it stores energy when supply is available and releases it during periods of high demand, helping improve power-system stability and reliability while reducing energy costs and emissions.

Against this background, efficient storage technologies and safe, economical products are developing rapidly. A complete electrochemical storage system primarily comprises batteries, a battery management system (BMS), an energy management system (EMS), a power conversion system (PCS) and other electrical equipment.

The following sections explain the three frequently discussed storage-control modules, sometimes called the 3S systems: BMS, EMS and PCS.

BMS performs the sensing role, monitoring, evaluating, protecting and balancing batteries. EMS performs the decision-making role, including data acquisition, network monitoring and energy dispatch. PCS performs the execution role, controlling battery charging and discharging and converting between direct and alternating current.

BMS: battery management system

A battery management system monitors battery condition and works with the cells as part of the complete battery system. It is sometimes described as the battery's caretaker or manager.

BMS measures basic parameters including voltage, current and temperature, helping prevent overcharge and excessive discharge and supporting battery life.

It also calculates and analyses state of charge (SOC) and state of health (SOH), reporting abnormal conditions promptly.

Many BMS designs use a three-level architecture, with hardware divided into subordinate monitoring units, master controllers and an overall controller.

Lower level: cell monitoring

The subordinate battery monitoring unit (BMU) manages individual cells. Battery-monitoring chips and associated circuits acquire cell information, calculate and analyse SOC and SOH, provide active cell balancing and report abnormal cell conditions to the master controller.

Middle level: battery-group management

The master battery control unit (BCU) collects cell information from the BMUs and other battery-group measurements. It calculates and analyses SOC and SOH at battery-group level.

Upper level: cluster coordination

The overall controller manages the battery cluster. It coordinates the internal system and exchanges information with EMS and PCS, controlling BMS operation in response to external requests.

EMS: energy management system

EMS is a central component of the complete storage system.

It is responsible for operating strategies that affect battery degradation and cycle life and therefore storage economics. It also monitors faults and abnormal conditions so that equipment can be protected promptly and safely.

EMS is commonly described in terms of equipment, communication and application layers. The more detailed breakdown below also identifies an information layer.

Equipment layer: supported by energy acquisition and conversion equipment, including PCS and BMS.

Communication layer: links, protocols, transmission and associated communications functions.

Information layer: caching middleware, databases and servers. The database processes and stores data, records real-time and important historical information and supports historical queries.

Application layer: apps and web interfaces provide visual monitoring and operating controls. Functions include energy-conversion decisions, energy-data transmission and acquisition, real-time monitoring and control, O&M analysis, power and energy visualisation and remote control.

Main EMS functions

System overview: the main monitoring page displays charging and discharging energy, real-time power, SOC, revenue, energy diagrams and operating-power charts.

Equipment monitoring: real-time operating information and controls for equipment including PCS, BMS, air conditioning, meters, intelligent circuit breakers, fire-control equipment and sensors.

Operating returns: storage revenue and energy information, which are particularly important to owners.

Fault alarms: a consolidated view of equipment alarms, searchable by time, status and severity.

Statistical analysis: historical equipment operating data and reports, with data export.

Energy management: the core EMS function configures storage strategies in manual and automatic modes for commissioning, inspection, routine operation and maintenance.

PCS: power conversion system

PCS, also described as a bidirectional storage inverter, enables energy to flow in both directions between the storage system and the grid. It controls battery charging and discharging and converts DC to AC and AC to DC.

A PCS includes hardware such as insulated-gate bipolar transistors (IGBTs), printed circuit boards, wires and cables. Its functions include smoothing power, exchanging information and providing protection. PCS performance determines output power quality and dynamic behaviour and can materially affect battery life.

PCS application examples in this edition

For storage power stations, the article describes configurations above 10 MW using cascaded multilevel topologies and IGBT modules. Multiple converters are installed in containers and can operate in parallel, with a transformer stepping up voltage for grid connection.

For centralised PCS above 250 kW, the edition describes widespread use of two-level topologies and modular IGBT designs. These use relatively few power devices, with individual units reaching megawatt scale and demanding high reliability.

For C&I PCS below 250 kW, the edition describes three-level topologies combined with distributed PV for self-consumption and operation around peak/off-peak electricity-price differences.

For household PCS below 10 kW, applications include pairing with residential PV for emergency power and electricity-bill management, with particular attention to safety and noise. As the market expands, PCS is expected to move beyond simple conversion toward greater integration. Software development, upgrades and optimisation can support intelligent control and safety functions and improve the use of storage within the grid.

In summary, the battery provides condition information to BMS, which shares it with EMS and PCS. EMS sends instructions to PCS and BMS based on optimisation and dispatch decisions, controlling charging and discharging of cells and battery groups. The three systems communicate, collect information and report their status so that storage can operate efficiently, accurately and safely according to the selected strategy.

Daqin Shuneng's DH200Y combines BMS and EMS, removing the separate communication process between those modules. The company states that this makes strategy execution more efficient and accurate and eliminates faults in that inter-module communication path.

China's carbon-peaking and carbon-neutrality objectives, together with development across the supply chain, are supporting the growth of large-scale storage markets.

As the market grows, residential, large power-system and C&I storage applications are becoming more diverse and can complement each other. The article sees broad prospects for new storage development, with electrochemical storage among the fastest-growing and more consistently developing fields, and expects wider adoption.