1 Application scenarios of large-scale energy storage systems
In order to smooth out fluctuations in output power, more and more power plants, including new energy power plants such as wind power or solar power plants, are starting to equip themselves with energy storage systems.
Independent energy storage power stations, which make a living by reselling electricity, have gradually emerged as power system reforms come into view.
A microgrid is a small power distribution network that includes distributed generation sources, electrical loads, energy storage systems, and a grid management system. To ensure the continuity and stability of power supply to the loads, each microgrid is equipped with an energy storage system.
Indoor energy storage power station
2. Differences between Energy Storage Battery Management System (ESBMS) and Power Lithium Battery Management System (BMS)
Energy storage battery management systems are very similar to power lithium battery management systems. However, power lithium battery systems are used in high-speed electric vehicles, which place higher demands on the battery's power response speed and power characteristics, SOC estimation accuracy, and the number of state parameters calculated.
Energy storage systems are extremely large in scale, and there are significant differences between centralized battery management systems and energy storage battery management systems. Here, we will only compare them with the distributed battery management system for power lithium batteries.
2.1 The position of the battery and its management system in their respective systems differs.
In an energy storage system, the energy storage battery only interacts with the energy storage converter at high voltage. The converter draws power from the AC grid to charge the battery pack; or the battery pack supplies power to the converter, and the electrical energy is converted into AC and sent to the AC grid through the converter.
In energy storage systems, the battery management system (BMS) has crucial communication relationships with the inverter and the energy storage power station's dispatch system. On one hand, the BMS sends important status information to the inverter to determine the high-voltage power interaction status; on the other hand, the BMS sends comprehensive monitoring information to the energy storage power station's dispatch system (PCS). (See diagram below.)
Basic topology of energy storage system
The BMS (Battery Management System) of an electric vehicle exchanges energy with both the electric motor and the charger at high voltage; in terms of communication, it exchanges information with the charger during the charging process; and throughout the entire application process, it has the most detailed information exchange with the vehicle controller. (See diagram below.)
Electric vehicle electrical topology
2.2 Different hardware logic structures
Energy storage management systems typically employ a two- or three-layer hardware architecture, with larger-scale systems tending towards a three-layer architecture, as shown in the diagram below.
Block diagram of a three-layer energy storage battery management system
Power lithium battery management systems typically have only one layer (centralized) or two layers (distributed), and three layers are extremely rare. Small cars primarily use a single-layer centralized battery management system. A two-layer distributed power lithium battery management system is shown in the diagram below.
Block diagram of distributed electric vehicle battery management system
Functionally, the first and second layers of the energy storage battery management system are essentially equivalent to the first-layer acquisition module and the second-layer main control module of a power lithium battery. The third layer of the energy storage battery management system is an additional layer built upon this foundation to handle the massive scale of energy storage batteries.
To use a somewhat imperfect analogy, the optimal number of subordinates for a manager is seven. If the department continues to expand to 49 people, then the manager must select one team leader from each of the seven team leaders, and then appoint a manager to oversee these seven team leaders. Exceeding an individual's capabilities can easily lead to management chaos.
Mapped to the energy storage battery management system, this management capability is the computing power of the chip and the complexity of the software program.
2.3 The communication protocols are different.
The communication between the energy storage battery management system and its internal systems mainly uses the CAN protocol, but its communication with external systems, especially the energy storage power station dispatch system (PCS), often uses the TCP/IP protocol.
The electric vehicle environment, in which power lithium batteries are used, all adopt the CAN protocol. The difference lies in the fact that the internal CAN is used between components within the battery pack, while the vehicle CAN is used between the battery pack and the whole vehicle.
2.4 The management system parameters vary greatly depending on the type of battery cells used in the energy storage power station.
For safety and economic reasons, energy storage power stations often choose lithium iron phosphate batteries when selecting lithium-ion batteries, and some even use lead-acid or lead-carbon batteries. The mainstream battery types for electric vehicles are currently lithium iron phosphate batteries and ternary lithium-ion batteries.
Different battery types have vastly different external characteristics, making battery models completely incompatible. Furthermore, the battery management system and cell parameters must have a one-to-one correspondence. Even for the same type of cell from different manufacturers, the detailed parameter settings will not be the same.
2.5 Different Threshold Setting Tendencies
Energy storage power stations generally have ample space to accommodate a large number of batteries. However, some stations are located in remote areas with difficult transportation, making large-scale battery replacement challenging. Energy storage power stations prioritize long-lasting battery cells to prevent failures. Therefore, their operating current limits are set relatively low to avoid overloading the cells. Requirements for the energy and power characteristics of the cells are not particularly high; cost-effectiveness is paramount.
Lithium-ion batteries for power applications are different. Within the limited space of a vehicle, the battery, which has been painstakingly installed, needs to be used to its fullest potential. Therefore, system parameters are designed to meet the battery's extreme limits, resulting in harsh operating conditions.
2.6 The two methods require different numbers of state parameters to be calculated.
State of Charge (SOC) is a state parameter that both systems need to calculate. However, to date, there is no unified requirement for energy storage systems regarding the specific state parameter calculation capabilities that an energy storage battery management system must possess. Furthermore, the application environment of energy storage batteries is relatively spacious and stable, making small deviations less noticeable within a large system. Therefore, the computational requirements for energy storage battery management systems are relatively lower than those for power lithium battery management systems, and consequently, the management cost per string of batteries is also lower than that of power lithium batteries.
2.7 The passive balancing conditions are relatively good for energy storage battery management systems.
Energy storage power stations urgently require efficient balancing capabilities from their management systems. Energy storage battery modules are relatively large, with multiple batteries connected in series. Significant voltage differences between individual cells will cause a decrease in the overall capacity of the storage enclosure; the more batteries connected in series, the greater the capacity loss. From an economic efficiency perspective, adequate balancing is crucial for energy storage power stations.
Furthermore, passive balancing can function more effectively due to ample space and good heat dissipation conditions. Even with a relatively large balancing current, there's no need to worry about excessive temperature rise. Low-cost passive balancing can play a significant role in energy storage power stations. Regarding the secondary use of power lithium batteries, what can retired power lithium batteries actually be used for? The reason for stipulating that power lithium batteries should be retired when they have 80% remaining capacity, besides the reduced driving range leading to a worse user experience, is that their consistency deteriorates and system reliability decreases after aging, which is a major reason for ceasing their application in high-speed vehicles. From this perspective, we can consider the general direction for the secondary use of power lithium batteries. This applies to scenarios where energy density requirements are not so high, reliability can be improved through other means, or reliability requirements are not high at all, cost is a major concern, and a desire for low-cost energy storage devices is desired.
