AC2C-C2P-based energy storage lithium ion battery active equalization method

Abstract

The invention relates to an active equalization method for an energy storage lithium ion battery based on AC2C-C2P, and belongs to the field of battery pack equalization of a container type energy storage system. The state of charge (SOC) of the battery is used as an equalization variable, a two-value equalization strategy is adopted, and the difference of individual battery characteristics is fully considered, so that the condition of local imbalance of the battery pack is avoided, and the available capacity of the battery pack is improved. According to the method, the effectiveness and the high efficiency of the battery pack during equalization are ensured by optimizing the energy flow of the battery. In order to verify the superiority of the method, a contrast experiment is carried out, and a plurality of evaluation indexes such as standard deviation, balance degree and balance speed are adopted for analysis. Experimental results show that the provided equalization topology structure and strategy improve the equalization efficiency and precision of the battery pack, have high practical value and popularization potential, and provide an effective solution for a future battery management system.

Description

Technical Field

[0001] The invention belongs to the field of battery pack balancing of container-type energy storage systems, and relates to an active balancing method for energy storage lithium-ion batteries based on AC2C-C2P. Background Art

[0002] As the use of fossil fuels leads to more severe environmental problems, lithium batteries are widely used in energy storage systems due to their high power density, long life and low pollution. However, due to the low quality of power generation from renewable energy sources and poor power supply stability, containerized energy storage technology has gradually become a research hotspot. Containerized energy storage systems are composed of multiple battery cells connected in series and parallel, but due to the influence of production processes and the environment, battery cells have differences in internal resistance, capacity and other characteristics, resulting in decreased performance and shortened life of the battery pack. Therefore, the development of an efficient battery balancing strategy has become a key issue.

[0003] Battery balancing technology is mainly divided into two categories: passive balancing and active balancing. Passive balancing consumes excess energy through resistors, with low cost and simple design, but it will shorten the battery life. Active balancing has higher energy efficiency and balancing speed, and will become the focus of future development. Active balancing methods can be divided into technologies based on capacitors, inductors and transformers according to different balancing elements. The capacitor-based method makes the balancing current uncontrollable due to voltage differences; the inductor and transformer-based technology provides a controllable current solution with higher efficiency. In terms of energy paths, AC2AC, AC2C, DC2C, P2C and other methods have their own advantages and disadvantages. The DC2C method has high efficiency in energy transfer, but it is only applicable to a few monomers; the C2P method transfers energy from the battery to the battery pack, which is efficient but costly. At present, balancing control strategies include maximum balancing method, average value comparison method and fuzzy logic control method. The maximum balancing method achieves fast balancing by discharging the battery with the highest SOC and charging the battery with the lowest SOC, but it may cause control confusion; the average value comparison method uses the average SOC of the battery pack as a reference, which is suitable for small-scale systems, but the balancing speed is slow in large-scale systems. The fuzzy logic control method adjusts the balance through the fuzzy control system, which has good real-time performance and accuracy, but the process is complex and there is uncertainty. In order to solve the problem of slow balancing speed of the existing balancing topology structure and balancing strategy, an active balancing method for energy storage batteries based on AC2C-C2P is proposed. Summary of the invention

[0004] In view of this, an object of the present invention is to provide an active equalization method for energy storage lithium-ion batteries based on AC2C-C2P.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] An active equalization method for energy storage lithium-ion batteries based on AC2C-C2P, the method comprising the following steps:

[0007] S1: Understand the structure of lithium-ion batteries in containerized energy storage systems and build balancing circuits; use Matlab / simulink to build a battery pack balancing topology based on AC2C-C2P;

[0008] S2: Analyze the energy flow of the AC2C-C2P balancing topology when balancing the battery pack, use SOC as the balancing variable, adopt a dual-value balancing strategy, and select standard deviation as the criterion for judging balancing;

[0009] S3: Compare the balanced topology of AC2C-C2P with other topologies and analyze them using standard deviation, balance degree, and balance speed as evaluation indicators.

[0010] Further, in S1, the containerized energy storage system is composed of a battery module, a battery management system BMS, an energy storage converter PCS, a fire protection system and an air conditioning system; specifically, the containerized energy storage system is composed of LiFePO4 batteries, and the batteries are connected in series and parallel in IP12S. The specific composition structure of the batteries in the containerized energy storage system is that 260 cells are connected in series to a battery cluster, and a battery cluster is composed of 5 battery packs connected in series, and a battery pack is composed of 4 battery modules connected in series, and a battery module is composed of 13 battery cells connected in series; the rated operating voltage of the battery is 883.2V, the operating voltage range of the battery is 772.8V-993.6V, the system output voltage range is 342V-418V, the rated voltage is 380V, and the battery capacity is 1.114MWh;

[0011] A transformer is used to quickly balance the overall energy level of the battery pack. At the same time, the adjacent batteries are balanced and accurately adjusted by using the CUK circuit. The balancing model includes a battery module, a balancing circuit module, a balancing current control module, and a switch control module. The battery module uses a lithium-ion battery, and the battery parameters used for simulation are 3.7V / 3.5Ah. A balancing topology of 13 series-connected batteries based on AC2C-C2P is built in simulink. The balancing current control module consists of a Matlabfunction module and a PWM output module. The switch control module uses a MOSFET control module to turn on and off the switch according to the state of each cell in the battery pack.

[0012] Furthermore, in the S2, when the AC2C-C2P balancing topology is balancing the battery pack and the circuit works in C2P mode, there are two steady states in each switching cycle; State I: When the average SOC of the batteries at both ends of the transformer is less than the average SOC of the battery pack, the power of the batteries at both ends of the transformer will be converted into magnetic energy and stored in the primary excitation inductance of the transformer; State II: In this working state, the magnetic energy of the transformer core is converted into electrical energy, and the transformer charges the entire battery pack; then the switch repeats the operation, and as the switch is continuously turned on and off, the energy of the battery and the transformer is continuously exchanged, and finally a balance is achieved;

[0013] AC2C-C2P balancing topology When the battery pack is balanced, when the circuit works in AC2C mode, there are two steady states;

[0014] State I: When the charge of battery B1 is greater than that of battery B2, battery B1 charges inductor L1, capacitor C1 forms a loop with battery B2 and inductor L2, and capacitor C1 discharges battery B2 and inductor L2;

[0015] State II: Battery B1, inductor L1, and capacitor C1 form a closed loop, inductor L2 and battery B2 form a loop, and inductor L2 charges battery B2;

[0016] Taking SOC as the balancing variable, the standard deviation is selected as the balancing standard; the SOC of each battery cell is estimated, and the average SOC of the entire battery pack and the difference between adjacent battery cells are calculated respectively. When the SOC of the battery cells at both ends of the transformer is greater than the average SOC of the battery pack and the SOC standard deviation of the entire battery pack is greater than the balancing threshold of 0.1%, the C2P transformer balancing is turned on. When there is a difference in the SOC of adjacent cells and the SOC standard deviation of the entire battery pack is greater than the balancing threshold of 0.1%, the AC2C inductor balancing is turned on. The proposed balancing topology and balancing strategy eliminate the SOC imbalance between batteries in turn.

[0017] Furthermore, in S3, the balanced topology of AC2C-C2P is compared with other topologies under the same initial SOC distribution, and the standard deviation, balance degree and balance speed are used as evaluation indicators for analysis.

[0018] The beneficial effects of the present invention are:

[0019] 1. The present invention proposes an active balancing method for energy storage lithium-ion batteries based on AC2C-C2P. The AC2C-C2P balancing topology is used to balance adjacent single cells while completing non-adjacent single cells, analyzing the energy flow of the balancing circuit and accelerating the battery balancing speed.

[0020] 2. Taking the battery state of charge SOC as the balancing variable, a dual-value balancing strategy is proposed, which takes into account the balancing requirements between single cells and battery packs, fully considers the local imbalance caused by individual battery characteristics, and uses the proposed balancing topology and balancing strategy to eliminate the SOC imbalance between batteries in turn. The experimental results show that the proposed battery balancing method significantly improves the speed and accuracy of balancing.

[0021] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0023] Figure 1 The battery structure of the actual containerized energy storage system;

[0024] Figure 2 This is the AC2C-C2P balanced topology diagram;

[0025] Figure 3 It is the energy flow diagram of the balanced circuit;

[0026] Figure 4 It is a flow chart of the equilibrium strategy;

[0027] Figure 5 This is a comparative analysis chart of different equalization methods. DETAILED DESCRIPTION

[0028] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0029] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0030] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0031] The active equalization method for energy storage lithium-ion batteries based on AC2C-C2P of the present invention mainly includes the following three steps:

[0032] The first part is to understand the structure of lithium-ion batteries in containerized energy storage systems and build a balancing circuit; use Matlab / simulink to build a battery pack balancing topology based on AC2C-C2P;

[0033] The second part is to analyze the energy flow of the AC2C-C2P balancing topology when balancing the battery pack, using SOC as the balancing variable, adopting a dual-value balancing strategy, and selecting standard deviation as the criterion for judging balancing;

[0034] The third part is to analyze the effectiveness of the proposed equalization method, compare the AC2C-C2P equalization topology with other topologies, and use standard deviation, equalization degree, and equalization speed as evaluation indicators to analyze the efficiency and accuracy of the proposed method. The specific simulation experiment steps are as follows.

[0035] Step 1: Build based on AC2C-C2P balanced topology

[0036] For containerized energy storage systems, they are usually composed of battery modules, battery management systems (BMS), power storage converters (PCS), fire protection systems, and air conditioning systems. The containerized energy storage system is composed of LiFePO4 batteries, which are connected in series and parallel in IP12S. Figure 1The figure shows the specific structure of the battery in the containerized energy storage system. 260 cells are integrated into a battery cluster in series. A battery cluster consists of 5 battery packs in series, a battery pack consists of 4 battery modules in series, and a battery module consists of 13 cells in series. The rated operating voltage of the battery is 883.2V, and the operating voltage range of the battery is 772.8V-993.6V. The system has an output voltage range of 342V-418V with a rated voltage of 380V and a battery capacity of 1.114MWh.

[0037] like Figure 2 The figure shows the balancing topology structure diagram. The transformer is used to quickly balance the overall energy level of the battery pack. At the same time, the adjacent batteries are balanced and accurately adjusted by using the CUK circuit. The balancing model includes a battery module, a balancing circuit module, a balancing current control module, and a switch control module. The battery module uses a lithium-ion battery. The battery parameters used for simulation are 3.7V / 3.5Ah. A balancing topology structure of 13 series batteries based on AC2C-C2P is built in simulink. The balancing current control module consists of a Matalb function module and a PWM output module. The switch control module uses a MOSFET control module to turn on and off the switch according to the state of each cell in the battery pack.

[0038] Step 2: Design of equilibrium scheme based on dual-value equilibrium strategy

[0039] like Figure 3 The figure shows the energy flow diagram of the balancing circuit. When the AC2C-C2P balancing topology is balancing the battery pack, when the circuit works in C2P mode, there are two steady states in each switching cycle. State I: When the average SOC of the batteries at both ends of the transformer is less than the average SOC of the battery pack, the power of the batteries at both ends of the transformer will be converted into magnetic energy and stored in the primary excitation inductance of the transformer. State II: In this working state, the magnetic energy of the transformer core is converted into electrical energy, and the transformer charges the entire battery pack. Then the switch repeats the operation, and as the switch is continuously turned on and off, the energy of the battery and the transformer is continuously exchanged, and finally a balance is achieved.

[0040] AC2C-C2P balancing topology When the battery pack is balanced, when the circuit works in AC2C mode, there are two steady states. State I: When the power of battery B1 is greater than that of battery B2, battery B1 charges inductor L1, capacitor C1 forms a loop with battery B2 and inductor L2, and capacitor C1 discharges battery B2 and inductor L2. State II: Battery B1 forms a closed loop with inductor L1 and capacitor C1, inductor L2 forms a loop with battery B2, and inductor L2 charges battery B2.

[0041] like Figure 4The figure shows the flow chart of the balancing strategy, with SOC as the balancing variable and standard deviation as the balancing standard. The SOC of each battery cell is estimated, and the average SOC of the entire battery pack and the difference between adjacent battery cells are calculated respectively. When the SOC of the battery cells at both ends of the transformer is greater than the average SOC of the battery pack and the SOC standard deviation of the entire battery pack is greater than the balancing threshold of 0.1%, the C2P transformer balancing is turned on. When there is a difference in the SOC of adjacent cells and the SOC standard deviation of the entire battery pack is greater than the balancing threshold of 0.1%, the AC2C inductor balancing is turned on. The proposed balancing topology and balancing strategy eliminate the SOC imbalance between batteries one by one.

[0042] Step 3: Comparative Experimental Analysis

[0043] The AC2C-C2P balancing topology is compared with other topologies under the same initial SOC distribution, and the efficiency and accuracy of the proposed method are analyzed using standard deviation, balancing degree, and balancing speed as evaluation indicators. Considering factors such as the number of components and balancing speed, compared with other battery active balancing technologies, the costs of different topologies are calculated, the economic efficiency of each topology is analyzed, and their advantages and applicability in practical applications are comprehensively evaluated in combination with performance indicators.

[0044] Under static conditions, the initial SOC of these 13 batteries is first adjusted to [70%, 67%, 64%, 59%, 55%, 51%, 45%, 42%, 49%, 52%, 57%, 61%, 65%]. Figure 5 The figure shows the battery pack balancing result. From Table 1, it can be seen that the balancing scheme proposed in this paper takes about 124.3s to balance, the battery balancing degree is 98.88%, and the balancing speed is 0.222%. In contrast, the AC2C balancing topology scheme takes about 597.3s to complete the balancing, the battery balancing degree is 98.64%, and the balancing speed is 0.046%. At the same time, the C2P balancing topology scheme takes about 144.5s to balance, the battery balancing degree is 98.26%, and the balancing speed is 0.19%. Analysis shows that under the same operating conditions, the battery pack balancing time of the AC2C-C2P balancing scheme designed in this paper is significantly shortened, and the balancing accuracy and speed are improved.

[0045] Considering factors such as the number of components and balancing speed, compared with other active battery balancing technologies, the costs of different topologies are calculated, and the economic feasibility of each topology is analyzed. The cost of MOSFET is 0.2$, the cost of diode is 0.2$, the cost of inductor is 1$, the cost of capacitor is 0.8$, and the cost of transformer is 5$. n represents the number of battery cells in the battery string. It can be seen from Table 2 that the AC2AC and C2P structures have faster balancing speeds, but their component costs are higher. The AC2C topology has a lower cost, but a slower balancing speed. The AC2C-C2P balancing topology has a faster balancing speed while having a lower component cost.

[0046] Table 1

[0047]

[0048] Table 2

[0049]

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. An active equalization method for energy storage lithium-ion batteries based on AC2C-C2P, characterized in that: The method comprises the following steps: S1: Understand the structure of lithium-ion batteries in containerized energy storage systems and build balancing circuits; use Matlab / simulink to build a battery pack balancing topology based on AC2C-C2P; S2: Analyze the energy flow of the AC2C-C2P balancing topology when balancing the battery pack, use SOC as the balancing variable, adopt a dual-value balancing strategy, and select standard deviation as the criterion for judging balancing; S3: Compare the balanced topology of AC2C-C2P with other topologies and analyze them using standard deviation, balance degree, and balance speed as evaluation indicators.

2. The active equalization method for energy storage lithium-ion batteries based on AC2C-C2P according to claim 1 is characterized in that: In S1, the containerized energy storage system is composed of a battery module, a battery management system BMS, an energy storage converter PCS, a fire protection system, and an air conditioning system; specifically, the containerized energy storage system is composed of LiFePO4 batteries, and the batteries are connected in series and parallel in IP12S. The specific composition structure of the batteries in the containerized energy storage system is that 260 cells are connected in series to a battery cluster, and a battery cluster is composed of 5 battery packs connected in series, and a battery pack is composed of 4 battery modules connected in series, and a battery module is composed of 13 battery cells connected in series; the rated operating voltage of the battery is 883.2V, the operating voltage range of the battery is 772.8V-993.6V, the system output voltage range is 342V-418V, the rated voltage is 380V, and the battery capacity is 1.114MWh; A transformer is used to quickly balance the overall energy level of the battery pack. At the same time, the adjacent batteries are balanced and accurately adjusted by using the CUK circuit. The balancing model includes a battery module, a balancing circuit module, a balancing current control module, and a switch control module. The battery module uses a lithium-ion battery, and the battery parameters used for simulation are 3.7V / 3.5Ah. A balancing topology of 13 series-connected batteries based on AC2C-C2P is built in simulink. The balancing current control module consists of a Matlab function module and a PWM output module. The switch control module uses a MOSFET control module to turn on and off the switch according to the state of each cell in the battery pack.

3. The active equalization method for energy storage lithium-ion batteries based on AC2C-C2P according to claim 2 is characterized in that: In S2, when the AC2C-C2P balancing topology is balancing the battery pack and the circuit works in C2P mode, there are two steady states in each switching cycle; State I: When the average SOC of the batteries at both ends of the transformer is less than the average SOC of the battery pack, the power of the batteries at both ends of the transformer will be converted into magnetic energy and stored in the primary excitation inductance of the transformer; State II: In this working state, the magnetic energy of the transformer core is converted into electrical energy, and the transformer charges the entire battery pack; then the switch repeats the operation, and as the switch is continuously turned on and off, the energy of the battery and the transformer is continuously exchanged, and finally a balance is achieved; AC2C-C2P balancing topology When the battery pack is balanced, when the circuit works in AC2C mode, there are two steady states; State I: When the charge of battery B1 is greater than that of battery B2, battery B1 charges inductor L1, capacitor C1 forms a loop with battery B2 and inductor L2, and capacitor C1 discharges battery B2 and inductor L2; State II: Battery B1, inductor L1, and capacitor C1 form a closed loop, inductor L2 and battery B2 form a loop, and inductor L2 charges battery B2; Taking SOC as the balancing variable, the standard deviation is selected as the balancing standard; the SOC of each battery cell is estimated, and the average SOC of the entire battery pack and the difference between adjacent battery cells are calculated respectively. When the SOC of the battery cells at both ends of the transformer is greater than the average SOC of the battery pack and the SOC standard deviation of the entire battery pack is greater than the balancing threshold of 0.1%, the C2P transformer balancing is turned on. When there is a difference in the SOC of adjacent cells and the SOC standard deviation of the entire battery pack is greater than the balancing threshold of 0.1%, the AC2C inductor balancing is turned on. The proposed balancing topology and balancing strategy eliminate the SOC imbalance between batteries in turn.

4. The active equalization method for energy storage lithium-ion batteries based on AC2C-C2P according to claim 3 is characterized in that: In S3, the balanced topology of AC2C-C2P is compared with other topologies under the same initial SOC distribution, and the standard deviation, balance degree and balance speed are used as evaluation indicators for analysis.

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