Hybrid battery pack active equalization method

Abstract

The invention discloses a hybrid battery pack active equalization method, which comprises the following steps of: for intra-pack equalization, connecting n single batteries in series, and connecting a bidirectional Cuk equalizer between every two batteries; step A-3, the SOC state of the next moment is obtained through the balance system model; for component equalization, n single batteries are connected in series to obtain battery packs, and one side of each battery pack is connected with one bidirectional Cuk converter in parallel; obtaining the SOC change process of the ith battery pack at the next moment; and finally, realizing intra-group equalization through a bidirectional Cuk equalizer by taking the SOC of the single battery at the (k + 1) moment as an equalization target, and meanwhile, realizing inter-group equalization through a bidirectional Cuk converter and a switch selection module by taking the SOC of the battery pack at the (k + 1) moment as an equalization target. According to the method, the equalization efficiency is greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery balancing control, and in particular to a hybrid battery pack active balancing method. Background Art

[0002] In order to alleviate the energy crisis and environmental pollution, sustainable energy has attracted more and more attention. Lithium batteries have the characteristics of high energy density, low self-discharge rate, zero memory effect, and excellent performance. They have been widely used in many fields such as electric vehicles and energy storage systems. Since the voltage of a single battery is inherently limited by its electrochemical characteristics, in order to meet the requirements of higher voltage and power in practical applications, a large number of batteries are usually connected in series and parallel. However, differences in battery manufacturing processes and usage environments can lead to an imbalance in the state of charge (SOC) between batteries. The state of charge refers to the remaining available capacity of the battery. Since the charging and discharging process must be limited by the upper and lower limits of the SOC, the batteries with the highest and lowest SOC limit the available capacity of the battery pack. In addition, the charging imbalance problem of series-connected batteries may lead to overcharging and over-discharging, shortening the battery life, and even causing safety accidents such as fire and explosion. Therefore, battery balancing technology for series-connected batteries is an important way to prevent these phenomena.

[0003] At present, battery balancing methods are mainly divided into two categories: passive balancing (i.e., dissipative balancing) and active balancing (i.e., non-dissipative balancing). Compared with passive balancing methods, active balancing methods have higher energy efficiency and shorter balancing time. According to the energy balancing path and the topological structure of the balancer, the active balancing structure can be divided into AC2C balancing (adjacent Cell-to-Cell), DC2C balancing (direct Cell-to-Cell), MC2MC balancing (multiple batteries to multiple batteries), C2P / P2C balancing (Cell-to-Pack or Pack-to-Cell) and modular hybrid balancing. In these technical solutions, faster balancing rates and higher energy conversion rates are accompanied by larger structural volumes and higher costs. However, there is currently almost no single structure that can simultaneously have the advantages of fast balancing rates, high energy conversion rates, small volume and mass, and low component costs. The modular hybrid balancing solution can combine the characteristics of various structures to meet more battery balancing scenarios. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention proposes a hybrid battery pack active balancing method, which combines the characteristics of the MC2MC and AC2C structures, and adopts a multi-agent consensus algorithm in the inter-group balancing, so that the overall performance of the battery pack balancing system is more comprehensive. The balancing efficiency is greatly improved under the premise of adding a small amount of inter-group balancer cost, and can meet the SOC balancing of the battery in three conditions: charging, discharging, and static state.

[0005] In order to solve the above technical problems, the technical solution of the present invention is:

[0006] A hybrid battery pack active balancing method comprises the following steps:

[0007] A. Balance within the group

[0008] Step A-1, connecting n single cells in series, and connecting a bidirectional Cuk balancer between every two cells, and the conduction circuit controls the MOSFET in the bidirectional Cuk balancer to alternately switch to adjust the balancing current;

[0009] Step A-2: Construct a balancing system model of a battery pack connected in series consisting of n single cells:

[0010]

[0011] Where x i (1≤i≤n) is the SOC of the i-th single cell, u j (1≤j≤n-1) is the jth equalizer I eqj The balancing current, d i The battery terminal current I flowing from other power sources or going to other loads S

[0012] Step A-3, obtaining the SOC state at the next moment through the balance system model;

[0013] B. Balance between groups

[0014] Step B-1, connect n single cells in series to obtain a battery pack, connect a bidirectional Cuk converter in parallel on one side of each battery pack, and connect to the energy bus through a switch selection module, the bidirectional Cuk converter corresponding to each battery pack is respectively connected to four bidirectional reverse-connected MOSFET switches, and connected to both ends of two bus capacitors, and the charge and discharge state of each battery pack can be selected by selecting the switch;

[0015] Step B-2: Assume that the number of overcharged batteries is X and the discharge rate is r a , the number of undercharged batteries is Y, and the charging rate is r b According to the law of conservation of energy, the system should satisfy

[0016] X·r a =Y·r b

[0017] During the balancing process, the maximum SOC change per unit time interval is r a or b , the maximum battery SOC change rate is represented by R, and the maximum balancing rate of the two groups of balancers is as follows:

[0018]

[0019] Let x i (k) is the total SOC of n cells in the ith battery pack at time k. The SOC change process of the ith battery pack at the next moment is as follows:

[0020]

[0021] The Comp(u,v) function is a comparison function that compares the number of overcharged battery groups and undercharged battery groups:

[0022]

[0023] Among them, δ (δ < < 1) is the convergence radius, denoted by N eq is the minimum number of equilibrium cycles for the system to reach equilibrium;

[0024] C. Taking the SOC of the single battery at time k+1 as the balancing target, the bidirectional Cuk equalizer is used to achieve intra-group balancing. At the same time, taking the SOC of the battery group at time k+1 as the balancing target, the bidirectional Cuk converter and the switch selection module are used to achieve inter-group balancing.

[0025] Preferably, the bidirectional Cuk equalizer includes two uncoupled inductors L1 and L2, an energy transfer capacitor C and two symmetrically structured MOSFET tubes Q1 and Q2.

[0026] Preferably, in step A-1, the circuit controls the alternating switching of the MOSFET by controlling the duty cycle of the PWM signal to adjust the balanced current.

[0027] Preferably, the method for obtaining the duty cycle is as follows:

[0028] Collect the voltage V of each single battery separately B and the capacitor voltage V in the bidirectional Cuk equalizer C , determine the switching period as T S and the balancing current I eq , the duty cycle formula corresponding to the controlled balancing current of the single cells on both sides of the bidirectional Cuk balancer is calculated as follows:

[0029]

[0030] Preferably, the method for constructing the equilibrium system model is:

[0031] First, n-1 bidirectional Cuk equalizers are needed. The overall topology can be represented by an undirected graph G = (ν, ε), where ν = {Βat1, Βat2, ..., Βatn} represents a node set, Represents the edge set. According to the undirected graph, the system's association matrix H = [h ij ]∈R n×(n-1) ;

[0032] For the i-th battery, the terminal current at time k is:

[0033]

[0034] The relationship between SOC at different times and the previous time can be obtained by using the ampere-hour integration method:

[0035]

[0036] Where η represents the coulombic efficiency and Q represents the battery capacity.

[0037] Discretization of the ampere-hour integration method formula yields:

[0038]

[0039] Thus, the equilibrium system model is obtained by the correlation matrix H, the terminal current at time k and the discretization formula of the ampere-hour integration method.

[0040] Preferably, the method for obtaining the SOC state at the next moment through the balance system model is:

[0041] The balancing system model is designed according to the multi-agent consensus algorithm. For the i-th converter, its balancing current is designed as

[0042] u i (k) = α(x i (k)-x i+1 (k))

[0043] Where α is a constant, satisfying α<Q / Tη. Taking the above formula as the system input, the equilibrium system model can be expressed as:

[0044] x(k+1)=x(k)-k1Hu(k)-B(k)

[0045] u(k)=αH T x(k)

[0046] where x(k)=[x1(k),x2(k),…,x n (k)] T ∈R n is the estimated SOC value of all single cells at time k; u(k)=[u1(k),u2(k),…,u n-1 (k)] T ∈R n-1is the balancing current of all cells at time k, where k1 = ηT / Q; B(k) = [B1(k), B2(k)…, B n (k)] T =[λd1(k),λd2(k),…,λd n (k)] T is the external interference term;

[0047] Thus, combined with the Laplace matrix L = HH T , we get x(k+1) refers to the SOC at the next moment:

[0048]

[0049] Preferably, the bidirectional Cuk converter comprises two inductors and two capacitors which are symmetrically distributed.

[0050] As a preference, the switch selection module is specifically as follows: assuming that the four switches corresponding to the i-th battery pack are S 4i-3 , S 4i-2 , S 4i-1 and S 4i When the battery is in a static state, neither discharging nor charging, all four switches are closed; when the battery pack is in a discharging state, switch S is turned on. 4i-3 and switch S 4i , turn off switch S 4i-2 and switch S 4i-1 ; When the battery pack is in charging state, turn on switch S 4i-2 and switch S 4i-1 , turn off switch S 4i-3 and switch S 4i .

[0051] Preferably, in the formula for the SOC change process of the i-th battery pack at the next moment, the Comp(u,v) function is a comparison function for comparing the number of overcharged battery packs and undercharged battery packs:

[0052]

[0053] Among them, δ (δ<<1) is the convergence radius.

[0054] As a preference, during the SOC change process of the i-th battery pack at the next moment, record N eq is the minimum number of equilibrium cycles when the system reaches equilibrium, then the equilibrium time of the system is T eq We can get:

[0055] T eq =N eq ×τ.

[0056] The present invention has the following characteristics and beneficial effects:

[0057] Using the above technical solution, a hybrid battery pack balancing solution combining intra-group balancing based on consistency control and bus-type inter-group balancing is proposed. The proposed solution can control the structure scale and component cost while taking into account the balancing efficiency. The overall balancing system can be divided into two parts: intra-group balancing and inter-group balancing.

[0058] Intra-group balancing refers to measures taken to address the SOC imbalance problem between single cells in a battery pack. In the present invention, the single cells in the battery pack are connected in series, and an improved bidirectional Cuk equalizer is installed between each two adjacent cells. The equalizer can be driven by a PWM signal, which controls the alternating switching of the MOSFET inside the equalizer to adjust the balancing current, thereby realizing energy conversion between two adjacent single cells. From the perspective of graph theory, each single cell in a group of series-connected batteries is regarded as a node, and the equalizer connecting the single cells is regarded as an edge, and the topology can be regarded as an undirected graph. Through the above analysis, a battery pack can be regarded as a multi-agent system, and the battery SOC balancing problem can be equivalent to the consistency problem. Therefore, in the present invention, a multi-agent consistency algorithm is used to solve the problem of inconsistent SOC between single cells in the battery pack. The beneficial effect is that compared with the structure of incorporating all single cells into a bus-type distribution, the solution adopted by the present invention requires fewer components, the cost is much lower than the former, and it is sufficient to complete the SOC balance of the batteries in the group during a single charge and discharge process.

[0059] Inter-group balancing refers to measures taken to address the problem of overall remaining power imbalance between battery groups. As a typical multi-battery to multi-battery balancing structure, the bus-type balancing solution can simultaneously select batteries at any position and number to charge the remaining batteries at any position and number through the bus. This method is simple and efficient, with a fast balancing rate and less energy loss. However, if all single batteries are taken as the main body and bus-type balancing is used uniformly, its huge circuit volume and mass, as well as the extremely high cost of components, are not suitable for most scenarios. The present invention only uses the bus-type balancing circuit for inter-group balancing, and its beneficial effect is that while it can also greatly improve the balancing efficiency of the overall balancing system, it greatly saves component costs. And the bus-type structure adopted is different from the traditional bidirectional Cuk converter, but adopts a combination of converter and switch selection, and replaces part of the large inductance of the bidirectional Cuk converter with a low-performance bidirectional reverse MOSFET switch, thereby further compressing the cost of components. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0061] Figure 1 Schematic diagram of the structure of a battery pack balancing circuit in an embodiment of the present invention.

[0062] Figure 2 Schematic diagram of the circuit structure of the improved bidirectional Cuk equalizer within the group in this embodiment.

[0063] Figure 3 It is an equivalent undirected graph of the series structure topology in this embodiment.

[0064] Figure 4 Schematic diagram of the consistency control loop in this embodiment.

[0065] Figure 5 Schematic diagram of the structure of the inter-group bus type equalization circuit in this embodiment.

[0066] Figure 6 Schematic diagram of the structure of a bidirectional Cuk equalizer including a controllable switch module in this embodiment.

[0067] Figure 7 This is a circuit simulation verification effect diagram of the present invention. DETAILED DESCRIPTION

[0068] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0069] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings.

[0070] On the contrary, the present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims. Further, in order to make the public have a better understanding of the present invention, some specific details are described in detail in the detailed description of the present invention below. Those skilled in the art can fully understand the present invention without the description of these details.

[0071] The present invention provides a hybrid battery pack active balancing method, such as Figure 4 As shown, the following steps are included:

[0072] A. Balance within the group

[0073] Step A-1, connect n single cells in series, and connect a bidirectional Cuk balancer between every two cells. The conduction circuit adjusts the balancing current by controlling the MOSFET in the bidirectional Cuk balancer to switch alternately.

[0074] Specifically, Figure 1 and Figure 3 As shown, in this embodiment, four single cells are connected in series to form a series structure, which can also be called a chain structure. An improved bidirectional Cuk equalizer is connected between every two cells. Figure 2 As shown, the bidirectional Cuk equalizer is a symmetrical structure composed of two uncoupled inductors L1 and L2, an energy transfer capacitor C and two MOSFET tubes Q1 and Q2. Specifically, the drain of Q1 is connected to one end of the inductor L1 and the capacitor C, the other end of the inductor L1 is connected to the positive electrode of the single battery on one side, the source of Q1 is connected to the negative electrode of the single battery on one side, the source of Q2 is connected to one end of the inductor L2 and the other end of the capacitor C; the other end of the inductor L2 is connected to the negative electrode of the single battery on the other side, the drain of Q2 is connected to the positive electrode of the single battery on the other side, and the source of Q1 is connected to the drain of Q2.

[0075] Furthermore, in this embodiment, the duty cycle of the PWM signal is controlled to control the alternating switching of the MOSFET to adjust the balanced current.

[0076] Specifically, the method for obtaining the duty cycle is as follows:

[0077] Collect the voltage V of each single battery separately B and the capacitor voltage V in the bidirectional Cuk equalizer C , determine the switching period as T S and the balancing current I eq , the duty cycle formula corresponding to the controlled balancing current of the single cells on both sides of the bidirectional Cuk balancer is calculated as follows:

[0078]

[0079] Step A-2: Construct a balancing system model of a battery pack connected in series consisting of n single cells:

[0080]

[0081] Where x i (1≤i≤n) is the SOC of the i-th single cell, u j (1≤j≤n-1) is the jth equalizer I eqj The balancing current, d i The battery terminal current I flowing from other power sources or going to other loads S .

[0082] Specifically, topological analysis. Figure 3 As shown, for a series structure consisting of n single cells, n-1 bidirectional Cuk equalizers are required, and the overall topology can be represented by an undirected graph G = (ν, ε), where ν = {Βat1, Βat2, ..., Βatn} represents a node set, Represents the edge set. According to the undirected graph, the system's association matrix H = [h ij ]∈R n×(n-1) , through L(G) = HH T The relationship further leads to the Laplace matrix L(G)∈R of the graph n×n as follows:

[0083]

[0084] In this embodiment, the number of single cells in the battery pack is 4. Obviously, the Laplace matrix here is as follows:

[0085]

[0086] The above matrix will be used in subsequent derivations.

[0087] Furthermore, for the ith battery, the terminal current at time k is:

[0088]

[0089] When the battery is discharging, the current is negative, and when it is charging, the current is positive. S (k) is the current discharged from the external load or charged by the power supply to the battery pack. The current passing through each battery is the same and remains constant in each sampling interval.

[0090] The SOC of each single cell is defined as the ratio of the remaining capacity to its nominal capacity. Assuming that the total capacity of each single cell is the same, the relationship between the SOC at different times and the previous time can be obtained by using the ampere-hour integration method:

[0091]

[0092] Where η represents the coulomb efficiency and Q represents the battery capacity. Discretizing it, we can get the following formula:

[0093]

[0094] Where T is the sampling time, which is much longer than the switching period T S .

[0095] Thus, the equilibrium system model is obtained by the correlation matrix H, the terminal current at time k and the discretization formula of the ampere-hour integration method. The equilibrium system model of a series battery pack consisting of n single cells is:

[0096]

[0097] Where x i (1≤i≤n) is the SOC of the i-th single cell, u j (1≤j≤n-1) is the jth equalizer I eqj The balancing current, d i The battery terminal current I flowing from other power sources or going to other loads S .

[0098] Step A-3: Obtain the SOC state at the next moment through the balancing system model.

[0099] Specifically, this embodiment mainly focuses on illustrating the feasibility of the equalization system constructed by the present invention. Therefore, the algorithm is based on the assumption that the SOC of the single battery in the series battery pack is known. SOC estimation is a major topic in battery research, and the SOC estimation method will not be described in detail here. The control loop of the system is as follows Figure 4 shown.

[0100] The balancing system model is designed according to the multi-agent consensus algorithm. For the i-th converter, its balancing current is designed as

[0101] u i (k) = α(x i (k)-x i+1 (k))

[0102] Where α is a constant, satisfying α<Q / Tη. Taking the above formula as the system input, the equilibrium system model can be expressed as:

[0103] x(k+1)=x(k)-k1Hu(k)-B(k)

[0104] u(k)=αH T x(k)

[0105] where x(k)=[x1(k),x2(k),…,x n (k)] T ∈R n is the estimated SOC value of all single cells at time k; u(k)=[u1(k),u2(k),…,u n-1 (k)] T ∈R n-1 is the balancing current of all cells at time k, where k1 = ηT / Q; B(k) = [B1(k), B2(k)…, B n (k)] T =[λd1(k),λd2(k),…,λd n (k)]T is the external interference term;

[0106] Thus, combined with the Laplace matrix L = HH T , we get x(k+1) refers to the SOC at the next moment:

[0107]

[0108] B. Inter-group balancing: balancing the SOC differences between the three series-connected battery groups in the embodiment.

[0109] Step B-1, connect n single cells in series to obtain a battery pack, connect a bidirectional Cuk converter in parallel on one side of each battery pack, and connect to the energy bus through a switch selection module, the bidirectional Cuk converter corresponding to each battery pack is respectively connected to four bidirectional reverse-connected MOSFET switches, and connected to both ends of two bus capacitors, and the charge and discharge state of each battery pack can be selected by selecting the switch.

[0110] The principle of bus balancing is to transfer and store the excess power of the overcharged battery to the energy bus through the converter connected between the battery and the energy bus (usually using a large capacitor as the energy storage medium), and then transfer the energy on the bus to the undercharged battery through the converter to achieve energy redistribution. This balancing structure can achieve multi-battery to multi-battery balancing, which can significantly reduce the balancing path of the battery balancing system. The overall structure of the system is shown in the figure below. Figure 5 shown.

[0111] In this embodiment, an improved bidirectional Cuk converter is connected in parallel to one side of each battery pack and connected to the energy bus through a switch selection module. Figure 6 As shown in Figure 1, the converter contains two inductors and two capacitors that are symmetrically distributed. The MOSFET tube is also controlled by controlling the duty cycle of the PWM signal. The converter corresponding to each battery pack is connected to four bidirectional reverse-connected MOSFET switches and connected to the two bus capacitors. The charge and discharge state of each battery pack can be selected by selecting the switch. Let the four switches corresponding to the i-th battery pack be S 4i-3 , S 4i-2 , S 4i-1 and S 4i When the battery is in a static state, neither discharging nor charging, all four switches are closed; when the battery pack is in a discharging state, switch S is turned on. 4i-3 and switch S 4i , turn off switch S 4i-2 and switch S 4i-1 ; When the battery pack is in charging state, turn on switch S 4i-2 and switch S 4i-1 , turn off switch S4i-3 and switch S 4i .

[0112] Step B-2: Assume that the number of overcharged batteries is X and the discharge rate is r a , the number of undercharged batteries is Y, and the charging rate is r b According to the law of conservation of energy, the system should satisfy

[0113] X·r a =Y·r b

[0114] During the balancing process, the maximum SOC change per unit time interval is r a or b , the maximum battery SOC change rate is represented by R, and the maximum balancing rate of the two groups of balancers is as follows:

[0115]

[0116] Construction of mathematical model of bus system.

[0117] Let x i (k) is the SOC of the ith battery pack at time k. The SOC change process of the system is as follows:

[0118]

[0119] definition

[0120]

[0121] The Comp(u,v) function is a comparison function that compares the number of overcharged battery groups and undercharged battery groups.

[0122]

[0123] Where δ (δ<<1) is the convergence radius. eq is the minimum number of equilibrium cycles when the system reaches equilibrium. Then the equilibrium time of the system is T eq Available

[0124] T eq =N eq ×τ

[0125] C. Taking the SOC of the single battery at time k+1 as the balancing target, the intra-group balancing is achieved through the bidirectional Cuk equalizer. At the same time, taking the SOC of the battery group at time k+1 as the balancing target, the inter-group balancing is achieved through the bidirectional Cuk converter and the switch selection module.

[0126] Specifically, in this embodiment, equalization verification is performed through simulation.

[0127] For this embodiment, a hardware circuit model is built to carry out simulation experiments, and the three states of the battery system, namely, charging state, external discharge state and static state, are verified. The different situations of not adding bus-type inter-group balancing and adding bus-type inter-group balancing in the three states are recorded respectively, and the differences between the two situations are compared.

[0128] Specifications of the Cuk bidirectional converter within the group: inductance 100μH, capacitance 470μF.

[0129] Specifications of the inter-group bus converter: inductance 9600μH, conversion capacitor 33μF, bus capacitor 33μF.

[0130] The initial SOC value of the single cell in the charging state is set to 10%, 11%, 12%, 13%, 16%, 17%, 18%, 19%, 22%, 23%, 24%, 25%; the initial SOC value of the single cell in the discharging state is set to 90%, 89%, 88%, 87%, 84%, 83%, 82%, 81%, 78%, 77%, 76%, 75%; the initial SOC value of the single cell in the static state is set to 57%, 56%, 55%, 54%, 51%, 50%, 49%, 48%, 45%, 44%, 43%, 42%. The sampling time is set to 50μs and the running time is 200s.

[0131] like Figure 7 As shown, Figure 7 (a) Figure 7 (b) and Figure 7 (c) respectively represent the SOC curves of the 12 cells in the embodiment under the three states of charging, discharging and resting when the intra-group balancing is performed independently. It can be observed that although the multi-agent system in each battery pack has played a role in balancing the SOC curves of the four cells in the group, the SOC differences between the three battery packs cannot be balanced, and the overall efficiency of balancing is low and takes a long time.

[0132] Figure 7 (d) Figure 7 (e) and Figure 7 (f) shows the SOC curves of 12 cells in the three states of charging, discharging and resting after adding bus-type inter-group balancing. Comparing the three groups of curves (a), (b) and (c), this improvement enables the SOC difference between battery groups to be quickly balanced and the balancing efficiency is greatly improved. Figure 7 (d), 7(e), and 7(f), when the three battery groups are in the three states of charging, discharging, and resting, the maximum SOC difference is reduced to less than 2% at 45s, 65s, and 62s, respectively, and the overall balancing work has been basically completed. Then the curves enter a slow convergence state and gradually eliminate the SOC difference.

[0133] In summary, the feasibility of the present invention can be verified through simulation experiments of the embodiments. The proposed hybrid battery pack balancing technology solution has a higher balancing efficiency with less increase in component costs, and can support battery SOC balancing in a static state.

[0134] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions and variations of these embodiments including components are made without departing from the principles and spirit of the present invention, and still fall within the scope of protection of the present invention.

Claims

1. A hybrid battery pack active balancing method, characterized in that: The steps include: A. Balance within the group Step A-1, connecting n single cells in series, and connecting a bidirectional Cuk balancer between every two cells, and the conduction circuit controls the MOSFET in the bidirectional Cuk balancer to alternately switch to adjust the balancing current; Step A-2: Construct a balancing system model of a battery pack connected in series consisting of n single cells: Where x i (1≤i≤n) is the SOC of the i-th single cell, u j (1≤j≤n-1) is the jth equalizer I eqj The balancing current, d i The battery terminal current I flowing from other power sources or going to other loads S Step A-3, obtaining the SOC state at the next moment through the balance system model; B. Balance between groups Step B-1, connect n single cells in series to obtain a battery pack, connect a bidirectional Cuk converter in parallel on one side of each battery pack, and connect to the energy bus through a switch selection module, the bidirectional Cuk converter corresponding to each battery pack is respectively connected to four bidirectional reverse-connected MOSFET switches, and connected to both ends of two bus capacitors, and the charge and discharge state of each battery pack can be selected by selecting the switch; Step B-2: Assume that the number of overcharged batteries is X and the discharge rate is r a , the number of undercharged batteries is Y, and the charging rate is r b According to the law of conservation of energy, the system should satisfy X·r a =Y·r b During the balancing process, the maximum SOC change per unit time interval is r a or b , the maximum battery SOC change rate is represented by R, and the maximum balancing rate of the two groups of balancers is as follows: Let x i (k) is the total SOC of n cells in the ith battery pack at time k. The SOC change process of the ith battery pack at the next moment is as follows: The Comp(u,v) function is a comparison function that compares the number of overcharged battery groups and undercharged battery groups: Among them, δ (δ < < 1) is the convergence radius, denoted by N eq is the minimum number of equilibrium cycles for the system to reach equilibrium; C. Taking the SOC of the single battery at time k+1 as the balancing target, the bidirectional Cuk equalizer is used to achieve intra-group balancing. At the same time, taking the SOC of the battery group at time k+1 as the balancing target, the bidirectional Cuk converter and the switch selection module are used to achieve inter-group balancing.

2. A hybrid battery pack active balancing method according to claim 1, characterized in that: The bidirectional Cuk equalizer includes two uncoupled inductors L1 and L2, an energy transfer capacitor C, and two MOSFET tubes Q1 and Q2 in a symmetrical structure.

3. The method for active balancing of a hybrid battery pack according to claim 1, characterized in that: In step A-1, the circuit controls the MOSFET to alternately switch on and off to adjust the balanced current by controlling the duty cycle of the PWM signal.

4. The method for active balancing of a hybrid battery pack according to claim 1, characterized in that: The method for obtaining the duty cycle is as follows: Collect the voltage V of each single battery separately B and the capacitor voltage V in the bidirectional Cuk equalizer C , determine the switching period as T S and the balancing current I eq , the duty cycle formula corresponding to the controlled balancing current of the single cells on both sides of the bidirectional Cuk balancer is calculated as follows:

5. The method for active balancing of a hybrid battery pack according to claim 1, characterized in that: The method for constructing the equilibrium system model: First, n-1 bidirectional Cuk equalizers are needed. The overall topology can be represented by an undirected graph G = (ν, ε), where ν = {Βat1, Βat2, ..., Βatn} represents a node set, Represents the edge set. According to the undirected graph, the system's association matrix H = [h ij ]∈R n×(n-1) ; For the i-th battery, the terminal current at time k is: The relationship between SOC at different times and the previous time can be obtained by using the ampere-hour integration method: Where η represents the coulombic efficiency, Q represents the battery capacity, Discretization of the ampere-hour integration method formula yields: Thus, the equilibrium system model is obtained by the correlation matrix H, the terminal current at time k and the discretization formula of the ampere-hour integration method.

6. A hybrid battery pack active balancing method according to claim 5, characterized in that: The method of obtaining the SOC state at the next moment through the balance system model is: The balancing system model is designed according to the multi-agent consensus algorithm. For the i-th converter, its balancing current is designed as u i (k)=α(x i (k)-x i+1 (k)) Where α is a constant, satisfying α<Q / Tη. Taking the above formula as the system input, the equilibrium system model can be expressed as: x(k+1)=x(k)-k1Hu(k)-B(k) u(k)=αH T x(k) where x(k)=[x1(k),x2(k),…,x n (k)] T ∈R n is the estimated SOC value of all single cells at time k; u(k)=[u1(k),u2(k),…,u n-1 (k)] T ∈R n-1 is the balancing current of all cells at time k, where k1=etaT / Q; B(k)=[B1(k),B2(k)…,B n (k)] T =[λd1(k),λd2(k),…,λd n (k)] T is the external interference term; Thus, combined with the Laplace matrix L = HH T , we get x(k+1) refers to the SOC at the next moment:

7. The method for active balancing of a hybrid battery pack according to claim 1, characterized in that: The bidirectional Cuk converter includes two inductors and two capacitors which are symmetrically distributed.

8. The method for active balancing of a hybrid battery pack according to claim 1, characterized in that: The switch selection module is specifically as follows: let the four switches corresponding to the i-th battery pack be S 4i-3 , S 4i-2 , S 4i-1 and S 4i When the battery is in a static state, neither discharging nor charging, all four switches are closed; when the battery pack is in a discharging state, switch S is turned on. 4i-3 and switch S 4i , turn off switch S 4i-2 and switch S 4i-1 ; When the battery pack is in charging state, turn on switch S 4i-2 and switch S 4i-1 , turn off switch S 4i-3 and switch S 4i .

9. A hybrid battery pack active balancing method according to claim 8, characterized in that: In the formula of the SOC change process of the i-th battery pack at the next moment, the Comp(u,v) function is a comparison function for comparing the number of overcharged battery packs and undercharged battery packs: Among them, δ (δ<<1) is the convergence radius.

10. A hybrid battery pack active balancing method according to claim 9, characterized in that: During the SOC change process of the i-th battery pack at the next moment, record N eq is the minimum number of equilibrium cycles when the system reaches equilibrium, then the equilibrium time of the system is T eq We can get: T eq =N eq ×τ。

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