CN111193306B

CN111193306B - Battery health state balancing method and system of modular energy storage battery grid-connected system

Info

Publication number: CN111193306B
Application number: CN202010106047.1A
Authority: CN
Inventors: 高峰; 马展
Original assignee: Shandong University
Current assignee: Shandong University
Priority date: 2020-02-20
Filing date: 2020-02-20
Publication date: 2021-04-13
Anticipated expiration: 2040-02-20
Also published as: CN111193306A

Abstract

The invention discloses a battery health state equalization method and system for a modular energy storage battery grid-connected system, including: performing a relative health state SOH detection on a retired battery pack to determine the relative SOH of each single cell in the battery pack; , using the active or passive balancing circuit of the battery management system BMS to perform SOH balancing between single cells; based on the coupling relationship between intra-group and inter-group SOH balancing, use the power conversion system PCS to coordinate intra- and inter-group SOH balancing , so that the single cell and the battery pack achieve SOH equilibrium at the same time. Compared with only considering the SOH balance between groups, the present invention can further extend the overall life of the energy storage system.

Description

Battery health state balancing method and system of modular energy storage battery grid-connected system

Technical Field

The invention belongs to the technical field of optimization control of a battery energy storage system, and particularly relates to a battery health state balancing method and system of a modular energy storage battery grid-connected system.

Background

The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.

With the rapid development of electric vehicles, the demand of power batteries is rapidly increasing. According to the industry standard, the power battery in the electric automobile needs to be eliminated from the electric automobile when the available capacity of the power battery is reduced to 80% of the rated capacity. The number of retired batteries will proliferate in the coming years. How to safely and efficiently process the retired battery will gradually become a main contradiction between resources, environment and new energy automobile development. Under the conditions that the ex-service battery has good appearance and effective functions of components, the reasonable echelon utilization of the ex-service battery can reduce the number of batteries directly entering a recovery stage, and the ex-service power battery can fully exert residual value in other fields. Therefore, the development of the echelon utilization and recovery of the power battery is significant.

Because the demand of the grid-connected energy storage system for instantaneous power is lower than that of an electric automobile, the retired battery of the electric automobile has enough energy and power output capacity to be applied to the auxiliary service of a power grid. Therefore, the cost of the battery energy storage system can be effectively reduced, and the further development of renewable energy sources is promoted.

A Battery Management System (BMS), which is a core unit in a large-scale battery use and maintenance process, mainly includes three contents: data acquisition, state estimation and performance management. State-of-charge (SOC) and state-of-health (SOH) estimation in state estimation and battery equalization in performance management are current research difficulties.

The SOC reflects the remaining battery capacity during battery operation, and the most common definition is based on the battery capacity, i.e., the ratio of the remaining battery capacity to the current dischargeable maximum capacity or the ratio of the used capacity to the current dischargeable maximum capacity subtracted from the initial SOC used, as shown in equation (1).

The SOH is a state quantity that determines when the battery replacement is performed, and the SOH gradually decreases as the battery is used. The capacity-based definition is the most common definition of SOH, as shown in equation (2), Q_maxIs the maximum capacity, Q, currently dischargeable by the battery_ratedIs the rated capacity of the battery.

In the aspect of battery balancing, the traditional BMS is balanced based on the SOC, the main balancing strategy comprises passive balancing and active balancing, and the commercial BMS system is developed based on the BMS chip of the mainstream manufacturer at present. Two typical passive and active equalization topologies are shown in fig. 2(a) -2 (b).

However, the inventor finds that the charging and discharging modes, the ambient temperature and the working road conditions of the power battery before the power battery is retired cannot be completely the same. Therefore, the retired batteries mutually present an obvious SOH inconsistency phenomenon, and the integrated reuse of the retired batteries is seriously influenced. In large-scale energy storage applications, batteries are combined in series and parallel to achieve the desired high voltage or high capacity. For the parallel batteries, because the positive electrode is connected with the positive electrode, and the negative electrode is connected with the negative electrode, if the SOC of a certain battery drops faster in the discharging process, the terminal voltage of the single battery is lower than that of other batteries, and the other batteries can spontaneously charge the battery. However, when used in series, the imbalance causes a problem of limiting the overall capacity. For the retired battery pack, when the same charge-discharge multiplying power is used, the SOC change speed of the retired battery with better SOH in the pack is smaller, and the SOC change speed of the retired battery with worse SOH is larger, which may cause the battery with lower SOH to reach the SOC lower limit value earlier and quit the operation, resulting in the barrel effect. Since the SOH imbalance exists not only between the battery packs but also within the battery packs, it is urgently required to perform the SOH equalization on the energy storage batteries simultaneously from two levels between the battery packs and within the battery packs.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a battery health state balancing method of a modular energy storage battery grid-connected system, which can realize simultaneous SOH balancing of energy storage batteries from two layers of a battery pack and a battery pack.

In order to achieve the above object, one or more embodiments of the present invention provide the following technical solutions:

the battery health state balancing method of the modular energy storage battery grid-connected system comprises the following steps:

detecting the relative SOH of the retired battery pack to determine the relative SOH of each single battery in the battery pack;

in the retired battery pack, an active or passive balancing circuit of a battery management system BMS is utilized to balance SOH among single batteries;

based on the coupling relation of the SOH balance between the battery packs, a power conversion system (PCS for short, comprising a DC-DC part and a DC-AC part) is utilized to carry out coordination and coordination of the SOH balance between the battery packs, so that the single batteries and the battery packs can simultaneously achieve the SOH balance.

According to the further technical scheme, the relative SOH of each single battery in the battery pack is obtained by monitoring the SOC change rate of the single battery.

According to the further technical scheme, the health state of the whole battery pack is used as a reference when the relative SOH of each single battery in the battery pack is detected.

According to the further technical scheme, the battery management system monitors the voltage, the current and the temperature of each single battery, provides useful information for SOC estimation, and utilizes the open-circuit voltage OCV to carry out SOC estimation.

According to the further technical scheme, the maximum dischargeable capacity of each single battery is calculated through relative SOH estimation of each single battery in the battery pack.

In a further technical scheme, the SOC estimation using the open circuit voltage OCV specifically comprises:

performing constant-current discharge on the battery pack by using standard discharge current until the voltage of the battery pack reaches a discharge cut-off voltage, and then standing the battery;

using standard charging current to perform constant current charging on the battery pack until the voltage of the battery pack reaches a charging cut-off voltage, then performing constant voltage charging at the voltage until the charging current is attenuated to a required value, and then standing the battery;

reading the OCV of each single battery, and obtaining an initial SOC value of the battery according to the OCV-SOC table;

carrying out constant-current discharge on the battery pack by using standard discharge current until the voltage of the battery pack reaches a discharge cut-off voltage, recording the released capacity in the process, obtaining the health state of the whole battery pack, and standing the battery again;

and reading the OCV of each single battery, obtaining a termination SOC value according to the OCV-SOC table, and then subtracting the termination SOC value from the initial SOC value to obtain the SOC variation of each battery in the discharging process.

In a further technical scheme, when the SOH among the single batteries is balanced, the batteries are attenuated at different rates and finally reach the same SOH, the battery pack with a larger SOH is applied with a larger depth-of-discharge (DOD), and the battery pack with a smaller SOH is applied with a smaller DOD.

According to a further technical scheme, the SOH equalization among the single batteries comprises the following steps:

the DOD of the battery with the maximum SOH is set as the upper limit value of the DOD;

the direct current component of the equalizing current is set to a maximum value within a power allowable range;

calculating a charging or discharging current flowing through a series path of the battery pack;

under the conditions, two single batteries with the maximum SOH and the minimum SOH in the battery pack are selected to calculate the minimum allowable value of the accumulated cycle number in the SOH balance between the single batteries;

according to the requirement of the equalization time, selecting an accumulated cycle number which is greater than or equal to the minimum allowable value to calculate the SOH equalization among the cells;

calculating DOD of each single battery;

calculating the actual balance current of each single battery during charging and discharging respectively;

a proportional-integral controller is used to generate the duty cycle of the switches in the equalization circuit for each cell.

According to the further technical scheme, based on the coupling relation of the SOH balance between the battery packs, the coordination and cooperation of the SOH balance between the battery packs are carried out by utilizing the PCS, so that the single batteries and the battery packs simultaneously achieve the SOH balance:

charging or discharging current i flowing through the series path of the ith battery pack_app,iThe current is directly related to the required power of each battery pack and is obtained by integrating the current flowing into the battery packs;

according to the SOH imbalance degree of the single batteries in the battery pack, i of the ith battery pack_app,iThe modulation depth of (a) is controlled independently;

describing the SOH imbalance degree of the ith battery pack by using discrete coefficients, calculating the average value of the discrete coefficients of the N battery packs, and obtaining a modulation depth adjusting parameter of the ith battery pack;

the initial required power of each battery pack needs to be multiplied by the regulating parameter of the battery pack to be corrected;

and tracking the regulated required power of each battery pack by using a PI controller, and generating the modulation depth and the duty ratio of the grid-connected converter switch.

In another aspect, the present invention provides a battery state of health balancing system for a modular energy storage battery grid-connected system, including a control system configured to:

based on the coupling relation of the SOH balance between the battery packs, the coordination and cooperation of the SOH balance between the battery packs are carried out by utilizing the PCS, so that the single batteries and the battery packs can achieve the SOH balance at the same time.

The above one or more technical solutions have the following beneficial effects:

1. in a traditional modularized energy storage battery grid-connected system, only the SOH balance problem among battery packs is considered, and the negative influence of the SOH inconsistency of single batteries in the battery packs on the normal work and the whole service life of the system is not considered. By designing a health state multi-layer balancing method, based on the coupling relation of the SOH balancing between the groups, the coordination and cooperation of the SOH balancing between the groups are carried out by utilizing the PCS, and finally all single batteries can reach the similar SOH at a higher speed.

2. Compared with the situation that only the SOH balance among the groups is considered, the invention can further prolong the whole service life of the energy storage system.

3. The relative SOH detection method for the retired battery pack of the electric automobile can conveniently and quickly determine the relative SOH of each single battery without disassembling the battery pack.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.

FIG. 1 is a schematic diagram of a topological structure of a modular energy storage battery grid-connected system according to an embodiment of the invention;

FIG. 2(a) is a diagram of a passive equalization topology using parallel resistors according to an embodiment of the present invention;

FIG. 2(b) is an active equalization topology using a flyback converter according to an embodiment of the present invention;

FIG. 3 is a control block diagram of SOH equalization among the individual cells in the pack in accordance with an embodiment of the present invention;

fig. 4 is a flow chart of a battery SOH multi-layer balancing method of a modular energy storage battery grid-connected system according to an embodiment of the present invention.

Detailed Description

It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.

The embodiments and features of the embodiments of the present invention may be combined with each other without conflict.

The general idea provided by the invention is as follows:

firstly, relative SOH detection is carried out on the collected electric automobile retired battery pack, and the relative SOH of each single battery in the battery pack is determined. The method does not need to disassemble the retired power battery pack, and is convenient and fast.

Next, SOH equalization between the unit cells is performed in the battery pack by using an active or passive equalization circuit of the BMS.

And finally, accelerating the SOH balancing speed among the single batteries by utilizing the coupling relation of the SOH balancing between the group and the inter-group, and enabling the single batteries and the battery pack to achieve the SOH balancing at the same time.

Example one

The embodiment discloses a battery health state balancing method of a modular energy storage battery grid-connected system, which can conveniently and quickly determine the relative SOH of each single battery in a battery pack without disassembling the battery pack by performing relative SOH detection; balancing the single batteries based on SOH by using an active or passive balancing circuit of the BMS; aiming at the coupling relation of the SOH balance between the group and the group, an SOH balance coordination control method between the group and the group is provided to accelerate the SOH balance speed of the single battery and enable the single battery and the battery pack to achieve the SOH balance at the same time.

The method can ensure that the SOH of each battery pack tends to be consistent, balance the SOH of each single battery in the battery pack, and balance and coordinate the SOH balance between the battery packs.

The method specifically comprises the following steps:

step (1): carrying out relative SOH detection on the battery pack, and determining the relative SOH of each single battery in the battery pack;

step (2): SOH equalization among the single batteries is carried out in the battery pack by utilizing an active or passive equalization circuit of the BMS;

and (3): the charge and discharge power/current of the battery pack is determined by the coupling relation of the SOH balance between the group and the group, namely the SOH balance between the groups, the overall charge and discharge power/current of the battery pack is an important factor for determining the SOH balance speed of the single batteries in the group, and the SOH balance coordination control between the group and the group is carried out.

Fig. 1 is a schematic diagram of a topological structure of a modular energy storage battery grid-connected system. A large number of single batteries are connected in series and parallel and then are subjected to alternating current-direct current conversion through the PCS, alternating current outputs of all the submodules are connected to a common grid-connected point together, and then grid connection is realized through a transformer and a filter. In the battery pack, the BMS and the balancing circuit are used for intra-pack management.

The specific method for detecting the relative SOH of the retired single battery comprises the following steps:

the relative SOH may be obtained by monitoring the rate of change of SOC of the unit cells. Battery with poor SOH when charging and discharging with same charging and discharging currentMay be greater than the SOC rate of change of a battery with better SOH. Obtaining the SOC variation (delta SOC) of the j-th single battery in unit time delta t according to the SOC definition in the formula (1)_j) As shown in equation (3):

wherein Q is_max,jIs the maximum dischargeable capacity, t, of the jth cell₀Is the charge and discharge initiation time.

According to the requirements of national standards, the SOH of each retired power battery pack needs to be detected before echelon utilization. The entire battery pack can be used as a reference cell in the relative SOH detection. According to the formulas (2) and (3), the SOH is calculated according to the state of health of the whole battery pack_packFor reference, the relative SOH value of the jth battery may be calculated according to equation (4).

In the formula, Q_max,packIs the maximum dischargeable capacity of the battery in ampere-hours (Ah); delta SOC_packIs the amount of change in SOC of the entire battery pack during the relative SOH test. The BMS may monitor the voltage, current, and temperature of each cell and may provide useful information for SOC estimation. SOC estimation is performed using an open-circuit-voltage (OCV) (this method does not require knowledge of the maximum dischargeable capacity of the unit cell). The procedure of the relative SOH test is shown in table 1. Through relative SOH estimation, the maximum dischargeable capacity of each single battery can be calculated by the formula (2).

TABLE 1 relative SOH estimation procedure for decommissioned cells

The specific content of the SOH balancing strategy among the retired single batteries is as follows:

for battery energy storage systems, DOD has been shown to be the most dominant factor affecting SOH of batteries therein, and SOH, DOD and cumulative cycle times (C)_acu) The relationship therebetween is shown in formula (5).

Wherein SOH (t) is the cycle C of the battery at a specific DOD_acuNext real-time SOH value, SOH_iniIs its initial SOH value. a and b are constant parameters obtained by fitting such a cell decay curve. Specifically, to allow the cells to decay at different rates and eventually reach the same SOH, the larger DOD will be applied to the cells with the larger SOH, while the smaller DOD will be applied to the cells with the smaller SOH.

Assuming that the initial SOH of the two batteries (j-th and k-th) are different, their real-time SOH (SOH) can be obtained according to the formula (5)_j(t) and SOH_k(t)) is represented by formula (6).

If the SOH (t) of the two batteries can pass through the same C_acuAfter equilibrium is reached, their DOD relationships are shown in equation (7).

Since the DOD of each single battery is not only related to the equalizing current of the single battery, but also related to the DOD of other single batteries, C required for the equalization of SOH among the single batteries_acuHaving a minimum value C determined by the degree of SOH inconsistency among cells, DOD demand value and operating state (charge or discharge)_acu,min。

Most of the existing active equalization circuits are large in size and high in cost, so that the BMS adopts passive equalization more frequently. The following description will be made specifically by taking the passive equalization topology using parallel resistors as an example in fig. 2 (a).

Selecting two batteries with the maximum SOH and the minimum SOH in the battery pack to calculate C_acu,min. DOD of the battery with the largest SOH is set as the DOD upper limit value DOD_max. To enable the DOD (DOD) of the battery with the worst SOH_worst) Equalizing the direct component I of the current as small as possible_balIs set to a maximum value (I) within a power allowable range_bal,max). From the same charge and discharge time, equation (8) and equation (9) can be derived.

In the formula, Q_max,bestMaximum dischargeable capacity, Q, of the most healthy cell_max,worsThe maximum dischargeable capacity of the battery with the worst state of health can be obtained by the relative SOH detection method of the retired single battery. i.e. i_chAnd i_disIs a charging or discharging current (i) flowing through the series path of the ith battery pack_app,i) As shown in fig. 2(a) and 2(b), the calculation can be performed by the formula (10).

In the formula, P^* _pack，iIs a required power reference value, v, of the ith battery pack_pack,iIs the real-time voltage value of the battery pack. DOD_worst,chAnd DOD_worst,disThe DOD of the single battery with the worst state of health in charging and discharging can be calculated by the formulas (8) and (9); and in order to ensure that the DOD values in the charging and discharging processes are the same, the DOD value with the larger median value is selected as the DOD_worst,min. Then, C_acu,minThe calculation can be performed by equation (11).

Thus, C is preset_acuMay be equal to or greater than C_acu,min. Once C is selected_acuBy setting the cell with the largest SOH as the kth cell and setting the DOD_kSet to DOD upper limit value DOD_maxThe DOD of each unit cell can be calculated according to equation (7).

Then, the equalization current of each unit cell during charging and discharging can be calculated separately. During charging, the equalizing resistors are not connected to the two ends of the battery with the maximum SOH in parallel, so that the charged electric quantity of the battery is as much as possible; and the two ends of other batteries are connected with the equalizing resistor in parallel. From the same charging time, equation (12) can be derived.

Then, the reference value I of the DC component of the equalizing current of the j battery during charging^* _bal，ch，jThis can be obtained by solving equation (12), as shown below.

Similarly, the equalizing resistor will not be connected in parallel to the two ends of the battery with the smallest SOH during discharging, so that the discharged electric quantity is as small as possible; and the two ends of other batteries are connected with the equalizing resistor in parallel. A similar equation (14) can then be derived from the same discharge time.

J-th battery during dischargingReference value I of DC component of equalizing current^* _{bal，dis，j}This can be obtained by solving equation (14), as shown below.

In summary, the control block diagram of SOH equalization of the single battery is shown in fig. 3, and the equalization current in the equalization circuit in the battery pack is taken as the control target, wherein a proportional-integral controller (PI) is used to generate the duty ratio d of the switch in the equalization circuit of the jth battery_bal,j。

The specific contents of the SOH balance coordination control method in the group and between the groups are as follows:

the SOH balance between the groups needs to consider the SOH conditions of all the single batteries, so that all the batteries can reach the SOH balance within the set accumulated cycle number. Therefore, the health state multi-layer balancing architecture of the modular energy storage battery grid-connected system is shown in fig. 4.

As can be seen from equations (8) and (9), the smaller i_app,i(i_chOr i_dis) Will cause DOD_worstFurther reduction, which can make the worst SOH cell decay more slowly, thereby speeding up the SOH equalization rate between cells. So that for a battery pack having a very high degree of imbalance in SOH among cells, i is small_app,iThe SOH balancing speed of the single batteries in the battery pack can be improved, so that the SOH level consistent with other battery packs can be reached as soon as possible.

As can be seen from formula (10), i_app,iDirectly related to the required power of each battery pack, can be obtained by integrating the current flowing into the battery pack, as shown in equation (16).

Wherein d is_iIndicating the duty cycle of the power electronic switch in the grid-connected converter of the ith battery, T being the power frequency period, i_iTo enter intoThe current of the grid-connected converter of the i battery packs.

Therefore, the i-th battery pack can be subjected to the i-th battery pack according to the SOH imbalance degree of the single batteries in the battery pack_app,iModulation depth (M) of_i) Independently controlled. And the degree of SOH imbalance of the ith battery pack may use a dispersion coefficient (c)_i) The description is given as follows.

Wherein σ_iRepresents the standard deviation, mu, of SOH of each single cell in the group_iIt is the average value thereof. Then, the average value of the N battery dispersion coefficients may be calculated by equation (18).

Then, the manipulated variable of the ith battery pack can be obtained by equation (19).

Initial power demand P of each battery pack_dem,iNeed to cooperate with u_adj,xiThe correction is performed by multiplication as shown in equation (20).

Finally, the PI controller is used for tracking the regulated required power P of each battery pack^* _pack，iGenerating modulation depth M of its grid-connected converter switch_iAnd duty ratio d_i。

Example two

The present embodiment aims to provide a battery state of health balancing system of a modular energy storage battery grid-connected system, which includes a control system configured to:

The steps involved in the apparatus of the above embodiment correspond to the first embodiment of the method, and the detailed description thereof can be found in the relevant description of the first embodiment.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The battery health state balancing method of the modularized energy storage battery grid-connected system is characterized by comprising the following steps:

in the retired battery pack, an active or passive balancing circuit of a battery management system BMS is utilized to balance SOH among single batteries; the SOH equalization steps among the single batteries are as follows:

selecting two single batteries with the maximum SOH and the minimum SOH in the battery pack to calculate the minimum allowable value of the accumulated cycle number in the SOH balance among the single batteries;

calculating DOD of each single battery;

generating a duty ratio of a switch in an equalization circuit of each unit cell by using a proportional-integral controller;

2. The battery state of health balancing method of the modular energy storage battery grid-connected system according to claim 1, characterized in that the relative SOH of each battery cell in the battery pack is obtained by monitoring the SOC change rate of the battery cell.

3. The battery state of health balancing method of the modular energy storage battery grid-connected system according to claim 2, characterized in that the state of health of the entire battery pack is used as a reference when detecting the relative SOH of each battery cell in the battery pack.

4. The battery state of health balancing method of the modular energy storage battery grid-connected system as claimed in claim 2, characterized in that the battery management system monitors the voltage, current and temperature of each single battery, provides useful information for SOC estimation, and performs SOC estimation using the open circuit voltage OCV.

5. The battery state of health balancing method of a modular energy storage battery grid-connected system as claimed in claim 4, characterized in that the maximum dischargeable capacity of each battery cell is calculated by estimating the relative SOH of each battery cell in the battery pack.

6. The battery state of health balancing method of the modular energy storage battery grid-connected system according to claim 4, wherein the SOC estimation using the open-circuit voltage OCV specifically comprises:

reading the OCV of each single battery, obtaining a termination SOC value according to an OCV-SOC table, and then subtracting the termination SOC value from the initial SOC value to obtain the SOC variation of each battery in the discharging process;

the SOC variation of the whole battery pack is set to be 100%, and the relative SOH of each single battery in the battery pack can be obtained only through one-time complete charging and discharging on the premise that the battery pack does not need to be disassembled by considering the relation that the ratio of the SOH of each single battery to the SOH of the whole battery pack is equal to the ratio of the SOC variation (100%) of the battery pack to the SOC variation of each single battery.

7. The battery state of health equalizing method of the modular energy storage battery grid-connected system of claim 1, characterized in that, when the SOH among the single batteries is equalized, the batteries are attenuated at different rates and finally reach the same SOH, the battery pack with larger SOH will be applied with larger DOD, and the battery pack with smaller SOH will be applied with smaller DOD.

8. The battery health state balancing method of the modular energy storage battery grid-connected system according to claim 1, wherein based on the coupling relationship of the inter-group and inter-group SOH balancing, the coordination and cooperation of the inter-group and inter-group SOH balancing are performed by using a PCS, so that the single batteries and the battery pack simultaneously achieve the SOH balancing:

describing the SOH imbalance degree of the ith battery pack by using discrete coefficients, and calculating the average value of the discrete coefficients of the N battery packs to obtain an adjustment parameter of the ith battery pack;

9. The battery health state balancing system of the modularized energy storage battery grid-connected system is characterized by comprising a control system, wherein the control system is configured to:

calculating DOD of each single battery;