CN117937701A

CN117937701A - Energy storage converter, energy storage system and control method for active equalization of battery

Info

Publication number: CN117937701A
Application number: CN202410329960.6A
Authority: CN
Inventors: 茹永刚; 王立; 骆辉; 贾奔; 贾淑文
Original assignee: Xi'an Xingyuan Borui New Energy Technology Co ltd
Current assignee: Xi'an Xingyuan Borui New Energy Technology Co ltd
Priority date: 2024-03-22
Filing date: 2024-03-22
Publication date: 2024-04-26

Abstract

The application discloses an energy storage converter, an energy storage system and a control method for active equalization of a battery, wherein the energy storage converter comprises the following components: the switching unit is connected with the battery unit, the battery unit comprises a plurality of batteries which are connected in series, and the switching unit is used for switching and charging among the batteries; the direct-current switch unit is connected with the switching unit; the first power conversion unit is connected with the direct current switch unit; the first power conversion unit realizes bidirectional conversion of alternating current and direct current; the second power conversion unit is connected with the switching unit, the direct current switch unit and the first power conversion unit; the second power conversion unit is used for reducing the direct current. The scheme can effectively prolong the service life of the battery and increase the return on investment.

Description

Energy storage converter, energy storage system and control method for active equalization of battery

Technical Field

The application relates to the technical field of energy storage converters, in particular to an energy storage converter for active equalization of batteries, an energy storage system and a control method.

Background

In an energy storage system, since several batteries are often connected in series, one battery may affect the whole energy storage system, so as to cause a "barrel effect", i.e. the battery with the lowest capacity determines the charge capacity and discharge capacity of the energy storage system. The influence of consistency on the cycle life and the charge-discharge characteristics of the energy storage system also follows the wooden barrel effect, and the group of batteries with the worst cycle life and charge-discharge characteristics in the batteries determine the overall performance of the whole energy storage system.

Different batteries of the energy storage system are connected in series, and when the energy storage system is charged, part of the energy storage system is full, and the other part of the energy storage system is not full; also, when discharged, some of the electricity is already exhausted, and some is not. Due to the short plate effect of the wooden barrel, some batteries cannot be fully charged during charging, and the electric quantity of some batteries is not used up during discharging, so that the battery utilization rate is low.

However, if the battery in the existing energy storage system is excessively charged and discharged, the service life of the battery and return on investment are affected.

Disclosure of Invention

The embodiment of the application provides an energy storage converter, an energy storage system and a control method for active equalization of batteries.

In a first aspect, embodiments of the present application provide an energy storage converter for active equalization of batteries, the energy storage converter comprising: the switching unit is connected with the battery unit, the battery unit comprises a plurality of batteries which are connected in series, and the switching unit is used for switching and charging among the batteries;

the direct-current switch unit is connected with the switching unit;

The first power conversion unit is connected with the direct current switch unit; the first power conversion unit realizes bidirectional conversion of alternating current and direct current;

The second power conversion unit is connected with the switching unit, the direct current switch unit and the first power conversion unit; the second power conversion unit is used for reducing the direct current.

In one embodiment, the switching unit comprises a first switch and a second switch, wherein a first end of the first switch is connected with the positive electrode of the first battery, and a second end of the first switch is connected with the direct current switching unit; the first battery is the first battery in a plurality of batteries connected in series;

the first end of the second switch is connected with the cathode of the second battery, and the second end of the second switch is connected with the second power conversion unit; the second battery is the last battery in a plurality of batteries connected in series;

the switching unit further comprises a third switch, and the first end of the third switch is connected with the second power conversion unit;

The switching unit further comprises at least two fourth switches, and the number of the fourth switches is 1 less than that of the batteries;

The first ends of the fourth switches are respectively connected between two adjacent batteries, and the second ends of all the fourth switches are connected with the second ends of the first switches and the second ends of the third switches; the third terminals of all the fourth switches are connected with the second terminal of the second switch.

In one embodiment, the energy storage converter further comprises: and the EMI unit is connected with the switching unit and the direct current switch unit.

In one embodiment, the energy storage converter further comprises: and the filtering unit is connected with the first power conversion unit and is also connected with a three-phase power line.

In one embodiment, the energy storage converter further comprises: and the alternating current switch unit is connected with the filtering unit and the three-phase power line.

In one embodiment, the first power conversion unit adopts a three-level topology or a full-bridge topology.

In one embodiment, the second power conversion unit employs a BUCK step-down circuit.

In a second aspect, embodiments of the present application provide an energy storage system comprising: the battery unit comprises a plurality of batteries connected in series;

The energy storage converter for active equalization of a battery according to the first aspect, wherein the switching unit is connected to the battery unit.

In a third aspect, an embodiment of the present application provides a control method for active equalization of a battery of an energy storage system as in the second aspect, where the control method includes: acquiring the voltage of each battery in the battery unit;

And according to all the voltages, controlling the switching unit to switch to charge the battery to be charged, wherein the voltage of the battery to be charged meets the preset condition.

In one embodiment, the preset conditions are: the voltage difference between the voltage of the battery to be charged and the maximum voltage in all the voltages is larger than a preset threshold value;

or the voltage of the battery to be charged is the minimum voltage among all voltages.

Compared with the prior art, the application has the following beneficial effects: the battery is switched and charged through the switching unit, so that the active equalization of the batteries is realized, the capacity of the battery pack is maximized, namely, the voltages of all batteries in the battery unit are the same or within a preset pressure difference range after the charging is finished, and the batteries can be discharged simultaneously during discharging, so that the service life of the batteries can be effectively prolonged, and the return on investment is increased.

Drawings

In order to more clearly illustrate the embodiments of the application or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the application, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

Fig. 1 is a schematic structural diagram of an energy storage converter for active equalization of a battery according to an embodiment of the present application;

Fig. 2 (a) is a schematic diagram of a first power conversion unit according to an embodiment of the present application; fig. 2 (b) is a schematic diagram of a second power conversion unit according to an embodiment of the present application; fig. 2 (c) is a schematic diagram III of a first power conversion unit according to an embodiment of the present application;

fig. 3 is a schematic structural diagram of a second power conversion unit according to an embodiment of the present application.

Detailed Description

The following description of the embodiments of the present application will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present application, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.

In the description of the present application, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like indicate orientations or positional relationships based on the orientation or positional relationships shown in the drawings, merely to facilitate describing the present application and simplify the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the present application.

In the description of the present application, it should be noted that, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be either fixedly connected, detachably connected, or integrally connected, for example; the specific meaning of the above terms in the present application can be understood as appropriate by those of ordinary skill in the art.

The terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include one or more such feature. In the description of the present application, unless otherwise indicated, the meaning of "a plurality" is two or more.

For example: a Battery Pack in an energy storage system is composed of 5 Pack batteries, where Pack1 is 80%, pack2 is 70%, pack3 is 60%, pack4 is 50%, pack5 is 40%, and then the BMS (Battery MANAGEMENT SYSTEM) must display the minimum value as SOC (State of Charge) of the whole energy storage system. The reason is that if the discharge is continued, 40% of electricity is discharged, the least Pack5 battery is completely dead, and if the discharge is continued, overdischarge is caused, so that the battery is greatly lost. However, at most 40% of the electricity in the battery is not discharged, which results in waste and lower charge and discharge utilization.

Existing energy storage systems can affect battery life and return on investment if the battery is overcharged.

The active equalization can maximize the capacity of the batteries in the group according to the barrel effect, and the batteries are fully charged and discharged at the same time, so that the service life of the batteries in the group is effectively prolonged. Therefore, the application provides an energy storage converter and an energy storage system for active equalization of batteries.

Referring to fig. 1, an energy storage converter for active equalization of a battery according to an embodiment of the present application is shown.

As shown in fig. 1, the energy storage converter for active equalization of a battery may include: the switching unit 11 is connected with the battery unit 2, the battery unit 2 comprises a plurality of batteries connected in series, and the switching unit 11 is used for switching charging among the batteries;

A dc switch unit 12, the dc switch unit 12 being connected to the switching unit 11;

A first power conversion unit 13, the first power conversion unit 13 being connected to the dc switching unit 12; the first power conversion unit 13 realizes bidirectional conversion of alternating current and direct current;

the second power conversion unit 14, the second power conversion unit 14 is connected with the switching unit 11, the direct current switch unit 12 and the first power conversion unit 13; the second power conversion unit 14 is used for step-down of the direct current.

Specifically, the switching unit 11 is configured to switch charging between the batteries in the battery unit 2, that is, when the voltage of a certain battery in the battery unit 2 is low, the battery with low voltage is charged by the switching unit, and other batteries are not charged, so that the above steps are repeated until the voltages of all the batteries are consistent with the maximum voltage in the battery unit 2 or within a preset voltage difference range. The dc switch unit 12 is used for controlling connection and disconnection of the dc circuit and adjusting the magnitude and direction of the current to meet the requirements of the energy storage system. The first power conversion unit 13 is a bidirectional AC/DC (ALTERNATING CURRENT/Direct Current) converter, and is configured to implement a bidirectional conversion function from AC to DC to convert DC from an AC power source to a desired voltage, store the DC into the battery unit 2, and convert the electric energy of the battery unit 2 into AC and output the AC to a load or a power grid. The second power conversion unit 14 is a unidirectional DC/DC converter for implementing a step-down function of direct current, so as to implement that the battery unit 2 needs a specific voltage and current to ensure safety and efficiency of the charging process when charging.

The switching unit in the energy storage converter for active equalization of the battery can switch and charge the battery so as to realize active equalization of the battery and maximize the capacity of the group battery, namely, the voltages of all batteries in the battery unit are the same or within a preset pressure difference range after the charging is finished, and the batteries can be discharged simultaneously when discharged, so that the service life of the battery can be effectively prolonged, and the return on investment is increased.

In one embodiment, the switching unit 11 includes a first switch 111 and a second switch 112, a first end of the first switch 111 is connected to the positive electrode of the first battery, and a second end of the first switch 111 is connected to the dc switching unit 12; the first battery is the first battery in a plurality of batteries connected in series;

A first end of the second switch 112 is connected to a negative electrode of the second battery, and a second end of the second switch 112 is connected to the second power conversion unit 14; the second battery is the last battery in a plurality of batteries connected in series;

The switching unit 11 further includes a third switch 113, a first end of the third switch 113 being connected to the second power conversion unit 14;

the switching unit 11 further includes at least two fourth switches 114, the number of the fourth switches 114 being 1 less than the number of the batteries;

The first ends of the fourth switches 114 are respectively connected between two adjacent batteries, and the second ends of all the fourth switches 114 are connected with the second ends of the first switches 111 and the second ends of the third switches 113; the third terminals of all fourth switches 114 are connected to the second terminal of the second switch 112.

Specifically, the first switch, the second switch, the third switch and the fourth switch are all devices capable of realizing on-off functions so as to realize switching charging among the batteries. For example, the first switch, the second switch, the third switch, and the fourth switch may be power electronic devices, relays, contactors, and the like, and are not limited herein, as long as the first switch, the second switch, and the third switch include two contacts (i.e., a first end and a second end), and the fourth switch includes three contacts (i.e., a first end, a second end, and a third end).

The number of the fourth switches is 1 less than that of the batteries in the battery unit 2, the first ends of the fourth switches are respectively connected between two adjacent batteries, the second ends of all the fourth switches are connected with the second ends of the first switches and the second ends of the third switches, and the third ends of all the fourth switches are connected with the second ends of the second switches. The battery to be charged is charged only by the on-off combination of different switches.

For example, with continued reference to fig. 1, in fig. 1, the battery unit 2 includes n series-connected batteries as an example, and assuming that n=5, the corresponding n batteries are set to be Pack1-Pack5, and the number of corresponding fourth switches is 4, and first ends of the 4 fourth switches, named as a fourth switch a, a fourth switch b, a fourth switch c, and a fourth switch d, are connected between Pack1 and Pack2, pack2 and Pack3, pack3 and Pack4, and Pack5, respectively.

The logic for each switch when charging 5 batteries in series is as follows: pack1 charge: and opening the fourth switch b, the fourth switch c, the fourth switch d and the second switch, closing the first switch and the third switch, closing the 1 contact and the 3 contact of the fourth switch a, and starting the first power conversion unit DC/AC and the second power conversion unit DC/DC.

Pack2 charging: the first switch, the fourth switch c, the fourth switch d and the second switch are opened, the third switch is closed, the 1 contact and the 2 contact of the fourth switch a are closed, the 1 contact and the 3 contact of the fourth switch b are closed, and the DC/AC and the DC/DC are started.

Pack3 charging: the first switch, the fourth switch a, the fourth switch d and the second switch are opened, the third switch is closed, the 1 contact and the 2 contact of the fourth switch b are closed, the 1 contact and the 3 contact of the fourth switch c are closed, and the DC/AC and the DC/DC are started.

Pack4 charge: the first switch, the fourth switch a, the fourth switch b and the second switch are opened, the third switch is closed, the 1 contact and the 2 contact of the fourth switch c are closed, the 1 contact and the 3 contact of the fourth switch d are closed, and the DC/AC and the DC/DC are started.

Pack5 charge: the first switch, the fourth switch a, the fourth switch b and the fourth switch c are opened, the second switch and the third switch are closed, the 1 contact and the 2 contact of the fourth switch d are closed, and the DC/AC and the DC/DC are started.

According to the switching unit provided by the embodiment, through the combination of the first switch, the second switch, the third switch and the fourth switches, any one battery in the battery unit can be switched to be charged independently, so that the effect that the voltages of all the batteries in the battery unit are consistent or within a preset pressure difference range is achieved, and the service life of the battery unit is effectively prolonged.

In one embodiment, with continued reference to fig. 1, the energy storage converter for active equalization of batteries further comprises: and an EMI unit 15, wherein the EMI unit 15 is connected to the switching unit 11 and the dc switching unit 12.

In particular, an EMI (Electro MAGNETIC INTERFERENCE ) unit is used to deal with problems related to electromagnetic interference to ensure the normal operation of the system and to avoid interference with the surrounding environment and other electronic devices.

In one embodiment, with continued reference to fig. 1, the energy storage converter for active equalization of batteries further comprises: the filtering unit 16, the filtering unit 16 is connected with the first power conversion unit 13, and the filtering unit 16 is also connected with a three-phase power line.

Specifically, the filtering unit 16 is configured to perform filtering processing on the electrical signal in the system to remove spurious signals and harmonics, ensure good power quality output by the system, and reduce interference to other electronic devices. Illustratively, the filtering unit 16 may employ an LCL filter.

In one embodiment, with continued reference to fig. 1, the energy storage converter for active equalization of batteries further comprises: and an ac switching unit 17, the ac switching unit 17 being connected to the filtering unit 16 and the three-phase power line.

Specifically, the ac switching unit 17 is used to control connection and disconnection of an ac circuit. For example, the ac switching unit 17 may employ an ac contactor, an ac circuit breaker, or the like.

In one embodiment, the first power conversion unit 13 adopts a three-level topology or a full-bridge topology. Exemplary, fig. 2 (a) -2 (c) show schematic structural diagrams of the first power conversion unit, wherein fig. 2 (a) and 2 (b) are schematic three-level topologies, and fig. 2 (c) is a schematic full-bridge topology. The topology shown in fig. 2 is adopted to realize the bidirectional conversion function from alternating current to direct current.

In one embodiment, the second power conversion unit 14 employs a BUCK voltage circuit. A common topology of the second power conversion unit is shown in fig. 3. The topology shown in fig. 3 is adopted to realize the step-down function of direct current.

With continued reference to fig. 1, an embodiment of the present application further provides an energy storage system, including: the battery unit 2, the battery unit 2 includes several batteries connected in series;

The energy storage converter 1 for active equalization of a battery provided in any of the above embodiments, the switching unit 11 in the energy storage converter 1 is connected with the battery unit 2.

According to the energy storage system provided by the application, the batteries are switched and charged through the switching unit in the energy storage converter, so that the active balance of the batteries is realized, the capacity of the grouped batteries can be maximized, namely, the voltages of all the batteries in the battery units are the same or within a preset pressure difference range after the charging is finished, the batteries can be discharged simultaneously during discharging, the service life of the batteries is effectively prolonged, and the return on investment can be increased.

The embodiment of the application also provides a control method for the active equalization of the battery of the energy storage system, which comprises the following steps: acquiring the voltage of each battery in the battery unit;

Optionally, the preset conditions are: the voltage difference between the voltage of the battery to be charged and the maximum voltage in all the voltages is larger than a preset threshold value;

Specifically, the voltage of each battery in the battery unit may be measured by a voltage sensor, a voltage measuring device, an integrated circuit having a voltage measuring function, or the like.

The preset threshold may be set according to actual requirements, and may also be referred to as a standard pressure difference.

The battery unit in the energy storage system is composed of a plurality of Pack batteries in series, and is assumed to be Pack1-Packn. The voltage range of a single Pack cell is Vmin1-Vmax1 (48 cells typically: DC 130-175V). The designed unidirectional DC/DC output voltage range is Vmin2-Vmax2. Wherein the DC/DC output voltage range covers the voltage range of the battery Pack, namely: vmin1 is larger than or equal to Vmin2, and Vmax1 is smaller than or equal to Vmax2. The voltages of the n batteries Pack are V1, V2, V3 … … Vn, respectively. Assume that the maximum voltage is Vmax3.

If the preset conditions are: the voltage difference between the voltage of the battery to be charged and the maximum voltage of all the voltages is greater than a preset threshold. The preset threshold is set to be Vh, that is, the standard voltage difference between each Pack voltage and Vmax3 in the battery unit is set to be Vh.

When the system detects that the voltage of a certain Pack is larger than Vmax3 in an intermittent period, corresponding switches in the DC/AC unit and the DC/DC unit and the switching unit are started to be switched to Pack charging until the voltage reaches the voltage of Vmax 3.

It will be appreciated that after repeated cycling through several groups, the voltage across each Pack and the voltage across Vmax3 can be made less than Vh, ending the charge.

It is further understood that when a voltage of a certain Pack is detected to be less than or equal to Vh with a Vmax3 voltage difference, the second power conversion unit is not activated, nor is the switch in the switching unit switched.

If the preset conditions are: the voltage of the battery to be charged is the minimum voltage among all voltages. And starting the second power conversion unit and switching the corresponding switch to be switched to Pack charging corresponding to the minimum voltage until the voltage reaches the maximum voltage.

For example, taking a 215KWH energy storage system as an example, the battery Pack is composed of 5 Pack batteries (Pack 1-Pack5 respectively), when the system needs to be charged, the PCS closes the first switch, the second switch, the ac switch unit and other switches are opened, and the system is charged, and due to the fact that the Pack1 voltage is DC160v, the Pack2 voltage is DC175v, the Pack3 voltage is DC175v, the Pack4 voltage is DC175v, and the Pack5 voltage is DC175v after the system is charged, the BMS must display the minimum voltage value as the SOC of the whole battery system. After the BMS compares the voltages of 5 Pack, the lowest voltage of Pack1 is confirmed, a charging instruction is sent to the PCS, after the PCS receives the charging instruction, the fourth switch b, the fourth switch c, the fourth switch d and the second switch are opened, the first switch and the third switch are closed, the 1 contact and the 3 contact of the fourth switch a are closed, the DC/AC and the DC/DC unit are started to charge the Pack1, and when the voltage of the Pack1 is charged to the maximum voltage, the charging is stopped. Therefore, the voltage and the electric quantity of the 5 packs are consistent, and the wooden barrel effect can not be generated when the discharge is carried out.

Similarly, the remaining Pack charging logic is as follows: pack2 charging: the first switch is opened, the fourth switch c, the fourth switch d, the second switch is closed, the third switch is closed, the 1 contact and the 2 contact of the fourth switch a are closed, the 1 contact and the 3 contact of the fourth switch b are closed, and the charging of the DC/AC and DC/DC units is started.

Pack3 charging: the first switch is opened, the fourth switch a, the fourth switch d, the second switch is closed, the third switch is closed, the 1 contact and the 2 contact of the fourth switch b are closed, the 1 contact and the 3 contact of the fourth switch c are closed, and the charging of the DC/AC and DC/DC units is started.

Pack4 charge: the first switch, the fourth switch a, the fourth switch b and the second switch are opened, the third switch is closed, the 1 contact and the 2 contact of the fourth switch c are closed, the 1 contact and the 3 contact of the fourth switch d are closed, and the DC/AC and DC/DC unit charging is started.

Pack5 charge: the first switch, the fourth switch a, the fourth switch b and the fourth switch c are opened, the second switch and the third switch are closed, the 1 contact and the 2 contact of the fourth switch d are closed, and the DC/AC and DC/DC unit charging is started.

The present application is not limited to the above embodiments, and any changes or substitutions within the technical scope of the present application should be covered by the scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. An energy storage converter for active equalization of a battery, the energy storage converter comprising:

the switching unit (11), the said switching unit (11) is connected with battery unit (2), the said battery unit (2) includes several batteries connected in series, the said switching unit (11) is used for switching and charging between the said batteries;

A direct current switch unit (12), wherein the direct current switch unit (12) is connected with the switch unit (11);

A first power conversion unit (13), wherein the first power conversion unit (13) is connected with the direct current switch unit (12); the first power conversion unit (13) realizes bidirectional conversion of alternating current and direct current;

The second power conversion unit (14), the said second power conversion unit (14) is connected with said switching unit (11), said direct-current switch unit (12), said first power conversion unit (13); the second power conversion unit (14) is used for reducing the voltage of direct current.

2. Energy storage converter for active equalization of batteries according to claim 1, characterized in that said switching unit (11) comprises a first switch (111) and a second switch (112), a first end of said first switch (111) being connected to the positive pole of the first battery, a second end of said first switch (111) being connected to said direct current switching unit (12); the first battery is the first battery in the plurality of batteries connected in series;

A first end of the second switch (112) is connected with a negative electrode of a second battery, and a second end of the second switch (112) is connected with the second power conversion unit (14); the second battery is the last battery in the plurality of batteries connected in series;

The switching unit (11) further comprises a third switch (113), a first end of the third switch (113) is connected with the second power conversion unit (14);

the switching unit (11) further comprises at least two fourth switches (114), the number of the fourth switches (114) being 1 less than the number of the batteries;

The first ends of the fourth switches (114) are respectively connected between two adjacent batteries, and the second ends of all the fourth switches (114) are connected with the second ends of the first switches (111) and the second ends of the third switches (113); all third terminals of the fourth switch (114) are connected to the second terminal of the second switch (112).

3. The energy storage converter for active equalization of a battery of claim 1, further comprising:

And an EMI unit (15), wherein the EMI unit (15) is connected with the switching unit (11) and the direct current switching unit (12).

4. The energy storage converter for active equalization of a battery of claim 1, further comprising:

And the filtering unit (16) is connected with the first power conversion unit (13), and the filtering unit (16) is also connected with a three-phase power line.

5. The energy storage converter for active equalization of a battery of claim 4, further comprising:

And the alternating current switch unit (17), wherein the alternating current switch unit (17) is connected with the filtering unit (16) and the three-phase power line.

6. Energy storage converter for active equalization of batteries according to claim 1, characterized in that said first power conversion unit (13) adopts a three-level topology or a full bridge topology.

7. The energy storage converter for active equalization of battery of claim 6, wherein said second power conversion unit (14) employs a BUCK circuit.

8. An energy storage system, the energy storage system comprising:

The battery unit (2), the said battery unit (2) includes several batteries connected in series;

Energy storage converter (1) for active equalization of a battery according to any of the claims 1-7, a switching unit (11) in the energy storage converter (1) being connected to the battery unit (2).

9. A method of controlling active equalization of a battery of an energy storage system of claim 8, said method comprising:

Acquiring the voltage of each battery in the battery unit;

10. The control method according to claim 9, wherein the preset condition is:

the voltage difference between the voltage of the battery to be charged and the maximum voltage in all the voltages is larger than a preset threshold value;

or the voltage of the battery to be charged is the minimum voltage among all the voltages.