Battery with full-cycle voltage stabilization

Abstract

The application discloses a self-reconfigurable battery capable of realizing full-cycle voltage stability. In order to solve the technical problems of relative instability of the voltage of the self-reconfigurable battery and energy loss, the application adaptively bypasses or accesses the battery unit according to the state of charge, and selects a suitable preset threshold according to the overall state of charge of the battery pack, so that the switching frequency can be reduced while the battery unit is fully discharged. The application adaptively incorporates the battery unit as a technical means, solves the technical problems of unstable battery voltage and capacity loss, and obtains the technical effects of low energy loss, high stability, high available capacity and high applicable voltage range. The application is suitable for the field of lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a battery that can achieve voltage stability throughout the entire battery life cycle. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles, energy storage systems, and other fields due to their high energy density, long cycle life, and low self-discharge rate. In these applications, many batteries are typically connected in series to provide higher voltage and power to the load. Although significant efforts are made in engineering practice to select batteries that are as compatible as possible, battery mismatch issues still exist in real-world applications due to manufacturing tolerances, varying self-discharge rates, uneven operating temperatures, and uneven aging processes. These unavoidable differences within the battery gradually disperse during cycling and can lead to overcharging or over-discharging. Clearly, this non-uniformity limits battery capacity and may even cause safety issues. Therefore, properly maintaining the balance of each battery cell is crucial for improving battery life.

[0003] In existing technologies, passive balancing strategies dissipate excess energy as heat through resistance, while active balancing strategies achieve cell balance by transferring excess energy between cell cells. Furthermore, self-reconfigurable batteries are widely considered an effective solution in academia.

[0004] However, the voltage of self-reconfigurable batteries is relatively unstable, which is determined by the characteristics of self-reconfigurable batteries. The voltage of a single cell / cell varies with the state of charge (SOC). For example, the voltage range of an NCM lithium-ion battery is 3.0V to 4.2V, while if the load is powered by 100 cells in series, the load voltage range is 300V to 420V.

[0005] Because the voltage of a self-reconfigurable battery gradually decreases with state of charge (SOC), even when all batteries are connected to the battery pack, the voltages between the batteries are different. This exacerbates voltage fluctuations, making it nearly impossible to power loads that require a higher voltage range. Therefore, balancing circuits such as self-reconfigurable batteries typically need to be connected to a DC-DC converter to maintain the battery pack's voltage. Using a DC-DC converter is a relatively simple method, but it has drawbacks such as energy loss.

[0006] To stabilize the voltage of a battery pack without a DC-DC converter, this invention proposes a battery with full-cycle voltage stability based on a novel self-reconfigurable battery topology, along with a corresponding control strategy. Summary of the Invention

[0007] To solve or alleviate some or all of the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0008] A battery with stable voltage throughout its entire lifecycle includes at least one battery pack, the battery pack comprising a plurality of battery cells, the battery cells being divided into a power supply battery pack or a bypass battery pack according to whether they participate in supplying power to a load; each of the battery cells has a corresponding switch or switch group that can integrate the battery cell into the power supply battery pack and the bypass battery pack; when the state of charge difference between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is greater than a preset threshold: the battery cell with the lowest state of charge in the power supply battery pack is added to the bypass battery pack by means of the corresponding switch or switch group, and the battery cell with the highest state of charge in the bypass battery pack is added to the power supply battery pack.

[0009] In one embodiment, when the state of charge difference between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is not greater than a preset threshold: it is determined whether the voltage of the battery pack is closer to the set value after the battery cell with the highest state of charge in the bypass battery pack is added to the power supply battery pack; if so, the battery cell with the highest state of charge in the bypass battery pack is added to the power supply battery pack.

[0010] In one embodiment, during the initialization of the battery pack, the battery cells in the battery pack are integrated into the power supply battery pack according to the order of their state of charge from high to low, until the voltage of the battery pack is closest to the set value.

[0011] In one embodiment, before the battery management system stops working, it is cyclically determined whether the state of charge difference between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is greater than a preset threshold.

[0012] In one embodiment, when the battery pack is supplying power to a load, the battery cells are connected in series.

[0013] In one embodiment, the battery pack is determined to be in a high-charge state or a low-charge state by the relationship between the value of ...

[0014] In one embodiment, any one of the plurality of battery cells is coupled to at least two switches. By controlling the two switches to be turned on or off, the battery cell can be bypassed or connected to the power supply battery pack to achieve battery cell balancing.

[0015] Some or all of the embodiments of the present invention have the following beneficial technical effects:

[0016] 1) Low energy loss: No DC-DC converter is required, avoiding the drawback of energy loss;

[0017] 2) Good stability: It can achieve full-cycle voltage stability and keep the battery pack with low voltage fluctuations and stable within the set range without a DC-DC converter;

[0018] 3) High battery capacity and low switching frequency: The battery topology of this invention has excellent circuit balancing performance. Without a DC-DC converter, it can ensure that the battery pack capacity utilization rate reaches 99.8%, maximizing the battery's usable capacity. Furthermore, the preset segmented SOC threshold scheme can reduce the switching frequency.

[0019] 4) High applicable voltage range: The topology battery and its control strategy of this invention enable the battery pack circuit to be applied to loads that require a higher voltage range without affecting the performance of the battery pack.

[0020] Further beneficial effects will be described in the preferred embodiments.

[0021] The technical solutions / features disclosed above are intended to summarize the technical solutions and features described in the Detailed Embodiments section, and therefore the scope of the description may not be entirely the same. However, these new technical solutions disclosed in this section are also part of the numerous technical solutions disclosed in this invention document. The technical features disclosed in this section, together with the technical features disclosed in the subsequent Detailed Embodiments section and some contents in the drawings not explicitly described in the specification, disclose more technical solutions in a reasonable combination.

[0022] The technical solution formed by combining all the technical features disclosed at any position in this invention is used to support the summary of the technical solution, the modification of the patent document, and the disclosure of the technical solution. Attached Figure Description

[0023] Figure 1 This is a comparison diagram of the topology proposed in this invention and a conventional self-reconfigurable battery structure;

[0024] Figure 2 Schematic diagrams of a traditional self-reconfigurable battery (30S3P) and the topology (90S1P) proposed in this invention;

[0025] Figure 3 This is a schematic diagram of a topology voltage regulator.

[0026] Figure 4 The diagram shows the initial state and equalization process of the circuit based on the control strategy.

[0027] Figure 5For the control strategy flowchart;

[0028] Figure 6 Diagram showing the state of some batteries reaching their discharge cutoff voltage;

[0029] Figure 7 This is a schematic diagram of the SOC topology proposed in this invention;

[0030] Figure 8 This is a voltage diagram corresponding to the SOC topology proposed in this invention;

[0031] Figure 9 A schematic diagram of the state of charge (SOC) of a self-reconfigurable battery;

[0032] Figure 10 This is a schematic diagram showing the voltage corresponding to the SOC of a self-reconfigurable battery.

[0033] Figure 11 This is a schematic diagram of the SOC of the topology proposed in this invention when the battery pack drops to the cutoff voltage;

[0034] Figure 12 This is a schematic diagram of the SOC corresponding to the topology proposed in this invention when the battery pack drops to the cutoff voltage;

[0035] Figure 13 This is a schematic diagram of the SOC of the topology proposed in this invention when the battery pack drops to the cutoff voltage (balance at A);

[0036] Figure 14 For the present invention Figure 11 Examples and Figure 13 Voltage diagram in the embodiment;

[0037] Figure 15 This diagram illustrates the remaining capacity and initial available capacity. Detailed Implementation

[0038] Since it is impossible to exhaustively describe all alternative solutions, the key points of the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Other technical solutions and details not disclosed in detail below generally belong to technical objectives or features that can be achieved by conventional means in the art, and due to space limitations, they will not be described in detail here.

[0039] Unless it refers to division, the " / " in any position in this invention represents logical "OR". The serial numbers "first", "second", etc., in any position in this invention are merely descriptive distinguishing marks and do not imply an absolute temporal or spatial order, nor do they imply that terms prefixed with such serial numbers necessarily refer to different things than the same terms prefixed with other modifiers.

[0040] This invention describes various key points used to combine into various specific embodiments, which will be incorporated into various methods and products. In this invention, even if a key point is described only when introducing a method / product solution, it means that the corresponding product / method solution also explicitly includes that technical feature.

[0041] The description of the existence or inclusion of a step, module, or feature at any location in this invention does not imply that such existence is exclusive or unique. Those skilled in the art can obtain other embodiments by supplementing the technical solutions disclosed in this invention with other technical means. Based on the key points described in the specific embodiments of this invention, those skilled in the art can substitute, delete, add, combine, or change the order of certain technical features to obtain a technical solution that still follows the concept of this invention. These solutions that do not depart from the technical concept of this invention are also within the protection scope of this invention.

[0042] The novel self-reconfigurable battery with a novel topology proposed in this invention (hereinafter referred to as topology or topology) differs from ordinary self-reconfigurable batteries primarily in that it changes the connection method of the battery cells, connecting parallel cells in series. It is important to note that this solution does not require adding or reducing the number of batteries, such as... Figure 1 As shown.

[0043] Taking a battery pack composed of 90 battery cells (also called individual cells, cells / units, batteries, or simply cells) as an example, when the DC-DC converter performs three voltage conversions, the reconfigurable battery with the DC-DC converter is a 30S3P, such as... Figure 2 As shown in (a) of the diagram. To obtain a higher voltage, the topology proposed in this invention requires connecting 90 batteries in series, i.e., 90S1P, as shown in [a]. Figure 2 As shown in (b), connecting all 60 batteries originally used in parallel into series achieves a higher voltage. Figure 1 In this case, m = 60.

[0044] The principle of voltage stabilization in this invention is as follows: Figure 3 As shown, to stabilize the battery pack voltage at 120V, the battery pack consists of 40 cells connected in series, assuming they are identical. Figure 3 The individual cell voltages of (a) to (d) are 4.0V, 3.7V, 3.4V and 3.0V, respectively.

[0045] like Figure 3 As shown in (a), the battery voltage is high, requiring only 30 batteries to power the load. Since all batteries in the battery pack have the same voltage, only the first 30 batteries need to be discharged, while the last 10 are bypassed. Figure 3As shown in (b), maintaining the voltage requires 32 batteries. Similar to the previous analysis, the first 32 batteries are selected for discharge. This can be used as an analogy to deduce... Figure 3 (c) and (d) in the text.

[0046] The topology proposed in this invention requires the number of series-connected m batteries to be m ≥ (Ut) / Uc, where Ut is the target voltage of the battery pack and Uc is the discharge cutoff voltage of a single battery cell. For NCM batteries, the discharge cutoff voltage is 3.0V, i.e., Uc is 3.0V.

[0047] The above analysis only introduced the principle of circuit voltage stabilization, but ignored the balancing between units. Next, we will... Figure 4 and Figure 5 This invention introduces the control strategy of the circuit proposed in this invention, which enables the battery pack to maintain the consistency of the SOC (State of Charge) of the individual battery cells in the battery pack while stabilizing the voltage.

[0048] The control strategy of this invention is: when the battery pack is discharging, such as Figure 4 As shown, batteries 1 to 3 have the same and highest SOC, batteries 4 to 6 have the same and medium SOC, and batteries 7 and 8 have the same and lowest SOC.

[0049] Combination Figure 4 and Figure 5 ,like Figure 4 As shown in (a), assuming that batteries 1 to 6 can meet the battery pack voltage requirements, they are connected in series to the battery pack, and the other two batteries with lower SOC are bypassed.

[0050] During battery pack initialization, the battery cells in the battery pack are connected to the power supply battery pack in descending order of their state of charge, until the voltage of the battery pack is closest to the set value.

[0051] like Figure 4 As shown in (b), after a period of time, the SOC difference between battery 5 (battery 6) and battery 7 (battery 8) satisfies SOC7 - SOC5 ≥ SOCset, reaching the set condition, where SOCset is the preset SOC threshold. At this time, Figure 5 If the first condition in the condition is true, bypass batteries 5 and 6, and connect batteries 7 and 8 to the battery pack (e.g. Figure 4 (c) in the middle.

[0052] Similarly, as Figure 4 As shown in (d), after a period of discharge, when the SOC difference between battery 5 and battery 4 exceeds a preset SOC threshold, battery 4 is bypassed, and battery 5 is connected to the battery pack. Figure 4As shown in (e), battery 6 is connected to the battery pack because connecting battery 6 to the battery pack brings the voltage of the battery pack closer to the voltage ( ). Figure 5 (The second judgment condition in the process). It should be noted that this is merely an example provided for ease of understanding, to illustrate the working principle of the circuit. This invention does not limit the number of batteries in each battery pack.

[0053] from Figure 4 and Figure 5 Analysis shows that even if there is a significant inconsistency in the initial state of charge of the individual cells in the battery pack (such as...), Figure 4 (a) in the text, but after a period of time, the consistency of the state of charge of the battery pack significantly improved (e.g., ...). Figure 4 (e) This is because in an additional series battery, when a battery with a low SOC is bypassed, a battery with a high SOC can replace the bypassed battery to power the battery load, thus keeping the battery pack voltage within the set range.

[0054] Before the battery management system stops working, it continuously checks whether the difference in state of charge between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is greater than a preset threshold.

[0055] Since the topology proposed in this invention does not have a DC-DC converter, the voltage provided by the battery pack to the load is the sum of the voltages of all available batteries. For example... Figure 6 As shown, when batteries 6 to 8 reach the discharge cutoff voltage, batteries 1 to 5 become unusable. If the total voltage of batteries 1 to 5 cannot meet the minimum voltage required by the load, the battery capacity cannot be released, which will lead to a reduction in the battery pack capacity utilization rate.

[0056] Refer to SOC j -SOC i ≥SOCset, Figure 6 The maximum SOC in the circuit should not exceed SOCset. This means that while a larger SOCset reduces the number of switching operations in the circuit, it may result in a lower usable battery capacity. Conversely, a smaller SOCset, while allowing for better utilization of the battery's available capacity, increases the number of switching operations.

[0057] In order to solve Figure 6 To address the identified shortcomings, the present invention employs a segmented SOCset: when the battery pack's SOC is high, the SOCset is increased, thereby reducing the number of switching operations; when the battery pack's SOC is low, the SOCset is decreased, fully utilizing the battery pack's available capacity. The high / low SOC of the battery pack can be determined by its relationship with a second threshold.

[0058] Continue to refer to Figure 2 ,Although Figure 2 The battery cells in (a) and (b) are connected differently, but since the number of battery cells used in the battery pack is 90, the total energy stored in the battery pack is the same, and the total energy available for use is the same.

[0059] If the power required by the load is P, then Figure 2 The currents I1 and I2 provided by each unit in (a) and (b) are respectively: I1=(P / η) / (90×Ucell), I2=P / (90×Ucell), where η is the conversion efficiency of the DC-DC converter and Ucell is the battery voltage. We assume that all batteries have the same voltage. Figure 2 In (a), the current of each battery is slightly higher than Figure 2 (b) is due to the power loss of the DC-DC converter, so the battery pack needs to provide higher power to meet the power demand of the load.

[0060] Although the topology proposed in this invention connects batteries that were originally connected in parallel into series, as can be seen from the above analysis, this has little impact on the performance of the battery pack. However, it should be noted that the topology proposed in this invention typically supplies power to all battery cells together only when the battery pack is close to its discharge cutoff voltage, but some battery cells usually do not supply power to the load. When the SOC is 100%, Figure 2 In (a) of this paper, the battery pack is powered by 90 cells (battery voltage is 270V), while the topology proposed in this invention is powered by only 64 cells (64 × 4.2V = 268.8V). The maximum power that the battery pack can provide is only [missing information]. Figure 2 (a) 64 / 90. However, the total energy provided by the battery pack is the same because the battery pack consists of 90 batteries.

[0061] Therefore, when the power supplied by the battery pack has a certain degree of redundancy, the topology proposed in this invention can better achieve the goal of self-reconfiguration of the battery in the DC-DC converter and avoid energy loss caused by the DC-DC converter.

[0062] Actual test results of the present invention:

[0063] (1) Battery balance and output voltage performance

[0064] Using the topology and control algorithm proposed in this project, and considering the consistency of the battery pack, the target voltage of the battery pack is 27V, and SOCset = 2%. The SOC of each battery and the voltage of the battery pack are as follows: Figure 7 and 8 As shown.

[0065] like Figure 7As shown, at 60 seconds, batteries 1 to 7 discharge. Batteries 8 and 9 are bypassed due to their lower SOC. The voltage of batteries 1 to 7 is approximately 25.5V, close to 27V. At approximately 480 seconds, when battery 7 (lowest SOC in the power battery pack) is 2% lower than battery 8 (highest SOC in the bypass battery pack) (SOCset), battery 7 is bypassed, and battery 8 is connected to the battery pack. Since the SOCs of batteries 7 and 8 are similar, the battery pack voltage does not change at this time. At approximately 1140 seconds, the battery pack changes from 7 batteries in series to 8 batteries, and the battery pack voltage rises to approximately 28.9V. Figure 8 As shown, the voltage of the battery pack is closer to 27V when eight batteries are connected in series compared to seven.

[0066] For easier comparison, we conducted the experiment again using a self-reconfigurable battery, as shown below. Figure 9 and Figure 10 As shown.

[0067] like Figure 9 As shown, the battery pack begins to equalize after 60 seconds. Because batteries 7, 8, and 9 have lower State of Charge (SOC), these batteries are bypassed. That is, between 60 and 420 seconds, only batteries 1 through 6 discharge. Figure 10 As shown, the voltage of the battery pack is approximately 22V, while the voltage of all individual cells is approximately 33V. After 420 seconds, the SOC of battery cells 7 to 9 reaches the set condition. Battery cells 7 to 9 are connected to the battery pack, and battery cells 4-6 are bypassed. As analyzed above, the voltage of the battery pack does not change.

[0068] from Figure 9 As can be seen, the self-reconfigurable battery's range decreased from 2.5% in 60s to 0.7% in 600s, which better balances the battery pack. Figure 7 As can be seen, the battery pack's range decreased from 2.5% in 60 seconds to 1.6% in 1200 seconds. This is because the battery pack voltage needs to be considered when bypassing the battery (self-reconfigurable battery bypassing 3 batteries). Figure 9 The present invention proposes a topology that bypasses two batteries. Figure 7 Therefore, the number of bypass batteries is limited. Furthermore, the topology proposed in this invention cannot balance the battery pack like a self-reconfigurable battery.

[0069] Because the topology proposed in this invention is similar to that of a self-reconfigurable battery, it possesses both the advantages and disadvantages of self-reconfigurable batteries. However, as... Figure 8 and Figure 10 As shown, when the battery pack is discharging, compared with a self-reconfigurable battery, the topology proposed in this invention can better stabilize the battery pack voltage even when bypassing the battery, and maintain the battery pack voltage within a set range.

[0070] (2) Capacity efficiency of the battery pack

[0071] like Figure 6 As shown, when the battery pack voltage is insufficient to meet the load voltage requirements, the battery pack cannot continue to supply power to the load. When the battery voltage in the battery pack is below 3.0V, the battery is considered to be unable to discharge further. Increasing the usable capacity of the battery pack by 1% with a SOCset will change the SOC of each battery and the battery pack's overall capacity during discharge. Figure 11 and Figure 12 As shown.

[0072] Figure 11 Analysis and Figure 7 The analysis is the same, so I won't go into too much detail here. However, it can be seen that when the battery pack increases from 0 seconds to about 1800 seconds, the SOC of the nine batteries is almost the same, and the same situation occurs at about 3400 seconds. Figure 11 Point A is shown. Regarding voltage, since one of the nine SOC cells has a lower voltage, initially, eight cells discharge, and the battery pack voltage is approximately 28.3V. Figure 13 and 14 As shown.

[0073] like Figure 13 As shown, the battery cell voltage drops below 3 volts at approximately 3400 seconds, so even though all batteries discharge early, the available battery capacity can be better utilized.

[0074] Figure 14 and 15 This indicates the remaining capacity of each battery cell in the battery pack and its initial available capacity. For example... Figure 15 As shown, the initial usable battery pack capacity is approximately 3960 mAh, with most cells' SOC dropping to 0, and each cell's capacity less than 1% (SOCset). Figure 11 In the middle, the remaining usable capacity of the 9 batteries is approximately 48.4 mAh, with a capacity utilization rate of approximately 98.8%; Figure 15 In the initial setup, the remaining battery capacity is approximately 48.4 mAh, and the usable capacity of the nine batteries is approximately 8.8 mAh, resulting in a capacity utilization rate of approximately 99.8%. However, it should be noted that if the battery pack begins charging with an initial SOC of approximately 20%, discharging from 100% will result in a remaining usable capacity of approximately 48.4 mAh for the nine batteries, but the total usable capacity will increase from 3960 mAh to 19800 mAh, with a capacity utilization rate of 99.8% instead of 98.8%. When the battery pack reaches a uniform capacity, all batteries can be discharged to maximize the usable capacity of the battery pack.

[0075] Although the invention has been described with reference to specific features and embodiments, various modifications, combinations, and substitutions can be made therein without departing from the invention. The scope of protection of this invention is not limited to the specific embodiments of processes, machines, manufactures, material compositions, apparatuses, methods, and steps described in the specification, and these methods and modules may also be implemented in one or more related, interdependent, cooperative, or upstream / downstream products or methods.

[0076] Therefore, the specification and drawings should be simply regarded as a description of some embodiments of the technical solutions defined by the appended claims, and thus the appended claims should be interpreted in accordance with the principle of the greatest reasonable interpretation, and are intended to cover as much as possible all modifications, variations, combinations or equivalents within the scope of the invention, while avoiding unreasonable interpretations.

[0077] To achieve better technical effects or for the needs of certain applications, those skilled in the art may make further improvements to the technical solution based on this invention. However, even if such improvements / designs are inventive and / or progressive, as long as they rely on the technical concept of this invention and cover the technical features defined in the claims, the technical solution should also fall within the protection scope of this invention.

[0078] The technical features mentioned in the appended claims may have alternative technical features, or the order of certain technical processes or material organization may be rearranged. Those skilled in the art, upon learning of this invention, will readily conceive of these alternative means, or alter the order of the technical processes or material organization, and then employ substantially the same means to solve substantially the same technical problems and achieve substantially the same technical effects. Therefore, even if the claims explicitly define the aforementioned means and / or order, these modifications, alterations, and substitutions should all fall within the scope of protection of the claims based on the principle of equivalents.

[0079] The method steps or modules described in the embodiments disclosed in this invention can be implemented in hardware, software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the steps and components of each embodiment have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application or design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered outside the scope of protection claimed by this invention.

Claims

1. A battery with stable voltage throughout its entire lifecycle, comprising at least one battery pack, characterized in that: The battery pack includes several battery cells, which are divided into a power supply battery pack or a bypass battery pack depending on whether they participate in supplying power to the load. Each of the plurality of battery cells has a corresponding switch or switch group that can connect the battery cell to the power supply battery pack and the bypass battery pack. When the state of charge difference between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is greater than a preset threshold: the battery cell with the lowest state of charge in the power supply battery pack is added to the bypass battery pack and the battery cell with the highest state of charge in the bypass battery pack is added to the power supply battery pack through the corresponding switch or switch group. When the state of charge difference between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is not greater than a preset threshold: determine whether the voltage of the battery pack is closer to the set value after the battery cell with the highest state of charge in the bypass battery pack is added to the power supply battery pack. If so, add the battery cell with the highest state of charge in the bypass battery pack to the power supply battery pack.

2. The battery with stable voltage throughout its entire life cycle according to claim 1, characterized in that: During the initialization of the battery pack, the battery cells in the battery pack are connected to the power supply battery pack in descending order of their state of charge, until the voltage of the battery pack is closest to the set value.

3. The battery with stable voltage throughout its entire life cycle according to claim 2, characterized in that: Before the battery management system stops working, it continuously checks whether the difference in state of charge between the battery cell with the highest state of charge in the bypass battery pack and the battery cell with the lowest state of charge in the power supply battery pack is greater than a preset threshold.

4. The battery with stable voltage throughout its entire life cycle according to claim 1, characterized in that: When the battery pack is supplying power to a load, the battery cells are connected in series.

5. The battery with stable voltage throughout its entire life cycle according to claim 1, characterized in that: The battery pack is determined to be in a high-charge or low-charge state by the relationship between its value and a second threshold; when the battery pack changes from a high-charge state to a low-charge state, the preset threshold is lowered.

6. The battery with stable voltage throughout its entire life cycle according to claim 1, characterized in that: Any one of the battery cells is coupled to at least two switches. By controlling the two switches to be turned on or off, the battery cell can be bypassed or connected to the power supply battery pack to achieve battery cell balancing.

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