A lithium battery pack distributed dynamic equalization charging system and method

Abstract

The present application relates to a kind of lithium battery pack distributed dynamic equalization charging system and method, belong to battery technical field.The charging system includes lithium battery pack, energy transfer circuit, isolated controller, control system, drive circuit and charging bidirectional switch;The input side of the energy transfer module is connected to the positive and negative end of single battery, and the output end is connected to the positive and negative end of lithium battery pack;The control system controls each energy transfer module by isolated controller, so that single battery energy is transferred to lithium battery pack;Charging bidirectional switch is provided in the lithium battery pack charging loop, and control system cooperates drive circuit to modulate it, controls the energy of lithium battery pack.The method is at the end of charging, and each battery is in the same preset equalization condition, and the electrical performance consistency is similar, the overall use capacity of lithium battery pack is maximized, and the service life of battery is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a distributed dynamic equalization charging system and method for lithium battery packs. Background Technology

[0002] In practical applications, lithium batteries typically require multiple individual lithium batteries to be connected in series to form a battery pack to meet the voltage levels and power requirements of different electrical devices. Due to differences in battery manufacturing processes and external operating environments, inconsistencies can occur between the batteries within a pack. When used in series, this can lead to a reduction in the usable capacity of the lithium battery pack, a shortened cycle life, and potential safety hazards. Therefore, it is necessary to implement balanced and safe management for series-connected lithium battery packs.

[0003] Currently, lithium battery pack balancing solutions are divided into passive balancing and active balancing based on different energy processing methods. Passive balancing dissipates excess energy from a battery by connecting a dissipation resistor in parallel across each individual cell. It has a simple structure and is easy to control, but the balancing current is small, the balancing speed is slow, and the parallel balancing resistor generates a large amount of heat, posing thermal management safety issues. Active balancing, on the other hand, has advantages such as high energy utilization and fast balancing speed. It typically uses capacitors, inductors, transformers, and DC-DC converters to construct energy transfer circuits to achieve energy transfer between cells within the pack.

[0004] Active balancing can be broadly categorized into centralized balancing and distributed balancing based on the energy transfer circuit structure. In a centralized balancing structure, the entire lithium battery pack shares a single energy transfer circuit, resulting in high circuit utilization. However, the circuit design is more complex, and the overall balancing time increases with the number of batteries. This approach is suitable for lithium battery packs with a limited number of cells in series and is difficult to expand and maintain. In a distributed balancing structure, each battery cell corresponds to its own energy transfer circuit, allowing for simultaneous energy transfer across multiple cells. This results in a higher energy transfer rate, more flexible control, and easier implementation of various balancing control strategies. It is also easily expandable and is currently the more commonly used balancing method.

[0005] Capacitor-based distributed equalization methods use capacitors as energy transfer carriers, achieving charge balance based on the voltage difference between individual cells within a group. When the voltage difference between adjacent cells is large, the equalization current is large, and after each equalization, the capacitor retains residual transferred energy, which can easily cause heat generation issues and result in low reliability. Inductor-based distributed equalization methods offer controllable equalization current, but lead to a larger energy transfer circuit size. Transformer-based distributed equalization methods become increasingly complex with the number of cells, increasing both size and cost. DC-DC converter-based distributed equalization methods allow converters to be designed with the same input and output parameters, simplifying converter circuit reuse, resulting in higher operational reliability, easier maintenance, and the ability for multiple converters to operate simultaneously with a high equalization rate, making them more widely used in engineering applications.

[0006] Currently, most distributed equalization methods based on DC-DC converters equalize the lithium battery pack while it is charging, transferring energy from the balanced individual cells to the entire pack. However, this method still allows energy to flow into the balanced individual cells, causing their voltage to rise further and posing a safety hazard of overcharging. Furthermore, it cannot ensure that all cells are fully charged, failing to maximize the usable capacity of the lithium battery pack. Alternatively, some methods involve real-time communication with the charger during the equalization process to regulate the input energy to the lithium battery pack, ensuring that the individual cells within the pack have the same charge level at the end of charging. However, real-time communication with the charger increases control complexity and cost. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a distributed dynamic equalization charging system and method for lithium battery packs, so as to achieve control of the charging energy of lithium battery packs without controlling the charger, avoiding the safety hazards of charging lithium battery packs for a long time without disconnecting the charger. It can also achieve dynamic equalization charging of lithium battery packs, keeping the charge level of fully charged batteries unchanged, while continuing equalization charging of partially charged batteries. At the end of charging, the entire battery pack is in a fully charged state, increasing the usable capacity of the lithium battery pack and extending battery life.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A distributed dynamic equalization charging system for lithium battery packs, the charging system comprising a lithium battery pack, an energy transfer circuit, an isolation controller, a control system, a drive circuit, and a bidirectional charging switch;

[0010] The lithium battery pack consists of n cells labeled Cell1, Cell2, ..., Cell... n The circuit consists of individual battery cells connected in series; each individual battery cell is connected to both ends with an energy transfer module, and there are a total of n modules, denoted as energy transfer module 1, energy transfer module 2, ..., energy transfer module n; the n energy transfer modules form an energy transfer circuit.

[0011] The input side of the energy transfer module is connected to the positive and negative terminals of a single battery cell, and the output side is connected to the positive and negative terminals of the lithium battery pack.

[0012] The control system controls each energy transfer module through an isolation controller, enabling the energy of individual batteries to be transferred to the lithium battery pack.

[0013] The lithium battery pack charging circuit is equipped with a bidirectional charging switch, which is modulated by the control system and the drive circuit to control the charging energy of the lithium battery pack.

[0014] Optionally, when the equalization function of the charging system is not enabled, the charger charges the lithium battery pack according to the preset settings, the energy transfer circuit does not work, and the charging bidirectional switch is in the fully open state.

[0015] When a single cell in the lithium battery pack reaches the equalization threshold, the corresponding single cell is transferred to the lithium battery pack through the energy transfer circuit. At the same time, the input energy is adjusted to cooperate with the energy transfer circuit of the single cell by controlling the duty cycle of the charging bidirectional switch. This ensures that the equalization variable of the single cell in the lithium battery pack that has reached the threshold remains unchanged after the energy transfer circuit is turned on. When the equalization ends, the equalization variables of all single cells tend to be consistent.

[0016] During the charging process of the lithium battery pack, the terminal voltage of each individual cell reacts with energy. The battery is considered fully charged when the voltage of a single cell reaches its maximum voltage, and a battery terminal voltage balancing variable is selected. At the end of the charging process of the power lithium battery pack, the input energy of the lithium battery pack is controlled to coordinate with the energy transfer of individual cells. This ensures that the energy of individual cells whose terminal voltage has reached the balancing threshold no longer increases and their terminal voltage remains unchanged, while the energy of the remaining individual cells continues to increase and their terminal voltage rises until they reach the balancing threshold. Ultimately, all individual cells reach the balancing threshold, achieving the goal of consistent terminal voltage across all individual cells at the end of charging.

[0017] Suppose that the terminal voltage of a single cell Cell1 in the lithium battery pack first reaches the equalization threshold, activating equalization. The energy transfer circuit of Cell1 then starts, and energy flows from Cell1 to the entire battery pack through this circuit. Let the efficiency of all energy transfer circuits be η, then:

[0018] ΔE'1=η·ΔE1 (1)

[0019] In a series-connected lithium battery pack, given the same charging current and relatively small differences in the terminal voltages of individual cells, the energy input to a single cell Cell1 is E1:

[0020]

[0021] Assuming that the energy outflow of a single cell Cell1 is equal to its energy input, and that the energy retained by a single cell Cell1 remains constant, we can obtain:

[0022] ΔE1=E1 (3)

[0023] Combining equations (1), (2), and (3), we get:

[0024] E in =E-ΔE'1=nE1-ηΔE1=(n-η)ΔE1 (4)

[0025] The required energy input to the charger, E, is obtained by measuring the equalizer input energy ΔE1. inAdjust the input energy E in To ensure that the energy of the single-cell battery remains constant throughout the remaining charge process, the average input energy is maintained. <E in The formula for calculating > is:

[0026]

[0027] In the formula E in The input energy remains constant over a period of time, where T is one PWM cycle, and t on D represents the on-time of the bidirectional charging switch within one switching cycle, and D represents the duty cycle of the bidirectional charging switch within one cycle. By adjusting the on-time of the charging switch, the average value of the input energy is controlled. The longer the on-time, the larger the average value of the input energy, and vice versa.

[0028] By controlling the enable switch of the energy transfer circuit, the terminal voltage of the single cell Cell1 is kept at V. ON With V OFF Fluctuations between E and E, energy in E ON With E OFF Intermittent fluctuations ensure the energy E input to the single cell Cell1 is maintained. 1_in satisfy:

[0029] E 1_in ≤ΔE1 (6)

[0030] Under the conditions of equation (6), when the energy transfer circuit of cell 1 is turned on, the energy of cell 1 decreases and the terminal voltage drops; when the energy transfer circuit of cell 1 is turned off, the energy of cell 1 increases and the terminal voltage rises; by changing the enable signal of the energy transfer circuit, the terminal voltage of cell 1 can be kept at V. ON With V OFF Fluctuations between E and E, energy in E ON With E OFF Intermittent fluctuations;

[0031] V ON -V OFF <V σ (7)

[0032] E ON -E OFF <E σ (8)

[0033] When V in equations (7) and (8) σ E σ When it is a minimum value, it is assumed that the single cell Cell1 in the equilibrium process t0~t mDuring the process, the terminal voltage and energy remain constant, thus ensuring that the energy of the single cell Cell1 remains relatively constant during the balancing process.

[0034] E n_in ≤ΔE n (9)

[0035] Under the condition of satisfying equation (9), all individual cells reach this energy, thus completing the balancing of all individual cells;

[0036] The energy transfer circuit consists of a DC-DC converter, which is a two-stage structure consisting of a front-stage Boost circuit and a rear-stage flyback circuit. The front-stage Boost circuit initially boosts the voltage of a single battery cell to 10V, which serves as the input to the rear-stage flyback circuit. The rear-stage flyback circuit performs circuit isolation and raises the input voltage to the lithium battery pack voltage. Combined with the intermittent flyback gain formula, it performs constant power output control, with a maximum rated output power of 5W.

[0037] A distributed dynamic equalization charging method for lithium battery packs based on the aforementioned system includes the following steps:

[0038] S1: Determine whether the lithium battery pack is currently in a protection state;

[0039] Before the charger is connected to the lithium battery pack, the charging two-way switch is open and the charging circuit is in an open circuit state; the control system detects the terminal voltage of each individual battery in the pack. When any battery is in an under-voltage or over-voltage state, the charging two-way switch is always open and the lithium battery pack is not charged. At the same time, the energy transfer circuit does not work.

[0040] S2: Lithium battery pack charging begins.

[0041] After determining that the lithium battery pack has not entered the protection state, turn on the two-way charging switch to charge the lithium battery pack according to the preset charging rate.

[0042] S3: Determine whether equalization charging is enabled.

[0043] During the charging process of the lithium battery pack, the control system detects whether the terminal voltage of a single battery reaches the equalization start threshold. If the terminal voltage of a single battery reaches the equalization start threshold, S4 is executed; otherwise, the current charging state is maintained and charging continues until the terminal voltage of a battery is detected to reach the equalization start threshold.

[0044] S4: Balanced Charging

[0045] Enable equalization charging; the control system queries and filters individual cells that have reached the equalization threshold, activates the energy transfer circuit, then queries the terminal voltage of the individual cells with the energy transfer circuit activated, calculates the difference between the current terminal voltage and the terminal voltage of the individual cell at the previous sampling time, i.e., the equalization terminal cell terminal voltage increment; if all equalization terminal cell terminal voltage increments are not positive, the charging switch duty cycle is increased, and the input energy power is increased; if there are equalization terminal cell terminal voltage increments that are positive, the charging switch duty cycle is decreased, and the input energy power is decreased.

[0046] S5: Determine if equalization charging is complete

[0047] Repeat the query and control method of S4 until all individual cells reach the equalization threshold, then turn off the charging switch and all energy transfer circuits. At this time, the terminal voltage of all individual cells in the lithium battery pack is between the equalization start threshold and the equalization stop threshold, and the equalization charging is completed.

[0048] S6: Charging complete

[0049] Report a charging completion signal, record voltage change data during the entire charging process, and end charging.

[0050] The beneficial effects of this invention are as follows:

[0051] (1) It does not need to communicate with the charger. The charging energy of the lithium battery pack is controlled by controlling the opening and closing time of the bidirectional charging switch, which simplifies the control.

[0052] (2) Ensure that each cell in the lithium battery pack is fully charged and no further charging is required, while other cells in the lithium battery pack can continue to charge without being affected by the equalization cells. This charging method can extend battery life and avoid the risk of overcharging.

[0053] (3) When the charging is finished, each battery is under the same preset equalization conditions, and the electrical performance is similar, which maximizes the overall capacity of the lithium battery pack and extends the battery life.

[0054] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0055] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0056] Figure 1It is a distributed dynamic full-balanced charging system structure;

[0057] Figure 2 To balance the energy flow relationship during the charging process;

[0058] Figure 3 This is a diagram showing the balance control of individual cells;

[0059] Figure 4 This is the energy transfer circuit for a DC-DC converter.

[0060] Figure 5 To balance the voltage changes of each individual battery cell during charging. Detailed Implementation

[0061] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0062] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0063] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0064] Equalization charging system structure: The distributed dynamic full equalization charging system structure for lithium battery packs provided by this invention Figure 1 As shown.

[0065] The lithium battery pack consists of n cells labeled Cell1, Cell2, ..., Cell.n The battery pack consists of individual battery cells connected in series. Each cell has an energy transfer module connected to both ends, with a total of n modules. The input of each energy transfer module is connected to the positive and negative terminals of the individual battery cell, and the output is connected to the positive and negative terminals of the entire battery pack. The control system uses an isolation controller to control the normal operation of each energy transfer module, enabling the transfer of energy from the individual battery cells to the lithium battery pack. A bidirectional charging switch is added to the lithium battery pack charging circuit, and the control system, in conjunction with the drive circuit, modulates this switch to control the energy input to the lithium battery pack.

[0066] The principle of equalization charging: The energy flow relationship during the equalization charging process of a lithium battery pack is as follows: Figure 2 As shown.

[0067] When the balancing function is not activated, the charger charges the lithium battery pack according to the preset settings, the energy transfer circuit does not work, and the bidirectional charging switch is in the fully open state. When a single cell in the pack reaches the balancing activation threshold, the corresponding single cell transfers energy to the lithium battery pack through the energy transfer circuit. At the same time, the input energy is adjusted to coordinate with the energy transfer circuit of the single cell by controlling the duty cycle of the bidirectional charging switch. This ensures that the balancing variables of the single cells in the lithium battery pack that have reached the threshold remain unchanged after the energy transfer circuit is activated. When balancing ends, the balancing variables of all single cells tend to be consistent.

[0068] During the charging process of a lithium battery pack, the terminal voltage of each individual cell reflects its energy. The battery is considered fully charged when its voltage reaches its maximum. Therefore, a battery terminal voltage balancing variable is used. Towards the end of the charging process, the input energy of the lithium battery pack is controlled in conjunction with the energy transfer between individual cells. This ensures that the energy of cells whose terminal voltage has reached the balancing threshold no longer increases, and their terminal voltage remains constant. Meanwhile, the energy of the remaining cells continues to increase, causing their terminal voltages to rise until they reach the balancing threshold. Ultimately, all individual cells reach the balancing threshold, achieving the goal of consistent terminal voltage across all cells at the end of charging.

[0069] Now assume that the terminal voltage of a single cell Cell1 in the lithium battery pack first reaches the equalization threshold, and equalization is initiated. Figure 2 As shown. At this time, the energy transfer circuit of the single cell Cell1 is activated, and energy flows from the single cell Cell1 to the entire battery pack through the energy transfer circuit. Let the efficiency of all energy transfer circuits be η, then we have:

[0070] ΔE'1=η·ΔE1 (1)

[0071] In a series-connected lithium battery pack, given the same charging current and relatively small differences in the terminal voltages of individual cells, the energy input to a single cell Cell1 is E1:

[0072]

[0073] Assuming that the energy outflow from cell 1 equals the energy input, the energy stored in cell 1 remains constant. Therefore:

[0074] ΔE1=E1 (3)

[0075] By combining equations (1), (2), and (3), we can obtain:

[0076] E in =E-ΔE'1=nE1-ηΔE1=(n-η)ΔE1 (4)

[0077] The required energy input to the charger, E, can be obtained by measuring the equalizer input energy ΔE1. in Adjust the input energy E in This allows the energy of a single cell to remain constant throughout the remaining charge process, representing an absolute energy balance. (Input energy average value) <E in The formula for calculating > is:

[0078]

[0079] In the formula E in The input energy is approximately constant over a short period of time, where T is one PWM cycle, and t on Let D be the on-time of the bidirectional charging switch within one switching cycle, and D be the duty cycle of the bidirectional charging switch within one cycle. By adjusting the on-time of the charging switch, the average value of the input energy can be controlled. The longer the on-time, the larger the average input energy, and vice versa. This method achieves input energy control based on the existing switching mechanism of power lithium battery pack management systems without adding additional circuitry or controlling the charger.

[0080] In practical applications, the efficiency of each energy transfer circuit varies, and the energy of each individual cell in a power lithium battery pack is not evenly distributed, making accurate quantitative analysis of the energy of all individual cells extremely difficult. Therefore, this paper improves the above-mentioned absolute energy balance into a relative energy balance, such as... Figure 3 As shown.

[0081] By controlling the enable switch of the energy transfer circuit, the Cell1 terminal of the single cell is pressed at V. ON With V OFF Fluctuations between E and E, energy in E ON With E OFF Fluctuations. At this point, it is only necessary to ensure that the energy E input to the single cell Cell1 is maintained. 1_in satisfy:

[0082] E 1_in ≤ΔE1 (6)

[0083] Under the conditions of equation (6), when the energy transfer circuit of cell 1 is turned on, the energy of cell 1 decreases and the terminal voltage drops. When the energy transfer circuit of cell 1 is turned off, the energy of cell 1 increases and the terminal voltage rises. By changing the enable signal of the energy transfer circuit, the terminal voltage of cell 1 can be kept at V. ON With V OFF Fluctuations between E and E, energy in E ON With E OFF Fluctuations.

[0084] V ON -V OFF <V σ (7)

[0085] E ON -E OFF <E σ (8)

[0086] When V in equations (7) and (8) σ E σ When it is a minimum value, it can be considered that the single cell Cell1 is in the equilibrium process t0~t m During the process, the terminal voltage and energy remain approximately constant. This achieves the goal of keeping the energy of the single cell Cell1 relatively constant during the equalization process.

[0087] This method can be extended to all single-cell batteries.

[0088] E n_in ≤ΔE n (9)

[0089] Under the condition of satisfying equation (9), the energy dynamic matching scheme can keep the energy of the individual cells relatively unchanged during the balancing process, so that all individual cells eventually reach the energy range and the balancing of all individual cells is completed.

[0090] Energy transfer circuits composed of DC-DC converters, such as Figure 4 As shown, the circuit consists of a two-stage structure comprising a pre-stage Boost circuit and a post-stage flyback circuit. The pre-stage Boost circuit initially boosts the voltage of a single battery cell to 10V, which serves as the input to the post-stage flyback circuit. The post-stage flyback circuit provides circuit isolation and raises the input voltage to the lithium battery pack voltage. Using the discontinuous flyback gain formula, it performs constant power output control, with a maximum rated output power of 5W.

[0091] The implementation steps of the plan are as follows:

[0092] Step 1: Determine if the lithium battery pack is currently in a protection state.

[0093] Before the charger is connected to the lithium battery pack, the bidirectional charging switch is open, and the charging circuit is in an open circuit state. The control system detects the terminal voltage of each individual battery in the pack. If any battery is in an undervoltage or overvoltage state, the bidirectional charging switch remains open, the lithium battery pack is not charged, and the energy transfer circuit does not work.

[0094] Step 2: Start charging the lithium battery pack.

[0095] After determining that the lithium battery pack has not entered the protection state, turn on the two-way charging switch to charge the lithium battery pack according to the preset charging rate.

[0096] Step 3: Determine whether equalization charging is enabled.

[0097] During the lithium battery pack charging process, the control system detects whether any individual battery's terminal voltage reaches the equalization activation threshold. If any individual battery's terminal voltage reaches the equalization activation threshold, step 4 is executed. Otherwise, the current charging state is maintained and charging continues until a battery's terminal voltage is detected to reach the equalization activation threshold.

[0098] Step 4: Equalizing charging

[0099] Enable equalization charging. The control system queries and filters individual cells that have reached the equalization threshold, activates the energy transfer circuit, and then queries the terminal voltage of the individual cells with the energy transfer circuit activated. It calculates the difference between the current terminal voltage and the terminal voltage of that individual cell at the previous sampling time, i.e., the equalization terminal cell terminal voltage increment. If all equalization terminal cell terminal voltage increments are not positive, the charging switch duty cycle is increased, increasing the input energy power; if any equalization terminal cell terminal voltage increment is positive, the charging switch duty cycle is decreased, decreasing the input energy power.

[0100] Step 5: Determine if equalization charging is complete.

[0101] Repeat this query and control method until all individual cells reach the equalization threshold, then turn off the charging switch and all energy transfer circuits. At this time, the terminal voltage of all individual cells in the lithium battery pack is between the equalization start threshold and the equalization stop threshold, and the terminal voltage difference is small, thus the equalization charging is complete.

[0102] Step 6: Charging complete

[0103] Report a charging completion signal, record voltage change data during the entire charging process, and end charging.

[0104] A case study is conducted using a 24-cell lithium iron phosphate battery pack as the experimental subject.

[0105] The single lithium iron phosphate battery model is IFR32135-15Ah.

[0106] Specific experimental procedures:

[0107] Step 1: Determine if the lithium battery pack is currently in a protection state.

[0108] Before the charger is connected to the lithium battery pack, the charging bidirectional switch is open, and the charging circuit is in an open circuit state. The control system detects the terminal voltage of each individual battery in the pack. If any battery is in an undervoltage (2.0V) or overvoltage (3.7V) state, the charging bidirectional switch remains open, the lithium battery pack is not charged, and the energy transfer circuit does not work.

[0109] Step 2: Start charging the lithium battery pack.

[0110] After determining that the lithium battery pack has not entered the protection state, turn on the two-way charging switch to charge the lithium battery pack at the preset charging rate of 0.5C (7.5A).

[0111] Step 3: Determine whether equalization charging is enabled.

[0112] During the lithium battery pack charging process, the control system detects whether any individual battery's terminal voltage reaches the equalization activation threshold (3.50V). If any individual battery's terminal voltage reaches the equalization activation threshold (3.50V), step 4 is executed. Otherwise, the current charging state is maintained and charging continues until a battery's terminal voltage is detected to reach the equalization activation threshold (3.50V).

[0113] Step 4: Equalizing charging

[0114] The equalization charging function is activated. The control system first adjusts the bidirectional charging switch to charge the lithium battery pack at a charging rate of 0.1C (1.5A). It then queries and filters individual cells that have reached the equalization activation threshold (3.50V) and activates the energy transfer circuit. Next, it queries the terminal voltage of the activated individual cells and calculates the difference between the current terminal voltage and the terminal voltage at the previous sampling time, i.e., the equalization terminal voltage increment. If all equalization terminal voltage increments are not positive, the charging switch duty cycle is increased, increasing the input energy power; if any equalization terminal voltage increment is positive, the charging switch duty cycle is decreased, decreasing the input energy power.

[0115] The current voltage of batteries 1 and 2 is set to reach the equalization threshold (3.50V), at which point energy transfer circuit 1 and energy transfer circuit 2 will start working. The energy transfer circuit is designed to output a constant power of 5W.

[0116] The formula for the output gain of the front-end Boost circuit in the energy transfer circuit is:

[0117]

[0118] In the formula V g V is the input voltage of the Boost circuit. o1 For the output voltage, D BoostThe duty cycle of the switch is the on-state operation during one switching cycle.

[0119] The formula for the output gain of the subsequent flyback circuit in the energy transfer circuit is:

[0120]

[0121] The expression for the output power P can be obtained as follows:

[0122]

[0123] In the formula L p V is the magnetizing inductance of the transformer in the flyback circuit. o1 Where is the input voltage, T is one switching cycle, and D1 is the on-duty cycle of one switching cycle.

[0124] At this point, the control system queries batteries 1 and 2 to calculate the current terminal voltage of each battery and the increase in terminal voltage of that battery at the previous sampling time. If the terminal voltage increase is not positive, the charging switch duty cycle is increased, increasing the input energy power. If there is a balanced battery with a positive terminal voltage increase, the charging switch duty cycle is decreased, decreasing the input energy power. The remaining batteries continue to be charged using the energy transferred through the balancing process.

[0125] Step 5: Determine if equalization charging is complete.

[0126] Repeat this query and control method until all individual cells reach the equalization threshold, then turn off the charging switch and all energy transfer circuits. At this time, the terminal voltage of all individual cells in the lithium battery pack is between the equalization start threshold and the equalization stop threshold, and the terminal voltage difference is small, thus the equalization charging is complete.

[0127] Step 6: Charging complete

[0128] Report a charging completion signal and end charging.

[0129] The battery terminal voltage change was collected throughout the charging process, and the results are as follows: Figure 5 As shown, at the end of charging, the terminal voltage of each battery is the same, and the voltage difference is 39mV, which completes the equalization charging of the lithium battery pack.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A distributed dynamic equalization charging system for lithium battery packs, characterized by: The charging system includes a lithium battery pack, an energy transfer circuit, an isolation controller, a control system, a drive circuit, and a bidirectional charging switch; The lithium battery pack is composed of n a plurality of single batteries respectively marked as Cell1, Cell2, …, Cell n N, each single battery is connected with an energy transfer module at both ends, and there are n N energy transfer modules, which are marked as energy transfer module 1, energy transfer module 2, …, energy transfer module n N, n N energy transfer modules form an energy transfer circuit; The input side of the energy transfer module is connected to the positive and negative terminals of a single battery cell, and the output side is connected to the positive and negative terminals of the lithium battery pack. The control system controls each energy transfer module through an isolation controller, enabling the energy of individual batteries to be transferred to the lithium battery pack. The lithium battery pack charging circuit is equipped with a bidirectional charging switch. The control system, in conjunction with the drive circuit, modulates the bidirectional charging switch to control the charging energy of the lithium battery pack. When a single cell in the lithium battery pack reaches the equalization activation threshold, the energy of the corresponding single cell is transferred to the lithium battery pack through the energy transfer circuit. At the same time, the input energy is adjusted by controlling the duty cycle of the bidirectional charging switch and cooperating with the single cell energy transfer circuit. This ensures that the equalization variable of the single cell in the lithium battery pack that has reached the equalization activation threshold remains unchanged after the energy transfer circuit is activated. When the equalization ends, the equalization variables of all single cells tend to be consistent. The equalization variable is the terminal voltage of the single cell. For Cell1, the single cell in the lithium battery pack whose voltage first reaches the equalization threshold and then initiates equalization, the input energy of the equalizer is measured. The energy input required by the charger. By adjusting the duty cycle of the charging bidirectional switch D Adjust input energy This ensures that the energy of the single-cell battery remains constant throughout the remaining charging process, with the average input energy remaining constant. satisfy: （1） In the formula, For a given period of time, the input energy remains constant. , T One pulse width modulation (PWM) cycle The on-time of the bidirectional charging switch within one switching cycle. D The duty cycle of the bidirectional charging switch within one cycle is used to control the average value of the input energy by adjusting the on-time of the bidirectional charging switch. By controlling the enable switch of the energy transfer circuit, the terminal voltage of the single cell Cell1 is set to... V ON and V OFF Fluctuations between, energy in E ON and E OFF Fluctuations between these ranges, and the energy input to the single cell Cell1 satisfy: （2） Under the conditions of equation (2), when the energy transfer circuit of cell 1 is turned on, the energy of cell 1 decreases and the terminal voltage drops; when the energy transfer circuit of cell 1 is turned off, the energy of cell 1 increases and the terminal voltage rises. By changing the enable signal of the energy transfer circuit, the terminal voltage of cell 1 can be maintained at... V ON and V OFF Fluctuations and energy levels E ON and E OFF Fluctuating between, and satisfying: （3） （4） in, and It is a local minimum.

2. The distributed dynamic equalization charging system for lithium battery packs according to claim 1, characterized in that: When the equalization function of the charging system is not activated, the charger charges the lithium battery pack according to the preset settings, the energy transfer circuit does not work, and the charging bidirectional switch is in the fully open state. During the charging process, the energy of a single cell is reflected by its terminal voltage. The battery is considered fully charged when the voltage of a single cell reaches its maximum voltage. Towards the end of the charging process, the input energy of the lithium battery pack is controlled to coordinate with the energy transfer of the individual cells. This ensures that the energy of cells whose terminal voltage has reached the equilibrium threshold no longer increases and their terminal voltage remains constant. Meanwhile, the energy of the remaining cells continues to increase and their terminal voltage rises until they reach the equilibrium threshold. Ultimately, all cells reach the equilibrium threshold, achieving the goal of consistent terminal voltage across all cells at the end of charging. Suppose that the terminal voltage of a single cell Cell1 in the lithium battery pack first reaches the equalization threshold, balancing is initiated, the energy transfer circuit of single cell Cell1 starts, and energy flows from single cell Cell1 to the entire battery pack through the energy transfer circuit. Let the efficiency of all energy transfer circuits be 1. Then we have: （5） In a series-connected lithium battery pack, given the same charging current and relatively similar terminal voltages of individual cells, the energy input to cell 1 is... : （6） Assuming that the energy outflow of a single cell Cell1 is equal to its energy input, and that the energy retained by a single cell Cell1 remains constant, we can obtain: （7） Combining equations (5), (6), and (7), we get: （8） For individual cells in lithium battery packs n Input single cell battery Cell n Energy in satisfy: （9） Under the condition of satisfying equation (9), all individual cells reach this energy, thus completing the balancing of all individual cells; The energy transfer circuit consists of a DC-DC converter, which is a two-stage structure composed of a front-stage Boost circuit and a rear-stage flyback circuit. The front-stage Boost circuit initially boosts the voltage of the individual battery cell to 10V, which serves as the input to the rear-stage flyback circuit. The rear-stage flyback circuit performs circuit isolation and raises the input voltage to the lithium battery pack voltage. Combined with the intermittent flyback gain formula, it performs constant power output control, and its maximum rated output power is 5W.

3. A distributed dynamic equalization charging method for lithium battery packs based on the system described in claim 1, characterized in that: The method includes the following steps: S1: Determine whether the lithium battery pack is currently in a protection state; Before the charger is connected to the lithium battery pack, the charging two-way switch is open and the charging circuit is in an open circuit state; the control system detects the terminal voltage of each individual battery in the pack. When any battery is in an under-voltage or over-voltage state, the charging two-way switch is always open and the lithium battery pack is not charged. At the same time, the energy transfer circuit does not work. S2: Lithium battery pack charging begins. After determining that the lithium battery pack has not entered the protection state, turn on the two-way charging switch to charge the lithium battery pack according to the preset charging rate. S3: Determine whether equalization charging is enabled. During the charging process of the lithium battery pack, the control system detects whether the terminal voltage of a single battery reaches the equalization start threshold. If the terminal voltage of a single battery reaches the equalization start threshold, S4 is executed; otherwise, the current charging state is maintained and charging continues until the terminal voltage of a battery is detected to reach the equalization start threshold. S4: Balanced Charging Enable equalization charging; the control system queries and filters individual cells that have reached the equalization threshold, activates the energy transfer circuit, then queries the terminal voltage of the individual cells with the energy transfer circuit activated, calculates the difference between the current terminal voltage and the terminal voltage of the individual cell at the previous sampling time, i.e., the equalization terminal cell terminal voltage increment; if all equalization terminal cell terminal voltage increments are not positive, the charging switch duty cycle is increased, and the input energy power is increased; if there are equalization terminal cell terminal voltage increments that are positive, the charging switch duty cycle is decreased, and the input energy power is decreased. S5: Determine if equalization charging is complete Repeat the query and control method of S4 until all individual cells reach the equalization threshold, then turn off the charging switch and all energy transfer circuits. At this time, the terminal voltage of all individual cells in the lithium battery pack is between the equalization start threshold and the equalization stop threshold, and the equalization charging is completed. S6: Charging complete Report a charging completion signal, record voltage change data during the entire charging process, and end charging.

Images