Fast battery active equalization method based on flyback converter

Abstract

The invention relates to a quick battery active equalization method based on a flyback converter, and belongs to the technical field of lithium battery equalization. According to the method, a bidirectional flyback converter is adopted as an energy transfer circuit, and a finite-state machine algorithm FSM and an external auxiliary power supply are introduced, so that the following functions are realized: when the battery with the highest electric quantity reaches a set value, the flyback converter is utilized to transfer energy to the battery with the low electric quantity, the charging speed is accelerated, and the charging time of the whole battery pack is shortened. By limiting the charging and discharging times of the high-energy battery, the aging speed is effectively slowed down, and the service life of the battery is prolonged. The optimal energy transfer algorithm judges the optimal energy transfer direction, efficient utilization of energy is ensured, and energy loss is reduced. The method is suitable for series battery packs, can be widely applied to the fields of electric vehicles, energy storage systems and the like, and effectively improves the performance and service life of the battery packs.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium battery equalization, and relates to a fast battery active equalization method based on a flyback converter. Background Art

[0002] In actual production and work, due to the limitations of the production characteristics of lithium batteries themselves, the energy storage of single cells is usually insufficient. In order to obtain the energy and voltage required for production activities, lithium batteries need to be connected in series or in parallel to form a battery pack for use. However, due to the manufacturing process and other uncontrollable factors, the performance of each battery may be inconsistent, and different battery energies will appear during the charging and discharging process. Overcharging may cause an explosion, and over-discharging may cause a decrease in battery performance or even loss of charging ability. Unbalanced charging and discharging will cause the entire battery pack to behave like a barrel, resulting in loss of energy and battery pack. In summary, it is necessary to balance the battery pack. Currently, common solutions include passive balancing and active balancing.

[0003] Nowadays, many methods are used for active balancing, such as fast balancing of lithium-ion battery packs based on model predictive control, which realizes energy transmission through fuzzy control algorithm, fast active battery balancing strategy for lithium-ion batteries, which transfers energy to the target battery in the battery pack through a non-isolated DC-DC converter, and dynamically adjusts the balancing current to achieve the best balancing speed and energy efficiency, active balancing based on dual controllable flyback converters, which realizes controllable charging of the battery pack through controllable flyback converter charging current, and efficient and high-speed active balancing method for lithium-ion batteries, which improves energy efficiency by minimizing energy loss during the balancing process, and solves the balancing speed related to low balancing current. To solve the problem of slow charging, the modular balancing strategy for lithium battery packs based on adaptive fuzzy logic control and energy path optimization uses AFLC within the group to adjust the balancing current between battery cells and optimize the energy transfer path within the group. It is used to realize flexible active balancing of multi-single-port converters in lithium-ion batteries. By configuring a multi-port active half-bridge converter, it can control the bidirectional power flow between two or more arbitrary batteries according to the charging state of each battery. The optimal control of active battery balancing: expands the range and useful life of the battery pack, maximizes the balancing efficiency by using the MPC model, and dynamically adjusts the balancing current according to the overall condition of the battery pack to achieve the optimal distribution of energy.

[0004] Among the many battery balancing solutions, the voltage-based balancing strategy is relatively simpler, but the terminal voltage is greatly affected by the internal resistance of the capacitor, so it is more appropriate to use the state of charge or as a balancing variable. In addition, most studies have focused on the supplementation of low-energy batteries, ignoring the aging of high-energy batteries caused by multiple charging and discharging of high-energy batteries. Therefore, the present invention mainly studies the balancing solution of the series battery pack under external power supply, using the optimal energy transfer algorithm as the control strategy, the battery SOC as the balancing variable, and the bidirectional flyback converter as the energy transfer circuit. It has the characteristics of judging the optimal energy transfer direction of the circuit and reducing the number of charge and discharge times of high-energy batteries. It can effectively reduce the number of charge and discharge times of high-energy batteries, and use the optimal energy transfer algorithm to effectively speed up the charging of low-energy batteries, thereby speeding up the charging efficiency and better protecting the healthy life and usage of the overall battery pack, avoiding battery health aging due to multiple charging and discharging.

[0005] According to the shortcomings of previous studies, since many balancing strategies mainly mention balancing batteries with different charges in the battery pack to the same charge, and rarely mention speeding up the charging speed of the battery with the lowest charge and reducing the number of charge and discharge times of the battery with a high charge, the focus of this technology is to use the optimal energy transfer algorithm to ensure that the batteries of the entire battery pack are set to a charged state. After the battery with the highest charge reaches the set value, the energy is transferred to the battery with a low charge, thereby speeding up the charging speed of the battery with a low charge, and ensuring that the battery with a high charge does not decrease, thereby speeding up the charging of the battery with a low charge and reducing the number of charge and discharge times of the battery with a high charge. Summary of the invention

[0006] In view of this, an object of the present invention is to provide a fast battery active balancing method based on a flyback converter.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A fast battery active balancing method based on a flyback converter comprises the following steps:

[0009] Connect the cells in the battery pack in series;

[0010] Connect a bidirectional flyback converter in parallel on both sides of each battery;

[0011] Set up an external auxiliary power supply to keep the battery pack in a charging state at all times;

[0012] Use the finite state machine algorithm FSM to determine whether the maximum battery capacity has reached the set value;

[0013] When the battery with the highest power reaches the set value, the flyback converter energy conversion circuit between it and the battery with the lowest power is turned on to transfer energy to the battery with the lowest power, thus speeding up its charging speed.

[0014] After the power of the low-power battery reaches the set value, the flyback converter energy conversion circuits of other batteries whose power is lower than the set value are simultaneously turned on to continue to accelerate their charging speed.

[0015] Connect the cells in the battery pack in series;

[0016] Connect a bidirectional flyback converter in parallel on both sides of each battery;

[0017] Set up an external auxiliary power supply to keep the battery pack in a charging state at all times;

[0018] Use the finite state machine algorithm FSM to determine whether the maximum battery capacity has reached the set value;

[0019] When the battery with the highest power reaches the set value, the flyback converter energy conversion circuit between it and the battery with the lowest power is turned on to transfer energy to the battery with the lowest power, thus speeding up its charging speed.

[0020] After the battery power of the lowest battery reaches the battery power of other batteries lower than the set value, the flyback converter energy conversion circuits of other batteries lower than the set value are synchronously turned on to continue to accelerate their charging speed.

[0021] Furthermore, the finite state machine algorithm FSM is used to determine whether the battery with the highest power reaches a set value, and control the switch state of the flyback converter energy conversion circuit of the corresponding battery according to the determination result, and its state conversion is:

[0022] At the beginning, the maximum battery capacity does not reach the set value, and the battery is charged;

[0023] When the battery with the highest power reaches the set value, balancing and energy transfer are turned on;

[0024] The lowest energy cell balancing circuit is turned on synchronously to accept energy transfer;

[0025] When the battery with the lowest power reaches the other batteries with power lower than the set value, the balancing circuit of other batteries with power lower than the set value is turned on simultaneously;

[0026] The energy of the battery pack tends to be uniform and the balance is completed.

[0027] Furthermore, the external auxiliary power supply is in a disconnected state when the balancing circuit is not turned on, and is in a connected state when the balancing circuit is turned on, so as to ensure that the battery pack is always in a charging state.

[0028] Furthermore, the set value is adjusted according to the specific conditions of the battery pack and is set according to the battery type, capacity and use environment factors.

[0029] Furthermore, in the energy transfer, the SOC of each battery in the battery pack is detected, the battery SOC is detected in real time and balancing is started according to the SOC, the energy transfer state is monitored in real time, and adjustments are made as needed.

[0030] Furthermore, in the energy transfer, the energy transfer strategy is optimized, that is, energy is preferentially transferred to the battery with the lowest charge.

[0031] The beneficial effects of the present invention are as follows: by adding a finite state machine algorithm FSM (Finite state machine) and an auxiliary voltage source, the battery pack is successfully set to a charging state, so that after the battery with the highest power reaches the set charging value, part of the energy is transferred to the low-power battery, thereby speeding up the charging speed of the low-power battery, reducing the number of charge and discharge times of the high-power battery, reducing the health loss of the high-power battery, and compared with the circuit before improvement, speeding up the charging speed of the entire battery pack.

[0032] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0034] Figure 1 It is the energy transfer topology diagram;

[0035] Figure 2 Store energy for W1 converter;

[0036] Figure 3 The W1 converter transmits energy to the W6 converter;

[0037] Figure 4 Transmit energy to the W6 converter;

[0038] Figure 5 It is a flow chart for determining the optimal energy transfer direction;

[0039] Figure 6 This is the balanced SOC diagram of the lithium battery under the optimal energy transfer strategy;

[0040] Figure 7 This is the balanced SOC diagram of the lithium battery without the optimal energy transfer strategy. DETAILED DESCRIPTION

[0041] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways 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 only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0042] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0043] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0044] A terminal voltage identification module is added to the circuit that has been realized, that is, each battery is controlled by an auxiliary power supply to be in a charging state, and the final battery charging capacity is set to 51%. After the charging experiment is started, when the battery with the highest power reaches the set value, its power no longer increases. At the same time, the finite state machine algorithm FSM (Finite state machine) determines to turn on the flyback converter energy conversion circuit of the low-power battery and the high-power battery, so that energy is supplied to the low-power battery through the flyback converter circuit to speed up its charging speed. After the power of the lowest-power battery is consistent with that of other batteries below the set power value, the flyback converter energy conversion circuits of other batteries below the set power value are synchronously turned on to speed up the charging speed of these batteries.

[0045] The specific control topology and algorithm structure are as follows Figure 1As shown, it consists of a series battery pack and an energy transfer circuit. The series battery pack consists of six series batteries, and the energy transfer circuit consists of a series of bidirectional flyback converters connected in parallel on the right side of each battery. The flyback converter is set to a boost state to ensure that the current increases after passing through the flyback converter. The switch of each bidirectional flyback converter is jointly operated by an N-type MOSFET and a diode, and the balancing switch is turned on after being judged by the finite state machine algorithm FSM (Finite state machine).

[0046] By adding an external auxiliary power supply to the high-voltage side of the flyback converter, the series-connected B1-B6 battery pack is controlled to be in a charging state at all times (mainly to ensure the charging state when the balancing circuit is turned on. When the balancing is not turned on, the external auxiliary power supply is disconnected). The power of the B1 battery is set to 50%, B2 to 45%, B3 to 40%, B4 to 35%, B5 to 30%, and B6 to 25%.

[0047] The following will take the energy transfer process from the highest energy battery to the lowest energy battery as an example to elaborate on the role of the finite state machine algorithm FSM (Finite state machine) in energy transfer.

[0048] When the battery B1 with the highest power reaches the set value of 51%, the algorithm calculates and determines that the battery B1 has reached the balance value at this time, outputs the balance opening signal, and turns on the N-type MOSFET to start the balance. At the same time, due to the addition of an external auxiliary power supply, all batteries are forced to remain in the charging state. At this time, the balance circuit of the battery B1 with the highest power and the balance circuit of the battery B6 with the lowest energy will be turned on synchronously, and the battery B1 will remain at 51% without decreasing. Part of the current acting on the series battery group is directly added to the B6 battery through the transformer. Although it will cause the charging current of the other four batteries to decrease, the charging current of the B6 battery is still increased as a whole, speeding up its charging speed. (The following mainly describes the energy transfer from B1 to the B6 battery. The energy transfer of the battery group can refer to the following balance plan).

[0049] Step 1: Run as Figure 2 When the B1 battery opens its balancing circuit, the W1 converter stores energy first because complementary PWM pulses are used. At this time, the N-type MOSFET on the D1 side is closed, the N-type MOSFET on the D2 side is disconnected, and the W1 primary side inductor stores energy. At this time, due to the appearance of two loops, the current on the original series battery will decrease, and energy is stored in the inductor, based on the following formula and topology display:

[0050] Among them, U1 is the B1 battery voltage, L1 is the inductance, D on is the duty cycle of turning on the primary side of the W1 converter, T is the period, I 储The current measured at the primary side of the W1 converter is:

[0051]

[0052] In this process, the primary current magnetizes the transformer core, and the transformer magnetic flux △Φ + Increase linearly, the increase is:

[0053]

[0054] Step 2: When the energy storage time reaches the set time, the N-type MOSFET on the D1 side is disconnected, and the N-type MOSFET on the D2 side and the N-type MOSFET on the D12 side are closed, forming a U2 voltage instantaneously, and at the same time transferring the stored energy to the energy transfer circuit of the B6 battery for energy storage, such as Figure 3 As shown. At this time, since D12 receives the superposition of two currents, I 储 To be greater than I 放 , realizing the transmission of energy.

[0055] At this time, the secondary current of W1 converter decreases, I 放 The current on the secondary side of the W1 converter gradually decreases:

[0056]

[0057] At the same time, the transformer magnetic flux △Φ - It will also reduce:

[0058]

[0059] But at the same time, it will transfer current to the secondary side of the converter W6 and store energy for the secondary side inductor of W6, I 放 is the W1 converter current, I 电源 After turning on the N-type MOSFET on the secondary side of W6, part of the current supplied by the wire power supply is:

[0060] I 储 =I 放 +I 电源

[0061]

[0062] At the same time the magnetic flux will also increase:

[0063]

[0064] Step 3: After reaching the energy transfer time setting, the N-type MOSFET on the D2 side and the N-type MOSFET on the D12 side are disconnected, and the N-type MOSFET on the D1 side and the N-type MOSFET on the D12 side are closed, thereby transferring the energy stored in the previous cycle to the B6 battery, and continuing to store energy in the energy transfer circuit of the B1 battery in the next cycle. At this time, the primary side current of W6 increases, speeding up the overall charging speed of the B6 battery:

[0065]

[0066] like Figure 4 As shown, the trigger signal of the flyback converter is set to complementary rising edge and falling edge triggering, thereby ensuring complete energy transfer.

[0067] In the experiment, the charges of the six batteries were set to 50%, 45%, 40%, 35%, 30%, and 25% respectively. The scheme consists of six flyback converters, and the batteries are connected through switches. When the equalizer does not start working, the six batteries are charged at the same speed, and their charges show the same rising trend, which shows the series nature of the circuit. After the battery with the highest charge reaches the set value, when the equalizer starts working, the energy will be transferred from the high-charge battery to the low-energy battery, which will cause the charging current of the series battery group to decrease slightly, but the overall current given to the low-charge battery will increase. As the battery charge of the battery group tends to be consistent, the current will rise to a normal value. Figure 6 As can be seen in the figure. The judgment algorithm flow chart is as follows Figure 5 As shown in the figure, the six batteries are charged with a constant working current, and the on / off signal of the equalizer is independently controlled by the PWM generator. Since the switch tube of the equalizer is designed as the controlled object in this experiment, the most energy transfer algorithm is used as the discrimination function at the input end of the switch tube of the equalizer to control the corresponding balancing switch. The converter is connected in parallel to each battery. This paper adopts the average SOC as the control object and the terminal voltage as the balancing scheme for charging and discharging discrimination. After detecting that the battery pack is charged, when the battery with the highest power reaches the maximum value set in the experiment, the charging of the battery will be stopped.

[0068] Example of formula to speed up energy transfer: Figure 6 This is the balanced SOC diagram of the lithium battery under the optimal energy transfer strategy; Figure 7 This is the balanced SOC diagram of the lithium battery without the optimal energy transfer strategy.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A fast battery active balancing method based on flyback converter, Features: The following steps are involved: Connect the cells in the battery pack in series; Connect a bidirectional flyback converter in parallel on both sides of each battery; Set up an external auxiliary power supply to keep the battery pack in a charging state at all times; Use the finite state machine algorithm FSM to determine whether the maximum battery capacity has reached the set value; When the battery with the highest power reaches the set value, the flyback converter energy conversion circuit between it and the battery with the lowest power is turned on to transfer energy to the battery with the lowest power, thus speeding up its charging speed. After the battery power of the lowest battery reaches the battery power of other batteries lower than the set value, the flyback converter energy conversion circuits of other batteries lower than the set value are synchronously turned on to continue to accelerate their charging speed.

2. A fast battery active balancing method based on a flyback converter according to claim 1, Features: The finite state machine algorithm FSM is used to determine whether the battery with the highest power reaches the set value, and controls the switch state of the flyback converter energy conversion circuit of the corresponding battery according to the determination result, and its state conversion is: At the beginning, the maximum battery capacity does not reach the set value, and the battery is charged; When the battery with the highest power reaches the set value, balancing and energy transfer are turned on; The lowest energy cell balancing circuit is turned on synchronously to accept energy transfer; When the battery with the lowest power reaches the other batteries with power lower than the set value, the balancing circuit of other batteries with power lower than the set value is turned on simultaneously; The energy of the battery pack tends to be uniform and the balance is completed.

3. The fast battery active balancing method based on a flyback converter according to claim 1, Features: The external auxiliary power supply is in a disconnected state when the equalizing circuit is not turned on, and is in a connected state when the equalizing circuit is turned on, so as to ensure that the battery pack is always in a charging state.

4. A fast battery active balancing method based on a flyback converter according to claim 2, Features: The set value is adjusted according to the specific conditions of the battery pack and is set according to the battery type, capacity and use environment factors.

5. A fast battery active balancing method based on a flyback converter according to claim 2, Features: In the energy transfer, the SOC of each battery in the battery pack is detected, the battery SOC is detected in real time and balancing is started according to the SOC, the energy transfer state is monitored in real time, and adjustments are made as needed.

6. A fast battery active balancing method based on a flyback converter according to claim 2, Features: In the energy transfer, the energy transfer strategy is optimized, that is, energy is preferentially transferred to the battery with the lowest charge.

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