Lithium battery equalization circuit based on multi-winding transformer and flyback converter

Abstract

The invention belongs to the technical field of equalization circuits, and particularly relates to a lithium battery equalization circuit based on a multi-winding transformer and a flyback converter, which comprises a series battery pack formed by n batteries, a multi-winding transformer with n primary windings and one secondary winding, 2n switches arranged at the primary winding end, n buffer resistors and n capacitors, an energy storage capacitor and a diode are arranged at the secondary winding end; the positive electrode of each battery is connected with one end of a corresponding primary winding of the multi-winding transformer, and the negative electrode is connected with the other end of the primary winding through a group of switches; the RC protection circuit is connected in parallel with the corresponding primary winding; two ends of a secondary winding of the multi-winding transformer are connected with the energy storage capacitor to form a loop and then connected with the series battery pack through the diode. According to the invention, equalization between any battery and any battery, equalization between any battery and any battery pack, equalization between any battery pack and any battery pack and equalization between battery packs can be realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of balancing circuits, and particularly relates to a lithium battery balancing circuit based on a multi-winding transformer and a flyback converter. Background Art

[0002] In recent years, with the rapid development of the electric vehicle market, lithium batteries, as an energy storage unit for electric vehicles with high energy density and long service life, have been widely used. However, during the charging and discharging process and daily use of lithium batteries, due to differences in manufacturing processes, material properties, and usage environments, voltage and capacity imbalances will occur among individual lithium battery cells. Working in this unbalanced state for a long time will cause the performance of the battery pack, such as capacity retention, voltage consistency, charge and discharge efficiency, and safety, to decline, shorten the service life of the lithium battery, and even pose potential safety hazards. To solve the inconsistency between batteries, the research on lithium battery balancing technology has become a research hotspot in recent years.

[0003] Currently, lithium battery balancing technologies can be divided into two types: passive balancing and active balancing, also known as passive equalization and active equalization. Passive balancing usually adopts the form of resistor discharge, releasing the excess energy of the battery with higher power through a resistor in the form of heat energy, without the need for a complex control circuit, and having obvious advantages in terms of cost. However, during the balancing process, the energy loss is large, the efficiency is low, and the heat generated during discharge poses additional requirements for the temperature management of the battery pack. If too much heat accumulates, it may affect the safety of the entire battery pack. Compared with passive balancing, active balancing actively transfers energy between individual battery cells through an external circuit, transferring the energy of the battery cell with higher energy to the battery cell with lower energy, with high energy utilization efficiency and significantly improved balancing efficiency, and having a broader application prospect.

[0004] Active balancing methods can generally be divided into inductor-based balancing, capacitor-based balancing, and transformer-based balancing according to different energy storage elements. Among them, the balancing topology with an inductor as the energy storage unit usually has a high energy conversion efficiency, and the balancing speed is not affected by the voltage difference between the batteries. However, this method often requires more switching devices and is relatively complex to control. In contrast, the balancing topology with a capacitor as the energy storage unit is easier to control, but the balancing speed is affected by the voltage difference between the capacitor and the balancing target. The active balancing method based on a transformer uses the transformer winding as the energy transfer carrier, further improving the balancing speed and efficiency. However, the balancing circuit control strategy is relatively more complex, and the balancing current will decrease at the end of the balancing stage, affecting the overall balancing efficiency. Summary of the Invention

[0005] In view of the deficiencies in the above prior art, the object of the present invention is to provide a lithium battery equalization circuit based on a multi-winding transformer and a flyback converter, which can achieve equalization between any battery and any other battery, equalization between any battery and a battery pack, equalization between a battery pack and any battery, and equalization between battery packs.

[0006] To achieve the above object, the technical solution adopted by the present invention is a lithium battery equalization circuit based on a multi-winding transformer and a flyback converter. The equalization circuit includes a series battery pack composed of n batteries, a multi-winding transformer with n primary windings and 1 secondary winding. At the primary winding end, there are 2n MOSFET switches, n buffer resistors, and n capacitors. At the secondary winding end, there is 1 energy storage capacitor C s and 1 diode D, and the battery is a lithium battery; The batteries in the series battery pack are respectively B1, B2,..., B n , and every two of the 2n MOSFET switches form a group, which are respectively S 1.1 , S 1.2 , S 2.1 , S 2.2 , …, S n.1 , S n.2 , and the n buffer resistors are respectively R1, R2,..., R n , and the n capacitors are respectively C1, C2,..., C n ; The positive electrode of each battery is connected to one end of the corresponding primary winding of the multi-winding transformer, and the negative electrode is connected to the other end of the primary winding after passing through a group of MOSFET switches to form a loop; The RC protection circuit is connected in parallel with the corresponding primary winding, and the RC protection circuit is composed of a buffer resistor and a capacitor; Both ends of the secondary winding of the multi-winding transformer are connected to the energy storage capacitor C s to form a loop, and then connected to the series battery pack through the diode D.

[0007] As a preferred solution of the present invention, the equalization circuit has 4 working modes, namely the cell-to-cell equalization mode between any battery and any other battery, the cell-to-pack equalization mode between any battery and a battery pack, the pack-to-cell equalization mode between a battery pack and any battery, and the pack-to-pack equalization mode between battery packs; where a battery pack refers to a battery pack composed of several batteries with the same voltage; Assume that the battery with the maximum voltage is B i , and the battery with the minimum voltage is B j , i, j = 1, 2,..., n and i ≠ j, calculate the maximum voltage difference ΔV, and set the threshold F; Judge the number of cells with the maximum voltage and the number of cells with the minimum voltage in the series battery pack. When the number of cells with the maximum voltage and the number of cells with the minimum voltage are both 1, enter the cell-to-cell equalization mode; when the number of cells with the maximum voltage is 1 and the number of cells with the minimum voltage is not 1, enter the cell-to-pack equalization mode; when the number of cells with the maximum voltage is not 1 and the number of cells with the minimum voltage is 1, enter the pack-to-cell equalization mode; when the number of cells with the maximum voltage and the number of cells with the minimum voltage are both not 1, enter the pack-to-pack equalization mode.

[0008] As a preferred solution of the present invention, the equalization process of the cell-to-cell equalization mode is as follows: S1. In each equalization period, judge the voltage magnitudes of each cell, find the cells with the maximum and minimum voltages, and judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2; otherwise, return to S1. S2. Control the MOSFET switch S i connected to the cell with the maximum voltage B i.1 to have a high-level PWM signal with S i.2 . At this time, the MOSFET switch S i.1 and S i.2 conduct to form a loop, and the cell with the maximum voltage B i discharges, and the cell energy is transferred to the primary winding. S3. Control the MOSFET switch S i.1 and S i.2 to have a low-level PWM signal. The MOSFET switch S i.1 and S i.2 disconnect. The energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s . S4. Control the MOSFET switch S j connected to the cell with the minimum voltage B j.1 to have a high-level PWM signal with S j.2 . The MOSFET switch S j.1 and S j.2 conduct to form a loop. The energy in the secondary winding is coupled to the primary winding to charge the cell B j . The energy storage capacitor C s charges the entire series battery pack through the diode D. S5. When the discharge of the secondary winding ends, control the MOSFET switch S j.1 and S j.2The PWM signal is at a low level; at this time, it is judged whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, then return to S2; if ΔV<F, the energy transfer stage of one equalization period is completed, and return to S1 to continue judging the voltages of each battery, and so on until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0009] As a preferred solution of the present invention, the equalization process of the cell-to-pack equalization mode is as follows: S1. In each equalization period, judge the voltage magnitudes of each battery, find the battery with the maximum voltage and the battery with the minimum voltage. Assume the battery with the maximum voltage is B i , and the battery with the minimum voltage is B j and B k , k = 1, 2,..., n and k≠i, j. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2; otherwise, return to S1; S2. Control the MOSFET switch S i connected to the battery B with the maximum voltage i.1 and S i.2 to have a high-level PWM signal. At this time, the MOSFET switches S i.1 and S i.2 are turned on to form a loop, and the battery B i discharges, and the battery energy is transmitted to the primary winding of the transformer; S3. Control the MOSFET switches S i.1 and S i.2 to have a low-level PWM signal. The MOSFET switches S i.1 and S i.2 are disconnected, and the energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transmitted to the energy storage capacitor C s ; S4. Control the MOSFET switches S j and B k connected to the battery B with the minimum voltage j.1 , S j.2 and S k.1 , S k.2 to have a high-level PWM signal. The MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 are turned on to form a loop. The energy in the secondary winding is coupled to the primary winding to charge the batteries B j and B k , and the energy storage capacitor C s charges the entire series battery pack through the diode D; S5. The discharge of the secondary winding ends, and the MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 have their PWM signals at low level. At this time, it is judged whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and so on until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0010] When there are more than two batteries with the minimum voltage, the equalization is carried out in the same way, and the subsequent equalization modes are the same.

[0011] As a preferred solution of the present invention, the equalization process of the pack-to-cell equalization mode is as follows: S1. In each equalization cycle, judge the voltage magnitudes of each battery, find the battery with the maximum and minimum voltages. Assume the battery with the maximum voltage is B i and B g , the battery with the minimum voltage is B j , g = 1, 2,..., n and g≠i, j. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2, otherwise return to S1; S2. Control the MOSFET switches S i and B g connected to the battery with the maximum voltage to have their PWM signals at high level. At this time, the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 are turned on to form a loop, and the batteries B i.1 , S i.2 and S g.1 , S g.2 discharge, and the battery energy is transferred to the primary winding of the transformer; i and B g discharge, and the battery energy is transferred to the primary winding of the transformer; S3. Control the PWM signals of the MOSFET switches S i.1 and S i.2 to be at low level, and the MOSFET switches S i.1 and S i.2 are disconnected. The energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switches S j connected to the battery with the minimum voltage to have their PWM signals at high level. At this time, the MOSFET switches S j.1 and S j.2The PWM signal is at a high level, and the MOSFET switch S j.1 and S j.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the battery B j The energy storage capacitor C s charges the entire series battery pack through the diode D; S5. When the discharge of the secondary winding ends, control the MOSFET switches S j.1 and S j.2 so that the PWM signals are at a low level; at this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of one cycle is completed, return to S1 to continue to judge the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0012] As a preferred solution of the present invention, the equalization process of the pack-to-pack equalization mode is as follows: S1. In each equalization cycle, judge the voltage magnitudes of each battery, find the batteries with the maximum and minimum voltages. Assume that the battery with the maximum voltage is B i and B g , and the battery with the minimum voltage is B j and B k , where k, g = 1, 2,..., n and k≠i, j, g≠i, j, k≠g. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2, otherwise return to S1; S2. Control the MOSFET switches S i and B g connected to the battery with the maximum voltage, so that the PWM signals of S i.1 , S i.2 and S g.1 , S g.2 are at a high level. At this time, the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 conduct to form a loop, and the batteries B i and B g discharge, and the battery energy is transferred to the primary winding of the transformer; S3. Control the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 so that the PWM signals are at a low level. The MOSFET switches S i.1 , S i.2 and S g.1 , S g.2Disconnect, the energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switches S j and B k connected to the minimum voltage batteries B j.1 , S j.2 and S k.1 , S k.2 such that the PWM signals are at high level. The MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the batteries B j and B k , and the energy storage capacitor C s charges the entire series battery pack through the diode D; S5. When the discharge of the secondary winding ends, control the MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 such that the PWM signals are at low level. At this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0013] As a preferred solution of the present invention, F is taken as 0.01.

[0014] As a preferred solution of the present invention, for the set threshold F, it is dynamically optimized according to the following formula: ; In the formula, is the initially set threshold; is the temperature sensitivity coefficient; is the maximum temperature difference of the battery; is the current suppression coefficient; is the current average charge and discharge current; is the frequency compensation coefficient; is the number of balancing triggers in the last 1 hour; is the preset maximum allowable balancing frequency; In addition, set the safety temperature threshold T of the battery. When the temperature of a battery exceeds the safety temperature threshold T, force F to be adjusted to 50% of its original value to accelerate balancing.

[0015] The beneficial effects of the present invention are: The present invention has four working modes, which can achieve the equalization between any battery and any other battery, the equalization between any battery and a battery pack, the equalization between a battery pack and any battery, and the equalization between battery packs; each battery unit only needs to be controlled by a pair of complementary MOSFET switches, and the entire equalization circuit only needs to be controlled by a pair of complementary PWM signals during equalization to achieve equalization, and the control is relatively simple.

[0016] The present invention can charge the entire series battery pack through the energy storage capacitor (auxiliary equalization path) on the secondary winding side, effectively alleviating the problem of the reduction in equalization speed at the end of the equalization stage; it can work in static, charging, and discharging states, and the applicable working conditions are more extensive.

[0017] The present invention adds an RC protection circuit to each battery unit on the primary winding side of the multi-winding transformer to absorb transient voltage during switch operation, prevent voltage spikes from damaging components, increase the service life of MOSFET switches, and improve the stability and reliability of the entire equalization circuit; using the multi-winding transformer as an energy transfer device, it has stronger adaptability to different battery quantity configurations and is more suitable for equalization of multiple battery packs. Description of the Drawings

[0018] Figure 1 is the equalization circuit diagram of the present invention based on the combination of a multi-winding transformer and a flyback converter; Figure 2 is the flow chart of the equalization control strategy of the present invention; Figure 3 is the structure diagram of the series battery pack and the equalization circuit in Embodiment 2 of the present invention; Figure 4 is the schematic diagram of the equalization circuit in the first stage of equalization in Embodiment 2 of the present invention; Figure 5 is the schematic diagram of the equalization circuit in the second stage of equalization in Embodiment 2 of the present invention; Figure 6 is the schematic diagram of the equalization circuit in the third stage of equalization in Embodiment 2 of the present invention; Figure 7 is the simulation diagram of the equalization result in Matlab / Simulink in Embodiment 2 of the present invention; Figure 8 is the PWM control signal diagram of the equalization circuit in Embodiment 2 of the present invention; Figure 9 is the experimental diagram of the equalization result in Embodiment 2 of the present invention. Detailed Embodiments

[0019] The following further describes the embodiments of the present invention with reference to the drawings: Embodiment 1: A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter, as Figure 1 shown. The equalization circuit includes a series battery pack composed of n batteries, a multi-winding transformer with n primary windings and 1 secondary winding. There are 2n MOSFET switches, n buffer resistors and n capacitors arranged at the primary winding end, and there is 1 energy storage capacitor C s and 1 diode D at the secondary winding end. The batteries are lithium batteries; The batteries in the series battery pack are B1, B2, …, B n , respectively. Every two of the 2n MOSFET switches form a group, which are S 1.1 、S 1.2 , S 2.1 、S 2.2 , …, S n.1 、S n.2 , respectively. The n buffer resistors are R1, R2, …, R n , respectively. The n capacitors are C1, C2, …, C n ; The positive electrode of each battery is connected to one end of the corresponding primary winding of the multi-winding transformer, and the negative electrode is connected to the other end of the primary winding after passing through a group of MOSFET switches to form a loop; The RC protection circuit is connected in parallel with the corresponding primary winding. The RC protection circuit consists of a buffer resistor and a capacitor, which absorbs the transient voltage during the switch operation to prevent the voltage spike from damaging the components; Both ends of the secondary winding of the multi-winding transformer are connected to the energy storage capacitor C s to form a loop, and then connected to the series battery pack through the diode D.

[0020] The equalization circuit has 4 working modes, namely the cell-to-cell equalization mode between any two batteries, the cell-to-pack equalization mode between any battery and the battery pack, the pack-to-cell equalization mode between the battery pack and any battery, and the pack-to-pack equalization mode between the battery packs; where the battery pack refers to a battery pack composed of several batteries with the same voltage; Assume that the battery with the maximum voltage is B i , and the battery with the minimum voltage is B j , i, j = 1, 2, …, n and i ≠ j. Calculate the maximum voltage difference ΔV, and set the threshold F, and the value of F is 0.01; Judge the number of cells with the maximum voltage and the number of cells with the minimum voltage in the series battery pack. When the number of cells with the maximum voltage and the number of cells with the minimum voltage are both 1, enter the cell-to-cell equalization mode; when the number of cells with the maximum voltage is 1 and the number of cells with the minimum voltage is not 1, enter the cell-to-pack equalization mode; when the number of cells with the maximum voltage is not 1 and the number of cells with the minimum voltage is 1, enter the pack-to-cell equalization mode; when the number of cells with the maximum voltage and the number of cells with the minimum voltage are both not 1, enter the pack-to-pack equalization mode.

[0021] In this embodiment, the MOSFET switch, capacitor, energy storage capacitor, diode, etc. constitute a flyback converter, which is the core circuit for realizing the energy equalization of lithium batteries.

[0022] As Figure 2 shown, the overall equalization principle of each equalization mode is as follows: S1. In each equalization cycle, judge the voltage magnitudes of each cell, find the cells with the maximum and minimum voltages, and judge whether the maximum voltage difference ΔV (the voltages of the cells with the maximum and minimum voltages are V max and V min , ΔV = V max -V min ) in the series battery pack is greater than or equal to the set threshold F. When ΔV ≥ F, enter S2; otherwise, return to S1. S2. Control the PWM signal of a group of MOSFET switches connected to the cell with the maximum voltage to be high level. This group of MOSFET switches conducts to form a loop, and the cell with the maximum voltage discharges, and the cell energy is transferred to the primary winding. S3. Control the PWM signal of this group of MOSFET switches to be low level. This group of MOSFET switches disconnects, and the energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s . S4. Control the PWM signal of a group of MOSFET switches connected to the cell with the minimum voltage to be high level. This group of MOSFET switches conducts to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the cell with the minimum voltage, and the energy storage capacitor C s charges the entire series battery pack through the diode D. S5. When the discharge of the secondary winding ends, the PWM signal of the MOSFET switch corresponding to the cell with the minimum voltage is low level; at this time, judge whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV ≥ F, return to S2; if ΔV < F, the energy transfer stage of one equalization cycle is completed, and return to S1 to continue judging the voltages of each cell. Repeat this process until the maximum voltage difference ΔV of all cells in the series battery pack is less than the set threshold F.

[0023] Embodiment 2: The number of the maximum-voltage battery and the minimum-voltage battery is both 1. As Figure 3 shown, set n = 4. The maximum-voltage battery is B1 with a voltage of 3.58 V, and the minimum-voltage battery is B4 with a voltage of 3.03 V. The difference is ΔV = 0.55 V, which is greater than the set balancing threshold of 0.01 V. At this time, the balancing process of the cell-to-cell balancing mode is as follows: First balancing stage: As Figure 4 shown, control the PWM signals of the MOSFET switches S 1.1 and S 1.2 connected to the maximum-voltage battery B1 to be at a high level. At this time, the MOSFET switches S 1.1 and S 1.2 conduct to form a loop, and the maximum-voltage battery B1 discharges, and the battery energy is transferred to the primary winding; until the voltage of the battery B1 tends to be the same as the voltage of the primary winding, enter the second balancing stage. The arrow dotted line is the current flow direction, and the same below.

[0024] Second balancing stage: As Figure 5 shown, control the PWM signals of the MOSFET switches S 1.1 and S 1.2 to be at a low level. The MOSFET switches S 1.1 and S 1.2 are disconnected, and all the switches in the circuit are in the off state, that is, in the dead time of the PWM signal. The energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; the energy storage capacitor Cs discharges the entire series battery pack through the diode D.

[0025] Third balancing stage: As Figure 6 shown, after the dead time ends, control the PWM signals of the MOSFET switches S 4.1 and S 4.2 connected to the minimum-voltage battery B4 to be at a high level. The MOSFET switches S 4.1 and S 4.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the battery B4, and the energy storage capacitor C s charges the entire series battery pack through the diode D; until the voltage of the battery B4 tends to be the same as the voltage of the primary winding.

[0026] After the discharge of the secondary winding ends, control the MOSFET switches S 4.1 and S 4.2The PWM signal is at a low level; at this time, it is judged whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV ≥ F, then return to S2; if ΔV < F, the energy transfer stage of one equalization period is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0027] During the above equalization process, there may be slight charge and discharge or energy transfer of the battery. If equalization is continuously performed, it may cause the battery to work excessively, increasing unnecessary heat generation or battery burden. By introducing an interval time, after the end of each equalization period and until the start of the next equalization period, a short interval time is set, which can allow the battery to rest and recover, avoid overoperation or battery overheating, and improve the stability of the equalization circuit operation.

[0028] As Figure 7 shown, it is the simulation diagram of the equalization result of this embodiment in Matlab / Simulink. According to the Figure 3 topological structure, a simulation model is built in Matlab / Simulink. The parameters of each device in the circuit are shown in Table 1. Eight capacitor units with a capacitance value of 1F are connected in series to simulate the series battery pack. The values of the RC circuit are set to 1kΩ and 47μF. The energy storage capacitance value at the secondary winding end is 10μF. The switching frequency is 10kHz, and the turns ratio of the transformer is 20:1. It can be seen from Figure 7 that during the equalization process, the voltage difference between each battery gradually decreases and finally tends to be the same.

[0029] Table 1 Component parameters

[0030] To further verify the feasibility and effectiveness of this equalization circuit in practice, an experimental platform of a four-series battery pack (B1 to B4) is built. The battery uses Panasonic battery NCR18650B, and the battery parameters are shown in Table 2. A complementary PWM signal (PWM1 and PWM2, when PWM1 is at a high level, PWM2 is at a low level; when PWM1 is at a low level, PWM2 is at a high level) is used to control the equalization circuit, as Figure 8 shown.

[0031] Table 2 Battery parameters

[0032] As Figure 9 shown, it is the experimental diagram of the equalization result of the above experimental platform. It can be seen from Figure 9It can be seen that around 1200 seconds, the voltages of all the batteries tend to be the same, and the voltage difference drops from the maximum value of 550 mV at the start of balancing to 90 mV. The experimental results prove the effectiveness and feasibility of this embodiment in practice.

[0033] Embodiment 3: The number of the batteries with the maximum voltage is 1, and the number of the batteries with the minimum voltage is not 1. At this time, the balancing process of the cell-to-pack balancing mode is as follows: S1. Within each balancing cycle, judge the voltage magnitudes of each battery, and find out the battery with the maximum voltage and the battery with the minimum voltage. Assume that the battery with the maximum voltage is B i , the battery with the minimum voltage is B j and B k , k = 1, 2,..., n and k ≠ i, j. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV ≥ F, enter S2; otherwise, return to S1. S2. Control the MOSFET switch S i connected to the battery B with the maximum voltage i.1 and S i.2 to have a high-level PWM signal. At this time, the MOSFET switch S i.1 and S i.2 conduct to form a loop, and the battery B i discharges, and the battery energy is transferred to the primary winding of the transformer. S3. Control the MOSFET switch S i.1 and S i.2 to have a low-level PWM signal. The MOSFET switch S i.1 and S i.2 disconnect. The energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s . S4. Control the MOSFET switches S j and B k connected to the battery B with the minimum voltage j.1 , S j.2 and S k.1 , S k.2 to have a high-level PWM signal. The MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 conduct to form a loop. The energy in the secondary winding is coupled to the primary winding to charge the batteries B j and B k , and the energy storage capacitor C s charges the entire series battery pack through the diode D. When the discharge of the secondary winding ends, control the MOSFET switch Sj.1 , S j.2 and S k.1 , S k.2 The PWM signals of S are at low level; at this time, it is judged whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and so on until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0034] Embodiment 4: The number of batteries with the maximum voltage is not 1, and the number of batteries with the lowest voltage is 1. At this time, the equalization process of the pack-to-cell equalization mode is as follows: S1. In each equalization cycle, judge the voltage magnitudes of each battery, find the batteries with the maximum and minimum voltages. Assume the battery with the maximum voltage is B i and B g , and the battery with the minimum voltage is B j , g = 1, 2,..., n and g≠i, j. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2, otherwise return to S1; S2. Control the MOSFET switches S i and B g connected to the battery with the maximum voltage B i.1 , S i.2 and S g.1 , S g.2 to have high-level PWM signals. At this time, the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 conduct to form a loop, and the batteries B i and B g discharge, and the battery energy is transferred to the primary winding of the transformer; S3. Control the PWM signals of the MOSFET switches S i.1 and S i.2 to be at low level, and the MOSFET switches S i.1 and S i.2 disconnect. The energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switches S j connected to the battery with the minimum voltage B j.1 and S j.2 to have high-level PWM signals. The MOSFET switches S j.1 and S j.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding for the battery Bj Charging, energy storage capacitor C s Charge the entire series battery pack through diode D; S5. When the discharge of the secondary winding ends, control the PWM signals of MOSFET switches S j.1 and S j.2 to be at a low level; at this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0035] Embodiment 5: The number of batteries with the maximum voltage and the number of batteries with the minimum voltage are both not 1. At this time, the equalization process of the pack-to-pack equalization mode is as follows: S1. In each equalization cycle, judge the voltage magnitudes of each battery, find the batteries with the maximum and minimum voltages. Assume the battery with the maximum voltage is B i and B g , and the battery with the minimum voltage is B j and B k , where k, g = 1, 2,..., n and k≠i, j, g≠i, j, k≠g. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2; otherwise, return to S1; S2. Control the PWM signals of the MOSFET switches S i and B g connected to the batteries with the maximum voltage to be at a high level. At this time, the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 conduct to form a loop, and the batteries B i.1 , S i.2 and S g.1 , S g.2 discharge, and the battery energy is transferred to the primary winding of the transformer; i and B g discharge, and the battery energy is transferred to the primary winding of the transformer; S3. Control the PWM signals of the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 to be at a low level. The MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 disconnect, and the energy stored in the primary winding is coupled to the secondary winding. A part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switches S j and B k connected to the minimum voltage battery B j.1 、S j.2 and S k.1 、S k.2 to have a high-level PWM signal, so that the MOSFET switches S j.1 、S j.2 and S k.1 、S k.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the batteries B j and B k , and the energy storage capacitor C s charges the entire series battery pack through the diode D; S5. When the discharge of the secondary winding ends, control the MOSFET switches S j.1 、S j.2 and S k.1 、S k.2 to have a low-level PWM signal; at this time, judge whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

[0036] Embodiment 6: For the set threshold F, dynamically optimize it according to the following formula: ; In the formula, is the initially set threshold (0.01); is the temperature sensitivity coefficient; is the maximum temperature difference of the battery; is the current suppression coefficient; is the current average charge and discharge current; is the frequency compensation coefficient; is the number of balancing times triggered within the last 1 hour; is the preset maximum allowable balancing frequency; e is the natural constant.

[0037] In addition, set the safety temperature threshold T of the battery (for example, 55°C). When the temperature of a battery exceeds the safety temperature threshold T, forcefully adjust F to 50% of its original value to accelerate balancing.

[0038] Each coefficient can be set according to requirements. For example, determine the base value through an offline calibration experiment: =0.005~0.015 / °C (when the temperature increases by 1°C, the threshold increases by 0.5%~1.5%); = 0.02~0.1 / A (for every 1A increase in current, the exponential decay coefficient increases by 2%~10%); 0.1~0.3 (the compensation coefficient linearly increases for every 10% of the maximum equalization frequency reached).

[0039] This dynamic threshold mechanism can be implemented through an embedded controller. Temperature and current monitoring are achieved through existing technologies. While maintaining the voltage equalization accuracy, it automatically suppresses ineffective equalization under abnormal conditions according to the real-time working conditions. When the battery pack is in a high-rate charge and discharge state, the threshold is increased to reduce the equalization trigger frequency to avoid energy conflicts, and fine equalization is automatically restored during the stationary state.

Claims

1. A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter, characterized in that: The balancing circuit includes a series battery pack composed of n batteries, a multi-winding transformer with n primary windings and 1 secondary winding. There are 2n MOSFET switches, n buffer resistors and n capacitors provided at the primary winding ends, and there is 1 energy storage capacitor C and 1 diode D at the secondary winding end. The battery is a lithium battery; s ​ The batteries in the series battery pack are B1, B2, …, B n , and the 2n MOSFET switches are grouped in pairs as S 1.1 、S 1.2 , S 2.1 、S 2.2 , …, S n.1 、S n.2 , and the n buffer resistors are R1, R2, …, R n , and the n capacitors are C1, C2, …, C n ; The positive electrode of each battery is connected to one end of the corresponding primary winding of the multi-winding transformer, and the negative electrode is connected to the other end of the primary winding through a group of MOSFET switches to form a loop; The RC protection circuit is connected in parallel with the corresponding primary winding, and the RC protection circuit consists of a buffer resistor and a capacitor; Both ends of the secondary winding of the multi-winding transformer are connected to the energy storage capacitor C s to form a loop, and then connected to the series battery pack through the diode D.

2. The lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 1, wherein: The equalization circuit has 4 working modes, namely the cell-to-cell equalization mode (any battery to any battery), the cell-to-pack equalization mode (any battery to the battery pack), the pack-to-cell equalization mode (the battery pack to any battery), and the pack-to-pack equalization mode (the battery pack to the battery pack); where the battery pack refers to a battery pack composed of several batteries with the same voltage; Assume that the battery with the maximum voltage is B i , and the battery with the minimum voltage is B j , where i, j = 1, 2, …, n and i ≠ j, calculate the maximum voltage difference ΔV, and set the threshold F; Judge the number of batteries with the maximum voltage and the number of batteries with the minimum voltage in the series battery pack. When the number of batteries with the maximum voltage and the number of batteries with the minimum voltage are both 1, enter the cell-to-cell equalization mode; when the number of batteries with the maximum voltage is 1 and the number of batteries with the minimum voltage is not 1, enter the cell-to-pack equalization mode; when the number of batteries with the maximum voltage is not 1 and the number of batteries with the minimum voltage is 1, enter the pack-to-cell equalization mode; when the number of batteries with the maximum voltage and the number of batteries with the minimum voltage are both not 1, enter the pack-to-pack equalization mode.

3. The lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 2, wherein The equalization process of the cell-to-cell equalization mode is as follows: S1. In each equalization period, judge the voltage magnitudes of each battery, find the battery with the maximum and minimum voltages, and judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV≥F, enter S2, otherwise return to S1; S2, control and maximum voltage battery B i The connected MOSFET switch S i.1 With S i.2 The PWM signal is high level, and the MOSFET switch S i.1 With S i.2 Conductivity forms a loop, the maximum voltage battery B i Discharging, the battery energy is transferred to the primary winding; S3. Control the MOSFET switch S i.1 With S i.2 The PWM signal is at a low level, and the MOSFET switch S i.1 With S i.2 is turned off, and the energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switch S connected to the minimum voltage battery B j When the PWM signal of S j.1 is high, the MOSFET switch S j.2 conducts to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the battery B j.1 The energy storage capacitor C j.2 charges the entire series battery pack through the diode D; j s ​​ S5. When the discharge of the secondary winding ends, control the MOSFET switch S j.1 and S j.2 The PWM signal of is at a low level; at this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV≥F, return to S2; if ΔV<F, the energy transfer stage of an equalization cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

4. A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 2, characterized in that, The equalization process of the cell-to-pack equalization mode is as follows: S1. During each equalization cycle, determine the voltage magnitudes of each battery, and find the batteries with the maximum and minimum voltages. Assume the battery with the maximum voltage is B i , the battery with the minimum voltage is B j and B k , where k = 1, 2, …, n and k ≠ i, j. Determine whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV ≥ F, proceed to S2; otherwise, return to S1 S2, control and maximum voltage battery B i The connected MOSFET switch S i.1 With S i.2 The PWM signal is high level, and the MOSFET switch S i.1 With S i.2 The circuit is formed, and battery B i Discharging, the battery energy is transferred to the primary winding of the transformer; S3. Control the MOSFET switch S i.1 and S i.2 the PWM signal of is at a low level, and the MOSFET switch S i.1 and S i.2 is turned off, and the energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switches S connected to the minimum voltage batteries B j and B k such that the PWM signals of the MOSFET switches S j.1 、S j.2 and S k.1 、S k.2 are at a high level, and the MOSFET switches S j.1 、S j.2 and S k.1 、S k.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the batteries B j and B k , and the energy storage capacitor C s charges the entire series battery pack through the diode D; S5. When the discharge of the secondary winding ends, control the MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 to have low-level PWM signals. At this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV ≥ F, return to S2; if ΔV < F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

5. A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 2, characterized in that, The equalization process of the pack-to-cell equalization mode is as follows: S1. In each balancing cycle, determine the voltage magnitudes of each battery, and find the battery with the maximum and minimum voltages. Assume the battery with the maximum voltage is B i and B g , and the battery with the minimum voltage is B j , where g = 1, 2, …, n and g ≠ i, j. Determine whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV ≥ F, proceed to S2; otherwise, return to S1; S2. Control the MOSFET switches S i connected to the maximum voltage batteries B g and B i.1 to be at a high level for the PWM signals of S i.2 , S g.1 , and S g.2 . At this time, the MOSFET switches S i.1 , S i.2 , and S g.1 , S g.2 conduct to form a loop, and the batteries B i and B g discharge, and the battery energy is transferred to the primary winding of the transformer; S3. Control the MOSFET switch S i.1 With S i.2 The PWM signal is at a low level, and the MOSFET switch S i.1 With S i.2 Is turned off, and the energy stored in the primary winding is coupled to the secondary winding. A part of the energy in the secondary winding is transferred to the energy storage capacitor C s ; S4. Control the MOSFET switch S connected to the minimum voltage battery B j When the PWM signal of S j.1 is at a high level, the MOSFET switch S j.2 conducts to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the battery B j.1 The energy in the secondary winding is coupled to the primary winding to charge the battery B j.2 The energy in the secondary winding is coupled to the primary winding to charge the battery B j The energy in the secondary winding is coupled to the primary winding to charge the battery B, and the energy storage capacitor C s charges the entire series battery pack through the diode D; S5. When the discharge of the secondary winding ends, control the MOSFET switches S j.1 and S j.2 to have a low-level PWM signal. At this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV ≥ F, return to S2. If ΔV < F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

6. A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 2, characterized in that, The equalization process of the pack-to-pack equalization mode is as follows: S1. During each equalization cycle, judge the voltage magnitudes of each battery, find the battery with the maximum and minimum voltages. Assume the battery with the maximum voltage is B i and B g , and the battery with the minimum voltage is B j and B k . Let k, g = 1, 2,..., n and k ≠ i, j, g ≠ i, j, k ≠ g. Judge whether the maximum voltage difference ΔV in the series battery pack is greater than or equal to the set threshold F. When ΔV ≥ F, go to S2; otherwise, return to S1 S2. Control the MOSFET switches S i connected to the maximum voltage batteries B g and B i.1 to be at a high level for the PWM signals of S i.2 , S g.1 , and S g.2 . At this time, the MOSFET switches S i.1 , S i.2 , and S g.1 , S g.2 conduct to form a loop, and the batteries B i and B g discharge, and the battery energy is transferred to the primary winding of the transformer; S3. Control the MOSFET switches S i.1 , S i.2 and S g.1 , S g.2 The PWM signals of S i.1 , S i.2 and S g.1 , S g.2 are at low level, and the MOSFET switches S s open. The energy stored in the primary winding is coupled to the secondary winding, and a part of the energy in the secondary winding is transferred to the energy storage capacitor C S4. Control the MOSFET switches S connected to the minimum voltage batteries B j and B k such that the PWM signals of the MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 are at high level, and the MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 conduct to form a loop, and the energy in the secondary winding is coupled to the primary winding to charge the batteries B j and B k . The energy storage capacitor C s charges the entire series battery pack through the diode D; S5. When the discharge of the secondary winding ends, control the MOSFET switches S j.1 , S j.2 and S k.1 , S k.2 to have a low-level PWM signal. At this time, determine whether the maximum voltage difference ΔV is greater than or equal to the set threshold F. If ΔV ≥ F, return to S2. If ΔV < F, the energy transfer stage of one cycle is completed, return to S1 to continue judging the voltages of each battery, and repeat this process until the maximum voltage difference ΔV of all batteries in the series battery pack is less than the set threshold F.

7. A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 2, characterized in that: F takes a value of 0.

01.

8. A lithium battery equalization circuit based on a multi-winding transformer and a flyback converter according to claim 2, characterized in that, For the set threshold F, it is dynamically optimized according to the following formula: ； Wherein, is the initially set threshold value; is the temperature sensitivity coefficient; is the maximum temperature difference of the battery; is the current suppression coefficient; is the current average charge and discharge current; is the frequency compensation coefficient; is the number of balancing times triggered within the last 1 hour; is the preset maximum allowable balancing frequency; In addition, set the safe temperature threshold T of the battery. When the temperature of a battery exceeds the safe temperature threshold T, forcefully adjust F to 50% of its original value to accelerate equalization.

Images