Cuk-based lithium battery pack SOC equalization circuit

Abstract

The invention discloses a Cuk-based lithium battery pack SOC equalization circuit. The circuit comprises a basic Cuk converter, (n-1) bridging capacitors, (n-1) output expansion units and n lithium ion batteries, the basic Cuk converter comprises two inductors L1 and Lin, capacitors C1 and Cin, a switch S1 and a diode D1; the (n-1) th output expansion unit comprises an inductor Ln, a capacitor Cn and a diode Dn; wherein one end of the inductor Ln is connected with the anode of the diode Dn, the other end of the inductor Ln is connected with one end of the capacitor Cn, and the other end of the capacitor Cn is connected with the cathode of the diode Dn. According to the invention, a multi-output Cuk converter equalization topological structure is adopted, and the terminal voltage equalization of the battery units in the battery pack is realized by adding a bridging capacitor between the output units. According to the equalization circuit, automatic voltage equalization between the battery units is achieved through the bridging capacitors, and complex sampling and control algorithms are not needed; and moreover, the problem of terminal voltage imbalance caused by inconsistent parasitic parameters of devices during multi-output of the converter is avoided, the voltage deviation is effectively reduced, and the battery equalization precision is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery balancing, and particularly to a Cuk-based SOC balancing circuit for a lithium battery pack. Background Art

[0002] To meet the performance requirements of energy storage systems, lithium-ion batteries are usually used in groups. However, there are inconsistencies among different battery monomers, such as differences in performance like capacity, internal resistance, and voltage. These differences will reduce the capacity utilization rate of the battery pack, and at the same time, will also cause overcharging and over-discharging of the battery, accelerating battery aging and endangering the safety of the battery system. The current battery balancing technology is most significant in optimizing the inconsistency effect. The battery balancing technology is mainly divided into passive balancing and active balancing. Passive balancing, also known as energy-consuming balancing, means that the excess energy of the battery monomer is all consumed in the form of heat. However, this method will cause large energy loss of the battery, and the high-temperature environment caused by improper heat dissipation treatment will damage the battery performance. Active balancing, as a non-energy-consuming balancing, means that energy is transferred through energy storage elements, thereby reducing the inconsistency of the battery pack and achieving the balance of battery energy. Due to its high energy utilization rate and flexible energy transfer, active balancing is a hot spot in battery balancing technology.

[0003] For example, the patent document with the application publication number CN116247925A discloses a control method for a battery balancing bidirectional DC-DC converter. This method adds multiple balancing control switches on the basis of a lithium-ion battery pack and integrates multiple working modes. By controlling the closing of the switches, the circuit topology is reconstructed, thereby changing the working modes, including charging mode, external power supply mode, battery pack function mode, battery pack voltage balancing mode, and energy recovery mode. It can not only achieve bidirectional energy flow, but also achieve step-up and step-down functions, and there are multiple different combinations of power supply methods to increase backup and improve fault tolerance; the voltage balance of the battery pack is achieved through the energy storage inductor and switch tube of the bidirectional DC-DC converter, thereby improving the utilization rate of the device. However, the balancing circuit adopted by this control method uses fourteen switch tubes for only three battery packs, and the logic control is very complex. Summary of the Invention

[0004] To address the deficiencies of existing battery pack balancing technologies, the present invention proposes a Cuk-based lithium battery pack SOC balancing circuit. Based on the balancing circuit of a converter, it adopts a multi-output Cuk converter balancing topology structure and realizes the equalization of the terminal voltages (equality) of the battery cells inside the battery pack by adding a bridging capacitor between each output unit. Compared with the traditional DC / DC converter circuit balancing structure, the balancing circuit of the present invention achieves automatic voltage equalization between battery cells through the bridging capacitor, without the need for complex sampling and control algorithms; and it avoids the problem of unequal terminal voltages caused by inconsistent parasitic parameters of devices in the case of multi-output converters, effectively reducing the voltage deviation and improving the battery balancing accuracy.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A Cuk-based lithium battery pack SOC balancing circuit, which includes a basic Cuk converter, n - 1 bridging capacitors, n - 1 output expansion units, and n lithium-ion batteries;

[0007] The basic Cuk converter includes two inductors L1, L in , capacitors C1, C in , switch S1 and diode D1;

[0008] The positive pole of the DC input source u in is connected to one end of inductor L in , and the other end of inductor L in is respectively connected to the drain of switch tube S1 and one end of capacitor C in , and the other end of capacitor C in is respectively connected to the anode of diode D1 and one end of inductor L1. The other end of inductor L1 is connected to one end of capacitor C1, and the other end of capacitor C1 is respectively connected to the cathode of diode D1, the source of switch S1, and the cathode of DC input source u in ;

[0009] Among the n - 1 output expansion units:

[0010] The first output expansion unit includes inductor L2, capacitor C2, and diode D2; where one end of inductor L2 is connected to the anode of diode D2, and the other end of inductor L2 is connected to one end of capacitor C2, and the other end of capacitor C2 is connected to the cathode of diode D2;

[0011] The second output expansion unit includes inductor L3, capacitor C3, and diode D3; where one end of inductor L3 is connected to the anode of diode D3, and the other end of inductor L3 is connected to one end of capacitor C3, and the other end of capacitor C3 is connected to the cathode of diode D3;

[0012] The third output expansion unit includes an inductor L4, a capacitor C4, and a diode D4; wherein, one end of the inductor L4 is connected to the anode of the diode D4, the other end of the inductor L4 is connected to one end of the capacitor C4, and the other end of the capacitor C4 is connected to the cathode of the diode D4;

[0013] And so on,

[0014] The (n - 1)-th output expansion unit includes an inductor L n , a capacitor C n , and a diode D n ; wherein, one end of the inductor L n is connected to the anode of the diode D n , the other end of the inductor L n is connected to one end of the capacitor C n , and the other end of the capacitor C n is connected to the cathode of the diode D n ;

[0015] Among the (n - 1) output expansion units:

[0016] One end of the bridging capacitor C VB1 , one end of the bridging capacitor C VB2 , one end of the bridging capacitor C VB3 , and so on, one end of the bridging capacitor C VB(n-1) is all connected to the drain of the switching transistor S1;

[0017] The other end of the bridging capacitor C VB1 is connected to the anode of the diode D2;

[0018] The other end of the bridging capacitor C VB2 is connected to the anode of the diode D3;

[0019] The other end of the bridging capacitor C VB3 is connected to the anode of the diode D4;

[0020] And so on,

[0021] The other end of the bridging capacitor C VB(n-1) is connected to the anode of the diode D n ;

[0022] Among the n lithium-ion batteries:

[0023] The negative electrode of the lithium-ion battery B1 is connected to one end of the capacitor C1; the positive electrode of the lithium-ion battery B1 is connected to the other end of the capacitor C1;

[0024] The negative electrode of the lithium-ion battery B2 is connected to one end of the capacitor C2; the positive electrode of the lithium-ion battery B2 is connected to the other end of the capacitor C2;

[0025] The negative electrode of the lithium-ion battery B3 is connected to one end of the capacitor C3; the positive electrode of the lithium-ion battery B3 is connected to the other end of the capacitor C3;

[0026] and so on

[0027] Lithium-ion battery B n The negative electrode is connected to capacitor C n One end; Lithium-ion battery B n The positive electrode is connected to capacitor C n The other end;

[0028] The positive electrode of lithium-ion battery B1 is connected to the negative electrode of lithium-ion battery B2,

[0029] The positive electrode of lithium-ion battery B2 is connected to the negative electrode of lithium-ion battery B3,

[0030] The positive electrode of lithium-ion battery B3 is connected to the negative electrode of lithium-ion battery B4,

[0031] and so on

[0032] Lithium-ion battery B n-1 The positive electrode is connected to lithium-ion battery B n The negative electrode.

[0033] The gate of the switch S1 is connected to the controller, and its duty cycle can vary between 0 and 1.

[0034] A lithium battery pack SOC balancing circuit containing 4 battery cells, the circuit includes a basic Cuk converter, 3 bridging capacitors, 3 output expansion units, and 4 lithium-ion batteries;

[0035] The basic Cuk converter includes two inductors L1, L in , capacitor C1, C in , switch S1 and diode D1;

[0036] DC input source u in The positive electrode is connected to inductor L in One end, the other end of inductor L in The other end is respectively connected to the drain of switch tube S1, capacitor C in One end, the other end of capacitor C in The other end is respectively connected to the anode of diode D1, one end of inductor L1, the other end of inductor L1 is connected to one end of capacitor C1, and the other end of capacitor C1 is respectively connected to the cathode of diode D1, the source of switch S1 and the DC input source u in Cathode;

[0037] Among the 3 output expansion units:

[0038] The first output expansion unit includes inductor L2, capacitor C2, and diode D2; among them, one end of inductor L2 is connected to the anode of diode D2, the other end of inductor L2 is connected to one end of capacitor C2, and the other end of capacitor C2 is connected to the cathode of diode D2;

[0039] The second output expansion unit includes an inductor L3, a capacitor C3, and a diode D3. One end of the inductor L3 is connected to the anode of the diode D3, the other end of the inductor L3 is connected to one end of the capacitor C3, and the other end of the capacitor C3 is connected to the cathode of the diode D3.

[0040] The third output expansion unit includes an inductor L4, a capacitor C4, and a diode D4. One end of the inductor L4 is connected to the anode of the diode D4, the other end of the inductor L4 is connected to one end of the capacitor C4, and the other end of the capacitor C4 is connected to the cathode of the diode D4.

[0041] Among the three output expansion units:

[0042] One end of the bridging capacitor C VB1 One end of the bridging capacitor C VB2 One end of the bridging capacitor C VB3 All are connected to the drain of the switching transistor S1.

[0043] The other end of the bridging capacitor C VB1 is connected to the anode of the diode D2.

[0044] The other end of the bridging capacitor C VB2 is connected to the anode of the diode D3.

[0045] The other end of the bridging capacitor C VB3 is connected to the anode of the diode D4.

[0046] Among the n lithium-ion batteries:

[0047] The negative electrode of the lithium-ion battery B1 is connected to one end of the capacitor C1; the positive electrode of the lithium-ion battery B1 is connected to the other end of the capacitor C1.

[0048] The negative electrode of the lithium-ion battery B2 is connected to one end of the capacitor C2; the positive electrode of the lithium-ion battery B2 is connected to the other end of the capacitor C2.

[0049] The negative electrode of the lithium-ion battery B3 is connected to one end of the capacitor C3; the positive electrode of the lithium-ion battery B3 is connected to the other end of the capacitor C3.

[0050] The negative electrode of the lithium-ion battery B4 is connected to one end of the capacitor C4; the positive electrode of the lithium-ion battery B4 is connected to the other end of the capacitor C4.

[0051] The positive electrode of the lithium-ion battery B1 is connected to the negative electrode of the lithium-ion battery B2,

[0052] The positive electrode of the lithium-ion battery B2 is connected to the negative electrode of the lithium-ion battery B3,

[0053] The positive electrode of the lithium-ion battery B3 is connected to the negative electrode of the lithium-ion battery B4.

[0054] When the switch S1 is turned on, the diodes D1, D2, D3, and D4 are reverse-biased and cut off, and the capacitor C in passes through the loop Cin →L2→C VB1 →C in to the bridging capacitor C VB1 charges. Similarly, the capacitor C in charges the bridging capacitor C VB2 and C VB3 charges. The capacitors C1, C2, C3, and C4 supply energy to the lithium-ion battery pack.

[0055] When the switch S1 is turned off, the diodes D1, D2, D3, and D4 conduct, and the input source u in charges the capacitor C in through the inductor L in while the bridging capacitor C VB1 and C VB2 and C VB3 discharge. The capacitors C1, C2, C3, and C4 charge, and the inductors L1, L2, L3, and L4 supply energy to the lithium-ion battery pack through the diodes D1, D2, D3, and D4 respectively.

[0056] Assume that the SOC of the single lithium-ion battery B1 in the series lithium-ion battery pack is higher than the average value. Then the initial voltage u B1 > u B2 . When the switch S1 is turned off, due to the effect of the bridging capacitor C VB1 , the diode D1 in the circuit does not conduct, and the current first discharges the capacitor C VB1 until the voltage u VB1 of the bridging capacitor C CVB1 rises to be equal to the output voltage u B1 of this circuit, at which point the diode D1 starts to conduct.

[0057] Assume that the output capacitor C1 is large enough. Then the output voltage ripple is much smaller than the voltage ripple of the bridging capacitor C VB1 . It can be considered that the average voltage of u B1 is approximately equal to the peak voltage of the bridging capacitor C VB1 during the off stage of the switch S1. Therefore, the voltage deviation Δu between u B1 and u B2 is equal to the difference between the peak voltage and the average value, that is, half of the peak-to-peak voltage ripple of the bridging capacitor C VB1 . As the capacitance value of the capacitor C VB1 increases, the voltage ripple of the bridging capacitor C VB1 will become smaller and smaller. Therefore, the output voltage deviation will decrease as the capacitance value of the bridging capacitor C VB1 increases, and the voltages of u B1 and u B2 will be closer. When the bridging capacitors C VB1 and C VB2 and C VB3When it is large enough, the voltages of each lithium-ion battery cell are equal, achieving voltage balance among the lithium-ion battery cells of the lithium-ion battery pack.

[0058] The SOC balancing circuit of the lithium battery pack based on Cuk in the present invention has the following technical effects:

[0059] 1) For the SOC balancing circuit of the lithium battery pack based on Cuk in the present invention, when there are multiple outputs, the problem of unbalanced terminal voltages caused by parasitic parameters of devices is effectively reduced, and the voltage deviation is reduced, improving the battery balancing accuracy.

[0060] 2) Compared with the traditional DC / DC converter circuit balancing structure, the SOC balancing circuit of the lithium battery pack based on Cuk in the present invention has a simple circuit. The automatic voltage equalization between battery cells is achieved through a bridging capacitor, without the need for complex sampling and control algorithms.

[0061] 3) The SOC balancing circuit of the lithium battery pack based on Cuk in the present invention is simple, and only one switch is used to achieve the balance of the battery pack, reducing the control difficulty and cost of the circuit. Description of the Drawings

[0062] The present invention will be further described below in conjunction with the drawings and examples;

[0063] Figure 1 is the schematic diagram of the balancing circuit of the present invention;

[0064] Figure 2 is the schematic diagram of the balancing circuit of the series battery pack with four battery cells of the present invention;

[0065] Figure 3 is the simulation diagram of the balancing circuit of the series battery pack with four battery cells of the present invention. Detailed Embodiment

[0066] As Figure 2 shown, it is the SOC balancing circuit of the lithium battery pack based on Cuk with 4 battery cells of the present invention. This circuit includes a basic Cuk converter, 3 bridging capacitors, 3 output expansion units, and 4 lithium-ion batteries;

[0067] The basic Cuk converter includes two inductors L1, L in , capacitors C1, C in , a switch S1 and a diode D1;

[0068] The positive pole of the DC input source u in is connected to one end of the inductor L in , and the other end of the inductor L in is respectively connected to the drain of the switch tube S1 and one end of the capacitor C in , and one end of the capacitor C inThe other ends are respectively connected to the anode of diode D1 and one end of inductor L1. The other end of inductor L1 is connected to one end of capacitor C1. The other end of capacitor C1 is respectively connected to the cathode of diode D1, the source electrode of switch S1 and the DC input source u in Cathode;

[0069] Among the 3 output expansion units:

[0070] The first output expansion unit includes inductor L2, capacitor C2 and diode D2. Among them, one end of inductor L2 is connected to the anode of diode D2, the other end of inductor L2 is connected to one end of capacitor C2, and the other end of capacitor C2 is connected to the cathode of diode D2;

[0071] The second output expansion unit includes inductor L3, capacitor C3 and diode D3. Among them, one end of inductor L3 is connected to the anode of diode D3, the other end of inductor L3 is connected to one end of capacitor C3, and the other end of capacitor C3 is connected to the cathode of diode D3;

[0072] The third output expansion unit includes inductor L4, capacitor C4 and diode D4. Among them, one end of inductor L4 is connected to the anode of diode D4, the other end of inductor L4 is connected to one end of capacitor C4, and the other end of capacitor C4 is connected to the cathode of diode D4;

[0073] Among the 3 output expansion units:

[0074] The bridging capacitor C VB1 One end, the bridging capacitor C VB2 One end, the bridging capacitor C VB3 One end is all connected to the drain electrode of switch tube S1;

[0075] The bridging capacitor C VB1 The other end is connected to the anode of diode D2;

[0076] The bridging capacitor C VB2 The other end is connected to the anode of diode D3;

[0077] The bridging capacitor C VB3 The other end is connected to the anode of diode D4;

[0078] Among the n lithium-ion batteries:

[0079] The negative electrode of lithium-ion battery B1 is connected to one end of capacitor C1; the positive electrode of lithium-ion battery B1 is connected to the other end of capacitor C1;

[0080] The negative electrode of lithium-ion battery B2 is connected to one end of capacitor C2; the positive electrode of lithium-ion battery B2 is connected to the other end of capacitor C2;

[0081] The negative electrode of lithium-ion battery B3 is connected to one end of capacitor C3; the positive electrode of lithium-ion battery B3 is connected to the other end of capacitor C3;

[0082] The negative electrode of lithium-ion battery B4 is connected to one end of capacitor C4; the positive electrode of lithium-ion battery B4 is connected to the other end of capacitor C4;

[0083] The positive electrode of lithium-ion battery B1 is connected to the negative electrode of lithium-ion battery B2,

[0084] The positive electrode of lithium-ion battery B2 is connected to the negative electrode of lithium-ion battery B3,

[0085] The positive electrode of lithium-ion battery B3 is connected to the negative electrode of lithium-ion battery B4.

[0086] When switch S1 is turned on, diodes D1, D2, D3, and D4 are reverse-biased and cut off, and capacitor C in charges the bridging capacitor C in →L2→C VB1 →C in through the loop C VB1 Similarly, capacitor C in charges the bridging capacitor C VB2 、C VB3 Capacitors C1, C2, C3, and C4 provide energy for the lithium-ion battery pack.

[0087] When switch S1 is turned off, diodes D1, D2, D3, and D4 conduct, and the input source u in charges capacitor C in through inductor L in The bridging capacitors C VB1 、C VB2 、C VB3 discharge, and capacitors C1, C2, C3, and C4 charge. Inductors L1, L2, L3, and L4 provide energy for the lithium-ion battery pack through diodes D1, D2, D3, and D4 respectively.

[0088] The principle of battery equalization is to keep each lithium-ion battery cell in a series lithium-ion battery pack in an equalized state. Currently, the equalization degree of the entire lithium-ion battery pack is mainly judged based on whether the terminal voltages of the lithium-ion battery cells are equal (the same). Assume that the SOC of the single lithium-ion battery B1 in the series lithium-ion battery pack is higher than the average value, then the initial voltage u B1 >u B2 。When switch S1 is turned off, due to the effect of the bridging capacitor C VB1 , diode D1 in the loop does not conduct, and the current first discharges capacitor C VB1 until the voltage u VB1 of capacitor C CVB1 rises to be equal to the output voltage u B1 of this loop, and then diode D1 starts to conduct. Assume that the output capacitor C1 is large enough, then the output voltage ripple is much smaller than the voltage ripple of the bridging capacitor C VB1 and u can be consideredB1 The average voltage approximately equals the peak voltage across the capacitor C VB1 during the turn-off phase of switch S1. Therefore, u B1 and u B2 The voltage deviation Δu equals the difference between the peak voltage and the average value, that is, half of the peak-to-peak voltage ripple of the capacitor C VB1 As the capacitance value of the capacitor C VB1 increases, the voltage ripple of the capacitor C VB1 becomes smaller and smaller. Therefore, the output voltage deviation will decrease as the capacitance value of C VB1 increases, and the voltages of u B1 and u B2 will be closer. When the capacitances of capacitors C VB1 、C VB2 、C VB3 are large enough, the voltages of each lithium-ion battery cell are equal, achieving voltage equalization among the lithium-ion battery cells in the lithium-ion battery pack.

[0089] It can be seen from Figure 3 that the SOCs set for the four batteries in the series lithium-ion battery pack are different, and other parameters are the same. Among them, the SOC of lithium-ion battery B1 is 88, the SOC of lithium-ion battery B2 is 84, the SOC of lithium-ion battery B3 is 80, and the SOC of lithium-ion battery B4 is 76. The Cuk-based SOC equalization circuit for lithium battery packs of the present invention can achieve the SOC equalization of the four lithium-ion battery cells.

[0090] Compared with the equalization structure of the traditional DC / DC converter circuit, the present invention realizes voltage equalization among lithium-ion battery cells by adding a capacitor across each output unit, without complex sampling and control algorithms, effectively reducing the voltage deviation and improving the battery equalization accuracy. Moreover, the present invention realizes the equalization of the lithium-ion battery pack by using only one switch, reducing the control difficulty and cost of the circuit.

Claims

1. A lithium battery pack SOC equalization circuit based on Cuk, characterized by: The circuit includes a basic Cuk converter, n-1 cross-capacitors, n-1 output expansion units, and n lithium-ion batteries; The basic Cuk converter consists of two inductors L1 and L in , capacitors C1, C in , switch S1 and diode D1; DC input source u in Positive connection inductor L in One end, inductor L in The other end is connected to the drain of switch tube S1 and capacitor C in One end, capacitor C in The other end is connected to the anode of diode D1 and one end of inductor L1. The other end of inductor L1 is connected to one end of capacitor C1. The other end of capacitor C1 is connected to the cathode of diode D1, the source of switch S1 and the DC input source u. in cathode; In n-1 output expansion units: The first output expansion unit includes an inductor L2, a capacitor C2, and a diode D2; wherein one end of the inductor L2 is connected to the anode of the diode D2, the other end of the inductor L2 is connected to one end of the capacitor C2, and the other end of the capacitor C2 is connected to the cathode of the diode D2; The second output expansion unit includes an inductor L3, a capacitor C3, and a diode D3; wherein one end of the inductor L3 is connected to the anode of the diode D3, the other end of the inductor L3 is connected to one end of the capacitor C3, and the other end of the capacitor C3 is connected to the cathode of the diode D3; The third output expansion unit includes an inductor L4, a capacitor C4, and a diode D4; wherein one end of the inductor L4 is connected to the anode of the diode D4, the other end of the inductor L4 is connected to one end of the capacitor C4, and the other end of the capacitor C4 is connected to the cathode of the diode D4; ... and so on. The n-1th output expansion unit includes an inductor L n , capacitor C n 、Diode D n ; Among them, the inductor L n One end is connected to diode D n Anode, inductance L n The other end is connected to capacitor C n One end, capacitor C n The other end is connected to diode D n cathode; In n-1 output expansion units: Cross-over capacitor C VB1 One end, cross-capacitor C VB2 One end, cross-capacitor C VB3 One end, ... and so on, the cross-capacitor C VB(n-1) One end is connected to the drain of the switch tube S1; Cross-over capacitor C VB1 The other end is connected to the anode of diode D2; Cross-over capacitor C VB2 The other end is connected to the anode of diode D3; Cross-over capacitor C VB3 The other end is connected to the anode of diode D4; ... and so on. Cross-over capacitor C VB(n-1) The other end is connected to diode D n anode; Among n lithium-ion batteries: The negative electrode of the lithium-ion battery B1 is connected to one end of the capacitor C1; the positive electrode of the lithium-ion battery B1 is connected to the other end of the capacitor C1; The negative electrode of the lithium-ion battery B2 is connected to one end of the capacitor C2; the positive electrode of the lithium-ion battery B2 is connected to the other end of the capacitor C2; The negative electrode of the lithium-ion battery B3 is connected to one end of the capacitor C3; the positive electrode of the lithium-ion battery B3 is connected to the other end of the capacitor C3; ... and so on. Lithium-ion battery B n Negative connection capacitor C n One end: Lithium-ion battery B n The positive terminal is connected to the capacitor C n The other end; The positive electrode of lithium-ion battery B1 is connected to the negative electrode of lithium-ion battery B2. The positive electrode of lithium-ion battery B2 is connected to the negative electrode of lithium-ion battery B3. The positive electrode of lithium-ion battery B3 is connected to the negative electrode of lithium-ion battery B4. ... and so on. Lithium-ion battery B n-1 Positive electrode connected to lithium-ion battery B n negative electrode.

2. The Cuk-based lithium battery pack SOC equalization circuit according to claim 1, characterized in that: The gate of the switch S1 is connected to a controller, and its duty cycle can be changed between 0 and 1.

3. A SOC equalization circuit for a lithium battery pack containing four battery cells, characterized in that: The circuit includes a basic Cuk converter, three cross-capacitors, three output expansion units, and four lithium-ion batteries; The basic Cuk converter consists of two inductors L1 and L in , capacitors C1, C in , switch S1 and diode D1; DC input source u in Positive connection inductor L in One end, inductor L in The other end is connected to the drain of switch tube S1 and capacitor C in One end, capacitor C in The other end is connected to the anode of diode D1 and one end of inductor L1. The other end of inductor L1 is connected to one end of capacitor C1. The other end of capacitor C1 is connected to the cathode of diode D1, the source of switch S1 and the DC input source u. in cathode; 3 output expansion units: The first output expansion unit includes an inductor L2, a capacitor C2, and a diode D2; wherein one end of the inductor L2 is connected to the anode of the diode D2, the other end of the inductor L2 is connected to one end of the capacitor C2, and the other end of the capacitor C2 is connected to the cathode of the diode D2; The second output expansion unit includes an inductor L3, a capacitor C3, and a diode D3; wherein one end of the inductor L3 is connected to the anode of the diode D3, the other end of the inductor L3 is connected to one end of the capacitor C3, and the other end of the capacitor C3 is connected to the cathode of the diode D3; The third output expansion unit includes an inductor L4, a capacitor C4, and a diode D4; wherein one end of the inductor L4 is connected to the anode of the diode D4, the other end of the inductor L4 is connected to one end of the capacitor C4, and the other end of the capacitor C4 is connected to the cathode of the diode D4; 3 output expansion units: Cross-over capacitor C VB1 One end, cross-capacitor C VB2 One end, cross-capacitor C VB3 One end is connected to the drain of the switch tube S1; Cross-over capacitor C VB1 The other end is connected to the anode of diode D2; Cross-over capacitor C VB2 The other end is connected to the anode of diode D3; Cross-over capacitor C VB3 The other end is connected to the anode of diode D4; Of the 4 lithium-ion batteries: The negative electrode of the lithium-ion battery B1 is connected to one end of the capacitor C1; the positive electrode of the lithium-ion battery B1 is connected to the other end of the capacitor C1; The negative electrode of the lithium-ion battery B2 is connected to one end of the capacitor C2; the positive electrode of the lithium-ion battery B2 is connected to the other end of the capacitor C2; The negative electrode of the lithium-ion battery B3 is connected to one end of the capacitor C3; the positive electrode of the lithium-ion battery B3 is connected to the other end of the capacitor C3; The negative electrode of the lithium-ion battery B4 is connected to one end of the capacitor C4; the positive electrode of the lithium-ion battery B4 is connected to the other end of the capacitor C4; The positive electrode of lithium-ion battery B1 is connected to the negative electrode of lithium-ion battery B2. The positive electrode of lithium-ion battery B2 is connected to the negative electrode of lithium-ion battery B3. The positive electrode of lithium-ion battery B3 is connected to the negative electrode of lithium-ion battery B4.

4. A SOC equalization circuit for a lithium battery pack having four battery cells according to claim 3, characterized in that: When the switch S1 is turned on, the diodes D1, D2, D3, and D4 are cut off due to the reverse voltage, and the capacitor C in Through loop C in →L2→C VB1 →C in To the jumper capacitor C VB1 Charging, similarly, capacitor C in To the jumper capacitor C VB2 , C VB3 Charging, capacitors C1, C2, C3, and C4 provide energy for the lithium-ion battery pack.

5. A SOC equalization circuit for a lithium battery pack having four battery cells according to claim 4, characterized in that: When the switch S1 is turned off, the diodes D1, D2, D3, and D4 are turned on, and the input source u in Through inductor L in To capacitor C in Charge, across capacitor C VB1 , C VB2 , C VB3 Discharging, capacitors C1, C2, C3, and C4 are charged, and inductors L1, L2, L3, and L4 provide energy to the lithium-ion battery pack through diodes D1, D2, D3, and D4 respectively.

6. A SOC equalization circuit for a lithium battery pack having four battery cells according to claim 5, characterized in that: Assume that the SOC of the single lithium-ion battery B1 in the series lithium-ion battery pack is higher than the average value, then the initial voltage u B1 >u B2 ; When switch S1 is turned off, due to the cross-connected capacitor C VB1 The diode D1 in the circuit is not turned on, and the current is first fed to the capacitor C VB1 Discharge until the cross-capacitor C VB1 The voltage u CVB1 rises to the output voltage u of the circuit B1 When they are equal, diode D1 starts conducting.

7. A SOC equalization circuit for a lithium battery pack having four battery cells according to claim 6, characterized in that: If the output capacitor C1 is large enough, the output voltage ripple is much smaller than the cross-capacitor C VB1 The voltage ripple can be considered as u B1 The average voltage across the capacitor C VB1 The peak voltage during the turn-off phase of switch S1; therefore, u B1 and u B2 The voltage deviation Δu is equal to the difference between the peak voltage and the average value, that is, the cross-capacitor C VB1 Half of the peak-to-peak voltage ripple; with the capacitance C VB1 As the capacitance increases, the cross-capacitor C VB1 The voltage ripple will become smaller and smaller, so the output voltage deviation will increase with the cross-connect capacitor C VB1 As the capacitance increases, u decreases. B1 and u B2 The voltage will be closer; when the capacitor C VB1 , C VB2 , C VB3 When the voltage is large enough, the voltages of the lithium-ion battery cells are equal, thus achieving voltage balance among the lithium-ion battery cells in the lithium-ion battery pack.

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