Buck-boost multi-lithium battery SOC equalization circuit

Abstract

The invention relates to the technical field of lithium battery equalization, in particular to a buck-boost multi-lithium battery SOC equalization circuit which comprises n paths of output buck-boost type converters, n-1 interstage capacitors Cp and n lithium battery units B. The n paths of output buck-boost type converters comprise power switches S1 and n output circuits, each output circuit is composed of an inductor L, an output capacitor C and a diode D, the n inductors L are respectively an inductor L1, an inductor L2,..., and an inductor Ln, the n output capacitors C are respectively an output capacitor C1, an output capacitor C2,..., and an output capacitor Cn, and the n diodes D are respectively a diode D1, a diode D2,..., and a diode Dn. According to the buck-boost converter, the inter-stage capacitors are additionally arranged between the outputs of the buck-boost converter, the effect of automatically equalizing the output voltage is achieved, the battery equalization degree is improved, SOC equalization of the lithium battery unit group is achieved through the power switch devices, the equalization circuit topology is simple, and the design difficulty of a control system is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery balancing, and specifically provides a buck-boost multi-lithium battery SOC balancing circuit. Background Technique

[0002] Lithium batteries have many advantages such as high energy density, long life, high efficiency, small size, and no pollution. Considering their comprehensive performance in terms of safety, practicality, and cost, they play a crucial role in the fields of power and energy storage. Due to the characteristics of single lithium batteries, such as low voltage and small capacity, multiple batteries are often connected in series. However, due to manufacturing process differences and the influence of the environment, charge and discharge modes, and chemical factors during use, the battery pack will have differences in state of charge, voltage, internal resistance, and capacity. These differences will cause problems in the durability, reliability, and safety of the battery pack.

[0003] Patent No. CN113489083A discloses a hierarchical balancing control method for a lithium-ion battery pack based on a buck-boost converter. This method adds multiple energy storage inductance elements, diodes, and balancing control switches to the lithium-ion battery pack. By controlling the closing of the switches, the single battery with higher energy storage stores the excess energy in the corresponding inductor, and then controls the switches to turn off, transferring the energy in the inductor to the single battery with lower energy storage, forming the balancing control of the series lithium-ion battery pack. The proposed balancing control method can not only achieve the balance between two adjacent single batteries, but also indirectly achieve the balance between two non-adjacent single batteries through the balancing control between adjacent modules, shortening the energy transmission path and improving the efficiency of the battery pack balancing system.

[0004] However, the balancing circuit adopted by the hierarchical balancing control method of the lithium-ion battery pack contains multiple control switches, and the logic control is very complex. Therefore, it does not meet the existing requirements, and for this reason, we propose a buck-boost multi-lithium battery SOC balancing circuit. Summary of the Invention

[0005] The purpose of the present invention is to provide a buck-boost multi-lithium battery SOC balancing circuit to solve the problem that the balancing circuit adopted by the hierarchical balancing control method of the lithium-ion battery pack contains multiple control switches and the logic control is very complex as mentioned in the above background technique.

[0006] To achieve the above purpose, the present invention provides the following technical solution: A buck-boost multi-lithium battery SOC balancing circuit includes an n-channel output buck-boost converter and n - 1 inter-stage capacitors C pand n lithium battery units B, the n-way output buck-boost converter includes a power switch S1 and n output circuits, the output circuit is composed of an inductor L, an output capacitor C and a diode D, and the n inductors L are respectively inductor L1, inductor L2, ..., inductor L n , the n output capacitors C are output capacitor C1, output capacitor C2, ..., output capacitor C n , the n diodes D are diode D1, diode D2, ..., diode D n ;

[0007] The inter-stage capacitance C p1 , interstage capacitance C p2 ,…, interstage capacitance C p(n-1) One end of each is connected to the anode of diode D1, the interstage capacitor C p1 , interstage capacitance C p2 ,…, interstage capacitance C p(n-1) The other end is connected to the inductor L2, inductor L3, ..., inductor L n One end and diode D2, diode D3, ..., diode D n cathode connection;

[0008] The lithium battery unit B1, lithium battery unit B2, ..., lithium battery unit B n The two ends of the output capacitor C1, output capacitor C2, ..., output capacitor C n Connect both ends of .

[0009] Preferably, the gate of the power switch S1 is connected to a controller, the gate duty cycle of the power switch S1 is floating between 0 and 1 and in phase, and the drain of the switch S1 is connected to an input power supply u in The positive electrode of the switch S1 is connected to one end of the inductor L1 and the cathode of the diode D1 respectively, the anode of the diode D1 is connected to one end of the output capacitor C1, and the other end of the output capacitor C1 is connected to the other end of the inductor L1 and the input power supply u in negative connection.

[0010] Preferably, the output capacitor C1, output capacitor C2, ..., output capacitor C n The two ends of the diode D2, diode D3, ..., diode D n Anode and inductor L2, inductor L3, ..., inductor L n The other end of each output capacitor C is connected, and each adjacent two output capacitors C are connected end to end.

[0011] Preferably, when n = 4, the buck-boost multi-lithium battery SOC balancing circuit includes a four-output buck-boost converter, three inter-stage capacitors C p and four lithium battery cells B. The four lithium battery cells B are respectively lithium battery cell B1, lithium battery cell B2, lithium battery cell B3 and lithium battery cell B4. The lithium battery cells B1, B2, B3 and B4 form a lithium battery cell group.

[0012] Preferably, in the buck-boost multi-lithium battery SOC balancing circuit, the average voltages of inductors L1, L2, L3 and L4 based on the volt-second balance of the inductor are 0 within the switching period. The inductor L1, the inter-stage capacitor C p1 , the inductor L2 and the capacitor C2 form a first loop and are obtained based on the KVL equation:

[0013] The voltage U p1 of the inter-stage capacitor C cp1 is equal to the output voltage U2. The voltage U p2 of the inter-stage capacitor C cp2 is equal to the output voltage U3. The voltage U p3 of the inter-stage capacitor C cp3 is equal to the output voltage U4.

[0014] Preferably, when the power switch S1 is turned off, the energies of the inductors L1, L2, L3 and L4 are transferred to the interiors of the output capacitors C1, C2, C3 and C4 through the conducting diodes D1, D2, D3 and D4. The inter-stage capacitor C p1 , the inductor L1, the output capacitor C2 and the inductor L2 form a second loop. The inter-stage capacitor C p1 charges the output capacitor C2 based on the unbalanced voltage within the second loop. The output capacitors C1, C2, C3 and C4 all supply power to the load.

[0015] Preferably, when the power switch S1 is turned on, the inter-stage capacitor C p1 , the inductor L2 and the output capacitor C2 form a third loop. The input power supply u in charges the output capacitor C2 through the third loop. The inter-stage capacitor C p1 , the diode D1, the output capacitor C1 and the diode D1 are arranged in sequence and form a fourth loop. The inter-stage capacitor C p1 is obtained through the fourth loop and the KVL equation:

[0016] The voltage U p1 of the inter-stage capacitor C cp1equal to the output voltage U1, the interstage capacitor C p2 Voltage U cp2 equal to the output voltage U2, the interstage capacitance C p2 Voltage U cp3 Equal to the output voltage U3.

[0017] Preferably, the SOC of the lithium battery cell B1 in the lithium battery cell group is higher than the average value, and when the power switch S1 is disconnected, the interstage capacitor C p The capacitor voltage ripple Δ is connected in parallel with the lithium battery cell B1, the lithium battery cell B2, the lithium battery cell B3 and the lithium battery cell B4 and transmits energy to the inside of the lithium battery cell B1, the lithium battery cell B2, the lithium battery cell B3 and the lithium battery cell B4. uCp and interstage capacitance C p The relationship is:

[0018]

[0019] The voltage deviation Δu is:

[0020]

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. This invention achieves SOC balancing of series-connected lithium battery packs by adding interstage capacitors between the outputs of the buck-boost converter and redistributing the charge of the interstage capacitors. This eliminates the need for additional voltage sampling and closed-loop control, supports mixed use of new and old batteries or batteries of different capacities, and compensates for capacity differences through dynamic charge distribution in the interstage capacitors.

[0023] 2. This invention achieves SOC balancing of lithium battery cells using a single power switch device. This single switch uses fixed-duty-cycle, in-phase control, requiring only a low-cost MCU. This not only reduces the complexity of the drive circuit but also effectively addresses the inherent voltage deviation problem caused by parasitic parameter differences in traditional multi-output topologies, improving cell balancing accuracy and significantly reducing circuit costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the buck-boost multi-lithium battery SOC balancing circuit of the present invention;

[0025] Figure 2 This is a circuit diagram of the lithium battery cell group of the present invention;

[0026] Figure 3 This is a circuit simulation diagram of the lithium battery cell group of the present invention;. DETAILED DESCRIPTION

[0027] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0028] Please refer to Figure 1 , an embodiment provided by the present invention: a buck-boost multi-lithium battery SOC balancing circuit, including an n-channel output buck-boost converter, n-1 inter-stage capacitors C p and n lithium battery units B. The n-channel output buck-boost converter includes a power switch S1 and n output circuits. The output circuit is composed of an inductor L, an output capacitor C, and a diode D. The n inductors L are respectively inductor L1, inductor L2,..., inductor L n , the n output capacitors C are respectively output capacitor C1, output capacitor C2,..., output capacitor C n , the n diodes D are respectively diode D1, diode D2,..., diode D n , the gate of the power switch S1 is connected to a controller. The duty cycle of the gate of the power switch S1 floats between 0 and 1 and is in the same phase. The drain of the switch S1 is connected to the positive pole of the input power supply u in , the source of the switch S1 is respectively connected to one end of the inductor L1 and the cathode of the diode D1. The anode of the diode D1 is connected to one end of the output capacitor C1. The other end of the output capacitor C1 is respectively connected to the other end of the inductor L1 and the negative pole of the input power supply u in , the negative pole of the input power supply u n , the two ends of the output capacitors C1, output capacitors C2,..., output capacitors C n are respectively connected to the anodes of the diodes D2, D3,..., diodes D n , and the other ends of the inductors L2, L3,..., inductors L n , the two ends of the lithium battery units B1, lithium battery units B2,..., lithium battery units B n are respectively connected to the two ends of the output capacitors C1, output capacitors C2,..., output capacitors C

[0029] The inter-stage capacitor C p1 , the inter-stage capacitor C p2 ,..., the inter-stage capacitor C p(n-1) One end of each is connected to the anode of the diode D1. The inter-stage capacitor C p1 , the inter-stage capacitor C p2 ,..., the inter-stage capacitor C p(n-1) The other ends are respectively connected to the inductors L2, L3,..., inductors L nOne end and diodes D2, D3, …, D n The cathode is connected, and the SOC balance of the series lithium battery pack is achieved through the charge redistribution mechanism of the inter-stage capacitor.

[0030] Please refer to Figure 2 , when n is 4, the buck-boost multi-lithium battery SOC balance circuit includes a four-way output buck-boost converter, three inter-stage capacitors C p And four lithium battery cells B. The four lithium battery cells B are respectively lithium battery cell B1, lithium battery cell B2, lithium battery cell B3 and lithium battery cell B4. The lithium battery cells B1, B2, B3 and B4 form a lithium battery cell group. In the buck-boost multi-lithium battery SOC balance circuit, the average voltages of the inductors L1, L2, L3 and L4 based on the inductor volt-second balance are 0 within the switching period. The inductor L1, the inter-stage capacitor C p1 , the inductor L2 and the capacitor C2 form the first loop and are obtained based on the KVL equation:

[0031] The voltage U of the inter-stage capacitor C p1 Is equal to the output voltage U2, and the voltage U of the inter-stage capacitor C cp1 Is equal to the output voltage U3, and the voltage U of the inter-stage capacitor C p2 Is equal to the output voltage U4; cp2 The voltage U of the inter-stage capacitor C p3 Is equal to the output voltage U4; cp3 Is equal to the output voltage U4;

[0032] When the power switch S1 is turned off, the energies of the inductors L1, L2, L3 and L4 are transferred to the interiors of the output capacitors C1, C2, C3 and C4 through the conducting diodes D1, D2, D3 and D4. The inter-stage capacitor C p1 , the inductor L1, the output capacitor C2 and the inductor L2 form the second loop. The inter-stage capacitor C p1 Charges the output capacitor C2 based on the unbalanced voltage within the second loop. The output capacitors C1, C2, C3 and C4 all supply power to the load. When the power switch S1 is closed, the inter-stage capacitor C p1 , the inductor L2 and the output capacitor C2 form the third loop. The input power supply u in Charges the output capacitor C2 through the third loop. The inter-stage capacitor C p1 , the diode D1, the output capacitor C1 and the diode D1 are arranged in sequence and form the fourth loop. The inter-stage capacitor C p1 Is obtained through the fourth loop and the KVL equation:

[0033] The voltage U of the inter-stage capacitor C p1The voltage U cp1 is equal to the output voltage U1, and the inter-stage capacitor C p2 The voltage U cp2 is equal to the output voltage U2, and the inter-stage capacitor C p2 The voltage U cp3 is equal to the output voltage U3. When the inter-stage capacitor C p1 and the inter-stage capacitor C p2 and the inter-stage capacitor C p3 are large enough, the voltages of each lithium battery cell B are equal.

[0034] When the SOC of the lithium battery cell B1 in the lithium battery cell group is higher than the average value, when the power switch S1 is turned off, the inter-stage capacitor C p is connected in parallel with the lithium battery cells B1, B2, B3, and B4 and transfers energy to the inside of the lithium battery cells B1, B2, B3, and B4, realizing the SOC balance of the lithium battery cell group. By introducing the inter-stage capacitor C p the pole voltage deviation caused by imbalance can be greatly reduced.

[0035] When the capacitance value of the inter-stage capacitor C p is large enough and Δu Cp is small enough, during the turn-off stage of the switch S1, all unbalanced charges ΔQ approximately flow into the capacitor C p and then the capacitor voltage ripple Δu Cp and the relationship between the inter-stage capacitor C p is:

[0036]

[0037] The voltage deviation Δu is:

[0038]

[0039] From the above formula, it can be seen that as the capacitance value of the inter-stage capacitor C p increases, the voltage ripple of the inter-stage capacitor C p will become smaller and smaller. Therefore, the voltage deviation of each lithium battery cell B will decrease as the capacitance value of the inter-stage capacitor C p increases. The operating frequency of the buck-boost multi-lithium battery SOC balancing circuit is 50 kHz, the power is 20W - 60W, and the capacitance value of the inter-stage capacitor is 1000uF.

[0040] Please refer to Figure 3, the initial SOCs of the four lithium batteries are 79%, 82%, 86% and 88% respectively. After a period of time, the SOCs of the four lithium batteries are consistent, and the standard deviation of the SOCs does not exceed 1%, which can effectively achieve the SOC balance of the four lithium battery units.

[0041] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced by the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

Claims

1. A buck-boost multi-lithium battery SOC balancing circuit, including an n-channel output buck-boost converter, n-1 inter-stage capacitors C p and n lithium battery cells B, characterized in that: The n-output buck-boost converter includes a power switch S1 and n output circuits, and each output circuit consists of an inductor L, an output capacitor C, and a diode D. The n inductors L are respectively inductor L1, inductor L2, …, inductor L n , and the n output capacitors C are respectively output capacitor C1, output capacitor C2, …, output capacitor C n , and the n diodes D are respectively diode D1, diode D2, …, diode D n ; The inter-stage capacitor C p1 , the inter-stage capacitor C p2 , …, one ends of the inter-stage capacitors C p(n-1) are all connected to the anode of the diode D1. The other ends of the inter-stage capacitors C p1 , the inter-stage capacitors C p2 , …, the inter-stage capacitors C p(n-1) are respectively connected to one ends of the inductors L2, L3, …, L n and the cathodes of the diodes D2, D3, …, D n ; The lithium battery cells B1, B2, …, B n are respectively connected at both ends to the output capacitors C1, C2, …, C n at both ends.

2. The buck-boost multi-lithium battery SOC equalization circuit according to claim 1, characterized in that: The gate of the power switch S1 is connected to a controller. The duty cycle of the gate of the power switch S1 floats between 0 and 1 and is in the same phase. The drain of the switch S1 is connected to the positive pole of the input power supply u in The source of the switch S1 is respectively connected to one end of the inductor L1 and the cathode of the diode D1. The anode of the diode D1 is connected to one end of the output capacitor C1. The other end of the output capacitor C1 is respectively connected to the other end of the inductor L1 and the negative pole of the input power supply u in is connected.

3. A buck-boost multi-lithium battery SOC balancing circuit according to claim 2, characterized in that: The output capacitors C1, C2, …, C n have their two ends respectively connected to the anodes of the diodes D2, D3, …, D n and the other ends of the inductors L2, L3, …, L n , and every two adjacent ones of the output capacitors C are connected end to end.

4. A buck-boost multi-lithium battery SOC equalization circuit according to claim 3, characterized in that: When n is 4, the buck-boost multi-lithium battery SOC balancing circuit includes a four-output buck-boost converter, three inter-stage capacitors C p and four lithium battery cells B. The four lithium battery cells B are respectively a lithium battery cell B1, a lithium battery cell B2, a lithium battery cell B3, and a lithium battery cell B4. The lithium battery cell B1, the lithium battery cell B2, the lithium battery cell B3, and the lithium battery cell B4 form a lithium battery cell group.

5. The buck-boost multi-lithium battery SOC balancing circuit according to claim 4, characterized in that: In the buck-boost multi-lithium battery SOC balancing circuit, the average voltages of inductors L1, L2, L3, and L4 based on the volt-second balance of the inductor are 0 within the switching period. The inductor L1, the inter-stage capacitor C p1 , the inductor L2, and the capacitor C2 form a first loop and are obtained based on the KVL equation as follows: The inter-stage capacitance C p1 has a voltage U cp1 equal to the output voltage U2. The inter-stage capacitance C p2 has a voltage U cp2 equal to the output voltage U3. The inter-stage capacitance C p3 has a voltage U cp3 equal to the output voltage U4.

6. The buck-boost multi-lithium battery SOC balancing circuit according to claim 5, characterized in that: When the power switch S1 is turned off, the energy of the inductors L1, L2, L3, and L4 is transferred to the inside of the output capacitors C1, C2, C3, and C4 through the diodes D1, D2, D3, and D4 in the conducting state. The inter-stage capacitor C p1 , inductor L1, output capacitor C2, and inductor L2 form a second loop. The inter-stage capacitor C p1 charges the output capacitor C2 based on the voltage in the second loop being in an unbalanced state. The output capacitors C1, C2, C3, and C4 all supply power to the load.

7. A buck-boost multi-lithium battery SOC equalization circuit according to claim 5, characterized in that: When the power switch S1 is closed, the inter-stage capacitor C p1 , the inductor L2 and the output capacitor C2 form a third loop, and the input power supply u in charges the output capacitor C2 through the third loop. The inter-stage capacitor C p1 , the diode D1, the output capacitor C1 and the diode D1 are arranged in sequence and form a fourth loop. The inter-stage capacitor C p1 is obtained through the fourth loop and the KVL equation: The inter-stage capacitance C p1 has a voltage U cp1 equal to the output voltage U1. The inter-stage capacitance C p2 has a voltage U cp2 equal to the output voltage U2. The inter-stage capacitance C p2 has a voltage U cp3 equal to the output voltage U3.

8. A buck-boost multi-lithium battery SOC equalization circuit according to claim 5, characterized in that: The SOC of the lithium battery cell B1 in the lithium battery cell group is higher than the average value. When the power switch S1 is turned off, the inter-stage capacitor C p is connected in parallel with the lithium battery cells B1, B2, B3, and B4 and transfers energy to the inside of the lithium battery cells B1, B2, B3, and B4. The capacitor voltage ripple Δ uCp and the inter-stage capacitor C p are related as follows: The voltage deviation Δu is as follows:

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