A circuit and a battery pack equalization method for implementing active equalization of a battery pack

Abstract

The present application relates to the technical field of battery, and aims to provide a circuit for realizing active balancing of a battery pack and a battery pack balancing method.The circuit comprises a battery pack, an equalization channel circuit, an inductor, an equalization conversion circuit, a voltage sensor and a current sensor; wherein the battery pack is composed of N battery monomers in series; the equalization channel circuit comprises N+1 independent channel circuits composed of two back-to-back switching tubes, and the equalization conversion circuit comprises four independent conversion paths composed of two back-to-back switching tubes; the voltage sensor is shared by N, and is connected in parallel across two ends of each battery monomer; and the current sensor is connected in series at any one end of the inductor.The present application greatly simplifies the active balancing circuit, can effectively reduce the number of switching tubes, and accordingly reduce the number of switching tube driving circuits, thereby reducing the circuit cost; can realize various battery grouping and battery balancing modes, and has flexible balancing process and fast balancing speed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a circuit for realizing active balancing of battery pack and a battery pack balancing method. BACKGROUND

[0002] Battery has high energy density, long cycle life and low self-discharge rate, and is an important energy storage device in current renewable energy systems (such as solar and wind energy) and a key component of electric vehicles (EV) and hybrid electric vehicles (HEV).

[0003] In practical applications, in order to obtain higher capacity and terminal voltage, a battery pack is often formed by connecting multiple battery cells in series. Due to individual differences, temperature differences and other reasons, the voltage and capacity of the battery cells in the battery pack will be unbalanced during production and charging and discharging. If this imbalance is not controlled, it will gradually worsen, seriously affecting the performance and service life of the battery pack. Therefore, capacity balancing between the battery cells in the series-connected battery pack is an urgent problem to be solved.

[0004] Currently, there are two ways to handle capacity balancing between battery cells in a series-connected battery pack: passive balancing and active balancing. Passive balancing refers to dissipating the excess energy of a battery cell with higher remaining capacity in the form of heat through a resistor, so as to balance the voltages of the battery cells; active balancing refers to transferring the energy of a battery cell with higher remaining capacity to a battery cell with lower remaining capacity through charge transfer, so as to achieve consistency of the capacities of the battery cells in the battery pack. However, passive balancing leads to energy waste in the form of heat dissipation. Active balancing circuits often use a large number of switching tubes to provide discharging and charging paths for each battery cell, which is costly.

[0005] Therefore, it is necessary to propose a more optimized active balancing technology for battery. SUMMARY

[0006] The technical problem to be solved by the present application is to overcome the deficiencies in the prior art and to propose a circuit for realizing active balancing of battery pack and a battery pack balancing method. The technology can effectively reduce the complexity of the balancing circuit and improve the balancing speed of the battery pack.

[0007] To solve the technical problem, the present application adopts the following solutions:

[0008] A circuit for realizing active balancing of battery pack is provided, which comprises a battery pack, a balancing channel circuit, an inductor, a balancing conversion circuit, a voltage sensor and a current sensor; wherein,

[0009] The battery pack is composed of N battery monomers in series, N≥2; the negative electrode of the first battery monomer is the negative electrode of the battery pack, and the positive electrode of the Nth battery monomer is the positive electrode of the battery pack;

[0010] The equalization channel circuit comprises N+1 independent channel circuits composed of two back-to-back switch tubes; the first ends of the first to Nth channel circuits are connected to the negative electrodes of the battery monomers in correspondence, respectively, the first end of the N+1th channel circuit is connected to the positive electrode of the Nth battery monomer; the second ends of all the channel circuits connected to the negative electrodes of the odd-numbered battery monomers are connected together and defined as odd poles; the second ends of all the channel circuits connected to the negative electrodes of the even-numbered battery monomers are connected together and defined as even poles;

[0011] The inductor is defined as an inductor A pole at the first end and an inductor B pole at the second end;

[0012] The equalization conversion circuit comprises four independent conversion paths composed of two back-to-back switch tubes; wherein the first end of the first conversion path is connected to the odd pole, and the second end is connected to the inductor A pole; the first end of the second conversion path is connected to the odd pole, and the second end is connected to the inductor B pole; the first end of the third conversion path is connected to the even pole, and the second end is connected to the inductor A pole; the first end of the fourth conversion path is connected to the even pole, and the second end is connected to the inductor B pole;

[0013] The voltage sensor is N in total and is connected in parallel across each battery monomer;

[0014] The current sensor is connected in series at any end of the inductor.

[0015] The application further provides a battery pack active equalization method using the foregoing circuit, comprising the following steps:

[0016] (1) measuring the terminal voltage values of the N battery monomers using the voltage sensor, denoted as V1, V2…VN;

[0017] (2) estimating the SOC (State of Charge) values of the N battery monomers using the open circuit voltage method and the ampere-hour integration method, denoted as SOC1, SOC2…SOCN;

[0018] (3) calculating the average SOC value of the battery pack;

[0019] (4) selecting an equalization threshold Δ between 1% and 3% according to actual equalization requirements;

[0020] The SOC of each battery cell is checked one by one from the first battery cell, and if the SOC value difference of n consecutive battery cells is less than the equalization threshold Δ, the n battery cells are grouped into a battery group, n≥3 and n is an odd number;

[0021] That is, the SOC relationship of the n battery cells that can form a battery group is as follows:

[0022] |SOCi-SOCj|≤Δ (3)

[0023] Wherein, SOCi, SOCj is the SOC of any two battery cells in the battery group;

[0024] If a battery cell cannot form a battery group because the difference between its SOC and the SOC of the battery cells on both sides is greater than the equalization threshold Δ, it is regarded as a battery group by itself; then the next battery cell from the battery group is searched until the last battery cell is found;

[0025] (5) The SOC values of each battery group are calculated, and the one with the highest SOC value is recorded as group A and the one with the lowest SOC value is recorded as group B;

[0026] (6) The battery groups recorded as group A and group B are equalized using inductance until the SOC of one of the groups is equal to the average SOC of the battery group;

[0027] The specific equalization method is as follows:

[0028] (6.1) The serial number of the first battery cell in group A is recorded as Aa, and the serial number of the last battery cell is recorded as Ab; the two switch tubes in the channel circuit connected to the negative electrode of battery cell Aa are KA1 and KA2, and the two switch tubes in the channel circuit connected to the positive electrode of battery cell Ab are KA3 and KA4; the serial number of the first battery cell in group B is recorded as Ba, and the serial number of the last battery cell is recorded as Bb; the two switch tubes in the channel circuit connected to the negative electrode of battery cell Ba are KB1 and KB2, and the two switch tubes in the channel circuit connected to the negative electrode of battery cell Bb are KB3 and KB4; the two switch tubes in the switching path between the odd electrode and the inductor A electrode are recorded as Ka and Kb, the two switch tubes in the switching path between the even electrode and the inductor B electrode are recorded as Kc and Kd, the two switch tubes in the switching path between the odd electrode and the inductor B electrode are recorded as Ke and Kf, and the two switch tubes in the switching path between the even electrode and the inductor A electrode are recorded as Kg and Kh;

[0029] (6.2) Apply PWM control signal PWMA to switch tubes KA1, KA2, KA3, KA4, the frequency of PWMA is f, the duty cycle is D, and the delay time is 0; apply PWM control signal PWMB to switch tubes KB1, KB2, KB3, KB4, and PWMB is complementary to PWMA;

[0030] (6.3) If Aa is odd and Ba is odd, apply PWMB to switch tubes Ka, Kb, Kc, and Kd, and apply PWMA to switch tubes Ke, Kf, Kg, and Kh; if Aa is odd and Ba is even, apply a constant off signal to switch tubes Ka, Kb, Kc, and Kd, and apply a constant on signal to switch tubes Ke, Kf, Kg, and Kh; if Aa is even and Ba is odd, apply a constant on signal to switch tubes Ka, Kb, Kc, and Kd, and apply a constant off signal to switch tubes Ke, Kf, Kg, and Kh; if Aa is even and Ba is even, apply PWMA to switch tubes Ka, Kb, Kc, and Kd, and apply PWMB to switch tubes Ke, Kf, Kg, and Kh;

[0031] (7) After the equalization control, if the SOC difference of all battery monomers in the battery pack is less than the selected equalization threshold Δ, it indicates that the equalization is completed; otherwise, steps (1) to (7) are cyclically executed until the SOC difference of all battery monomers in the battery pack is less than the equalization threshold Δ.

[0032] Compared with the prior art, the beneficial effects of the present application are:

[0033] 1. The present application greatly simplifies the active equalization circuit. From the circuit topology structure, the battery pack composed of N battery monomers only needs to additionally configure 2N+10 switch tubes for forming an equalization channel circuit and an equalization conversion circuit. Compared with the conventional technology, the present application can effectively reduce the number of switch tubes and correspondingly reduce the number of switch tube driving circuits, thereby reducing the circuit cost.

[0034] 2. The battery active equalization circuit of the present application can realize various battery grouping and battery equalization modes, including one-to-one, one-to-many, many-to-one, and many-to-many, and the equalization process is flexible.

[0035] 3. In the equalization process of the battery active equalization circuit of the present application, in one period of a control signal, when PWMA is high, the energy of the discharging battery (group) is transferred to the inductor, and when PWMB is high, the energy in the inductor is transferred to the charging battery (group), that is, the energy transfer process from the discharging battery (group) to the charging battery (group) is completed in one period, the equalization target is achieved, and the equalization speed is fast. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1The battery active equalization circuit topology of the present application (taking the example of the battery monomer number N being odd).

[0037] Figure 2 The battery active equalization circuit topology of the present application (taking the example of the battery monomer number N being even).

[0038] Figure 3 The battery equalization method flow chart of the present application.

[0039] Figure 4 The schematic diagram of step six of example 1.

[0040] Figure 5 The schematic diagram of the second execution of step six of example 1. DETAILED DESCRIPTION

[0041] The present application is described in detail below according to the drawings.

[0042] The first part of the implementation scheme of the present application

[0043] The present application first provides a circuit for realizing the active equalization of a battery pack, which includes a battery pack, an equalization channel circuit, an inductor, an equalization conversion circuit, a voltage sensor, a current sensor and the like structure, wherein:

[0044] Structure one: the battery pack is composed of N battery monomers in series, N≥2. The negative electrode of the first battery monomer is the negative electrode of the battery pack, the positive electrode of the Nth battery monomer is the positive electrode of the battery pack, and the positive electrode of the ith battery monomer is connected to the negative electrode of the i+1th battery monomer (1≤i≤N-1). For the jth battery monomer (1≤j≤N), when j is odd, the battery monomer is defined as an odd bit battery monomer, and when j is even, the battery monomer is defined as an even bit battery monomer.

[0045] Structure two: the equalization channel circuit includes N+1 independent channel circuits composed of two back-to-back switch tubes, and the first end of each channel circuit is respectively connected to the positive and negative electrodes of each battery monomer in structure one. If the positive electrode of a battery A is connected to the negative electrode of another battery B, then only the first end of one channel circuit is connected at the positive electrode of battery A and the negative electrode of battery B. The second ends of all channel circuits connected to odd bit battery monomers are connected together and defined as odd poles. The second ends of all channel circuits connected to even bit battery monomers are connected together and defined as even poles.

[0046] Figure 1 、 2 The difference between the "structure two" part in the above two structures is that the wiring sequence under BATN-1 and BATN is different, Figure 1 N is odd in the above structure, Figure 2 N is even in the above structure.

[0047] Structure 3: Inductor, its first end is defined as inductor A, and its second end is defined as inductor B.

[0048] Structure 4: Equalization conversion circuit, comprising four independent conversion paths composed of two back-to-back switching transistors. The first terminal of the first conversion path is connected to the odd-numbered terminal in Structure 2, and the second terminal is connected to the inductor A terminal in Structure 3. The first terminal of the second conversion path is connected to the odd-numbered terminal in Structure 2, and the second terminal is connected to the inductor B terminal in Structure 3. The first terminal of the third conversion path is connected to the even-numbered terminal in Structure 2, and the second terminal is connected to the inductor A terminal in Structure 3. The first terminal of the fourth conversion path is connected to the even-numbered terminal in Structure 2, and the second terminal is connected to the inductor B terminal in Structure 3.

[0049] Structure 5: Voltage sensors, N in total, connected in parallel to both ends of each battery cell, used to measure the terminal voltage of each battery cell.

[0050] Structure 6: Current sensor, connected in series at any end of the inductor (for example, connected in series between the inductor's A terminal and the previous circuit node), is used to measure the current flowing through the inductor.

[0051] Based on the above circuit, the active battery pack balancing method of the present invention includes the following steps:

[0052] Step 1: Measure the terminal voltage of each of the N battery cells using a voltage sensor, and record them as V1, V2...VN.

[0053] Step 2: Estimate the SOC (State of Charge) value of each of the N battery cells using the open-circuit voltage method and the ampere-hour integration method, and denote them as SOC1, SOC2...SOCN.

[0054] Combining the open-circuit voltage method and the ampere-hour integration method, the SOC value of a single battery cell is calculated according to the following formula:

[0055]

[0056] Among them, SOC i Let V be the state of charge (SOC) of the i-th battery cell, and let f() be the function representing the one-to-one correspondence between the battery cell's terminal voltage and its SOC. i Let C be the terminal voltage of the i-th battery. N I represents the rated capacity of a single battery cell. i Let be the current of the i-th battery.

[0057] Step 3: Calculate the average SOC of the battery pack, as shown in the following formula:

[0058]

[0059] in, is the average SOC of the battery pack, N is the number of battery cells of the battery pack, SOC i is the SOC of the i-th battery cell.

[0060] Step four: select a balancing threshold Δ between 1% and 3% according to the actual balancing demand;

[0061] Check the SOC of each battery cell one by one from the first battery cell. If the difference between the SOC values of n (n > 3, n is an odd number) consecutive battery cells is less than the balancing threshold Δ, then these n battery cells form a battery group.

[0062] The SOC relationship of the n battery cells that can form a battery group is as follows:

[0063] | SOC i - SOC j | ≤ Δ (3)

[0064] Wherein, SOC i, SOC j is the SOC of any two battery cells in the battery group.

[0065] If a battery cell cannot form a battery group because the difference between its SOC and the SOC of the battery cells on both sides is greater than the balancing threshold Δ, then it is considered as a battery group by itself. Then continue to search from the next battery cell of this battery group until the last battery cell is found.

[0066] Step five: find the highest SOC in all battery groups, denoted as group A, and find the lowest SOC, denoted as group B.

[0067] The SOC value of the battery group is defined as the average of the SOC of the n battery cells in the group, as shown in the following formula:

[0068]

[0069] Wherein SOC gi is the SOC value of the i-th battery group.

[0070] Step six: use inductance to balance the batteries in group A and group B until the SOC of one of the groups is equal to the average SOC of the battery pack. The specific balancing method is as follows:

[0071] (1) Denote the battery serial number of the first battery cell in group A as Aa, the battery serial number of the last battery cell as Ab, the battery serial number of the first battery cell in group B as Ba, and the battery serial number of the last battery cell as Bb.

[0072] Let KA1 and KA2 be the two switch tubes in the channel circuit connected with the negative pole of battery cell Aa. Let KA3 and KA4 be the two switch tubes in the channel circuit connected with the positive pole of battery cell Ab. Let KB1 and KB2 be the two switch tubes in the channel circuit connected with the negative pole of battery cell Ba. Let KB3 and KB4 be the two switch tubes in the channel circuit connected with the negative pole of battery cell Bb. Let Ka and Kb be the two switch tubes in the conversion path between the odd pole and the pole of inductor A. Let Kc and Kd be the two switch tubes in the conversion path between the even pole and the pole of inductor B. Let Ke and Kf be the two switch tubes in the conversion path between the odd pole and the pole of inductor B. Let Kg and Kh be the two switch tubes in the conversion path between the even pole and the pole of inductor A. Apply PWM control signal PWMA to switch tubes KA1, KA2, KA3 and KA4, the frequency of PWMA being f, the duty cycle being D, and the delay time being 0. Apply PWM control signal PWMB to switch tubes KB1, KB2, KB3 and KB4, PWMB being complementary to PWMA.

[0073] If Aa is odd and Ba is odd, apply PWMB to switch tubes Ka, Kb, Kc and Kd, and apply PWMA to switch tubes Ke, Kf, Kg and Kh. If Aa is odd and Ba is even, apply always-off signal to switch tubes Ka, Kb, Kc and Kd, and apply always-on signal to switch tubes Ke, Kf, Kg and Kh. If Aa is even and Ba is odd, apply always-on signal to switch tubes Ka, Kb, Kc and Kd, and apply always-off signal to switch tubes Ke, Kf, Kg and Kh. If Aa is even and Ba is even, apply PWMA to switch tubes Ka, Kb, Kc and Kd, and apply PWMB to switch tubes Ke, Kf, Kg and Kh.

[0074] wherein the frequency f and the duty cycle D of the PWM control signal PWMA are subject to the following conditions:

[0075] The inductor current during the conduction of switch tubes KA1, KA2, KA3 and KA4 is:

[0076]

[0077] wherein i 0DCH is the initial current at the beginning of the conduction stage, equal to the minimum current i Lmin on the inductor, U A is the terminal voltage of the battery in group A, L is the inductance size; R on1 is the conduction resistance of the circuit in this stage, R on1 = 8R K + n A R Bat , R K is the conduction resistance of a single switch tube, RBat R is the internal resistance of the single battery cell, n A is the number of battery cells in the A group battery.

[0078] In this process, when i LDCH reaches the maximum value:

[0079]

[0080] The inductor current during the conduction of the switch tubes KB1, KB2, KB3, and KB4 is:

[0081]

[0082] wherein i 0CHG is the initial current at the beginning of the conduction stage, equal to the maximum current i LMAX on the inductor, U B is the terminal voltage of the B group battery, and L is the inductance size; R on2 is the conduction resistance of the circuit at this stage, R on2 = 8R K + n B R Bat , R K is the conduction resistance of a single switch tube, r Bat is the internal resistance of the single battery cell, n B is the number of battery cells in the A group battery.

[0083] In this process, when i LCHG reaches the minimum value:

[0084]

[0085] According to formulas (5), (6), (7), and (8), the following can be solved:

[0086]

[0087] wherein,

[0088] (c) According to the principle of energy transfer from the A group battery to the B group battery, i Lmin ≥ 0, i LMAX ≥ 0, and when the equalization speed is required, i Lmin ≥ I setmin , i LMAX ≥ I setmin , I seymin is the minimum value of the equalization current;

[0089] According to the safety principle, the maximum current must not exceed the safe charging current I safe, i Lmin ≤I safe 、i LMAX ≤I safe ;

[0090] The constraint conditions of the frequency f and the duty cycle D of the PWMA are obtained as follows:

[0091] I setmin ≤i LMAX ≤I safe (11)

[0092] I setmin ≤i Lmin ≤I safe (12)

[0093] In the case of satisfying the above condition constraints, f is selected as a value in 10 kHZ to 100 kHz according to experience; the difference between the desired balanced current I se t and the inductance current I L is taken as the input of the PI control loop, and the output is the duty cycle D, as shown in the following formula:

[0094]

[0095] Where s is a complex variable in the complex frequency domain; the proportional coefficient K p , the integral coefficient K i can be set by theoretical calculation methods such as frequency response method, root locus method, or engineering methods such as empirical adjustment method.

[0096] Step seven: After the balanced control, if the SOC difference of all battery monomers in the battery pack is less than the selected balancing threshold Δ, then it means that the balancing is completed; otherwise, the operations or calculations of steps one to seven are cycled until the SOC difference of all battery monomers in the battery pack is less than the balancing threshold Δ.

[0097] Second part Specific implementation example

[0098] In the battery pack in this example, the number of battery monomers N is an odd number, N = 7, and the circuit connection mode is as shown in Figure 4 The voltages of the 7 battery monomers are 3.689V, 3.693V, 3.685V, 3.737V, 3.621V, 3.619V, and 3.617V, respectively.

[0099] Step one: measure the terminal voltage values of each of the N battery monomers by the voltage sensor, denoted as V1, V2… VN.

[0100] V1 = 3.689 V, V2 = 3.693 V, V3 = 3.685 V, V4 = 3.737 V, V5 = 3.621 V, V6 = 3.619 V, V7 = 3.617 V

[0101] Step two: Estimate the SOC (State of Charge) value of each of the N battery monomers by open circuit voltage method and ampere-hour integral method, denoted as SOC1, SOC2...SOCN.

[0102] SOC1 = 76.2%, SOC2 = 76.6%, SOC3 = 75.8%, SOC4 = 81.0%, SOC5 = 69.1%, SOC6 = 68.8%, SOC7 = 68.6%

[0103] Step three: Calculate the average SOC of the battery pack, as shown in the following formula:

[0104]

[0105] wherein, is the average SOC of the battery pack, N is the number of battery monomers of the battery pack, and SOCi is the SOC of the i-th battery monomer.

[0106] Step four: Select the equalization threshold Δ as 1%, and check the SOC of each battery monomer one by one from the first battery monomer. If the difference between the SOC values of the n (n≥3, n is an odd number) consecutive battery monomers is within 1%, then the n battery monomers form a battery group.

[0107] If a battery monomer cannot form a battery group, it is considered as a battery group itself. Then continue to search from the next battery monomer of this battery group until the last battery monomer is found. The SOC relationship of the n battery monomers that can form a battery group is as shown in the following formula:

[0108] |SOCi-SOCj|≤1%

[0109] wherein, SOCi and SOCj are the SOC of any two battery monomers in the battery group.

[0110] The SOC value of the battery group is defined as the average of the SOC of the n battery monomers in the group, as shown in the following formula:

[0111]

[0112] wherein SOCgi is the SOC value of the i-th battery group.

[0113] The SOC difference of the battery cells 1, 2, 3 is within 1%, which constitutes a battery subgroup 1, the SOC difference of the battery cell 4 and the battery cell after it is greater than 1%, the battery cell 4 itself constitutes a battery subgroup 2, the SOC difference of the battery cells 5, 6, 7 is within 1%, which constitutes a battery subgroup 3. The SOC values of the three battery subgroups are respectively:

[0114]

[0115] Step five: find the highest SOC in all battery subgroups, record as group A, find the lowest SOC, record as group B. Then record the battery subgroup 2 as group A and the battery subgroup 3 as group B.

[0116] Step six: the A group battery and the B group battery are balanced through the inductor until the SOC of one of the groups is equal to the average SOC of the battery pack. The balancing method is as follows:

[0117] Record the battery serial number of the first battery cell in the A group battery as Aa, the battery serial number of the last battery cell as Ab, the battery serial number of the first battery cell in the B group battery as Ba, and the battery serial number of the last battery cell as Bb.

[0118] Then Aa=4, Ab=4, Ba=5, Bb=7

[0119] Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected to the negative electrode of the battery cell Aa as KA1 and KA2. Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected to the positive electrode of the battery cell Ab as KA3 and KA4. Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected to the negative electrode of the battery cell Ba as KB1 and KB2. Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected to the negative electrode of the battery cell Bb as KB3 and KB4, as shown in Figure 4 .

[0120] Among them, KA3 and KB1 are the same switching tube, KA4 and KB2 are the same switching tube.

[0121] Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the odd pole and the inductor A pole as Ka and Kb. Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the even pole and the inductor B pole as Kc and Kd. Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the odd pole and the inductor B pole as Ke and Kf. Record the two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the even pole and the inductor A pole as Kg and Kh.

[0122] Apply PWM control signal PWMA to switching transistors KA1, KA2, KA3, and KA4. PWMA has a frequency of f, a duty cycle of D, and a delay time of 0. Apply PWM control signal PWMB to switching transistors KB1, KB2, KB3, and KB4. PWMB is complementary to PWMA.

[0123] Aa is an even number and Ba is an odd number. Apply normally on signals to switches Ka, Kb, Kc, and Kd, and normally off signals to switches Ke, Kf, Kg, and Kh.

[0124] like Figure 4 As shown, during one control cycle, when the PWMA signal is high, battery group 2 (cell 4) charges the inductor through switching transistors KA1, KA2, KA3, KA4, Ka, Kb, Kc, and Kd, while the other switching transistors are off. Current flows from terminal A of the inductor to terminal B. The current starts from the positive terminal of battery group 2 (cell 4), passes sequentially through: KA3, KA4, Ka, Kb, inductor, Kd, ​​Kc, KA2, and KA1, and returns to the negative terminal of battery group 2 (cell 4).

[0125] When the PWMB signal is high, the inductor charges battery group 3 (cells 5, 6, and 7) through switching transistors KB1, KB2, KB3, KB4, Ka, Kb, Kc, and Kd. The other switching transistors are off, and the current flows from terminal A of the inductor to terminal B. The current flows sequentially from terminal B through: Kd, Kc, KB4, KB3, battery group 3, KB1, KB2, Ka, and Kb back to terminal A of the inductor.

[0126] This is to achieve a balance between battery group 2 and battery group 3.

[0127] The rated charging current of each battery cell in the battery pack is 2A, the desired equalization current is set to 1.8A, the minimum equalization current is 1.7A, the inductance is 1mH, the internal resistance of each battery cell is 35mΩ, and the internal resistance of the switching transistor is 10mΩ. The constraints on the PWM signal frequency and duty cycle are calculated as follows:

[0128]

[0129]

[0130] Based on experience, we choose f = 15kHz and T = 1 / f, combined with the constraints:

[0131] 1.7A≤i LMAX ≤2A

[0132] 1.7A≤i Lmin ≤2A

[0133] Substitute i LMAX ILmin The expression is calculated as follows:

[0134] 0.7601≤D≤07611

[0135] The difference between the inductance current and the battery safe charging current is taken as the input of the PI control loop, and the output is the duty ratio D.

[0136]

[0137] After this equalization step is completed, the SOC value of the battery group 2 (battery cells 4) becomes the average SOC of the battery group, i.e. 73.72%, and the SOC value of the battery group 3 (battery cells 5, 6, 7) becomes:

[0138]

[0139] Step seven: At this time, the equalization is still not completed, and steps one to seven are continued to be executed in a loop.

[0140] Step one second execution: Measure the respective terminal voltage values of the N battery cells through the voltage sensor, denoted as V1, V2… VN.

[0141] V1=3.689V, V2=3.693V, V3=3.685V, V4=3.663V, V5=3.641V, V6=3.641V, V7=3.641V

[0142] Step two second execution: Estimate the respective SOC (State of Charge) values of the N battery cells through the open circuit voltage method and the ampere-hour integration method, denoted as SOC1, SOC2…SOCN.

[0143] SOC1=76.2%, SOC2=76.6%, SOC3=75.8%, SOC4=73.72%, SOC5=71.257%, SOC6=71.257%, SOC7=71.257%

[0144] Step three second execution: Calculate the average SOC of the battery group, as shown in the following formula:

[0145]

[0146] wherein, is the average SOC of the battery group, N is the number of battery cells of the battery group, and SOCi is the SOC of the i-th battery cell.

[0147] Step four second execution: select the equalization threshold Δ as 1%, from the first battery monomer, check each battery monomer SOC one by one, if the difference of the SOC value of the continuous n (n≥3, n is odd) battery monomers is within 1%, then the n battery monomers form a battery group.

[0148] If a battery monomer cannot form a battery group, it is regarded as a battery group itself. Then continue to find from the next battery monomer of the battery group until the last battery monomer is found. The SOC relationship of n battery monomers that can form a battery group is as follows:

[0149] |SOCi-SOCj|≤1%

[0150] Wherein, SOCi, SOCj is the SOC of any two battery monomers in the battery group.

[0151] The SOC value of the battery group is defined as the average value of the SOC of the n battery monomers in the group, as shown in the following formula:

[0152]

[0153] Wherein SOCgi is the SOC value of the i-th battery group.

[0154] The SOC difference of battery monomers 1, 2 and 3 is within 1%, forming battery group 1, battery monomer 4 and the following battery monomers have a SOC difference greater than 1%, battery monomer 4 itself forms battery group 2, battery monomers 5, 6 and 7 have a SOC difference within 1%, forming battery group 3. The SOC values of the three battery groups are respectively:

[0155]

[0156] Step five second execution: find the highest SOC in all battery groups, record as A group, find the lowest SOC, record as B group. Record battery group 1 as A group and battery group 3 as B group.

[0157] Step six second execution: A group battery and B group battery are balanced by inductance until the SOC of one of them is equal to the average SOC of the battery group. The equalization method is as follows:

[0158] Record the battery serial number of the first battery monomer in A group battery as Aa, and the battery serial number of the last battery monomer as Ab. Record the battery serial number of the first battery monomer in B group battery as Ba, and the battery serial number of the last battery monomer as Bb.

[0159] Then Aa=1, Ab=3, Ba=5, Bb=7

[0160] The two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected with the negative electrode of the battery monomer Aa are KA1 and KA2. The two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected with the positive electrode of the battery monomer Ab are KA3 and KA4. The two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected with the negative electrode of the battery monomer Ba are KB1 and KB2. The two switching tubes in the independent circuit path composed of two back-to-back switching tubes connected with the negative electrode of the battery monomer Bb are KB3 and KB4, as shown in Figure 5 .

[0161] In Figure 4 , 5 , the identification positions of KA1-KA4 and KB1-KB4 are different. Since the batteries at different positions are charged and discharged, the positions of the MOSFETs renamed as KA1-KA4 and KB1-KB4 according to the equalization process are different.

[0162] The two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the odd electrode and the inductor A pole are Ka and Kb. The two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the even electrode and the inductor B pole are Kc and Kd. The two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the odd electrode and the inductor B pole are Ke and Kf. The two switching tubes in the independent circuit path composed of two back-to-back switching tubes between the even electrode and the inductor A pole are Kg and Kh.

[0163] The switching tubes KA1, KA2, KA3, and KA4 are applied with a PWM control signal PWMA, the frequency of PWMA is f, the duty cycle is D, and the delay time is 0. The switching tubes KB1, KB2, KB3, and KB4 are applied with a PWM control signal PWMB, which is complementary to PWMA.

[0164] If Aa is odd and Ba is odd, the switching tubes Ka, Kb, Kc, and Kd are applied with PWMB, and the switching tubes Ke, Kf, Kg, and Kh are applied with PWMA.

[0165] As shown in Figure 5 , in one control cycle, when the PWMA signal is high, the battery group 1 (battery monomers 1, 2, and 3) charges the inductor through the switching tubes KA1, KA2, KA3, KA4, Ke, Kf, Kg, and Kh, and other switching tubes are closed. The current flows from the positive electrode of the battery group 1 (battery monomers 1, 2, and 3) to the inductor B pole through KA3, KA4, Ke, Kf, the inductor, Kh, Kg, KA2, and KA1 in turn, and returns to the negative electrode of the battery group 1 (battery monomers 1, 2, and 3).

[0166] When the PWMB signal is high, the inductor charges the battery group 3 (battery cells 5, 6, 7) through the switch tubes KB1, KB2, KB3, KB4, Ka, Kb, Kc, Kd, and other switch tubes are closed, and the current flows from the inductor A pole to the inductor B pole. The current flows from the inductor B pole in turn through: Kd, Kc, KB4, KB3, battery group 3, KB1, KB2, Ka, Kb, and returns to the inductor A pole.

[0167] In this way, the balance between the battery group 1 and the battery group 3 is completed.

[0168] The rated charging current of each battery cell in the battery group is 2A, the expected balancing current is set to 1.8A, the minimum balancing current is 1.7A, the inductance value is 1mH, the battery cell internal resistance is 35mΩ, and the switch tube internal resistance is 10mΩ. The constraints of the PWM signal frequency and duty cycle are calculated as follows:

[0169]

[0170]

[0171] According to experience, f = 15kHz, T = 1 / f, and combined with the constraint conditions:

[0172] 1.7A≤i Lmin ≤2A

[0173] 1.7A≤i Lmin ≤2A

[0174] Substituting i LMAX , i Lmin into the expression, the calculation result is:

[0175] 0.5126≤D≤0.5120

[0176] It is not achievable, adjust the frequency to f = 30kHz, and substitute i LMAX , i Lmin into the expression, the calculation result is:

[0177] 0.5118≤D≤0.5127

[0178] The difference between the inductor current and the safe charging current of the battery is taken as the input of the PI control loop, and the output is the duty cycle D.

[0179]

[0180] After this balancing step, the SOC value of the battery group 1 (battery cells 1, 2, 3) becomes the average SOC of the battery group, i.e. 73.72%, and the SOC value of the battery group 3 (battery cells 5, 6, 7) also becomes the average SOC of the battery group, i.e. 73.72%.

[0181] Step seven second execution: At this time, the SOC value of all battery monomers is 73.72%, and the equalization is completed.

Claims

1. A circuit for implementing active balancing of a battery pack, characterized by, The circuit comprises a battery pack, an equalization channel circuit, an inductor, an equalization conversion circuit, a voltage sensor and a current sensor, wherein The battery pack is composed of N battery monomers connected in series, and N≥2; the negative electrode of the first battery monomer is the negative electrode of the battery pack, and the positive electrode of the Nth battery monomer is the positive electrode of the battery pack; The equalization channel circuit comprises N+1 independent channel circuits composed of two back-to-back switch tubes; the first ends of the first to Nth channel circuits are connected to the negative electrodes of the battery monomers, respectively, and the first end of the N+1th channel circuit is connected to the positive electrode of the Nth battery monomer; the second ends of all the channel circuits connected to the negative electrodes of the odd-numbered battery monomers are connected together and defined as odd poles; the second ends of all the channel circuits connected to the negative electrodes of the even-numbered battery monomers are connected together and defined as even poles; The inductor has a first end defined as an inductor A pole and a second end defined as an inductor B pole; The equalization conversion circuit comprises four independent conversion paths composed of two back-to-back switch tubes; the first end of the first conversion path is connected to the odd poles, and the second end is connected to the inductor A pole; the first end of the second conversion path is connected to the odd poles, and the second end is connected to the inductor B pole; the first end of the third conversion path is connected to the even poles, and the second end is connected to the inductor A pole; the first end of the fourth conversion path is connected to the even poles, and the second end is connected to the inductor B pole; The voltage sensor has N in total and is connected in parallel across each battery monomer; The current sensor is connected in series at either end of the inductor.

2. A method for actively balancing a battery pack using the circuit of claim 1, wherein, The method comprises the following steps: (1) measuring the terminal voltage values of the N battery monomers by using the voltage sensor, and recording as V1, V2…VN; (2) estimating the SOC values of the N battery monomers by using the open circuit voltage method and the ampere-hour integration method, and recording as SOC1, SOC2…SOCN; (3) calculating the average SOC value of the battery pack; (4) selecting an equalization threshold Δ between 1% and 3% according to the actual equalization requirement; Starting from the first battery monomer, checking the SOC of each battery monomer one by one, if the SOC value difference of the SOC of the n battery monomers in succession is less than the equalization threshold Δ, then the n battery monomers form a battery group, n≥3 and n is an odd number; That is, the SOC relationship of the n battery monomers capable of forming a battery group is as shown in the following formula: |SOCi-SOCj|≤Δ (3) Wherein, SOCi and SOCj are the SOC of any two battery monomers in the battery group; If a battery monomer cannot form a battery group due to the reason that the difference between its SOC and the SOC of the battery monomers on both sides is greater than the equalization threshold Δ, then it is regarded as a battery group by itself; then the next battery monomer of the battery group is started to continue to be searched until the last battery monomer is found; (5) calculating the SOC value of each battery group, and recording the one with the highest SOC value as group A and the one with the lowest SOC value as group B; (6) using the inductor to equalize the battery groups recorded as group A and group B until the SOC of one of the groups is equal to the average SOC of the battery pack. The specific balancing method is as follows: (6.1) the serial number of the first battery cell in the A group is recorded as Aa, and the serial number of the last battery cell is recorded as Ab; the two switch tubes in the channel circuit connected with the negative electrode of the battery cell Aa are KA1 and KA2, and the two switch tubes in the channel circuit connected with the positive electrode of the battery cell Ab are KA3 and KA4; the serial number of the first battery cell in the B group is recorded as Ba, and the serial number of the last battery cell is recorded as Bb; the two switch tubes in the channel circuit connected with the negative electrode of the battery cell Ba are KB1 and KB2, and the two switch tubes in the channel circuit connected with the negative electrode of the battery cell Bb are KB3 and KB4; the two switch tubes in the conversion path between the odd pole and the inductor A pole are recorded as Ka and Kb, the two switch tubes in the conversion path between the even pole and the inductor B pole are recorded as Kc and Kd, and the two switch tubes in the conversion path between the odd pole and the inductor B pole are recorded as Ke and Kf; the two switch tubes in the conversion path between the even pole and the inductor A pole are recorded as Kg and Kh; (6.2) PWM control signals PWMA are applied to the switch tubes KA1, KA2, KA3 and KA4, the frequency of PWMA is f, the duty ratio is D, and the delay time is 0; PWM control signals PWMB are applied to the switch tubes KB1, KB2, KB3 and KB4, and PWMB is complementary to PWMA; (6.3) if Aa is odd and Ba is odd, PWMB is applied to the switch tubes Ka, Kb, Kc and Kd, and PWMA is applied to the switch tubes Ke, Kf, Kg and Kh; if Aa is odd and Ba is even, a constant off signal is applied to the switch tubes Ka, Kb, Kc and Kd, and a constant on signal is applied to the switch tubes Ke, Kf, Kg and Kh; if Aa is even and Ba is odd, a constant on signal is applied to the switch tubes Ka, Kb, Kc and Kd, and a constant off signal is applied to the switch tubes Ke, Kf, Kg and Kh; if Aa is even and Ba is even, PWMA is applied to the switch tubes Ka, Kb, Kc and Kd, and PWMB is applied to the switch tubes Ke, Kf, Kg and Kh; (7) after the balancing control, if the SOC difference of all battery cells in the battery group is less than the selected balancing threshold Δ, it indicates that the balancing is completed; Otherwise, steps (1) to (7) are repeatedly executed until the SOC difference of all battery cells in the battery group is less than the balancing threshold Δ.

3. The method of claim 2, wherein, In the step (2), the SOC value of the battery cell is calculated according to the following formula: wherein SOC i is the SOC of the i-th battery cell, f() is a one-to-one correspondence between the battery cell terminal voltage and the SOC, V i is the terminal voltage of the i-th battery, C N is the rated capacity of the battery cell, I i is the current of the i-th battery.

4. The method of claim 2, wherein, In the step (3), the average SOC of the battery group is calculated according to the following formula: wherein, is the average SOC of the battery pack, N is the number of battery cells of the battery pack, SOC i is the SOC of the i-th battery cell.

5. The method of claim 2, wherein, In the step (5), the SOC value of the battery group is defined as the average value of the SOC of the n battery cells in the group, and the calculation formula is as follows: Wherein SOCgi is the SOC value of the i-th battery group.

6. The method of claim 2, wherein, In the step (6), the frequency f and the duty ratio D of the PWM control signal PWMA are subject to the following conditions: (a) the inductor current of the switch tubes KA1, KA2, KA3 and KA4 during conduction is: Among them, i 0DCH The initial current at the start of the conduction phase is equal to the minimum current i in the inductor. Lmin ;U A R is the terminal voltage of battery group A, L is the inductance value; on1 R is the on-resistance of the circuit during the conduction phase. on1 =8R K +n A R Bat R k R is the on-resistance of a single switching transistor. Bat n is the internal resistance of a single battery cell. A This represents the number of individual battery cells in group A. During this process, when i LDCH reaches a maximum: (b) the inductor current of the switch tubes KB1, KB2, KB3 and KB4 during conduction is: Among them, i 0CHG The initial current at the start of the conduction phase is equal to the maximum current i in the inductor. LMAX U B R is the terminal voltage of battery group B, L is the inductance value; on2 For the on-resistance of the circuit at this stage, R on2 =8R K +n B R Bat R K R is the on-resistance of a single switching transistor. Bat n is the internal resistance of a single battery cell. B This represents the number of individual battery cells in group A. During this process, when i LCHG reaches a minimum: According to the formula (5), (6), (7), (8), the solution is: wherein, (c) According to the principle of energy transfer from Group A to Group B, i Lmin ≥ 0, i LMAX ≥ 0, i Lmin ≥ I setmin , i LMAX ≥ I setmin , I setmin is the minimum balancing current; According to the safety principle, the maximum current must not exceed the battery safety charging current I safe , i Lmin ≤I safe 、i LMAX ≤I safe ; Thus, the constraint condition of the frequency f and the duty cycle D of the PWM A is obtained: I setmin ≤i LMAX ≤I safe (11) I setmin ≤i Lmin ≤I safe (12) In the case of meeting the above condition constraints, f is selected as a value in 10 kHZ to 100 kHz according to experience; in order to expect the balanced current I set The difference between the inductance current I L and the reference current Iref is taken as the input of the PI control loop, and the output is the duty ratio D, as shown in the following formula: Wherein, s is a complex variable in the complex frequency domain; the proportional coefficient K p , the integral coefficient K i is set by theoretical calculation or empirical adjustment method.

Images