Charging and discharging control method of lithium ion battery pack, battery management system and charging and discharging method

Abstract

The application belongs to the field of lithium battery charging and discharging, and particularly relates to a charging and discharging control method of a lithium ion battery pack, a battery management system and a charging and discharging method. The control method comprises the following steps: 1) according to a battery internal resistance-SOC curve, a low SOC interval, a middle SOC interval and a high SOC interval are divided according to the change of the battery internal resistance; 2) the new current I1' of the low SOC interval, the new current I3' of the high SOC interval and the new charging current I2' of the middle SOC interval are calculated according to the relationship between the resistance and the SOC. The application provides a charging and discharging current control method with simple algorithm based on the model of the change of the battery internal resistance with the SOC, which can improve the heating of the battery pack in the low SOC interval and the high SOC interval, and is beneficial to delaying the aging of the battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery charging and discharging, specifically relating to a charging and discharging control method, a battery management system, and a charging and discharging method for a lithium-ion battery pack. Background Technology

[0002] As is well known, the cycle life of lithium-ion battery packs is strongly correlated with temperature; the cycle life at high temperatures is significantly lower than that at room temperature. Therefore, reducing or eliminating the heat generation of the battery pack can effectively extend its lifespan. For battery systems that rely on passive natural cooling (without active cooling such as liquid cooling devices), improving battery pack heat generation is even more crucial.

[0003] Traditional battery management systems (BMS) typically employ charge / discharge control strategies that pre-determine the corresponding charge / discharge currents at different states of charge (SOC) and temperatures (e.g., using range MAP tables or ammeters), generally without considering changes in the battery's internal electrochemical states. However, due to inherent characteristics such as polarization, lithium batteries exhibit significant changes in internal resistance between low SOC (0-30%) and high SOC (80-100%) ranges. Adjusting the charge / discharge current based on these internal resistance changes can mitigate battery pack heating, thus slowing capacity decay and extending battery pack lifespan. Summary of the Invention

[0004] The purpose of this invention is to provide a charging and discharging control method, a battery management system, and a charging and discharging method for a lithium-ion battery pack.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for controlling the charging and discharging of a lithium-ion battery pack includes the following steps:

[0007] 1) Based on the battery internal resistance-SOC curve, divide the battery into low SOC range, medium SOC range and high SOC range according to the change of battery internal resistance.

[0008] 2) Calculate the new current I1' in the low SOC range, the new current I3' in the high SOC range, and the new charging current I2' in the medium SOC range based on the relationship between resistance and SOC.

[0009] In step 2), the new current I1' in the low SOC range and the new current I3' in the high SOC range are calculated first. Then, the new charging current I2' in the middle SOC range is adjusted based on the new current I1' in the low SOC range and the new current I3' in the high SOC range.

[0010] In step S1), the battery internal resistance-SOC curve presents a U-shape with high resistance at both ends and low resistance in the middle. The part with low resistance in the middle is the medium SOC range, and the parts with high resistance at both ends are the low SOC range and the high SOC range. The low SOC range is [0, X1%], the medium SOC range is [X1%, X2%], and the high SOC range is [X2%, 100%], where X1≤30 and X2≥80.

[0011] In step S1), the rate of change of internal resistance is generally greater than 0.01-0.2mΩ / 1% in the low SOC range and high SOC range, and less than 0.01-0.5mΩ / 10% in the medium SOC range.

[0012] The relationship between resistance and SOC was fitted in both the low SOC and high SOC ranges; the relationship between resistance and SOC is as follows:

[0013] R(soc) = A n soc n +A n-1 soc n-1 +…+A1soc 1 +A0 (1);

[0014] Where R(soc) represents the internal resistance of the battery, and A i These are the polynomial coefficients (i from n to 0), where n represents the polynomial degree, and r is the coefficient of determination for the fit. 2 =1.

[0015] Assuming the coefficient of determination r 2 When n=1, this is achieved when n=2; therefore, the relationship between resistance and SOC can be fitted as y=ax 2 +bx+c.

[0016] If we define that the heat generation does not increase when the battery internal resistance increases, then we can obtain equations (2) and (3):

[0017] △Q=I×I×y×△t=I×I×y×(△x / I)=I'×I'×y'×(△x / I')(2)

[0018] I×(ax 2 +bx+c)=I'[a(x 2 +2x△x+△x 2 )+bx+b△x)+c](3);

[0019] In the formula, ΔQ represents the system heat generation, I represents the initial charging / discharging current, I' represents the current adjusted between the low and high SOC ranges, y represents the current battery internal resistance, y' represents the battery internal resistance corresponding to the measured SOC at the next moment, x represents the current system SOC, and Δx represents the change in SOC at the next moment, which is also the change in battery capacity. During charging, capacity increases, Δx > 0; during discharging, capacity decreases, Δx < 0; Δt represents the time corresponding to the infinitesimal change in SOC, and a, b, and c are the polynomial coefficients obtained by fitting when n = 2.

[0020] Based on equations (2) and (3), we can obtain the new current I1' in the low SOC range (4) and the new current I3' in the high SOC range (5).

[0021] I1'=I×y1 / y1'=I×(a1x 2 +b1x+c1) / [a1(x 2 +2x△x+△x 2 )+b1x+b1△x1)+c3](4);

[0022] In the formula, I is the original charging and discharging current, I1' is the new current in the adjusted low SOC range; y is the current battery internal resistance, y1' is the battery internal resistance corresponding to the SOC at the next moment in the low SOC range, x is the current system SOC, and Δx is the measured change in SOC at the next moment, which is also the change in battery capacity; during charging, the capacity increases, Δx > 0; during discharging, the capacity decreases, Δx < 0; a1, b1, and c1 are the polynomial coefficients obtained by fitting when n = 2.

[0023] I3'=I×y3 / y3'=I×(a3x 2 +b3x+c3) / [a3(x 2 +2x△x+△x 2 )+b3x+b3△x3)+c3](5);

[0024] In the formula, I is the original charging and discharging current, I3' is the new current in the adjusted high SOC range; y is the current battery internal resistance, y3' is the battery internal resistance corresponding to the SOC at the next moment in the high SOC range, x is the current system SOC, and Δx is the measured change in SOC at the next moment, which is also the change in battery capacity; during charging, the capacity increases, Δx > 0; during discharging, the capacity decreases, Δx < 0; a3, b3, and c3 are the polynomial coefficients obtained by fitting when n = 2.

[0025] Optionally, during the discharge process, the discharge current control threshold I2 in the SOC range is not adjusted and increased.

[0026] Optionally, for the charging process, assuming that the total charging time does not increase, a larger charging current I2' than the original I2 can be requested from the charging equipment (e.g., charging pile) in the middle SOC range where the battery internal resistance is relatively small. Then, formula (8) is obtained according to the following formulas (6) and (7).

[0027]

[0028] t 总 =t 1总 +t 2总 +t 3总 =t 1总 '+t 2总 '+t 3总 (7)

[0029] I2'=(C 总 -∑I 1i ' ×t 1i '-∑I 3i ' ×t 3i ') / (t 总 -t 1总 '-t 3总 ') (8)

[0030] Among them, C 总 C represents the total charging capacity of the battery system. 1总 C 2总 C 3总 The total charging capacity for the low SOC range, medium SOC range, and high SOC range are respectively represented by I. 1i I 2i I 3i These represent the charging currents of cell i in the original low SOC, medium SOC, and high SOC ranges, respectively (i = 1, 2, ..., n, where n is a natural number), and t. 1i t 2i t 3i These represent the charging times for unit i in the original low SOC, medium SOC, and high SOC ranges, respectively (i = 1, 2, ..., n, where n is a natural number). 1总 t 2总 t 3总 These represent the total charging time for the original low SOC range, medium SOC range, and high SOC range, respectively. 1i '、I 2i '、I 3i 'These represent the new charging currents for the i-cell in the adjusted low SOC, medium SOC, and high SOC ranges, respectively, t 1i '、t 2i '、t 3i'These represent the charging times for unit i in the adjusted low SOC, medium SOC, and high SOC ranges, respectively (i = 1, 2, ..., n, where n is a natural number), t 1总 '、t 2总 '、t 3总 These represent the total charging time for the original low SOC range, medium SOC range, and high SOC range, respectively.

[0031] The present invention also includes a charging and discharging method, which uses the charging and discharging control method of the lithium-ion battery pack to perform charging and discharging.

[0032] The present invention also includes a battery management system, which is used to perform charging and discharging using the charging and discharging control method of the lithium-ion battery pack.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] This application provides a simple charging and discharging current control method based on a model of battery internal resistance varying with SOC. This method can improve battery pack heating in both low and high SOC ranges, thus helping to delay battery aging. The technical solution of this application can be implemented using existing conventional battery internal resistance, the R(SOC) function polynomial fitting is easy, and the current control algorithm is simple. Attached Figure Description

[0035] Figure 1 This is a flowchart of a lithium-ion battery pack charging and discharging control method according to one embodiment of the present invention.

[0036] Figure 2 This is a graph showing the resistance versus SOC curve used for fitting calculations according to one embodiment of the present invention.

[0037] Figure 3-4 These are fitting graphs of the low SOC interval and the high SOC interval obtained when n=2 according to one embodiment of the present invention. Detailed Implementation

[0038] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and preferred embodiments.

[0039] This application provides a charging and discharging control method for a lithium-ion battery pack, such as... Figure 1 The steps shown are as follows:

[0040] In the equivalent circuit of a lithium-ion battery, the ohmic internal resistance is generally set to R0, and the polarization internal resistance is set to R. p Under steady-state conditions, the battery's internal resistance R can be considered as the sum of its ohmic internal resistance and polarization internal resistance, i.e., R = R0 + R pImpedance tests were conducted on lithium-ion batteries at different charge / discharge rates to determine the trend of the battery's charging and discharging impedance R as a function of the state of charge (SOC) curve.

[0041] Based on existing battery internal resistance-SOC curves and the trend of internal resistance changes, the entire SOC is divided into a low SOC range [0, X1%] and a high SOC range [X2, 100%] where resistance increases significantly, and a medium SOC range [X1%, X2%] where resistance is relatively small. X1 is generally ≤30%, and X2 is generally ≥80%. Typically, the rate of change of internal resistance in the low and high SOC ranges is greater than 0.01-0.2 mΩ / 1% (i.e., for every 1% change in SOC, the resistance changes by 0.01-0.2 mΩ), while the rate of change of resistance in the medium SOC range is generally less than 0.01-0.5 mΩ / 10%.

[0042] Figure 2 Here is an example of how the resistance of a lithium battery changes with SOC during charging and discharging at a 1C rate (C = rated capacity): the internal resistance of the battery increases significantly in the low SOC range [0, 10%] and the high SOC range [95, 100%], while the internal resistance of the battery is relatively small in the medium SOC range [10%, 95%].

[0043] Suppose that the relationship between the resistance R and the SOC in the low SOC range and the high SOC range satisfies the polynomial relationship shown in the following formula (1):

[0044] R(soc) = A n soc n +A n-1 soc n-1 +…+A1soc 1 +A0 (1)

[0045] Where R(soc) represents the internal resistance of the battery, and A i These are the polynomial coefficients (i from n to 0), where n represents the polynomial degree, and r is the coefficient of determination for the fit. 2 =1.

[0046] Figure 3 The polynomial function R(soc) showing the variation of the ordinate y (resistance R) with the abscissa x (SOC) in the low SOC range has a coefficient of determination R when n=2. 2 =1, at this time R(soc) is expressed as y1 = a1x 2 +b1x+c1=127.74x 2 -23.461x+2.1936.

[0047] Figure 4The polynomial function R(soc) showing the variation of the ordinate y (resistance R) with the abscissa x (SOC) in the high SOC interval has a coefficient of determination R when n=2. 2 =1, at this time R(soc) is expressed as y3 = a3x 2 +b³x + c³ = 103.28x 2 -191.58x+89.753.

[0048] The BMS calculates the new current I1' in the low SOC range, the new current I3' in the high SOC range, and the new charging current I2' in the medium SOC range based on R(soc).

[0049] Let the initial charge / discharge current be I, and the BMS-adjusted charge / discharge current be I'. Considering maintaining the same heat generation even as the battery's internal resistance increases, then according to...

[0050] △Q=I×I×y×(△x / I)=I'×I'×y'×(△x / I') (2)

[0051] I×(ax 2 +bx+c)=I' [a(x 2 +2x△x+△x 2 (3)

[0052] In the formula, ΔQ represents the system heat generation, y is the current battery internal resistance, y' is the battery internal resistance corresponding to the SOC at the next measured moment, x is the current system SOC, and Δx is the change in SOC at the next measured moment, which is also the change in capacity. During charging, the capacity increases, Δx > 0; during discharging, the capacity decreases, Δx < 0; Δt is the time corresponding to the infinitesimal change in SOC.

[0053] From equations (2) and (3), it can be seen that the current I' changes inversely with the resistance y, and equation (4) is obtained for the new current I1' in the low SOC range and equation (5) is obtained for the new current I3' in the high SOC range.

[0054] I1'=I×y1 / y1'=I×(a1x 2 +b1x+c1) / [a1(x 2 +2x△x+△x 2 )+b1x+b1△x1)+c3](4);

[0055] I3'=I×y3 / y3'=I×(a3x 2 +b3x+c3) / [a3(x 2 +2x△x+△x 2 )+b3x+b3△x3)+c3](5);

[0056] With the low SOC range of this lithium battery ( Figure 3 Taking the example shown, assuming the upper limit of the original charge / discharge current control threshold is 0.4C (C = rated capacity value), then according to the fitted y1 = a1x 2 +b1x+c1=127.74x 2 Substituting -23.461x+2.1936 (a1=127.74,b1=-23.46,c1=2.1936) into formula (4), we can obtain the upper limit of the threshold of the new charge and discharge current I1' in the low SOC range as shown in Table 1 below (taking the 1% SOC range divided into 10 units as an example, this application does not limit the division of the SOC range, and the division can be further refined).

[0057] Table 1

[0058] SOC / % 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 <![CDATA[Original current I1 / C]]> 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 <![CDATA[New current I1' / C]]> 0.21 0.23 0.25 0.28 0.31 0.34 0.36 0.38 0.39 0.40

[0059] With the high SOC range of this lithium battery ( Figure 4 Taking the example shown, assuming the upper limit of the original charge / discharge current control threshold is 0.4C (C = rated capacity value), then according to the fitted y3 = a3x 2 +b³x + c³ = 103.28x 2 Substituting -191.58x+89.753 (a3=103.28,b3=-191.58,c3=89.753) into formula (5), we can obtain the upper limit of the threshold of the new charge and discharge current I3' in the high SOC range as shown in Table 2 below (taking the 1% SOC range divided into 5 units as an example).

[0060] Table 2

[0061]

[0062]

[0063] Optionally, for the charging process, assuming that the total charging time does not increase, a larger charging current I2' than the original I2 can be requested from the charging equipment (e.g., charging pile) in the middle SOC range where the battery internal resistance is relatively small. Then, formula (8) is obtained according to the following formulas (6) and (7).

[0064]

[0065] t 总 =t 1总 +t 2总 +t 3总 =t 1总 '+t 2总 '+t 3总 (7)

[0066] I2'=(C总 -∑I 1i ' ×t 1i '-∑I 3i ' ×t 3i ') / (t 总 -t 1总 '-t 3总 ') (8)

[0067] Among them, C 总 C represents the total charging capacity of the battery system. 1总 C 2总 C 3总 The total charging capacity for the low SOC range, medium SOC range, and high SOC range are respectively represented by I. 1i I 2i I 3i These represent the charging currents of cell i in the original low SOC, medium SOC, and high SOC ranges, respectively (i = 1, 2, ..., n, where n is a natural number), and t. 1i t 2i t 3i These represent the charging times for unit i in the original low SOC, medium SOC, and high SOC ranges, respectively (i = 1, 2, ..., n, where n is a natural number). 1总 t 2总 t 3总 These represent the total charging time for the original low SOC range, medium SOC range, and high SOC range, respectively. 1i '、I 2i '、I 3i 'These represent the new charging currents for the i-cell in the adjusted low SOC, medium SOC, and high SOC ranges, respectively, t 1i '、t 2i '、t 3i 'These represent the charging times for unit i in the adjusted low SOC, medium SOC, and high SOC ranges, respectively (i = 1, 2, ..., n, where n is a natural number), t 1总 '、t 2总 '、t 3总 These represent the total charging time for the original low SOC range, medium SOC range, and high SOC range, respectively.

[0068] Table 3 can be obtained from the data in Tables 1 and 2.

[0069] Table 3

[0070] Original charging time t / h 0.25 2.125 0.125 New charging time t / h 0.334 2.017 0.149

[0071] Calculate t 总=t1+t2+t3=0.25+2.125+0.125=2.5(h, in hours), t 2总 = t1 + t2 + t3 - t 1总 '-t 3总 = 2.017 (h, in hours), I2' = 85% C / t 2总 = 0.4958C;

[0072] In summary, during charging and discharging, the BMS determines the current battery internal resistance (whether it falls within a low, medium, or high SOC range) based on the initial SOC state of the lithium-ion battery pack. It then calculates the corresponding charging current control thresholds I1', I2', or I3', or the discharging current control thresholds I1', I3', and dynamically adjusts the control current according to SOC changes during charging and discharging. By using smaller charging and discharging currents in the low and high SOC ranges (where internal resistance is high), the system's heat generation does not increase as internal resistance increases, thus improving the battery pack's heat dissipation in both low and high SOC ranges and helping to delay battery aging.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for controlling the charging and discharging of a lithium-ion battery pack, characterized in that, Includes the following steps: 1) Based on the battery internal resistance-SOC curve, divide the battery into low SOC range, medium SOC range and high SOC range according to the change in battery internal resistance. 2) Calculate the new current I1' in the low SOC range, the new current I3' in the high SOC range, and the new charging current I2' in the medium SOC range based on the relationship between resistance and SOC. First, calculate the new current I1' in the low SOC range and the new current I3' in the high SOC range, and then adjust the new charging current I2' in the medium SOC range based on the new current I1' in the low SOC range and the new current I3' in the high SOC range. Polynomial fitting was performed on the relationship between resistance and SOC in both the low and high SOC ranges; the polynomial relationship between resistance and SOC is as follows: R(soc)=A n soc n +A n-1 soc n-1 +…+A1soc 1 +A0 (1); Where R(soc) represents the internal resistance of the battery, and A i These are polynomial coefficients, where i ranges from n to 0, and n represents the polynomial degree. The coefficient of determination r for the fit is... 2 =1; When calculating the new current I' in the low or high SOC range, if we define that the heat generation does not increase when the battery internal resistance increases, then: △Q=I×I×y×△t=I×I×y×(△x / I)=I'×I'×y'×(△x / I') (2) I×(ax 2 +bx+c)=I’[a(x 2 +2x△x+△x 2 )+bx+b△x)+c](3); △Q represents the system heat generation, I represents the initial charging / discharging current, I' represents the adjusted current in the low or high SOC range, y represents the current battery internal resistance, y' represents the battery internal resistance corresponding to the measured SOC at the next moment, x represents the current system SOC, and △x represents the change in SOC at the next moment. During charging, the capacity increases, △x > 0; during discharging, the capacity decreases, △x < 0; △t represents the time corresponding to the infinitesimal change in SOC, and a, b, and c are the polynomial coefficients obtained by fitting when n=2. Based on equations (2) and (3), we can obtain equation (4) for the low SOC interval and equation (5) for the high SOC interval. Coefficient of determination r 2 =1 is achieved when n=2, and the relationship between resistance and SOC is fitted as y=ax 2 +bx+c; The calculation method for the new current I1' in the low SOC range is as follows: I1'=I×y1 / y1'=I×(a1x1 2 +b1x1+c1) / [a1(x1 2 +2x1△x+△x 2 )+b1x1+b1△x+c1](4); Where I is the original charging and discharging current, I1' is the new current in the adjusted low SOC range; y1 is the current battery internal resistance, y1' is the battery internal resistance corresponding to the SOC at the next moment in the low SOC range, x1 is the current system SOC, and Δx is the measured change in SOC at the next moment. During charging, the capacity increases, Δx > 0; during discharging, the capacity decreases, Δx < 0; a1, b1, and c1 are the polynomial coefficients obtained by fitting when n=2. The calculation method for the new current I3' in the high SOC range is as follows: I3'=I×y3 / y3'=I×(a3x3 2 +b3x3+c3) / [a3(x3) 2 +2x3△x+△x 2 (+b3x3+b3△x+c3)(5) Where I is the original charging and discharging current, I3' is the new current in the high SOC range after adjustment; y3 is the current battery internal resistance, y3' is the battery internal resistance corresponding to the SOC at the next moment in the high SOC range, x3 is the current system SOC, and Δx is the measured change in SOC at the next moment; during charging, the capacity increases, Δx > 0; during discharging, the capacity decreases, Δx < 0; a3, b3, and c3 are the polynomial coefficients obtained by fitting when n=2.

2. The charging and discharging control method for a lithium-ion battery pack according to claim 1, characterized in that, In step S1), the battery internal resistance-SOC curve presents a U-shape with high ends and low middle; the low middle part is the medium SOC range, and the high ends are the low SOC range and the high SOC range; in step S1), the rate of change of internal resistance in the low SOC range and the high SOC range is greater than 0.01-0.2mΩ / 1%, and the rate of change of internal resistance in the medium SOC range is less than 0.01-0.5mΩ / 10%.

3. The charging and discharging control method for a lithium-ion battery pack according to claim 1, characterized in that... The low SOC range is [0, X1%], the medium SOC range is [X1%, X2%], and the high SOC range is [X2, 100%], where X1 ≤ 30 and X2 ≥ 80.

4. The charging and discharging control method for a lithium-ion battery pack according to claim 1, characterized in that, The new charging current I2' in the middle SOC range after adjustment by formula (8) can be obtained by calculating the following formulas (6) and (7); C 总 =C 1总 +C 2总 +C 3总 = + + = + + (6) t 总 =t 1总 +t 2总 +t 3总 =t 1总 ’+t 2总 ’+t 3总 ’(7) I2’=(C 总 -∑I 1i ’×t 1i ’-∑I 3i ’×t 3i ’) / (t 总 -t 1总 ’-t 3总 ’)(8) Among them, C 总 C represents the total charging capacity of the battery system. 1总 C 2总 C 3总 These represent the total charging capacity for the low SOC range, medium SOC range, and high SOC range, respectively. I 1i I 2i I 3i These are the charging currents of unit i in the original low SOC range, medium SOC range, and high SOC range, respectively, where i = 1, 2, ..., n, and n is a natural number. t 1i t 2i t 3i These are the charging times for unit i in the original low SOC, medium SOC, and high SOC ranges, respectively, where i = 1, 2, ..., n; n is a natural number. t 1总 t 2总 t 3总 These are the total charging times for the original low SOC range, medium SOC range, and high SOC range, respectively. I 1i '、I 2i '、I 3i 'These are the new charging currents for the i-cells in the adjusted low SOC, medium SOC, and high SOC ranges, respectively; t 1i '、t 2i '、t 3i 'These represent the charging times for unit i in the adjusted low SOC, medium SOC, and high SOC ranges, respectively, where i = 1, 2, ..., n; n is a natural number. t 1总 '、t 2总 '、t 3总 These represent the total charging time for the original low SOC range, medium SOC range, and high SOC range, respectively.

5. A charging and discharging method, characterized in that, The lithium-ion battery pack is charged and discharged using the charge and discharge control method according to any one of claims 1-4.

6. A battery management system, characterized in that, The battery management system described above is used to implement the charging and discharging control method of the lithium-ion battery pack according to any one of claims 1-4.

Images