A lithium ion battery pack thermal state control method based on BMS

Abstract

The application provides a lithium ion battery pack thermal state control method based on a BMS, comprising the following steps: calculating the thickness growth rate of each battery cell according to the smoothed thickness expansion sequence, identifying the permanent deformation area of the gasket by combining the local high-temperature area proportion obtained based on the temperature spatial distribution map, and obtaining a potential air gap formation position list; calculating the rebound force difference value of adjacent sampling time according to the compression rebound force data corresponding to each position in the potential air gap formation position list, distinguishing between abnormal attenuation and normal attenuation, and obtaining the rebound force attenuation mark of each position; and generating a cooling power distribution scheme for each battery cell according to the degraded battery cell position set and the rebound force attenuation mark of each position.

Description

Technical Field

[0001] This invention relates to the field of information technology, and in particular to a method for controlling the thermal state of lithium-ion battery packs based on a battery management system (BMS). Background Technology

[0002] In the rapid development of new energy vehicles and energy storage systems, thermal state control of lithium-ion battery packs based on BMS (Battery Management System) is crucial. Battery pack temperature management directly affects system safety, performance, and lifespan, especially under high-load operation or extreme environmental conditions, where the risk of thermal runaway increases significantly. Therefore, ensuring a stable temperature distribution throughout the battery pack's lifespan has become a pressing challenge for the industry. Currently, although many thermal management solutions dissipate heat through cooling plates and thermally conductive materials, in practical applications, the battery cells gradually expand with each charge and discharge cycle. Existing thermal management strategies generally ignore the degradation in heat conduction efficiency caused by this physical deformation, leading to a significant decrease in the accuracy of temperature control in later stages of operation. Initially, cell expansion may compress the thermally conductive material, seemingly aiding heat transfer. However, over time, under continuous pressure, the thermally conductive material undergoes permanent deformation and may even lose its original elastic resilience, failing to maintain a tight fit to the cell surface. This loss of elasticity leads to tiny air gaps between the cell and the thermally conductive material. Air has extremely poor thermal conductivity, preventing effective heat transfer and ultimately causing the contact thermal resistance to increase rather than decrease. Specifically, in a battery pack, if a cell's thickness increases due to long-term cycling, the pressure distribution between it and the thermally conductive material becomes uneven. Some areas may have formed air gaps, while others remain in contact. This uneven contact state causes uneven temperature distribution on the cell surface, increasing the risk of localized overheating. Worse still, because the loss of elasticity in the thermally conductive material and the expansion of air gaps accumulate gradually with charge-discharge cycles, these changes are often gradual and difficult to detect in daily operation until the risk of thermal runaway becomes apparent. Therefore, dynamically identifying and addressing the contradiction between cell expansion and changes in the contact thermal resistance of the thermally conductive material throughout the battery pack's entire lifespan has become a key issue in the field of thermal state control. Summary of the Invention

[0003] This invention provides a method for controlling the thermal state of a lithium-ion battery pack based on a battery management system (BMS), mainly comprising: Collect the current thickness expansion data of each cell, the temperature values ​​of each temperature measuring node at the interface between the gasket and the cell, and the compression and rebound force data at each measuring point. Interpolate and stitch the temperature values ​​of each temperature measuring node according to spatial coordinates to generate a temperature gradient distribution image. Then, perform noise filtering and fusion processing on the thickness expansion data and the temperature gradient distribution image to obtain a smoothed thickness expansion sequence and temperature spatial distribution map. The thickness growth rate of each cell is calculated based on the smoothed thickness expansion sequence. The proportion of local high-temperature areas obtained based on the temperature spatial distribution map is used to identify the areas where permanent deformation of the gasket occurs, and a list of potential air gap formation locations is obtained. Calculate the difference in rebound force between adjacent sampling times based on the compression rebound force data corresponding to each location in the potential air gap formation location list, distinguish between abnormal attenuation and normal attenuation, and obtain the rebound force attenuation mark for each location. After matching the rebound force attenuation markers at each location with the potential air gap formation location list according to spatial coordinates, a step degradation assessment is performed to obtain the contact thermal resistance degradation value and determine the set of degraded cell locations. A cooling power allocation scheme for each battery cell is generated based on the set of locations of degraded cells and the rebound force attenuation markers at each location.

[0004] Furthermore, the process involves collecting current thickness expansion data for each battery cell, temperature values ​​at each temperature measurement node at the interface between the gasket and the battery cell, and compression and rebound force data at each measurement point. The temperature values ​​at each temperature measurement node are then interpolated and stitched together according to spatial coordinates to generate a temperature gradient distribution image. Noise filtering and fusion processing are then applied to the thickness expansion data and the temperature gradient distribution image to obtain a smoothed thickness expansion sequence and a temperature spatial distribution map, including: Obtain the current displacement value of each cell, and determine the thickness expansion data based on the difference between the current displacement value and the initial thickness reference value; Temperature values ​​of each cell interface are obtained by temperature measurement nodes arranged in a preset grid, and an interpolation method is used to generate a temperature gradient distribution image covering the entire interface based on the temperature values ​​and spatial coordinates. The compression and rebound force data at each temperature measurement node is obtained through a pressure sensor. The thickness expansion data is smoothed using a sliding window mean filtering method to obtain the smoothed thickness expansion sequence. The temperature gradient distribution image is filtered to suppress high-frequency noise, resulting in the temperature spatial distribution map.

[0005] Furthermore, the process involves calculating the thickness growth rate of each cell based on the smoothed thickness expansion sequence, identifying the permanent deformation areas of the gasket by combining the proportion of local high-temperature regions obtained from the temperature spatial distribution map, and obtaining a list of potential air gap formation locations, including: Based on the smoothed thickness expansion sequence, the thickness expansion data of adjacent sampling times are extracted, the thickness expansion difference is calculated, and the thickness growth rate of each cell is determined in combination with the sampling time interval. For battery cells whose thickness growth rate exceeds a preset growth rate threshold, the temperature distribution data of the corresponding region is obtained according to the temperature spatial distribution map, and the temperature value of each pixel in the region is traversed and high-temperature pixels are marked. The proportion of local high-temperature regions is determined based on the ratio of the number of high-temperature pixels to the total number of pixels in the region. If the proportion of local high-temperature regions exceeds a preset proportion threshold, it is determined that the region has undergone permanent deformation. The spatial coordinates are recorded and summarized to obtain the list of potential air gap formation locations.

[0006] Furthermore, the step of calculating the rebound force difference between adjacent sampling times based on the compression rebound force data corresponding to each location in the potential air gap formation location list, distinguishing between abnormal and normal attenuation, and obtaining the rebound force attenuation marker for each location includes: Extract the compression rebound force data corresponding to each location from the potential air gap formation location list, obtain the rebound force value at adjacent sampling times, and calculate the rebound force difference; The rebound force difference is compared with a preset normal attenuation threshold. If the rebound force difference does not exceed the preset normal attenuation threshold, the position is determined to be under normal attenuation and marked as normal attenuation. If the difference in rebound force exceeds the preset normal attenuation threshold, then the position is determined to be an abnormal attenuation and marked as an abnormal attenuation marker; By associating the spatial coordinates of each location with the corresponding attenuation marker, the rebound force attenuation marker is obtained.

[0007] Furthermore, after matching the rebound force attenuation markers at each location with the potential air gap formation location list according to spatial coordinates, a step degradation assessment is performed to obtain the contact thermal resistance degradation value, and the set of degraded cell locations is determined, including: The spatial coordinates of each position are extracted based on the rebound force attenuation mark, and compared with the coordinates in the potential air gap formation position list to determine the matching position attenuation mark set. For the matched set of position attenuation markers, abnormal attenuation marker positions are filtered out, the number of abnormal attenuation positions is counted and the ratio to the total number of positions is calculated to obtain the air gap area ratio of each cell. The degree of contact thermal resistance degradation is determined based on the air gap area ratio and the preset degradation level threshold. If the contact thermal resistance degradation value exceeds the preset degradation judgment threshold, the corresponding cell location identifier is added to the degraded cell location set.

[0008] Furthermore, the step of generating a cooling power allocation scheme for each battery cell based on the set of degraded cell locations and the rebound force attenuation markers at each location includes: Based on the set of locations of the degraded cells, the location identifier and degree of deterioration of each degraded cell are extracted, and the reference cooling power of each cell is obtained. The total additional cooling power is determined based on the number and degree of degradation of the battery cells; For each cell in the set of degraded cell locations, the number of abnormal attenuation marker locations is counted based on the resilience attenuation marker, and the power increment weight is calculated in combination with the degree of deterioration. The additional cooling power is allocated according to the weighting ratio of the power increment to obtain the cooling power increment of each degraded cell. The target cooling power value is determined by combining the benchmark cooling power and summarizing to obtain the cooling power allocation scheme for each cell.

[0009] Furthermore, the method also includes: adjusting the overall cooling power of the battery pack according to the cooling power distribution scheme of each cell, re-collecting the temperature distribution data of each cell after adjustment, and using a temperature uniformity evaluation method to compare the deviation between the adjusted temperature distribution data and the temperature spatial distribution map to obtain the thermal state stability control result throughout the entire life cycle.

[0010] Furthermore, the process of adjusting the overall cooling power of the battery pack according to the cooling power distribution scheme of each cell, re-collecting the temperature distribution data of each cell after adjustment, and comparing the deviation between the adjusted temperature distribution data and the temperature spatial distribution map using a temperature uniformity evaluation method to obtain the thermal state stability control results throughout the entire life cycle includes: According to the cooling power allocation scheme of each cell, a power adjustment command is sent to the cooling device to adjust the flow rate of the cooling medium in each cooling channel and complete the adjustment of the overall cooling power of the battery pack. By re-collecting the temperature values ​​of each cell interface and interpolating and stitching them together, the adjusted temperature distribution data is obtained. The adjusted temperature distribution data is subtracted point by point from the temperature spatial distribution map to obtain the temperature deviation value at each location; The temperature distribution uniformity index is determined based on the dispersion of the temperature deviation value. If the temperature distribution uniformity index is lower than the preset uniformity threshold, the thermal state stability control is deemed to have met the standard, and the thermal state stability control result is obtained.

[0011] The technical solutions provided by the embodiments of the present invention may include the following beneficial effects: This invention discloses a BMS-based thermal state control method for lithium-ion battery packs, proposing a complete solution to address the risks of contact thermal resistance degradation and thermal runaway caused by thickness expansion, uneven temperature gradient distribution, and air gap formation during the use of lithium-ion battery packs. This invention collects cell thickness expansion data, temperature distribution, and compression-rebound force data to generate a smoothed thickness expansion sequence and temperature spatial distribution map. It calculates the thickness growth rate and the proportion of local high-temperature regions, identifies potential air gap formation locations, and assesses the air gap area ratio and contact thermal resistance degradation degree using rebound force attenuation markers, accurately locating the degraded cell. Based on this, the invention generates a targeted cooling power allocation scheme, adjusts the overall cooling power of the battery pack, and compares the temperature distribution deviation before and after adjustment using a temperature uniformity evaluation method to ensure stable thermal state control throughout the entire lifespan. The core innovation of this invention lies in achieving accurate prediction and control of the battery pack's thermal state through multi-dimensional data fusion and dynamic adjustment, effectively reducing the risk of thermal runaway and improving battery safety and lifespan. Attached Figure Description

[0012] Figure 1 This is a flowchart of a lithium-ion battery pack thermal state control method based on BMS according to the present invention.

[0013] Figure 2 This is a schematic diagram of a lithium-ion battery pack thermal state control method based on BMS according to the present invention.

[0014] Figure 3 This is another schematic diagram of a lithium-ion battery pack thermal state control method based on BMS according to the present invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0016] like Figures 1-3 This embodiment of a lithium-ion battery pack thermal state control method based on BMS may specifically include: Step S101: Collect the current thickness expansion data of each cell, the temperature values ​​of each temperature measuring node at the interface between the gasket and the cell, and the compression and rebound force data at each measuring point. Interpolate and stitch the temperature values ​​of each temperature measuring node according to spatial coordinates to generate a temperature gradient distribution image. Perform noise filtering and fusion processing on the thickness expansion data and temperature gradient distribution image to obtain a smoothed thickness expansion sequence and temperature spatial distribution map.

[0017] The current displacement value output by the displacement sensors arranged on the surface of each battery cell is obtained. The initial thickness reference value of each battery cell is subtracted from the current displacement value to obtain the thickness expansion data of each battery cell. Simultaneously, the temperature values ​​output by each temperature measuring node arranged in a preset grid on the interface between the gasket and the battery cell, as well as the compression and rebound force data output by the pressure sensor at the corresponding position of each temperature measuring node, are collected. Based on the spatial coordinates of each temperature measuring node, a bilinear interpolation method is used to interpolate the temperature values ​​in the region between adjacent temperature measuring nodes. The interpolated temperature values ​​are then stitched together according to two-dimensional plane coordinates to generate a temperature gradient distribution image covering the entire battery cell interface. The value of each pixel in the temperature gradient distribution image corresponds to the temperature value at that position. For the thickness expansion data, a sliding window mean filtering method is used to smooth the thickness expansion data at consecutive sampling times, eliminating random noise interference to obtain a smoothed thickness expansion sequence. For the temperature gradient distribution image, a Gaussian filtering method is used to suppress high-frequency noise in the image, resulting in a temperature spatial distribution map.

[0018] In one embodiment, for monitoring the thermal state of a lithium-ion battery pack, displacement sensors are arranged at preset positions on the surface of each cell, temperature measuring nodes are arranged in a grid pattern on the interface between the pad and the cell, and pressure sensors are arranged at the corresponding positions of each temperature measuring node, forming a multi-source sensor array covering the cell interface.

[0019] Specifically, the displacement sensor uses contact or non-contact measurement methods to monitor the displacement change of the cell surface relative to its initial installation state in real time. After the battery pack is assembled, the thickness of each cell in its uncharged state is recorded as an initial thickness reference value, which is stored in the battery management unit. During battery pack operation, the displacement sensor continuously outputs the current displacement value. The difference between the current displacement value and the corresponding cell's initial thickness reference value is calculated. A positive difference indicates that the cell has expanded, and the absolute value of the difference is the thickness expansion data of that cell.

[0020] For example, if the initial thickness of a battery cell is 10 mm and the current displacement sensor output is 10.3 mm, then the thickness expansion data of the battery cell is 0.3 mm.

[0021] In one embodiment, the temperature measuring nodes are thermocouples or thermistors, and are evenly distributed on the bonding interface between the pad and the battery cell according to a preset row and column spacing. Each temperature measuring node synchronously outputs the temperature value at that location.

[0022] It should be noted that the bilinear interpolation method is used to spatially complete the temperature values ​​between discrete temperature measurement nodes. This method selects four adjacent temperature measurement nodes around the location to be interpolated. Based on the horizontal and vertical distances between the location to be interpolated and the four nodes, two linear interpolations are performed in the horizontal direction to obtain two intermediate temperature values. Then, a single linear interpolation is performed in the vertical direction on these two intermediate temperature values ​​to obtain the estimated temperature value for the location to be interpolated. By sequentially performing the above interpolation operation on all grid gaps within the bonding interface, the estimated temperature values ​​at each location are arranged and stitched together according to two-dimensional planar coordinates to form a continuous temperature gradient distribution image. The value of each pixel in the temperature gradient distribution image corresponds to the temperature value at that spatial location.

[0023] In one possible implementation, to address the sampling noise in the thickness expansion data, a sliding window mean filtering method is used for smoothing. This involves selecting thickness expansion data from several consecutive sampling times, calculating the arithmetic mean, and replacing the original data at the center of the window with this average. The window slides along the time axis until all sampling points are traversed, resulting in a smoothed thickness expansion sequence. To suppress high-frequency noise in the temperature gradient distribution image, a Gaussian filtering method is used. A Gaussian kernel function is used to perform a weighted average of each pixel and its neighboring pixels in the image, yielding a temperature spatial distribution map.

[0024] Step S102: Calculate the thickness growth rate of each cell based on the smoothed thickness expansion sequence, identify the permanent deformation area of ​​the gasket by combining the proportion of local high temperature area obtained based on the temperature spatial distribution map, and obtain a list of potential air gap formation locations.

[0025] Based on the smoothed thickness expansion sequence, the thickness expansion data of each cell at adjacent sampling times are extracted, and the difference Δd = d between the thickness expansion data at the current time and the thickness expansion data at the previous time is calculated. t -d t-1 (d) t For the current thickness, d t-1 (The thickness is the previous thickness). Divide the difference by the sampling time interval Δt to obtain the thickness growth rate r = Δd / Δt (Δt is in hours). For cells whose thickness growth rate exceeds the preset growth rate threshold of 0.1 mm / h, the temperature distribution data of the corresponding area of ​​the cell is directly referenced from the temperature spatial distribution map collected in S101. The temperature values ​​of each pixel in the area are traversed. If the temperature value of a pixel exceeds the preset temperature threshold of 60°C, the pixel is marked as a high-temperature pixel. The ratio of the number of high-temperature pixels to the total number of pixels in the area is calculated to obtain the proportion of local high-temperature areas. If the proportion of local high-temperature areas exceeds the preset proportion threshold of 50%, it is determined that the gasket in the area has undergone permanent deformation. The spatial coordinates of the permanently deformed area are recorded and summarized to obtain a list of potential air gap formation locations.

[0026] In one implementation, to address the dynamic changes in cell thickness during the operation of a lithium-ion battery pack, thickness expansion data at adjacent sampling times are extracted based on a smoothed thickness expansion sequence. The thickness growth rate of each cell is then obtained through difference calculation and time interval normalization.

[0027] Specifically, the thickness growth rate reflects the expansion rate of the battery cell per unit time, a value closely related to the intensity of the electrochemical reaction within the cell and the number of charge-discharge cycles. During the long-term operation of the battery pack, the cell's electrode volume changes due to lithium-ion insertion and extraction, and this change gradually accumulates, resulting in thickness expansion. When the thickness growth rate of a battery cell exceeds a preset growth rate threshold, it indicates that the cell's expansion rate is abnormally accelerated, and the mechanical pressure distribution between it and adjacent pads has changed.

[0028] For example, if the thickness growth rate of a certain battery cell is consistently high during a continuous sampling period, the compressive force on the gasket in the corresponding area of ​​the battery cell will increase significantly, and the gasket material is prone to plastic deformation and loss of elastic recovery ability under continuous high pressure.

[0029] It should be noted that for battery cells whose thickness growth rate exceeds the preset growth rate threshold, the temperature distribution data of the corresponding area of ​​the battery cell is extracted from the temperature spatial distribution map, and the temperature value of each pixel in the area is compared with the preset temperature threshold one by one.

[0030] In one embodiment, the statistical process for determining the proportion of locally high-temperature areas is achieved by dividing the number of high-temperature pixels by the total number of pixels in that area. When the gasket undergoes permanent deformation, a tiny air gap forms between it and the cell surface. Since the thermal conductivity of air is much lower than that of the gasket material, heat in this area cannot be transferred to the cooling plate in time, and the continuous accumulation of heat causes the temperature value of the corresponding pixel to rise. Therefore, when both conditions are met simultaneously—that the proportion of locally high-temperature areas exceeds a preset proportion threshold and that the thickness growth rate exceeds a preset growth rate threshold—it is determined that the gasket in that area has undergone permanent deformation. This determination method combines the mechanical expansion characteristics of the cell with the phenomenon of thermal conductivity degradation.

[0031] It is understandable that the spatial coordinates of the permanent deformation area are recorded and summarized according to the cell number and location information to form a list of potential air gap formation locations. The list contains the two-dimensional plane coordinates of each potential air gap location and the corresponding cell identification information.

[0032] Step S103: Calculate the rebound force difference between adjacent sampling times based on the compression rebound force data corresponding to each position in the potential air gap formation location list, distinguish between abnormal attenuation and normal attenuation, and obtain the rebound force attenuation mark for each position.

[0033] Based on the list of potential air gap formation locations, the compression and rebound force data corresponding to each location are extracted. For each location, the rebound force value at the current sampling time and the rebound force value at the previous sampling time are obtained. The rebound force difference at each location is obtained by subtracting the rebound force value at the current sampling time from the rebound force value at the previous sampling time. A preset normal attenuation threshold is set for the rebound force difference. If the rebound force difference at a location does not exceed the preset normal attenuation threshold, the attenuation type at that location is determined to be normal attenuation. If the rebound force difference at a location exceeds the preset normal attenuation threshold, the attenuation type at that location is determined to be abnormal attenuation. Each location is marked according to the attenuation type. Locations determined to have normal attenuation are marked as normal attenuation markers, and locations determined to have abnormal attenuation are marked as abnormal attenuation markers. The spatial coordinates of each location are associated with the corresponding attenuation markers to obtain the rebound force attenuation marker for each location.

[0034] In one implementation, for each location recorded in the potential air gap formation location list, a time-series rebound force value sequence corresponding to the location is extracted from the compression rebound force data. This sequence contains rebound force measurements at multiple consecutive sampling times.

[0035] Specifically, the rebound force difference is calculated by subtracting the rebound force value at the current sampling time from the rebound force value at the previous sampling time. Under the expansion pressure of the battery cell, the gasket material undergoes compressive deformation. When the pressure is released, the gasket should return to its original thickness and generate rebound force. If the gasket material performance is normal, the rebound force value between adjacent sampling times will show a small and stable change. If the gasket material undergoes permanent deformation or aging, its elastic modulus decreases, and the rebound force value will show a significant downward trend, manifested as a significant increase in the rebound force difference. By calculating the rebound force difference between adjacent times, the dynamic changes in the elastic properties of the gasket material can be captured.

[0036] It should be noted that the preset normal degradation threshold is set based on the physical properties of the gasket material and the operating conditions of the battery pack. This threshold reflects the natural degradation of the gasket's resilience during normal aging.

[0037] In one embodiment, when the rebound force difference at a certain location does not exceed a preset normal attenuation threshold, it indicates that the gasket at that location is still within the normal elastic decay range, its adhesion to the cell surface is good, and the heat conduction path is not significantly affected. In this case, the attenuation type at that location is determined to be normal attenuation. When the rebound force difference at a certain location exceeds the preset normal attenuation threshold, it indicates that the elastic performance of the gasket at that location has decreased abnormally rapidly. The gasket may have undergone plastic deformation or material structure damage, and it can no longer provide sufficient contact pressure to maintain tight adhesion to the cell surface. In this case, the attenuation type at that location is determined to be abnormal attenuation.

[0038] Understandably, each location is marked according to the determined attenuation type. Locations with normal attenuation are marked with normal attenuation, and locations with abnormal attenuation are marked with abnormal attenuation. The spatial coordinate information of each location is associated with the corresponding attenuation mark and stored to form a complete record of rebound force attenuation markings for each location.

[0039] Step S104: Match the rebound force attenuation markers at each location with the potential air gap formation location list according to spatial coordinates. Calculate the air gap area ratio based on the ratio of the area marked as abnormal attenuation to the total contact area of ​​the cell in the matching results. Perform a step degradation assessment to obtain the contact thermal resistance degradation value and determine the set of degraded cell locations.

[0040] Based on the rebound force attenuation markers at each location, the spatial coordinate information corresponding to each location is extracted. These spatial coordinates are then compared one by one with the coordinates recorded in the potential air gap formation location list. If the spatial coordinates match, the location is considered a successful match, resulting in a set of matched location attenuation markers. For the matched location attenuation marker set, locations marked as abnormal attenuation markers are selected, and the number of abnormal attenuation locations is counted. The total number of locations corresponding to each cell in the potential air gap formation location list is obtained, and the ratio of the number of abnormal attenuation locations to the total number of locations is calculated to obtain the air gap area ratio of each cell. Based on the air gap area ratio, a degradation level is determined. Multiple preset degradation level thresholds are set to divide the air gap area ratio into several degradation intervals. If the air gap area ratio of a cell falls into a certain degradation interval, the contact thermal resistance degradation value corresponding to that cell is determined. The contact thermal resistance degradation value increases sequentially from low to high according to the degradation interval. For the contact thermal resistance degradation value, a preset degradation judgment threshold is set. If the contact thermal resistance degradation value of a certain cell exceeds the preset degradation judgment threshold, the location identifier of the cell is added to the degraded cell location set. After traversing all cells to complete the judgment, the degraded cell location set is determined.

[0041] In one implementation, the degradation level is determined based on the air gap area ratio. First, a threshold is calculated based on the safety boundary SB of the battery pack thermal management and the performance characteristics PC of the gasket material. The threshold calculation formula is T1=0.1*SB / PC, T2=0.3*SB / PC, where SB is the upper limit of thermal resistance and PC is the elastic modulus. The air gap area ratio is divided into degradation intervals. For example, a ratio less than T1 indicates low degradation, T1 to T2 indicates medium degradation, and a ratio greater than T2 indicates high degradation. If the air gap area ratio of a certain cell falls into the high degradation interval, the cell's location identifier is added to the set of degraded cell locations. After traversing all cells to complete the determination, the set of degraded cell locations is finalized. In another implementation, for the rebound force attenuation marker at each location, the spatial coordinate information corresponding to each location is extracted. The spatial coordinate information includes the horizontal and vertical coordinates of the location on the two-dimensional plane of the cell interface. The spatial coordinates are compared one by one with the coordinates recorded in the potential air gap formation location list.

[0042] Specifically, the matching process employs a coordinate consistency determination method. If the spatial coordinates corresponding to a certain rebound force attenuation mark are completely consistent with the spatial coordinates of a record in the potential air gap formation location list, then the location is considered to be successfully matched. By performing the above matching operation on all rebound force attenuation marks one by one, a set of matched location attenuation marks is obtained, which contains the spatial coordinates of each location and its corresponding attenuation mark type.

[0043] It should be noted that the calculation of the air gap area ratio is based on a statistical method of discrete locations. For the matched set of location attenuation markers, each location record in the set is traversed, and locations marked as abnormal attenuation markers are selected and their numbers are counted. Since each location in the potential air gap formation location list is evenly distributed on the cell interface according to a preset grid, and each location represents a grid area of ​​the same size, the ratio of the number of abnormal attenuation locations to the total number of locations reflects the proportion of the air gap area to the total contact area of ​​the cell.

[0044] For example, if the total number of positions corresponding to a certain battery cell is one hundred, and the number of positions marked as abnormal attenuation is fifteen, then the air gap area of ​​the battery cell accounts for fifteen percent.

[0045] In one embodiment, the degradation level thresholds are determined based on the safety boundaries of battery pack thermal management and the performance characteristics of the gasket materials. Multiple degradation level thresholds divide the range of air gap area ratio into several continuous and non-overlapping degradation intervals. Furthermore, a one-to-one mapping relationship is established between the degradation intervals and the contact thermal resistance degradation values. When the air gap area ratio of a cell falls into a certain degradation interval, the corresponding contact thermal resistance degradation value is the level value mapped to that degradation interval. The contact thermal resistance degradation value increases sequentially from low to high according to the degradation interval, reflecting the physical law that a larger air gap area ratio results in more severe contact thermal resistance degradation.

[0046] For example, if the degradation range is divided into four levels, the cell with the lowest air gap area ratio corresponds to a level one level of contact thermal resistance degradation, while the cell with the highest air gap area ratio corresponds to a level four level of contact thermal resistance degradation. This step-like classification method converts the continuously changing air gap area ratio into discrete degradation level levels, which facilitates the implementation of differentiated thermal management measures for different degradation levels.

[0047] Understandably, the preset degradation judgment threshold is used to distinguish between cells in normal operating condition and cells in deteriorated condition. The setting of this threshold takes into account both the overall thermal safety margin of the battery pack and the tolerance for thermal runaway risk of individual cells.

[0048] In one possible implementation, the contact thermal resistance degradation value of each cell is sequentially compared with a preset degradation judgment threshold. If the contact thermal resistance degradation value of a cell exceeds the preset degradation judgment threshold, the cell's location identifier is extracted and added to the degraded cell location set. By performing the above judgment operation on all cells in the battery pack, the degradation state of all cells is identified, and the degraded cell location set is finally determined.

[0049] Preferably, the set of degraded cell locations is stored in the form of a list. Each record in the list includes the location identifier of the degraded cell, the corresponding air gap area ratio, and the degree of contact thermal resistance degradation, so that the battery management unit can formulate a targeted cooling power adjustment scheme based on the distribution location and degree of degradation of the degraded cells.

[0050] For example, through the above-mentioned degradation assessment process, the location of the battery cell with abnormally increased contact thermal resistance due to permanent deformation of the gasket can be dynamically identified during the entire life cycle of the battery pack, providing a positioning basis for subsequent precise control of thermal status.

[0051] Step S105: Generate a cooling power allocation scheme for each battery cell based on the set of degraded cell locations and the rebound force attenuation markers at each location.

[0052] Based on the set of degraded cell locations, the location identifier and corresponding contact thermal resistance degradation value of each degraded cell in the set are extracted. The baseline cooling power of each cell under normal conditions is obtained, and this baseline cooling power is used as the cooling power allocation value for non-degraded cells. A preset total additional cooling power is determined based on the number and degradation degree of the degraded cells. For each cell in the set of degraded cell locations, the number of locations marked with abnormal attenuation markers within each degraded cell is counted based on the rebound force attenuation markers at each location. The number of abnormal attenuation marker locations is multiplied by the contact thermal resistance degradation value of the cell to obtain the power increment weight of that cell. The proportion of the power increment weight of each degraded cell to the sum of the power increment weights of all degraded cells is calculated. The preset total additional cooling power is allocated according to this proportion to obtain the cooling power increment of each degraded cell. In one implementation, the number of degraded cells *n* and the average degree of deterioration *a* are first calculated, where *a* is the arithmetic mean of all deterioration degrees. Then, the total additional cooling power *P* is calculated as: base cooling power *b* × *n* × *a*. For example, when *b* = 10 watts, *n* = 5, and *a* = 0.3, *P* = 15 watts. For each cell in the set of degraded cell locations, the number of locations marked with abnormal attenuation markers within each degraded cell is counted based on the rebound force attenuation markers at each location. The number of abnormal attenuation marker locations is multiplied by the contact thermal resistance deterioration degree of the cell and then multiplied by an adjustment coefficient *k* to obtain the power increment weight of the cell. Here, *k* = 1.2 is used to consider the nonlinear thermal resistance effect under high deterioration scenarios to improve the rationality of allocation. The proportion of the power increment weight of each degraded cell to the sum of the power increment weights of all degraded cells is calculated. Based on this proportion, the preset total additional cooling power is allocated to obtain the cooling power increment of each degraded cell. The target cooling power value for each degraded cell is obtained by adding the baseline cooling power to the cooling power increment of each degraded cell. The cooling power allocation values ​​for non-degraded cells and the target cooling power values ​​for each degraded cell are then summarized and organized according to the cell location identifier to obtain the cooling power allocation scheme for each cell.

[0053] In one embodiment, for a set of degraded cell locations, the location identifier and corresponding contact thermal resistance degradation value of each degraded cell in the set are extracted. At the same time, the reference cooling power of each cell under normal operating conditions is obtained. The reference cooling power is predetermined based on the rated capacity of the cell and the heat generation power under standard operating conditions.

[0054] Specifically, for cells not included in the set of degraded cell locations, the cooling power allocation value is directly adopted as the baseline cooling power. For degraded cells, a preset total additional cooling power is determined based on the number of degraded cells and the sum of the degraded degree values ​​of each degraded cell. The total additional cooling power represents the supplementary cooling capacity that the cooling device can provide on top of the baseline cooling power.

[0055] It should be noted that the calculation of the power increment weight comprehensively considers two factors: the distribution range of the air gap within the degraded cell and the severity of contact thermal resistance degradation. For each cell in the set of degraded cell locations, the number of locations marked with abnormal attenuation is counted based on the rebound force attenuation markers at each location. This number reflects the breadth of the air gap distribution on the cell interface. The power increment weight of the cell is obtained by multiplying the number of abnormal attenuation marker locations by the contact thermal resistance degradation value of the cell. This multiplication operation gives a larger power increment weight to cells with a wide air gap distribution range and a high degree of degradation.

[0056] For example, if the number of abnormal attenuation markers in a deteriorated battery cell is twenty and the degree of contact thermal resistance deterioration is level three, then the power increment weight of the battery cell is sixty.

[0057] In one embodiment, the proportion of the power increment weight of each degraded cell to the sum of the power increment weights of all degraded cells is calculated, and a preset total amount of additional cooling power is allocated according to the proportion to obtain the cooling power increment of each degraded cell.

[0058] It is understood that the target cooling power value of each degraded cell is obtained by adding the base cooling power to the cooling power increment of each degraded cell. The cooling power allocation value of the non-degraded cells and the target cooling power value of each degraded cell are summarized and organized according to the cell location identifier to form a cooling power allocation scheme covering all cells of the battery pack. The scheme records the cooling power value corresponding to each cell.

[0059] Step S106: Adjust the overall cooling power of the battery pack according to the cooling power distribution scheme of each cell, re-collect the temperature distribution data of each cell after adjustment, and use the temperature uniformity evaluation method to compare the deviation between the adjusted temperature distribution data and the temperature spatial distribution map to obtain the thermal state stability control result throughout the entire life cycle.

[0060] According to the cooling power allocation scheme for each cell, a power adjustment command is sent to the battery pack cooling device. This command includes the target power value for the corresponding cooling channel of each cell. The cooling device adjusts the cooling medium flow rate of each cooling channel according to the target power value, thus adjusting the overall cooling power of the battery pack. After a preset stabilization time interval following the cooling power adjustment, the temperature values ​​output by each temperature measurement node on each cell interface are re-acquired. The temperature values ​​are interpolated and stitched according to spatial coordinates to obtain adjusted temperature distribution data. The adjusted temperature distribution data is then subtracted point-by-point from the temperature spatial distribution map before adjustment, according to the corresponding pixel positions, to obtain the temperature deviation value at each location. Temperature uniformity is evaluated based on the temperature deviation values ​​at each location. The dispersion of the temperature deviation values ​​at all locations is statistically analyzed to obtain a temperature distribution uniformity index. If the temperature distribution uniformity index is lower than a preset uniformity threshold, the thermal state stability control is deemed satisfactory; if the temperature distribution uniformity index exceeds the preset uniformity threshold, the thermal state stability control is deemed unsatisfactory, thus obtaining the thermal state stability control result over the entire life cycle.

[0061] In one implementation, a power adjustment command is sent to the battery pack cooling device according to the cooling power allocation scheme of each cell. The power adjustment command includes the target power value of the cooling channel corresponding to each cell. The control unit of the cooling device receives and parses the power adjustment command.

[0062] Specifically, the cooling device adjusts the flow rate of the cooling medium or the cooling power output of each cooling channel according to the target power value. For liquid cooling devices, the flow rate of the cooling fluid is changed by adjusting the speed of the cooling fluid circulation pump. For air cooling devices, the air flow rate is changed by adjusting the fan speed, thereby adjusting the overall cooling power of the battery pack.

[0063] It should be noted that after the cooling power adjustment is completed, the battery pack temperature field reaches a new thermal equilibrium state after a preset stabilization time interval. Then, the temperature values ​​output by each temperature measurement node on each cell interface are re-acquired. These temperature values ​​are then subjected to bilinear interpolation and stitching processing using the same spatial coordinates as described above to obtain the adjusted temperature distribution data. The adjusted temperature distribution data is then subtracted point-by-point from the temperature spatial distribution map before adjustment, according to the corresponding pixel positions, to obtain the temperature deviation value at each location. A positive temperature deviation value indicates that the temperature at that location has increased, while a negative value indicates that the temperature at that location has decreased; the absolute value reflects the magnitude of the temperature change.

[0064] In one embodiment, temperature uniformity assessment uses the standard deviation index in statistics to measure the dispersion of temperature deviation values ​​at each location. The arithmetic mean of the temperature deviation values ​​at all locations is calculated, then the square of the difference between each location's temperature deviation value and the mean is calculated. The sum of these squared values ​​across all locations is then divided by the total number of locations. The square root of the quotient is taken to obtain the temperature distribution uniformity index. A smaller temperature distribution uniformity index indicates that the temperature variation at each location tends to be more uniform, and the temperature distribution is more even.

[0065] It is understood that the temperature distribution uniformity index is compared with a preset uniformity threshold. If the temperature distribution uniformity index is lower than the preset uniformity threshold, the thermal state stability control is determined to be up to standard. If the temperature distribution uniformity index exceeds the preset uniformity threshold, the thermal state stability control is determined to be down to standard. Thus, the thermal state stability control result over the entire life cycle is obtained.

[0066] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A method for controlling the thermal state of a lithium-ion battery pack based on a battery management system (BMS), characterized in that, The method includes: Collect the current thickness expansion data of each cell, the temperature values ​​of each temperature measuring node at the interface between the gasket and the cell, and the compression and rebound force data at each measuring point. Interpolate and stitch the temperature values ​​of each temperature measuring node according to spatial coordinates to generate a temperature gradient distribution image. Then, perform noise filtering and fusion processing on the thickness expansion data and the temperature gradient distribution image to obtain a smoothed thickness expansion sequence and temperature spatial distribution map. The thickness growth rate of each cell is calculated based on the smoothed thickness expansion sequence. The proportion of local high-temperature areas obtained based on the temperature spatial distribution map is used to identify the areas where permanent deformation of the gasket occurs, and a list of potential air gap formation locations is obtained. Calculate the difference in rebound force between adjacent sampling times based on the compression rebound force data corresponding to each location in the potential air gap formation location list, distinguish between abnormal attenuation and normal attenuation, and obtain the rebound force attenuation mark for each location. After matching the rebound force attenuation markers at each location with the potential air gap formation location list according to spatial coordinates, a step degradation assessment is performed to obtain the contact thermal resistance degradation value and determine the set of degraded cell locations. A cooling power allocation scheme for each battery cell is generated based on the set of locations of degraded cells and the rebound force attenuation markers at each location.

2. The method for controlling the thermal state of a lithium-ion battery pack based on a BMS according to claim 1, characterized in that, The process involves collecting current thickness expansion data for each battery cell, temperature values ​​at each temperature measurement node at the interface between the gasket and the battery cell, and compression and rebound force data at each measurement point. The temperature values ​​at each temperature measurement node are then interpolated and stitched together according to spatial coordinates to generate a temperature gradient distribution image. Noise filtering and fusion processing are then applied to the thickness expansion data and the temperature gradient distribution image to obtain a smoothed thickness expansion sequence and a spatial temperature distribution map, including: Obtain the current displacement value of each cell, and determine the thickness expansion data based on the difference between the current displacement value and the initial thickness reference value; Temperature values ​​of each cell interface are obtained by temperature measurement nodes arranged in a preset grid, and an interpolation method is used to generate a temperature gradient distribution image covering the entire interface based on the temperature values ​​and spatial coordinates. The compression and rebound force data at each temperature measurement node is obtained through a pressure sensor. The thickness expansion data is smoothed using a sliding window mean filtering method to obtain the smoothed thickness expansion sequence. The temperature gradient distribution image is filtered to suppress high-frequency noise, resulting in the temperature spatial distribution map.

3. The method for controlling the thermal state of a lithium-ion battery pack based on a BMS according to claim 1, characterized in that, The process involves calculating the thickness growth rate of each cell based on the smoothed thickness expansion sequence, identifying the permanent deformation areas of the gasket based on the proportion of local high-temperature regions obtained from the temperature spatial distribution map, and obtaining a list of potential air gap formation locations, including: Based on the smoothed thickness expansion sequence, the thickness expansion data of adjacent sampling times are extracted, the thickness expansion difference is calculated, and the thickness growth rate of each cell is determined in combination with the sampling time interval. For battery cells whose thickness growth rate exceeds a preset growth rate threshold, the temperature distribution data of the corresponding region is obtained according to the temperature spatial distribution map, and the temperature value of each pixel in the region is traversed and high-temperature pixels are marked. The proportion of local high-temperature regions is determined based on the ratio of the number of high-temperature pixels to the total number of pixels in the region. If the proportion of local high-temperature regions exceeds a preset proportion threshold, it is determined that the region has undergone permanent deformation. The spatial coordinates are recorded and summarized to obtain the list of potential air gap formation locations.

4. The method for thermal state control of a lithium-ion battery pack based on a BMS according to claim 1, characterized in that, The step of calculating the rebound force difference between adjacent sampling times based on the compression rebound force data corresponding to each position in the potential air gap formation location list, distinguishing between abnormal and normal attenuation, and obtaining the rebound force attenuation marker for each position includes: Extract the compression rebound force data corresponding to each location from the potential air gap formation location list, obtain the rebound force value at adjacent sampling times, and calculate the rebound force difference; The rebound force difference is compared with a preset normal attenuation threshold. If the rebound force difference does not exceed the preset normal attenuation threshold, the position is determined to be under normal attenuation and marked as normal attenuation. If the difference in rebound force exceeds the preset normal attenuation threshold, then the position is determined to be an abnormal attenuation and marked as an abnormal attenuation marker; By associating the spatial coordinates of each location with the corresponding attenuation marker, the rebound force attenuation marker is obtained.

5. The method for controlling the thermal state of a lithium-ion battery pack based on a BMS according to claim 1, characterized in that, After matching the rebound force attenuation markers at each location with the potential air gap formation location list according to their spatial coordinates, a step degradation assessment is performed to obtain the contact thermal resistance degradation value, and the set of degraded cell locations is determined, including: The spatial coordinates of each position are extracted based on the rebound force attenuation mark, and compared with the coordinates in the potential air gap formation position list to determine the matching position attenuation mark set. For the matched set of position attenuation markers, abnormal attenuation marker positions are filtered out, the number of abnormal attenuation positions is counted and the ratio to the total number of positions is calculated to obtain the air gap area ratio of each cell. The degree of contact thermal resistance degradation is determined based on the air gap area ratio and the preset degradation level threshold. If the contact thermal resistance degradation value exceeds the preset degradation judgment threshold, the corresponding cell location identifier is added to the degraded cell location set.

6. The method for controlling the thermal state of a lithium-ion battery pack based on a BMS according to claim 1, characterized in that, The step of generating a cooling power allocation scheme for each battery cell based on the set of degraded cell locations and the rebound force attenuation markers at each location includes: Based on the set of locations of the degraded cells, the location identifier and degree of deterioration of each degraded cell are extracted, and the reference cooling power of each cell is obtained. The total additional cooling power is determined based on the number and degree of degradation of the battery cells; For each cell in the set of degraded cell locations, the number of abnormal attenuation marker locations is counted based on the resilience attenuation marker, and the power increment weight is calculated in combination with the degree of deterioration. The additional cooling power is allocated according to the weighting ratio of the power increment to obtain the cooling power increment of each degraded cell. The target cooling power value is determined by combining the benchmark cooling power and summarizing to obtain the cooling power allocation scheme for each cell.

7. The method for controlling the thermal state of a lithium-ion battery pack based on a BMS according to claim 1, characterized in that, The method further includes: adjusting the overall cooling power of the battery pack according to the cooling power distribution scheme of each cell, re-collecting the temperature distribution data of each cell after adjustment, and using the temperature uniformity evaluation method to compare the deviation between the adjusted temperature distribution data and the temperature spatial distribution map to obtain the thermal state stability control result throughout the entire life cycle.

8. The method for thermal state control of a lithium-ion battery pack based on a BMS according to claim 7, characterized in that, The process involves adjusting the overall cooling power of the battery pack according to the cooling power distribution scheme of each cell, re-collecting the temperature distribution data of each cell after adjustment, and comparing the deviation between the adjusted temperature distribution data and the temperature spatial distribution map using a temperature uniformity evaluation method to obtain the thermal stability control results throughout the entire life cycle, including: According to the cooling power allocation scheme of each cell, a power adjustment command is sent to the cooling device to adjust the flow rate of the cooling medium in each cooling channel and complete the adjustment of the overall cooling power of the battery pack. By re-collecting the temperature values ​​of each cell interface and interpolating and stitching them together, the adjusted temperature distribution data is obtained. The adjusted temperature distribution data is subtracted point by point from the temperature spatial distribution map to obtain the temperature deviation value at each location; The temperature distribution uniformity index is determined based on the dispersion of the temperature deviation value. If the temperature distribution uniformity index is lower than the preset uniformity threshold, the thermal state stability control is deemed to have met the standard, and the thermal state stability control result is obtained.

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