Optimization design method of lithium ion battery structural member

Abstract

The invention discloses a lithium ion battery structural member optimization design method, which comprises the following steps: measuring the internal temperature of an existing structural cell under different SOC working conditions to obtain the temperature data of each part of an existing structural cell pole group under different SOC working conditions; according to the temperature cloth data of each part of the existing structural battery cell, correcting the heat production power of the battery cell pole group under different SOC working conditions obtained by simulation in the three-dimensional model of the existing structural battery cell; on the basis of the corrected heat production power of the battery cell pole group under different SOC working conditions and the heat production power of a metal structural part in the target structural battery cell, temperature data of all parts of the target structural battery cell are obtained through true calculation in the standard structural battery cell refining simulation model; and correcting the design of each component of the target structure cell according to the temperature data of each component of the target structure cell so as to meet the requirements of over-current and short-circuit test fusing. The method can provide more accurate data support for the design of the metal structural member of the lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery structural component optimization design, and in particular to a lithium ion battery structural component optimization design method. Background Art

[0002] Lithium-ion power batteries are widely used in the field of electric transportation. However, range anxiety and charging anxiety are recognized as the core reasons that restrict the large-scale promotion and popularization of electric vehicles. By increasing the battery charging rate, fast charging can eliminate the range anxiety of pure electric vehicles, which is the current direction of battery development. As important metal components of lithium-ion batteries, the heating characteristics of metal connectors and tabs have a great influence on the temperature distribution of the battery cell. Reasonable structural design of metal connectors and tabs should consider their influence on the temperature distribution of the battery cell. Especially for fast-charging batteries, as the charging speed increases, the heat generation of the battery pole group and the metal structural parts will increase, affecting the life of the battery. At the same time, some areas are in a high temperature environment for a long time, which will cause safety hazards to the battery. Therefore, reasonable metal structural parts design is one of the key factors in designing fast-charging batteries. In addition, by changing the size of the aluminum connector to design the fuse structure, it can be melted in the event of an external short circuit to quickly cut off the current for protection, which is one of the keys to designing battery structural parts.

[0003] At present, Ansys Fluent thermal simulation method is used to predict the temperature performance of power batteries. The simulation method is widely used in battery temperature performance research due to its advantages such as short calculation cycle and low cost. Simulation can calculate the temperature distribution of batteries under different operating scenarios by inputting a series of information data without making actual products. Another method is to prepare multiple groups of physical batteries through the DOE method, and obtain the heating characteristics of the batteries by measuring the battery temperature under actual working conditions.

[0004] For Ansys Fluent thermal simulation, the accuracy of the thermal model solution is directly affected by the actual heat generation power of the battery cell, assembly ratio, pole ear structure of the pole group and thermal resistance of the structural parts. The actual heat generation power of the square lithium-ion battery is highly correlated with the system design and assembly ratio design. The traditional direct DC internal resistance conversion cannot accurately input the actual pole group heat generation power simulation parameters, making it impossible for the thermal simulation results to accurately reproduce the temperature distribution inside the battery cell and the metal connecting piece and the pole ear, and cannot accurately simulate the temperature distribution of each component of the battery during the charge and discharge process. The DOE method is used to prepare multiple groups of physical batteries, and the battery development cost is high and the development cycle is long. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings and defects of the prior art and to provide a method for optimizing the design of metal structural parts of lithium-ion batteries. The method of the present invention can accurately simulate the temperature of various parts of the lithium battery, facilitate the analysis of various components inside the battery cell under different working conditions, including metal connecting pieces, tabs and temperature distribution of various parts of the battery cell, and provide solid data support for the design and development of structural parts.

[0006] A lithium-ion battery structural component optimization design method comprises the following steps:

[0007] S1. Establish a three-dimensional model of the existing structural battery cell;

[0008] S2. Measure the internal temperature of the existing structure battery cell under different SOC conditions to obtain the temperature data of each part of the existing battery electrode group under different SOC conditions;

[0009] S3. According to the temperature distribution data of each part of the existing battery electrode group, the heat generation power of the battery electrode group under different SOC conditions obtained by simulation calculation in the existing structural battery three-dimensional model is corrected to match the model simulation data with the measured data, and the heat generation power of the battery electrode group under different SOC conditions is obtained after correction;

[0010] S4. Establish a target structure cell refinement simulation model;

[0011] S5. Based on the corrected heat generation power of the cell pole group under different SOC conditions and the heat generation power of the metal structural parts in the target structure cell, in the target structure cell refinement simulation model, the temperature data of each component of the target structure cell is obtained by simulation calculation;

[0012] S6. In the target structure battery cell refinement simulation model, the design of each component of the target structure battery cell is modified according to the temperature data of each component of the target structure battery cell to meet the requirements of overcurrent and short circuit testing and fusing.

[0013] The internal temperature of the existing structure battery cell is measured under different SOC conditions to obtain the temperature data of each part of the existing battery electrode group under different SOC conditions, including:

[0014] Arrange thermocouples at predetermined positions of existing structural cells;

[0015] Use the preset test strategy to perform charging tests on the existing structural battery cells, record the thermocouple temperatures at the preset positions, and obtain the temperature distribution data of the battery electrode group under different SOC conditions.

[0016] Among them, the metal structural parts in the target structure battery cell include metal connecting pieces, pole ears, pole columns, connecting bars, welding points and short-circuit devices of different shapes and structures.

[0017] The target structure cell refinement simulation model is established through the online simulation platform Ansys Fluent.

[0018] The heat generation power of the metal structural parts in the target structure battery cell is calculated by the Ansys Fluent potential module to obtain the internal resistance of the metal structural parts welding, combined with Joule's law.

[0019] Wherein, the three-dimensional model of the existing structural battery cell is established through the Comsol modeling and simulation platform.

[0020] Among them, the heat generation power of the battery cell electrode group under different SOCs obtained by simulation calculation in the three-dimensional model of the existing structure battery cell is calculated by the electrochemical model of Comsol.

[0021] Wherein, after obtaining the corrected heat generation power of the battery cell electrode group under different working conditions, it also includes:

[0022] The variables of the heat generation power of the battery cell pole group obtained by simulation calculation, which vary with non-uniform time, are processed into variables with uniform time intervals through programming, so as to keep consistent with the calculation time step of the target structure battery cell refinement simulation model; the heat generation power of the battery cell pole group and the heat generation power of the metal component are imported into the target structure battery cell refinement simulation model, and the temperature data of each component of the target structure battery cell are simulated and calculated.

[0023] Among them, when simulating and designing the short-circuit device structure, different short-circuit device structure designs are substituted into the target structure battery cell refinement simulation model, and simulation is performed using the current under external short-circuit conditions to obtain a curve of the maximum temperature of each component of the battery cell changing with the short-circuit time under the external short-circuit condition; the short-circuit device structure design is optimized until the maximum temperature of each component of the target structure battery cell meets the design requirements.

[0024] Among them, when simulating and calculating in the target structure battery cell refinement simulation model, the boundary conditions of natural heat dissipation and liquid cooling are respectively adopted, among which natural heat dissipation is achieved by setting the convection heat transfer coefficient, and liquid cooling is achieved by setting the coolant temperature and mass flow rate; the liquid cooling position is the bottom surface of the battery cell, and the natural heat dissipation position is all exposed surfaces except the bottom surface of the battery cell.

[0025] The present invention corrects the heat generation power of the cell pole group under different SOC conditions of the existing structural cell three-dimensional model through the measured data of the existing structural cell, and then uses the obtained corrected heat generation power of the cell pole group under different SOC conditions and the heat generation power of the metal structural parts in the target structural cell. In the target structural cell refinement simulation model, the temperature data of each component of the target structural cell is obtained through simulation calculation. According to the temperature data of each component of the target structural cell, the design of each component of the target structural cell is corrected to meet the requirements of overcurrent and short-circuit test fusing, and can accurately simulate the temperature of each part of the lithium battery, which is convenient for analyzing the internal components of the cell under different working conditions, including metal connecting pieces, pole ears, and temperature distribution of various parts of the cell, and provides more accurate data support for the design and development of structural parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of a lithium-ion battery structural component optimization design method according to an embodiment of the present invention.

[0027] Figure 2 It is a schematic diagram of a three-dimensional model of an existing structural battery cell according to an embodiment of the present invention.

[0028] Figure 3A , Figure 3B They are respectively a top view and a side view of a conventional structure battery cell with a built-in thermocouple according to an embodiment of the present invention.

[0029] Figure 4 2 is a schematic diagram comparing the heat generation simulation and experimental data of the battery cell according to an embodiment of the present invention.

[0030] Figure 5 It is a schematic diagram of establishing a three-dimensional model of a target structural battery cell according to an embodiment of the present invention.

[0031] Figure 6 It is a schematic diagram of the temperature rise of various components with different design thicknesses in an embodiment of the present invention.

[0032] Figure 7 It is a schematic diagram of the temperature rise of the fuse device according to the embodiment of the present invention under the condition of 2mΩ external short circuit.

[0033] Figure 8 It is a schematic diagram of the temperature rise of various components in an embodiment of the present invention under a 2mΩ external short-circuit condition. DETAILED DESCRIPTION

[0034] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0035] The lithium-ion battery structural component optimization design method of the present invention establishes a three-dimensional model of an existing structural battery cell of the same system, performs a built-in thermocouple temperature test on the existing structural battery cell of the same system under different SOC conditions, and corrects the heat generation power of the battery cell pole group under different SOCs in the simulation model based on the temperature data of each part, so that the simulation data matches the measured data, and the pole group heat generation power of the existing structural battery cell under different SOC conditions can be obtained, and the pole group heat generation power is the target structure battery cell pole group heat generation power; the target structure battery cell metal structure heat generation power is calculated by combining the resistance calculated by the Ansys Fluent potential module in the simulation model with Joule's law; the target structure battery cell pole group heat generation power and the metal structure heat generation power are brought into the Ansys Fluent solver for solution and calculation, so that the temperature change of each component of the battery cell with the SOC under different SOC conditions can be obtained efficiently, and the design of each metal structure of the target structure battery cell is corrected according to the temperature change to meet the overcurrent design and short-circuit test fusing requirements.

[0036] See also Figure 1 As shown, a lithium-ion battery structural component optimization design method includes the following steps:

[0037] S1. Establish a three-dimensional model of the existing structural battery cell;

[0038] S2. Measure the internal temperature of the existing structure battery cell under different SOC conditions to obtain the temperature data of each part of the existing battery electrode group under different SOC conditions;

[0039] S3. According to the temperature distribution data of each part of the existing battery electrode group, the heat generation power of the battery electrode group under different SOC conditions obtained by simulation calculation in the existing structural battery three-dimensional model is corrected to match the model simulation data with the measured data, and the heat generation power of the battery electrode group under different SOC conditions is obtained after correction;

[0040] S4. Establish a target structure cell refinement simulation model;

[0041] S5. Based on the corrected heat generation power of the cell pole group under different SOC conditions and the heat generation power of the metal structural parts in the target structure cell, in the target structure cell refinement simulation model, the temperature data of each component of the target structure cell is obtained by simulation calculation;

[0042] S6. In the target structure battery cell refinement simulation model, the design of each component of the target structure battery cell is modified according to the temperature data of each component of the target structure battery cell to meet the requirements of overcurrent and short circuit testing and fusing.

[0043] In the embodiment of the present invention, the established three-dimensional model of the existing structure battery cell includes the clamping plate 10, the bus bar 20 and the battery cell 30 used for the test, such as Figure 2As shown in the figure, the battery cell model includes physical structures such as battery cell shell, pole group, positive and negative pole ears, positive and negative connecting bars, positive and negative pole columns, cover plate, plastic bracket under the cover plate, insulating sleeve, sealing ring, explosion-proof valve, lower gasket, etc. When doing simulation modeling, the blue film wrapped around the outside of the battery cell shell and the pole group protective film between the pole group inside the battery cell and the inner wall of the shell are reflected by setting Boundary Condition-Wall in Ansys Fluent. The remaining components are retained for the requirements of simulation accuracy.

[0044] In some embodiments, the internal temperature of the existing structure battery cell is measured under different SOC conditions to obtain the temperature data of each part of the existing battery electrode group under different SOC conditions, including:

[0045] Arrange thermocouples at predetermined positions of existing structural cells; use a preset test strategy to perform charging tests on existing structural cells, record the temperature of the thermocouples at the preset positions, and obtain the temperature distribution data of the battery electrode group under different SOC conditions. The thermocouples can collect temperature at corresponding positions respectively: Figure 3A , Figure 3B As shown in the figure, the thermocouples are arranged at the positive electrode laser weld center 1, the positive electrode short circuit device (FUSE) 2, the positive electrode ultrasonic weld center 3, the negative electrode laser weld center 4, the negative electrode ultrasonic weld center 5, the upper edge of the positive electrode of the electrode group 6, the upper edge of the middle of the electrode group 7, the upper edge of the negative electrode of the electrode group 8, the middle of the electrode group 9, the positive column and the negative column 11, as shown by the red dots in the figure. The target charging strategy shown in Table 1 is used to test the existing structure battery cell and record the temperature of the thermocouples at various locations.

[0046] Table 1

[0047]

[0048] In the present invention, the preferred implementation is to use the Comsol electrochemical model to preliminarily calculate the heat generation power of the existing structure battery cell group: according to the material parameters required for the provided electrochemical simulation, the Comsol electrochemical model can be used to obtain the current value, battery capacity and heat generation power of the battery cell group that change with time under different strategies. Because the electrochemical simulation studies the microscopic behavior of the battery from the material level and clarifies the multi-phenomenon mechanism inside the battery, the calculation result is more accurate than the pure internal resistance calculation of heat generation. However, for complex, multi-layered, and large-scale metal structural parts, Comsol cannot draw a grid that can be effectively calculated, and the heat generation calculation has a large deviation or even cannot be calculated. Therefore, the embodiment of the present application preferably uses the Comsol electrochemical model to preliminarily calculate the heat generation power of the existing structure battery cell group.

[0049] In the present invention, after obtaining the corrected heat generation power of the battery cell pole group under different working conditions, the following steps are also included: the variables of the heat generation power of the battery cell pole group obtained by simulation calculation that change with non-uniform time are processed into variables with uniform time intervals through programming (such as programming using Visual Studio) so as to keep consistent with the calculation time step of the target structure battery cell refinement simulation model; the heat generation power of the battery cell pole group and the heat generation power of the metal component are imported into the target structure battery cell refinement simulation model, and the temperature data of each component of the target structure battery cell are simulated and calculated.

[0050] Among them, in the three-dimensional model of the existing structural battery cell, the heat generation power of the existing structural battery cell can be corrected according to the comparison between the simulation results of the model and the test results of the experiment, which can be specifically reflected in the temperature rise of each component, see Figure 4 shown. Figure 4 The dashed line is the experimental data, and the solid line is the simulation data.

[0051] In this application, in the Ansys Fluent platform, Ansys Fluent is used to correct the heat generation power of existing battery cells: first, the variable values ​​​​that change with non-uniform time obtained by the Comsol electrochemical model must be processed into variables with uniform time intervals through Visual Studio programming to ensure consistency with the time step of the subsequent Ansys Fluent solution calculation; then, the heat generation power of the battery cell pole group and the heat generation power of the metal structural parts obtained by solving the Ansys Fluent potential module combined with Joule's law are imported into the target structure battery cell refinement simulation model established by Ansys Fluent, and subsequent solution calculations are performed through Ansys Fluent, so as to efficiently obtain the metal connecting pieces of different designs inside the target structure battery cell, the pole ear structures of different designs, and the temperature distribution of the battery cell pole group.

[0052] In the embodiment of the present application, the heat generation power of the metal structural parts of the target structure battery cell is preferably calculated by the Ansys Fluent potential module, and then calculated by applying current density and combining with Joule's law; for complex, multi-layered, and large-scale metal structural parts, Ansys Fluent can quickly and efficiently draw high-quality grids, and the heat generation calculation accuracy is high. Therefore, the embodiment of the present application uses the Ansys Fluent potential module to calculate the internal resistance of the metal structural parts and combines Joule's law to calculate the heat generation power of the metal structural parts of the target structure battery cell.

[0053] In an embodiment of the present application, when designing the metal parts of the target structure battery cell, a detailed simulation model of the target structure battery cell is first established, and the corrected heat generation power of the existing battery cell pole group in the same system and the heat generation power of each metal structural part in the target battery cell are brought into the Ansys Fluent solver for solution and calculation. In this way, the temperature changes of each component of the battery cell, including internal metal connecting plates with different designs, pole ear structures with different designs, and fuse devices (FUSE) can be obtained efficiently, and the design of each metal structural part and pole ear can be modified according to this temperature change to meet the overcurrent design and short-circuit test fusing requirements.

[0054] The target structure battery cell three-dimensional model established includes a battery cell 40, a cold plate 50, and a thermal conductive adhesive 60, such as Figure 5 As shown in the figure, the battery cell model includes physical structures such as battery cell shell, pole group, positive and negative pole ears, positive and negative connecting bars, positive and negative pole columns, cover plate, plastic bracket under the cover plate, insulating sleeve, sealing ring, explosion-proof valve, lower gasket, etc. During simulation modeling, the blue film wrapped around the outside of the battery cell shell and the pole group protective film between the pole group inside the battery cell and the inner wall of the shell are reflected by setting Boundary Condition-Wall in Ansys Fluent. The remaining components are retained for the requirements of simulation accuracy.

[0055] During model simulation, the heat generation power of the electrode group of the target structural battery cell in different SOC segments is obtained by calibrating the simulation and experimental data of the existing structural battery cell. The heat generation power of the metal components including the upper positive and negative poles, welding points, positive and negative connecting bars, positive and negative pole ears, etc. is calculated by the resistance calculated by the Ansys Fluent potential module combined with Joule's law.

[0056] When simulating the model, the boundary conditions of natural heat dissipation and liquid cooling are used respectively. Natural heat dissipation is achieved by setting the convection heat transfer coefficient, and liquid cooling is achieved by setting the coolant temperature and mass flow rate. The liquid cooling position is the bottom surface of the battery cell, and the natural heat dissipation position is all other exposed surfaces.

[0057] Specifically, during the simulation, the target charging strategy is used to design and simulate the metal components of the target structure battery cell, including the positive and negative ears, positive and negative connecting pieces, fuse protection devices at the positive connecting pieces, positive and negative poles, and welding points. If the temperature of the metal component does not meet the design requirements, the design of the metal component is adjusted to meet the design requirements. For example, when the thickness of the positive connecting piece of the target structure battery cell is designed to be 0.6mm, the simulation results of the working condition show that the highest temperature occurs on the positive connecting piece, and the temperature is 52.6℃, which is higher than the upper limit of the design temperature of 50℃ for this model; the thickness of the positive connecting piece is adjusted to 1mm in the simulation model. At this time, the simulation results show that the maximum temperature at the positive connecting piece is reduced to 47.8℃, which meets the design requirements.

[0058] Among them, the fuse device (FUSE) design simulation: different FUSE structure designs are substituted into the simulation model of the target structure battery cell, and the current under a certain external short-circuit condition is used for simulation to obtain the maximum temperature variation curve of each component of the battery cell, including the pole group, structural parts and FUSE part, as a function of the short-circuit time; the FUSE structure design is optimized until the temperature of each component of the battery cell meets the design requirements. Figure 7 The temperature rise of the designed fuse device (FUSE) structure under 2mΩ external short circuit condition is shown. Figure 8 This is a schematic diagram of the temperature rise of each component of the embodiment of the present invention under the condition of a 2mΩ external short circuit. After 100s, the temperature of the positive FUSE is greater than 600°C and reaches the melting temperature, meeting the short circuit test design requirements.

[0059] The basic principles and main features of the present invention and the advantages of the present invention are shown and described above. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0060] Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is therefore intended that all changes falling within the meaning and range of equivalent elements of the claims are included in the present invention.

[0061] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A lithium-ion battery structural component optimization design method, characterized in that: Includes steps: S1. Establish a three-dimensional model of the existing structural battery cell; S2. Measure the internal temperature of the existing structure battery cell under different SOC conditions to obtain the temperature data of each part of the existing battery electrode group under different SOC conditions; S3. According to the temperature distribution data of each part of the existing battery electrode group, the heat generation power of the battery electrode group under different SOC conditions obtained by simulation calculation in the existing structural battery three-dimensional model is corrected to match the model simulation data with the measured data, and the heat generation power of the battery electrode group under different SOC conditions is obtained after correction; S4. Establish a target structure cell refinement simulation model; S5. Based on the corrected heat generation power of the cell pole group under different SOC conditions and the heat generation power of the metal structural parts in the target structure cell, in the target structure cell refinement simulation model, the temperature data of each component of the target structure cell is obtained by simulation calculation; S6. In the target structure battery cell refinement simulation model, the design of each component of the target structure battery cell is modified according to the temperature data of each component of the target structure battery cell to meet the requirements of overcurrent and short circuit testing and fusing.

2. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that: The internal temperature of the existing structure battery cell is measured under different SOC conditions to obtain the temperature data of each part of the existing battery electrode group under different SOC conditions, including: Arrange thermocouples at predetermined positions of existing structural cells; Use the preset test strategy to perform charging tests on the existing structural battery cells, record the thermocouple temperatures at the preset positions, and obtain the temperature distribution data of the battery electrode group under different SOC conditions.

3. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that: The metal structural parts in the target structure battery cell include metal connecting pieces, pole ears, pole columns, connecting bars, welding points and short-circuit devices of different shapes and structures.

4. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that The target structure battery cell refinement simulation model is established through the online simulation platform Ansys Fluent.

5. The lithium-ion battery structural component optimization design method according to claim 4, characterized in that: The heat generation power of the metal structural parts in the target structure battery cell is calculated by the Ansys Fluent potential module to obtain the internal resistance of the metal structural parts welding, combined with Joule's law.

6. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that: The three-dimensional model of the existing structural battery cell is established through the Comsol modeling and simulation platform.

7. The lithium-ion battery structural component optimization design method according to claim 6, characterized in that: The heat generation power of the battery cell electrode group under different SOCs obtained by simulation calculation in the three-dimensional model of the existing structure battery cell is calculated by the electrochemical model of Comsol.

8. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that: After obtaining the corrected heat generation power of the battery cell electrode group under different working conditions, the method further includes: The variables of the heat generation power of the battery cell pole group obtained by simulation calculation, which vary with non-uniform time, are processed into variables with uniform time intervals through programming, so as to keep consistent with the calculation time step of the target structure battery cell refinement simulation model; the heat generation power of the battery cell pole group and the heat generation power of the metal component are imported into the target structure battery cell refinement simulation model, and the temperature data of each component of the target structure battery cell are simulated and calculated.

9. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that: During the simulation design of the short-circuit device structure, different short-circuit device structure designs are substituted into the target structure battery cell refinement simulation model, and the current under the external short-circuit condition is used for simulation to obtain the maximum temperature variation curve of each component of the battery cell with the short-circuit time under the external short-circuit condition; the short-circuit device structure design is optimized until the maximum temperature of each component of the target structure battery cell meets the design requirements.

10. The lithium-ion battery structural component optimization design method according to claim 1, characterized in that: During simulation calculation in the target structure battery cell refinement simulation model, the boundary conditions of natural heat dissipation and liquid cooling are respectively adopted. Natural heat dissipation is achieved by setting the convection heat transfer coefficient, and liquid cooling is achieved by setting the coolant temperature and mass flow rate. The liquid cooling position is the bottom surface of the battery cell, and the natural heat dissipation position is all exposed surfaces except the bottom surface of the battery cell.

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