Heat transfer in battery module assembly

Abstract

A battery module assembly (100) comprises a first battery cell (101) having at least one cell unit (102), the at least one cell unit (102) comprising a first set of cells (202) and a second set of cells (204) electrically connected. The first cell holder (206) holds the cells (202, 204). The first set of cells (202) is disposed in the first cell holder (206) at a distance from the second set of cells (204). The first case (104) surrounds the cell unit (102). A first thermally conductive pad interface (226) is positioned between the cell (202, 204) and the first cell holder (206). The first thermally conductive pad interface (226) is in contact with the first housing (104) and transfers heat generated by the cells (202, 204) to the housing (104). The thermally conductive pad interface (226) includes a continuous first plurality of hollow portions (228).

Description

Technical Field

[0001] The present subject matter relates generally to battery module assemblies and, more particularly, to heat transfer in battery module assemblies. Background Art

[0002] Battery modules, such as lithium-ion (Li-ion) battery modules, are used to power components in applications such as electric vehicles, hybrid vehicles, mobile phones, notebook computers, etc. The battery module has a plurality of single cells electrically connected to each other.

[0003] Typically, in a battery module such as a Li-ion battery module, during operation, a cell or region within a cell may reach a high temperature due to various factors such as thermal failure, mechanical failure, internal short circuit, external short circuit, electrochemical abuse, etc. This may lead to thermal runaway of one or more cells, resulting in fire or explosion of the battery module. Therefore, during operation, in order to avoid thermal runaway and ensure that the battery module operates normally without failure, the heat generated by each cell is dissipated from the battery module.

[0004] In conventional battery modules, heat exchange from cell to cell is performed by using liquid filling materials in the battery module to maintain the temperature of the entire battery module. In some conventional battery modules, phase change materials (PCM) are also filled in the battery module, which absorb heat and become solid. However, such liquid filling materials and PCM cause problems such as handling problems, assembly time, and no heat exchange between cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The detailed description is provided with reference to the accompanying drawings. In the drawings, the leftmost digit of a reference number indicates the drawing in which the reference number first appears. Throughout the drawings, the same reference numbers are used to refer to the same features and components.

[0006] Figure 1 A battery module assembly according to an embodiment of the present subject matter is shown;

[0007] Figure 2 shows an exploded view of a battery module assembly according to an embodiment of the present subject matter;

[0008] Figure 3 shows a thermally conductive pad interface surrounding a plurality of battery cells of a battery module assembly according to an embodiment of the present subject matter;

[0009] Figure 4 shows a subassembly view of a battery module assembly depicting a thermal pad interface contacting a housing according to an embodiment of the present subject matter; and

[0010] Figure 5A thermally conductive pad interface of a battery module assembly according to an embodiment of the present subject matter is shown. DETAILED DESCRIPTION

[0011] A battery module has a plurality of cells electrically connected to each other. During operation, each cell generates heat, which must be dissipated from the battery module to ensure that the battery module operates normally without failure. In particular, battery modules such as lithium-ion (Li-ion) battery modules have thermal runaway problems. For example, when a cell, an area within a cell, or multiple cells of a Li-ion battery module reaches a high temperature due to thermal failure, mechanical failure, internal or external short circuit, or electrochemical abuse, a large amount of heat is generated. When the generated heating is greater than the heat dissipation, various side reactions are induced between the components inside the battery module. This may cause further heat generation, and the pressure and temperature of the battery module may rise sharply. This may cause the battery module to catch fire and / or explode. This process is called thermal runaway.

[0012] Conventionally, in order to dissipate the heat generated in the single cells of the battery module and overcome the thermal runaway problem, the battery module includes a liquid filling material, such as a coolant. However, the use of such a material causes handling problems for the battery module. In some conventional battery modules, a phase change material (PCM) is filled in the battery module for heat transfer. The PCM absorbs the heat generated by the single cell and becomes solid. However, the conductivity of the PCM is very low. Further, filling materials such as liquid filling materials and PCM are only used at the tips of the single cells. Therefore, heat exchange only occurs at the tips of the single cells, and heat exchange that occurs through the surfaces of the single cells is eliminated. Further, the process of using the filling material is cumbersome and a time-consuming process. Therefore, the use of the filling material in the battery module increases the assembly time.

[0013] The present subject matter relates to heat transfer in a battery module assembly. Through various embodiments of the present subject matter, heat generated by cells of a battery module assembly can be efficiently dissipated from the battery module assembly, thereby eliminating thermal runaway.

[0014] According to example embodiments, a battery module assembly may include a first battery. The first battery may include at least one single cell unit. The single cell unit may include a first group of single cells and a second group of single cells. The first group of single cells and the second group of single cells may include a first plurality of single cells connected to each other. The first group of single cells and the second group of single cells may be electrically connected to each other. A first single cell holder may hold the first group of single cells and the second group of single cells. The first single cell holder may include a first plurality of grooves. Each groove may be designed to hold a single cell from the first plurality of single cells. The first group of single cells may be arranged at a distance from the second group of single cells in the first single cell holder.

[0015] Further, the first battery cell may include a first shell surrounding the single battery cell. The first thermal pad interface may be positioned between the first group of single batteries, the second group of single batteries, and the first single battery holder. The first thermal pad interface may contact the first shell to cause the heat generated by the first plurality of single batteries to dissipate. In other words, the first thermal pad interface may transfer heat from the first plurality of single batteries to the first shell, and the heat dissipates from the first shell. The first thermal pad interface may include a continuous first plurality of hollow portions. In the example, each hollow portion may surround a single battery in the first plurality of single batteries, and may be arranged between a groove in the first plurality of grooves and a corresponding single battery. Each hollow portion may contact the corresponding single battery.

[0016] In an example, the first thermal pad interface may include a first rib and a second rib extending from both ends thereof. The first rib and the second rib may be disposed in contact with the housing to transfer heat generated by the first plurality of battery cells to the first housing.

[0017] In the present subject matter, a thermal pad interface enables the heat generated by multiple single cells to be dissipated. Therefore, the present subject matter eliminates thermal runaway, thereby eliminating the explosion of the battery module assembly. By efficiently dissipating heat from multiple single cells, the present subject matter improves the performance of the battery module assembly. Further, unlike conventional battery modules that use PCM or liquid filling materials for heat dissipation, the present subject matter uses a thermal pad interface for heat dissipation. The thermal pad interface is positioned between the first group of single cells, the second group of single cells, and the single cell retainer. Therefore, compared to conventional battery modules, the present subject matter enables the battery module assembly to be easily assembled. In addition, the present subject matter also reduces the costs associated with the cooling of the battery module assembly, improves the handling of the battery module assembly, and enhances the safety of the battery module assembly.

[0018] Combination Figures 1 to 5 The present subject matter is further described. It should be noted that the description and drawings illustrate only the principles of the present subject matter. Various arrangements that are not explicitly described or shown herein but that encompass the principles of the present subject matter may be designed. Moreover, all statements herein reciting the principles, aspects, and examples of the present subject matter and specific examples thereof are intended to encompass their equivalents.

[0019] Figure 1 A battery module assembly 100 according to an embodiment of the present subject matter is shown. The battery module assembly 100 may include a first battery unit 101. The first battery unit 101 may include at least one single battery unit. The at least one single battery unit may be explained with reference to the single battery unit 102. The single battery unit 102 may include a first group of single batteries ( Figure 1 not shown) and a second group of cells ( Figure 1). The first group of cells and the second group of cells may include a first plurality of cells connected to each other. The first plurality of cells may be referred to as a plurality of cells hereinafter. The first group of cells and the second group of cells are electrically connected to each other. The first group of cells and the second group of cells may be, for example, lithium-ion (Li-ion) batteries, nickel-metal hydride batteries, etc. Thus, the battery module assembly 100 may be a Li-ion battery module assembly, a nickel-metal hydride battery module assembly, etc. The battery module assembly 100 will be explained below with reference to the Li-ion battery module assembly 100.

[0020] The battery module assembly 100 may include a first housing 104 to provide coverage and protection for the components of the battery module assembly 100. In particular, the first housing 104 may surround the single battery cells 102. In addition to providing coverage and protection for the components of the battery module assembly 100, the first housing 104 may also help dissipate heat from the battery module assembly 100 to the surrounding environment. Thus, in an example, the first housing 104 may be made of a thermally conductive material. The first housing 104 may be referred to as the housing 104 hereinafter.

[0021] Figure 2 An exploded view of a battery module assembly 100 according to an embodiment of the present subject matter is shown. A first group of cells 202 and a second group of cells 204 may be disposed at a distance from each other. Further, each of the plurality of cells in the first group of cells 202 and each of the plurality of cells in the second group of cells 204 may be disposed at a distance from each other. In an example, each of the first group of cells 202 and the second group of cells 204 may be cylindrical. In another example, the first group of cells 202 and the second group of cells 204 may be brick-shaped. In yet another example, the first group of cells 202 and the second group of cells 204 may be flat rectangular. In an example, the diameter of each cell 202, 204 and the height of each cell 202, 204 may be the same. The first group of cells 202 and the second group of cells 204 may be explained below with reference to the cylindrical shape.

[0022] The first battery unit 101 may include a first single cell holder 206 for holding a first group of single cells 202 and a second group of single cells 204. The first single cell holder 206 may be referred to as the single cell holder 206 hereinafter. In an example, the single cell holder 206 may include a first single cell holding portion 208 and a second single cell holding portion 210. The first single cell holding portion 208 and the second single cell holding portion 210 may be connected together to hold the first group of single cells 202 and the second group of single cells 204. Although in the above example, the battery module assembly 100 is shown as including the single cell holder 206, in an example, the battery module assembly may include a plurality of single cell holders ( Figure 2 206, and may include a first cell holding portion 208 and a second cell holding portion 210.

[0023] Further, the cell holder 206 may include a first plurality of grooves 212. The first plurality of grooves 212 may be referred to hereinafter as a plurality of grooves 212. Each groove 212 is designed to hold a cell from a plurality of cells. As will be appreciated, the first group of cells 202 and the second group of cells 204 are disposed at a distance from each other in the cell holder 206. In an example, each groove 212 may be cylindrical. In another example, each groove may be brick-shaped. In yet another example, each groove 212 may be flat and rectangular. Further, in an example, the diameter of each groove 212 and the height of each groove 212 may be the same. The shape of the groove 212 may correspond to the shape of the first group of cells 202 and the second group of cells 204. The groove 212 may be cylindrical, brick-shaped, flat and rectangular, etc. The groove 212 may be explained below with reference to the cylindrical shape.

[0024] In order to enable the first group of cells 202 and the second group of cells 204 to be electrically connected to each other, the first battery unit 101 may include a first interconnect 214 and a second interconnect 216. The first interconnect 214 may be connected at one end of the first group of cells 202 and the second group of cells 204. The second interconnect 216 may be connected at the other end of the first group of cells 202 and the second group of cells 204. In an example, the first interconnect 214 and the second interconnect 216 may be rectangular sheets. The first interconnect 214 and the second interconnect 216 may be welded to the first group of cells 202 and the second group of cells 204. The first interconnect 214 and the second interconnect 216 may have contact points 218 therein, and the contact points 218 contact the first group of cells 202 and the second group of cells 204, respectively, to establish an electrical connection between the first group of cells 202 and the second group of cells 204.

[0025] like Figure 2 As depicted in FIG. 1 , the first interconnect 214 may be disposed adjacent to the first cell holding portion 208, and the second interconnect 216 may be disposed adjacent to the second cell holding portion 210. In an example, the first interconnect 214 may be positioned adjacent to the inner surface 220 of the first cell holding portion 208, rather than adjacent to the outer surface ( Figure 2 The second interconnecting member 216 may be positioned adjacent to the outer surface 222 of the second battery holding portion 210, rather than adjacent to the inner surface ( Figure 2As will be appreciated, in a top view, the first interconnect 214 may be positioned above the first cell holding portion 208, while the second interconnect 216 may be positioned above the second cell holding portion 210.

[0026] During operation of the battery module assembly 100, the cells in the first group of cells 202 and the second group of cells 204 may generate heat. In order to prevent thermal runaway and to prevent the battery module assembly 100 from exploding, the heat may have to be dissipated from the battery module assembly 100. Thus, in order to enable heat dissipation from the cells, the first battery unit 101 may include a first thermal pad interface 226. The first thermal pad interface 226 may be referred to as the thermal pad interface 226 hereinafter. The thermal pad interface 226 may be disposed between the first group of cells 202, the second group of cells 204, and the cell holder 206. The thermal pad interface 226 may be disposed between each cell in the first group of cells 202 and the second group of cells 204. Further, the thermal pad interface 226 may be in contact with the housing 104, as will be referred to in detail in detail below. Figure 3 and Figure 4 Described.

[0027] The thermal pad interface 226 may include a first plurality of hollows 228. The first plurality of hollows 228 may be referred to hereinafter as a plurality of hollows 228. Each hollow in the plurality of hollows 228 may be continuous. In an example, each hollow 228 may be cylindrical. In another example, each hollow 228 may be brick-shaped. In yet another example, each hollow 228 may be flat and rectangular. The plurality of hollows 228 may be explained below with reference to a cylindrical shape. In an example, each hollow 228 may be coaxial with a plurality of grooves 212. Each hollow 228 surrounds a single cell in a plurality of single cells 202, 204. As previously described, the thermal pad interface 226 may be disposed between a first group of single cells 202, a second group of single cells 204, and a single cell holder 206. Therefore, the diameter of each hollow portion 228 may be greater than the diameter of the battery cells 202 , 204 , and the diameter of each groove 212 of the battery cell holder 206 may be greater than the diameter of each hollow portion 228 .

[0028] Figure 3 A thermal pad interface 226 is shown surrounding a plurality of battery cells 202, 204 of a battery module assembly 100 according to an embodiment of the present subject matter. Each hollow portion 228 of the thermal pad interface 226 may be in contact with a corresponding battery cell 202, 204 surrounded by the hollow portion 228.

[0029] The thermal pad interface 226 may include a first end 302-1 and a second end 302-2. The second end 302-2 may be opposite to the first end 302-1. The thermal pad interface 226 may include a first rib 304 extending from the first end 302-1 and a second rib 306 extending from the second end 302-2. In other words, the first rib 304 contacts some of the plurality of hollows 228 in the first end 302-1, and the second rib 306 contacts some of the plurality of hollows 228 in the second end 302-2.

[0030] Further, the first rib 304 may be arranged to contact the housing 104 to transfer the heat generated by the plurality of battery cells 202, 204 to the housing 104. Thus, the length of the first rib 304 is designed to ensure that the first rib 304 is in contact with the housing 104. Similarly, the second rib 306 is arranged to contact the housing 104 to dissipate the heat generated by the plurality of battery cells 202, 204 to the housing 104. Thus, the length of the second rib 306 is designed to ensure that the second rib 306 is in contact with the housing 104.

[0031] During operation, when the plurality of cells 202, 204 generate heat, heat is transferred from the cells 202, 204 to the hollow portions 228 of the thermal pad interface 226 because each hollow portion 228 is in contact with the corresponding cell 202, 204 of the thermal pad interface 226. Since the ribs 304, 306 extend from some of the hollow portions 228, heat is transferred from the hollow portions 228 to the first rib 304 and the second rib 306.

[0032] Figure 4 A subassembly view of a battery module assembly 100 is shown, depicting a thermal pad interface 226 contacting the housing 104, according to an embodiment of the present subject matter. Here, the second interconnect 216, the housing 104, and the ribs 304, 306 of the thermal pad interface 226 are depicted. During operation of the battery module assembly 100, when the first rib 304 and the second rib 306 are in contact with the housing 104, the thermal conductivity of the plurality of cells 202, 204 ( Figure 4 The heat transferred to the housing 104 can be dissipated into the surrounding environment.

[0033] Figure 5 The thermal pad interface 226 of the battery module assembly 100 according to an embodiment of the present subject matter is shown. To prevent electrical conduction and enhance thermal conduction, the thermal pad interface 226 can be made of an electrically insulating material and a thermally conductive material. In an example, the thermal pad interface 226 can be made of a silicon-based material. In an example, each hollow portion 228 can have the same height and the same diameter. Further, the height of each hollow portion 228 of the thermal pad interface 226 is the same as the corresponding single battery 202, 204 ( Figure 5 The height of the single cells 202 and 204 is substantially the same so as to be dissipated over the entire outer surface (not shown) of the single cells 202 and 204. Figure 5 Thus, unlike conventional battery modules that dissipate heat only from the tips or corners of the cells, the present subject matter enables heat to be dissipated from the entire outer surface of each cell 202, 204.

[0034] The shapes and positions of the first rib 304 and the second rib 306 are designed to ensure that heat from the hollow portion 228 is transferred to the housing 104. In an example, the first rib 304 and the second rib 306 can be positioned at a central position along the height direction H of the thermal pad interface 226 and positioned at a central position along the width direction W. Further, in an example, the first rib 304 and the second rib 306 can be rectangular parallelepiped, wherein one face of the first rib 304 and one face of the second rib 306 are wavy. The wavy shape of the first rib 304 can correspond to the shape of a portion of some of the hollow portions 228 from which the first rib 304 can extend. Similarly, the wavy shape of the second rib 306 can correspond to the shape of a portion of some of the hollow portions 228 from which the second rib 306 can extend. In other examples, the first rib 304 and the second rib 306 can be other shapes, such as cubic, cylindrical, ellipsoidal, hemispherical, etc.

[0035] Further, the dimensions of the first rib 304 and the second rib 306 may be designed so as to ensure efficient heat transfer to the housing 104 while also ensuring that the weight of the first rib 304 and the second rib 306 does not increase to a higher value. In an example, the width of the first rib 304 and the width of the second rib 306 may be greater than the combined diameter of the two hollows 228. However, in another example, the width may be less than or equal to the combined diameter of the two hollows 228.

[0036] In an example, the hollow portion 228 may be displaced to be a distance away from another hollow portion 228. In an example, the distance between the hollow portions 228 may be determined by the plurality of grooves 212 ( Figure 5 Thus, the diameter of each groove 212 of the battery holder 206 may be greater than the diameter of each hollow portion 228 of the thermal pad interface 226 .

[0037] In the above example, the battery module assembly 100 is explained as having only one battery cell (i.e., the first battery cell 101). In another example, the battery module assembly 100 may include additional battery cells based on the power requirements of the application using the battery module assembly 100. Each battery cell may be connected in series or in parallel. For example, the battery module assembly may include a second battery cell ( Figures 1 to 5101). The second battery cell may be electrically connected to the first battery cell 101 in series or in parallel. The second battery cell may correspond to the first battery cell 101. The second battery cell may include at least one single battery cell. The at least one single battery cell may include a third group of single batteries and a fourth group of single batteries. The third group of single batteries and the fourth group of single batteries may each include a second plurality of single batteries connected to each other. Further, the third group of single batteries and the fourth group of single batteries may be electrically connected to each other.

[0038] The second battery cell may include a third cell holder for holding a third group of cells and a fourth group of cells. The third cell holder may include a second plurality of grooves. Each groove may be designed to hold a cell from the second plurality of cells. The third group of cells may be arranged at a distance from the fourth group of cells in the third cell holder. The second battery cell may further include a second housing surrounding at least one cell unit, and a second thermal pad interface positioned between the third group of cells, the fourth group of cells and the third cell holder. The second thermal pad interface may contact the second housing to transfer the heat generated by the second plurality of cells to the second housing. The second thermal pad interface may include a second plurality of continuous hollow portions.

[0039] In the present subject matter, a thermal pad interface enables the heat generated by multiple single cells to be dissipated. Therefore, the present subject matter eliminates thermal runaway, thereby eliminating the explosion of the battery module assembly. By efficiently dissipating heat from multiple single cells, the present subject matter improves the performance of the battery module assembly. Further, unlike conventional battery modules that use PCM or liquid filling materials for heat dissipation, the present subject matter uses a thermal pad interface for heat dissipation. The thermal pad interface is positioned between the first group of single cells, the second group of single cells, and the single cell retainer. Therefore, compared with traditional battery modules, the present subject matter enables the battery module assembly to be easily assembled. Further, the present subject matter also reduces the costs associated with the cooling of the battery module assembly, improves the handling of the battery module assembly, and enhances the safety of the battery module assembly.

[0040] Although the subject matter has been described in conjunction with specific embodiments, this description should not be construed as limiting. Various modifications of the disclosed embodiments as well as alternative embodiments of the subject matter will be apparent to those skilled in the art after reference to the description of the subject matter.

Claims

1. A battery module assembly (100), the battery module assembly (100) comprising: A first battery unit (101), wherein the first battery unit (101) comprises: At least one single battery unit (102), the at least one single battery unit (102) comprising: A first group of battery cells (202) and a second group of battery cells (204), wherein the first group of battery cells (202) and the second group of battery cells (204) each include a first plurality of battery cells connected to each other, and the first group of battery cells (202) and the second group of battery cells (204) are electrically connected to each other; and A first single cell holder (206), the first single cell holder (206) being used to hold the first group of single cells (202) and the second group of single cells (204), wherein the first single cell holder (206) comprises a first plurality of grooves (212), each groove (212) being designed to hold a single cell from the first plurality of single cells, The first group of single cells (202) is in the first single cell holder (206). The second group of single batteries (204) is arranged at a distance from each other; A first housing (104) enclosing the at least one battery cell (102); and A first thermal pad interface (226), wherein the first thermal pad interface (226) is positioned between the first group of single cells (202), the second group of single cells (204) and the first single cell retainer (206), and the first thermal pad interface (226) contacts the first shell (104) to transfer heat generated by the first plurality of single cells to the first shell (104), wherein the first thermal pad interface (226) includes a continuous first plurality of hollow portions (228).

2. The battery module assembly (100) according to claim 1, wherein: Each hollow portion (228) surrounds a single cell in the first plurality of single cells and is disposed between a groove in the first plurality of grooves (212) and a corresponding single cell (202, 204), and each hollow portion (228) contacts the corresponding single cell (202, 204).

3. The battery module assembly (100) according to claim 1, wherein: The shape of each groove in the first plurality of grooves (212) corresponds to the shape of the first group of battery cells (202) and the second group of battery cells (204).

4. The battery module assembly (100) according to claim 1, wherein: Each battery cell (202, 204), each groove in the first plurality of grooves (212), and each hollow portion (228) is one of the following shapes: cylindrical, brick-shaped, and flat rectangular.

5. The battery module assembly (100) according to claim 4, wherein: Each single cell (202, 204), each groove in the first plurality of grooves (212), and each hollow portion (228) are cylindrical, and wherein each groove in the first plurality of grooves (212) and each hollow portion in the first plurality of hollow portions (228) are coaxial with the corresponding single cell (202, 204).

6. The battery module assembly (100) according to claim 1, wherein: The first thermal pad interface (226) comprises: A first rib (304), the first rib (304) extending from a first end (302-1) of the first thermal pad interface (226), the first rib (304) being arranged in contact with the first shell (104) to transfer heat generated by the first plurality of battery cells to the first shell (104); and A second rib (306), the second rib (306) extending from a second end (302-2) of the first thermal pad interface (226), wherein the second end (302-2) is opposite to the first end (302-1), and wherein the second rib (306) is arranged in contact with the first shell (104) to dissipate the heat generated by the first plurality of single cells to the first shell (104).

7. The battery module assembly (100) according to claim 6, wherein: The first rib (304) and the second rib (306) are in one of the following shapes: a rectangular parallelepiped, wherein one face of the first rib and one face of the second rib are wavy; a cube; a cylinder; an ellipsoid; and a hemisphere.

8. The battery module assembly (100) according to claim 1, wherein: The first thermal pad interface (226) is made of a thermally conductive and electrically insulating material.

9. The battery module assembly (100) according to claim 1, wherein: The first thermal pad interface (226) is made of silicon-based material.

10. The battery module assembly (100) according to claim 1, wherein: The height of each hollow portion (228) is substantially the same as the height of the corresponding single cell (202, 204) disposed therein, so as to dissipate heat generated on the entire outer surface of the single cell (202, 204).

11. The battery module assembly (100) according to claim 1, wherein: The at least one single cell unit comprises a second single cell holder for holding the first group of single cells (202) and the second group of single cells (204), wherein the second single cell holder comprises a second plurality of grooves, each groove of the second plurality of grooves being designed to hold a single cell from the first plurality of single cells.

12. The battery module assembly (100) according to claim 1, comprising: A second battery unit, the second battery unit is electrically connected to the first battery unit (101) in one of a series connection and a parallel connection, and the second battery unit comprises: At least one single battery unit, the at least one single battery unit comprising: a third group of battery cells and a fourth group of battery cells (204), each of the third group of battery cells and the fourth group of battery cells comprising a second plurality of battery cells connected to each other, and the third group of battery cells and the fourth group of battery cells being electrically connected to each other; and A third single cell holder is used to hold the first three groups of cells and the fourth group of cells, wherein the third cell holder comprises a second plurality of recesses, each recess being configured to retain a cell from the second plurality of cells, wherein the third group of cells is disposed in the third cell holder at a distance from the fourth group of cells; a second housing that surrounds the at least one single battery cell; and A second thermal pad interface, wherein the second thermal pad interface is positioned between the third group of cells, the fourth group of cells and the third cell retainer, the second thermal pad interface contacts the second shell to transfer heat generated by the second plurality of cells to the second shell, wherein the second thermal pad interface includes a continuous second plurality of hollow portions.

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