Battery unit

Abstract

There is provided a battery unit. The battery unit comprises a housing; a battery cell inside the housing; and a thermally conductive layer arrangement to conduct heat from the battery cell to the housing. The thermally conductive layer arrangement comprises a primary conduction layer and a secondary conduction layer. A cross-plane thermal conductivity of the primary conduction layer is higher than an in-plane thermal conductivity of the primary conduction layer, and an in-plane thermal conductivity of the secondary conduction layer is higher than a cross-plane thermal conductivity of the secondary conduction layer.

Description

[0001] BATTERY UNIT

[0002] BACKGROUND

[0003] Many known appliances, such as handheld vacuum cleaners, are powered by battery units. A battery unit comprises one or more battery cells for providing electrical power to the appliance.

[0004] SUMMARY

[0005] According to a first aspect, there is provided a battery unit comprising: a housing; a battery cell inside the housing; and a thermally conductive layer arrangement to transfer heat from the battery cell to the housing; wherein the thermally conductive layer arrangement comprises a primary conduction layer and a secondary conduction layer; wherein a crossplane thermal conductivity of the primary conduction layer is higher than an in-plane thermal conductivity of the primary conduction layer; and wherein an in-plane thermal conductivity of the secondary conduction layer is higher than a cross-plane thermal conductivity of the secondary conduction layer.

[0006] Thermal conductivity is typically specified as a scalar physical property independent of direction, in which case a single number is used to quantify heat flow in any direction. Thermal conductivity can be dependent on direction, i.e. anisotropic, for example as a result of the microscopic structure of a thermal conductor or the macroscopic arrangement of a thermal conductor. In the context of anisotropic thermal conductivity, it is common to quantify directional thermal conductivity in terms of an in-plane thermal conductivity and a cross-plane thermal conductivity. The in-plane thermal conductivity quantifies thermal conductivity within a (geometric) plane of reference. The cross-plane thermal conductivity quantifies thermal conductivity in a transverse direction relative to the plane of reference, e.g. perpendicular thereto.

[0007] The battery unit according to the first aspect utilises a combination of conduction layers with comparatively high cross-plane thermal conductivity and comparatively high in-plane thermal conductivity, i.e. the primary conduction layer and the secondary conduction layer. The combination of the primary conduction layer and the secondary conduction layer in the battery unit may in use improve conduction of heat from the battery cell to the housing. For example, the secondary conduction layer may reduce formation of thermal hotspots on the housing, which refers to localised areas of peak temperature, when the battery unit is in use.

[0008] Examples of the primary conduction layer and the secondary conduction layer are described in detail below, which the skilled person will be able to implement. Here the following examples are briefly noted. An exemplary primary conduction layer is a thermal pad arranged so that its in-plane thermal conductivity is limited by air adjacent to the thermal pad while the cross-plane thermal conductivity is not so limited, meaning that the cross-plane thermal conductivity is higher than the in-plane thermal conductivity. An exemplary in-plane conduction layer is a pyrolytic graphite sheet, which is known to have a high in-plane thermal conductivity. Another example of an in-plane conduction layer is a metal clip (as described below) arranged so that its cross-plane thermal conductivity is limited by being interfaced with, e.g., the primary conduction layer or the housing of lower cross-plane thermal conductivity. As such, the skilled person will be able to arrange even a material with isotropic thermal conductivity to provide anisotropic thermal conductivity, for example by suitably arranging the material in the thermally conductive layer arrangement or by arranging the material adjacent to air.

[0009] It is noted that the primary conduction layer as well as the secondary conduction layer may be adjacent to air, e.g. with an an air gap around each layer. In use, heat received into the primary conduction layer at a location adjacent to air may to a greater extent flow crossplane, whereas heat received into the secondary conduction layer at a location adjacent to air may to a greater extent flow in-plane (e.g. as a result of a comparatively high in-plane thermal conductivity of the secondary conduction layer). As such, the secondary conduction layer may spread heat flow even when bounded by an air gap around the secondary conduction layer. The cross-plane thermal conductivity of the primary conduction layer and the cross-plane thermal conductivity of the secondary conduction layer may be with respect to the same direction. For example, the thermally conductive layer arrangement may define a primary heat flow direction. The primary conduction layer and the secondary conduction layer may be configured to conduct the heat in the primary heat flow direction according to their respective cross-plane thermal conductivities.

[0010] The in-plane thermal conductivity of the primary conduction layer and the in-plane thermal conductivity of the secondary conduction layer may be with respect to the same direction. For example, the primary conduction layer and the secondary conduction layer may be configured to conduct the heat in a secondary heat flow direction according to their respective cross-plane thermal conductivities. The secondary heat flow direction may be perpendicular to the cross-plane heat flow direction.

[0011] The thermally conductive layer arrangement may include at least one primary conduction layer. The at least one primary conduction layer may include at least one of a cell-side primary conduction layer and a housing-side primary conduction layer. The cell-side primary conduction layer may be interfaced with the battery cell. The housing-side primary conduction layer may be interfaced with the housing.

[0012] The thermally conductive layer arrangement may include at least one secondary conduction layer. The at least one secondary conduction layer may include at least one of a cell-side secondary conduction layer and a housing-side secondary conduction layer. The cell-side secondary conduction layer may be interfaced with the battery cell. The housingside secondary conduction layer may be interfaced with the housing.

[0013] Here, “interfacing” of a structure with another structure, such as interfacing the cell-side primary conduction layer and the battery cell, may mean arranging the structures in physical contact for heat conduction between the structures. Examples may include abutment of the structures; the structures being joined together by means of adhesive; one structure may be a coating on the other structure; one structure may be received into the other structure. The primary conduction layer and the secondary conduction layer may be interfaced.

[0014] Where the primary conduction layer and the secondary conduction layer are interfaced such that their cross-plane thermal conductivities are in the same direction and / or their inplane thermal conductivities are in the same direction, heat transfer from the battery cell to the housing may be particularly improved. For example, formation of hotspots at the housing may be particularly reduced as conduction layers may spread the heat flow towards the housing whilst conducting the heat flow towards the housing. Hotspots are regions of the housing where the housing temperature in use spikes.

[0015] Where the primary conduction layer and the secondary conduction layer are interfaced such that their cross-plane thermal conductivities are in the primary heat flow direction and said primary heat flow direction is towards the housing, this may particularly reduce formation of hotspots at the housing.

[0016] The battery cell may have a tab for electrical connection. The tab may be interfaced with the cell-side primary conduction layer.

[0017] When the battery unit supplies electrical power from the battery cell, the tab of the battery cell may generate heat as a result of electrical current therethrough. As such, interfacing the tab with the cell-side primary conduction layer may improve heat flow from the tab into the cell-side primary conduction layer.

[0018] The tab of the battery cell may be received into the cell-side primary conduction layer. For example, the tab may be surrounded by or encased in the cell-side primary conduction layer.

[0019] By receiving the tab into the cell-side primary conduction layer, heat flow from the tab into the cell-side primary conduction layer may be further improved. The thermally conductive layer arrangement may comprise the cell-side primary conduction layer and the cell-side secondary conduction layer. For example, the cell-side primary conduction layer may be interfaced with the tab of the battery cell, while the cellside secondary conduction layer may be interfaced with another portion of the battery cell.

[0020] The battery unit may comprise a further battery cell. The battery unit may comprise a plurality of battery cells, which may include the battery cell and the further battery cell.

[0021] The further battery cell may have a tab for electrical connection. The tab of the further battery cell may be interfaced with the cell-side primary conduction layer.

[0022] The tab of the battery cell may be welded to the tab of the further battery cell for electrical connection. For example, the tab of the battery cell may be welded to the tab of the further at an elongate welded portion. The elongate welded portion may be a continuous elongate welded portion.

[0023] When the battery unit supplies electrical power from the battery cell, electrical current between the battery cells via the tabs may generate heat at the tabs of the battery cell and the further battery cell. Interfacing the tab of the battery cell and the further battery cells with the cell-side primary conduction layer may improve heat flow from the tabs into the cell-side primary conduction layer.

[0024] The elongate welded portion may provide an improved physical connection between the tabs of the battery cell and the further battery cell compared to a physical connection resulting from one or multiple discrete spot welds. The improved physical connection may in use improve electrical connection between the tabs and reduce formation of hotspots on the tabs.

[0025] The elongate welded portion may be interfaced with, e.g. received into, the cell-side primary conduction layer. The improved physical connection provided by the elongate welded portion may, when received into the cell-side primary conduction layer, improve interfacing with the cell-side primary conduction layer for improved heat conduction into the cell-side primary conduction layer, for example by ensuring a close contact between the tabs joined by the elongate welded portion.

[0026] The battery unit may comprise a cell enclosure. The cell enclosure may comprise at least one enclosure clip and at least one enclosure plate to house the battery cell. The at least one enclosure clip and the at least one enclosure plate may be configured to cooperate to compress the battery cell or, where provided, the battery cell and the further battery cell or, where provided, the plurality of battery cells.

[0027] The enclosure clip may provide the secondary conduction layer.

[0028] The enclosure clip may be interfaced with the cell-side primary conduction layer.

[0029] The enclosure plate of the cell enclosure may provide the cell-side secondary conduction layer.

[0030] The thermally conductive layer arrangement may comprise at least two secondary conduction layers. The primary conduction layer may be arranged between and interfaced with the two secondary conduction layers. For example, the primary conduction layer may be arranged between and interfaced with the cell-side secondary conduction layer and the housing-side secondary conduction layer.

[0031] The thermally conductive layer arrangement may comprise at least two primary conduction layers.

[0032] One of the two secondary conduction layers may be arranged between and interfaced with the two primary conduction layers. For example, a first of the two secondary conduction layers may be arranged between the cell-side primary conduction layer and another primary conduction layer, and the other primary conduction layer may be arranged between the first and a second of the two secondary conduction layers. The second of the two secondary conduction layers may be a housing-side secondary conduction layer.

[0033] The or each secondary conduction layer may have a thickness which is less than a thickness of the primary conduction layer. The thicknesses of the conduction layers may be measured in the same direction.

[0034] The in-plane thermal conductivity of the or each secondary conduction layer may be at least 100 W / mK.

[0035] The cross-plane thermal conductivity of the or each secondary conduction layer may be at most 20% of the in-plane thermal conductivity of the or each secondary conduction layer.

[0036] The or each primary conduction layer may have a cross-plane thermal conductivity not exceeding 10 W / mK.

[0037] Where the cross-plane thermal conductivity of the primary conduction layer is lower than the cross-plane thermal conductivity of secondary conduction layer, this may promote heat flow in the secondary heat flow direction (e.g. where the primary conduction layer is farther from the battery cell and is interfaced with the secondary conduction layer). As such, a difference in cross-plane thermal conductivity of the primary conduction layer and the secondary conduction layer may reduce formation of thermal hotspots on the housing by making heat flow through the conductive layer arrangement in the primary heat flow direction more uniform.

[0038] The primary conduction layer may comprise any suitable material, which may include silicone / non-silicone thermal interface materials, such as silicone / non-silicone pads and silicone thermal gel. For example, where the cell-side primary conduction layer is to receive the tab of the battery cell (or the tabs of the battery cell and the further battery cell, or the tabs of the plurality of battery cells), the cell-side primary conduction layer may be a silicone thermal gel. The secondary conduction layer may comprise any suitable material, which may include metals, such as aluminium or pyrolytic graphite. For example, where the enclosure plate or the enclosure clip provides the secondary conduction layer, the secondary conduction layer may be made of metal, such as aluminium. For example, the housing-side secondary conduction layer may be a pyrolytic graphite sheet.

[0039] The thermally conductive layer arrangement may comprise the housing-side primary conduction layer and the housing-side secondary conduction layer. For example, the housing-side primary conduction layer and the housing-side secondary conduction layer may be interfaced with different portions of the housing.

[0040] The housing may comprise a heat dissipation arrangement for dissipation of the heat from the battery cell to ambient air.

[0041] The heat dissipation arrangement may comprise one or more protrusions, e.g. cooling fins, arranged on an outside surface of the housing.

[0042] The one or more protrusions may be arranged to form an airflow path about the housing between an underside of the housing and a topside of the housing. The underside of the housing may in use face downwards, whereas the topside of the housing may in use face upwards.

[0043] The housing may be aerodynamically shaped to promote laminar airflow about the housing from the underside of the housing to the topside of the housing, optionally via the airflow path. The housing may be aerodynamically shaped to promote laminar airflow about the housing along at least part of the topside of the housing.

[0044] The housing may be made from any suitable material, e.g. plastic or a thermally conductive polymer.

[0045] The housing may have a thermal conductivity which is less than the thermal conductivity of the conduction layer arrangement. For example, the thermal conductivity of the housing may be less than the in-plane and cross-plane thermal conductivity of each conduction layer.

[0046] The housing may have a melting point higher than 180 degrees Celsius.

[0047] According to a second aspect, there is provided an electrical appliance comprising a battery unit as described above.

[0048] BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure l is a perspective view of a conventional battery unit.

[0050] Figure 2 is an exploded view of the conventional battery unit.

[0051] Figure 3 is a first cross-sectional view of the conventional battery unit.

[0052] Figure 4 is a second cross-sectional view of the conventional battery unit.

[0053] Figure 5 is an illustration of an exemplary battery unit.

[0054] Figure 6 is a cross-sectional view of the exemplary battery unit.

[0055] Figure 7 is another cross-sectional view of the exemplary battery unit.

[0056] Figure 8 is another cross-sectional view of the exemplary battery unit.

[0057] Figure 9 is a plan view of a battery cell for use in the exemplary battery unit.

[0058] Figure 10 is a perspective view of the exemplary battery unit.

[0059] Figure 11 is a graph illustrating thermal performance of the exemplary battery unit.

[0060] Figure 12 is a perspective view of an electrical appliance comprising the exemplary battery unit.

[0061] DETAILED DESCRIPTION

[0062] For reference, Figures 1 to 4 illustrate a conventional battery unit 10 for an electrical appliance, such as a handheld vacuum cleaner. Figure l is a perspective view of the battery unit 10; Figure 2 is an exploded view; Figures 3 and 4 are cross-sectional views taken lengthwise and sideways. The battery unit 10 comprises a housing 11 (or ‘cover’). The housing 11 shown in Figures 1 to 4 has multiple housing portions 12, 13, 14 which, when assembled, define the housing 11. The housing 11 may generally comprise any number of housing portions.

[0063] The battery unit 10 comprises a plurality of battery cells 15 arranged inside the housing 11. The battery cells 15 may have any suitable shape and configuration. For example, the battery cells 15 may be provided as so-called stick cells of generally cylindrical shape. In this example, the battery cells 15 are provided as pouch cells which have a generally (flattened) cuboid shape, with two opposite major faces (providing upper and lower surfaces of each pouch cell as illustrated). The pouch cells are stacked on top of one another such that their major faces bear against one another. To enable electrical connection between the battery cells 15, each battery cell 15 includes two terminals 16 for electrical connection. In this example, the terminals 16 are arranged at opposite ends of the battery cell 15 and in the form of a tab which extends internally into its respective battery cell 15 so as to electrically connect to an internal structure of the battery cell 15.

[0064] The battery unit 10 comprises a cell enclosure 17 to house the battery cells 15. The battery cells 15 are partly enclosed by the cell enclosure 17. The cell enclosure 17 comprises two enclosure plates 18 and two enclosure clips 19 to house the battery cells 15. The enclosure plates 18 are spaced apart in a cell stack direction so as to define a (substantially cuboid) cell-receiving space therebetween, in which the battery cells 15 are received. The enclosure clips 19 are arranged on opposite sides of the cell-receiving space and engage the enclosure plates 18 to compress the battery cells 15 via the enclosure plates 18.

[0065] The battery unit 10 may also include further components, such as a printed circuit board assembly, e.g. to provide a battery management system.

[0066] It has been found that the conventional battery unit 10 has a relatively high thermal gradient towards the housing 11, indicating relatively low heat transfer from heat generating bodies, such as the battery cells 15, busbars, battery management system components. This may lead to relatively high internal temperatures, e.g. at the battery cells 15 and other heat generating bodies, and relatively low temperatures at the housing 11. In use, the battery cells 15 may reach a temperature of 75 degrees Celsius while, at the same time, the housing may reach a temperature of around 27 degrees Celsius. As a result, runtime and performance of the battery unit 10, especially at peak power, may be reduced. For example, the battery unit 10 may be configured to turn off upon reaching a cutoff temperature measured at an internal location, and may reach said cutoff temperature relatively quickly due to the low heat transfer to the housing 11.

[0067] Figure 5 is a schematic view of an exemplary battery unit 100. The battery unit 100 comprises a housing 110 and a battery cell 120 inside the housing 110.

[0068] The battery unit 100 comprises a thermally conductive layer arrangement 130 to conduct heat from the battery cell 120 to the housing 110. The thermally conductive layer arrangement 130 defines a primary heat flow direction 132 and a secondary heat flow direction 134. The primary heat flow direction 132 may be indicative of the shortest path from a given point on the battery cell 120 to the housing 110. As such, the primary heat flow direction 132 may be different when considering different points, e.g. on different sides, of the battery cell 120. The secondary heat flow direction 134 is a direction which is transverse to the primary heat flow direction 132, for example perpendicular thereto.

[0069] The thermally conductive layer arrangement 130 comprises a plurality of conduction layers 140, 150 which are configured to conduct the heat in the primary heat flow direction 132 and in the secondary heat flow direction 134. In this example, the conduction layers 140, 150 are sequentially arranged with respect to the primary heat flow direction 132.

[0070] The thermally conductive layer arrangement 130 is arranged such that the cross-plane thermal conductivity of each conduction layer 140, 150 is aligned with the primary heat flow direction 132, and the in-plane thermal conductivity is aligned with the secondary heat flow direction 134.

[0071] The thermally conductive layer arrangement 130 comprises at least one primary conduction layer 140 and at least one secondary conduction layer 150. The at least one primary conduction layer 140 has a cross-plane thermal conductivity and an in-plane thermal conductivity where the cross-plane thermal conductivity is higher than the in-plane thermal conductivity. Similarly, the least one secondary conduction layer 150 has a crossplane thermal conductivity and an in-plane thermal conductivity, but for at least one the secondary conduction layer 150 the in-plane thermal conductivity is higher than the crossplane thermal conductivity. In this example, the cross-plane thermal conductivity quantifies heat flow with respect to the primary heat flow direction 132, while the in-plane thermal conductivity quantifies heat flow with respect to the secondary heat flow direction 134. As such, the primary conduction layer 140 conducts more heat in the primary heat flow direction 132 than in the secondary heat flow direction 134, whereas the secondary conduction layer 134 conducts more heat in the secondary heat flow direction 134 than in the primary heat flow direction 134. In Figure 5, arrows indicate the heat flow in the primary conduction layer 140 and the secondary conduction layer 150.

[0072] In Figure 5, the at least one primary conduction layer 140 and the least one secondary conduction layer 150 are illustrated as separated from each other, and separated from both the housing 110 and the battery cell 120. However, the primary conduction layer 140 and the secondary conduction layer 150 may be interfaced with each other; and either conduction layer 140, 150 (or both) may be interfaced with the housing 110 and / or the battery cell 120.

[0073] In Figure 5, the at least one primary conduction layer 140 is positioned towards the housing 110, while the at least one secondary conduction layer 150 is positioned towards the battery cell 120. Other arrangements are also possible, such as the primary conduction layer 140 towards the battery cell 120 and the secondary conduction layer 150 towards the housing 110.

[0074] Other arrangements of the thermally conductive layer arrangement 130 may allocate the primary conduction layer 140 and the secondary conduction layer 150 to separate portions of the housing 110. For example, the secondary conduction layer 150 may be provided at a portion of the housing 110 which a user is more likely to contact, e.g. because a handle is provided thereat, while the primary conduction layer 140 may be provided at a different portion of the housing 110. This may provide a particularly spatially-efficient arrangement of the conduction layers 140, 150, whilst also improving user experience and operational safety by reducing formation of hotspots where users are likely to contact the housing 110.

[0075] With reference to Figures 6, 7 and 8, further examples of the thermal conductivity layer arrangement 130 are described.

[0076] Figure 6 is a cross-sectional view of part of the battery unit 100. In Figure 6, a portion of the frame 110, multiple stacked battery cells 120 and a cell enclosure 160 of the battery unit 100 are shown. The cell enclosure 160 comprises a pair of enclosure plates 162 (with one enclosure plate depicted in Figure 6), and a pair of enclosure clips 164.

[0077] In Figure 6, an underside of the battery unit 100 and, in particular, an underside 112 of the housing 110 is shown. The thermally conductive layer arrangement 130 is provided between the battery cells 120 and the underside 112 of the housing 110 to conduct heat towards the underside 112 of the housing 110. As such, the primary heat flow direction 132 is towards the underside 112 of the housing 110.

[0078] The at least one primary conduction layer 140 includes a housing-side primary conduction layer 142. The housing-side primary conduction layer 142 is interfaced with the underside 112 of the housing 110.

[0079] The at least one secondary conduction layer 150 includes a cell-side secondary conduction layer 152. The cell-side secondary conduction layer 152 is interfaced with at least one of the battery cells 120.

[0080] The cell-side secondary conduction layer 152 and the housing-side primary conduction layer 142 are interfaced. The cell-side secondary conduction layer 152 has a relatively high thermal conductivity, but heat flow across the cell-side secondary conduction layer 152 in the primary heat flow direction 132 is limited by the housing-side primary conduction layer 142. In particular, the housing-side primary conduction layer has a thermal conductivity in the primary heat flow direction 132 which is lower than that of the cell-side secondary conduction layer 152. As such, heat flow in the cell-side secondary conduction layer 152 in the secondary heat flow direction 134 is greater than in the primary heat flow direction 132.

[0081] The enclosure plate 162 of the cell enclosure 160 is the cell-side secondary conduction layer 152. Suitably, the enclosure plate 162 is made from a suitable material with high thermal conductivity exceeding that of the housing-side primary conduction layer 152. For example, the enclosure plate 162 may be made from aluminium, while the housing-side primary conduction layer 152 may be made from a material with lower thermal conductivity.

[0082] The housing-side primary conduction layer 152 may be chosen from materials which exhibit suitable thermal conductivity and pliability to accommodate the expansion of the battery cells 120, such as silicone / non-silicone thermal pads.

[0083] Figure 7 is a cross-sectional view of part of the battery unit 100. In Figure 7, a portion of the frame 110, multiple stacked battery cells 120 and the cell enclosure 160 of the battery unit 100 are shown. The cell enclosure 160 comprises a pair of enclosure plates 162 (with two enclosure plates 162 depicted, and one enclosure clip 164 depicted in Figure 6).

[0084] In Figure 7, a side of the battery unit 100 and, in particular, a side 114 of the housing 110 is shown. The thermally conductive layer arrangement 130 is provided between the battery cells 120 and the side 114 of the housing 110 to conduct heat flow towards the side 114 of the housing 110. As such, the primary heat flow direction 132 is towards the side 114 of the housing 110.

[0085] As shown in Figure 7, the at least one primary conduction layer 140 includes a cell-side primary conduction layer 144 and an intermediate primary conduction layer 146. The cellside primary conduction layer 144 is interfaced with at least one of the battery cells 120. The intermediate primary conduction layer 146 is positioned between and interfaced with two of the at least one secondary conduction layers 150. As shown in Figure 7, the at least one secondary conduction layer 150 includes an intermediate secondary conduction layer 154 and a housing-side secondary conduction layer 156. The intermediate secondary conduction layer 152 is positioned between and interfaced with the cell-side primary conduction layer 144 and the intermediate primary conduction layer 146. The intermediate primary conduction layer 146 is positioned between and interfaced with intermediate secondary conduction layer 154 and the housingside secondary conduction layer 156.

[0086] The housing-side secondary conduction layer 156 may be thin, e.g. having a thickness of up to 1mm or up to 0.1mm, as measured in the primary heat flow direction 132. Suitable material choices, such as pyrolytic graphite sheets, may be provide sufficient in-plane thermal conductivity whilst allowing for a thin secondary layer.

[0087] As already noted, the intermediate secondary conduction layer 156 is positioned between and interfaced with the cell-side primary conduction layer 144 and the intermediate primary conduction layer 146. The intermediate secondary conduction layer 156 has a comparatively high thermal conductivity, such that heat flow across the intermediate secondary conduction layer 156 in the primary heat flow direction 132 is limited by the cross-plane thermal conductivity of the intermediate primary conduction layer 146. As such, the heat flow in the intermediate secondary conduction layer 156 in the secondary heat flow direction 134 is greater than in the primary heat flow direction 132.

[0088] The enclosure clip 164 of the cell enclosure 160 is the intermediate secondary conduction layer 156. Suitably, the enclosure clip 164 is made from a suitable material with high thermal conductivity exceeding that of the intermediate primary conduction layer 146. For example, the enclosure clip 164 may be made from aluminium, while the intermediate primary conduction layer 146 may be made from a material with lower thermal conductivity.

[0089] The enclosure clip 164, which provides the intermediate secondary conduction layer 156, is resilient to compress the stack of battery cells 120 and accommodates for expansion of the battery cells 120 in use. Figure 8 is a cross-sectional view of part of the battery unit 100. In Figure 8, a portion of the frame 110 and the tabs of multiple stacked battery cells 120 are shown.

[0090] In Figure 8, an end of the battery unit 100 and, in particular, an end 116 of the housing 110 is shown. The thermally conductive layer arrangement 130 is provided between the battery cells 120 and the end 116 of the housing 110 to conduct heat flow towards the end 116 of the housing 110. As such, the primary heat flow direction 132 is towards the end 116 of the housing 110.

[0091] As shown in Figure 8, the at least one primary conduction layer 140 includes a second cellside primary conduction layer 148. The second cell-side primary conduction layer 148 is interfaced with at least one of the battery cells 120. More particularly, in this example tabs 122 of the battery cells 120 are received into the second cell-side primary conduction layer 148. It is noted that the second cell-side primary conduction layer 148 may be provided additionally to the cell-side primary conduction layer 144 or alternatively to the cell-side primary conduction layer 144.

[0092] As shown in Figure 8, the at least one secondary conduction layer 150 includes a second housing-side secondary conduction layer 158. The second housing-side secondary conduction layer 158 is interfaced with the end 116 of the housing 110. It is noted that the second housing-side secondary conduction layer 158 may be provided additionally to the housing-side secondary conduction layer 156 or alternatively to the housing-side secondary conduction layer 156.

[0093] The second housing-side secondary conduction layer 158 is provided as a pyrolytic graphite sheet which has a comparatively high in-plane thermal conductivity and a comparatively low cross-plane thermal conductivity, where the in-plane thermal conductivity relates to heat flow in the secondary heat flow direction 134 and the crossplane thermal conductivity relates to heat flow in the primary heat flow direction 132. The tabs 122 of the battery cells 120 are pairwise joined for electrical connection between adjacent battery cells 120. More particularly, the tabs 122 are welded together at an elongate welded portion 124 (shown in Figure 9), and the elongate welded portion 124 is received into the cell-side primary conduction layer 148.

[0094] Figure 9 is a view of part of the battery cell 120 and, in particular, shows the tab 122 for electrical connection. Said tab 122 is joined to the tab 122 of an adjacent battery cell 120 by the elongate welded portion 124 indicated in Figure 9. The elongate welded portion 124 may be made, for example, by laser-welding together the tabs 122.

[0095] Figure 10 is a perspective view of the housing 110. The housing 110 comprises a heat dissipation arrangement 118 for dissipation of the heat from the battery cells 120 to ambient air. The heat dissipation arrangement comprises multiple protrusions 119, which are provided as cooling fins, arranged on an outside surface of the housing 110.

[0096] Adjacent protrusions 119 are arranged to form an airflow path about the housing 110 therebetween, the airflow path extending from the underside of the housing to the topside of the housing 110. As such, the housing 110 is aerodynamically shaped to promote laminar airflow about the housing 110 from the underside to the topside.

[0097] Figure 11 is a graph illustrating measured battery cell temperature over time for the battery unit 100 in use.

[0098] In Figure 11, four lines are depicted. Line L0 (dashed) corresponds to a baseline temperature measurement without the thermally conductive layer arrangement 130, for example as performed on the conventional battery unit 10.

[0099] Lines LI, L2 and L3 correspond to various configurations of the thermally conductive layer arrangement 130. More particularly, line LI corresponds to an example where the thermally conductive layer arrangement 130 includes conduction layers arranged to improve heat flow from the battery cells 120 to the cell enclosure 160; line L2 corresponds to an example where the thermally conductive layer arrangement 130 additionally includes conduction layers arranged to improve heat flow from the cell enclosure 160 to the housing 110; line L3 corresponds to an example wherein additionally heat dissipation arrangement 118 is provided. Figure 12 is a perspective view of an electrical appliance 1000 including the battery unit 100. In this example, the appliance 1000 is a handheld vacuum cleaner powered by the battery unit 100.

Claims

CLAIMS1. A battery unit comprising: a housing; a battery cell inside the housing; and a thermally conductive layer arrangement to conduct heat from the battery cell to the housing; wherein the thermally conductive layer arrangement comprises a primary conduction layer and a secondary conduction layer; wherein a cross-plane thermal conductivity of the primary conduction layer is higher than an in-plane thermal conductivity of the primary conduction layer; and wherein an in-plane thermal conductivity of the secondary conduction layer is higher than a cross-plane thermal conductivity of the secondary conduction layer.

2. The battery unit of claim 1, wherein the primary conduction layer is a cell-side primary conduction layer interfaced with the battery.

3. The battery unit of claim 2, wherein the battery cell has a tab for electrical connection and the tab is interfaced with the cell-side primary conduction layer.

4. The battery unit of claim 3, further comprising a further battery cell having a tab for electrical connection; wherein the tab of the battery cell is welded to the tab of the further battery cell for electrical connection between the battery cell and the further battery cell; and wherein the tab of the further battery cell is interfaced with the cell-side primary conduction layer.

5. The batery unit of claim 4, wherein the tab of the battery cell is welded to the tab of the further battery cell at an elongate welded portion and the elongate welded portion is interfaced with the cell-side primary conduction layer.

6. The battery unit of any one of claims 2 to 5, further comprising a cell enclosure which comprises at least one enclosure clip and at least one enclosure plate to house the battery cell; wherein the enclosure clip provides the secondary conduction layer; and wherein the enclosure clip is interfaced with the cell-side primary conduction layer.

7. The battery unit of claim 1, wherein the primary conduction layer is a housing-side primary conduction layer interfaced with the housing.

8. The battery unit of any of claims 1 to 5 or 7, wherein the secondary conduction layer is a cell-side secondary conduction layer interfaced with the battery cell.

9. The batery unit of claim 8, further comprising a cell enclosure which comprises at least one enclosure clip and at least one enclosure plate to house the battery cell; wherein the enclosure plate provides the cell-side secondary conduction layer.

10. The battery unit of any of claims 1 to 7, wherein the secondary conduction layer is a housing-side secondary conduction layer interfaced with the housing.

11. The batery unit of any of claim 1, wherein the thermally conductive layer arrangement comprises at least two secondary conduction layers; and wherein the primary conduction layer is arranged between and interfaced with the two secondary conduction layers.

12. The battery unit of claim 11, wherein the thermally conductive layer arrangement comprises at least two primary conduction layers; and one of the two secondary conduction layers is arranged between and interfaced with the two primary conduction layers.

13. The battery unit of any preceding claim, wherein the secondary conduction layer has a thickness which is less than a thickness of the primary conduction layer.

14. The battery unit of any preceding claim, wherein the in-plane thermal conductivity of the secondary conduction layer is at least 100 W / mK; and wherein the cross-plane thermal conductivity of each secondary conduction layer is at most 20% of the in-plane thermal conductivity of the secondary conduction layer.

15. The battery unit of any preceding claim, wherein the primary conduction layer has a cross-plane thermal conductivity not exceeding 10 W / mK.

16. The battery unit of any preceding claim, wherein the housing comprises a heat dissipation arrangement for dissipation of the heat from the battery cell to ambient air; wherein the heat dissipation arrangement comprises cooling fins arranged on an outside surface of the housing.

17. The batery unit of claim 16, wherein the cooling fins are arranged to form an airflow path about the housing between an underside of the housing and a topside of the housing.

18. The batery unit of claim 17, wherein the housing is aerodynamically shaped to promote laminar airflow about the housing from the underside of the housing to the topside of the housing via the airflow path and along at least part of the topside of the housing.

19. The battery unit of any one of claims 1 to 18, wherein the housing is made from plastic or a thermally conductive polymer; wherein the housing has a thermal conductivity which is less than the thermal conductivity of the conduction layer arrangement; and wherein the housing has a melting point higher than 180 degrees Celsius.

20. An electrical appliance comprising a battery unit of any one of claims 1 to 19.

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