A battery enclosure

Abstract

A battery 100 comprising one or more pouch cells 102 and a pouch cell enclosure 101 comprising a pair of plates 104 spaced apart in a stack and moveable relative to one another, and a biasing arrangement engaged with at least one of the plates to urge the plates together. At least one of the plates is a compression plate comprising one or more first regions in contact with the biasing mechanism and one or more second regions having a lower stiffness than the first region(s). The second region(s) may not engage with the biasing apparatus. The compression plate(s) may be non-planar in a natural state, being compressed to planar upon assembly, or thicker in the first region(s) than the second, or planar on an outer surface (1974, Fig. 19) and non-planar on an inner surface (1975, Fig. 19), or made of a fibre-reinforced composite, or form a wave-like profile. The varying stiffness may be because of varying material properties. The biasing arrangement may comprise a plurality of spring elements 108, possibly extending along opposite edges 106 of each compression plate.

Description

BACKGROUND Batteries, such as those used in consumer devices, are typically formed of a plurality of battery cells (or just “cells”). Battery cells (such as lithium-ion cells) come in various formats. Known battery cells often have a cylindrical shape or a rectangular (or cuboid) shape. One type of rectangular battery cell that is increasingly being used in devices is a pouch cell. A pouch cell includes a laminated battery architecture contained within a flexible (i.e. non-rigid) pouch, which is commonly formed of a plastic-coated aluminium film. Tabs are provided at one end (or two opposite ends) of the pouch cell to provide terminals that allow electrical connection of the pouch cell to other pouch cells in a pouch cell stack or to other electrical components of a device. The use of a pouch rather than a rigid housing (as is the case with other types of battery cell) reduces the overall weight and volume of the cell. Likewise, the rectangular (or cuboid) shape provides a more efficient use of space than a cylindrical shape when multiple cells are packaged together, (i.e. in a cell stack) The flexible nature of the pouch of a pouch cell, however, means that an enclosure must be provided to protect the pouch cell (or a stack of pouch cells) of a battery against damage. One characteristic of pouch cells (such as lithium-ion pouch cells) is that each charge and discharge cycle of the pouch cell can result in expansion and contraction of the pouch cell. Likewise, for performance reasons, it is desirable to maintain compression of pouch cells through each charge and discharge cycle. Thus, pouch cells must be packaged in a way that accommodates this expansion and contraction while maintaining compression. To achieve this, pouch cell stacks are typically packaged in a rigid enclosure with foam provided between each cell, and between the outermost cells and the enclosure (i.e. the foam it typically arranged in series with the pouch cells). As the cells expand, the foam is compressed, which provides a reactionary force to maintain compression of the cells. SUMMARY Generally, the disclosure provides a battery including a compression plate that is non-uniform (e.g. in thickness, material properties, and / or shape), the non-uniformity being such that a substantially uniform pressure distribution is provided by the compression plate on pouch cells in response to a non-uniform force (i.e. a non-evenly distributed force, such as may be applied by a biasing arrangement) applied on an opposite side of the compression plate. In a first aspect, there is provided a battery comprising: one or more pouch cells; and a pouch cell enclosure comprising: a pair of plates spaced apart in a cell stack direction so as to define a cellreceiving space therebetween in which the one or more pouch cells are received, the plates being moveable relative to one another in the cell stack direction; and a biasing arrangement in engagement with at least one of said plates to urge it towards the other plate of the pair; wherein at least one of the plates is a compression plate comprising one or more first regions within which the biasing arrangement engages the compression plate and one or more second regions having a lower stiffness than the one or more first regions. It is desirable to maintain a compressive force on pouch cells received within an enclosure. This is achieved in the battery of the first aspect by the plates and the biasing arrangement that urges the plates together. One issue with compressing pouch cells in this way, however, is that it can result in an uneven distribution of pressure on the pouch cells. This is because more pressure may be applied at locations where the biasing arrangement engages the plates, which can result in local deformation of the plates. As may be appreciated, to maintain an even distribution of pressure it can be desirable to maintain the plates (or at least surfaces of the plates engaging the pouch cells) as planar as possible. This could be achieved by selecting a plate thickness that resists any deformation, but this can increase the weight and overall size of the battery. By providing at least one of the plates (i.e. the at least one compression plate) with a stiffness that varies in different regions, a more even distribution of pressure can be applied to the pouch cells of the battery while minimising the weight and size of the plates. In particular, configuring the plate to be stiffer in regions where the biasing arrangement engages the plate, can help to limit deformation of the plate in those regions (which would otherwise be particularly susceptible to such deformation due to the engagement by the biasing arrangement). The term “stiffness” refers to the extent to which a region resists deformation in response to a given force. Thus, stiffness of a region of the compression plate can be determined by applying a given force to the region (in the compression direction) and measuring the amount of deformation of the region (i.e. in the direction of compression) in response to the force. References to variations of stiffness (across the compression plate) herein are references to the stiffness varying in a plane that is substantially parallel to major faces of the plate. That is, the regions are regions in a plane substantially parallel to the major faces of the plate (as opposite to e.g. the stiffness varying in a direction of the thickness of the plate). Optional features of the first aspect are set out below. These can be combined with any other aspect (including optional features of such an aspect) described herein. The stiffness may be a stiffness in response to a compressive force. That is, the stiffness (i.e. as referred to herein) may be the ability of the respective region to resist deformation in response to a force directed in cell stack direction towards the other of the plates (i.e. a compression direction). Thus, the one or more second regions may have a lower stiffness in the compression direction than the one or more first regions. The stiffness referred to herein may be an in-use (or in-situ) stiffnesses. That is, such stiffness may be present (in some cases, only present) when the compression plate is assembled as part of the battery (and is thus under the force of the biasing arrangement). As will be described further below, at least some optional embodiments rely on the plate being deformed (e.g. bent) from its natural state in order to generate regions of different resistance to deformation. As should be appreciated, in such embodiments, the plate may not necessarily have regions of different stiffness when it is in an unassembled (i.e. predeformed) state. In-use (or in-situ) stiffness may be determined, for example, using finite element analysis (FEA). Such in-use stiffness may also be determined by, for example, by compressing the compression plate (in an unassembled position) between two stiff planar plates. Pressure sensing pads can be provided between each stiff planar plate and a respective side of the compression plate to measure the reactionary force of the compression plate. The pressures measured by the pressure sensing pads can be used to determine regions of first (e.g. where the reactionary force is higher) and regions of low stiffness (i.e. where the reactionary force is lower). The stiffnesses may be an inherent stiffnesses. That is, the high and low stiffness regions may be present even when the compression plate is in its natural state. In this case, the stiffness may be measured, for example, in a more traditional manner (e.g. by applying a force to each region and measuring the deflection resulting from the applied force). The engagement of the compression plates involves the transfer of force from the biasing arrangement to the compression plate (i.e. the force urging the compression plate towards the other of the plates). Thus, for example, engagement may be in the form of direct contact between the biasing arrangement and the compression plate. Alternatively, the engagement may be indirect, via an intermediate element transfers the force from the biasing arrangement to the compression plate. Even further, engagement may be provided by an integral connection (for example, the compression plate may be integral with the biasing arrangement), in which case the engagement occurs at the integral j oin between the compression plate (which performs the compression function) and the biasing arrangement. For completeness, the term “biasing arrangement” not only encompasses the feature that directly generates the compressive force but can also encompass features that transfer that force to the respective compression plate. The one or more second regions may be arranged such that the biasing arrangement does not engage the compression plate within the one or more second regions. Thus, regions of the plate that are not engaged by the biasing arrangement may be lower weight and / or volume. These regions may be more susceptible to deformation due to their lower stiffness, but because they are not engaged by the biasing arrangement such deformation is less likely to occur in use. Accordingly, the weight and / or volume of the battery may be minimised without significant detriment to the evenness of the pressure distribution on the pouch cells. In some embodiments the pair of plates may both be compression plates. Each plate may comprise one or more first regions within which the biasing arrangement engages the respective plate and one or more second regions. Each second region may have a lower stiffness than the one or more first regions. The second regions may be arranged such that the biasing arrangement does not engage the respective plate therein. For conciseness only one compression plate will be referred to below. It should be appreciated, however, that where both plates are compression plates they may both be as described below (i.e. one or both of the plates may take any one or more of the optional features described below). The compression plate may have a substantially rectangular shape. Thus, the compression plate may have at least first and second opposite edges. The compression plate may have opposite major faces, (i.e. parallel to the plane of the plate). Various distributions of first and second regions can be provided on the compression plate. Likewise, the first and second regions may have various shapes. At least one first region may be elongate. For example, the compression plate may comprise a first region extending along an edge of the plate. The at least one first region may extend for substantially the entire edge of the plate. The at least one first region may be inwardly spaced from the edge of the plate or may be at the edge of the plate. The compression plate may comprise two first regions, each extending along a respective opposite edge of the plate (optionally inwardly spaced from the edge of the plate). A low stiffness region may be provided between the two first regions. Other distributions of first and second regions may be provided. For example a first region may be provided that extends substantially fully about the periphery (e.g. four edges) of the compression plate so as to surround a second region. Alternatively, discrete first regions may be distributed across the compression plate. Variable stiffness (i.e. in first and second regions) may be introduced into the compression plate in various ways. In particular, the compression plate may, for example, vary (i.e. in a direction across the compression plate) in thickness, material properties, and / or may be non-planar in its unassembled state to provide varying stiffness. That is, the compression plate may: have an increased thickness in the one or more first regions; have a greater material stiffness in the one or more first regions; and / or in its natural state, be non-planar in the one or more first regions (and may be deformed from its natural state when assembled). A first way to achieve variable stiffness (as mentioned above) is to provide a compression plate that, when in an assembled state, is deformed from its natural shape. In such embodiments, the compression plate may be resilient or elastic (may be formed of a resilient / elastic material). The compression plate may comprise e.g. steel (e.g. spring steel), aluminium, plastic (e.g. polycarbonate), fibre-reinforced plastic, etc. The compression plate may be non-planar in its natural, unassembled, state. In particular, the compression plate may be non-planar in the one or more first regions. The compression plate may be compressed by the biasing arrangement to be substantially planar (or closer to planar) when assembled in the pouch cell enclosure. Thus, the compression plate may be considered a biasing member that is under compression when in the assembled state (i.e. the bias of the compression plate being such that it counteracts the bias of the biasing arrangement (at least in the first regions)). Accordingly the higher stiffness of the one or more first regions (i.e. the resistance of each region to deformation) may be a result of the elastic or resilient nature of the plate and the fact that it is deformed from its natural state when assembled in the plate. The one or more first regions may provide increased resistance to deformation in the compression direction because in attempting to return to their original shape the one or more first regions may generate a force that opposes a force in the compression direction. The compression plate may, in its unassembled state, extend out of plane in the one or more first regions. The compression plate may be arranged in the battery such that the first regions are biased in a direction away from the other of the plates. In other words, the compression plate may be arranged such that, in the absence of the biasing arrangement, the one or more first regions would extend outwardly in a direction away from the pouch cells (and away from the other of the plates). The compression plate may be curved in its unassembled state. The compression plate may be curved about a single axis (e.g. may be bowed). In such a compression plate, first regions may be formed at opposite edges of the plate when the plate is assembled. The compression plate may be curved about two axes that are normal to one another. The compression plate may comprise an undulating profile (e.g. may be wave shaped). Thus, the compression plate may comprise a profile that has one or more crests interspersed between troughs (e.g. a trough interposed between two crests). In this case, first regions may be formed as e.g. elongate regions along the crests of the plate when the compression plate is assembled. A second way of providing the compression plate with a variation in stiffness is by varying the thickness of the compression plate between the first and second regions. Thus, for example, the compression plate may be thicker in regions engaged by the biasing arrangement (i.e. may be thicker in the one or more first regions than in the one or more second regions). The thickness of the plate may change gradually between the one or more first regions and the one or more second regions. That is, there may be a smooth transition between each of the one or more thicker regions of the compression plate and the one or more thinner regions of the compression plate. The difference between the minimum thickness of the compression plate and the maximum thickness of the compression plate may be e.g. between 0.1 mm and 0.8 mm, or e.g. between 0.2 and 0.5 mm. In some embodiments, the compression plate may comprise a first (i.e. major) surface that is substantially planar and an opposite second (major) surface that is non-planar (i.e. resulting variations in thickness of the compression plate). For example, the compression plate may comprise a substantially planar inner surface facing the pouch cells, and an opposite outer non-planar surface (to provide the greater thickness in the one or more first regions than in the one or more second regions). The second (outer) surface may have a concave profile or e.g. an undulating profile (i.e. wave-shaped profile). The compression plate may comprise a layered structure. One or more of the layers may vary in thickness to provide greater thickness in the one or more first regions than in the one or more second regions. For example, the compression plate may comprise a metal layer (e.g. a metal plate) and a rubber layer (e.g. a rubber pad). The material plate may be of substantially constant thickness. The rubber layer may, on the other hand, have one or more regions that are thicker than one or more other regions (the one or more thicker regions providing the first regions of the plate). The thickness of the plate may vary in a direction extending between opposite edges of the plate. For example, the maximum thickness of the compression plate may be at a location that is at, or near to, an edge of the plate. The minimum thickness of the compression plate may be at a location that is at or near to a centre of the compression plate. Such arrangements may be suitable where the biasing arrangement engages the compression plate near to or at its edges. The compression plate may have a thicker (i.e. first) region that is elongate and extends along an edge of the plate. The thicker region may be inwardly spaced from the edge of the plate. The compression plate may comprise two thicker regions, each extending along a respective opposite edge of the plate. Thus, the thickness of the compression plate may taper inwardly in a direction from opposite edges of the plate towards a central axis of the plate between the opposite edges (such tapering may be gradual). A third way of providing the compression plate with first and second regions is to provide different material properties in different regions of the plate. Thus, the material properties of the compression plate may vary between the one or more second regions and the one or more first regions to provide greater stiffness in the one or more first regions than in the one or more second regions. In one example, the compression plate may comprise a fibre-reinforced composite material. The one or more first regions may have a different fibre architecture to the one or more second regions. For example, the compression plate may comprise a greater density of fibres in the one or more first regions compared to the one or more second regions. The compression plate may comprise fibres of a first type in the one or more first regions and fibres of a second type (different to the first type) in the one or more second regions. The fibres of the first type may have a different length, thickness, shape, and / or may be formed of a different material to the fibres of the second type. Likewise, fibres in the one or more first regions may have a first orientation and fibres in the one or more second regions may have a second orientation (that is different to the first orientation). The fibres of the reinforced composite may, for example, comprise carbon fibres, glass fibres, polymer fibres, ceramic fibres, and mixtures thereof. As set forth above, the compression plate may have a layered structure. Thus, the composite material may form one layer of the compression plate. The compression plate may further comprise e.g. a metal layer (e.g. a metal plate) overlying (or underlying) the composite material. It should be appreciated that a compression plate having first and second regions may be provided by a combination of two or more of varying thickness, varying material properties or a non-planar plate (in the unassembled configuration) as described above. In some embodiments, the biasing arrangement may comprise a plurality of spring elements arranged to engage an outer surface of the compression plate to urge the compression plate towards the other plate of the pair of plates. The compression plate may comprise a plurality of first regions arranged such that each spring element engages (i.e. directly or indirectly, as discussed above) the compression plate in a respective first region of the plurality of first regions. In some embodiments, the biasing arrangement may connect the pair of plates (for example, as described in the first aspect). The biasing arrangement may comprise first and second resilient spring members extending along first and second opposite edges of the compression plate. Each spring member may, for example, be as described in the first aspect (e.g. and may include one or more of the optional features of the spring members of the first aspect). The compression plate may comprise a primary first region in which the first resilient spring member engages the compression plate, and a secondary first region in which the second resilient spring member engages the compression plate. As noted above, the primary and secondary first regions may be elongate and may extend respectively along the first and second edges of the or each compression plate. A second region may thus be provided between the primary and secondary first regions. In such embodiments, the first regions may be provided by a compression plate that, in its natural, unassembled state, is bowed about an axis extending substantially parallel to first and second edges of the compression plate. In other words, the compression plate (in its unassembled state) may have a profile, taken perpendicular to the first and second edges, that is concave. When assembled, the compression plate may be deformed to become more planar (i.e. may be flattened). One result of this is that regions at the edges of the compression plate (i.e. the extremities of the concave profile) may be more resistant to deformation in a direction towards the other of the plates (i.e. they will be stiffer). In some embodiments, the primary and secondary first regions may respectively extend substantially parallel to, and inward of, the first and second edges of the compression plate. In such embodiments, the compression plate may (in its natural, unassembled state) comprises a wave-shaped or undulating profile taken perpendicular to the first and second edges. The primary first region and secondary first regions may be provided by crests of the wave-shaped profile. The compression plate may comprise two crests spaced that extend along (but are spaced inwardly from) the first and second edges. The compression plate may comprise a trough between the two crests. In a second aspect there is provided a battery comprising: one or more pouch cells; and a pouch cell enclosure comprising: a pair of plates spaced apart in a cell stack direction so as to define a cellreceiving space therebetween in which the one or more pouch cells are received, the plates being moveable relative to one another in the cell stack direction; and a biasing arrangement in engagement with at least one of said plates to urge the plate towards the other plate of the pair of plates; wherein at least one of the plates is a compression plate that is engaged by the biasing arrangement in one or more first regions, and wherein: the compression plate has an increased thickness in the one or more first regions; the compression plate has a greater material stiffness in the one or more first regions; and / or the compression plate, in its natural state, is non-planar in the one or more first regions. The battery of the second aspect may be as otherwise described with respect to the first aspect. That is, the battery of the second aspect may include one or more features of the first aspect, including one or more optional features of the first aspect. For the avoidance of doubt, the term “material stiffness” describes the inherent stiffness of the material from which the respective region of the compression plate is formed. In a third aspect there is disclosed a method of forming a battery of the first or second aspect, the method comprising providing deforming the compression plate upon assembly of the compression plate in the battery, wherein the one or more first regions are deformed to a greater extent than the one or more second regions. Optional features of the third aspect will now be set out. These are applicable singly or in any combination with any aspect. The deformation may be in the form of bending. The step of deforming the compression plate may be such that the compression plate becomes more planar (i.e. flatter). For the avoidance of doubt, the compression plate (and the remaining features of the battery) may be as described in the first aspect. In a fourth aspect there is disclosed a pouch cell enclosure comprising: first and second compression members spaced apart in a cell stack direction so as to define a cell-receiving space therebetween for receipt of one or more pouch cells, the first and second compression members being moveable relative to one another in the cell stack direction; and a biasing arrangement connecting the first and second compression members and configured to urge the first and second compression members towards one another. The pouch cell enclosure of the fourth aspect can be of reduced dimension in the cell stack direction compared to known arrangements that make use of foam for compression as discussed above. One problem that is inherent in the use of foam is that it is only able to compress to a certain strain for a given cell expansion. In other words, at maximum compression of the foam (being maximum expansion of the cell(s)), the foam still has some residual thickness. This residual thickness must be accounted for in the overall dimension of the battery in the cell stack direction, resulting in a larger battery (at least in the cell stack direction). This issue is exacerbated by the fact that pouch cell batteries typically contain a plurality of cells in a stack with several layers of foam. Another potential problem with the use of foam is that at least some types of foam material are susceptible to decomposition in thermal runaway events. A thermal runaway event occurs when a battery reaches an elevated temperature, causing a chain reaction that ultimately results in a very rapid rise in the temperature of the battery. Not only is decomposition of the foam undesirable but, in some cases, the use of foam between cells can also increase the likelihood of reaching those elevated temperatures because it can act as an insulator. The arrangement of the fourth aspect removes the need to use foam in the enclosure. This can result in an enclosure of reduced dimension in the cell stack direction (because the enclosure does not need to accommodate the additional foam layers). Likewise, the removal of foam can reduce the likelihood of a cell reaching the elevated temperatures required for a runaway event, because (in the absence of the foam layers) heat is able to be transferred from the cell to any surrounding cells and / or the enclosure. Accordingly, the pouch cell enclosure of the fourth aspect may be more compact (at least in the cell stack direction) and may be less likely to undergo a thermal runaway event. Optional features of the fourth aspect will now be set out. These are applicable singly or in any combination with any aspect. The biasing arrangement may be disposed at a periphery of the cell-receiving space. The biasing arrangement may be disposed at a periphery of the first and / or second compression members. The provision of the biasing arrangement at the periphery of the compression members rather than in series with the pouch cells may facilitate a reduction in the overall dimension of the pouch cell enclosure in the cell stack direction. The pouch cell enclosure may comprise first and second spaced lateral sides. The first and second lateral sides may extend between spaced ends of the pouch cell enclosure. One or both of the ends of the pouch cell enclosure may be at least partly open, providing for access to the tabs (i.e. terminals) of pouch cells when received in the cell-receiving space. The lateral sides and the ends of the enclosure may extend between the spaced apart compression members. In one orientation, for example, the compression members may provide upper and lower walls of the pouch cell enclosure (and the cell stack direction may be substantially vertical). The pouch cell enclosure may have a substantially cuboid shape. For the avoidance of doubt, the terms “lateral”, “upper” and “lower” are not intended to require the pouch cell enclosure to be used in any particular orientation. The biasing arrangement may comprise a resilient spring member. The spring member may have a first end connected to the first compression member and a second end connected to the second compression member. The spring member may therefore span the cell-receiving space or may at least partly define a boundary (e.g. a lateral boundary) of the cell-receiving space. The spring member may extend at least partly along respective peripheries of the first and second compression members. The spring member may extend at least partly along a periphery of the cell-receiving space (and e.g. along respective peripheries of the one or more pouch cells when received therein). For example, when the pouch cell enclosure comprises lateral sides, the spring member may extend partly along a lateral side of the pouch cell enclosure. The spring member may extend for substantially the entirety of a lateral side of the pouch cell enclosure (and may e.g. extend beyond at least one end of the lateral side so as to be longer than the lateral side). The spring member may extend for substantially the entirety of the length of a pouch cell when received in the cell-receiving space. In this respect, the spring member may define a wall or boundary of the pouch cell enclosure (e.g. aiding in enclosing pouch cell(s) within the cell-receiving space). In such embodiments (whether the wall or boundary extends for an entire side), the spring member may provide dual functionality: providing a protective wall of the pouch cell enclosure while also urging the compression plates towards one another. The spring member may be configured to flex between an expanded configuration and a contracted configuration. In the contracted configuration, the first and second ends of the spring member may be closer to one another in the cell stack direction than when the spring member is in the expanded configuration. The spring member may be biased towards the contracted configuration. The contracted configuration may, for example, be a natural configuration of the spring member (i.e. may represent the natural shape of the spring member). Flexing of the spring member may comprise bending of the spring member in a plane that is substantially perpendicular to the direction of extension of the spring member along the periphery of the cell-receiving space (e.g. perpendicular to a direction along the peripheries of the compression members). For, example when the pouch cell enclosure includes lateral sides, the flexing of the spring member may comprise bending of the spring member in a plane that extends perpendicularly to the lateral sides. Such bending may be promoted by the shape of the spring member. Thus, for example, the spring member may have a thickness dimension that is significantly smaller than length and height dimensions. The height dimension may be taken in a direction between the compression members (the cell stack direction). The width dimension may be taken in a direction along the peripheries of the compression plates. The thickness may be a dimension taken between outer and inner surfaces of the spring member (the outer surface facing away from the cell-receiving space and the inner surface facing towards the cell-receiving space). Thus, the spring member may, for example, be in the form of a sheet material (e.g. a sheet metal). The spring member may be formed of a thermally conductive material (e.g. copper or aluminium). In this respect, the spring member may be configured to transfer heat away from pouch cells received in the cell-receiving space. In some embodiments, the spring member may be formed of a composite material (e.g. a composite sheet material). The spring member may be formed of a plurality of layers. For example, the composite sheet material may comprise a thermally and / or electrically conductive layer that is combined with a resilient layer. As an example, the spring member may be formed of steel sheet (e.g. spring steel sheet) and may be lined e.g. with a copper layer. In another example, the spring member may be formed of carbon fibre of glass fibre. In some embodiments, the spring member may be formed of an electrically conductive material. In some embodiments, the spring member may be formed of a thermally and / or electrically insulating material. The spring member may comprise a body. The body may be a portion of the spring member that extends between the first and second compression members. The spring member may be configured such that the body has concave profile (in at least the contracted configuration). In some embodiments, the body may have a substantially serpentine profile (or another profile that permits collapsing of the body, such as a corrugated or concertina profile). The body may alternatively be substantially planar (e.g. may have a linear profile). For the avoidance of doubt, the term profile is used here to describe a cross-sectional shape, the cross section being taken along a plane perpendicular to the direction of extension of the spring member along the peripheries of the compression members and / or cell-receiving space. The body may be configured to flex such that a radius of curvature of the (e.g. concave) profile of the body of the spring member is greater in the expanded configuration than in the contracted configuration. In some embodiments, the body may be expandable from the contracted configuration to a position in which the body is substantially planar (e.g. the body may be substantially planar in the expanded configuration). In some embodiments the body may be expandable from the contracted configuration to a position in which the body is convex towards the cell-receiving space. Accordingly, the body may sit more closely to the compression members and the cell-receiving space when expanded (e.g. in the expanded configuration) than would otherwise be the case if the body were to be concave / curved. This may help minimise the volume of the pouch cell enclosure in use. The spring member may comprise an arm extending laterally from the body. The arm may engage the first compression member. The arm may define the first end of the spring member. The arm may be configured to pivot towards and away from the body when the spring member flexes between the contracted and expanded configurations. The arm may be closer to the body in the contracted configuration than in the expanded configuration. The arm may be biased towards the contracted configuration. An arm extension angle a may be defined between the body and the arm (of the spring member). For the avoidance of doubt, the arm extension angle a is an internal angle (not the external angle) between the arm and the body. The arm may be configured to bend relative to the body so that the arm extension angle a is larger in the expanded configuration than in the contracted configuration. As above, the arm may be biased towards the contracted configuration. The arm extension angle a may be an acute angle (e.g. in one or both of the contracted and expanded configurations). In the expanded configuration, the arm extension angle a may be approximately 90 degrees (e.g. may be between 70 and 120 degrees or between 80 and 100 degrees). In some embodiments, the arm may be a first arm and the spring member may comprise a second arm extending laterally from the body. The second arm may engage the second compression member. The second arm may define the second end of the spring member. The second arm may function in the manner described with respect to the first arm. Thus, the second arm may be configured to pivot towards and away from the body when the spring member flexes between the contracted and expanded configurations. The second arm may be closer to the body in the contracted configuration than in the expanded configuration. The second arm may function in the manner described above with respect to the first arm. A second arm extension angle P may be defined between the body and the second arm. The second arm may be configured to bend relative to the body so that the second arm extension angle P is larger in the expanded configuration than in the contracted configuration. The second arm may be biased towards the contracted configuration. The second arm extension angle P may be an acute angle (e.g. in one or both of the contracted and expanded configurations). In the expanded configuration, the arm extension angle a may be approximately 90 degrees. The spring member may have a substantially C-shaped profile (e.g. the body providing the spine of the “C”, and the arms providing the two lateral projections). In this way, the spring member may be configured to wrap around a side of the pouch cell enclosure (e.g. so as to define a lateral wall of the enclosure). At least one of the first and second arms may engage the respective compression plate and an engagement region that is inward of a lateral edge of the compression plate. The engagement region may, for example, be a distance from the lateral edge that is between 20% and 40% of a width of the compression plate (e.g. about a third of the way across the compression plate in the lateral direction). In embodiments in which the second arm is not present, the second end of the spring member may instead be engaged with, or may be integral with, the second compression member. The pouch cell enclosure may comprise a strut (e.g. plate) that extends from one lateral side of the enclosure to another lateral side of the enclosure across the cell-receiving space. The strut may be arranged such that, in use, it is received between two pouch cells in a stack of pouch cells received in the cell-receiving space. The strut may be substantially parallel to the first and second compression members. The body of the spring member may comprise a strut engagement portion for retaining the strut. The strut engagement portion may be a recess for engagement with a corresponding protrusion or edge of the strut. The spring member may comprise one or more shock absorbing elements (e.g. integrally formed as part of the spring member). For example, the spring member may comprise one or more tabs projecting out of plane from the spring member. The one or more tabs may project from the body of the spring member (e.g. laterally outwardly from the body). The spring member may comprise one or more resilient protrusions for engaging a portion of a device in which the enclosure may be received. The one or more resilient protrusions may, for example, project outwards from the first or second arm. The one or more resilient protrusions may be integrally formed as part of the spring member. Each of the one or more resilient protrusions may, for example, be in the form of a lance bridge feature. In some embodiments, the enclosure may comprise a plurality of spring members (i.e. each as described above). For example, the enclosure may comprise a plurality of spring members spaced along the periphery (e.g. a lateral side) of the pouch cell enclosure. When the enclosure includes opposite lateral sides, the enclosure may include two spring members, one provided on each of the lateral sides (or each lateral side may be provided with a plurality of spring members spaced therealong). In some embodiments, the spring member may be configured to flex between the expanded and contracted configurations by bending in a plane that is substantially parallel to the direction of extension of the spring member along the periphery of the cell-receiving space (e.g. parallel to the peripheries of the compression members). For, example when the pouch cell enclosure includes lateral sides, the flexing of the spring member may comprise bending of the spring member in a plane that is parallel to the lateral sides. The spring member may comprise a flexible structure extending between the first and second ends of the spring member. The flexible structure may be collapsible in the stack direction to provide movement of the spring member between the expanded and contracted positions. The flexible structure may be provided with apertures (or openings) therethrough, arranged to promote bending of the flexible structure as described above. The flexible structure may comprise a first elongate member extending obliquely from the first end of the spring member towards the second compression plate. The spring structure may further comprise a second elongate member extending obliquely from the second end of the spring member towards the first compression plate. The first and second elongate members may join intermediate the first and second ends (and intermediate the first and second compression members) to form an elbow that flexes in a hinged manner to permit movement of the flexible structure (and thus spring member) between the contracted and expanded configurations. The spring member may comprise two such flexible structures. The two flexible structures may be arranged such that they partly overlap. The two flexible structures may be reversed in orientation with respect to one another (i.e. one may be a mirror image of the other). In this way, the elbows of the two flexible structures may point in opposite directions along the periphery of the cell-receiving space. Alternatively, the flexible structure may be in the form of a lattice. That is, the flexible structure may comprise a plurality of interconnected elongate elements that extend obliquely between the first and second ends of the spring member. The elongate element may, for example, be arranged to form a plurality of connected triangular units. In further embodiments, the flexible structure may, for example, comprise a plurality of spaced apart serpentine elements extending between the first and second ends of the spring member. As may be appreciated, in each of the above-described embodiments, a plurality of spring members (i.e. each as described above) may be provided. For example, the plurality of spring members may be spaced along a periphery of the pouch cell enclosure (e.g. spaced along respective peripheries of the first and second compression members). Where the pouch cell enclosure (and e.g. each compression member) has opposite lateral sides, at least one spring member may be provided on each lateral side. Each lateral side may comprise a spring member that extends for substantially the entire length of the lateral side. In this way, the pouch cell enclosure may comprise two spring members forming opposite lateral side walls thereof. In some embodiments, the biasing arrangement may be configured to provide a substantially constant restoring force as it is expanded (e.g. by expansion of the pouch cells). As may be appreciated, this may translate to the provision of a substantially constant amount of compression being applied to pouch cells received in the cell-receiving space regardless of the state of expansion of the pouch cells. In other words, the biasing arrangement may be configured to move between expanded and contracted configurations, with the biasing arrangement being biased towards the contracted configuration (in which the first and second compression members are closer together in the stack direction than in the expanded configuration). Such a biasing arrangement may be configured to provide a substantially constant restoring force as it is moved from the contracted configuration to the expanded configuration. The biasing arrangement may comprise a spring (e.g. a leaf spring or compression spring) connected to one of the first and second compression members and a cam surface connected to the other of the first and second compression members. A portion of the spring may move across the cam surface as the biasing arrangement is moved from the contracted configuration to the expanded configuration. The biasing arrangement may comprise a spring (e.g. a leaf spring or compression spring) connected to one of the first and second compression members and a pivotably mounted linkage that is connected to the other of the first and second compression members. The spring may be engaged by the linkage as the biasing arrangement is moved from the contracted configuration to the expanded configuration so as to be bent and / or compressed by pivoting of the linkage. In one example, the biasing arrangement may comprise first and second portions. The first portion may extend from the first compression member towards the second compression member. The first portion may have a proximal end at the first compression member and an opposite distal end (i.e. distal from the first compression member). The second portion may extend from the second compression member towards the first compression member. The second portion may have a proximal end at the second compression member and an opposite distal end (i.e. distal from the second compression member). The biasing arrangement may comprise a spring element arranged between the distal end of the first portion and the distal end of the second portion. The spring element may be biased to urge the distal ends of the first and second portions away from one another in the cell stack direction. The spring element may be a leaf spring. The spring element may have a first end at (e.g. mounted to or integral with) the distal end of the second portion. The spring element may extend from the first end to a free second end in contact with the distal end of the first portion. The distal end of the first portion may comprise a cam surface. The free second end of the spring element may move across the cam surface as the biasing arrangement is moved from the contracted configuration to the expanded configuration. Such movement may cause deflection of the spring element. For example, the spring element (e.g. leaf spring) may be caused to hinge about the first end of the spring element. In this way, the second end of the spring element may be deflected in a direction away from the second portion (and / or the first portion). The cam surface may be convex. The cam surface may be shaped such that as the biasing arrangement is moved from the contracted configuration to the expanded configuration the deflection of the leaf spring caused by the cam surface provides a substantially constant restoring force. The spring element may be a first spring element and the biasing arrangement may comprise a second spring element having a first end mounted to the distal end of the second portion and a second free end in contact with the distal end of the first portion. The first and second spring elements may be provided on opposite sides of the first and / or second portions. The cam surface may be a first cam surface and a second cam surface may be provided with which the free second end of the second spring element contacts. The first and second portions and the spring member(s) may define a biasing device of the biasing arrangement. As may be appreciated, the biasing arrangement may comprise a plurality of such biasing devices, which may be spaced along a periphery of the cellreceiving space. In some embodiments, when the pouch cell enclosure comprises lateral sides, the biasing devices may be spaced along one or both lateral sides (i.e. connecting respective lateral sides of the compression members). In some embodiments, the biasing arrangement may comprise an elastic member arranged to urge the first and second compression members towards one another. In some embodiments, as will now be described, the second compression member may form a portion of the elastic member. The elastic member may, for example, comprise a laterally-extending portion that extends laterally across an interior of the pouch cell enclosure and this laterally-extending portion may define the second compression member. The elastic member may comprise end portions (either side of the laterally-extending portion) that connect the laterally- extending portion to the first compression member. The cellreceiving space may be defined between the laterally-extending portion and the first compression member. The pouch cell enclosure may comprise lateral sidewalls (to which the end portions may be fixed). Each sidewall may comprise an aperture through which the elastic member extends. Each aperture may be sized to allow movement of the laterally-extending portion of the elastic member in the cell-stack direction. In any of the above-described embodiments, at least one of the first and second walls may comprise a fastener for mounting an external component to the first or second wall. The fastener may comprise a head pressed into the first wall such that the first or second wall is deformed around the fastener to retain the fastener in the first or second wall. The fastener may comprise a shaft extending from the head so as to project from, and beyond, the outer surface of the first or second wall. The shaft or head of the fastener may comprise (e.g. radially or axially extending) teeth. Deformed material may be received between the teeth. The shaft or head of the fastener may comprise a recess extending circumferentially (e.g. fully) around the shaft or head. Deformed material may be received in the recess. The shaft of the fastener may be threaded. The head of the fastener may be fully received within the first or second wall (i.e. may not project therefrom). In any one of the above-described embodiments of the fourth aspect, at least one of the compression members may be in the form of a compression plate as is described in the first aspect. Thus at least one of the compression members may be in the form of a compression plate comprising one or more first regions within which the biasing arrangement engages the compression plate and one or more second regions, having a lower stiffness than the one or more first regions. Likewise, the variation in stiffness may be provided by providing the compression plate in a deformed state when assembled, providing a compression plate of varying thickness and / or providing a compression plate of varying material properties (e.g. varying material stiffness). Similarly, in any one of the above-described embodiments of the fourth aspect, at least one of the compression members may be in the form of a compression plate as is described in the second aspect. For example, in embodiments where the biasing arrangement (e.g. spring member) engages an edge of the compression plate, the compression plate may comprise a first region (or engagement region) that extends along the edge of the compression plate. This could be provided by a compression plate that, in its natural unassembled state, is bowed about an axis extending substantially parallel to the edge. Likewise, in embodiments where the biasing arrangement (e.g. spring member) engages the compression plate inwardly of an edge of the compression plate, the first region (or engagement region) may be elongate in a direction along the edge (and may be inward of the edge of the compression plate). In such embodiments, for example, the first region may be provided by a compression plate with a wave-shaped (i.e. undulating) profile in its natural unassembled state. A crest of the wave-shaped profile may provide the elongate first region. In variations where the biasing arrangement extends along two opposite edges (e.g. spring members extending along two opposite edges) a first region as described above may be provided along each edge (i.e. either at or inward of the edge). Of course, as set forth above, one or more first regions may be as otherwise described in the first aspect or second aspect. In a fifth aspect, there is disclosed battery comprising: one or more pouch cells; and a pouch cell enclosure comprising: first and second compression members spaced apart in a cell stack direction so as to define a cell-receiving space therebetween in which the one or more pouch cells are received, the first and second compression members being moveable relative to one another in the cell stack direction; and a biasing arrangement connecting the first and second compression members and configured to urge the first and second compression members towards one another. The battery of the fifth aspect is advantageous for the same reasons as discussed above with respect to the first aspect. Optional features of the fifth aspect will now be set out. These are applicable singly or in any combination with any aspect. The pouch cell enclosure of the fifth aspect may be as described above with respect to the fourth aspect and may include one or more of the optional features described above with respect to the fourth aspect. Likewise, the battery of the fifth aspect may be as described with respect to the first or second aspect (e.g. the at least one of the first and second compression members may be a compression plate as described with respect to the first or second aspect). Each pouch cell may have opposite first and second major faces, which may respectively face towards the first and second compression members. The major faces of each pouch cell may be substantially perpendicular to the cell stack direction. The layers of an internal layered structure of each pouch cell may be substantially parallel to the major faces of the pouch cell. Each pouch cell may comprise opposite lateral sides (e.g. aligned with the lateral sides of the enclosure) and may comprise opposite ends. Each pouch cell may comprise at least two terminals, which may be at the same end or at opposite ends of the pouch cell. In a sixth aspect there is disclosed an appliance comprising the battery of the first, second or fourth aspects. Optional features of the sixth aspect will now be set out. These are applicable singly or in any combination with any aspect. The appliance may be, for example, a vacuum cleaner. The appliance may comprise an appliance housing, which may house the battery. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a perspective view of a pouch cell enclosure according to a first embodiment; Figure IB is a perspective view of a spring member of the pouch cell enclosure of Figure 1A; Figure IC is a front view of the spring member of Figure IB; Figure 2 is a section view of a pouch cell enclosure according to a second embodiment; Figures 3A and 3B are section views of a pouch cell enclosure according to a third embodiment; Figure 4 is a perspective view of a spring member for a pouch cell enclosure according to a fourth embodiment; Figure 5 is a section view of part of a pouch cell enclosure according to a fifth embodiment; Figure 6 is a side view of a pouch cell enclosure according to a sixth embodiment; Figure 7 is a perspective view of a spring member for a pouch cell enclosure according to a seventh embodiment; Figure 8 is a perspective view of a spring member for a pouch cell enclosure according to an eighth embodiment; Figure 9A is a perspective view of a pouch cell enclosure according to a ninth embodiment; Figure 9B is a section view of a biasing device of the pouch cell enclosure of Figure 9 A; Figure 10 is a perspective view of a biasing device of a pouch cell enclosure according to a tenth embodiment; Figures 11 A, 11B, 11C and 11D provide variations of the biasing device according to an eleventh embodiment; Figure 12A is a section view of a pouch cell enclosure according to a twelfth embodiment; Figure 12B is a side view of the pouch cell enclosure of Figure 12A; Figure 13 is a schematic top view of a compression plate according to a thirteenth embodiment; Figure 14 is a schematic top view of a compression plate according to a fourteenth embodiment; Figure 15 is a schematic top view of a compression plate according to a fifteenth embodiment; Figure 16 is a schematic top view of a compression plate according to a sixteenth embodiment; Figure 17A and 17B are section views of a battery in an unassembled state and an assembled state according to a seventieth embodiment; Figure 18 is a front view of a compression plate according to an eighteenth embodiment; Figure 19 is a section view of a battery according to a nineteenth embodiment; Figure 20 is a section view of a battery according to a twentieth embodiment; Figure 21 is a section view of a battery according to a twenty-first embodiment. DETAILED DESCRIPTION Figure 1A depicts a battery 100 that includes a pouch cell enclosure 101 and a plurality of (in this case four) pouch cells 102 stacked within the pouch cell enclosure 101. Each pouch cell 102 includes an internal layered structure (which provides the function of the battery) housed with a flexible pouch, and two tabs 103 that act as terminals so as to allow electrical connection to the internal layered structure. In the presently illustrated embodiment, each pouch cell 102 has a generally rectangular shape, so as to include two opposite lateral sides extending between two opposite ends. The two terminals 103 of each pouch cell 102 are provided at the opposite ends of the pouch cell 102. Each pouch cell 102 also as two opposite (substantially rectangular) major faces, and the pouch cells 102 are stacked so that their major faces are in contact. The pouch cell enclosure 101 includes first 104a and second 104b substantially parallel compression members (only one of which is visible) spaced apart in a cell stack direction (vertical as illustrated) so as to define a cell-receiving space 105 therebetween for receipt of the pouch cells 102. Each compression member 104a, 104b is a substantially rectangular and planar plate, so as to have two opposite lateral sides 106 that extend between two opposite ends 107 of the compression member 104a, 104b. The cell stack direction extends in a direction that is normal to each of the first 104a and second 104b compression members. As illustrated, the first compression member 104a provides an upper wall of the pouch cell enclosure 101 and the second compression member 104b provides a lower wall of the pouch cell enclosure 101 (it should be appreciated, however, that the pouch cell enclosure 101 may be used in any orientation). The pouch cells 102 are stacked within the cell-receiving space 105 so as to be held between the first 104a and second 104b compression members. An inner surface of the first compression member 104a contacts an uppermost pouch cell 102 of the stack of pouch cells 102, and an inner surface of the second compression member 104b contacts a lowermost pouch cell 102 of the stack of pouch cells 102. The first 104a and second 104b compression members are moveable relative to one another in the cell stack direction. In this way, the first 104a and second 104b compression members are able to move as the pouch cells 102 expand and contract during use. As has been explained further above, however, it can also be beneficial to maintain some compression of the pouch cells 102 throughout such expansion and contraction. To provide this compressive force, the pouch cell enclosure 101 further comprises a biasing arrangement in the form of two spring members 108, connecting the first 104a and second 104b compression members. Each spring member 108 comprises a first end 109 that engages the first compression member 104a and a second end 110 that engages the second compression member 104b. The spring members 108 are configured to urge the first 104a and second 104b compression members towards one another (i.e. in the cell stack direction) to apply a compressive force to the pouch cells 102 in the cell-receiving space 105. As should be apparent from Figure 1A, each spring member 108 extends along a periphery of the cell-receiving space 105, and peripheries of the first 104a and second 104b compression members. In particular, each spring member 108 extends substantially entirely along the respective lateral sides 106 of the first 104a and second 104b compression members (although in other embodiments, each spring member 108 may extend only partway along a respective lateral side 106). As a result, the pouch cell enclosure 101 has a substantially cuboid shape, with each spring member 108 forming a lateral side of the enclosure 101 (i.e. providing side walls of the enclosure 101). In effect, the spring members 108 thus provide dual functionality: they provide the biasing force that urges the compression members 104a, 104b together, while also acting as walls to protect the pouch cells 102 held within the cell-receiving space 105. Ultimately, this can reduce the volume of the battery 100 (i.e. increasing energy density of the battery 100). Figures IB and IC illustrate one of the spring members 108 of the battery 100. As may be appreciated, both spring members 108 of the battery 100 are identical but are arranged in reversed orientation. Figure IB shows the underside of the spring member 108 (i.e. the spring member 108 is oriented with the second arm 114b provided on top and the first arm 114a on the bottom). The spring member 108 is configured to flex between an expanded configuration (as is shown in Figure 1A) and a contracted configuration (as is shown in Figures IB and IC). In the contracted configuration, the first 109 and second ends 110 of the spring member 108 are closer to one another in the cell stack direction than when the spring member 108 is in the expanded configuration. The spring member 108 is biased towards the contracted configuration (i.e. the contracted configuration is the natural configuration of the spring member 108). The flexing of the spring member 108 is facilitated by its shape. The spring member 108 is formed of sheet metal, such that a thickness dimension (between inner 111 and outer 112 surfaces) of the spring member 108 is much smaller than height and length dimensions of the spring member 108. The spring member 108 also has a substantially uniform cross-sectional shape along its length. The spring member 108 includes a body 113, and first 114a and second 114b arms that extend laterally from the body 113 to respectively engage the first 104a and second 104b compression members (in particular, outer surfaces of the first 104a and second 104b compression members). In this way, the spring member 108 has a C-shaped profile and acts like a clip that wraps around a side of the battery 100 (as per Figure 1A). The body 113 and arms 114a, 114b flex in use as the pouch cells 102 expand and contract causing the spring member 108 to move between the expanded and contracted configurations. In particular, each arm 114a, 114b pivots (or hinges) towards and away from the body 113 when the spring member 108 moves between the contracted and expanded configurations. In this way, each arm 114a, 114b is closer to the body 113 when the spring member 108 is in the contracted configuration. In other words, an arm extension angle a defined between the body 113 and each arm 114a, 114b is larger in the expanded configuration than in the contracted configuration. The body 113 of the spring member 108 also flexes in use. In particular, the body 113 flexes between having a substantially concave profile (as shown in Figure IC) in the contracted position, to having substantially linear profile (as per Figure 1A). In other words, the radius of curvature of the (e.g. concave) profile of the body 113 of the spring member 108 is greater in the expanded configuration than in the contracted configuration. Accordingly, the body 113 can sit more closely to the pouch cells 102 when expanded. As should be apparent from the figures, the flexing of the spring member 108 (including flexing of the arms 114a, 114b and the body 113) comprises bending in a plane that is substantially perpendicular to the length direction of the spring member 108 (i.e. a plane that extends in a direction laterally across the battery 100). Thus, the bending generally includes movement of the body 113 of the spring member 108 towards and away from the pouch cells 102 in the cell-receiving space 105. Flexing of each spring member 108 in this way ensures that the pouch cells 102 remain compressed as they expand and contract in use. This can prevent, for example, delamination of the internal layers of each pouch cells 102. Each spring member 108 also includes further features that help to protect the pouch cells 102. For example, each spring member 108 includes two shock absorbing tabs 116 that project laterally outwardly from the body 113 of the spring member 108 to act as shock absorbers for lateral impact. Each tab 116 is formed as a cut-out from the sheet metal of the respective spring member 108 that has been bent out of plane. Each spring member 108 also includes four outwardly projecting resilient protrusions 117 on the second arm 114b thereof, each protrusion 117 formed as a bridge lance feature (i.e. an out of plane bridge connected at both ends to the second arm 114b). As may be appreciated, such protrusions 117 may additionally or alternatively be provided on the first arm 114a. These protrusions 117 engage a housing of a device in which the battery 100 is received in use to help retain the battery 100 in position within the housing. Although not illustrated, it should be appreciated that each spring member 108 may further include shock absorbing tabs (or other shock absorbing feature) that bend to absorb shock loading that is applied in a direction along a length of the spring member 108. Such shock absorbing tabs could, for example, extend obliquely from side edges 62 of the spring member 108. Figure 2 shows a battery 200 that is variation of the battery 100 described above (and shown in Figures 1A, IB and IC). Similar reference numerals have been used for corresponding features. The illustrated battery 200 includes a strut 218 that extends across the cell-receiving space 205 between the first 214a and second 214b compression members. The strut 218 is a plate that is substantially parallel to the first 214a and second 214b compression members and that is disposed between two pouch cells 202 of the stack of pouch cells 202. The strut 218 is therefore in contact (but may be in indirect engagement via e.g. an adhesive layer) with two of the pouch cells 202 (and, in particular, is in contact with substantially the entirety of a respective major face of each of those pouch cells 202). As is apparent from Figure 2, lateral sides 219 of the strut engage each of the two spring members 208. To enable this, each spring member 208 comprises an elongate strutreceiving recess 220 (i.e. a groove) extending in a direction along a length of the spring member 208 (into the page as illustrated). The lateral sides 219 of the strut 218 are received in the strut-receiving recess 220 of each spring member 208. The strut 218 aids in supporting the pouch cells 202 within the cell-receiving space 205. Figures 3A and 3B illustrate part of battery 300 that is a further variation of the embodiment shown in Figures 1A, IB and IC. Again, similar reference numerals have been used to identify similar features. Figure 3A shows the battery 300 in a compressed or contracted configuration and Figure 3B shows the battery 300 in an expanded configuration (in which the pouch cells 302 have expanded). In this variation of the battery 300, each spring member 308 (only one of which is shown) includes a single arm 314 extending laterally from the body 313. Instead of providing a second arm, the body 313 is provided with a hook-shaped engagement member 321 that defines the second end 310 of the spring member 308 and that that engages a recess 322 of the second compression member 304b. As may be appreciated, in other embodiments the second compression member 304b and spring member 308 may be integrally formed (i.e. may be a unitary piece). Figure 4 illustrates an embodiment in which the spring member 408 is, again, formed of sheet metal, and bends in a plane perpendicular to the length of the spring member 408 (i.e. a laterally extending plane), but that includes apertures 423 to promote such bending. In particular, the spring member 408 includes a plurality of apertures 423 spaced along a length thereof, which form a plurality of elongate (and resilient) portions 424 connecting first 409 and second 410 ends of the spring member 408, which are defined by elongate bars 425 extending in the length direction (i.e. extending along lateral sides of the compression members when connected thereto). Each elongate portion 424 is curved between the first 409 and second 410 ends to further promote bending thereof. Although not illustrated, it should be appreciated that the first 409 and second 410 ends of the spring member 408 may connect to first and second compression member via various engagement means (including e.g. arms as described above). The spring member 508 of Figure 5 (forming part of a pouch cell enclosure 501) is, instead, able to bend by way of its serpentine profile. The spring member 508, which connects first 514a and second 514b compression members, includes a plurality of curves alternating in direction so as to form a serpentine profile that promotes bending (i.e. the curves allowing the spring member 508 to collapse and expand in the cell stack direction). As should be apparent from the figure, the curves on the inner side 526 of the spring member 508 (closer to the cell-receiving space 505) have a smaller radius of curvature than those on the outer side 527 of the spring member 508. This can provide more (vertical) space on the inner side 526 of the spring member 508 for receipt of portions of the pouch cells (when received in the cell-receiving space 505), which may allow a reduction in overall volume of the pouch cell enclosure 501. The battery 600 of Figure 6 differs from those previously described in that each spring member 608 (the battery 600 includes two spring members 608 on opposite lateral sides thereof, but only one is shown) is configured to flex between the expanded and contracted configurations by bending in a plane that is substantially parallel to the direction of extension of the spring member 608 along the periphery of the cell-receiving space 605. In other words, each spring member 608 bends in a plane that extends along a lateral side of the battery 600. Each spring member 608 includes two flexible structures 628 that each extend between the first 609 and second 610 ends of the spring member 608 and that are collapsible in the stack direction to provide movement of the spring member 608 between the expanded and contracted positions. Each flexible structure 628 includes first 629a and second 629b elongate members. Each first elongate member 629a extends obliquely from the first end 609 of the respective spring member 608 towards the second compression plate 604b. The second elongate member 629b extends obliquely from the second end 610 of the spring member 608 towards the first compression plate 604a. The first 629a and second 629b elongate members join intermediate the first 609 and second 610 ends (and intermediate the first 604a and second 604b compression members) to form an elbow 630 that flexes in a hinged manner to permit movement of each flexible structure 628 between the contracted and expanded configurations. The two flexible structures 628 of each spring member 608 are arranged in a reversed manner such that they overlap and their respective elbows 630 point in opposite directions along the lateral sides of the battery 600. In this way, the first elongate members 629a of the two flexible structures 628 extend across one another and the second elongate members 629b of the two flexible structures 628 extend across one another. Each flexible structure 628, at each end thereof, may comprise a laterally extending connecting member 631 (extending into the page as illustrated) that extends laterally across the outer surface of a respective compression member 604a, 604b. Each connecting member 631 may connect a respective flexible structure 628 to a flexible structure 628 provided on the opposite lateral side of the battery 600 (i.e. forming part of the other of the two spring members 608). In this way, the spring members 608 may form a cage that extends about the enclosure 601 and provides compression of the pouch cells 602 by urging the compression members 604a, 604b towards one another. Figure 7 illustrates a further embodiment in which the spring member 708 is configured to bend in a plane that is parallel to the length direction of the battery. In this case, the flexible structure 728 of the spring member 708 (which is formed as a unitary piece) comprises a plurality of interconnected elongate elements 732 (only some of which are labelled for clarity) that extend obliquely between the first 709 and second 710 ends of the spring member 708. The elongate elements 732 form a plurality of connected triangular units such that the flexible structure 728 has a lattice formation. Elongate bars 725 extend in the lengthwise direction of the spring member 708 at the first 709 and second 710 ends (the flexible structure 728 extending between these elongate bars 725). Elongate slots 733 are spaced along each bar 725, which allows for connection of the spring member 708 to compression plates in use. Figure 8 illustrates yet another embodiment in which the spring member 808 is configured to bend in a plane that is parallel to the length direction of the battery. The spring member, which is formed as a unitary piece 808, of this embodiment includes a flexible structure 828 provided by a plurality of spaced apart serpentine elements 834 extending between the first 809 and second 810 ends of the spring member 808. These bend to permit expansion and contraction of the spring member 808. The serpentine elements 834 are connected within an obround-shaped peripheral element 835. The obround element 835 includes elongate bars 825 forming the first 809 and second 810 ends of the spring member 808 and oppositely arranged arcuate (semi-circular) connecting elements 836 that connect the elongate bars 825. Figures 9A and 9B illustrate a further embodiment in which the pouch cell enclosure 901 comprises a biasing arrangement configured to provide a substantially constant restoring force as it is expanded (e.g. by expansion of the pouch cells). As may be appreciated, this can give rise to the provision of a substantially constant amount of compression being applied to the pouch cells regardless of the state of expansion of the pouch cells. The biasing arrangement includes a plurality of biasing devices 937. The biasing devices 937 are spaced apart along lateral sides 906 of the compression members 904a, 904b, such that each biasing device 937 extends between the first 904a and second 904b compression members. A single biasing device 937 is shown in Figures 9B and 9C, which will be described below. While only one such biasing device 937 will be described, it should be appreciated that each biasing device 937 is the same, except for those on the ends of each row of biasing devices 937 (the end biasing devices 937 being modified so as to be half of a typical biasing device 937 - this is apparent from Figure 9A). Each biasing device 937 includes first 938 and second 939 portions. The first portion 938 includes a base 940 secured to the first compression member 904a and a stem 941 that extends from the first compression member 904a towards the second compression member 904b. The first portion 938 therefore has a proximal end 942 at the first compression member 904a (defined by the base 940) and an opposite distal end 943 (i.e. distal from the first compression member 904a), forming part of the stem 941. The second portion 939 also includes a base 944 secured to the second compression member 904b and a stem 945. In this case, the stem 945 extends from the second compression member 904b towards the first compression member 904a. The second portion 939 therefore has a proximal end 946 (defined by the base 944) at the second compression member 904b and an opposite distal end 947 (i.e. distal from the second compression member 904b) forming part of the stem 945. The stem 945 of the second portion 939 comprises a recess 948 (defined between two spaced apart walls 949 of the stem) along its length within which the stem 941 of the first portion 938 is slideably received. As may be appreciated, in other embodiments the first portion 938 may instead include the described recess. The biasing device 937 further includes two spring elements 950, each in the form of a leaf spring, that are arranged either side of the first 938 and second 939 portions (i.e. the first 938 and second 939 portions extend between the two spring elements 950). Each spring element 950 extends between the distal end 943 of the first portion 938 and the distal end 947 of the second portion 939, and (as will be described further) is biased to urge the distal ends 943, 947 of the first 938 and second 939 portions away from one another in the cell stack direction. In particular, each spring element 950 includes a first end 951 that is integral with the distal end 947 of the second portion 939, and extends from the first end 951 to a free second end 952 in contact with the distal end 943 of the first portion 938. The second end 952 of each spring element 950 is bulbous and rounded. The distal end 943 of the first portion 938 includes two convex (arcuate) cam surfaces 953 (facing back towards the proximal end 942 of the first portion 938) across which the free second end 952 of each spring element 950 moves as the biasing device 937 is moved from the contracted configuration to the expanded configuration. To allow movement between the first 938 and second 939 portions, the spaced apart walls of the second portion 939 terminate (at their proximal ends) at a location that is spaced from the convex cam surfaces 953. The movement of the spring elements 950 across the cam surfaces 953 causes deflection of each spring element 950. As the compression members 904a, 904b are moved away from one another (by expansion of pouch cells in the cell-receiving space 905) the second ends 952 of the two spring elements 950 are forced apart by way of their contact with the convex cam surfaces 953. The deflection of the spring elements 950 is mostly in the form of each spring element 950 hinging at its first end 951 (where it integrally connects to the second portion 939 via a bend). The nature of the deflection of each spring element 950 is such that the magnitude of the deflection changes in a non-linear manner (due to the convex shape of each cam surface 953) as the compression members 904a, 904b (and thus first 938 and second 939 portions) move apart in the cell stack direction. In particular, as the compression members 904a, 904b move apart, the rate of deflection (i.e. deflection per expansion distance) of each spring member 950 decreases. The non-linearity of the relationship between the deflection and amount of expansion counteracts the non-linear nature of the restoring force of each spring element 950, which is a result of its material properties. That is, typically, the nature of a spring element 950 (such as a leaf spring) is such that the further it is deflected, the greater the restoring force, but in the present embodiment this is counteracted by the reduced deflection of each spring element 950 as the compression members 904a, 904b are moved away from one another. Accordingly, the result is a biasing device 937 that provides a substantially constant force urging the compression members 904a, 904b towards one another, throughout the expansion of the pouch cell enclosure 901 between the contracted and expanded configurations. In this way, a substantially constant compressive force may be applied to pouch cells received in the cell-receiving space 905. Figure 10 illustrates a biasing device 1037 that is a variation of that shown in Figure 9B. In this variation, each spring element 1050 is a separate element mounted to the stem 1045 of the second portion 1039. In particular, the first end 1051 of each spring element 1050, which includes a bend, is received in a correspondingly curved slot 1054 of the stem 1045 of the second portion 1039. Each spring element 1050 may, for example, be formed of steel. Figures 11 A, 11B, 11C and 11D show variations of the biasing device 937 of Figure 9. Further variations (not illustrated) that achieve a similar (or the same function) will be apparent to the skilled person. Each of the illustrated variations provides a substantially constant restoring force as it is expanded (e.g. by expansion of the pouch cells). It should therefore be appreciated that each of the illustrated variations could provide a replacement to the biasing device 937 illustrated in Figure 9 (while providing substantially the same function). For brevity, the same reference numerals have been used for the same features in each of Figures 11A-11D. Each variation includes a biasing device 1137 that connects first 1104a and second 1104b spaced compression members and that is configured to urge the compression members 1104a, 1104b towards one another. Each biasing device 1137 includes either one spring element 1150 or two spring elements 1150. In Figure 11 A, the biasing device 1137 includes a leaf spring element 1150 that is mounted at one end to the second compression member 1104b and at another end to a pivoting arm 1163 (which is mounted to the first compression member 1104a). The spring element 1150 bends in response to movement of the compression members 1104a, 1104b away from one another. In the variation of Figure 11B, two leaf spring elements 1150 extend from the second compression member 1104b and are provided with rollers (or e.g. sliding elements) that move across respective convex cam surfaces 1153, which face away from the second compression member 1104b (and are mounted to and face towards the first compression member 1104a). In the variation of Figure 11C, two compression spring elements 1150 (mounted to the second compression member 1104b) are provided, and in operation are compressed by respective connecting members 1164 (connecting the spring elements 1150 to linkages 1165) that are constrained (e.g. by rails) to move perpendicularly to the cell stack direction (and away from one another). The connecting members 1164 are driven by the linkages 1165, which are pivotably mounted by a fixed stem 1166 mounted to the first compression member 1104a. In the variation of Figure 11D, two compression spring elements 1150 (mounted to the second compression member 1104b) are provided and, in operation are compressed in a direction perpendicular to the cell stack direction by rollers 1167 that are guided across respective convex cam surfaces 1153 (which are mounted to the first compression member 1104a and face the first compression member 1104a). Figures 12A and 12B depict a further embodiment in which the pouch cell enclosure 1201 includes lateral sidewalls 1255 extending between an upper wall (which forms a first compression member 1204a) and a lower wall 1256. Each sidewall 1255 has two apertures 1257 formed therein. The enclosure 1201 is also provided with two elastic members 1258, each having opposite ends 1259, that are fixed to a respective one of the sidewalls 1255 (at an outer surface of the sidewall 1255) at locations above the apertures 1257. Intermediate the ends, the elastic members each extend through the apertures 1257 in the sidewalls 1255 and across an interior of the pouch cell enclosure 1201. In this way, each elastic member 1258 includes two end portions 1259 extending in the cell-stack direction along the two sidewalls 1255, and an intermediate laterally-extending portion (which provides a second compression member 1204b) that extends across the pouch cell enclosure 1201 between the lateral sidewalls 1255. A cell-receiving space 1205 is defined between the second compression members 1204b (defined by the elastic members 1258) and the first compression member 1204a (provided by the upper wall). The biasing arrangement thus is provided by the two end portions 1260 of the elastic members 1258 which urge the second compression member 1204b towards the first compression member 1204a. As may be appreciated, in other embodiments, the laterally-extending portion of each elastic member may be replaced by a separate e.g. rigid component (such as a plate). To permit expansion and contraction of the pouch cells 1202 held in the cell-receiving space 1205, each aperture 1257 formed in the sidewalls 1255 is configured to allow movement of the part of the elastic member 1258 passing therethrough in the cell stack direction. In other words, the height of each aperture 1257 is significantly greater than the thickness of each elastic member 1258. The present embodiment also differs from those previously described in that it includes a central wall 1261 interposed between the sidewalls 1255 so as to divide the cell-receiving space 1205 into two regions (although, of course, the present embodiment could be provided without such a central wall and with a single stack of pouch cells). Each of these regions includes a respective stack of pouch cells 1202 (i.e. unlike previous embodiments two stacks of pouch cells 1202 are provided in a side-by side manner). The central wall 1261 includes apertures 1262 to accommodate the elastic member 1258. Figures 13 to 21 illustrate further variations of a battery. In these variations compression plates are provided that have varying stiffness in order to provide a more even distribution of pressure on the pouch cells of the battery. As will become apparent, the compression plates described in these figures can replace any of the first and / or second compression members in the embodiments described above. Figure 13 illustrates one such compression plate 1304. The compression plate 1304 is rectangular and has two opposite major surfaces 1369 (only one of which is visible in the figure). The rectangular shape of compression plate 1304 means that the compression plate 1304 includes first 1368a and second 1368b opposite parallel edges. The compression plate 1304 varies in stiffness in a plane that is parallel to its major surfaces 1369. In particular, the compression plate 1304 includes two first regions 1370, which are interspersed between three second regions 1371 that are of lower stiffness than the first regions 1370. Each of the first 1370 and second regions 1371 is elongate and extends in a direction parallel to the first 1368a and second 1368b edges. The first regions 1370 are spaced inwardly (but are located near to) the first 1368a and second 1368b edges. As will be described further below, the first regions 1370 are arranged at locations where the compression plate 1304 is engaged by a biasing arrangement, when assembled in a battery. This helps to reduce deformation of the compression plate 1304 (while minimising mass and volume), which could otherwise result in an uneven distribution of pressure applied by the compression plate 1304 to pouch cells in a battery. Thus, the compression plate 1304 of Figure 13 is particularly suited to a battery in which the biasing arrangement engages the compression plate 1304 inward of the first 1368a and second 1368b edges (such as in the batteries of Figures 1 to 3). The compression plate 1404 of Figure 14 includes two first regions 1470 that are provided at the first 1468a and second 1468b edges of the compression plate 1404. In this case, a single second region 1471 is provided between the two first regions 1470. As may be appreciated, this compression plate 1404 is particularly suited for batteries in which the biasing arrangement engages the compression plate 1404 at the first 1468a and second 1468b edges (e.g. as per Figures 4-5, 7-12). Figures 15 illustrates a further distribution of first 1570 and second 1571 regions. In this case, there are four discrete first regions 1570 spaced evenly across the compression plate 1504 and surrounded by a second region 1571. The first regions 1570 could be formed by providing the compression plate 1704 (in the unassembled state) with a wave-like profile in two dimensions (i.e. the crests forming bumps at the locations of the first regions 1570). Alternatively, the first regions 1570 may be discrete regions of greater thickness or higher material stiffness. This arrangement of regions 1570, 1571 may be particularly suitable for arrangements in which the biasing arrangement includes individual spring elements, e.g. four such spring elements, contact the compression plate 1504 in a distributed manner across the compression plate 1504 (i.e. at the first regions 1570). Figure 16 illustrates yet a further arrangement of first 1670 and second 1671 regions. In this case a peripheral first region 1670 is provided that extends along all four of the edges of the compression plate 1604 and that surrounds a central second region 1671. This arrangement of regions 1670, 1671 may be provided by providing a compression plate 1604 (in its natural, unassembled state) that is bowed along two axes that are normal to one another. Alternatively, as above, the regions 1670, 1671 may be provided by a thicker periphery and / or higher material stiffness at the periphery. This arrangement of regions 1670, 1671 may be particularly suited where the biasing arrangement engages all edges of the compression plate 1604. Figures 17A and 17B illustrate a battery 1700 that includes compression plates 1704 with first 1770 and second 1771 regions (for clarity these are only labelled in Figure 17A). The battery includes two compression plates 1704 that are spaced apart in the cell stack direction to define a cell-receiving space 1705. A plurality of pouch cells 1702 are received in the cell-receiving space 1705 so as to be compressed between the compression plates 1704. Figure 17A shows the battery 1700 in an unassembled state. In this state, the compression plates 1704 are each in a natural, undeformed state. In the present embodiment, the first regions 1770 are introduced into the compression plates 1704 by deformation of the compression plates 1704 when in the assembled state, which is shown in Figure 17B. In particular, the compression plates 1704 are deformed so as to be flattened (i.e. become more planar) when assembled in the battery 1700. In the unassembled state of Figure 17A each compression plate 1704 is non-planar and has a wave-shaped profile (taken in a direction extending between opposite first 1768a and second 1768b edges of the respective compression plate 1704). The wave-shaped profile includes two crests 1772 that each extends along (and near to) a respective one of the first 1768a and second 1768b edges. A trough 1773 is formed between the crests. When the compression plates 1704 assembled in the battery 1700, the crests 1772 are flattened. This flattening of the crests 1772 (and their inclination to rebound) means they provide regions of higher stiffness. The result is a distribution of first regions 1770 and second regions 1771 that is similar to that shown in Figure 13. As is apparent from Figure 17B, the first regions 1770 are provided at locations where a biasing arrangement of the battery 1700 engages the compression plates 1704. In the illustrated embodiment, the battery arrangement comprises two spring members 1708, each extending along one of two opposite lateral sides 1706 of the battery 1700. Each spring member 1708 includes a body 1713 and first 1714a and second 1714b arms that extend laterally from the body 1713 so as to engage a respective compression plate 1704. Each spring member 1708 is thus essentially the same as those already described above (for example with respect to Figures IB and IC). The spring members 1713 are each configured to flex and are biased such that the arms 1714a, 1714b urge the compression plates 1704 towards one another. By providing the crests 1772 at regions where the arms 1714a, 1714b engage the compression plates 1704 when assembled in the battery 1700, deformation of the compression plates 1704 by the arms 1714a, 1714b can be resisted more readily at those regions. As already discussed above, this is because the flattening of the crests 1772 in assembly creates the first regions 1770 where the compression plates 1704 are engaged by the arms 1714a, 1714b. In general, the first regions in such embodiments are dictated by the form of the compression plate in its natural, unassembled state. Thus, the unassembled form of the compression plates may differ from that shown in Figures 17A and 17B where a different distribution of first is desirable. One example of such a variation is shown in Figure 18. In this case, the compression plate 1804 has a concave profile (as opposite to a wave-shaped profile). In other words, the compression plate 1804 is bowed about an axis parallel to opposite first 1868a and second 1868b edges of the compression plate 1804 (extending into the page). As may be appreciated, when flattened, such a compression plate 1804 will provide a distribution of first and second regions that is similar to that shown in Figure 14. Figures 19 to 21 illustrate further ways that the stiffness of a compression plate can be varied. The general structure of the batteries (including the biasing arrangements) in these variations is the same as that shown in Figures 17A and 17B and will not be described again here (for the same reasons, similar reference numerals have been used for similar feature, albeit with replacement of the first two digits which identify the respective embodiment). In the embodiment of Figure 19, the compression plates 1904 vary in thickness in a direction extending between opposite first 1968a and second 1968b edges of each compression plate 1904. This variation in thickness is provided by each compression plate 1904 having an outer surface 1974 that has an undulating, wave-shaped profile and an inner surface 1975 that is substantially planar. The crests of the wave-shaped profile of the outer surfaces 1974 of the compression plates 1904 create thicker regions that provide first regions 1970 in the compression plates 1904. Likewise, troughs formed in the outer surfaces 1974 result in thinner regions that provide second regions 1971 in the compression plates 1904. As may be appreciated, the distribution of first 1970 and second 1971 regions is thus similar to that shown in Figure 13. The spring members 1908 engage the compression plates 1904 at the thicker first regions 1970 of the compression plates 1904 where the compression plates 1904 are less susceptible to local deformation that could otherwise cause an uneven distribution of pressure to the pouch cells 1902. The compression plates 2004 of the battery 2000 of Figure 20 have a layered structure. In particular, each compression plate 2004 includes an outer metal layer 2077 and an inner layer 2078 (i.e. closer to the pouch cells 2002) that comprises a rubber material. Each outer layer 2077 is substantially planar and of uniform thickness. Each inner rubber layer 2078 varies in thickness in a similar manner to the compression plates 1904 of Figure 19. In particular, each inner layer 2078 has an inner surface 2075 with a wave-like (undulating) profile, which creates first regions 2070 where the inner layer 2078 (and thus the compression plate 2004) is thicker and second regions 2071 where the inner layer 2078 (and thus the compression plate 2004) is thinner. Like previous embodiments, the first 2070 are located where each compression plate 2004 is engaged by spring members 2008 of the battery 2000. In Figure 21, the first 2170 and second 2171 regions are provided by varying the material properties of the compression plates 2104. Like the battery of Figure 20, the compression plates 2104, again, have a layered structure. In this case, each compression plate 2104 includes an outer metal layer 2177 and an inner layer 2178 (closer to the pouch cells 2102) that is formed of a fibre-reinforced composite material (e.g. glass fibre or carbon fibre). Each outer layer 2177 is a planer layer of uniform thickness and uniform material properties. The material properties of each inner layer 2178 vary in different regions of each inner layer 2178. In particular, each inner layer 2178 includes first regions 2170 provided by higher material stiffness (e.g. provided by a higher density of fibres) and second regions 2171 provided by a lower material stiffness (e.g. a lower density of fibres). As per previous embodiments, the first regions 2170 are provided where the spring members 2108 engage the compression plates 2104. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be 5 understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A battery comprising:one or more pouch cells; anda pouch cell enclosure comprising:a pair of plates spaced apart in a cell stack direction so as to define a cellreceiving space therebetween in which the one or more pouch cells are received, the plates being moveable relative to one another in the cell stack direction; anda biasing arrangement in engagement with at least one of said plates to urge the plate towards the other plate of the pair of plates,wherein at least one of the plates is a compression plate comprising one or more first regions within which the biasing arrangement engages the compression plate and one or more second regions, having a lower stiffness than the one or more first regions,2. A battery according to claim 1 wherein the biasing arrangement does not engage the compression plate in the one or more second regions.

3. A battery according to claim 1 or 2, wherein said pair of plates are both said compression plates, each comprising one or more first regions within which the biasing arrangement engages the respective plate and one or more second regions, having a lower stiffness than the one or more first regions.

4. A battery according to any one of the preceding claims, wherein the or each compression plate is non-planar in its natural, unassembled, state and is compressed by the biasing arrangement to be substantially planar, or closer to planar, when assembled in the pouch cell enclosure.

5. A battery according to claim 1, wherein the or each compression plate is deformed out of plane so as to extend outwardly in the one or more first regions in its unassembled state.

6. A battery according to any one of the preceding claims, wherein the or compression plate is thicker in the one or more first regions than in the one or more second regions.

7. A battery according to claim 6, wherein the or each compression plate has an inner substantially planar surface facing the pouch cells, and an opposite outer non-planar surface to provide the greater thickness in the one or more first regions than in the one or more second regions.

8. A battery according to any one of the preceding claims wherein the material properties of the or each compression plate varies between the one or more second regions and the one or more first regions to provide greater stiffness in the one or more first regions than in the one or more second regions.

9. A battery according to claim 8, wherein the or each compression plate comprises a fibre-reinforced composite material and the one or more first regions have a different fibre architecture to the one or more second regions.

10. A battery according to claim 9 wherein the biasing arrangement comprises a plurality of spring elements arranged to engage an outer surface of the or each compression plate to urge the compression plate towards the other compression plate of the pair; andwherein the or each compression plate comprises a plurality of first regions arranged such that each spring element engages the compression plate in a respective first region of the plurality of first regions.

11. A battery according to claim 10 wherein the biasing arrangement connects the pair of plates.

12. A battery according to claim 11 wherein the biasing arrangement comprises first and second resilient spring members extending along first and second opposite edges of the or each compression plate, and wherein the or each compression plate comprises:a primary first region in which the first resilient spring member engages the compression plate; anda secondary first region in which the second resilient spring member engages the compression plate.

13. A battery according to claim 12 wherein the primary first region and secondary first region extend respectively along the first and second edges of the or each compression plate.

14. A battery according to claim 13, when dependent on claim 4, wherein in its natural, unassembled state, the or each compression plate is bowed about an axis extending substantially parallel to first and second edges of the compression plate.

15. A battery according to claim 12, wherein the primary first region and secondary first region respectively extend substantially parallel to, and inward of, the first and second edges of the or each compression plate.

16. A battery according to claim 15, when dependent on claim 4, wherein in its natural, unassembled state, the or each compression plate comprises a wave-like profile taken in a cross-section extending between the first and second edges, and wherein the primary first region and secondary first region are provided by crests of the wave-like profile.

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