A battery enclosure

Abstract

A battery enclosure (200, figure 2) for housing an aircraft battery comprises an internal compartment 204 for receiving the battery assembly, and a plurality of layers at least partially defining the

Description

TECHNICAL FIELD This disclosure concerns a battery enclosure and is particularly, although not exclusively, concerned with a composite battery enclosure for an aircraft battery assembly. BACKGROUND Future aerospace applications such as more electric, hybrid electric and purely electric aircraft will likely use high voltage battery packs made up of a number of battery modules connected in series and / or parallel. These battery packs may, for example, power one or more motors used to propel the aircraft and / or to provide power to one or more ancillary systems. Heating of battery cells beyond their normal operating range can occur for various reasons. For example, a fault may develop in one or more of the battery packs, or the battery pack could be exposed to an external source of heat such as a fire, high solar irradiance or exhaust gas from a gas turbine engine. Heat from one battery pack can spread to others, which could lead to thermal runaway of an entire energy storage system. Excessive heating and thermal runaway create a risk of fire and with it damage to the battery, damage to the surrounding structure of the aircraft and risk of electric shock, for example by causing damage to electrical insulation and structures that support the battery pack mounting and current carrying conductors. Accordingly, it is desirable to protect against the consequences of excessive cell heating and thermal runaway propagation. SUMMARY While it is desirable to improve battery enclosures for all applications, the present inventor has determined that, for an aircraft battery enclosure, the design requirements may be particularly demanding and the design priorities may be different from other applications. For example, while the energy density of a battery assembly is important in all vehicles, it is particularly important for an aircraft. Similarly, aircraft battery assemblies may be subjected to more extreme environments than automotive battery assemblies. These aviation environments may include a greater external operating temperature range (e.g., -60 °C to 80 °C), greater exposure to electromagnetic fields (e.g., occurring naturally from lightning storms), greater exposure to variations in altitude and air dependant factors such as ionized particles, humidity, heat and pressure (impacting partial discharge, and breakdown voltage) and cyclical loading from multiple take-offs and landings (including associated pressure variations), as well as potential exposure to high mechanical stresses resulting from heavy landings. Additionally, the reliability and fire safety of components on board an aircraft are critical (e.g., more so than for those in terrestrial vehicles). Other important considerations for battery enclosures include the mechanical performance, electromagnetic interference protection, and fault tolerance. Accordingly, although the battery enclosure of the present disclosure may be particularly beneficial for aircraft battery assemblies, it will be understood that the present battery enclosure may also be beneficial for other applications, such as automotive applications. The following aspects were devised with the foregoing in mind. According to a first aspect of the present disclosure, there is provided a battery enclosure. The enclosure may comprise an enclosure for housing an energy storage system, such as a battery assembly, such as a high voltage (HV) battery assembly (e.g., for an aircraft, such as a more electric, a purely electric and / or a hybrid electric aircraft). The energy storage system (e.g., battery assembly) may include one or more battery packs or modules. The enclosure may comprise (e.g., define) an internal compartment, such as an internal compartment for receiving a battery assembly. The enclosure may have a wall or a side which at least partially defines the internal compartment. The enclosure (e.g., a wall or a side thereof) may comprise a plurality of layers (e.g., in the form of a multi-layer structure having stacked layers). The layers may at least partially define the internal compartment. The enclosure may thereby comprise a composite battery enclosure (e.g., the enclosure may be formed from multiple materials). The enclosure may define a thickness direction. The thickness direction may extend away from the internal compartment (e.g., perpendicularly from the internal compartment to an external surface of the enclosure). The layers may be stacked along the thickness direction. The dimensions of the layers perpendicular to the thickness direction may be significantly greater than the thickness of the layers. Accordingly, the layers may be substantially planar. The thickness dimension of a layer may be defined as its smallest dimension. The enclosure (e.g., at a first region, such as at a wall or a side of the enclosure) may comprise up to ten layers. The enclosure may comprise: Layer (A), optionally an ingress protection layer; Layer (B), optionally a fibre-reinforced polymer composite layer; Layer (C), optionally an electromagnetic interference (EMI) shielding layer; Layer (D), optionally a grounding layer; Layer (E), optionally a fibre-reinforced polymer composite layer; Layer (F), optionally a layer comprising polyurethane foam; Layer (G), optionally a layer having an array of cavities; Layer (H), optionally a layer comprising aramid fibres; Layer (I), optionally a layer comprising ceramic fibres; and / or Layer (J), optionally an electrically insulating layer. For brevity, the above layers (A)-(J) may be referenced by the labels in parentheses. For example, the ingress protection layer may be referenced by layer (A) rather than by ingress protection layer. The enclosure may comprise any combination of layers (A)-(J). The enclosure may comprise any at least one, any at least two, any at least three, any at least four, any at least five, any at least six, any at least seven, any at least eight, any at least nine, and / or all ten, of layers (A)-(J). For example, the enclosure may comprise any one, any two, any three, any four, any five, any six, any seven, any eight, any nine, or all ten, of layers (A)-(J). The plurality of layers may be provided relative to one another in the same order as above. For example, in an embodiment in which the enclosure comprises layers (C), (G), (I), the layers may be provided in alphabetical order relative to one another (e.g., layer (G) between layers (C) and (I)). Layers (F)-(H) may be provided adjacent one another in any order. The enclosure may comprise layer (G) and / or layer (I). The enclosure may comprise layer (H) (e.g., between layers (G) and (I)). The enclosure may comprise layer (C) and / or layer (D), such as between layers (B) and (E). The enclosure may comprise layer (F), optionally adjacent layer (G), such as between (e.g., directly between) layers (B)-(E) and layers (G)-(l) (e.g., the enclosure may comprise layers (B)-(l) in alphabetical order). Layer (A) may be provided outermost and / or layer (J) may be provided innermost. These layers may at least partially define the internal compartment (e.g., these layers may be stacked at the same portion of the enclosure to form a multi-layer structure). The enclosure may comprise layers (B), (C), (D), (G) and (I), such as in that order (from exterior to interior of the enclosure, optionally with additional layers interspersed therebetween). A first layer may be provided to the exterior of a second layer when the first layer is provided further from the internal compartment, and / or closer to an external surface of the enclosure (e.g., the atmosphere or external environment of the enclosure), than the second layer. Similarly, a first layer may be provided to the interior of a second layer when the first layer is provided closer to the internal compartment (e.g., closer to battery active components), and / or further from an external surface of the enclosure, than the second layer. The enclosure (e.g., a wall or side of the enclosure) may consist essentially of a combination of layers (A)-(J). For example, any portion of the structure of the enclosure may comprise no further layers than a combination of layers (A)-(J). Layer (A) may comprise (e.g., consist essentially of) a polymer. Layer (A) may have thickness between 50 micrometres and 1 mm. Layer (A) may be resistant to (e.g., seal the plurality of layers against) water or solvent penetration, chemical exposure and / or minor abrasion. Layer (A) may be applied as a coating (e.g., as a thin film polymer covering, such as a label, or as a paint). Layer (A) may be provided to the exterior of any combination of layers (B) to (J). For example, layer (A) may form an outermost surface of the enclosure (e.g., furthest from the internal compartment). Layers (B) and / or (E) may comprise a fibre reinforced polymer composite layer. Layers (B) and / or (E) may comprise carbon fibre and / or glass fibre, e.g., provided as a single ply or multiple plies in different orientations. Layers (B) and / or (E) may comprise prepreg carbon fibre and / or glass fibre. Layers (B) and / or (E) may have a thickness between 200 micrometres and 1 mm. Layers (B) and / or (E) may improve the mechanical strength (e.g., structural rigidity) of the plurality of layers. Layer (B) may be provided to the interior of (A), and / or to the exterior of any combination of layers (C) to (J). Layer (E) may be provided to the interior of any combination of layers (A) and (D), and / or to the exterior of any combination of layers (F) to (J). Layer (C) may comprise an electromagnetic interference (EMI) shielding and / or absorption layer. Layer (C) may comprise a conductive mesh (e.g., a grid of conductors), optionally made from nickel (e.g., heavily plated with tin), tungsten, titanium, stainless steel, mild steel, aluminium, copper, silver, gold or other metals. The material may be selected according to the desired attenuation performance at different frequency ranges. Layer (C) may comprise multiple meshes of different mesh gauges, mesh densities, conductivities and / or permeability properties e.g., according to the desired attenuation characteristics. The mesh may be electrically grounded. For example, layer (C) may be provided in electrical communication with layer (D). Layer (C) may be provided to the interior of any combination of layers (A) and (B), and / or to the exterior of any combination of layers (D) to (J). Specifically, layer (C) may be provided to the exterior of layer (D). Layer (C) may be incorporated into layer (B) and / or layer (E) as part of the multiple ply construction. Layers (C) and (D) may be electrically connected via a low electrical resistance path. Layer (D) may comprise a grounding layer. Layer (D) may comprise electrically conductive cables, e.g., high ampacity current carrying conductive cables (e.g., to handle fault loads). The cables may comprise nickel, copper, aluminium and / or steel. Copper may be preferred due to its coefficient of thermal expansion. The cables may be plated to improve corrosion resistance. The ampacity of layer (D) (e.g., the ampacity of the conductive cables) may exceed that of the mesh of layer (C). Layer (D) may be exposed at an interface of a component part of the enclosure e.g., so as to provide electrical continuity across the interface between adjacent component parts, such as a low electrical impedance connection. Layers (C) and / or (D) may be directly connected to an electrical terminal in the composite (e.g., a ground star point) to support the electrical connection of the enclosure. Layer (D) may be provided to the interior of any combination of layers (A) to (C), and / or to the exterior of any combination of layers (E) to (J). Specifically, layer (D) may be provided to the interior of layer (C) and / or to the exterior of layers (G), (H), and / or (I). Layers (C) and / or (D) may be provided between (e.g., directly between) layers (B) and (E) during manufacture, e.g., so as to form a subassembly of layers (B) to (E). Layers (C) and (D) may be contiguous or merged. For example, layer (C / D) may comprise allotropes of carbon, such as metal-nanocarbon based with graphite, graphene and carbon nanotubes, which may provide both EMI shielding and fault current carrying capacity. Layer (F) may comprise a thermal insulation layer. Layer (F) may comprise (e.g., consist essentially of) a polyurethane or a polyethylene foam, e.g., of thickness between 2 mm and 10 mm, such as 3 mm to 5 mm, optionally of polyethylene terephthalate foam. Layer (F) may be provided to the interior of any combination of layers (A) to (E), and / or to the exterior of any combination of layers (G) to (J). Specifically, layer (F) may be provided externally to (e.g., further from the internal compartment than) layer (G), such as adjacent (e.g., immediately adjacent) layer (G). Layer (G) may comprise an open structure, such as an array of cavities (e.g., a large number of compartments or cavities defined by a network of thin walls). The cavities may be distributed in three dimensions throughout layer (G). The cavities may have dimensions (e.g., long axis dimensions) between 2 and 12 mm (e.g., between 3 and 5 mm). Layer (G) may have a density of 10 to 80 kg / m3, such as 10 to 30 kg / m3 and a thickness between 0.5 mm and 10 mm, such as 1.5 mm to 2.5 mm The cavities may be prismatic (e.g., have a polygonal cross section extending uniformly along a long axis). The cavities may comprise hexagonal prisms. The cavities may be tessellated. Adjacent cavities may be adjoined along a long face. The long axis of the cavities may extend perpendicular to a thickness dimension of the enclosure. For example, layer (G) may comprise a honeycomb structure which cavities have longitudinal axes extending perpendicular to the thickness direction of the layer and / or the enclosure (e.g., perpendicular to the thickness of the wall or side of the enclosure). Layer (G) may comprise aluminium (e.g., an aluminium alloy) or an aramid such as Nomex (RTM). The cavities may be filled by air or another gas. The volumes of adjacent cavities may be in fluid communication (e.g., each cavity may comprise a pin hole or other small aperture extending through to an adjacent cavity). The cavity volumes may communicate with an exterior of the enclosure (e.g., to permit changes in gas pressure due to changes in altitude). A polyurethane foam in layer (F) may provide improved bonding between layer (G) and any exterior layers (A)-(E). Layer (G) may be provided to the interior of any combination of layers (A) to (F), and / or to the exterior of any combination of layers (H) to (J). Specifically, Layer (G) may be provided to the exterior of layer (I). Layer (H) may comprise aramid fibres. Layer (H) may comprise an aramid fibre resin matrix (e.g., Kevlar (RTM)). The layers of aramid fibre may form sheets perpendicular to the thickness direction. Layer (H) may comprise multiple plies, with different orientations, weaves, density, etc. Layer (H) may be provided to the interior of (e.g., closer to the internal compartment than) layer (G). Layer (H) may be provided adjacent (e.g., immediately adjacent) layer (G) and / or layer (I). Layer (H) may be provided between (e.g., immediately between) layers (G) and (I). Layer (H) may protect layer (G) from penetration, such as from external sources (e.g., runway debris) or internal sources, such as any ejecta of a thermal runaway event. Layer (H) may have a thickness between 0.2 mm and 0.5 mm. Layer (H) may be provided to the interior of any combination of layers (A) to (G), and / or to the exterior of any combination of layers (I) and (J). Specifically, layer (H) may be provided internally to layer (G) and / or externally to layer (I), such as between (e.g., immediately between) layers (G) and (I). Layer (I) may comprise a barrier layer, e.g., for protecting against ejecta from a runaway event. Layer (I) may comprise ceramic, such as ceramic fibres, such as in the form of a ceramic fibre cloth. Layer (I) may comprise a ceramic fibre reinforced polymer composite. The ceramic fibres may comprise calcium-magnesium silicate. Layer (I) may comprise ceramic in the form of mica (e.g., sheets or plates of mica, such as provided perpendicular to the thickness direction). For example, layer (I) may comprise layers of mica. Layer (I) may comprise natural cork. Layer (I) may comprise a thin sheet or layer of stainless steel. Layer (I) may have a high flame retardance, chemical resistance and / or surface hardness (e.g., any combination of flame retardance, chemical resistance and surface hardness). The flame retardance, chemical resistance and / or surface hardness of layer (I) may exceed that of layer (G). Layer (I) may have a thickness between 0.5 mm and 2 mm. Layer (I) may be provided to the interior of any combination of layers (A) to (H), and / or to the exterior of layer (J). Specifically, layer (I) may be provided to the interior of layer (G). Layer (J) may comprise an electrically insulating layer. Layer (J) may comprise (e.g., consist essentially of) a polymer, such as a polyimide or an aramid, such as Nomex (RTM). Layer (J) may have a thickness between 50 and 300 micrometres. Layer (J) may be provided to the interior of any combination of layers (A) to (I). For example, layer (J) may be provided innermost (e.g., immediately adjacent the internal compartment). The enclosure may comprise a plurality of component parts (e.g., a base component and a cover component), one or more of which may comprise a wall or a side. The enclosure may comprise a plurality of regions. A first region may comprise a major wall or a major side of the enclosure (e.g., as part of a cover component). A second region may comprise a connecting region, such as between component parts (e.g., a flange region of a cover component for engaging a flange region of a base component) or for attaching the enclosure to the aircraft. Upon assembly of the enclosure, an interface may be formed between component parts. For example, an interface between component parts may be formed between adjacent connecting regions. The first region and / or the second region may comprise a plurality of layers (A) to (J). The first region of the enclosure may comprise a combination of layers (A) to (J). The second region may comprise a subset of the layers comprised by the first region. For example, the first region may consist essentially of a combination of layers (A) to (J) and the second region may omit one or more of the combination of layers (A) to (J). The second region may comprise fewer layers than the first region. The second region may have a lesser thickness than the first region. At least one of layers (A) to (J) (e.g., layer (G)) in the second region may have a lesser thickness than a corresponding at least one layer in the first region. At least one of layers (A) to (J) (e.g., layers (B) and / or (E)) in the second region may have a greater thickness than a corresponding at least one layer in the first region. At least one of layers (A) to (J) may be continuous across an interface between component parts. For example, at least one of layers (A) to (J) may be oriented perpendicular to an interface such that the at least one layer intersects the interface in alignment with a corresponding layer on an opposing component of the interface. Layer (D) may be continuous across the interface. At least one of layers (A) to (J) may not intersect the interface between adjacent component parts. According to a second aspect of the present disclosure, there is provided an assembly comprising: an enclosure according to the first aspect; and a battery assembly (e.g., an HV battery assembly for an aircraft). According to an aspect of the present disclosure, there is provided an aircraft (e.g., a more electric, a purely electric or a hybrid aircraft) comprising the enclosure of the first aspect and / or the assembly of the second aspect. According to an aspect of the present disclosure, there is provided a method of forming a battery enclosure (e.g., a battery enclosure according to the first aspect). The method may comprise providing a plurality of layers. The method may comprise providing a layer having an array of cavities (e.g., layer (G)). The method may comprise providing a layer comprising ceramic fibres (e.g., layer (I)). The method may comprise providing an EMI shielding layer. The method may comprise providing a grounding layer. The method may comprise providing a layer comprising a fibre-reinforced polymer composite. The method may comprise forming an internal compartment at least partially defined by a plurality of layers (e.g., a plurality of layers comprising a combination of layers (A)-(J) including layers (G) and(l)). The method may comprise forming a sub-assembly of layers comprising an electromagnetic interference shielding layer (e.g., layer (C)) and / or a grounding layer (e.g., layer (D)). The electromagnetic interference shielding layer and / or the grounding layer may be provided between fibre reinforced polymer composite layers (e.g., layers (B) and / or (E)). According to an aspect, there is provided a battery enclosure comprising: an internal compartment for receiving a battery assembly; and a plurality of layers at least partially defining the internal compartment, the layers comprising a first layer having an array of cavities and a second layer comprising ceramic fibres. This aspect may form part of and / or be used in conjunction with any other aspect. The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive any feature described herein may be applied to any aspect and / or combined with any other feature described herein. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which: Figure 1 shows a general arrangement of a turbofan engine for an aircraft; Figure 2 shows a schematic section through a battery enclosure; Figure 3 shows a magnified view of the portion III-III of the enclosure of Figure 2; Figure 4 shows a magnified view of the portion IV-IV of the enclosure of Figure 2; and Figure 5 is a flowchart showing a method according to the present disclosure. DETAILED DESCRIPTION Figure 1 A general arrangement of an engine 101 for an aircraft is shown in Figure 1. The engine 101 is of turbofan configuration, and thus comprises a ducted fan 102 that receives intake air A and generates two pressurised airflows: a bypass flow B which passes axially through a bypass duct 103 and a core flow C which enters a core gas turbine. The core gas turbine comprises, in axial flow series, a low-pressure compressor 104, a high-pressure compressor 105, a combustor 106, a high-pressure turbine 107, and a low-pressure turbine 108. In operation, the core flow C is compressed by the low-pressure compressor 104 and is then directed into the high-pressure compressor 105 where further compression takes place. The compressed air exhausted from the high-pressure compressor 105 is directed into the combustor 106 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high-pressure turbine 107 and in turn the low-pressure turbine 108 before being exhausted to provide a small proportion of the overall thrust. The high-pressure turbine 107 drives the high-pressure compressor 105 via an interconnecting shaft. The low-pressure turbine 108 drives the low-pressure compressor 104 via another interconnecting shaft. Together, the high-pressure compressor 105, high-pressure turbine 107, and associated interconnecting shaft form part of a high-pressure spool of the engine 101. Similarly, the low-pressure compressor 104, low-pressure turbine 108, and associated interconnecting shaft form part of a low-pressure spool of the engine 101. Such nomenclature will be familiar to those skilled in the art. Those skilled in the art will also appreciate that whilst the illustrated engine has two spools, other gas turbine engines have a different number of spools, e.q., three spools. The fan 102 is driven by the low-pressure turbine 108 via a reduction gearbox in the form of a planetary-configuration epicyclic gearbox 109. Thus in this configuration, the low-pressure turbine 108 is connected with a sun gear of the gearbox 109. The sun gear is meshed with a plurality of planet gears located in a rotating carrier, which planet gears are in turn meshed with a static ring gear. The rotating carrier drives the fan 102 via a fan shaft 110. It will be appreciated that in alternative embodiments a star-configuration epicyclic gearbox (in which the planet carrier is static and the ring gear rotates and provides the output) may be used instead, and indeed that the gearbox 109 may be omitted entirely so that the fan 102 is driven directly by the low-pressure turbine 108. It is increasingly desirable to facilitate a greater degree of electrical functionality on the airframe and on the engine. To this end, the engine 101 of the present embodiment comprises one or more rotary electric machines, generally capable of operating both as a motor and as a generator. The number and arrangement of the rotary electric machines will depend to some extent on the desired functionality. Some embodiments of the engine 101 include a single rotary electric machine 111 driven by the high-pressure spool, for example by a core-mounted accessory drive 112 of conventional configuration. Such a configuration facilitates the generation of electrical power for the engine and the aircraft and the driving of the high-pressure spool to facilitate starting of the engine in place of an air turbine starter. Other embodiments, including the one shown in Figure 1, comprise both a first rotary electric machine 111 coupled with the high-pressure spool and a second rotary electric machine 113 coupled with the low pressure spool. In addition to generating electrical power and the starting the engine 101, having both first and second rotary machines 111, 113, connected by power electronics, can facilitate the transfer of mechanical power between the high and lower pressure spools to improve operability, fuel consumption etc. As mentioned above, in Figure 1 the first rotary electric machine 111 is driven by the high-pressure spool by a core-mounted accessory drive 112 of conventional configuration. In alternative embodiments, the first electric machine 111 may be mounted coaxially with the turbomachinery in the engine 101. For example, the first electric machine 111 may be mounted axially in line with the duct between the low- and high-pressure compressors 104 and 105. In Figure 1, the second electric machine 113 is mounted in the tail cone 114 of the engine 101 coaxially with the turbomachinery and is coupled to the low-pressure turbine 108. In alternative embodiments, the second rotary electric machine 113 may be located axially in line with low-pressure compressor 104, which may adopt a bladed disc or bladed drum configuration to provide space for the second rotary electric machine 113. It will of course be appreciated by those skilled in the art that any other suitable location for the first and (if present) second electric machines may be adopted. The first and second electric machines 111, 113 are connected with power electronics. Extraction of power from or application of power to the electric machines is performed by a power electronics module (PEM) 115. In the present embodiment, the PEM 115 is mounted on the fan case 116 of the engine 101, but it will be appreciated that it may be mounted elsewhere such as on the core of the gas turbine, or in the vehicle to which the engine 101 is attached, for example. Control of the PEM 115 and of the first and second electric machines 111 and 113 is in the present example performed by an engine electronic controller (EEC) 117. In the present embodiment the EEC 117 is a full-authority digital engine controller (FADEC), the configuration of which will be known and understood by those skilled in the art. It therefore controls all aspects of the engine 101, i.e., both of the core gas turbine and the first and second electric machines 111 and 113. In this way, the EEC 117 may holistically respond to both thrust demand and electrical power demand. The one or more rotary electric machines 111, 113 and the power electronics 115 may be configured to output to or receive electric power from one, two or more de busses. The de busses allow for the distribution of electrical power to other engine electrical loads and to electrical loads on the airframe. The de busses may further receive electrical power from, or deliver electrical power to, an energy storage system such as one or more battery modules or packs. Those skilled in the art will appreciate that the gas turbine engine 101 described above may be regarded as a ‘more electric’ gas turbine engine because of the increased role of the electric machines 111, 113 compared with those of conventional gas turbines. Figure 2 shows a schematic section through a battery enclosure 200. The enclosure 200 comprises a base component 201 and a cover component 202 which are configured to be joined together to define an internal compartment 204 therebetween. The internal compartment 204 is configured (e.g., shaped and / or dimensioned) to receive a battery assembly (not shown), such as one or more battery packs (e.g., the battery packs provided in electrical communication with the rotary electric machines 111, 113 and / or in communication with the power electronics 115 via the de busses). The base and cover 201, 202 each comprise a connecting region in the form of flanges 206, 207 for connecting the base and cover 201, 202 together and / or for fixing the enclosure 200 to the airframe (not shown). Accordingly, the connecting regions 206, 207 may comprise blind inserts or holes for receiving fasteners (not shown). Upon joining, the base and cover 201, 202 define an interface 203 extending from the internal compartment 204 to the exterior 205 of the enclosure 200. The enclosure 200 effectively isolates the internal compartment 204 from the exterior 205, such that the environment of the internal compartment 204 may be controlled relative to that of the exterior 205. Although in the illustrated example the base and the cover 201, 202 are generally symmetric and each comprise walls having a generally concave geometry, in alternative examples, a different enclosure geometry may be provided. For example, only one of the base and the cover 201, 202 may comprise a concave geometry (e.g., the base 201 may be substantially planar). The enclosure 200 may comprise features permitting communication between the internal compartment 204 and the exterior 205. For example, the enclosure 200 may comprise a vent (not shown) as part of a vent management exhaust system for relieving pressure and / or accumulated gases within the receiving portion 204, and / or an electrical conduit (not shown) for permitting electrical continuity with the battery assembly. The base and the cover 201, 202 have a thickness dimension extending perpendicularly from a local boundary of the internal compartment 204 to a local exterior boundary of the enclosure 200. The thicknesses of the base and cover 201, 202 vary in different parts of the enclosure 200. For example, the thickness of the cover 202 on a major face of the enclosure 200, such as at the location of portion III-III, exceeds the thickness of the cover 201 at the connecting regions 206, 207, such as at the location of portion IV-IV. Figure 3 shows a magnified view of portion III-III of the cover 202 of Figure 2. The enclosure 200 at portion III-III comprises a plurality of layers 301-310 extending perpendicularly to the thickness direction T, defined perpendicularly to the local innermost and outermost surfaces of the enclosure 200. In the illustrated example of Figure 3, the layers 301-310 extend into and out of the page, as well as across the page. At portion III-III, each layer 301-310 has a uniform thickness into and across the page, such that the layers are substantially coplanar and so ‘stack’. Although illustrated schematically as being of generally equal thickness, it will be understood that the thicknesses of the layers 301-310 may differ from one another, as described below. Further, as described below, the thicknesses of the layers 301-310 may vary in different parts of enclosure 200. On an outermost surface, the enclosure 200 comprises an ingress protection layer 301 of thickness between 50 micrometres and 1 mm. The ingress protection layer 301 comprises a polymer, such as acrylic resin, epoxies, urethanes, silicones, or a polyurethane (epoxy or polyester based), and is generally resistant to water or solvent penetration, chemical exposure and / or minor abrasion. The ingress protection layer 301 is thereby configured to improve the resistance of the enclosure 200 to external substances, such as solvents, which might otherwise negatively affect the performance of the enclosure 200, and thus the functioning of the battery assembly. The layer 301 may be applied to layer 302 as a coating (e.g., as a paint). Beneath the ingress protection layer 301, the enclosure 200 comprises a fibre-reinforced polymer composite layer 302 of thickness of between 200 micrometres and 1 mm. The layer 302 comprises a carbon fibre- (e.g., multi-ply, such as in perpendicular orientations) or glass fibre-(single or multiple layers) reinforced thermosetting polymer (e.g., epoxy) composite. A glass fibre reinforced plastic (GFRP) composite having an unsaturated polyester resins (UPRs) and / or a furan resin (e.g., PFA, poly(furfuryl alcohol)) matrix may comprise about 20 wt. % glass fibre, about 70 wt. % UPR and / or PFA, and about 10 wt. % flame retardants and additives. Layer 302 improves the mechanical strength of the enclosure 200, and may reduce the transfer of heat to the external surfaces of the enclosure 200 during a thermal runaway event. Beneath layer 302, the enclosure comprises an electromagnetic interference (EMI) shielding and absorption layer 303 having a conductive mesh, such as a fine conductive mesh or weave, made from nickel or another metal. The mesh layer 303 improves the electromagnetic compatibility of the enclosure, and thus the battery assembly in the operating environment, such as by improving radio frequency interference performance. For example, the mesh may act as a Faraday cage to restrict (e.g., absorb, attenuate and / or prevent) the penetration of radiofrequency energy through the walls of the enclosure 200 and into the internal compartment 204. The specification of the mesh (e.g., gauge and / or density) may be selected so as to provide different levels of EMI attenuation performance (70-100 dB of attenuation may be desirable) over a large range of frequencies (e.g., from 1Hz to 40 GHz), such as inwards from external sources (e.g., in the range of 10 kHz to 400 MHz from nearby motor drives) and outwards from the battery pack itself (e.g., in the range of 1 Hz to 4 kHz). The mesh is electrically bonded (e.g., by connection to layer 304) in order to discharge any currents generated during operation. The mesh may additionally provide heat dissipation performance. Beneath the EMI shielding layer 303, the enclosure 200 comprises a grounding layer 304. The grounding layer 304 comprises high ampacity conductive cables (not shown) (e.g., comprising nickel, copper, steel or other metals) for handling fault currents from the HV battery pack. The grounding layer 304 has a higher ampacity than the mesh layer 303 and so is provided internally to the mesh layer 303 (i.e., closer to the internal compartment 204 than the mesh layer) so that it is closer to the HV battery pack. Accordingly, the grounding layer 304 may carry a fault current away from the mesh layer 303 which might otherwise melt when carrying such high currents. Additionally, the grounding layer 304 may protect the battery pack from a lightning strike. Similarly, the mesh layer 303 may be provided externally to the grounding layer 304 as the grounding layer 304 has a lower conductor density than the mesh layer 303 and so, at certain frequency ranges, the mesh layer 303 may be better at EMI shielding than the grounding layer 304. The grounding layer 304 is exposed at the interface 203 between component parts 201, 202 such that corresponding layers 304 in parts 201, 202 are provided in electrical continuity with one another and with other parts of the airframe, such as ground connections. For example, at the interface 203, the grounding layer 304 may be connected to ground connections and other electrical connectors via conductive gaskets. Further, grounding layer 304 may comprise an electrical bonding network for conductive pipes which carry coolant fluids to, from and around the enclosure 200. Beneath grounding layer 304 is provided a further fibre-reinforced polymer composite layer 305. Layer 302 and layer 305 may have similar (e.g., identical) structures, compositions, thicknesses and forming methods. For example, layers 302 and 305 may be provided as prepregs either side of layers 303 and 304, to secure layers 303, 304 within the multi-layer structure of portion III-III and to allow the enclosure 200 to be formed into the desired shape, such as in a mould. For example, the mesh and ground layers 303, 304 may be formed as a sub-assembly of the enclosure 200 and the sub-assembly may then be shaped in a mould and finally cured into the final shape of the enclosure 200. A thermal insulation layer 306 is provided adjacent and to the interior of the fibre reinforced polymer composite layer 305. The thermal insulation layer 306 comprises a polyurethane foam of thickness 2mm to 10 mm, such as 3 mm to 5 mm. The thermal insulation layer 306 provides thermal insulation for the battery pack to improve operation in extreme thermal environments, e.g., the freezing air of high altitude and proximate the hot exhaust gases of a gas turbine engine. As described subsequently in relation to layer 307, the material of the thermal insulation layer 306 may additionally improve the bonding between layers (e.g., sub-assemblies of the multi-layer structure 301-310). To the interior of layer 306 is provided layer 307 having an array of cavities (e.g., cells or compartments) arranged in a honeycomb structure (not shown). The cavities are hexagonal prismatic cavities which are tessellated relative to one another such that the long axes of adjacent cavities are parallel. The cavities are hollow and have thin walls relative to the dimensions of their internal cavities such that, by volume, each cavity may largely consist of air or other gas. The layer 307 has a thickness of between 0.5 mm and 10 mm, such as between 1.5 mm and 2.5 mm. The cavities are small in relation to the thickness of the layer 307, such that a large number of cavities is provided along the thickness direction T. The cavities extend such that their long axes are perpendicular to the thickness direction. Accordingly, in order for heat to travel along the thickness direction T, it must pass from cavity to cavity rather than remain within a single cavity. In this manner, the transfer of heat along the thickness direction T towards the external surface of the enclosure 200 is reduced. Each cavity may have long axis dimensions between 2 mm and 12 mm, and may have short axis dimensions less than half of the long axis dimensions, such as about a quarter of the long axis dimensions. The layer 307 may be formed from aluminium (e.g., an aluminium alloy) or an aramid, (such as Kevlar, Nomex with a resin system (e.g., phenolic, polyimide), which may be preferred due to a higher temperature resistance and lower density. Glass fibre may also be used. Accordingly, the layer 307 has a high resistance to heat transfer and so is highly insulating in a thermal runaway event. Additionally, the layer 307 has high ballistic protection characteristics. In the aviation environment, this is particularly beneficial. Additionally, layer 307 enhances the mechanical properties of the enclosure 200 due to its high cross-sectional inertia increasing the torsional stiffness of the enclosure 200. Although described in relation to hexagonal cavities, it will be understood that an alternative arrangement of cavities may provide similar properties. For example, a tessellated array of prismatic cavities of alternative geometries (e.g., square, triangular, tubular, pentagonal, tetrahedral, pyramidal) may also be suitable. Hexagonal cavities may achieve an optimal balance between mechanical strength and degree of tessellation, making the material lightweight. Due to the long axes of the cavities being perpendicular to the thickness direction T, the boundaries of layer 307 with the layers 306 and 308 may not be flat or planar but may instead have a series of recesses (e.g., a series of triangular or trapezoidal recesses when the layer 307 has a honeycomb structure). During forming, the material of layer 306 may flow into the recesses at the boundary of layer 307 and so layer 306 may aid in bonding the layer 307 to the rest of the structure of the enclosure 200, such as by forming a relatively flat exterior surface, e.g., for bonding with layer 305. Similarly, the resin of layer 308 (described below) may flow into the recesses on the opposing boundary of layer 307. Accordingly, the provision of the layer 306 and / or the layer 308 adjacent layer 307 may be particularly advantageous. Internally to the layer 307 is provided a layer 308 comprising aramid fibres, such as part of an aramid fibre resin matrix, e.g., Kevlar (RTM). The layer 308 has a thickness between 0.2 mm and 0.5 mm. Kevlar and other aramid fibres are strong and heat resistant fibres and so provide good resistance to flames and high temperatures caused by a thermal runaway event. Further, layer 307 may improve the mechanical impact strength, penetration resistance, and / or ballistic protection characteristics of the enclosure 200, providing protection from external sources towards the battery pack as well as outwardly from the internal compartment 204, e.g., due to any ejecta produced by a thermal runaway event. Within the multi-layer structure at portion III-III, the layer 308 is provided towards the internal compartment 204 as, for aviation, the risk of penetration outwardly may be a greater risk to the aircraft than is inward penetration (e.g., unlike in automotive environments where inward penetration may be a greater risk). Accordingly, by providing the layer 308 closer to the compartment 204 than are layers 301-307, the effects on layers 301-307 of a thermal runaway event (e.g., including any ejecta) may be reduced. For example, physical damage or crumpling to layer 307 may be reduced or prevented, and damage to the grounding layer 304 may be prevented. It will be understood that alternative arrangements of layer 308 may be desirable in other environments. To the interior of the layer 308, the enclosure 200 comprises a barrier layer 309, which may comprise ceramic fibres, such as in a ceramic fibre cloth. The ceramic fibres may form part of a fibre-reinforced polymer composite, such as a resin (e.g., a thermosetting resin such as epoxy) reinforced with calcium-magnesium silicate fibres. The layer 309 has a thickness between 0.1 mm and 2 mm. The layer 309 is configured to act as a physical and chemical barrier to substances emitted by a battery during a thermal runaway event. The layer 309 comprising ceramic fibres may be chemically resistant to any substances ejected during a thermal runaway event, have a high surface hardness to mitigate the abrasive effects of any ejected debris and remain non-combustible even at the high flame temperatures. It may therefore be desirable to provide this layer 309 to the interior of layers 301-308 which may be more susceptible to such ejecta. Ceramic fibres in the form of a cloth, a paper or a felt (e.g., rather than a ceramic coating) may provide particularly beneficial ballistic properties during a thermal runaway event. Additionally or alternatively to ceramic fibres, the layer 309 may comprise ceramic in another form, such as mica, or may comprise natural cork and / or a thin layer or sheet of stainless steel. These materials may function in a similar way as the ceramic fibres to provide a barrier having high flame retardance, high temperature resistance, high chemical resistance and / or high surface hardness. Accordingly, layer 309 may protect layers 301-308 to the exterior from the effects of any thermal runaway event. On an innermost surface, the enclosure 200 comprises an electrical insulation layer 310. The electrical insulation layer 310 comprises a polymer, such as a polyimide or Nomex (RTM), of thickness between 50 and 300 micrometres. The layer 310 may be applied as a film, a coating or a label. The electrical insulation layer 310 may prevent short circuits from occurring if contact is made between the enclosure 200 and exposed HV conductors of the battery assembly. For example, the layer 310 may provide increased electrical insulation in crush or impact scenarios where the enclosure wall may be deformed inwards and could contact exposed live electrical conductors. Similarly, the layer 310 may minimise other effects of the HV battery assembly, such as partial discharge (arcing, corona, surface, void discharge) in the internal compartment 204. The layer 310 additionally acts as a moisture barrier to prevent condensation being wicked into the multilayer structure 301-310. Further, the layer 310 may provide a flat surface for gaskets and other seals. By providing an innermost electrical insulation layer 310, the clearance between the interior surface of the enclosure 200 and the battery pack may be reduced. For example, in previous enclosures, it may have been necessary to provide a large clearance between an interior surface of an enclosure and the battery pack. However, by providing an electrically insulating layer 310 innermost, this clearance can be reduced and so the energy density of the battery pack assembly may be increased. Although in a thermal runaway event the layer 310 may be burnt off, the electrical insulation properties of the innermost layer 310 may be prioritised over its resistance to high temperatures. It will be understood by the skilled person that layers 301-310 are inverted in base component 202 such that ingress protection layer 301 remains outermost and electrical insulation layer 310 remains innermost. Although described in relation to portion III-III of cover 201, layers 301-310 may extend continuously throughout the enclosure 200. For example, the layers 301-310 may extend continuously around corners or contours of the main body of the enclosure 200, and in both the cover and base 201, 202. Accordingly, a proportion of the layers 301-310 of either component 201, 202 may intersect the interface 203 such that, upon assembly of the enclosure 200, the layers 301-310 in the cover 202 may align with and abut a corresponding layer 301-310 provided in the base 201. For example, the conductive cables in the grounding layer 304 may form an electrical connection across the interface 203 (e.g., via conductive gaskets and / or mechanical connectors) such that the internal compartment 204 is enclosed by a continuous grounding layer 304. Additionally or alternatively, certain layers may not extend to or intersect the interface 203. For example, layers 306, 307, 308, 309, 310 may not be provided at a portion of the enclosure 200 where mechanical load transfer occurs (e.g., portion IV-IV). The extent to which mesh layer 303 extends across the interface 203 may depend on the connection to grounding layer 304 and / or gasket arrangement. Away from the main body of the enclosure, such as the connecting regions 206, 207, the number and thicknesses of the layers 301-310 may vary. For example, the thickness of the layer 307 comprising a honeycomb structure may be reduced (e.g., to zero) in the connecting regions 206, 207. Figure 4 shows a magnified view of portion IV-IV at the location of the flange 206 of cover 202. The layer structure of the enclosure 200 at portion IV-IV may be identical to the layer structure at portion III-III with the following exceptions. The overall thickness T of the layer structure of the flange 206 is less than the overall thickness T of the main body of the enclosure at portion Layer 307 has been omitted entirely as connecting regions 206, 207 may not be directly exposed to a thermal runaway event and so the functions of these layers are less critical in this region. Similarly, the EMI shielding layer 303 has been omitted as the continuous interface 203 between layers 303 of the base and cover 201, 202 may be considered to provide sufficient EMI shielding. Accordingly, in order to increase the energy density of an assembly including the enclosure, layers 303, 307 may be omitted from regions 206, 207. Conversely, the thicknesses of layers 302, 305 are greater in the connecting regions 206, 207 in order to provide improved mechanical properties where the enclosure may be fastened to the airframe. Additionally, the presence and thickness of the layers 301-310 may vary in other portions of the enclosure 200. These portions (not shown) may include mounting points, lifting points, connection interfaces, cable management features and / or zones of a battery pack. Figure 5 shows an example method 500 of forming a battery enclosure 200. The method 500 comprises 501 providing a first layer having an array of cavities, such as the layer 307. The method 500 additionally comprises 502 providing a second layer comprising ceramic fibres, such as the layer 309, and 503 forming an internal compartment 204 at least partially defined by a plurality of layers 301-310 comprising the first layer 307 and the second layer 309. The method 500 may additionally comprise forming a sub-assembly of layers comprising: an electromagnetic interference shielding layer, such as the layer 303; and a grounding layer, such as the layer 304, provided between fibre reinforced polymer composite layers, such as the layers 302, 305. Although not shown, the enclosure 200 may comprise a number of additional features, such as metal inserts at the connecting regions 206, 207 or in the major wall at portion III-III (e.g., as fixing supports or local reinforcement). Additionally, the enclosure 200 may comprise built-in sensors, such as temperature sensors, pressure sensors, conductivity sensors (for e.g., for fluid leaks), strain gauges (e.g., for deformation or stress identification), and / or lights / LEDs (not shown). Further, the enclosure may comprise an integrated thermal heat transfer plate cold plate for conducting heat away from the internal compartment 204. The cold plate may be provided within the multi-layer structure 301-310. The enclosure may also comprise cable management features (such as clips, routing guides, cable separation, strain relief, etc.) within the composite structure 301-310. The enclosure may comprise shielded interconnecting cables to connect to the ground and EMI layers. This may be done by making areas with different layers and / or additional conductive inserts, local to the mounting hole / feature for the connector(s). The enclosure 200 may additionally comprise a thermal management system and / or a vent management exhaust system. Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and subcombinations of one or more features described herein. It will also be appreciated that whilst the invention has been 5 described with reference to aircraft and aircraft propulsion systems, the electric machine drive techniques described herein could be used for many other applications. These include, but are not limited to, automotive, marine and land-based applications.

Claims

1. A battery enclosure for housing a battery assembly, the enclosure comprising:an internal compartment for receiving the battery assembly; anda plurality of layers at least partially defining the internal compartment, the layers comprising:(i) a layer comprising a fibre-reinforced polymer composite;(ii) an electromagnetic interference shielding layer;(iii) a grounding layer;(iv) a layer having an array of cavities; and(v) a layer comprising ceramic, natural cork or stainless steel.

2. The enclosure of claim 1, wherein layer (v) is provided closer to the internal compartment than is layer (iv).

3. The enclosure of claims 1 or 2, wherein layer (iv) comprises a honeycomb structure which hexagonal prismatic cavities have longitudinal axes extending perpendicular to a thickness dimension of the enclosure.

4. The enclosure of any preceding claim, wherein layer (v) comprises a ceramic fibre cloth.

5. The enclosure of any preceding claim, additionally comprising a layer (vi) comprising aramid fibres, layer (vi) being provided closer to the compartment than is layer (iv).

6. The enclosure of claim 5, wherein layer (vi) is provided between layer (iv) and layer (v).

7. The enclosure of any preceding claim, wherein layer (iii) is provided externally to layer (iv).

8. The enclosure of any preceding claim, wherein layer (ii) is provided externally to layer (iii).

9. The enclosure of any preceding claim, wherein layers (i) to (v) are provided relative to one another in the order (i), (ii), (iii), (iv), (v).

10. The enclosure of any preceding claim, wherein layer (ii) and layer (iii) are provided between a pair of layers (i).

11. The enclosure of any preceding claim, further comprising a layer (vii) comprising a polyurethane foam or a polyethylene foam, layer (vii) being provided externally to layer (iv).

12. The enclosure of any preceding claim, further comprising an electrically insulating layer (viii) provided immediately adjacent the internal compartment.13.The enclosure of any preceding claim, comprising an ingressprotection layer (ix) outermost.

14. The enclosure of any preceding claim, wherein the enclosure comprises a first component part, a second component part and an interface therebetween, wherein at least one layer is continuous across the interface.

15. The enclosure of any preceding claim, wherein the plurality of layers is a first plurality of layers, and the enclosure comprises:a first region comprising the first plurality of layers; anda second region having a subset of the first plurality of layers.

16. The enclosure of claim 15, wherein:in the first region, a thickness of a layer in the first plurality of layers is less than a thickness of a corresponding layer in the second region; and / orin the first region, a thickness of a layer in the first plurality of layers exceeds a thickness of a corresponding layer in the second region.

17. An assembly comprising: a battery assembly; and the enclosure of any preceding claim.

18. An electric or hybrid aircraft comprising the enclosure of any ofclaims 1 to 16, and / or the assembly of claim 17.

19. A method of forming a battery enclosure according to any of claims1 to 16, the method comprising:providing (i) a layer comprising a fibre-reinforced polymer composite;5 providing (ii) an electromagnetic interference shielding layer;providing (iii) a grounding layer;providing (iv) a layer having an array of cavities;providing (v) a layer comprising ceramic, natural cork or stainless steel; and10 forming an internal compartment at least partially defined by layers (i) to (v).

20. The method of claim 19, comprising forming a sub-assembly of layers comprising layer (ii) and layer (iii) provided between a pair of layers 15 (i).

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