A battery pack

Abstract

A battery pack 100 comprising a battery enclosure defining a cavity 52 housing one or more battery cells 101 therewithin; and a non-reflective geometry 54 disposed on an internal surface 53 of the bat

Description

Field of Invention The present disclosure relates to a battery pack, an aircraft propulsion system, and an aircraft. Background Interest in aircraft with electric, hybrid electric and ‘more electric’ propulsion systems is increasing because of the need to reduce carbon emissions and pollution, and because of developments in the facilitating electrical technologies. Hybrid electric propulsion systems include engines, for example gas turbine or diesel engines, energy storage, typically in the form of batteries, and / or fuel cells. Purely electric propulsion systems completely dispense with engines and use only batteries and, in some cases, fuel cells as an energy source for their propulsors. So-called ‘more electric’ aircraft may also include energy storage to provide additional functionality (e.g., electric starting without the use of ground equipment) and to improve the efficiency and operability of their engines. The batteries used in these applications take the form of battery packs that include multiple battery cells, for example lithium-ion cells, electrically connected to meet the charge capacity and power requirements of the application. It is known that a battery cell may enter a state known as thermal runaway. In this state, a chain reaction within the cell causes the internal temperature and pressure to build until, unless mitigated, the cell fails. Some cell types, including some cylindrical cells, include a venting arrangement that is designed to open in the event of the onset of thermal runaway. The vent allows the release of heat, gas, and fire from the cell, in a controlled manner, to prevent the internal pressure and temperature building to the point of explosion. Another cell failure phenomenon, which is considered a type of thermal runaway, is known as side wall rupture (SWR). This is when the cell pressure vessel (e.g., the casing of a cylindrical cell) fails arbitrarily and bursts open, releasing gas, heat, and fire from a location along the side wall of the cell. It is possible for the thermal runaway (e.g., SWR) of one battery cell to propagate and initiate failure events in neighbouring cells of a battery pack. In a worst-case-scenario, the failure of a single cell or a small group of cells can result in the total destruction of a battery pack, accompanied by the release of a large amount of energy (e.g., a fire or explosion). Battery cells may enter thermal runaway as a result of a detectable defect or because they are subject to extreme conditions (e.g., operation with physical, electrical, or thermal conditions outside of their design window). It is, however, also possible for a cell to experience a failure due to an undetectable defect, such as may result from the inherent variability that can exist in the cell manufacturing process. In the automotive industry, for example, for product safety and certification assessment, battery cells are generally assumed to be safe (e.g., will not enter thermal runaway) unless they have a detectable defect or are subject to extreme conditions. Thus, automotive design and manufacturing processes may focus on detecting faulty cells and on preventing operation outside of the design window of the cells. In aerospace, however, the potential consequence of a propagating thermal runaway may be much more severe, creating safety concerns. It can also be challenging to develop ways to prevent or manage thermal runaway in aerospace applications while also satisfying the strict weight requirements in these applications. It is therefore desired to develop a battery pack for aerospace applications which addresses at least some of the aforementioned problems. Summary According to a first aspect, there is provided a battery pack, comprising: a battery enclosure defining a cavity configured to house one or more battery cells therewithin; and a non-reflective geometry disposed on an internal surface of the battery enclosure, wherein the non-reflective geometry is configured to inhibit reflection of media expelled by the one or more battery cells during thermal runaway of the one or more battery cells when the media impinges on the internal surface. The non-reflective geometry may comprise a first surface and a second surface intersecting the first surface to form an angle therebetween, wherein the angle is up to 120°. The angle may preferable be up to 90°. The non-reflective geometry may comprise a plurality of projections extending into the cavity. The first surface and the second surface may form at least part of a trough defined between two adjacent projections of the plurality of projections. The plurality of projections may comprise a plurality of pyramidal projections. Each of the plurality of projections may extend in an elongate manner along a length of the inner surface of the enclosure. The plurality of projections may comprise a plurality of triangular prismatic projections. Each of the plurality of projections may extend into the cavity by a height. The height may be variable along the length of the inner surface of the enclosure. Each of the plurality of projections may comprise a variable cross-section along the length of the inner surface of the enclosure. The plurality of projections may form a channel structure, wherein the channel structure is configured to direct the media to an outlet of the battery enclosure. The one or more battery cells may each comprise a vent configured to expel media during thermal runaway of the one or more battery cells. The non-reflective geometry may be disposed on an internal surface of the enclosure that is closest to the vent of the one or more battery cells. The non-reflective geometry may be formed from a resilient or flexible material. The non-reflective geometry may be formed from a porous material. The porous material may be an open-cell foam. The battery pack may further comprise a flame arrestor layer disposed between the one or more battery cells and the non-reflective geometry. The battery pack may be an aircraft battery pack. According to a second aspect, there is provided an aircraft propulsion system comprising a battery pack according to the first aspect. The aircraft propulsion system may be an electric propulsion system or a hybrid electric propulsion system. According to a third aspect, there is provided an aircraft comprising: a battery pack according to the first aspect; or an aircraft propulsion system according to the second 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 Figures, in which: Figure 1 is a sectional side view of a gas turbine engine; Figure 2A is a schematic illustration of a hybrid electric aircraft propulsion system; Figure 2B is a schematic illustration of a purely electric aircraft propulsion system; Figure 3 is a perspective view of an aircraft incorporating a battery pack; Figure 4A is a perspective view of an example battery pack for an aircraft; Figure 4B shows a cylindrical battery cell; Figure 4C shows a prismatic battery cell; Figure 4D shows a pouch cell; Figure 5 is a sectional side view through a first example battery pack according to the present disclosure; Figure 6 shows the first example battery pack of Figure 5 during thermal runaway of one of the battery cells; Figure 7 is a perspective view of a first example non-reflective geometry, as used in the first example battery pack of Figures 5 and 6; Figure 8 is a sectional side view through a second example battery pack according to the present disclosure; Figure 9 is a perspective view of a second example non-reflective geometry; Figure 10 is a perspective view of a third example non-reflective geometry; Figure 11 is a perspective view of a fourth example non-reflective geometry; and Figure 12 is a sectional side view through a third example battery pack according to the present disclosure. Detailed Description With reference to Figure 1, a gas turbine engine is generally indicated at 10, having a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a low-pressure turbine 19 and an exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines both the intake 12 and the exhaust nozzle 20. The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the intermediate pressure compressor 14 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow directed into it before delivering that air to the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high 17, intermediate 18 and low 19 pressure turbines drive respectively the high pressure compressor 15, intermediate pressure compressor 14 and fan 13, each by suitable interconnecting shaft. Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g., two) and / or an alternative number of compressors and / or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and / or fan. The gas turbine engine may also incorporate or interface with one or more batteries, for example to power ancillary electrical systems and / or to cooperate with one or more electric machines involved in the transfer of mechanical power to and from one or more of the interconnecting shafts. Now referring to Figure 2A, the propulsion system of a hybrid electric aircraft is generally indicated at 200 and incorporates both an engine 210, such as the gas turbine engine 10 described above with reference to Figure 1, and a battery pack 100. Both the engine 210 and the battery pack 100 are used as energy sources to power a motor-driven propeller 216, as well as ancillary electrical systems (not shown). The propulsion system 200 of the hybrid electric aircraft will typically further comprise a generator 211, an AC / DC converter 212, a high voltage DC (HVDC) distribution bus 213, a DC / AC converter 214, a motor 215 that drives the propeller 216, and a DC / DC converter 217. A shaft of the engine 210 is coupled to and drives the rotation of a shaft of the generator 211 which thereby produces alternating current. The AC / DC converter 212, which faces the generator 211, converts the alternating current into direct current which is fed to various electrical systems via the HVDC distribution bus 213. These electrical systems include the motor 215 that drives the propeller 216. The motor 215 will typically be a synchronous motor that interfaces with the HVDC distribution bus 213 via the DC / AC converter 214. The battery pack 100, which may be made up of a number of lithium ion battery modules connected in series and / or parallel, is connected to the HVDC distribution bus 213 via the DC / DC converter217. The DC / DC converter 217 converts between a voltage of the battery pack 100 and a voltage of the HVDC distribution bus 213. In this way, the battery pack 100 can replace or supplement the power provided by the engine 210 (by discharging and thereby feeding the HVDC distribution bus 213) or can be charged using the power provided by the engine 210 (by being fed by the HVDC distribution bus 213). It will be understood that the hybrid electric propulsion system 200 of Figure 2A is of the series hybrid type. Parallel hybrid electric systems, turboelectric systems and systems having features of more than one type are also known and in accordance with the present disclosure. A battery pack will also appear in the propulsion system of a purely electric aircraft, generally indicated as 300 in Figure 2B. The battery pack 100 feeds a HVDC distribution bus 313, possibly via a DC / DC converter (not shown), which delivers power to one or more synchronous motors 315 via a DC / AC converter 314. The one or more motors 315 drive the one or more propellers 316 that propel that aircraft. Figure 3 illustrates a conventional take-off and landing (CTOL) aircraft 40 with a purely electric propulsion system. The front portion of the aircraft 10 is cutaway to show a battery pack 100, which in this example is located in what would be the engine bay of an engine-powered aircraft. The battery pack 100 supplies electrical power to the windings of an electric motor via an inverter, and the rotor of the motor drives rotation of a shaft of a propeller 42. It will be understood that the electric CTOL aircraft 40 of Figure 3 is only intended to be an example. Various other configurations are known, will occur to those skilled in the art and are in accordance with the present disclosure. Figure 4A shows the outer structure of a battery pack 100 such as may be used in the aircraft propulsion systems 200, 300 and the aircraft 40 of Figures 2A-2B and 3. The battery pack 100 includes a housing comprising first and second (upper and lower) housing portions 110a, 110b. Each of the housing portions 110a, 110b houses a plurality of battery cells, for example cylindrical lithium-ion battery cells. In this example, each housing portion 110a, 110b has a removable lid 111a, 111b that allows access to an internal region in which the cells are located. In this way, cells can be added or removed from the pack 100 during assembly or maintenance. The two housing portions 110a, 110b are further shown to be separated by, and located on opposite sides of, a cooling channel 160 through which a coolant (e.g., a liquid coolant or a gaseous coolant such as air) may pass to cool the cells. For example, heat produced by the cells of the pack 100 may be exchanged with the coolant flowing through the channel 160 via an internal surface of the channel 160. It will be appreciated that the structure of the battery pack 100 is merely one example and is not intended to limit the scope of the present disclosure. For example, a battery pack may not include two housing portions 110a, 110b, and could instead include only one or more than two housing portions. Further, a battery pack 100 may take a different approach to cell cooling, for example using direct cooling instead of indirect cooling, liquid cooling instead of air cooling, and / or cooling that makes use of phase change materials. Figure 4B shows a single cylindrical battery cell 101. The cell 101 comprises a cylindrical outer sidewall 1010 that extends in a z-direction, parallel to a longitudinal axis of the cylinder, between a first side 1011 and a second side 1012. The cell has a positive terminal 1014 and a negative terminal 1015. The two terminals 1014, 1015 are located at the same, first, end 1011 of the cell. The positive terminal 1014 occupies a central region of the cell and the negative terminal 1015 occupies a radially outer region of the cell (e.g., the cell rim). In other cylindrical cells, the positive and negative terminals may be located at axially opposite sides of the cell. The internal structure of the cell 101, which typically includes a rolled up layered structure, will be familiar to those skilled in the art and will not be described further. As described previously, some cylindrical cells can also include a vent 1016 for allowing the cell to expel gas, flames and other thermal products in a controlled manner. In some examples, the vent 1016 is configured to expel thermal products in the z-direction from one of the ends of the cell 101, for example, the first end 1011. In accordance with the present disclosure, the cylindrical cell may be of any suitable type, including cells of the 4680, 4690, 18650, 21700 and 26650 types. In some examples, a battery pack may include cells of more than one type (e.g., 4680 cells and 21700 cells). Figure 4C shows a single prismatic battery cell 10T. The cell 101’ comprises a prismatic shape, generally in the form of a cuboid. The cell 10T comprises four sidewalls 1010’ extending in a z-direction, between a top end 1011’ and a bottom end 1012’. The cell has a positive terminal 1014’ and a negative terminal 1015’. The two terminals 1014’, 1015’ are located at the same, top, end 1011’ of the cell 10T. In other prismatic cells, the positive and negative terminals may be located at axially opposite sides of the cell. Similar to cylindrical cells, prismatic cells can include a vent 1016’ for allowing the cell to expel gas, flames and other thermal products in a controlled manner. In some examples, the vent 1016’ is configured to vent or expel thermal products in the z-direction from one of the ends of the cell 10T, for example, the top end 101T. In accordance with the present disclosure, the prismatic cell 101’ may be of any suitable type. Figure 4D shows a single pouch cell 101”. The cell 101” has a generally flat shape, having a first side 1010” and an opposed second side (not shown), which are joined together to form an external casing 1017”. The first and second sides extend in a z-direction, between a first end 1011” and a second end 1012”. The cell 101” has a positive terminal 1014” and a negative terminal 1015”. The two terminals 1014”, 1015” are located at the same, first, end 1011” of the external casing 1017” and are formed as generally flat cell tabs. Pouch cells 101” do not generally have a vent in the manner of the cylindrical cells or the prismatic cells. Instead, in the event of thermal runaway, pouch cells can rupture at various points on the external casing 1017”, typically along the joining points of the first side 1010” and the second side. Figure 5 shows a portion of a first example battery pack 100 in accordance with the present disclosure. In examples, the battery pack 100 may correspond to one of the first and second housing portions 110a, 110b as described with reference to Figure 4A. The battery pack 100 comprises an enclosure or housing 50 and at least one battery cell 101. The enclosure 50 defines an internal cavity 52 configured to house the at least one battery cell 101. In the present example, the enclosure 50 houses a plurality of battery cells 101a-101c. The plurality of battery cells 101a-101c are arranged in an array within the enclosure 50. The battery cells 101 may be cylindrical cells or prismatic cells. The cells 101 may be located in their positions in the array by a carrier structure (not shown). The enclosure 50 comprises an internal surface 53 facing into the cavity 52. A portion of the internal surface 53 comprises a non-reflective geometry 54. The non-reflective geometry 54 is disposed on a portion of the internal surface 53 which is closest to a vent 1016a-1016c of the battery cells 101. In the present example, the non-reflective geometry 54 is disposed at a portion of the internal surface 53 which is closest to the top 1011a-1011c of the battery cells 101, as the respective vents 1016a-1016c of each of the cells 101 are disposed towards the top 1011a-1011c of each cell 101. The non-reflective geometry 54 is configured to inhibit reflection of media expelled by the at least one battery cell 101 during thermal runaway of the at least one battery cell 101. As described previously, during thermal runaway of a battery cell 101, the cell 101 may vent or rupture to expel media, including gas, fire, debris, and / or particles. The non-reflective geometry 54 is configured to inhibit or substantially prevent the media from being reflected or transmitted to neighbouring or adjacent cells 101 when the cell 101 undergoes thermal runaway. For this purpose, the non-reflective geometry 54 is configured to dissipate and / or absorb the media expelled by the cell 101 when it impinges on the internal surface 53 formed by the non-reflective geometry 54, before it can reach neighbouring or adjacent cells 101. The non-reflective geometry 54 may have an anechoic geometry. The non-reflective geometry 54 comprises a plurality of surfaces including a first surface 55 and a second surface 56. The first surface 55 and the second surface 56 intersect to define a trough 57 therebetween. The first surface 55 and the second surface 56 together define an angle 0 therebetween. The angle 0 can be any angle up to 120°. More preferably, the angle 0 can be any angle up to 90°. This angle 0 enables the non-reflective geometry 54 to inhibit reflection of media expelled by the cells 101 when the media impinges upon the internal surface 53 formed by the non-reflective geometry 54, as will be explained further below. The plurality of surfaces also comprises a third surface 59 and a fourth surface 60. The plurality of surfaces enable the non-reflective geometry to comprise a plurality of projections 61, with each two adjacent projections of the plurality of projections defining a trough 57 therebetween. The third surface 59 is disposed adjacent to the first surface 55. The third surface 59 and the first surface 55 intersect to define a first projection 611 of the plurality of projections 61. The fourth surface 60 is disposed adjacent to the second surface 56. The second surface 56 and the fourth surface 60 intersect to define a second projection 612 of the plurality of projections 61. The plurality of projections 61 extend into the cavity 52 from the internal surface 53. In the present example, each projection is pyramidal in shape, as shown in Figure 7. The trough 57 defined by the first surface 55 and the second surface 56 is formed in between the first projection 61 and the second projection 62. As shown in Figure 7, each trough 57 is formed by the space between adjacent projections 61. Each projection 61 extends into the cavity 52 by a height H from a bottom of the respective adjacent trough 57 to a tip of the projection 61. Similarly, each trough 57 has a depth D from a bottom of the trough 57 to the tips of the adjacent projections 61. The non-reflective geometry 54 can be formed from a fire-retardant or fire-resistant material. The non-reflective geometry 54 may be integrally formed with the enclosure 50 or formed as a separate component and subsequently attached or fixed within the enclosure 50 using any suitable means of attachment. Each cell 101 is positioned in the enclosure 50 such that its vent 1016 is disposed substantially directly below a trough 57 formed in the non-reflective geometry 54 of the internal surface 53. In present example, the cells 101 comprise a first cell 101a, a second cell 101b, and a third cell 101c. The cells 101a-c are arranged in series in the enclosure 50. The first cell 101a is positioned in the enclosure 50 such that its vent 1016a is positioned below a first trough 571 of the non-reflective geometry 54. The second cell 101b is positioned in the enclosure 50 such that its vent 1016b is positioned below a second trough 57 of the non-reflective geometry 54. The third cell 101c is positioned in the enclosure 50 such that its vent 1016c is positioned below a third trough 573 of the non-reflective geometry 54. Figure 6 shows the first example battery pack 100 during thermal runaway of one or more of the cells 101. In particular, the second cell 101b is undergoing thermal runaway. During thermal runaway, the second cell 101b expels media 70 via its vent 1016b at the top of the cell 101b. The media 70 expelled by the cell 101b can include one or more of gas, fire, debris, particles, or other thermal products. When the media 70 is expelled out of the vent 1016b of the cell 101b, the media tends to travel upwards towards non-reflective geometry 54 of the internal surface 53 at the inner top end of the enclosure 50, due to the vent 1016b being located at the top of the cell 101b. The media 70 enters the second trough 57 which is positioned above the top of the second cell 101b. When the media 70 enters the second trough 57, the media 70 impinges against one or both of the first surface 55 and the second surface 56 forming the trough 57. Due to the angle 0 between the first surface 55 and the second surface 56, the media 70 subsequently impinges against the other of the first surface 55 and the second surface 56, rather than being reflected directly back towards the cell 101b. The media 70 continues to impinge between the first surface 55 and the second surface 56. During this process, the kinetic energy carried by the media 70 is gradually dissipated by contact with the first surface 55 and the second surface 56, and due to contact with air in the trough 57. It will be understood that the depth D of the trough 57 can be varied to vary the non-reflective capability of the non-reflective geometry 54. In particular, a trough 57 having a relatively high depth D has a relatively high non-reflective capability as compared to a trough 57 having a relatively low depth D. This is because the increased depth D enables the trough to have a greater surface area, thus causing more kinetic energy of the media to be dissipated. The non-reflective geometry 54 can be formed from a resilient or flexible material which can increase the amount of energy absorbed from the media 70 as it impinges thereupon. In addition, or alternatively, the non-reflective geometry 54 may be formed from a porous material such that at least part of the media 70 can be absorbed by the non-reflective geometry 54 when the media 70 impinges on the first surface 55 and the second surface 56. For example, particles 72 in the media 70 may be absorbed by the non-reflective geometry 54 as the media 70 impinges on the first surface 55 and the second surface 56. A trough having a relatively high depth D has a greater surface area, enabling a greater proportion of the media 70 to be absorbed. The porous material may include an open cell foam. An open-cell foam may include one or more of the following: open-cell polymeric foams, e.g., fire-resistant Pll foam, PI foam, or TPU foam, where the foams may have additional fire-retardant additives; open-cell ceramic foams; and open-cell metal foams. In this way, the media 70 expelled by the cell 101b is prevented from spreading to any of the adjacent cells 101a, 101c. It will also be understood that instead of having a single trough disposed above a cell 101, more than one trough may be disposed above a cell 101. For example, two or more such troughs may be disposed above a cell 101, such that media expelled from the cell 101 can enter one or more of the troughs. Figure 8 shows a second example battery pack 100’ according to the present disclosure. The second example battery pack 100’ is substantially similar to the first example battery pack 100 but differs by the inclusion of a flame arrestor layer. The battery pack 100’ comprises a flame arrestor layer 80 disposed between the cells 101 and the non-reflective geometry 54. In particular, the flame arrestor layer 80 is disposed directly above the cells 101 and below the non-reflective geometry 54 of the internal surface 53 of the enclosure 50. This is so that any media 70 expelled by the cells 101 reaches the flame arrestor layer 80 before reaching the non-reflective geometry. The flame arrestor layer 80 is formed as a substantially planar layer. The flame arrestor layer 80 may be fixed in its position within the enclosure 50 by attaching it to one or more walls of the enclosure 50 or to the internal surface 53, including to the non-reflective geometry 54. In other examples, the flame arrestor layer 80 may be formed in any suitable shape, for example in a shape which corresponds to the shape of the non-reflective geometry 54. The flame arrestor layer 80 is configured to enable gas to pass therethrough but prevent fire or flames from passing therethrough. The flame arrestor layer 80 is configured to absorb heat from a flame, thereby causing the flame to be extinguished. The flame arrestor layer 80 comprises channels or passages configured to absorb the heat from a flame. For example, the flame arrestor layer 80 may be formed as a mesh (e.g., a metallic mesh) having channels defined in the gaps between the mesh material. Alternatively, the flame arrestor layer 80 may be formed from a crimped metal ribbon having channels formed therethrough by the crimping. In other examples, the flame arrestor layer 80 may be formed from an open-cell metal foam or an open-cell ceramic foam. During thermal runaway of a cell 101, for instance, the second cell 101b, media is expelled from its vent 1016b. The media 70 can include fire or flames, as well as gas or other particles. The media 70 is expelled upwards towards the flame arrestor layer 80. If flames are present, they are extinguished by the flame arrestor layer 80 as the media reaches the flame arrestor layer 80. Subsequently, only gas or other particles pass through the flame arrestor layer 80 and reach the trough 57. The flame arrestor layer 80 also acts to absorb heat from the media. In a similar manner as described for the first example battery pack 100, the non-reflective geometry 54 inhibits the reflection of the media 70 when it impinges upon the internal surface 53 formed by the non-reflective geometry 54. In particular, by striking the first and second surfaces 55, 56 of the trough 57, the kinetic energy of the media can absorbed. Gas or particles can also be absorbed by the non-reflective geometry 54 if its material is porous, as described above. Figure 9 shows a second example of a non-reflective geometry 54’ forming part of an internal surface of an enclosure of a battery pack, according to the present disclosure. The second example non-reflective geometry 54’ is similar to the first example non-reflective geometry 54, in that it also comprises a plurality of projections 61’ and troughs 57’ formed between adjacent projections. However, the second example non-reflective geometry 54’ comprises a plurality of triangular prismatic projections 61’, instead of pyramidal projections. Each of the triangular prismatic projections 61’ comprise a triangular cross section, and each projection 61’ extends along the length of the non-reflective geometry 54’. A trough 57’ is formed between two adjacent projections 61’. Each trough 57’ is defined by a respective first surface 55’ and a respective second surface 56’. The first surface 55’ and the second surface 56’ define an angle 9’ therebetween. The angle 0’ may be any angle up to 120°. More preferably, the angle 0’ may be any angle up to 90°. Each projection 61’ extends into the cavity 52 of the enclosure 50 by a respective height H’, from a bottom of the respective adjacent trough 57’ to a tip of the projection 61’. The height H’ is substantially constant along the length of each projection 6T. Due to each projection 61’ extending along the length of the non-reflective geometry 54’, each trough 57’ also extends along the length of the non-reflective geometry 54’, thereby having a substantially elongate profile. Each trough 57’ extends along the length of the non-reflective geometry 54’ in a substantially linear manner. A depth D’ of each trough 57’ is substantially constant. Each trough 57’ may be positioned in a manner such that it corresponds to one or more cells 101. For example, each trough 57’ may be positioned to correspond to a plurality of cells 101, such that media 70 expelled by any of the plurality of cells 101 can be received in the trough 57’. In use, when media 70 is expelled from a cell 101, the non-reflective geometry 54’ is configured to inhibit reflection of the media 70 in a similar manner as previously described for the first example battery pack 100. In addition, due to the elongate profile of each trough 57’, the troughs 57’ form a channel structure in which each trough 57’ can direct the media 70 as it is expelled from a cell 101. The media 70 may flow along the trough 57’ and the trough 57’ may have an outlet at an end of the trough 57’, where the outlet is configured to vent the media 70 to a region outside of the battery pack. The second example non-reflective geometry 54’ may be formed from the same materials as described for the first example non-reflective geometry 54. The second example non-reflective geometry may be combined with a flame arrestor layer, in a manner similar to the example described with reference to Figure 8. Figure 10 shows a third example of a non-reflective geometry 54” forming part of an internal surface of an enclosure of a battery pack, according to the present disclosure. The third example non-reflective geometry 54” is similar to the second example non-reflective geometry 54’. However, the third example non-reflective geometry 54” differs in that the height H” of each projection 61” is variable along its length. As shown in Figure 10, the non-reflective geometry 54’” is formed by a plurality of triangular prismatic projections 61”. Each of the projections 61” comprises a substantially triangular cross section, and each projection 61” extends along the length of the non-reflective geometry 54”. A trough 57” is formed between two adjacent projections 61”. Each trough 57” is defined by a respective first surface 55” and a respective second surface 56”. The first surface 55” and the second surface 56” define an angle 0” therebetween. The angle 0” may be up to 120°. More preferably, the angle 0” may be up to 90°. Each projection 61” extends into the cavity 52 of the enclosure 50 by a respective height H”, from a bottom of the respective adjacent trough 57” to a tip of the projection 61”. The height H” is variable along the length of the respective projection 61”. The height H” is variable along the length of the respective projection 61” in a substantially sinusoidal or undulating manner. However, in other examples, the height H” may be variable along the length of the respective projection 61” in any other manner, for example, in a zig-zag like manner. Due to each projection 61” extending along the length of the non-reflective geometry 54”, each trough 57” also extends along the length of the non-reflective geometry 54”, thereby having a substantially elongate profile. Each trough 57” extends along the length of the non-reflective geometry 54” in a substantially linear manner. A depth D” of each trough 57” can also vary along its length. Each trough 57” may be positioned in a manner such that it corresponds to one or more cells 101. For example, each trough 57” may be positioned to correspond to a plurality of cells 101, such that media 70 expelled by any of the plurality of cells 101 can be received in the trough 57”. Cells 101 may be positioned in a group orcluster of two or more cells within the enclosure 50. The group or cluster of cells may be positioned adjacent to a region of a trough 57” which has the greatest depth D”. As discussed above, it has been found that the non-reflective capability of the non-reflective geometry is increased when the depth of the trough is increased. This means that cells 101 can be positioned adjacent to regions of a trough 57” which have the greatest depth D”, and regions of the trough 57” which have the lowest depth D” may have fewer or no cells 101 positioned adjacent thereto. This can be particularly useful when cells are positioned in groups or clusters within the battery pack, such that the non-reflective geometry 54” has the greatest non-reflective capability in regions which correspond to the greatest number of cells 101. In use, when media 70 is expelled from a cell 101, the non-reflective geometry 54” is configured to inhibit reflection of the media 70 in a similar manner as previously described for the first example battery pack 100. In addition, due to the elongate profile of each trough 57”, the troughs 57” form a channel structure, where each trough 57” can direct the flow of the media 70 as it is expelled from a cell 101. The media 70 may flow along the trough 57” and the trough 57” may have an outlet at an end of the trough 57”, where the outlet is configured to vent the media 70 to a region outside of the battery pack. The third example non-reflective geometry 54” may be formed from the same materials as described for the first example non-reflective geometry 54. The third example non-reflective geometry may be combined with a flame arrestor layer, in a manner similar to the example described with reference to Figure 8. Figure 11 shows a fourth example of a non-reflective geometry 54”’ forming part of an internal surface of an enclosure of a battery pack, according to the present disclosure. The fourth example non-reflective geometry 54”’ comprises a plurality of projections 6T”. Each projection 6T” extends along the length of the non-reflective geometry 54’”. Each projection 6T” has a variable cross-section along its length. Each projection 61’” thereby has an undulating or zig-zag-like profile along its length. A trough 57’” is formed between two adjacent projections 6T”. Each trough 57’” is defined by a respective first surface 55’” and a respective second surface 56’”. The first surface 55’” and the second surface 56’” define an angle 0’” therebetween. The angle 0’” may be up to 120°. More preferably, the angle 0’” may be up to 90°. Due to each projection 6T” extending along the length of the non-reflective geometry 54’”, each trough 57’” also extends along the length of the non-reflective geometry 54”, thereby having a substantially elongate profile. Due to the variable cross-section of the respective adjacent projections 6T”, each trough 57’” extends along the length of the non-reflective geometry 54’” in an undulating or wave-like profile. The angle 0’” remains constant along the length of each trough 57’”. The profile of each trough 57”’ is dependent upon the profile of the first and second surfaces 55’”, 56’” of the adjacent projections 6T”. Each projection 6T” extends into the cavity 52 of the enclosure 50 by a respective height H’”, from a bottom of the respective adjacent trough 57’” to a tip of the projection 6T”. The height H’” is substantially constant along the length of the respective projection 6T”. The depth D’” of each trough is also substantially constant along its length. However, in other examples, the height H’” may be variable along the length of the respective projection 6T” and / or the depth D’” may be variable along the length of the respective trough 57’”. In use, when media 70 is expelled from a cell 101, the non-reflective geometry 54’” is configured to inhibit reflection of the media 70 in a similar manner as previously described for the first example battery pack 100. In addition, due to the elongate profile of each trough 57’”, the troughs 57’” form a channel structure such that each trough 57’” can direct the flow of media 70 as it is expelled from a cell 101. The media 70 may flow along the trough 57’” and the trough 57’” may have an outlet at an end of the trough 57’, where the outlet is configured to vent the media 70 to a region outside of the battery pack. The fourth example non-reflective geometry may be used when the cells 101 are positioned in a staggered arrangement within the enclosure 50. The staggered arrangement of the cells 101 may correspond to the undulating or wave-like profile of the troughs 57”’. In particular, the cells 101 may be positioned such that their arrangement follows a pattern defined by the profile of the troughs 57”’. This can ensure that when the cells 101 are positioned in a staggered arrangement, the non-reflective geometry 54’” can be configured to inhibit the reflection of media expelled by one or more of the cells 101 when the media impinges upon the internal surface 53. The fourth example non-reflective geometry 54’” may be formed from the same materials as described for the first example non-reflective geometry 54. The fourth example non-reflective geometry 54’” may be combined with a flame arrestor layer, in a manner similar to the example described with reference to Figure 8. Figure 12 is a portion of a third example battery pack according to the present disclosure. The third example battery pack 100” is substantially similar to the first example battery pack 100, in that it comprises a battery enclosure 150 having an internal surface 153 with a non-reflective geometry 154. However, the third example battery pack 100” differs in that the non-reflective 154 is provided on multiple internal surfaces 153 of the enclosure 150. The battery pack 100” comprises at least one battery cell 101”. The cell 101” may be a pouch cell 101”, having terminals 1014”, 1015” at one end of a casing 1017”. Being a pouch cell 101”, the cell 101” may not have any vents for release of media during thermal runaway of the cell 101”. Instead, during thermal runaway, the casing 1017” may expand and eventually rupture at any point on the casing 1017”. This means that the direction at which media is expelled by the cell 101” is not readily predictable. In other examples, the battery cell 101” may be any other type of cell, for example, a cylindrical cell or a prismatic cell. The enclosure 150 comprises a non-reflective geometry 154 on its top and side internal surfaces 153. In other examples, the non-reflective geometry 154 may be formed on only one of the top internal surfaces or the side internal surfaces 153. The non-reflective geometry 154 is configured to inhibit reflection of media expelled by the at least one battery cell 101” during thermal runaway of the at least one battery cell 101”. The non-reflective geometry 154 is substantially similar to the non-reflective geometry as described in the first and second example battery packs 100, 100’. The non-reflective geometry 154 comprises a series of surfaces which form a plurality of projections 161. A trough 157 is formed between adjacent projections 161 of the plurality of projections 161. The trough 157 comprises a first surface 155 and a second surface 156 which intersect. The first surface 155 and the second surface 156 form an angle a therebetween. The angle a may be any angle up to 120°. More preferably, the angle a may be any angle up to 90°. When the cell 101” undergoes thermal runaway, the cell 101” may rupture at any point along its casing 1017” to expel media 170. The media may include gas, fire, debris and / or particles. In the example shown in Figure 12, the media 170 is shown as being expelled from a sidewall of the casing 1017”. The media 170 subsequently travels towards the non-reflective geometry 154 formed on the side internal surface 153 of the enclosure 150. As discussed with respect to the first example battery pack 100, the media 170 enters the trough 157 and impinges against the first surface 155 and the second surface 156, causing the kinetic energy of the media 170 to be dissipated. This inhibits reflection of the media 170 back towards the cell 101” or to any surrounding cells 101”. As with the first example battery pack 100, the non-reflective geometry may be formed from a resilient or flexible material, and / or a porous material. When formed from a porous material, at least part of the media 170 can be absorbed by the non-reflective geometry 54 when the media 70 impinges on the first surface 55 and the second surface 56. Although the example of Figure 12 only shows a single cell 101” within the enclosure 150, it will be understood that a plurality of cells 101” could also be contained within the enclosure 150. It will also be appreciated that the non-reflective geometry 154 may also be combined with a flame arrestor layer, similar to that of the second example battery pack 100’, described with reference to Figure 8. The present disclosure provides a battery pack which can substantially inhibit or prevent the propagation of thermal runaway between cells within the battery pack. This is achieved by the non-reflective geometry provided on at least a portion of the internal surface of the enclosure of the battery pack. The projections and troughs of the non-reflective geometry substantially dissipates the kinetic energy of the media expelled by a battery cell undergoing thermal runaway when the media impinges upon the non-reflective geometry of the internal surface of the cavity. This means that the media is not reflected back towards the cell or towards any adjacent or surrounding cells, thus reducing the likelihood of other cells also undergoing thermal runaway. Thereby, the risk of catastrophic failure of the battery pack can be substantially reduced. The non-reflective properties of the non-reflective geometry can be enhanced if the geometry is formed from a resilient or flexible material and / or from a porous material. A resilient or flexible material enables a greater amount of kinetic energy of the expelled media to be absorbed by the non-reflective geometry. A porous material enables at least a portion of the expelled media to be absorbed by the non-reflective geometry. A flame arrestor layer can also be provided to substantially extinguish fire or flames expelled by the battery cell. The non-reflective geometry can also provide the battery pack with an insulating function. For example, if formed from a porous material, such as a foam, the non-reflective geometry forms a layer of insulation on at least a portion of the enclosure of the battery pack. This can make the battery pack less susceptible to being exposed to extreme temperatures outside of the battery pack, and also reduce the likelihood of regions outside the battery pack being exposed to extreme temperatures from within the battery pack (e.g., due to thermal runaway of one or more cells). It will be understood that the invention is not limited to the embodiments abovedescribed and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A battery pack, comprising:a battery enclosure defining a cavity configured to house one or more battery cells therewithin; and5 a non-reflective geometry disposed on an internal surface of the battery enclosure, wherein the non-reflective geometry is formed from a porous material and configured to inhibit reflection of media expelled by the one or more battery cells during thermal runaway of the one or more battery cells when the media impinges on the internal surface and wherein the non-reflective geometry comprises a first surface and a second surface10 intersecting the first surface to form an angle therebetween, wherein the angle is up to 120°.

2. A battery pack as claimed in claim 1, wherein the angle is up to 90°.

3. A battery pack as claimed in any of claims 1 or 2, wherein the non-reflectiveOJ . geometry comprises a plurality of projections extending into the cavity; and15 wherein the first surface and the second surface form at least part of a trough1 defined between two adjacent projections of the plurality of projections.1—r-") 4. A battery pack as claimed in claim 3, wherein the plurality of projections compriseCM a plurality of pyramidal projections.

5. A battery pack as claimed in claim 3, wherein each of the plurality of projections 20 extend in an elongate manner along a length of the inner surface of the enclosure.

6. A battery pack as claimed in claim 5, wherein the plurality of projections comprises a plurality of triangular prismatic projections.

7. A battery pack as claimed in claim 5 or 6, wherein each of the plurality of projections extends into the cavity by a height; and25 wherein the height is variable along the length of the inner surface of the enclosure.

8. A battery pack as claimed in any one of claims 5 to 7, wherein each of the plurality of projections comprises a variable cross-section along the length of the inner surface of the enclosure.

9. A battery pack as claimed in any one of claims 5 to 8, wherein the plurality of projections form a channel structure, wherein the channel structure is configured to direct the media to an outlet of the battery enclosure.

10. A battery pack as claimed in any preceding claim, wherein the one or more battery 5 cells each comprise a vent configured to expel media during thermal runaway of the one or more battery cells;wherein the non-reflective geometry is disposed on an internal surface of the enclosure that is closest to the vent of the one or more battery cells.

11. A battery pack as claimed in any preceding claim, wherein the non-reflective 10 geometry is formed from a resilient or flexible material.

12. A battery pack as claimed in any preceding claim, wherein the porous material is an open-cell foam.

13. A battery pack as claimed in any preceding claim, further comprising a flame <N arrestor layer disposed between the one or more battery cells and the non-reflectiveT— 15 geometry.1—14. A battery pack as claimed in any preceding claim, wherein the battery pack is an aircraft battery pack.

15. An aircraft propulsion system comprising a battery pack as claimed in any of the preceding claims.20 16. An aircraft propulsion system as claimed in claim 15, wherein the aircraftpropulsion system is an electric propulsion system or a hybrid electric propulsion system.

17. An aircraft comprising:a battery pack as claimed in any one of claims 1 to 14; oran aircraft propulsion system as claimed in claim 15 or claim 16.25

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