Patent Publication Number: US-2023155205-A1

Title: Duct and a method of manufacturing a duct

Description:
The present invention relates to duct for communicating a fluid therethrough and in particular to a duct for communicating a heat transfer fluid therethrough for acting as a heat exchanger. 
     A wide range of modern machinery has a thermal management requirement due to excessive heating and/or cooling of the machinery which, if left unchecked, results in machinery operating outside an optimal efficiency envelope. With the advent of electric and hybrid vehicles in the last number of decades, thermal management of the battery packs for these vehicles has emerged as a key functional requirement of the overall vehicle system architecture. The key requirements for next-generation battery packs, particularly lithium-ion battery packs for vehicular applications, are improved gravimetric and volumetric energy density, improved cycle life and fast-charging. Gravimetric and volumetric energy densities are largely improved through advances in cell electrochemistry and chemical engineering. However, improvements in the mechanical design of the battery pack also have an appreciable impact on the overall weight and size of the pack. Battery pack mechanical design impacts cycle life and fast-charging capability mainly through the thermal management system. The thermal management system can be used to minimise temperature variations within the pack to prevent differential cell aging which would ultimately result in reduced cycle life. Furthermore, it is important to maintain a relatively constant temperature of 25° C. throughout the battery pack to maximise cell lifetime. The latter is particularly challenging to maintain during fast-charging due to the high heat generation within the pack. 
     Thermal management systems in state-of-the-art battery packs typically include a heat exchanger in the form of a duct. The duct provides a conduit through which a heat transfer material can pass through the pack to cool or warm the individual cells. Rigid metallic ducts are one option used to communicate a thermal management fluid through the battery packs. However, the obvious problem with rigid metallic ducts is that they must be carefully shaped during manufacture to ensure that the surface area in contact with the battery cells is optimized for thermal transfer. Flexible ducts are another option and are particularly useful since they are lightweight and can closely conform to the shape of the cells when in a pressurised or inflated state. However, a significant drawback of using flexible ducts is that they are liable to bursting: a build-up of pressure within the duct causes the duct wall(s) to stretch and thin, reducing the strength of the walls and potentially leading to leakage of heat transfer fluid within the pack. While the risk of bursting can be mitigated by increasing the wall thickness of the duct, doing so also increases the thermal resistance of the duct and therefore the effectiveness of the thermal management system. 
     A further problem with state-of-the-art battery packs is their propensity to catch fire. The risk of fire within a battery pack is increased when the cells are exposed to high temperatures, when short circuits develop and/or when the internal structure of the cells are compromised. For example, lithium plating and/or crystal formation within a cell can puncture internal cell dielectric membranes, resulting in potentially catastrophic short circuits and explosion of the cell. Such an event can spread throughout the pack causing all of the cells to catch fire. There exists a need for a way of stopping or reducing the spread of excessive heat from a localised region within a battery pack using a suitable thermal management system. 
     A yet further problem with a battery pack array and the duct for communicating a fluid therethrough for acting as a heat exchanger is that the heat of the fluid varies along the length of the duct and consequently so too does the heat transfer capacity of the fluid along the length of the duct. This means that there is a non-uniform heating or cooling of the cells of the battery pack as the fluid travels further along the duct. As a result, it proves difficult to maintain a relatively constant temperature of 25° C. throughout the battery pack to maximise cell lifetime. The latter is particularly challenging to maintain during fast-charging due to the high heat generation within the pack. 
     It is an object of the invention to obviate or mitigate the problems outlined above. In particular, it is an object of the invention to provide a duct which is capable of providing uniform heat transfer between a heat source and the duct along the length of the engaging surface areas of the duct and the heat source. 
     According to a first aspect of the invention there is provided a duct capable of engaging at least part of a surface area of a heat source, the duct extendable along and engageable with at least part of the surface area of the heat source along all or part of the length of the heat source from a first engagement position to at least one final engagement position between the duct and heat source, a heat transfer fluid flowable along an internal conduit of the duct such that heat can be transferred between the duct and the heat source via the heat transfer fluid about the engageable surface areas of the duct and the heat source, the duct being adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the heat source. 
     Ideally, the duct being adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the heat source along the length of the duct. 
     Advantageously, the duct being adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the heat source along the length of the duct compensates for the variation in temperature of the heat transfer fluid as a result of ongoing thermal transfer as the heat transfer fluid flows along the length of the duct. This ensures uniform thermal transfer between the heat source and the duct via the heat transfer fluid along the length of the duct as other parameters such as fluid temperature vary. 
     Ideally, the heat source comprises a battery pack comprising: one or more cells. 
     Preferably, the duct is a flexible duct. 
     Alternatively, the duct is a rigid duct. 
     In one embodiment, the duct is a metal or metal alloy duct. 
     Ideally, the duct is positioned proximally to the surface of the heat source such that heat can be exchanged between the duct and the heat source. 
     Preferably, the duct is positioned proximally to the surface of the one or more cells such that heat can be exchanged between the duct and at least one of the one or more cells. 
     In one embodiment, where the duct is a flexible duct, a potting means is provided adapted to act as a support for at least a part of the duct. Advantageously the flexible duct can closely conform to the surface shape of the heat source/cells within the pack while being reinforced by the potting means which acts to prevent the flexible duct from over inflation and/or bursting. 
     Preferably, the duct is configured to carry the heat transfer fluid from an inlet to an outlet to transfer thermal energy between the heat source and the duct at their engageable contact surfaces via the heat transfer fluid and wherein the thermal resistance of the duct at the inlet is higher than the thermal resistance of the duct at the outlet. 
     Ideally, the duct is configured to carry the heat transfer fluid from an inlet to an outlet to transfer thermal energy between the one or more cells and the duct at their engageable contact surfaces via the heat transfer fluid and wherein the thermal resistance of the duct at the inlet is higher than the thermal resistance of the duct at the outlet. 
     This is advantageous as varying the thermal resistance of the duct along the length of the duct promotes a uniform temperature distribution across the heat source/battery pack. 
     In particular, having a higher thermal resistance at the inlet to the duct prevents over cooling or heating of heat source/cells located proximal to the inlet where the temperature differential between the heat transfer fluid and the heat source/cells is at its greatest. 
     Ideally, the thermal resistance of the duct is varied linearly or non-linearly along the length of the duct such that the thermal resistance of the duct decreases as the temperature differential between the heat transfer fluid and the heat source/cells also decreases, thereby promoting uniform power dissipation along the length of the duct. 
     In one embodiment the wall thickness of the duct may be thicker at the inlet compared to the outlet. This is advantageous as increasing the wall thickness also increases the thermal resistance of the duct. As such increasing the wall thickness of the duct at the inlet also increases the thermal resistance of the duct. 
     In an embodiment the wall thickness of the duct may vary linearly along the longitudinal length of the duct. In another embodiment the wall thickness of the duct may vary non-linearly along the longitudinal length of the duct. Varying the wall thickness of the duct along the longitudinal length of the duct has the effect of varying the thermal resistance of the duct along its longitudinal length. 
     In an embodiment the wall thickness may be varied such that a substantially constant power dissipation is achieved along the longitudinal length of the duct. This is advantageous as it promotes an even temperature distribution throughout the array of cells. This may be achieved by increasing the thermal resistance along the length of the duct. 
     Optionally the heat source/battery pack comprises a plurality of ducts. 
     Preferably the one or more ducts are serpentine ducts. 
     Optionally the one or more ducts are manifold ducts. 
     Optionally the heat source/battery pack comprises one or more substantially straight ducts. 
     Optionally the heat source/battery pack comprises one or more parallel ducts. 
     Preferably the or each duct comprises one or more substantially straight sections. 
     Preferably the or each duct is configured to carry a coolant fluid. 
     Preferably the or each duct is configured to carry a water-glycol mixture. 
     Preferably the or each duct is pressurised by the coolant fluid to an inflated state. 
     Preferably the or each duct, when in the inflated state, is in conformity with the surface of the heat source/one or more cells. Advantageously, inflating the flexible duct such that its shape conforms to the shape of the heat source/cells improves the thermal contact between the or each duct and the heat source/cells such that the coolant fluid may transfer thermal energy between the coolant fluid and the heat source/cells more efficiently. 
     Preferably the shape of the or each duct partially conforms to at least part of the surface of the heat source/one or more cells. 
     Preferably the cells are cylindrical cells. Advantageously, the flexible duct is well suited for use with cylindrical cells as the duct can expand and conform to the undulating surface of the cylindrical cells, ensuring good thermal contact between the cells and the duct. 
     Preferably the battery pack comprises an array of cells. 
     Preferably the array of cells are in a close-packed configuration. 
     Preferably the minimum separation between the cells is 2 mm. 
     Preferably the flexible duct is positioned adjacent to one or more cells. 
     Preferably the flexible duct is positioned between cells. 
     Preferably the flexible duct is in direct contact with side surface(s) of the one or more cells. 
     Preferably the flexible duct is in indirect contact with side surface(s) of the one or more cells via an interface region or interface material. 
     Preferably the flexible duct is in indirect contact with side surface(s) of the one or more cells via a casing sheath surrounding the cell(s). 
     Preferably the flexible duct is in indirect contact with side surface(s) of the one or more cells via a thermally conductive filler material such as a conductive paste or adhesive. 
     Ideally the flexible duct is formed from a polymer-based material. 
     Preferably the flexible duct is formed from an inflatable plastics material. An inflatable plastics material is advantageous as the material is intrinsically electrically insulating, lightweight and does not corrode or chemically interact with a coolant such as a glycol water mix. 
     Ideally the inflatable plastics material is low-density polyethylene (LDPE). 
     Ideally the inflatable plastics material is linear low-density polyethylene (LLDPE). 
     Ideally the inflatable plastics material is high-density polyethylene (HDPE). 
     Ideally the inflatable plastics material is polyester. 
     Ideally the walls of the flexible duct are between 10 μm and 150 μm thick. 
     Advantageously, the inflatable plastics material may be made very thin which allows for good thermal transfer properties between the or each duct and the cells. 
     Preferably the flexible duct is a single-lumen duct. 
     Optionally the flexible duct is a multi-lumen duct. A multi-lumen duct may be used in large battery packs where a single lumen duct is not capable of promoting an even temperature distribution. 
     Ideally the multi-lumen duct comprises two or more lumens along which coolant fluid may flow. 
     Preferably the battery pack comprises a battery pack housing. 
     Preferably the battery pack comprises a lower clamshell. 
     Preferably the battery pack comprises an upper clamshell. 
     Preferably the lower clamshell and/or upper clamshell comprises one or more recesses for receiving and retaining cell(s). 
     Preferably the lower clamshell and/or upper clamshell comprises one or more apertures for receiving electrical connections to the cells. 
     Preferably one or more busbars are provided on the upper clamshell and/or lower clamshell. 
     Preferably the upper clamshell and/or lower clamshell comprise one or more apertures for electrically connecting cell(s) to the busbar(s). 
     Preferably the battery pack comprises one or more sidewalls. 
     Preferably the one or more sidewalls are attached to the lower clamshell. 
     Preferably the one or more sidewalls are attached to the upper clamshell. 
     Preferably the battery pack comprises a fluid inlet means. 
     Preferably the fluid inlet means provides a fluid entrance to the or each duct. 
     Preferably the fluid inlet means comprises an inlet nozzle. 
     Preferably the battery pack comprises a fluid outlet means. 
     Preferably the fluid outlet means comprises an outlet nozzle. 
     Preferably the fluid outlet means provides a fluid exit from the or each duct. 
     Preferably the fluid inlet means and/or fluid outlet means pass through apertures in the side wall(s). 
     Preferably the battery pack comprises at least one further flexible duct which is positioned between further cells. 
     Preferably the potting means comprises a potting material. 
     Ideally the potting means comprises foam. Advantageously foam is lighter than other potting materials and therefore reduces the overall weight of the pack when compared with other potting materials. 
     Optionally the potting means comprises a thermosetting plastic, silicone rubber gel or epoxy resin. 
     Ideally the potting means comprises a thermally insulating foam. Beneficially thermally insulating foam can prevent a high energy thermal event propagating through the battery pack. Furthermore, thermally insulating foam can reduce the effect of external temperature fluctuations on the battery pack and helps to ensure that the duct is the primary controller of thermal energy within the battery pack. 
     Ideally the potting means comprises expanded foam. Advantageously use of an expandable foam within the battery pack means that the foam, when in the expanded state, can substantially fill any gaps within the battery pack. Coupled with the thermal insulation properties of the foam, the ability for thermal events to travel through the pack is significantly reduced. 
     Ideally the potting means comprises intumescent foam. 
     Ideally the potting means is a polyurethane foam. 
     Ideally the potting means acts as a support for at least a part of at least one duct. 
     Ideally the potting means acts as a rigid support for at least a part of at least one duct. 
     Ideally the potting means can be poured into the pack in a liquid state and sets, cures or hardens within the pack. 
     Ideally the potting means, in its set, cured or hardened state, is substantially rigid such that it secures the cell(s) and the duct(s) in position within the battery pack. This is advantageous as it reduces the effects of vibrations on components within the battery pack. 
     Preferably the potting means surrounds at least a part of at least one duct. 
     Preferably the potting means surrounds the duct and provides total external support to the at least one duct. Advantageously surrounding the duct with potting material prevents excessive expansion and/or bursting of the duct. 
     Preferably the potting means defines a cavity within which at least a part of at least one duct is located. 
     Preferably the volume within the battery pack housing is substantially filled with the cells, support structure, duct and potting means. Advantageously, substantially filling the battery pack ensures that moisture and/or corrosive agents are excluded from the space within the battery pack. 
     Preferably the potting means acts as an adhesive. 
     Preferably the potting means acts as an adhesive to secure the or each duct in position. 
     Preferably the or each duct has an open configuration such that a heat transfer material is able to flow through the duct. 
     Preferably the or each duct is maintained in an open configuration by pressurised heat transfer fluid within the or each duct and/or via adhesion to the potting means. 
     Preferably the potting means acts as an adhesive to maintain one or more duct(s) in an open configuration. 
     Preferably the potting means is adhesively attached to at least a part of one or more duct(s). 
     Preferably the potting means acts as an adhesive to secure the cell(s) in position. 
     Preferably the potting means acts as an adhesive to secure an outer casing to the battery pack. This beneficially negates the requirement for additional fixings or fasteners, reducing the complexity of the battery pack and improving the manufacturing process. 
     Preferably the battery pack comprises at least one support means configured to provide support to at least one duct. 
     Preferably the or each support means is locatable on the lower clamshell. 
     Preferably the one or more support means is located at the peripheral edge of the array of cells. 
     Preferably the or each support means is configured to provide support to a duct at a point where the duct changes and/or reverses direction. Advantageously, the support means prevents the duct from kinking at points where it reverses direction. Preventing kinking reduces blockages within the system, reduces pressure losses within the system and improves the flow rate of thermal transfer fluid through the duct(s). 
     Preferably the or each support means comprises a guide channel. 
     Ideally the guide channel is configured to guide the flexible duct. 
     Preferably at least part of the flexible duct is located within a support means channel. Locating the duct within a channel is advantageous as the channel guides the duct at points where the duct reverses direction thus preventing kinks. Furthermore, the channel provides support to the duct on both sides which prevents the duct bulging and potentially bursting. 
     Preferably the support means comprises at least one recess configured to partially receive the duct in an uninflated state such that slack is created in the duct. Beneficially, providing the duct with excess slack helps prevent the duct kinking when the duct is inflated. This is because as the duct is inflated it comes under tension and the excess slack helps to prevent excess tension building in the duct. 
     Preferably the support means is configured to provide a thermal barrier between at least one cell and the duct. This is beneficial because it is important to maintain a constant temperature distribution across the battery pack in order to prolong the life of the battery. By thermally insulating a cell at a location where there is too much thermal contact between the duct and the cell, the thermal contact between the duct and the cells is kept substantially constant throughout the battery pack. This in turns promotes a constant temperature distribution across the battery pack. 
     Preferably the battery pack is operably connected to a thermal management system. 
     Preferably the thermal management system comprises a reservoir. 
     Preferably the reservoir is in fluid communication with a heat transfer loop. 
     Ideally the reservoir comprises a heat transfer fluid. 
     Preferably the reservoir provides hydrostatic pressure to heat transfer fluid in the heat transfer loop. 
     Preferably the thermal management system comprises a pump configured to pump heat transfer fluid from the reservoir to the heat transfer loop to pressurise the heat transfer loop. Advantageously, heat transfer fluid in the reservoir may be used to pressurise the thermal management system. Beneficially this allows the pressure to be maintained within the thermal management system such that the pressure is maintained at a target operating pressure. Pressurising the duct via the reservoirs makes it self-supporting thus eliminating any of the hydrodynamic pressure loss from the pump and greatly reducing the pressure drop within the cooling system. 
     Ideally thermal management system comprises a pressure sensor to monitor the pressure of the heat transfer fluid such that a target operating pressure is maintained. 
     Preferably the duct material comprises a matrix and a filler. Ideally the thermal conductivity of the filler is greater than the thermal conductivity of the matrix. Advantageously, the inclusion of filler within the matrix increases the thermal conductivity of the duct material. 
     Preferably the matrix is a flexible matrix. 
     Preferably the matrix is electrically insulating. 
     Preferably the matrix is a plastic matrix. 
     Preferably the matrix is a polymer matrix. 
     Preferably the matrix is a low-density polyethylene (LDPE) matrix, linear low-density polyethylene (LLDPE) matrix. high-density polyethylene (HDPE) matrix, polyester, silicone or rubber matrix. 
     Preferably the matrix has a thermal conductivity less than 15 Wm −1  K −1 , less than 10 Wm −1  K −1 , less than 5 Wm −1  K −1  and/or less than 1 Wm −1  K −1 . 
     Preferably the filler comprises particles of a filler material. 
     Preferably the particles of filler material are dispersed throughout the matrix. 
     Preferably the particles of filler material have an average diameter of between 1 nm and 10 μm. 
     Preferably the particles of filler material have an elongate, tubular, fiber or substantially spherical shape. 
     Preferably the elongate particles of filler material have a diameter of 1-10 nm and optionally a length of 0.5-5 nm. 
     Preferably the filler comprises an organic filler material. Preferably the filler comprises a carbon-based filler material such as carbon, carbon black, graphite, graphite platelets graphene, multi-walled carbon nanotubes or single-wall carbon nanotubes. 
     Optionally the filler comprises an inorganic filler material. Optionally the filler comprises a ceramic filler material such as aluminium oxide, silicon carbide, boron nitride, silicon nitrate, alumina, aluminium nitride or zinc oxide. 
     Preferably the filler has a thermal conductivity greater than 10 Wm −1  K −1  and/or greater than 100 Wm −1  K −1 . 
     Preferably the duct material comprises less than 25% by volume of filler, 5-18% by volume of filler or 15% by volume of filler. Advantageously, incorporating a limited amount of filler into the matrix provides an increased thermal conductivity while maintaining a low electrical conductivity and suitable flexibility of the material. 
     Preferably the duct material has a thermal conductivity greater than 0.33 Wm −1  K −1  at room temperature, greater than 1 Wm −1  K −1  at room temperature and/or greater than 10 Wm −1  K −1  at room temperature. 
     According to a second aspect of the invention there is provided a method of managing the thermal transfer of a heat source, the method comprising engaging a heat transfer duct with at least part of a surface area of a heat source, extending the duct along and engaging the duct with at least part of the surface area of the heat source along all or part of the length of the heat source from a first engagement position to at least one final engagement position between the duct and heat source, passing a heat transfer fluid along an internal conduit of the duct such that heat can be transferred between the duct and the heat source via the heat transfer fluid about the engageable surface areas of the duct and the heat source, the method comprising adapting the duct to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the heat source. 
     Ideally, the method comprising adapting the duct to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the heat source along the length of the duct. 
     Advantageously, adapting the duct to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the heat source along the length of the duct compensates for the variation in temperature of the heat transfer fluid as a result of ongoing thermal transfer as the heat transfer fluid flows along the length of the duct. This ensures uniform thermal transfer between the heat source and the duct via the heat transfer fluid along the length of the duct as other parameters such as fluid temperature vary. 
     Ideally, the method comprising providing a heat source comprising a battery pack having: one or more cells. 
     Preferably, the method comprising providing the duct as a flexible duct. 
     Alternatively, the method comprising providing the duct as a rigid duct. 
     In one embodiment, the method comprising providing the duct as a metal or metal alloy duct. 
     Ideally, the method comprising positioning the duct proximally to the surface of the heat source such that heat can be exchanged between the duct and the heat source. 
     Preferably, the method comprising positioning the duct proximally to the surface of the one or more cells such that heat can be exchanged between the duct and at least one of the one or more cells. 
     In one embodiment, where the duct is a flexible duct, the method comprising providing a potting means adapted to act as a support for at least a part of the duct. Advantageously the flexible duct can closely conform to the surface shape of the heat source/cells within the pack while being reinforced by the potting means which acts to prevent the flexible duct from over inflation and/or bursting. 
     Preferably, the method comprising configuring the duct to carry the heat transfer fluid from an inlet to an outlet to transfer thermal energy between the heat source and the duct at their engageable contact surfaces via the heat transfer fluid and wherein the method comprising providing the duct with a higher thermal resistance at the inlet than the thermal resistance of the duct at the outlet. 
     Ideally, the method comprising configuring the duct to carry the heat transfer fluid from an inlet to an outlet to transfer thermal energy between the one or more cells and the duct at their engageable contact surfaces via the heat transfer fluid and the method comprising providing the duct with a higher thermal resistance at the inlet than the thermal resistance of the duct at the outlet. 
     This is advantageous as varying the thermal resistance of the duct along the length of the duct promotes a uniform temperature distribution across the heat source/battery pack. 
     In particular, having a higher thermal resistance at the inlet to the duct prevents over cooling or heating of heat source/cells located proximal to the inlet where the temperature differential between the heat transfer fluid and the heat source/cells is at its greatest. 
     Ideally, the method comprising varying the thermal resistance of the duct linearly or non-linearly along the length of the duct such that the thermal resistance of the duct decreases as the temperature differential between the heat transfer fluid and the heat source/cells also decreases, thereby promoting uniform power dissipation along the length of the duct. 
     In one embodiment the method comprising varying the wall thickness of the duct so that the duct wall thickness is thicker at the inlet compared to the outlet. This is advantageous as increasing the wall thickness also increases the thermal resistance of the duct. As such increasing the wall thickness of the duct at the inlet also increases the thermal resistance of the duct. 
     In an embodiment the method comprising varying the wall thickness of the duct linearly along the longitudinal length of the duct. In another embodiment the method comprising varying the wall thickness of the duct non-linearly along the longitudinal length of the duct. Varying the wall thickness of the duct along the longitudinal length of the duct has the effect of varying the thermal resistance of the duct along its longitudinal length. 
     In an embodiment the method comprising varying the wall thickness such that a substantially constant power dissipation is achieved along the longitudinal length of the duct. This is advantageous as it promotes an even temperature distribution throughout the array of cells. This may be achieved by increasing the thermal resistance along the length of the duct. 
     Optionally the method comprising providing the heat source/battery pack comprising a plurality of ducts. 
     Preferably the method comprising providing the one or more ducts as serpentine ducts. 
     Optionally the method comprising providing the one or more ducts as manifold ducts. 
     Optionally the method comprising providing the heat source/battery pack comprising one or more substantially straight ducts. 
     Optionally the method comprising providing the heat source/battery pack comprising one or more parallel ducts. 
     Preferably the method comprising providing the or each duct comprising one or more substantially straight sections. 
     Preferably the method comprises providing an array of cells. 
     Preferably the method comprises providing one or more cylindrical cells. 
     Preferably the method comprises providing an array of close-packed cylindrical cells wherein the minimum separation between the cells is 0.5-5 mm. 
     Preferably the method comprises providing an array of close-packed cylindrical cells wherein the minimum separation between the cells is 2 mm. 
     Preferably the method comprises constructing the battery pack housing. 
     Preferably the method comprises providing a lower clamshell. 
     Preferably the method comprises locating one or more cells in recesses in the lower clamshell. 
     Preferably the method comprises providing one or more sidewalls. 
     Preferably the method comprises attaching the one or more sidewalls to the lower clamshell. 
     Preferably the method comprises providing an upper clamshell. 
     Preferably the method comprises locating one or more cells in recesses in the upper clamshell. 
     Preferably the method comprises attaching the one or more sidewalls to the upper clamshell. 
     Preferably the method comprises attaching one or more busbars to the upper clamshell and/or lower clamshell. 
     Preferably the method comprises fitting fluid inlet means to the or each duct. 
     Preferably the method comprises fitting fluid outlet means to the or each duct. 
     Preferably the method comprises passing an inlet nozzle and an outlet nozzle through apertures in the side wall(s). 
     Preferably the method comprises positioning the or each flexible duct in position adjacent to one or more cells. 
     Preferably the method comprises positioning the or each flexible duct between cells. 
     Preferably the method comprises positioning one or more further flexible ducts proximally to the surface of one or more cells such that heat can be exchanged between the or each further flexible duct and at least one of the one or more cells. 
     Preferably the step of positioning the flexible duct(s) between cells is performed after the step of locating one or more cells in recesses in the lower clamshell. 
     Preferably the step of positioning the flexible duct(s) between two or more cells is performed before the step of locating one or more cells in recesses in the upper clamshell. 
     Preferably the method comprises positioning the or each flexible duct along a serpentine path within the battery pack. 
     Preferably the method comprises positioning the or each duct proximally to the surface of at least one of the one or more cells when the or each duct is in a substantially uninflated state. 
     Preferably the step of inserting fluid into the or each duct causes the duct(s) to expand. 
     Preferably the step of inserting fluid into the or each duct comprises substantially filling the duct(s) with fluid. 
     Preferably method comprises inflating the duct(s) with a fluid. 
     Preferably method comprises inflating the duct(s) with a working fluid such as air or a coolant fluid. 
     Preferably the step of inserting fluid into the or each flexible duct comprises pressurising the duct(s). 
     Preferably the step of inserting fluid into the or each flexible duct comprises pressurising the duct(s) such that fluid pressure within the duct(s) is greater than atmospheric pressure. 
     Preferably the method comprises inflating the or each flexible duct such that the shape of the or each duct conforms to at least a part of the surface shape of the one or more cell(s). Advantageously, this increases the thermal contact area between the duct and the cells which improves the transfer of thermal energy between coolant in the duct and the individual cells. 
     Ideally the method comprises securing, by the duct(s), the one or more cells in position. This is advantageous as it removes the requirement for an adhesive to secure the cells in place in the battery pack. Furthermore, when the battery pack is being used in an automotive or aerospace application where it is subject to vibration, the duct may reduce the effects of vibrations on the battery pack by securing the individual cells in place. 
     Preferably the method comprises positioning one or more support means on the lower clamshell. 
     Preferably the method comprises positioning one or more support means on the lower clamshell at the peripheral edge of the array of cells. 
     Preferably the method comprises locating a portion of the duct within a support means to provide support to at least a portion of the duct. Locating the duct in a support means is advantageous as it prevents the duct from kinking as the duct is expanded. 
     Preferably the method comprises locating a portion of the duct within a recess in the support means when the duct is in a substantially uninflated state. This is advantageous as locating the duct in the recess ensures that there is excess slack in the duct prior to inflation. Providing excess slack in the duct mitigates kinking of the duct during the inflation process. 
     Preferably the method comprises surrounding at least a part of one or more of the duct(s) with the potting means. 
     Preferably the method comprises surrounding substantially the or each entire duct with the potting means. 
     Preferably the method comprises inserting the potting means through the upper clamshell, lower clamshell and/or sidewall(s). 
     Preferably the method comprises injecting an expandable potting means into the battery pack. 
     Preferably the method comprises performing a pressure test on the flexible duct prior to inserting the potting means into the battery pack. 
     Preferably the method comprises inserting foam into the battery pack. 
     Preferably the method comprises inserting intumescent foam into the battery pack. 
     Preferably the method comprises inserting polyurethane foam into the battery pack. 
     Preferably the method comprises inserting a thermosetting plastic, silicone rubber gel or epoxy resin into the battery pack. 
     Preferably the method comprises inserting the potting means into the battery pack. 
     Preferably the method comprises inserting the potting means into the battery pack while the potting means is in a viscous or liquid state. 
     Preferably the method comprises inserting fluid into the or each duct prior to inserting the potting means into the battery pack. 
     Preferably the method comprises pressurising and/or inflating the or each flexible duct prior to inserting the potting means into the battery pack. 
     Preferably the method comprises curing or hardening the potting means within the battery pack. 
     Preferably the step of inserting fluid into the or each flexible duct causes the duct(s) to expand into an open configuration. 
     Preferably the method comprises maintaining, via adhesion to the potting means, the or each duct in the open configuration within the battery pack. 
     Preferably the method comprises curing or hardening the potting means within the battery pack while the or each duct is in a substantially inflated state and/or an open configuration. 
     Ideally the method comprises maintaining pressure within the or each duct until the potting means is set or hardened and enters a substantially rigid state. Beneficially, inflating the duct prior to injecting the potting means ensures that the duct has sufficient space to expand once the potting means is set rigid. 
     Preferably the method comprises expanding the potting means to fill gaps within the battery pack. Advantageously, expansion of the foam means that the foam fills any gaps within the battery pack. This improves the overall mechanical strength of the pack. 
     Preferably the method comprises thermally insulating the cells by surrounding the cells with a thermally insulating foam. Beneficially thermally insulating foam can prevent a high energy thermal event propagating through the battery pack. Furthermore, thermally insulating foam can reduce the effect of external temperature fluctuations on the battery pack and helps to ensure that the duct is the primary controller of thermal energy within the battery pack. 
     Preferably the method comprises securing, by the potting means, the duct and/or cells in position within the battery pack. 
     Preferably the method comprises maintaining, via adhesion to the potting means, the or each duct in an open configuration within the battery pack. 
     Preferably the method comprises securing, by the potting means, an outer casing to the battery pack. This beneficially negates the requirement for additional fixings or fasteners, reducing the complexity of the battery pack and improving the manufacturing process. 
     Preferably the method includes electrically connecting the cells to the busbars. 
     Preferably the method includes electrically connecting the cells to the busbars using ultrasonic bonding, laser welding, ultrasonic welding or resistance welding. 
     Preferably the method includes electrically connecting the cells to the busbars while the cells are held in place by the flexible duct. 
     Preferably the method includes electrically connecting the cells to the busbars before the potting material is inserted into the battery pack. 
     Preferably the method includes inserting the potting means into the battery pack after electrically connecting the cells to the busbars. Advantageously, the potting means serves to protect the aluminium ultrasonic wire bonds from external moisture thereby preventing galvanic corrosion of the wire bonds. 
     According to a further aspect of the present invention there is provided a method of electrically connecting a cell to a busbar, the method comprising: holding the cell in a desired position using an inflated flexible duct adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct and the cell; and providing an electrical connection between the cell and the busbar. Advantageously, the flexible duct can secure the cell(s) in position within the pack, removing the need for glue when electrically connecting the cell(s) to the busbar(s). 
     Preferably the step of providing an electrical connection between the cell and the busbar comprises ultrasonic bonding, laser welding, ultrasonic welding or resistance welding. 
     Preferably the step of providing an electrical connection between the cell and the busbar comprises connecting an aluminium wire bond to the cell and/or busbar. 
     Preferably the method includes potting the cell after connecting the cells to the busbars. 
     It will be appreciated that optional features applicable to one aspect of the invention can be used in any combination, and in any number. Moreover, they can also be used with any of the other aspects of the invention in any combination and in any number. This includes, but is not limited to, the dependent claims from any claim being used as dependent claims for any other claim in the claims of this application. 
    
    
     
       The invention will now be described with reference to the accompanying drawings which shows by way of example only one embodiment of an apparatus in accordance with the invention. 
         FIG.  1    is a diagram showing duct wall thickness varying along the length of the duct; 
         FIG.  2    is a top plan view of an array of cells and a serpentine duct with duct wall thickness varying along the length of the duct; 
         FIG.  3    is a section view of a part of the duct of  FIGS.  1  and  2   ; 
         FIG.  4    is a schematic diagram of a thermal management system and control module suitable for use with a battery pack in accordance with the invention. 
         FIG.  5    is a perspective view of a battery pack fitted with upper and lower clamshells and side walls; 
         FIG.  6    is a perspective view of the battery pack of  FIG.  5    with the side walls removed; 
         FIG.  7    is a perspective view of a lower clamshell component of the battery pack of  FIG.  5   . 
         FIG.  8    is a perspective view of the lower clamshell of  FIG.  7    fitted with an array of cells. 
         FIG.  9    is an enlarged perspective view showing the array of cells of  FIG.  8    fitted with a thermistor. 
         FIG.  10    is an enlarged perspective view showing a support structure fitted to the array of cells shown in  FIG.  8   . 
         FIG.  11    is a perspective view of a support structure suitable for use with embodiments of the invention. 
         FIG.  12    is a perspective view of a flexible duct being fitted to the array of cells shown in  FIG.  8   . 
         FIG.  13    is a perspective view of a multi-lumen flexible duct suitable for use with embodiments of the invention. 
         FIG.  14    is a top view of a heat exchanger comprising a plurality of flexible ducts suitable for use with embodiments of the invention. 
         FIG.  15    is a perspective view of the heat exchanger shown in  FIG.  14   . 
         FIG.  16    is a top view of a heat exchanger comprising a plurality of flexible multi-lumen ducts suitable for use with embodiments of the invention. 
         FIG.  17    is a perspective view of the heat exchanger shown in  FIG.  16   . 
         FIG.  18    is a top view of a heat exchanger comprising a plurality of flexible multi-lumen ducts suitable for use with embodiments of the invention. 
         FIG.  19    is a perspective view of the heat exchanger shown in  FIG.  18   . 
         FIG.  20    is a perspective view of a battery pack fitted with upper and lower clamshells, side walls and a pressurisation manifold. 
         FIG.  21    is a cross-sectional view showing the flexible duct located between the cells in an uninflated state. 
         FIG.  22    is a cross-sectional view of the flexible duct located between cells in an inflated state. 
         FIG.  23    is an enlarged plan view of the support structure and the flexible duct in an uninflated state. 
         FIG.  24    is an enlarged plan view of the support structure and the flexible duct being located in position. 
         FIG.  25    is an enlarged plan view of the support structure and the flexible duct in an inflated state. 
         FIG.  26    is a perspective view of the battery pack being filled with a potting material in an automated process. 
         FIG.  27    is a schematic diagram of a thermal management system suitable for use with embodiments of the invention comprising a reservoir. 
         FIG.  28    is a schematic diagram of the thermal management system of  FIG.  27    being pressurised. 
         FIG.  29    is a schematic diagram of the thermal management system of  FIG.  28    in an operating state. 
         FIG.  30    is a schematic diagram of an alternative thermal management system suitable for use with embodiments of the invention. 
         FIG.  31    is a cutaway view of a part of a battery pack showing the potting material. 
         FIG.  32    is a cross sectional schematic view of a duct wherein the duct material comprises a matrix and a filler. 
         FIG.  33    is a perspective view of a further support structure; 
         FIG.  34    is a plan view of the support structure of  FIG.  33   ; 
         FIG.  35    is a perspective view of a further support structure; 
         FIG.  36    is a perspective view of the support structure of  FIG.  35    installed within a battery pack; 
         FIG.  37    is an additional perspective view of the support structure of  FIG.  35    installed within a battery pack. 
     
    
    
     Referring to the drawings and initially to  FIGS.  1  to  3   , there is shown a duct  230  capable of engaging at least part of a surface area of a heat source  30 , the duct  230  extending along and engageable with at least part of the surface area of the heat source  30  along all or part of the length of the heat source  30  from a first engagement position after inlet  52  to at least one final engagement position after outlet  54  between the duct  230  and heat source  30 . A heat transfer fluid flows along an internal conduit of the duct  230  such that heat can be transferred between the duct  230  and the heat source  30  via the heat transfer fluid about the engageable surface areas of the duct  230  and the heat source  30 . The duct  230  is adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct  230  and the heat source  30 . 
     The duct  230  is adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct  230  and the heat source  30  along the length of the duct  230 . 
     Advantageously, the duct  230  being adapted to allow variable thermal transfer via the heat transfer fluid between the engageable surface areas of the duct  230  and the heat source  30  along the length of the duct  230  compensates for the variation in temperature of the heat transfer fluid as a result of ongoing thermal transfer as the heat transfer fluid flows along the length of the duct  230 . This ensures uniform thermal transfer between the heat source  30  and the duct  230  via the heat transfer fluid along the length of the duct  230  as other parameters such as fluid temperature vary. The heat source  30  comprises a battery pack  21  comprising a plurality of cells  30 . The duct  230  is a flexible duct although in some embodiments, the duct  230  is a rigid duct. In these rigid embodiments, the duct  230  is a metal or metal alloy duct. 
     The duct  230  is positioned proximally to the surface of the heat source  30  such that heat can be exchanged between the duct  230  and the heat source  30 . The duct  230  is positioned proximally to the surface of the cells  30  such that heat can be exchanged between the duct  230  and the cells  30 . 
     In one embodiment, where the duct  230  is a flexible duct although it will be appreciated that this is not necessary, a potting material  231  see  FIG.  31    is provided adapted to act as a support for at least a part of the duct  230 . Advantageously the flexible duct  230  can closely conform to the surface shape of the heat source/cells  30  within the pack  21  while being reinforced by the potting material  231  which acts to prevent the flexible duct  230  from over inflation and/or bursting. 
     The duct  230  is configured to carry the heat transfer fluid from an inlet  52  to an outlet  54  to transfer thermal energy between the heat source/cells  30  and the duct  230  at their engageable contact surfaces via the heat transfer fluid and wherein the thermal resistance of the duct  230  at the inlet  52  is higher than the thermal resistance of the duct at the outlet  54 . This is advantageous as varying the thermal resistance of the duct  230  along the length of the duct  230  promotes a uniform temperature distribution across the heat source/battery pack  21 . In particular, having a higher thermal resistance at the inlet to the duct  230  prevents over cooling or heating of heat source/cells  30  located proximal to the inlet  52  where the temperature differential between the heat transfer fluid and the heat source/cells  30  is at its greatest. The thermal resistance of the duct  230  is varied linearly as illustrated in  FIG.  1    or non-linearly along the length of the duct  230  such that the thermal resistance of the duct  230  decreases as the temperature differential between the heat transfer fluid and the heat source/cells  30  also decreases, thereby promoting uniform power dissipation along the length of the duct  230 . 
     In one embodiment the wall thickness of the duct  230  may be thicker at the inlet  52  compared to the outlet  54  as illustrated in  FIG.  3    where a vertical section through the duct  230  at the outlet and the inlet is shown illustrating the variation in duct wall thickness. This is advantageous as increasing the wall thickness also increases the thermal resistance of the duct  230 . As such increasing the wall thickness of the duct  230  at the inlet also increases the thermal resistance of the duct  230 . 
     In an embodiment the wall thickness of the duct may vary linearly along the longitudinal length of the duct  230 . In another embodiment the wall thickness of the duct  230  may vary non-linearly along the longitudinal length of the duct  230 . Varying the wall thickness of the duct  230  along the longitudinal length of the duct  230  has the effect of varying the thermal resistance of the duct  230  along its longitudinal length. 
     In an embodiment the wall thickness may be varied such that a substantially constant power dissipation is achieved along the longitudinal length of the duct  230 . This is advantageous as it promotes an even temperature distribution throughout the array of cells  30 . This may be achieved by increasing the thermal resistance along the length of the duct  230 . 
     In  FIG.  4    there is shown a thermal management system  18  for a battery pack  21 . The term “battery” is used herein to describe one or more individual cells, for example a group of cells arranged in an array. The term “cell” may be used to refer to any variety of cell, including but not limited to, lithium-ion or nickel metal hydride cells. The battery pack  21  comprises one or more cells  30 , a flexible duct  50 / 230  positioned proximally to the surface of at least one of the one or more cells  30  such that heat can be exchanged between the flexible duct  50 / 230  and at least one of the one or more cells  30  and a potting material adapted to act as a support for at least a part of the duct  50 / 230 . Any number of individual cells may be used to create the desired voltage and capacity of the battery pack  21 . 
     The thermal management system  18  is used to manage the thermal energy within the battery pack  21  so as to maintain the individual cells at an appropriate operating temperature, for example around 25° C. The individual cells within the battery pack  21  generate heat as they are charged and/or discharged. The thermal management system  18  manages the thermal energy within the battery pack  21  by circulating a thermal transfer fluid, such as a glycol-water mix, through a flexible duct  50 / 230  that is proximal to the surface of and/or in contact with individual cell(s)  30 . 
     The thermal management system  18  comprises a heat exchanger  23 , a pump  25  and a flexible duct (not shown) that carries a coolant through the battery pack  21 . The flexible duct is in fluid communication with the heat exchanger  23  and the pump  25  as part of the same coolant circuit  183 . The coolant in the thermal management system  18  is pressurised and the pump  25  causes a flow of the coolant through the coolant circuit  183 . Although the term coolant will be sued in the detailed description it will of course be appreciated that the heat transfer fluid can be used to heat the batteries as well as cool them. The pressure of the coolant fluid causes the flexible duct  50 / 230  to expand. As the flexible duct  50 / 230  expands, it conforms to the undulating surface presented by the shape of the cylindrical cells  30  thereby increasing the surface area of the flexible duct  50 / 230  that is in contact with each of the cylindrical cells  30 . This is advantageous as it increases the thermal contact area and contact pressure between the cells  30  and the flexible duct  50 / 230 , improving the transfer of thermal energy between the flexible duct  50 / 230  and the individual cells  30 . 
     By regulating the flow rate of coolant within the flexible duct  50 / 230 , the pump  25  is configured to maintain the temperature of the battery pack  21  at the desired operating temperature. The heat exchanger  23  can dissipate thermal energy from the coolant when the battery pack  21  requires cooling. The heat exchanger  23  can add thermal energy to the coolant when the battery pack  21  requires heating. A supplementary heating or cooling system may cooperate with the heat exchanger  23  as required. 
     The thermal management system  18  is connected to a control module  27 . The control module  27  receives input signals indicative of the temperature within the battery pack  21 . The control module  27  may output a control signal to the thermal management system  18  to regulate the thermal management system  18  in response to the received temperature input signals such that the desired operating temperature is maintained. 
     The battery pack  21  comprises an array or matrix of cylindrical cells  30 . The cells  30  are sandwiched between lower and upper clamshells  20 ,  80  that are joined by peripheral side walls  90 ,  92  shown in  FIG.  5   .  FIG.  6    shows the pack with a number of components removed (including sidewalls  90 , 92 ) in order to be able to view the cells  30  within the pack. The cells  30  are aligned along a parallel axes and are arranged in an array of straight, parallel rows. The lower and upper clamshells  20 ,  80  include bus bars that connect the individual cells  30  electrically to create the battery pack  21 . 
     The skilled reader will appreciate that the cells could be a shape other than cylindrical, for example, cuboidal, prismatic or pouch cells. However, cylindrical cells are relatively low cost and have a high energy density making them an attractive choice for use in battery packs. Furthermore, cylindrical cells are easier to make in mass production than other cell shapes such as pouch cells or cuboidal cells, and are self-supporting (pouch cells require a carrier or support while prismatics are also self-supporting). In example embodiments, the cells are 18650 or 2170 lithium-ion cells. 
       FIG.  7    shows a perspective view of the lower clamshell  20  of the battery pack  21 . The lower clamshell  20  is a plate with an array of recesses in the form of circular sockets  22 . The base of each socket  22  comprises an inwardly-projecting flange that surrounds an aperture penetrating the clamshell  20 . Each socket  22  is configured to receive an end portion of a respective cylindrical cell  30 . In the example shown, the sockets  22  are arranged in an array with sixteen parallel rows wherein each row is thirteen sockets in length. The sockets  22  of each row are staggered with respect to the sockets of the neighbouring row or rows so that most of the sockets  22  are each nested between a pair of sockets  22  of one or two neighbouring rows. This maximises space efficiency and power density but increases the challenge of maintaining the cells  30  at the correct operating temperature. 
     The skilled reader will appreciate that any number of rows of cells having any appropriate length may be used in battery pack  21 . Increasing the number of individual cells  30  in the battery pack  21  increases the overall capacity and/or voltage of the battery pack  21 . Furthermore, the cells  30  in the battery pack  21  may be arranged vertically in a vertically stacked battery pack. 
     Manufacture of the battery pack  21  involves providing one or more cells  30 , for example the array of cells shown in  FIG.  8   . In the example embodiment, a plurality of cells  30  are inserted into respective sockets  22  of the lower clamshell  20 . The cells  30  are located by the sockets  22  and bus bars positioned on the underside of the lower clamshell  20  (not shown) are connected to the individual cells  30 . 
     Many battery cell manufacturers recommend a minimum cell-to-cell spacing distance of 2 mm to prevent thermal propagation. The skilled reader will recognise that a staggered close-packed array of cylindrical cells is the most volumetrically efficient way to pack cylindrical cells into a given volume whilst maintaining the minimum recommended cell-to-cell spacing. The flexible duct  50  described herein has walls that are between 10 μm and 150 μm thick and the duct  50  can easily fit within the 2 mm staggered channel between adjacent cylindrical cells  30 . Prior art thermal management systems typically require increased cell-to-cell spacing to accommodate the duct, increasing the overall pack dimension and reducing volumetric energy density. The present invention offers a significant improvement over the current state of the art in this respect. Furthermore, the present invention allows neighbouring cells  30  to be separated by the minimum spacing limit recommended by cell manufacturers. 
       FIG.  9    shows how temperature sensors  40 , for example an array of thermistors, may be connected to a suitably-spaced selection of the cells  30  within the battery pack  21 . During assembly the cable  42  attached to the temperature sensor  40  is left free. This is to enable the cable  42  to be secured to the upper clamshell  80  when the upper clamshell  80  is secured to the battery pack  21 . The temperature sensors  40  monitor the temperature of the individual cells  30  within the battery pack  21  and provide a temperature reading to a control module  27 . If the temperature of the cells  30  deviates from a target operating temperature, the control module  27  may adjust the thermal management system  18  to maintain the target operating temperature. 
     As will be appreciated by the skilled reader, the present invention can be used in battery packs employing generally straight, parallel, manifold and/or serpentine heat exchangers/ducts. Serpentine ducts are typically utilised with prismatic cells because the planar surfaces of prismatic cells provide a large surface area for thermal contact with the duct. It is easy to wrap a flexible duct around prismatic cells in a serpentine manner while maintaining thermal contact in this way. However, serpentine ducts are susceptible to kinking at points of inflection where the duct reverses or changes direction. Kinking of the heat exchanger can cause blockages and a build-up of pressure in the duct which can hinder or prevent the flow of coolant. Kinking causes the flexible duct  50 / 230  to fold in on itself which may result in a blockage within the duct  50 / 230 . The pressure loss within the system due to kinking over a series of multiple bends may be significant, reducing the overall performance of the thermal management system  18 . Furthermore, build-up of pressure can result in stretching and thinning of the duct wall, which may ultimately result in bursting and loss of coolant. 
     Blockages due to kinking can be overcome by pressurising the coolant fluid within the duct  50 / 230  to a sufficient level which forces the flexible duct  50 / 230  to an open configuration even at the bends. However, use of a high pressure to overcome kinking may cause the flexible duct  50 / 230  to stretch, thin and burst. The pressure required to overcome kinks at each bend in the flexible duct  50 / 230  is often in excess of the pressure that the flexible duct  50 / 230  can withstand without bursting. 
     In embodiments where the flexible duct  50 / 230  is to follow a serpentine path between and/or around the cells  30  in the battery pack  21 , it is necessary to provide a way to safely support the duct  50 / 230  at the corners, to prevent kinking and/or collapse thereof. As shown in  FIG.  10   , support structures  70  are used within battery pack  21  as a guide at places where the flexible duct  50 / 230  changes direction i.e. where it is prone to kinking. The support structures  70  are located at the edge of the battery pack  21  where the flexible duct  50 / 230  emerges from the array of cells  30  and reverses direction.  FIG.  10    shows the support structures  70  positioned on the battery pack  21  at the periphery of the cells  30 . The support structures  70  are positioned along opposing sides of the battery pack  21  at each point where the duct  50 / 230  emerges from and re-enters the array of cells  30 . 
     Respective support structures  70  are positioned at opposing sides of the battery pack  21  to guide the flexible duct  50 / 230  where the flexible duct  50 / 230  emerges from the array of cells  30  and changes direction. For this purpose, as shown in  FIG.  11    the support structure  70  defines a guide path  74  for the flexible duct  50 / 230 . The guide path  74  is a slot or channel into which the flexible duct  50 / 230  may be inserted and that the flexible duct  50 / 230  then follows so as to change direction without kinking. The guide path  74  of the support structure  70  is defined between an inner supporting face  77  of an inner guide formation  72  and an outer supporting face  78  of an outer guide formation  79 . 
     The flexible duct  50 / 230  can be inserted into the support structure  70  in an uninflated state to follow the guide path  74 . The guide path  74  is shaped to accommodate an excess length of the flexible duct  50 / 230 . Providing the flexible duct  50 / 230  with excess length creates some slack that mitigates kinking when the flexible duct  50  is inflated and so comes under tension. The flexible duct  50 / 230  is inserted into the guide path  74  in an uninflated state for ease of assembly. However, the skilled reader will appreciate that a small amount of working fluid may be used to pressurise the flexible duct  50 / 230  to give the flexible duct  50 / 230  some stiffness to aid assembly. The working fluid may be, for example, air or a coolant fluid. 
     The inner guide formation  72  is dimensioned such that the bend radius of the inner supporting face  77  is large enough to guide the flexible duct  50 / 230  smoothly through 180° in successive 90° bends without the duct  50 / 230  kinking. As shown in  FIG.  11   , the inner supporting face  77  comprises a planar elongate facet  73  between two radiused edges  75 . The elongate facet  73  serves to straighten and support the flexible duct  50 / 230  at the point at which kinking would otherwise be most likely. 
     Notch-like recesses  76  in the outer guide formation  79  opposite the radiused edges  75  form part of the outer supporting face  78  to accommodate the slack defined by the excess length of the flexible duct  50 / 230 . Specifically, slack portions of the flexible duct  50 / 230  that bend around the radiused edges  75  can be pulled or pushed away from the radiused edges  75  and into the recesses  76 . Pressing the flexible duct  50 / 230  into the  35  recesses  76  in this way before inflating the flexible duct  50 / 230  creates slack in the flexible duct  50 / 230  at the radiused edges  75 . Providing this slack in the flexible duct  50 / 230  before inflation is advantageous as it helps to mitigate kinking of the duct  50 / 230  as it is inflated. The notch-like recesses  76  are recesses in the outer supporting face  78  of the outer guide formation  79  and may be any shape suitable for partially receiving the duct  50 / 230  to create slack around the radiussed edges  75 . 
     As will be understood by the skilled reader, where the battery pack  21  does not include substantial bends and/or is not likely to kink (such as where a non-serpentine or generally straight duct is used) then support structures are generally not required. 
     Manufacture of the battery pack  21  involves positioning the flexible duct  50 / 230  proximally to the surface of at least one of the one or more cells  30  such that heat can be exchanged between the flexible duct  50 / 230  and at least one of the one or more cells  30 .  FIG.  12    shows a flexible duct  50 / 230  being inserted into the array of cells  30 . The duct  50 / 230  is arranged in a serpentine manner within the battery pack  21  such that a coolant fluid  20  is carried through the battery pack  21 . Specifically, the duct  50 / 230  has a series of generally straight limbs that extend between adjacent rows of the cells  30 . The limbs of the duct  50 / 230  alternate with bends where the duct  50 / 230  emerges from the array of cells  30  and reverses in direction to extend along and between the next pair of rows of cells  30 . 
     The serpentine arrangement of the flexible duct  50 / 230  ensures that the flexible duct  50 / 230  is in thermal contact with all of the cells  30  within the battery pack  21 . The flexible duct  50 / 230  may, for example, be an inflatable ribbon of plastics material, such as polyester, LDPE, LLDPE, HDPE or any other plastics material or polymer-based material that is flexible and able to withstand the pressure of the coolant. An inflatable plastics material is advantageous as the material is intrinsically electrically insulating, lightweight and does not corrode or chemically interact with a coolant such as a glycol-water mix. 
     The flexible duct  50 / 230  is fitted with an inlet  52  and an outlet  54 . In use, the inlet  52  and outlet  54  are connected to the pump  25 . The pump  25  is configured to induce a flow in the coolant within the flexible duct  50 / 230  such that the coolant flows through the flexible duct  50 / 230 . Pressurising the coolant within the thermal management system  18  to a pressure above atmospheric pressure causes the flexible duct  50 / 230  to expand and conform to the shape of the cylindrical cells  30 . Details of how the coolant is pressurised are provided in further detail below. 
     As best viewed in  FIG.  12   , nozzles are connected to the flexible duct at the inlet  52  and the outlet  54  respectively. The nozzles are configured to be attached to the coolant loop  183  of the thermal management system  18  such that coolant fluid may be conveyed around the thermal management system  18 . The region of the flexible duct  50  connected to the inlet  10  and outlet nozzles may be reinforced to prevent the duct  50 / 230  bursting or expanding excessively. The duct  50 / 230  may be reinforced by manufacturing the end of the duct  50 / 230  from a stronger plastics material or by providing an external sleeve of material over the flexible duct  50 / 230  to prevent the flexible duct  50 / 230  expanding in the region of the inlet  52  or outlet  54 . 
     The duct  50 / 230  shown in  FIG.  9    is a single lumen expandable duct  50 / 230 . However, turning to  FIG.  10   , a multi-lumen expandable duct  223  may be used in the thermal management system  18 . The multilumen duct  223  comprises an inlet passage  221  and an outlet passage  220 . The inlet passage  221  and outlet passage  220  are configured to carry a coolant fluid through the battery pack  21 . This is advantageous for use in large battery packs  21  as it improves the distribution of thermal energy throughout the battery pack  21 . In large battery packs  21  the single lumen duct  50 / 230  may not be able to provide sufficient cooling or heating to cells  30  located downstream in the duct  50 / 230 . This problem is overcome through the use of a multi-lumen duct  223  which provides a more uniform temperature distribution throughout the battery pack  21 . 
     The multi-lumen duct  223  is manufactured from the same plastics material as the single lumen duct  50 / 230 . To create the multi-lumen duct  223 , a seal  222  is created between the inlet and outlet passages  221 ,  220 . The seal  222  may be created by melting the plastics material of the duct  223  to create a bond. The operation of the multi-lumen duct  223  is substantially the same as the single lumen duct  50 / 230  except the multi-lumen duct has a bi-directional coolant flow. The multilumen duct  223  may be located within the supporting structure  70  in a similar manner to the single lumen duct  50 / 230 . Furthermore, the multi-lumen duct  223  may be pressurised by the coolant fluid as described above such that the duct  223  expands to conform to the surface shape of the cells  30 . To implement the multi-lumen duct  223  the skilled reader will appreciate that a manifold would be located at an end of the duct  223  opposing the inlet to the duct  223 . The manifold would allow coolant fluid to transition from the inlet passage  221  to the outlet passage  220  thereby facilitating a bi-directional coolant flow in the duct  223 . 
     While the embodiment shown in  FIG.  12    uses a serpentine duct  50 / 230 , the skilled person will appreciate that other duct geometries are possible and can be used to implement the invention.  FIGS.  14  and  15    show a plurality of substantially straight, single-lumen ducts  550 / 230  in their expanded state (for clarity the cells are not shown). Each of the individual straight ducts  550 / 230  are connected via inlet  552  and the outlet  554  and are to be located between adjacent rows of cells.  FIGS.  16  and  17    show a plurality of substantially straight, multi-lumen ducts  650  in their expanded state (for clarity the cells are not shown). A first lumen of each duct  650  is connected to inlet  652  and outlet  654 . A second lumen of each duct  650  is connected to an inlet  651  and an outlet  653 .  FIGS.  18  and  19    show a plurality of substantially straight, multi-lumen ducts  750  in their expanded state (for clarity the cells are not shown). Each of the individual straight ducts  750  are connected via inlet  752  and the outlet  754  and are to be located between adjacent rows of cells. The lumens in each of the straight ducts  750  are connected at the end of the duct  750  which is opposite to the inlet and outlet. 
     Returning to the embodiment of  FIG.  12   , after the flexible duct  50 / 230  has been located in position within the battery pack  21  and between/adjacent to the cells  30  the construction of the battery pack housing is completed. The housing comprises lower and upper clamshells  20 ,  80  that are joined by four peripheral side walls including sidewalls  90 ,  92  shown in  FIG.  20   . Side wall  92  comprises two apertures corresponding to the inlet  52  and the outlet  54  of the flexible duct  50 / 230 . The inlet  52  and outlet  54  align with the respective apertures in the side wall  92  so that the flexible duct  50 / 230  may be connected to the pump  25  and the heat exchanger  23  of the thermal management system  18 . 
     As will be appreciated by the skilled person, it is possible for one or more of the sidewalls  90 ,  92  to be attached to the lower clamshell  20  before the cells  30  are inserted into respective sockets  22  of the lower clamshell  20  and/or before the flexible duct  50 / 230  is inserted between and around cells  30 . 
     The upper clamshell  80  is placed on top of the array of cells  30  within the battery pack  21  after the flexible duct  50 / 230  has been positioned as described above. Bus bars (not shown) are located within recesses  82  on top of the upper clamshell  80  to connect the individual cells  30  electrically. The aforementioned wires  42  connected to the thermistors  40  are fed through the upper clamshell  80  and run along grooves  84  located on the upper surface of the upper clamshell  80 . 
     As shown in  FIG.  20   , a pressurisation manifold  100  is coupled to the flexible duct  50 / 230  of the battery pack  21  via the inlet  52  and the outlet  54 . Shut-off valves  101  act between the pressurisation manifold  100  and the inlet  52  and the outlet  54 . The pressurisation manifold  100  pressurises the flexible duct  50 / 230  by delivering a working fluid such as air to the flexible duct  50 / 230  under higher-than-ambient pressure. For example, the pressurisation manifold  100  pressurises the flexible duct  50 / 230  to a gauge pressure of between 0.5 bar and 1.5 bar during the assembly process. This causes the flexible duct  50 / 230  to expand into an inflated state. 
     Pressurising the flexible duct  50 / 230  in this way causes the duct  50 / 230  to expand and conform to the shape of the cells  30  and in particular to the undulating shape of the rows of cells  30 . The pressure of the flexible duct  50 / 230  may be monitored for a pre-defined period of time during the manufacturing process to ensure that there are no leaks in the flexible duct  50 / 230 . 
     During assembly, the shut-off valves  101  may be closed and the pressurisation manifold  100  removed from the battery pack  21 . This is advantageous as assembly of the battery pack  21  may be continued with the flexible duct  50 / 230  in an inflated state. It is beneficial to carry out the steps of wiring the battery pack  21  and adding the potting material to the battery pack  21  when the flexible duct  50 / 230  is in an expanded state. This is because the flexible duct  50 / 230  secures the cells  30  in position when in the expanded state (as discussed below) and because adding the potting material when the flexible duct  50 / 230  is in the unexpanded state would prevent the duct  50 / 230  from subsequently being inflated. 
       FIG.  21    shows the flexible duct  50 / 230  in an unexpanded state when it is inserted into the battery pack  21  between adjacent rows of cells  30 . The flexible duct  50 / 230  is substantially straight when in the unexpanded state such that the area of contact between the flexible duct  50 / 230  and each cell  30  is relatively small, being essentially tangential to the surface of the cell, and extending as a narrow band along each cell  30  without significant circumferential extension. 
       FIG.  22    shows the flexible duct  50 / 230  in an expanded, operating state. When the flexible duct  50 / 230  is pressurised by the working fluid before use, or by the coolant during use, the flexible duct  50 / 230  expands and conforms to the undulating shape of the rows of cells  30 . As can be seen in  FIG.  22   , when in the expanded state, the flexible duct  50 / 230  more fully conforms to the shape of the individual cells  30  thereby increasing the thermal contact area between the duct  50 / 230  and the cells  30 . Pressurised coolant within the duct  50 / 230  also increases the contact pressure between the duct and each individual cell  30 , improving the thermal coupling therebetween. Furthermore, the natural flow impingement causes strong mixing of coolant flows within the duct  50 / 230 . 
       FIG.  23    shows the flexible duct  50 / 230  in an uninflated state located within the guide path  74  of the supporting structure  70 .  FIG.  24    shows the slack in the flexible duct being taken into the recesses  76 .  FIG.  25    shows the flexible duct  50 / 230  in an inflated state within the support structure  70  and cells  30 . 
     When the flexible duct  50 / 230  is first located within the guide path  74  an elongate rod or tool  120  may be used to locate the flexible duct  50 / 230  within the recesses  76  as shown in  FIG.  23   . The elongate rod or tool  120  pushes the flexible duct  50 / 230  within the recesses  76  such that slack is created in the flexible duct  50 / 230 . In particular the slack is created in the region of the radiussed edges  75  such that when the flexible duct  50 / 230  is inflated, thus coming under tension, the flexible duct  50 / 230  does not kink. 
     When the duct  50 / 230  is in the inflated state, tension in the flexible duct  50 / 230  takes up any excess slack in the duct  50 / 230 . As the excess slack is taken up in the duct  50 / 230 , the flexible duct  50 / 230  is pulled from the recessed notches  76  as shown in  FIG.  23   . In the inflated state the duct  50 / 230  contacts the radiussed edges  75  on the inner supporting face  77  and is supported by the elongate facet  73 . 
     The support structure  70  is dimensioned such that the cells  130  positioned on the end of each row of the array have substantially the same thermal contact area with the duct  50 / 230  as cells  30  located in the centre of the array. This is advantageous as it promotes a more even temperature distribution throughout the battery pack  21  thereby extending the life of the battery pack  21 . The support structure  70  achieves this by shielding or thermally insulating a portion of the end cells  130  from thermal contact with the duct  50 / 230  such that the duct  50 / 230  has substantially the same thermal contact area with the end cells  130  as cells  30  located within the array. 
     As shown in  FIGS.  23  to  25   , the ends of the outer supporting face  78  abut the end cells  130  such that the outer bend of the guide path  74  is defined by the outer supporting face  78  from the point the duct  50 / 230  emerges from the array to the point that the duct  50 / 230  re-enters the array. The outer supporting face  78  prevents the duct  50 / 230  expanding such that it wraps around the exterior of the cells  130  which would cause the end cell  130  to have an increased thermal contact with the duct  50 / 230 . 
     Similarly, one end of the inner supporting face  77  abuts an end cell  130 . The end of the inner supporting face  77  in abutment with the end cell  130  provides support to the duct  50 / 230  thereby preventing the duct  50 / 230  bulging and wrapping around the end cell  130 . The other end portion  110  of the inner supporting face  77  partially follows the surface of another end cell  130  such that the end portion  110  wraps around the end cell  130  to form a thermal insulating barrier. The end portion  110  of the inner supporting face  77  partially wraps around the exterior surface of the end cell  130  such that when the duct  50 / 230  is located within the guide path  74  the duct  50 / 230  does not contact the end cell  130  in the region of the end portion  110 . The skilled reader will understand that the extent to which the portion  110  extends around the end cell  130  is dependent upon the thermal contact between the duct  50 / 230  and the cells  30 . The portion  110  extends around the end cell  130  sufficiently to ensure that the duct  50 / 230  does not contact the end cell  130  more than any other cell  30  within the array. 
     The support structure  70  serves to mitigate kinking of the duct  50 / 230  at points where the duct  50 / 230  emerges from the array and reverses direction. The guide path  74  defines a channel for the duct  50 / 230  to follow from the point the duct  50 / 230  emerges from the array to the point that the duct  50 / 230  re-enters the array. The guide path  74  prevents excessive bulging and/or collapse of the duct  50 / 230 . 
     The sockets  22  on the lower and upper clamshells  20 ,  80  are dimensioned to have a clearance fit relative to the individual cells  30 . This is advantageous as it enables the individual cells  30  to easily be located in the sockets  22  by an automated manufacturing process; however, the cells  30  may move within their respective sockets  22  which is undesirable when wiring the battery pack  21  using e.g. ultrasonic wire bonding to connect the cells  30  to the bus bars. This is because both the cells and the bus bars should be mechanically stiff for the ultrasonic wire bonding process to create a high quality electrical connection therebetween. To overcome this problem, it is known in the prior art to glue the individual cells  30  in position to ensure a strong mechanical connection between the individual cells  30  and the lower and upper clamshells  20 ,  80 . However, this is an additional and inefficient step in the manufacturing process. Pressurising the flexible duct  50 / 230  not only causes the flexible duct  50 / 230  to expand and conform to the shape of the individual cells  30  but it also secures the individual cells  30  in position within the sockets  22 . Thus the inflated flexible duct  50 / 230  can be used to secure the cell(s)  30  in position while forming an electrical connection between the cell(s) and busbar(s). Securing the cells  30  in position using an inflated duct  50 / 230  negates the requirement for gluing the individual cells  30  in position on the clamshell  20 ,  80 . 
     The individual cells  30  may be wired via an automated ultrasonic wire bonding process. This process is performed on both the lower and upper clamshells  20 ,  80 . The skilled person will understand that the individual cells  30  may be wired via any other suitable process. Furthermore, the control module  27  is connected to the bus bars at this stage in the assembly process. An in-line electronic test of the battery pack  21  may be carried out at this stage in the assembly process as a quality assurance step to ensure that the connections have been produced correctly prior to continuing the assembly process. It is desirable to perform the wire bonding process when the flexible duct  50 / 230  is in an expanded state such that the individual cells  30  are secured in position as this improves the quality of the bond. Furthermore, the potting material serves to protect the aluminium ultrasonic wire bonds from external moisture thereby preventing galvanic corrosion of the wire bonds. 
     Manufacture of the battery pack  21  involves providing a potting material adapted to act as a support for at least a part of the duct  50 / 230 . In the preferred embodiment the potting material is intumescent foam such as expandable polyurethane foam although other potting materials such as thermosetting plastic, silicone rubber gel or epoxy resin may be used. 
     The potting material is injected into the battery pack  21  while in its liquid or viscous state after the housing has been completed and after the wire bonding has been completed. In the case of an expandable potting material such as intumescent foam, the expandable potting material expands to fill the gaps within the battery pack  21  such that the flexible duct  50 / 230  and individual cells  30  are surrounded by the potting material. Once fully expanded, the volume within the battery pack housing is substantially filled with the cells  30 , support structures  70 , duct  50 / 230  and potting means. The expandable potting material expands from a liquid state and sets rigid after injection such that can mitigate against and/or prevent thermal propagation through the battery pack  21 . The expandable potting material may be polyurethane foam that is designed to char when exposed to high temperatures, for example up to 1000° C. This is advantageous as the char layer of pure carbon acts as an excellent thermal insulator thereby preventing the propagation of a high energy thermal event through the battery pack  21 . In this way the battery pack is fire-retardant. 
     The potting material is injected into the battery pack  21  when the duct  50 / 230  is in an inflated state. The potting material sets rigid around the inflated duct  50 / 230  such that a cavity is provided within the potting material that the duct  50 / 230  is located within. The cavity provides total external support to the duct  50 / 230  thereby preventing the duct  50 / 230  being over inflated and/or bursting. The potting material sets substantially rigid to secure the duct  50 / 230  in position and also acts as an external support to the duct  50 / 230  to provide mechanical support to the duct  50 / 230 . The polyurethane foam is advantageous since it is extremely lightweight due its high air content when compared to other potting materials such as water based or silicone gels. 
     After the potting material has cured or hardened, the or each duct  50 / 230  is maintained in its open configuration via adhesion to the potting material. This means that the working fluid may be removed from the interior of the duct  50 / 230  and the duct would still be in its open configuration. 
     Use of potting material such as foam within the battery pack  21  also thermally insulates the battery pack  21  from the external environment. This is advantageous as it means the thermal management system  18  is the prominent thermal regulator of the battery pack  21  (as opposed to external environment factors) making overall control of the thermal management system  18  easier. Insulating the battery pack  21  improves the thermal “endurance” of the battery pack  21 , reducing the requirement for intermittent cooling of the battery pack  21  when the battery pack  21  is not being used in sustained low or high temperature environmental conditions. Foam within battery pack  21  also provides increased vibration and mechanical protection to the internal components of battery pack  21 . The foam sets rigid meaning that it serves to secure the cells  30  and flexible duct  50 / 230  in position within the pack  21 . This is particularly advantageous in automotive applications where the battery pack  21  is subject to periods of sustained vibration. 
       FIG.  26    shows the battery pack  21  being injected with the potting material. The potting material may be injected into the battery pack  21  by an automated process via holes in the clamshell  20 ,  80  using the nozzles  130 . The potting material flows into the battery pack  21  as a liquid thereby flooding the battery pack  21 . The potting material then sets rigid over time. As shown in  FIG.  23    the control module  27  is secured to the side wall  92  and the control module  27  is also flooded with potting material. 
     Once the battery pack  21  is flooded with the potting material, the lower and upper clamshells  20 ,  80  are covered by an outer casing. The outer casing is a sheet metal component that is positioned on the battery pack  21  prior to the potting material setting rigid. In the case of intumescent foam as potting material, as the foam sets it expands thus contacting the outer casing. The potting material acts as an adhesive once it is cured thereby securing the outer casing to the battery pack  21 . In an embodiment the outer casing is secured to the battery pack  21  by external fasteners and the potting material. In another embodiment the outer casing is secured to the battery pack  21  by the hardened/set/cured potting material only. 
       FIGS.  27  to  29    show schematic diagrams of the thermal management system  18 . The thermal management system  18  comprises a reservoir  150 , the pump  25 , the heat exchanger  23 , the battery pack  21 , a three-way control valve  180  and a switching module  181  connected to the control module  27 . The reservoir  150  is a tank configured to store coolant fluid  151 . The reservoir  150  is in selective fluid communication with the coolant loop  183  such that fluid within the reservoir  150  may be introduced to the coolant loop  183  to pressurise the coolant loop  183 . Similarly, coolant fluid may be removed from the coolant loop  183  to reduce the pressure in the coolant loop  183  if required. The reservoir  150  may further be in communication with the atmosphere such that a pocket of air  152  may be located above the coolant  151  when the reservoir  150  is not full. If the level of coolant  151  within the reservoir  150  drops below a threshold value a user of the battery pack  21  may introduce coolant  151  into the reservoir  150 . 
     The three-way control valve  180  is controllable to selectively engage the reservoir  150  in fluid communication with the coolant loop  183 . Furthermore, the three-way control valve  180  may be actuated to close the coolant loop  183  such that coolant cannot flow around the coolant loop  183  when the battery pack  21  is turned off. 
     The reservoir  150  is partially filled with coolant fluid  151  and partially filled with air  152 . The reservoir  150  may be positioned in fluid communication vertically above the coolant loop  183  such that coolant within the coolant loop  183  is under a hydrostatic pressure by the pressure of the coolant  151  in the reservoir  150 . Alternatively, the air  152  within the reservoir  150  may be pressurised such that a force is exerted on the coolant  151  within the reservoir  150  which in turn applies a force on the coolant within the coolant loop. 
       FIG.  27    shows the thermal management system  18  in a non-operating state where the three-way control valve  180  is closed. When in the non-operating state the control valve  180  is closed and pressure within the closed coolant loop is maintained at the desired operating pressure. 
     Turning to  FIG.  28   , the thermal management system  18  may be pressurised by running a pressurisation cycle wherein coolant fluid  151  from the reservoir  150  is drawn into the coolant loop  183  to increase the pressure of the coolant in the loop  183 . When running the pressurisation cycle the switching module  181  actuates the three-way control valve  180  to open two of the three valves such that a flow path is provided between the reservoir  150  and the pump  25 . The third valve member is closed such that the coolant loop  183  is blocked. Simultaneously, the pump  25  is driven to create a pressure differential across the pump  25  such that fluid is drawn from the reservoir  150  and into the coolant loop  183 . Drawing fluid  151  from the reservoir  150  into the coolant loop  183  causes the pressure within the coolant loop  183  to increase. Pressurising the duct via the reservoirs makes it self-supporting thus eliminating any of the hydrodynamic pressure loss from the pump and greatly reducing the pressure drop within the cooling system. 
     A pressure sensor (not shown) monitors the pressure within the coolant loop  183  during the pressurisation cycle and when the desired pressure within the coolant loop  183  is achieved the control valve  180  is actuated such that the path between the reservoir  150  and the coolant loop is closed. Simultaneously the pump  25  may be stopped being driven such that the thermal management system  18  is switched to a non-operating state or alternatively the pump  25  may be driven and the control valve  180  actuated to operate the thermal management system  18  in an operating state. 
       FIG.  29    shows the thermal management system  18  in an operating state. In the operating state the control valve  180  is actuated such that a flow path is provided across the control valve  180  to allow coolant fluid to circulate through the coolant loop  183 . When in the operating state the reservoir  150  is not in fluid communication with the coolant loop  183 . The control module  27  may monitor the pressure of the coolant within the coolant loop  183  to ensure that the coolant pressure is maintained at a desired operating pressure. If the pressure within the coolant loop  183  drops below a threshold value a pressurisation cycle may be run to increase the pressure within the coolant loop  183  to the target operational pressure, as described above. The target operational pressure may be between 0.5 bar and 1.5 bar for example. 
       FIG.  30    shows an alternative embodiment of the thermal management system  18 . As shown in  FIG.  30    the thermal management system  18  comprises a two-way control valve  182  positioned upstream from the reservoir  150 . A pump  210  is positioned between the reservoir  150  and the two-way control valve  182 . The pump  210  is configured to pressurise the reservoir  150  by pumping air from the atmosphere into the reservoir  150  when the two-way control valve  182  is in an open position. The two-way control valve  182  may be closed when the desired pressure within the reservoir  150  is achieved. This ensures that the pressure within the reservoir  150  is maintained. 
     The reservoir  150  illustrated in  FIG.  30    is in constant fluid communication with the coolant loop  183  such that the pressure in the coolant loop  183  may be maintained by the pressure of the air  152  within the reservoir  150 . The pressure within the reservoir  150  may be monitored and when the pressure within the reservoir drops below a target operating value the valve  182  and pump  210  may be actuated to pressurise the reservoir  150  and thus coolant loop  183  to the target operating pressure. 
     The skilled reader will appreciate that various changes and modifications can be made to the present invention without departing from the scope of the present application. The thermal management system described herein may be used with any kind of battery pack having one or more individual cells. For example, the present invention may be employed in a battery pack within an auxiliary power unit (APU) for a long-distance haulage truck, a traction battery of a battery electric or hybrid vehicle, an energy storage system or any other battery pack in the marine, aerospace, automotive, industrial and energy storage sectors requiring thermal management. 
     The flexible duct  50 / 230  may be in indirect contact with side surface(s) or any other surface(s) of the one or more cells  30  via an interface region or interface material such as a casing sheath surrounding the cell(s)  30  or a thermally conductive filler, paste or adhesive. In optional embodiments, the flexible duct may be at least partially in contact with end surface(s) of the cell(s). 
     In  FIG.  32    there is shown a schematic cross section of a battery pack indicated generally by the numeral  2000 . The battery pack  2000  includes a duct  2011  used to thermally manage cells  2020 . The duct  2011  comprises flexible duct material  2001  comprising a matrix  2002  and a filler  2003 . The flexible duct carries a fluid  2004  such as air, water or a water-glycol mixture. Heat is transferred between cells  2020  and the coolant  204  via the duct material  2001 . 
     The matrix  2002  is a flexible plastic or polymer material, in this case LDPE, LLDPE, HDPE polyester, silicone or rubber. The matrix  2002  is electrically insulating. The matrix  2002  has a thermal conductivity less than 15 Wm −1  K −1 , ideally less than 10 Wm −1  K −1 , 5 Wm −1  K −1  and/or 1 Wm −1  K −1 . 
     The filler  2003  comprises particles of a filler material and these are dispersed throughout the matrix  2002 . In preferred embodiments the filler  2003  comprises NANOCYL® NC7000 series thin multiwall carbon nanotubes however any suitable filler material may be used such as a carbon-based filler material such as carbon, carbon black, graphite, graphite platelets graphene, multi-walled carbon nanotubes or single-wall carbon nanotubes or a ceramic filler material such as aluminium oxide, silicon carbide, boron nitride, silicon nitrate, alumina, aluminium nitride or zinc oxide. The particles of filler material may be elongate and tubular having a diameter of 1-10 nm and a length of 0.5-5 nm. Alternatively the particles of filler may be substantially spherical with an average diameter of between 1 nm and 10 μm. 
     The thermal conductivity of the filler  2003  is greater than the thermal conductivity of the matrix  2002 . Ideally the filler  2003  has a thermal conductivity greater than 10 Wm −1  K −1  and/or greater than 100 Wm −1  K −1 . The duct material  2001  comprises less than 25% by volume of filler  2003 , ideally 5-18% by volume of filler or 15% by volume of filler  2003 . Incorporating a limited amount of filler  2003  into the matrix provides an increased thermal conductivity while maintaining a low electrical conductivity and favourable mechanical properties (i.e. suitable flexibility for an inflatable duct). 
     In this example, the duct material  2001  has a thermal conductivity greater than 0.33 Wm −1  K −1  at room temperature, ideally greater than 1 Wm −1  K −1  and/or 10 Wm −1  K −1 . This means that the heat transfer through the duct material  2011  is better than a conventional polymer duct. The duct material  2001  itself is electrically insulating, since the electrical conductivity of the duct material  2001  is dominated by the electrical properties of the non-conductive matrix  2002 . The electrically insulating nature of the duct material/matrix significantly reduces the risk of short circuits when compared with a metallic duct. 
     The duct  2011  is at least partially surrounded by a potting material  2005  which acts to reinforce the duct  2011  at places where it does not contact the wall of a cell  2020 . Incorporation of filler  2003  within matrix  2002  can alter the mechanical properties of the duct  2001 , particularly for high concentrations of filler  2003 . Where this leads to any reduction in mechanical strength the reinforcing material 5 can be used counteract such effects. This embodiment can be used as an alternative or in combination with the variable wall thickness embodiment. 
     Referring now to  FIGS.  33  and  34    there is shown a further support structure  1201  having an outer guide formation, an inner guide formation and a guide channel  1205  therebetween. The support structure  1201  is used to prevent a flexible duct from kinking, bulging and/or bursting when the duct changes direction. The support structure  1201  is dimensioned such that the cells positioned on the end of each row of the array have substantially the same thermal contact area with the duct as cells located in the centre of the array. The support structure  1201  achieves this by shielding or thermally insulating a portion of the end cells from thermal contact with the duct. 
     The outer guide formation of support structure  1201  is formed by the combination of an outer upstand  1208  and the inner surface  1211  of a wall  1210  of the outer pack casing (see  FIG.  34   ). The outer upstand  1208  is located adjacent to at least one cell at the edge of the array of cells. The outer upstand  1208  is a block that has a cell-abutting face  1235  which is curved to match the shape of a cell sidewall, and an outer supporting face  1209  which extends from the cell-abutting face  1235 . The outer upstand  1208  is integrally formed with the lower clamshell  1237  of the battery pack housing  1236 . 
     The inflatable duct is supported by both the outer supporting face  1209  of the upstand  1208  and the inner surface  1211  of the battery pack wall  1210 . Using the battery pack wall  1210  as part of the outer guide formation removes the need for a larger support structure and therefore reduces the width of, and eliminates dead-space within, the battery pack. 
     The inner guide formation of support structure  1201  is formed by a combination of an inner upstand  1206   a  and an interface portion  1206   b . The inner upstand  1206   a  is similar in construction to the outer upstand  1208 . The inner upstand  1206   a  is a block that is integrally formed with the lower clamshell  1237  of battery pack housing  1238 . The inner upstand  1206   a  is located on the opposing side of the guide channel  1205  to the outer guide formation. The inner upstand  1206   a  has two curved cell-abutting faces  1239   a ,  1239   b  for abutting two adjacent, spaced apart cells. 
     The inner upstand  1206   a  further has an inner supporting face  1207  that extends between the cell-abutting faces  1239   a ,  1239   b . The inner supporting face  1207  of the inner upstand  1206   a  has a substantially planar portion and a substantially curved portion that extends from the substantially planar portion towards the sidewall of a cell. The inner supporting face  1207  provides support to the duct thereby preventing the duct bulging and wrapping around an end cell. 
     The interface portion  1206   b  is provided by a compressible pad adhered to the surface of a cell. Specifically, the pad is open-cell polyvinyl chloride (PVC) tape. Alternatively, closed-cell PVC or polyurethane foam could be used, or other suitable compressible material. In use, the interface portion  1206   b  of the inner guide formation extends from a cell-abutting face  1239   a  of the first part  1206   a  and around a portion of the cell to which it is adhered. When the flexible duct (not shown) is inflated it presses against the inner upstand  1206   a  and an interface portion  1206   b  of the inner guide formation. 
     The interface portion  1206   b  is used to limit the thermal contact between the duct and the peripheral cell to which it is attached. The interface portion  1206   b  wraps around the exterior surface of an end cell such that when the duct is located within the guide path  1205  the duct does not contact the end cell in the region of the interface portion  1206   b . The skilled reader will understand that the extent to which the interface portion  1206   b  extends around the end cell is dependent upon the required thermal contact between the duct and the cells. The interface portion  1206   b  should extend around the end cell sufficiently to ensure that the duct does not contact the end cell more than any other cell within the array. 
     The skilled person will appreciate that both of the inner upstand  1206   a  and an interface portion  1206   b  may be compressible and/or may be integrally connected to one another. Interface portion  1206   b  may be integrally formed with the lower clamshell  1237 . 
     Referring now to  FIGS.  35  to  37    there is shown a yet further embodiment of a support structure, indicated generally by reference numeral  1301 . The support structure  1301  of this embodiment has an outer guide formation  1308 , an inner guide formation  1306  and a guide channel  1305  therebetween. The support structure  1301  is used to prevent a flexible duct from kinking, bulging and/or bursting when the duct changes direction. The support structure  1301  is dimensioned such that the cells positioned on the end of each row of the array have substantially the same thermal contact area with the duct as cells located in the centre of the array. The support structure  1301  achieves this by shielding or thermally insulating a portion of the end cells from thermal contact with the duct. 
     The outer guide formation of support structure  1301  is formed by the combination of a first outer upstand  1308   a , a second outer upstand  1308   b  and the inner surface  1311  of a wall  1310  of the outer pack casing (see  FIG.  35   ). The first and second upstanding structures  1308   a ,  1308   b  are spaced apart and both are connected to a support structure base  1312 . The lower clamshell  1337  of the battery pack housing  1338  may include appropriate recesses to accommodate the support structure base  1312  at the edge of the array of cells although in optional embodiments the support structure  1301  may be integrally formed with the lower clamshell  1337 . 
     The inflatable duct is supported by the first outer upstand  1308   a , the second outer upstand  1308   b  and the inner surface  1311  of the battery pack wall  1310 . Using the battery pack wall  1310  as part of the outer guide formation removes the need for a larger support structure and therefore reduces the width of, and eliminates dead-space within, the battery pack. 
     The upstanding structures  1308   a ,  1308   b  are curved and define corners of the guide formation  1305 . The gap between the upstanding structures  1308   a ,  1308   b  can be used to pull excess amounts of the duct through the support structure  1301  when arranging the duct in the battery pack. The first outer upstand  1308   a  the second outer upstand  1308   b  prevent the duct expanding such that it would wrap around the exterior of an end cell causing the end cell to have an increased thermal contact with the duct. 
     The support structures can be made from any suitable rigid, semi-rigid or compressible material which has sufficient rigidity to support a flexible duct, for example metal, plastic or rubber. In an important example, the support structures are made from the potting material used within the battery pack or possess similar thermal propagation prevention properties as the bulk potting compound. For example, the support structures can be manufactured by pouring a potting material into a suitable mould, or by cutting out a support structure from e.g. a block of pre-cured thermally insulating foam. In alternatives, the support structure(s) can be integrally formed with the walls of the battery pack, for example with either of the upper or lower clamshell. In such examples the support structures are extrusions from the plastic shells as opposed to an insert within the battery pack. 
     In the preceding discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, coupled with an indication that one of the values is more highly preferred than the other, is to be construed as an implied statement that each intermediate value of the parameter, lying between the more preferred and the less preferred of the alternatives, is itself preferred to the less preferred value and also to each value lying between the less preferred value and the intermediate value. 
     The features disclosed in the foregoing description or the following drawings, expressed in their specific forms or in terms of a means for performing a disclosed function, or a method or a process of attaining the disclosed result, as appropriate, may separately, or in any combination of such features be utilised for realising the invention in diverse forms thereof as defined in the appended claims.