GB2598534A

GB2598534A - Battery pack antipropagation techniques

Info

Publication number: GB2598534A
Application number: GB2003902.0A
Authority: GB
Inventors: P White Mark; Alexander Newnham Ian
Original assignee: Cummins Inc
Current assignee: Cummins Inc
Priority date: 2020-03-18
Filing date: 2020-03-18
Publication date: 2022-03-09
Also published as: CN214706141U; GB202003902D0; CN214706147U

Abstract

A battery module 12 for a battery pack (10, Figure 1) includes a plurality of stacked battery cell units (24, Figures 2-4). At least one side of the module is provided with a sheet of flame-retardant material 60. The stacked cells may be held together by module bands 28 and the sheet of flame-retardant material may be provided under at least one of the bands. This may restrict or prevent propagation of a thermal runaway event. The module may have end plates 26, which may act as anti-propagation barriers, and a cover 30. The sheet of flame-retardant material may be provided on sides of a battery module which are not provided with an end plate or a cover. A battery pack has a plurality of a plurality of clamping plates (52, Figure 8) for clamping a plurality of battery modules. An antipropagation sheet (62, Figure 8) is provided on the clamping plates. Alternatively, or additionally a battery pack has rows of battery modules and an antipropagation sheet (80, Figures 10-12) attached to a cross-member (15, Figures 10 and 12) situated between two rows of battery modules.

Description

BATTERY PACK ANTIPROPAGATION TECHNIQUES

The present invention relates to antipropagation techniques for preventing or restricting thermal runaway events in a battery pack. The present invention has particular, but not exclusive, application in battery packs for use in mobile applications such as electric or hybrid electric vehicles, construction equipment, and so forth.

Electric vehicles and hybrid electric vehicles, such as cars, buses, vans and trucks, use battery packs that are designed with a high ampere-hour capacity in order to give power over sustained periods of time. A battery pack typically comprises a large number of individual electrochemical cells connected in series and parallel to achieve the total voltage and current requirements. To assist in manufacturing, assembly and servicing, the cells in a battery pack may be grouped into modules. The modules may include a support structure and a battery management unit to manage cell charge and discharge.

In order to help with packing efficiency, some known battery modules use pouch cells. Pouch cells are electrochemical cells (typically lithium-ion) provided in a pouch of flexible material. Typically, a number of pouch cells are stacked together inside a support structure to form a battery module. The cells in the module are connected in series and parallel to achieve the target voltage.

Battery modules constructed from pouch cells can provide energy dense electrical storage, making them suitable for use in mobile applications. However, if a cell is short-circuited or exposed to high temperature, exothermic reactions can be triggered, which may result in the cell overheating or catching fire. The close proximity of the individual cells means that if one cell catches fire, the fire can easily spread through the module. Furthermore, due to the close proximity of the modules in a battery pack, the fire can potentially spread to other modules.

This may in turn lead to a thermal runaway event throughout the battery pack.

It would therefore be desirable to provide a battery pack with antipropagation measures that can help to prevent or restrict thermal runaway events.

According to one aspect of the present invention there is provided a battery module for a battery pack, the battery module comprising a plurality of stacked battery cell units, wherein at least one side of the module is provided with a sheet of flame-retardant material.

This aspect of the invention may provide the advantage that, by providing at least one side of the module with a sheet of flame-retardant material, the propagation of a thermal runaway event to an adjacent module may be restricted or prevented. Furthermore, this may be achieved without adding significantly to the size or weight of the module.

Preferably the module is a cuboid, although it may have any other appropriate shape. Preferably the sheet of flame-retardant material covers at least two sides of the battery module. In this case, the sheet may be bent into the required shape (e.g. [-shape or U-shape) before being added to the module.

Preferably each battery cell unit comprises a pouch cell and a cell tray. The cell tray may comprise a cell frame which surrounds an edge of the pouch cell. Such an arrangement may help to provide energy dense electrical storage and may allow pressure to be applied to the pouch cell, which may help to extend its life.

The stack of battery cell units may be held together by module bands. This may help to provide a compact and flexible battery module. Preferably the module bands apply a compressive force to the battery module. For example, the module bands may be under tension. This may help to maintain the size and shape of the module, and may compress a foam expansion pad between the cells to ensure that a suitable pressure is applied to the cells.

Where the battery module comprises module bands, the sheet of flame-retardant material may be provided under at least one of the module bands. This may allow the sheet to fit within the envelope of the module and/or be held in place using the module bands, and may allow antipropagation measures to be applied without adding significantly to the size, weight and cost of the module. Furthermore, it may be possible to slide the sheet under the module bands, which may facilitate assembly.

In a preferred embodiment, the sheet of flame-retardant material covers three sides of the battery module. In this case, the sheet of flame-retardant material may comprise a middle portion and two end portions, with each portion covering one or the three sides. The two end portions may be provided under module bands on opposite sides of the module. This may allow the sheet to cover the appropriate sides of the module and at the same time fit within the envelope of the module and/or be held in place using module bands on both sides of the module.

Preferably the sheet of flame-retardant material is U-shaped. For example, the sheet may be bent into a U-shape before being fitted to the module. In this case it may be possible to guide the two end portions of the U-shaped sheet under the module bands on each side of the module, which may facilitate assembly.

The battery module may comprise an end plate on each side of the stack of battery cell units. The end plates and the stack of battery cell units may be held together by the module bands. This may help to maintain the size and shape of the module, and to ensure that a suitable pressure is applied to the cells. In this case, the sheet of flame-retardant material may be provided on at least one side of the module which does not have an end plate.

Preferably, the end plates act as an anti-propagation barrier. Thus, the end plates together with the sheet of flame-retardant material may help to provide antipropagation measures where they are needed.

Preferably the dimensions of the end plates are (slightly) larger than those of the battery cell units. This may allow the sheet of flame-retardant material to be guided under the module bands.

The battery module may further comprise a removable cover, which may be used for example to cover a battery management unit. Thus, the module may comprise two end plates and a cover, which may be provided on respective sides of the battery module. In this case, the sheet of flame-retardant material may be provided on sides of the battery module which are not provided with an end plate or a cover. For example, the sheet of flame-retardant material may be wrapped around the three sides of the module which are not covered with an end plate or cover. This may help to ensure that all sides of the module are provided with some form of antipropagation barrier.

The sheet of flame-retardant material may comprise adhesive for adhering the material to at least one side of the module. This may help to ensure that it does not lift away during assembly or use.

The sheet of flame-retardant material may be made from any suitable material with the required flame retardant properties. In one embodiment, the sheet comprises a meta-aramid material.

In another aspect of the invention there is provided a battery pack comprising a plurality of battery modules in any of the forms described above.

In such a battery pack, a clamping arrangement may be used to hold the battery modules in place. The clamping arrangement may comprise a plurality of clamping plates for clamping the battery modules. For example, a clamping plate may be provided for each battery module. This may help to ensure that an even pressure is applied to the battery module.

In a preferred embodiment, a separate antipropagation sheet is provided on the clamping plates. This may help to provide the battery pack with additional antipropagation protection. The antipropagation sheet may be a card-like material and may be thicker than sheet of flame-retardant material used on the modules. For example, the antipropagation sheet may be made from a medium-density pressboard having the appropriate flame retarding properties.

This aspect of the invention may also be provided independently. Thus, according to another aspect of the invention there is provided a battery pack comprising: a plurality of battery modules; a plurality of clamping plates for clamping the battery modules; and an antipropagation sheet on the clamping plates.

Preferably the antipropagation sheet has at least one side which is located between a battery module and a frame of the battery pack. For example, the sheet may have a top and two sides, and each of the two sides may be located between a battery module and a frame of the battery pack. The sheet may be U-shaped, and may be arranged such that, during assembly, the sides can be guided into gaps between the modules and the frame. This may provide a convenient way of adding antipropagation measures to the battery pack.

The battery pack may comprise a plurality of rows of battery modules. In this case, the antipropagation sheet may cover a row of battery modules. For example, where the sheet has a top and two sides, the top may cover a row of battery modules and each of the two sides may be located between a battery module at one end of the row and a frame of the battery pack.

Preferably the battery pack comprises a plurality of rows of battery modules and cross members between rows of battery module. The cross members may be provided for structural support and/or to provide mounting points for battery pack covers and other components.

In a preferred embodiment, the clamping plates are attached to adjacent cross members. By attaching clamping plates to adjacent cross members, the appropriate pressure may be applied to the module, and the structural rigidity of the battery pack may be improved.

Each row may comprise a plurality of clamping plates (for example, one per module). In this case, an antipropagation sheet may be provided on a row of clamping plates (preferably one for each row).

Preferably the antipropagation sheet at least partially covers the top of a cross member. This may help to prevent or restrict the propagation of a thermal runaway event from one row to another.

The antipropagation sheet may comprise holes. The holes may be aligned with holes in the cross member, which holes may be used for securing a panel to the battery pack. This may allow the antipropagation sheet to be secured in place using bolts or other fastening means which are used to attach a battery pack panel to the cross members.

The cross members in the battery pack may be in the form of an open web, in order to maximise the stiffness to weight ratio. However, in such an arrangement, if gasses are vented from the top or bottom of one module during a thermal runaway event, then those gasses may be directed towards modules in adjacent rows. This may then trigger thermal runaway in those modules, leading in turn to a pack-wide thermal runaway event.

In a preferred embodiment, the battery pack further comprises an antipropagation sheet attached to each of the cross members. This may help to slow or arrest the spread of a thermal runaway event between rows of modules, while avoiding any significant increase in the size or weight of the battery pack.

This feature may also be provided independently. Thus, according to another aspect of the invention there is provided a battery pack comprising: a plurality of rows of battery modules; a cross member between two rows of battery modules; and an antipropagation sheet attached the cross member.

The battery pack may comprise a plurality of cross members, and an antipropagation sheet may be attached to each of the cross members.

The cross members may have openings in order to maximise their stiffness to weight ratio. In this case the antipropagation sheet may at least partially (and preferably fully) cover the openings.

Preferably the antipropagation sheet is arranged to divert gasses venting from one module away from a module in an adjacent row.

A suitable material for the antipropagation sheet is a flame-retardant fabric which has been pre-impregnated with a resin. In a preferred embodiment, the antipropagation sheet is of a type which turns into a thermally reflective material on the application of heat. This may help to slow thermal transfer between rows of modules during a thermal runaway event.

Thus, an advantage of this aspect of the invention is that an anti-propagation barrier capable of both slowing thermal transfer and blocking the flow of gasses from one row to another may be provided without adding substantially to the size or weight of the battery pack.

Corresponding methods may also be provided. Thus, according to another aspect of the invention there is provided a method of providing a battery module with antipropagation protection, the battery module comprising a plurality of stacked battery cell units held together by module bands, the method comprising sliding a sheet of flame-retardant material under at least one of the module bands.

Features of one aspect of the invention may be used with any other aspect. Any of the apparatus features may be provided as method features and vice versa.

Further details of the battery pack, battery module and clamping arrangement may be, for example, as disclosed in co-pending UK patent applications entitled "Battery Module Clamping Arrangement", "Battery Module" and "Battery Pack" in the name of the present applicant, the subject matter of each of which is incorporated herein by reference.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an overview of a battery pack; Figure 2 shows parts of a battery module; Figure 3 is an exploded view of a battery module; Figure 4 shows parts of a battery cell unit in more detail; Figure 5 shows parts of a battery module in one embodiment; Figure 6 shows parts of a battery pack with the battery modules in place; Figure 7 shows one embodiment of a clamp flame barrier for use with a row of modules; Figure 8 shows part of a battery pack with a flame barrier in place; Figures 9(A) and 9(B) two types of cross member which may be used in a battery pack; Figure 10 illustrates the principles of an antipropagation sheet in an embodiment of the invention; Figure 11 shows an example of an anti-propagation sheet; and Figure 12 shows how an anti-propagation sheet is attached to a cross member.

Battery pack Figure 1 shows an overview of a battery pack of a type with which embodiments of the invention may be used. The battery pack of Figure 1 is designed to be used with electric and hybrid vehicles, particularly in high horsepower applications as buses, trucks, vans, construction equipment, and so forth.

However, the principles of the present invention may be applied to any type of battery pack for use in any suitable application.

Referring to Figure 1, the battery pack 10 comprises a plurality of battery modules 12, a plurality of cooling plates 14, cross members 15, a battery management system 16, a module retainer 18, a surround frame 20, a top panel 21 and a bottom panel 22. In this example, fifteen battery modules 12 are provided in five rows of three modules. Each row of three battery modules 12 is located on a corresponding cooling plate 14. The cooling plates 14 are hollow to allow the flow of coolant. Cross members 15 are provided between rows of battery modules. The battery management system 16 is located at one end of the battery pack. In the assembled state, the cross members 15 are attached to the surround frame 20 and span the frame from one side to the other. The top panel 21 and the bottom panel 22 are attached to the top and bottom respectively of the frame 20 and the cross members 15. The battery modules 12, cooling plates 14, battery management system 16 and retainer 18 are housed inside the frame 20 and panels 21, 22. The module retainer 18 is used to hold the battery modules 12 and other components in place.

Battery module Figure 2 shows parts of a battery module in an embodiment of the present invention. Referring to Figure 2, in this example the battery module 12 comprises twenty-four battery cell units 24 stacked together side by side. The battery cell units 24 are electrically connected in series and/or parallel to achieve the target module voltage. End plates 26 are provided on each side of the module. The battery cell units 24 and end plates 26 are held together by steel bands 28. A removable cover 30 is provided at one end of the module. A battery management unit is integrated with the module 12 beneath the cover 30 to monitor and manage cell charge and other aspects of cell operation.

Figure 3 is an exploded view of a battery module. Referring to Figure 3, the battery module 12 is formed by stacking together a plurality of battery cell units 24. A compression foam expansion pad 36 is provided between adjacent cell units. Each of the battery cell units 24 is in the form of a pouch cell 32 held within a cell tray 34. In this example the cell trays 34 are made from a plastic polymer material such as a thermoplastic. Each of the battery cell units 24 includes electrical terminal blocks 38, which are connected to the electrical terminals of the pouch cell 32. Each of the battery cell units 24 also has a cooling sheet 40 which is used to conduct heat away from the pouch cell 32. The cooling sheets 40 are provided on the rear sides of the cells in Figure 3. Each cooling sheet includes tabs 41 which extend around the bottom of the cell tray 34. The tabs 41 are designed to contact cooling plates such as those shown in Figure 1, in order to conduct heat away from the cells. The cooling sheets 40 are made from a thermally conductive material such as aluminium or graphite. A thermally conductive electrically insulative film is provided on the cooling sheets.

In the arrangement of Figure 3, a laminated busbar 42 is used to make electrical connections to the various cell units 24. The laminated busbar 42 is connected to the cell units 24 by means of electrically conducting pins 44. The pins 44 pass through holes in the busbar 42 and into corresponding holes in the terminal blocks 38 of the cell units in order to provide electrical and mechanical connections between the two. The laminated busbar 42 includes electrical conductors (busbars) which connect the battery cell units 24 in the required series and/or parallel connections to achieve the target voltage. The laminated busbar 42 also connects to positive and negative terminals 45 which provide electrical connections to and from the battery module.

Also shown in Figure 3 is a battery management unit 46. The battery management unit 46 is used to monitor and manage cell charge and other aspects of cell operation, in cooperation with the battery management system shown in Figure 1. The battery management unit 46 is provided on a circuit board, which is mounted on the laminated busbar 42 via an electrically insulating barrier plate 48. A plurality of temperature sensors and voltage sensors are provided on the laminated busbar 42, and are used by the battery management unit 46 to monitor cell temperature and voltage. The battery management unit 46 is protected by removable cover 30. The removable cover 30 is made from a plastic polymer material such as a thermoplastic.

In order to assemble the battery module, the various battery cell units 24 (comprising pouch cell 32, cell tray 34, terminal blocks 38, cooling plate 40, and cell cover 50) are stacked together with a thermally conductive foam expansion pad 36 between each adjacent cell unit. The cell trays include location features such that the cell units can only be stacked in one orientation. The end plates 26 are then added to each side of the stack of cell units. The stack of cell units is then compressed to the required pressure. This ensures that the foam expansion pads 36 apply pressure to each of the pouch cells 32. The steel bands 28 are placed around the stack of cells as it is held under pressure. The ends of the steel bands are then crimped together. The steel bands ensure that the required pressure is maintained against the cells in the module, as well as maintaining the size and shape of the battery module.

Figure 4 shows parts of a battery cell unit in more detail. Referring to Figure 4, the battery cell unit 24 comprises pouch cell 32, cell tray 34, terminal blocks 38, cooling sheet 40, and cell cover 50. The pouch cell 32 is an electrochemical cell (typically lithium-ion) provided in a pouch of flexible material. Such pouch cells are commercially available and therefore not described further. The terminal blocks 38 are made from an electrically conductive material such as copper and are used to make the electrical connections to the pouch cell 32. The terminal blocks 38 are welded to the terminals 37 of the pouch cell, for example using ultrasonic welding. The cell tray 34 frames the cell pouch 32 and is used to hold the pouch cell in place. The cell tray 34 is made from a plastic polymer material such as a thermoplastic. The cooling sheet 40 attaches to the cell tray 34 so as to be in contact with the pouch cell 32. The cooling sheet 40 is made from a thermally conductive material such as aluminium. Where the cooling sheet is made from a metallic material, it may be coated with a high voltage isolating film. The cooling sheet 40 includes tabs 41 which extend around the bottom of the cell tray 34. The tabs 41 are designed to contact cooling plates in order to conduct heat away from the cells. The cell cover 50 attaches to the cell tray 34 such that the terminal blocks 38 and the top of the pouch cell 32 are clamped between the cell cover and the cell tray. The cell cover may be attached to the cell tray 34 using heat staking, although other techniques such as plastic rivets could be used instead.

Module bands The module design described above uses end plates 26 and steel bands 28 to fulfil the structural rigidity requirements and to provide the required pressure for the cells, while helping to minimise the size and weight of the module.

The primary function of the steel bands 28 is to retain the shape and size of the module and to compress the foam expansion pads to the desired pressure. Applying external pressure to the cells in this way can have a significant benefit on the operation of the cells. In particular, the application of pressure may provide an increase in cell capacity, and a decrease in the discharge ohmic resistance.

The steel bands also allow the module to be easily disassembled for servicing or repurposing the battery cells. A disassembled module can be easily reassembled by stacking together the battery cell units and end plates, compressing the stack and applying new steel bands.

In this embodiment the bands are made from steel, the ends of which are crimped together. However, module bands could be provided made from another material, such as another metal, or a plastic or fibre-based material.

Furthermore, any suitable technique could be used to connect the ends of the bands.

Anti-propagation techniques In the arrangements described above, the battery cells are typically lithium-ion cells held in a pouch. Compared with other types of rechargeable battery cell, lithium-ion cells have a high specific capacity, energy density and power density. These advantages make lithium-ion cells suitable for long-term operation and high-current usage in applications such as electric vehicles. However, if a lithium-ion cell is short-circuited or exposed to high temperature, exothermic reactions can be triggered. This may result in the cell overheating or catching fire. The close proximity of the individual cells means that if one cell catches fire, the fire can easily spread through the module. Furthermore, due to the close proximity of the modules in the battery pack, the fire can potentially spread to other modules, leading to a thermal runaway event throughout the battery pack.

Embodiments of the present invention provide preventative measures which may help to limit the spread of fire and/or increase the amount of time before a battery pack thermal runaway event, should one or more of the cells overheat or catch fire.

Module flame barrier Figure 5 shows parts of a battery module in one embodiment. Referring to Figure 5, the battery module comprises a stack of battery cell units 24 and end plates 26 held together with steel bands 28, in the manner described above with reference to Figures 2 and 3. The battery module also includes a removable cover 30 at one end of the module.

In the arrangement of Figure 5, the module is protected on two sides by the end plates 26 and on one side by the cover 30. The ends plates and cover are designed to offer flame resistance. In addition, the battery module includes a sheet of flame barrier material 60 wrapped around the module. The sheet of flame barrier material is wrapped around those sides of the stack which would otherwise be exposed, namely, the three sides which are not covered by either the end plates 26 or the cover 30. Thus, in the arrangement shown, all six sides of the stack of battery cell units are provided with a flame barrier.

An advantage of the arrangement shown in Figure 5 is that all sides of the stack can be provided with a flame barrier with very little additional size or weight and at minimal cost. The steel bands 28 are used hold the flame barrier material 60 in place, helping to ensure that it does not become detached. In addition, the flame barrier material 60 may be provided with an adhesive backing so that it can the stuck to the module.

It has been found that the sheet of flame barrier material 60 can be guided under the steel bands 28 after the module has been assembled. This is because the dimensions of the end plates 26 are slightly larger than those of the battery cell units 24, to ensure that the steel bands do not bear directly on the battery cell units. Thus, the flame barrier material can be easily added to an assembled module.

In one example, a single sheet of flame barrier material is used to cover all three sides of the module which would otherwise be exposed. For example, the sheet of material may be bent into a U-shape, and then slid onto the module under the steel bands on each side. In this case, it may be preferably to add adhesive only to the middle section of the flame barrier material (i.e. the section which will be opposite the cover 30). This can allow the material to be slid under the steel bands, and then held in place with the adhesive.

Alternatively, it would be possible to add the flame barrier material 60 to the battery module before the steel bands are applied. This method may be preferred if there is insufficient space to slide the material under the steel bands, or if the material is likely to catch on the battery cell units.

In other arrangements, separate sheets of flame barrier material could be added to one or more sides of the module.

A suitable material for the flame barrier has been found to be Nomex®. Nomex is a flame-resistant meta-aramid material produced and marketed by DuPont®. However, any other suitable flame-resistant material could be used instead.

Use of a module flame barrier in the way described above can help to isolate the module from its neighbours, thereby helping to prevent or restrict the propagation of a thermal runaway event from one module to another.

Clamp flame barrier Figure 6 shows parts of a battery pack with the battery modules in place. In the arrangement shown, a module clamp plate 52 is placed over each of the battery modules 12 and attached to the cross members 15. The clamp plates are used to apply downward pressure to the battery modules, to hold them in place.

In another embodiment of the invention, a flame barrier is used which fits over the top of a row of module clamps. In this embodiment, a pre-cut section is laid onto the pack to further improve the flame isolation.

Figure 7 shows one embodiment of a clamp flame barrier for use with a row of modules. Referring to Figure 7, the barrier 62 is formed from a card-like material bent into a U-shape with top portion 64 and two side portions 66. The top portion 64 is designed to sit on top of a row of module clamps. The side portions 66 are designed to slot in between the outer modules of a row and the battery pack surround frame. The top portion 64 is wide enough to cover a row of modules and the tops of the cross members on either side of the row of modules. Holes 68 are provided along the outer edges of the top portion 64. The holes 68 allow a battery pack top panel such as that shown in Figure 1 to be attached to the cross members using bolts which pass through the holes 68. The side portions 66 narrow as they extend away from the top portion 64. This allows the side portions to fit between adjacent cross members. The top portion 64 may have an adhesive backing which adheres the flame barrier 62 to the module clamps.

Figure 8 shows part of a battery pack with a flame barrier in place. Referring to Figure 8, the battery pack comprises a plurality of battery modules 12 inside a battery pack surround frame 20. A module clamp plate 52 is located over each of the battery modules 12 and is attached to a cross member 15 on either side of the battery module. The module clamp plates 52 clamp the modules against a bottom panel and/or a cooling system, such as those shown in Figure 1.

In the arrangement of Figure 8, once the module clamp plates 52 have been fully tightened, a flame barrier 62 is placed over a row of the module clamp plates. The flame barrier 62 spans the battery pack from one side to the other. The side portions 66 of the flame barrier slot in between the outer modules of the row and the surround frame 20. The top portion 64 covers the row of modules and the tops of the cross members 15 on either side of the modules. The holes 68 in the top portion are aligned with holes in the cross members, so as to allow a battery pack top panel to be attached to the cross members.

Further flame barriers 62 are then added to the other rows of battery modules.

The flame barriers of adjacent rows overlap with each other to increase the flame resistance. The holes 68 in one flame barrier 62 are aligned with corresponding holes 68 in the adjacent flame barriers, and with the bolt holes in the tops of the cross members. Thus, when a top panel 21 is added to the battery pack, the flame barriers 62 are held in place by virtue of the bolts which secure the top panel to the cross members. This can avoid the need to provide separate features for securing the flame barriers 62 in the battery pack.

A suitable material for the flame barrier 62 is a Nomex0 pressboard manufactured by DuPont®, although other similar materials could be used 25 instead.

Cross member flame barrier Figures 9(A) and 9(B) show in more detail two types of cross member which may be used in a battery pack such as that described above. Referring to Figures 9, the cross members 15, 15' are of a generally open construction, and are designed to provide rigidity to the surround frame 20 while minimising weight. The cross members include surfaces 70 with bolt holes for connecting the cross member to the surround frame; surfaces 72 with bolt holes for connecting the top panels to the cross members; and surfaces 74 with bolt holes for connecting the battery module clamps to the cross members.

Thermal runaway of a battery cell may be triggered, for example, if the cell has defects that lead to short-circuiting, if it is overheated, if it is subject to excessive power usage, or it is punctured. During thermal runaway, the electrolyte reacts with the electrode and releases flammable hydrocarbon gases. In a pouch cell, the release of gases will force open the pouch at its weakest point, which is usually the top of the cell where the electrodes are located and/or the bottom of the cell opposite the electrodes. Thus, during thermal runaway, hot, flammable gasses are typically expelled from the top and/or bottom of the pouch cells.

When the battery modules 12 described above are inserted into the battery pack, this is typically done with the pouch cells laid sideways so that they run from front to back inside the battery pack (in the direction of the x-axis shown in Figure 1). Therefore, if thermal runaway of a pouch cell occurs, it will tend to vent flammable gases in the direction of battery modules in adjacent rows.

The cross members in the battery pack are in the form of an open web, in order to maximise the stiffness to weight ratio. However, the use of open cross members means that if gasses are vented from the top or bottom of one module, the gasses will be directed towards the modules in adjacent rows. This may then trigger thermal runaway in those modules, leading in turn to a pack-wide thermal runaway event.

In embodiments of the invention, in order to slow or arrest the spread of a thermal runaway event between modules, an anti-propagation sheet is attached to each of the cross members.

Figure 10 illustrates the principles of the antipropagation sheet in this embodiment. Referring to Figure 10, hot and flammable gasses are vented from one of the modules 12 in the direction of the cross member 15 and a module in the adjacent row. In this arrangement, an anti-propagation sheet 80 is attached to the cross member 15. This causes the gasses to be directed away from the cross member 15 and the adjacent module, as shown by the arrows. This can slow down or prevent the spread of a thermal runaway event from one row of modules to another.

Figure 11 shows an example of the anti-propagation sheet 80. The antipropagation sheet in this embodiment is made from a flame-retardant fabric which has been pre-impregnated with a resin product ("prepreg"). The anti-propagation sheet includes holes 82 which can be used to attach it to one of the cross members. The anti-propagation sheet 80 is shaped so as to cover at least the openings in the cross member to which it is to be attached.

Figure 12 shows how the anti-propagation sheet of Figure 11 is attached to a cross member. Referring to Figure 12, the anti-propagation sheet 80 is attached to the cross member 15 using a plurality of snap rivets 84 which pass through the holes 82 in the sheet 80 and into the cross member 15. If desired, other techniques such as cable ties could be used as well or instead.

A suitable material for the anti-propagation sheet is a flame retardant prepreg such as PS200 supplied by SHD composites. This is a flame-retardant resin sheet with high service temperature designed for heat and flame shielding. If subjected to high temperatures such as those typically encountered during thermal runaway, the sheet turns into a ceramic-like heat reflective material. This helps to prevent the transfer of heat from one module to another. However, any other appropriate material may be used instead.

An anti-propagation sheet of the type described above is fitted to each of the cross members in the battery pack. The shape of the sheet may be adapted to correspond to the cross member to which it is to be fitted. In a preferred embodiment, each cross member in the pack has anti-propagation material covering at least 80% of its area, although of course other values could be used instead.

The anti-propagation material slows thermal transfer to neighbouring modules by turning into ceramic! thermally reflective material at high temperatures. The material also prevents the hot gasses, which usually come from the top or bottom edges of the cell, from hitting a neighbouring module. This can slow the thermal runaway of the pack as a whole significantly before the pack is consumed by fire, thereby allowing time for the appropriate safety measures to be taken.

An advantage of the arrangement described above is that the anti-propagation material can be thin and light weight. Thus, an anti-propagation barrier capable of both slowing thermal transfer and blocking the flow of gasses from one row to another can be provided without adding substantially to the size or weight of the battery pack.

In the above description, preferred features of the invention have been described with reference to various embodiments. It will be appreciated that features of one embodiment may be used together with those of any other embodiment. In particular, the various antipropagation techniques which have been described may be used in any combination. Furthermore, the invention is not limited to these embodiments, and variations in detail may be made within the scope of the appended claims.

Claims

CLAIMS1. A battery module for a battery pack, the battery module comprising a plurality of stacked battery cell units, wherein at least one side of the module is provided with a sheet of flame-retardant material.
2. A battery module according to claim 1, wherein each battery cell unit comprises a pouch cell and a cell tray.
3. A battery module according to claim 1 or 2, wherein the stacked battery cell units are held together by module bands, and the sheet of flame-retardant material is provided under at least one of the module bands.
4. A battery module according to claim 3, wherein the sheet of flame-retardant material comprises a middle portion and two end portions, and the two end portions are provided under module bands on opposite sides of the module.
5. A battery module according to claim 3 or 4, wherein the module comprises an end plate on each side of the stack of battery cell units, and the end plates and the stack of battery cell units are held together by the module bands.
6. A battery module according to claim 5, wherein the sheet of flame-retardant material is provided on at least one side of the module which does not have an end plate.
7. A battery module according to claim 5 or 6, wherein the dimensions of the end plates are larger than those of the battery cell units.
8. A battery module according to any of claims 5 to 7, wherein the end plates act as anti-propagation barriers.
9. A battery module according to any of the preceding claims, the module comprising two end plates and a cover, wherein the sheet of flame-retardant material is provided on sides of the battery module which are not provided with an end plate or a cover.
10. A battery module according to any of the preceding claims, wherein the sheet of flame-retardant material comprises adhesive for adhering the material to at least one side of the module.
11. A battery module according to any of the preceding claims, wherein the sheet of flame-retardant material comprises a meta-aramid material.
12. A battery pack comprising a plurality of battery modules according to any of the preceding claims.
13. A battery pack according to claim 12, further comprising: a plurality of clamping plates for clamping the battery modules; and an antipropagation sheet on the clamping plates.
14. A battery pack comprising: a plurality of battery modules; a plurality of clamping plates for clamping the battery modules; and an antipropagation sheet on the clamping plates.
15. A battery pack according to claim 13 or 14, wherein the antipropagation sheet has at least one side which is located between a battery module and a frame of the battery pack.
16. A battery pack according to any of claims 13 to 15, wherein the battery pack comprises a plurality of rows of battery modules, and the antipropagation sheet covers a row of battery modules.
17. A battery pack according to any of claims 13 to 16, wherein: the battery pack comprises a plurality of rows of battery modules and cross members between rows of battery modules; and the clamping plates are attached to adjacent cross members.
18. A battery pack according to claim 17, wherein the antipropagation sheet at least partially covers the top of a cross member.
19. A battery pack according to claim 18, wherein the antipropagation sheet comprises holes which are aligned with holes in the cross member for securing a panel to the battery pack.
20. A battery pack according to any of claims 17 to 19, further comprising a sheet of flame-retardant material attached to each of the cross members.
21. A battery pack comprising: a plurality of rows of battery modules; a cross member between two rows of battery modules; and an antipropagation sheet attached the cross member.
22. A battery pack according to claim 20 or 21, wherein the cross members have openings, and the antipropagation sheet covers the openings.
23. A battery pack according to any of claims 20 to 22, wherein the antipropagation sheet is arranged to divert gasses venting from one module away from a module in an adjacent row.
24. A battery pack according to any of claims 20 to 23, wherein the antipropagation sheet is of a type which turns into a thermally reflective material on the application of heat.
25. A method of providing a battery module with antipropagation protection, the battery module comprising a plurality of stacked battery cell units held together by module bands, the method comprising sliding a sheet of flame-retardant material under at least one of the module bands.