Battery assembly and method of detecting onset of thermal runaway in a battery assembly

Abstract

The present invention provides a battery assembly comprising a plurality of battery cells, a thermosensitive coating applied to a surface of the battery assembly, and a sensor, wherein the thermosensitive coating is configured to emit a substance within a predetermined temperature range indicative of conditions leading to thermal runaway of the battery, and wherein the sensor is configured to detect the emitted substance.

Description

BATTERY ASSEMBLY AND METHOD OF DETECTING ONSET OF THERMAL RUNAWAY IN A BATTERY ASSEMBLY

[0001] The invention relates to a battery assembly and a method of detecting onset of thermal runaway in a battery assembly.Background

[0002] Secondary batteries such as lithium-ion batteries can be repeatedly charged and discharged, making them useful in many electric applications such as in electric vehicles. The process of charging and discharging lithium-ion batteries during normal operation generates heat from both internal ohmic and ionic resistances. The rate of heat generation increases in line with charging and discharging rates of the battery. If the rate of heat generation is too high, or heat dissipation is too low, then the heat generated within the battery exceeds the amount of heat dissipated to its surroundings, which can lead to a process known as thermal runaway. Thermal runaway is a chain reaction of exothermic reactions that occurs in lithium-ion batteries, making them reach extreme temperatures which can result in major safety hazards including smoke, fire and explosion. Typically, thermal runaway has three stages: an onset stage in which self-heating of the battery is triggered; an acceleration stage in which the increased temperature causes numerous exothermic reactions to take place; and a runaway stage in which the battery combusts to a very high temperature and decomposes. Thermal runaway is particularly a problem in dense battery packs, where heat generated by one cell can propagate to nearby cells.

[0003] Typically, batteries are provided with a battery management system that monitors the key characteristics of the battery, such as voltage, temperature and current, and implements control mechanisms if certain thresholds are reached. Some battery management systems require many electronic components and sensors to be placed throughout a battery pack, which requires extensive computing power to operate. Some battery management systems aim to mitigate the need for many sensors by providing a single sensor to monitor a group of cells at a single location. However, this arrangement means that the majority of the battery surface is unmonitored, or some of the abnormal characteristics of the battery go completely undetected, such as an abnormal temperature. If this is undetected, then no preventative measures can be implemented against battery thermal runaway which can quickly propagate through a battery pack, leading to extreme safety hazards and in some cases, catastrophic failure.

[0004] It would be desirable to provide a battery assembly with improved safety. In particular, it would be desirable to provide a battery assembly with reduced risk of thermal runaway. It would also be desirable to provide a battery assembly that more sensitively and accurately monitors the battery. It is an object of the invention to provide a battery assembly that detects the early signs of potential thermal runaway.Summary of the Invention

[0005] In accordance with the present invention there is provided a battery assembly according to the appended claims.

[0006] According to an aspect of the present invention, there is provided a battery assembly comprising a plurality of battery cells, a thermosensitive coating applied to a surface of the battery assembly, and a sensor, wherein the thermosensitive coating is configured to emit a substance within a predetermined temperature range indicative of conditions leading to thermal runaway of the battery, and wherein the sensor is configured to detect the emitted substance.

[0007] Thus, when the predetermined temperature range is reached, the thermosensitive coating of the battery assembly emits a substance which can be detected by the sensor which indicates of the onset of thermal runaway. This provides a battery assembly with improved safety, and with reduced risk of thermal runaway. In addition, the battery assembly of the present invention gives an advanced warning of a dangerous or abnormal battery temperature, so that the potential of thermal runaway can be detected during onset and before thermal runaway accelerates. The battery assembly of the present invention sensitively and accurately detects the onset of thermal runaway.

[0008] In examples, the sensor may be in communication with a battery management system. The battery management system may activate a suppression system in response to detection of the onset of thermal runaway.

[0009] In examples, the thermosensitive coating comprises microcapsules containing the substance or a source of the substance and adapted to rupture at the predetermined temperature range. This is particularly advantageous because the emission of the substance is sudden (i.e. sharp) and can be more easily detected by the sensor. The rupture temperature of the microcapsules can be set based on the predetermined temperature range so that the microcapsules do not rupture at a lower temperature. Thus, the thermosensitive coating does not degrade during routine use of the batteryand the substance is only emitted when the predetermined temperature range is reached.

[0010] In examples, the microcapsules comprise a polymer material. In examples, the microcapsules comprise a core comprising the substance or a source of the substance, and a shell comprising the polymer material. In examples, the polymer material has a glass transition temperature that is within or lower than the predetermined temperature range.

[0011] In examples, the polymer material comprises polymethyl methacrylate.

[0012] In examples, the polymer material has a molecular weight of between 10,000 and 1 ,000,000 kg / mol by gel permeation chromatography (GPC), for example between 100,000 and 800,000 kg / mol by GPC, for example between 300,000 and 700,000 kg / mol by GPC, for example about 500,000 kg / mol by GPC.

[0013] In examples, the battery assembly comprises a single gas sensor unit.

[0014] In examples, the sensor comprises a sensor array having a plurality of gas sensor units. The plurality of gas sensor units may be arranged in the battery assembly at the same location. In other examples, the plurality of gas sensor units may be arranged in the battery assembly at different locations. In examples, each of the gas sensor units can detect the same or different emitted substances.

[0015] In examples, the thermosensitive coating comprises a primary agent configured to emit a first substance and a secondary agent configured to emit a second substance. In examples, the primary agent is configured to emit the first substance at a first subrange of the predetermined temperature range, and the secondary agent is configured to emit the second substance at a second subrange of the predetermined temperature range. The primary agent may contain a source of the first substance. The second agent may contain a source of the second substance.

[0016] In examples, the first subrange is different from the second subrange. In examples, the primary agent and the secondary agent are applied to the battery assembly in the same location. In this way, the primary agent and the secondary agent release a different substance at different temperature subranges. Accordingly, different stages of thermal runaway can be detected - a first stage at the first subrange of the predetermined temperature range and a second stage at the second subrange of the predetermined temperature range. After the detection of the first substance, the detection or non-detection of the second substance may indicate whether thermalrunaway has progressed, or if it has been prevented, for example by the battery management system.

[0017] In other examples, the primary agent and the secondary agent are applied to the battery assembly at different locations. The respective detected substance of the primary agent and / or the secondary agent may therefore indicate the location within the battery assembly at which there is onset of thermal runaway.

[0018] In examples, the substance is a hydrocarbon, for example an essential oil vapour. The coating may therefore comprise a hydrocarbon (e.g. essential oil) vapour. Alternatively, the coating may comprise a hydrocarbon (e.g. essential oil) that is liquid room at temperature but forms a vapour in the predetermined temperature range. In examples, the substance is emitted by the thermosensitive coating within the predetermined temperature range and would not otherwise be present in the battery pack during its usual operation. In examples, the substance may be encapsulated in a microcapsule that ruptures at the predetermined temperature range. In examples, a source of the substance may be encapsulated in a microcapsule that ruptures at the predetermined temperature range.

[0019] In examples, the substance may be any one of carbon dioxide, carbon monoxide, or ammonia gas. In one example, the thermosensitive coating may comprise ammonium carbonate ((NH^COs) that decomposes and releases ammonia within a predetermined temperature range (e.g., 40 to 100 degrees Celsius). In another example, the thermosensitive coating may comprise Urea (CH4N2O), that decomposes and releases ammonia within a predetermined temperature range (e.g., 80 to 100 degrees Celsius). The sensor may detect the released ammonia. The sensor may be a metal oxide semiconductor gas sensor.

[0020] In examples, the thermosensitive coating may comprise a thermo-responsive polymer that swells and creates pores at the predetermined temperature range. The substance may be emitted through the pores. The critical temperature at which the swelling of the thermo-responsive polymer takes place can be finely tuned by altering the ratio of co-polymers in the polymerisation process. Accordingly, a thermosensitive coating comprising a thermo-responsive polymer can be adapted to release a substance within a predetermined temperature range, and the substance can be detected by the sensor.

[0021] In examples, the predetermined temperature range is between 50 and 120 degrees Celsius, preferably between 60 and 100 degrees Celsius.

[0022] In examples, the thermosensitive coating is a paint. In examples, the paint is a water-based paint.

[0023] In examples, the thermosensitive coating is applied to a surface of at least one of the plurality of battery cells. In examples, the thermosensitive coating is applied to the surface of a majority of the plurality of battery cells, for example all of the battery cells. By applying the thermosensitive coating directly to the battery cells the temperature of the thermosensitive coating is more directly linked to the temperature of the battery cells where thermal runaway would potentially take place. In other examples, the thermosensitive coating may additionally or alternatively be applied to other surfaces of the battery assembly, for example a surface of a housing, casing or divider within the battery assembly.

[0024] According to another aspect of the present invention, there is provided a method of detecting onset of thermal runaway in a battery assembly, the method comprising detecting a substance emitted from a thermosensitive coating on the battery assembly, the thermosensitive coating being configured to emit the substance within a predetermined temperature range indicative of onset of thermal runaway.

[0025] In examples, the method comprises detecting a first substance emitted from the thermosensitive coating on a first part of the battery assembly and / or detecting a second substance emitted from the thermosensitive coating on a second part of the battery assembly, and determining a location of onset of thermal runaway based on the detected first substance and / or second substance.

[0026] In examples, the method comprises detecting a first substance emitted from the thermosensitive coating in a first subrange of the predetermined temperature range and / or detecting a second substance emitted from the thermosensitive coating in a second subrange of the predetermined temperature range, and determining the onset of thermal runaway based on the detected first substance and / or second substance.

[0027] According to a further aspect of the present invention, there is provided a paint comprising microcapsules, wherein the paint is water-based and the microcapsules comprise a substance or source of a substance encapsulated by a polymer material; wherein the microcapsules are adapted to rupture at a predetermined temperature range; and wherein the polymer material has a glass transition temperature that is within or lower than the predetermined temperature range.

[0028] In examples, the polymer material comprises polymethyl methacrylate.

[0029] In examples, the polymer material has a molecular weight of between 10,000 and 1 ,000,000 kg / mol by gel permeation chromatography (GPC), for example between 100,000 and 800,000 kg / mol by GPC, for example between 300,000 and 700,000 kg / mol by GPC, for example about 500,000 kg / mol by GPC.

[0030] The substance may be encapsulated in a vapour form and released in the predetermined temperature range. Alternatively, the substance may encapsulated in a liquid form that is converted to the vapour form in the predetermined temperature range. The substance may be a volatile liquid that releases a vapour, for example a liquid with a low vapour pressure such as a perfume or an essential oil.

[0031] The coating (e.g. the microcapsules) may comprise the substance.

[0032] The coating (e.g. the microcapsules) may comprise a source of the substance. The source of the substance may be a different chemical entity that decomposes in the predetermined temperature range to form the substance.

[0033] In examples, the substance is a hydrocarbon, for example an essential oil vapour. The microcapsules may therefore comprise a hydrocarbon (e.g. essential oil) vapour. Alternatively, the microcapsules may comprise a hydrocarbon (e.g. essential oil) that is liquid at room temperature but forms a vapour in the predetermined temperature range. In examples, the substance is emitted by the thermosensitive coating within the predetermined temperature range and would not otherwise be present in the battery pack during its usual operation. In examples, the substance may be any one of carbon dioxide, carbon monoxide, or ammonia gas.

[0034] In examples, the thermosensitive coating comprises a smart polymer (i.e. stimuli-responsive polymer). The smart polymer may be a thermo-responsive polymer that swells and creates pores at the predetermined temperature range to emit the substance through the pores. The critical temperature at which the swelling of the polymer takes place can be finely tuned by altering the ratio of co-polymers in the polymerisation process. Accordingly, a thermosensitive coating comprising a thermo- responsive polymer can be adapted to release a substance in a predetermined temperature range.Brief Description of the Drawings

[0035] Embodiments of the invention are now described, by way of example only, hereinafter with reference to the accompanying drawings, in which:Figure 1 (a) illustrates a perspective view of an example battery assembly, and (b) illustrates a perspective view of a different example battery assembly;Figure 2 illustrates a block diagram showing a method of detecting onset of thermal runaway in a battery assembly;Figure 3 illustrates a block diagram showing another method of detecting onset of thermal runaway in a battery assembly; andFigure 4 illustrates a schematic diagram showing an example of a microcapsule.Detailed Description

[0036] Certain terminology is used in the following description for convenience only and is not limiting. The words Tight’, ‘left’, ‘lower’, ‘upper’, ‘front’, Tear’, ‘upward’, ‘down’ and ‘downward’ designate directions in the drawings to which reference is made and are with respect to the described component when assembled and mounted. The words ‘inner’, ‘inwardly’ and ‘outer’, ‘outwardly’ refer to directions toward and away from, respectively, a designated centreline or a geometric centre of an element being described (e.g. central axis), the particular meaning being readily apparent from the context of the description.

[0037] Further, as used herein, the terms ‘connected’, ‘attached’, ‘coupled’, ‘mounted’ are intended to include direct connections between two members without any other members interposed therebetween, as well as, indirect connections between members in which one or more other members are interposed therebetween. The terminology includes the words specifically mentioned above, derivatives thereof, and words of similar import.

[0038] Further, unless otherwise specified, the use of ordinal adjectives, such as, “first”, “second”, “third” etc. merely indicate that different instances of like objects are being referred to and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner.

[0039] Like reference numerals are used to depict like features throughout.

[0040] Referring now to Figure 1 (a), there is shown a battery assembly 10. In various examples the battery assembly 10 may be termed a battery module, battery pack or battery unit. The battery assembly 10 includes a number of battery cells 12 held within a housing 18 and arranged within a matrix. The battery cells 12 are lithium-ion battery cells that can be charged and discharged. A thermosensitive coating 14 is applied to a surface of the battery assembly 10. In this particular example, the thermosensitivecoating 14 is applied to a surface of the battery cells 12, in particular a surface of every battery cell 12. However, it is envisaged that the coating 14 may be applied to any surface of the battery assembly 10, for example only one of, or a portion of, the battery cells 12 or onto a surface of the housing 18. The coating 14 is thermosensitive and when it reaches a predetermined temperature range it emits a substance which may be detected. The battery assembly 10 includes a sensor 16. The sensor 16 is arranged near the battery cells 12, and thus near the thermosensitive coating 14. The sensor 16 is arranged on the housing 18 in this particular example. The sensor 16 in this example is a gas sensor, and more specifically a metal oxide semiconductor (MOS) based gas sensor. The battery assembly 10 is provided with a battery management system 20 that is electrically connected to the sensor 16. The battery management system 20 includes a processor and control circuit, which are known and will not be described in detail.

[0041] The illustrated battery assembly 10 may be used on its own to power an electric device, or may be connected together to form a larger battery assembly 50 such as that shown in Figure 1 (b). Larger battery assemblies 50 may be used for higher power applications, for example for use in an electric vehicle. The battery assembly 10 may be used in operation on its own or connected to form the larger battery assembly 50 to provide greater capacity. The battery assembly 50 includes six of the battery assemblies 10A-F illustrated in Figure 1 (a) arranged in a side-by-side configuration and held within a tray 52. The tray 52 forms a part of a housing of the battery assembly 50.

[0042] In use, the battery assembly 10 is charged to store energy, and discharged to power an electric device. Both the charging and discharging processes generate heat in the battery assembly 10. Since the thermosensitive coating 14 is applied on a surface of the battery assembly 10 it also heats up during charging and discharging. When heated to a predetermined temperature range, the thermosensitive coating 14 emits a substance. In this example, the thermosensitive coating 14 emits an essential oil vapour when subject to a temperature range of between 50 and 120 degrees Celsius, which may be indicative of the onset of thermal runaway of the battery cells 12. The thermosensitive coating 14 includes a source of the essential oil vapour, for example an essential oil. The sensor 16 is placed such that it can detect the presence of the emitted substance. When the sensor 16 detects the presence of the essential oil vapour, this indicates that the battery assembly 10 has reached the temperature range of 50 to 120 degrees Celsius, which is indicative of the onset of thermal runaway of the battery cells 12. Although the predetermined temperature range is between 50 and 120 degreesCelsius in this example, other ranges are also envisaged, such as 60 to 100 degrees Celsius, or 70 to 90 degrees Celsius, for example.

[0043] It is also envisaged that the emitted substance may be a substance other than an essential oil vapour, for example the vapour of another hydrocarbon.

[0044] In alternative examples, the emitted substance may be carbon dioxide, carbon monoxide, or ammonia gas. In one example, the thermosensitive coating may comprise ammonium carbonate ((NH^COs) that decomposes and releases ammonia within a predetermined temperature range (e.g., 40 to 100 degrees Celsius). In another example, the thermosensitive coating may comprise Urea (CH4N2O), that decomposes and releases ammonia within a predetermined temperature range (e.g., 80 to 100 degrees Celsius). The sensor may detect the released ammonia. The sensor may be a metal oxide semiconductor gas sensor.

[0045] In other examples, the thermosensitive coating may comprise a thermo- responsive polymer that swells and creates pores at the predetermined temperature range. The substance may be emitted through the pores. The critical temperature at which the swelling of the thermo-responsive polymer takes place can be finely tuned by altering the ratio of co-polymers in the polymerisation process. Accordingly, a thermosensitive coating comprising a thermo-responsive polymer can be adapted to release a substance within a predetermined temperature range, and the substance can be detected by the sensor. . In examples, the thermo-responsive polymer may be created from two co-polymers; polyethylene glycol (PEG) and poly(2-(2- methoxyethoxy)ethyl meth-acrylate) (PMEO2MA) which exhibit critical temperatures of 26°C and 90°C respectively. By altering the ratio of the two polymers, the critical temperature of the smart polymer can be tuned between these two critical temperatures such that the swelling and pore creation occurs at the predetermined temperature range.

[0046] Referring now to Figure 2, a method of detecting thermal runaway in a specific location of a battery assembly 10 will now be described. In a simplified example shown, the battery assembly 10 has four battery cells 12A,12B,12C,12D which are positioned at different locations. Each of the battery cells 12A,12B,12C,12D has a surface that is coated with a respective thermosensitive coating 14A,14B,14C,14D. In this example, each coating is arranged to emit a different substance at the same predetermined temperature range, for example between 80 and 100 degrees Celsius. The sensor 16 in this example is a sensor array made up of a plurality of different gas sensor units, or a single sensor that can detect the different substances of the different coatings.

[0047] When the battery assembly 10 is in use the temperature across all four battery cells 12A,12B,12C,12D increases. However, the temperature of one of the battery cells 12B may increase significantly more than the other battery cells 12A,12C,12D, which may be due to non-uniformities in the structure of the battery cell 12B or damage of the battery cell 12B. For example, the temperature of the battery cell 12B increases to 90 degrees Celsius, while the other battery cells increase only to 60 degrees Celsius. The increase of temperature in the battery cell 12B is indicative of an abnormal temperature, which may lead to the thermal runaway of the battery cell 12B. The thermosensitive coating 14B that is applied to the battery cell 12B emits a substance (e.g. pine needle oil vapour) because it has reached the predetermined temperature range of 80 and 100 degrees Celsius. The thermosensitive coating 14 includes pine needle oil which releases pine needle oil vapour when it reaches the predetermined temperature range. As the emitted substance is a vapour, the vapour travels towards a sensor 16 in the battery assembly 10. The sensor 16 detects the emitted substance from the thermosensitive coating 14B applied to the battery cell 12B. The sensor 16 in this example is a gas sensor that detects the composition of the emitted substance. The sensor 16 sends a signal to a battery management system 20 which includes information about the composition of the emitted substance. The battery management system 20 determines the location of the battery cell 12B and thermosensitive coating 14B that the emitted substance came from. Therefore, the battery assembly 10 can determine the location of an abnormal temperature reached by a specific battery cell 12 within the battery assembly 10. The battery assembly 10 also includes a suppression system 22 which may suppress the risk of thermal runaway at the location subject to thermal runaway, for example by applying a cooling or extinguishing agent to the site which is at risk of thermal runaway.

[0048] Figure 3 illustrates another method of detecting onset of thermal runaway in a battery assembly. For simplicity, only a single battery cell 12 is shown. The battery cell 12 has a surface that is provided with a thermosensitive coating 14. The thermosensitive coating 14 contains a primary agent and a secondary agent. Each of the primary agent and the secondary agent within the thermosensitive coating 14 are designed to emit a different substance (i.e. first substance and second substance respectively) when they reach a certain predetermined temperature range. The primary agent is configured to emit a first substance at a first subrange of a predetermined temperature range, which in this example is 60 to 70 degrees Celsius. The secondary agent is configured to emit a second substance that is different from the first substance. The second substance isemitted by the secondary agent at a second subrange of the predetermined temperature range, which in this example is 90 to 100 degrees Celsius.

[0049] When the battery assembly is in use, the temperature of the battery cell 12 increases. The increase of temperature in the battery cell 12 may reach a temperature at which the primary agent emits the first substance, for example at a temperature of 60 degrees Celsius. At this temperature, which is within the first temperature subrange of 60 to 70 degrees Celsius, the first substance is emitted and is detected by the sensor 16. The composition of the emitted first substance is sent as a signal to the battery management system 20 which determines that the battery cell 12 has reached a temperature within the first subrange of 60 to 70 degrees Celsius. This temperature range may correspond to a low risk of thermal runaway of the battery 12, or to an early onset of thermal runaway.

[0050] As the battery assembly continues to be used, the temperature of the battery cell 12 continues to increase. The battery cell 12 may reach a temperature of 95 degrees Celsius, for example, which lies within the second subrange of 90 to 100 degrees Celsius. As such, the second substance is emitted by the secondary agent within the thermosensitive coating 14. The emitted second substance is detected by the sensor 16, and the composition of the detected second substance is sent as a signal to the battery management system 20. The battery management system 20 determines that the battery cell 12 has reached a temperature within the second subrange of 90 to 100 degrees Celsius. This second subrange may correspond to continuing onset of thermal runaway of the battery cell 12, and / or to acceleration of the thermal runaway. The battery assembly in this example has a suppression system 22 which may suppress the risk of thermal runaway of the battery cell 12, for example by isolating the battery cell 12 to reduce its temperature. By having a thermosensitive coating 14 that has different agents that emit a substance at different temperature subranges, the risks of thermal runaway (e.g. whether high, medium or low risk) can be assessed. It is envisaged that in addition to the primary and secondary agents, the thermosensitive coating 14 may include further agents that emit a further substance at a different temperature subrange.

[0051] Figure 4 shows an example of a microcapsule 100 that may be used in the previously described thermosensitive coating 14. The microcapsule has a core 104 containing the substance to be emitted or a source of that substance. The core 104 is encapsulated by an outer shell 102 that is designed to rupture when the predetermined temperature range is reached, to release the substance from the core. The rupturing of the outer shell 102 of the microcapsule 100 provides a sudden release of the substancefrom the core 104. The outer shell 102 in this example is made of a polymer material, in particular polymethyl methacrylate. The polymer material has a glass transition temperature that is within the predetermined temperature range. In examples, the glass transition temperature may be lower than the predetermined temperature range. In examples, the polymer material of the outer shell 102 has a molecularweight of between 10,000 and 1 ,000,000 by gel permeation chromatography (GPC). Other molecular weights are also envisaged, for example between 100,000 and 800,000 kg / mol by GPC, or between 300,000 and 700,000 kg / mol by GPC, in particular about 500,000 kg / mol by GPC.

[0052] More than one type of microcapsule may be provided in the thermosensitive coating, each type containing an agent material that releases a different substance at a different temperature subrange. In particular, the thermosensitive coating may contain a first type of microcapsule containing a primary agent that releases a first substance at a first subrange, and a second type of microcapsule containing a secondary agent that releases a second substance at a second subrange. The first subrange may be 80 to 90 degrees Celsius, for example. The second subrange may be greater than the first subrange, for example the second subrange may be 100 to 120 degrees Celsius. Therefore, a single thermosensitive coating may be applied to a specific location within the battery assembly 10 and depending on the emitted substance that is detected, the onset of thermal runaway can be assessed.

[0053] The thermosensitive coating is a paint that includes a binder, preferably a binder with a low level of volatile organic compounds (VOCs). In this example, the binder is an acrylic polymer. The paint includes a water-based solvent, and additives including the microcapsules. The paint may additionally include flow improvers and defoamers, which improve the properties of the paint to make it easier to apply. When the paint is applied to the surface of the battery assembly and dries, a microcapsule-binder matrix attaches to the surface. Providing a paint with a water-based solvent and preferably low-VOC binder is beneficial because such a paint does not leave excess volatile organic compounds (VOC’s) on the surface after application, which may impact the sensor(s) of the battery assembly.

[0054] It will be appreciated by persons skilled in the art that the above detailed examples have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims. Various modifications to the detailed examples described above are possible.

[0055] Through the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0056] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract or drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0057] It will be appreciated by persons skilled in the art that the above embodiment(s) have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims. Various modifications to the detailed designs as described above are possible.

Claims

CLAIMS1 . A battery assembly comprising a plurality of battery cells, a thermosensitive coating applied to a surface of the battery assembly, and a sensor, wherein the thermosensitive coating is configured to emit a substance within a predetermined temperature range indicative of conditions leading to thermal runaway of the battery, and wherein the sensor is configured to detect the emitted substance.

2. A battery assembly according to claim 1 , wherein the thermosensitive coating comprises microcapsules containing the substance or a source of the substance and adapted to rupture at the predetermined temperature range.

3. A battery assembly according to claim 2, wherein the microcapsules comprise a polymer material.

4. A battery assembly according to claim 3, wherein the polymer material has a glass transition temperature that is within or lower than the predetermined temperature range.

5. A battery assembly according to claim 3 or claim 4, wherein the polymer material comprises polymethyl methacrylate.

6. A battery assembly according to any one of claims 3 to 5, wherein the polymer material has a molecular weight of between 10,000 and 1 ,000,000 kg / mol by gel permeation chromatography (GPC).

7. A battery assembly according to any one of the preceding claims, comprising a single gas sensor unit.

8. A battery assembly according to any one of claims 1 to 6, wherein the sensor comprises a sensor array having a plurality of gas sensor units.

9. A battery assembly according to claim 8, wherein the plurality of gas sensor units are arranged in the battery assembly at the same location.

10. A battery assembly according to claim 8, wherein the plurality of gas sensor units are arranged in the battery assembly at different locations.

11. A battery assembly according to any one of the preceding claims, wherein the thermosensitive coating comprises a primary agent configured to emit a first substance and a secondary agent configured to emit a second substance.

12. A battery assembly according to claim 11 , wherein the primary agent is configured to emit the first substance at a first subrange of the predetermined temperature range, and the secondary agent is configured to emit the second substance at a second subrange of the predetermined temperature range.

13. A battery assembly according to claim 12, wherein the first subrange is different from the second subrange.

14. A battery assembly according to any one of claims 11 to 13, wherein the primary agent and the secondary agent are applied to the battery assembly in the same location.

15. A battery assembly according to any one of claims 11 to 13, wherein the primary agent and the secondary agent are applied to the battery assembly at different locations.

16. A battery assembly according to any one of the preceding claims, wherein the substance is a hydrocarbon, e.g. an essential oil.

17. A battery assembly according to any one of the preceding claims, wherein the predetermined temperature range is between 50 and 120 degrees Celsius, preferably between 60 and 100 degrees Celsius.

18. A battery assembly according to any one of the preceding claims, wherein the thermosensitive coating is a paint.

19. A battery assembly according to claim 18, wherein the paint is a water-based paint.

20. A method of detecting onset of thermal runaway in a battery assembly having a plurality of battery cells, the method comprising detecting a substance emitted from a thermosensitive coating on the battery assembly, the thermosensitive coating being configured to emit the substance within a predetermined temperature range indicative of conditions leading to thermal runaway.21 . A method of detecting onset of thermal runaway in a battery assembly according to claim 20, comprising detecting a first substance emitted from the thermosensitive coating on a first part of the battery assembly and / or detecting a second substance emitted from the thermosensitive coating on a second part of the battery assembly, and determining a location of onset of thermal runaway based on the detected first substance and / or second substance.

22. A method of detecting onset of thermal runaway in a battery assembly according to claim 20 or claim 21 , comprising detecting a first substance emitted from the thermosensitive coating in a first subrange of the predetermined temperature range and / or detecting a second substance emitted from the thermosensitive coating in a second subrange of the predetermined temperature range, and determining the onset of thermal runaway based on the detected first substance and / or second substance.

23. A paint comprising microcapsules, wherein the paint is water-based and the microcapsules comprise a substance or a source of the substance encapsulated by a polymer material; wherein the microcapsules are adapted to rupture at a predetermined temperature range; and wherein the polymer material has a glass transition temperature that is within or lower than the predetermined temperature range.

24. A paint according to claim 23, wherein the substance is a hydrocarbon, e.g. an essential oil.

25. A paint according to claim 23 or 24, wherein the polymer material comprises polymethyl methacrylate, preferably having a molecular weight of between 10,000 and 1 ,000,000.

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