Flexible battery expansion sensor for rechargeable batteries

Abstract

A battery set comprises a housing, at least one first battery cell, in particular a pouch cell arranged inside the housing, and at least one expansion sensor, wherein an outer shell of the first battery cell comprises an electrically conductive material.The at least one expansion sensor is arranged between the electrically conductive material of the outer casing of the first battery cell and a contact surface adjacent to the first battery cell and comprises at least one flat inductor circuit arranged on a flexible substrate, wherein the at least one flat inductor circuit is arranged in a contact zone between the first battery cell and the contact surface adjacent to the first battery cell, at least one first compressible buffer layer arranged between the flexible substrate with the inductor circuit and the electrically conductive material of the outer casing of the first battery cell, and a detection circuit coupled to the at least one flat inductor circuit by means of respective connecting leads arranged on the flexible substrate in order to detect a change in the inductance of the at least one flat inductor circuit.

Description

Technical field

[0001] The invention relates to a battery expansion sensor for use in a battery pack, wherein a battery pack comprises a plurality of linearly stacked battery cells and at least one such battery expansion sensor, and a method for at least one of the detection of an impending thermal instability state, supporting the evaluation of a safety state and supporting the evaluation of a health state of such a battery pack. Background of the invention

[0002] Modern batteries are used in a wide range of technological fields. For example, batteries are currently used in electrical devices, music systems, tools, and garden equipment as energy storage (buffer storage) (for household and industrial sites with solar panels and other generators), in vehicles or industrial plants on a large scale, or for remote storage (off-grid). Multiple batteries or battery cells, such as pouch cells, are typically arranged within a single casing to form a battery pack.

[0003] Particularly in light of today's mobility-related technologies, such battery sets represent key elements for storing and providing energy for electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid vehicles (PHEVs) and new energy vehicles (NEVs).

[0004] During its service life, a battery pack is exposed not only to harsh environmental conditions such as heat, cold, and humidity, but also to challenging dynamics, such as the frequency and number of charge and discharge cycles. These factors influence the overall and remaining service life and the condition of the battery pack. Consequently, battery cells and battery packs are subject to aging and degradation processes that can increase the likelihood of swelling or gas release.

[0005] Gas release generally refers to a phenomenon caused by gas generation within a battery (cell). Gas release can result from electrolyte decomposition within the battery and / or be caused by overheating and / or overcharging. A gas-releasing battery cell may swell, rupture, or even explode. Swelling generally refers to a change in the volume of the battery (cell). Swelling can be caused, for example, by processes of lithium ion storage and removal in and / or on the electrode. Swelling can also be caused by gas release. Swelling leads to mechanical deformation of the battery cell, causing compressive forces in and / or on the battery cell and / or battery pack casing.To compensate for swelling, battery manufacturers typically use rigid structures, such as metal or hard plastic casings, to counteract expansion. Furthermore, battery manufacturers typically incorporate elastic materials like foams into the stack to absorb the swelling.

[0006] Expansion, displacement, or thermal expansion caused by compressive forces during swelling can correlate with the so-called "state of health" (SOH) of a battery pack. State of health generally refers to the aging state of a battery pack, thus providing a measure or indicator of its ability to store and deliver electrical energy compared to a new battery pack. Thermal expansion is also used to determine and / or predict the end of life (EOL) of the battery pack. EOL is generally used to determine the period during which the battery pack can be safely charged and discharged. Like SOH, EOL can also be used as an indicator to show the remaining operating time or service life of the battery pack.

[0007] To improve the safety and reliability of the battery pack in its operating environment, a battery management system (BMS) is often used to determine or estimate the state of charge (SOC) of the individual battery cells within the battery pack, as well as the state of health (SOH) and end-of-life (EOL). The "state of charge" generally refers to the available capacity, which can be expressed or represented as a percentage of its predetermined capacity. In other words, the SOC, EOL, and SOH are indicators that can be determined by the BMS.

[0008] Furthermore, the BMS can be configured to measure or determine additional parameters of the battery pack and / or individual battery cells, such as cell temperature and / or voltage. The BMS can also access predefined and stored specific characteristics and measurements of the battery cell, taken from a reference cell and / or battery pack. Based on such data, the BMS can, for example, compare stored and / or measured values ​​of a cell with the reference values ​​to determine the various indicators more accurately. The BMS can also be configured to monitor the operation of the individual cells, as well as the charging and discharging processes. Consequently, the BMS can identify defective cells and shut them down.In most cases, cells identified as defective must be replaced; typically, the entire module or set is replaced.

[0009] The service life, operating time, or remaining operating time of a battery may be limited by a maximum pressure exerted on the mechanical enclosure or casing of the battery pack. Manufacturers typically know the value of this maximum withstandable pressure. Pressure (a force) exceeding the predetermined maximum withstandable pressure can lead to failure of the battery cell, the casing, or the entire battery pack. For example, pressure caused by swelling of a battery cell that exceeds the predetermined maximum withstandable pressure value can cause the battery cell to rupture. For this reason, battery management systems may also be configured to detect swelling.

[0010] To detect swelling, conventional battery management systems use algorithms or complex mechanical devices to make estimates of the current condition or status of the battery cell and / or battery pack. In most existing battery packs, the monitored parameters include voltage, charging currents, temperature, and similar variables. These parameters allow for an estimation of the battery's health. The use of such algorithms may be based on a more or less accurate estimate of the EOL (End of Life) indicator, the SOC (State of Charge) indicator, or the SOH (State of Health) indicator.

[0011] Estimating cell expansion based on parameters such as battery voltage, charging currents, and battery temperature is not always accurate enough. Several ideas have been proposed for directly measuring the pressure within the battery pack, such as a mechanical sensing mechanism, piezoelectric materials, or pressure-sensitive thin-film materials. A major hurdle for most of these approaches is that the sensor needs to be very flat to be integrated into a battery pack and cover a substantial area of ​​the pack, for example, approximately 10 x 30 cm for automotive battery packs. Additionally, despite this size, it must be very inexpensive for consumer applications. Furthermore, due to mechanical requirements, conventional inductive or capacitive sensors on rigid printed circuit boards (PCBs) are not practical. Object of the invention

[0012] The invention is therefore based on the objective of providing a flat, battery-compatible expansion sensor for use in a battery pack, comprising a plurality of linearly stacked battery cells, in particular pouch cells, for reliably sensing an expansion of a battery cell that is present in the battery pack to support the detection of an impending thermal instability state and / or the evaluation of the safety state (SOS) and / or health state (SOH) of the battery pack. General description of the invention

[0013] According to one aspect of the present invention, the problem is solved by a battery pack comprising a housing, at least one first battery cell arranged within the housing, and at least one expansion sensor, wherein an outer shell of the first battery cell comprises an electrically conductive material. The battery cell can be of any suitable type with an adapted design, such as, in particular, a pouch cell or, for example, a prismatic battery or the like. The at least one expansion sensor is arranged between the electrically conductive material of the outer shell of the first battery cell and a contact surface adjacent to the first battery cell.The at least one expansion sensor comprises at least one flat inductor circuit arranged on a flexible substrate, wherein the at least one flat inductor circuit is arranged in a contact zone between the first battery cell and the mounting surface adjacent to the first battery cell, at least one first compressible buffer layer is arranged between the flexible substrate with the inductor circuit and the electrically conductive material of the outer casing of the first battery cell, and a detection circuit is coupled to the at least one flat inductor circuit by means of respective connecting leads arranged on the flexible substrate in order to detect a change in the inductance of the at least one flat inductor circuit.

[0014] It should be noted that the outer casing of the battery cells in conventional battery packs, e.g., for electric vehicles, consists of a conductive material. For example, in most pouch cells, this is a thin layer of aluminum foil laminated onto PE or PT protective films. A finding of the present invention is that this conductive property of the pouch cell's outer casing can advantageously be used to detect the expansion of the battery cell in battery packs by measuring the displacement of a battery cell wall near the sensor using flexible electrical circuits that are sensitive to conductive material, such as flat inductor circuits.

[0015] Key aspects of the sensor architecture are a flat topography and a flexible circuit design suitable for placement between cells under high pressure without causing damage or impairment to the cells during the battery's lifetime. In this context, it should be noted that a "flat inductor circuit" refers to an inductor circuit in which the current bands, forming the coils or spirals of the inductor circuit, are arranged flat on the same plane on the substrate.

[0016] The first compressible buffer layer serves as a compressible spacer between the flexible substrate containing the inductor circuit and the electrically conductive material of the outer casing of the first battery cell. The compressible buffer layer, or preferably two compressible buffer layers, one on each side of the flexible substrate containing the inductor circuit, acts as a compression cushion to ensure slight compression in a newly assembled state and to further accommodate expansion and contraction during charging, discharging, and aging. For the material used in such a compressible buffer layer, a stress-strain curve exhibiting low compressive stress over a wide range of compressive strain is desirable. A known example of such a material is a fine-pored foam, such as a microcellular polyurethane foam.It is evident that the compressible buffer layer or layers also serve as insulating layers, preventing direct contact between the conductive material of the outer casing of the battery cell and the flat inductor circuits.

[0017] It is evident that the expansion sensor described above provides the prerequisites for continuous monitoring of the expansion of the battery cell(s) within the battery pack and can thus support the evaluation of the battery cell health status by applying a suitable evaluation method known in the world. Accordingly, the proposed sensor can enable early detection of a thermal instability condition and can support the implementation of preventative measures by accurately sensing the current compression load.

[0018] A thermal instability state is known to be one of the most problematic failure modes of a rechargeable traction battery. Details are described, for example, in Koch, Sascha et al., “Fast Thermal Runaway Detection for Lithium-Ion Cells in Large Scale Traction Batteries.” (Batteries 2018, 4(2), 16; DOI: 10.3390 / batteries4020016). A thermal instability state of individual cells within a large lithium-ion battery pack is a well-known risk that can lead to critical situations if no countermeasures are taken in today's lithium-ion traction batteries for battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (HEVs).Therefore, fast and reliable detection of faulty cells experiencing a thermal instability state within the lithium-ion battery is a key factor in the design of batteries for comprehensive passenger safety.

[0019] It is evident that the expansion sensor used in the invention is suitable for use in batteries in a wide range of technological fields, such as batteries used as energy storage devices (buffer storage) in electrical devices, music systems, tools, and garden equipment, or in large-scale industrial plants or for remote storage (off-grid). The expansion sensor used in the invention is particularly advantageous for use in battery packs for automotive applications. The term "motor vehicle," as used in this patent application, is understood to mean, in particular, suitable for use in vehicles, including passenger cars, trucks, semi-trailer trucks, and buses.

[0020] As previously stated, the at least one expansion sensor comprises a second compressible buffer layer arranged between the flexible substrate with the inductor circuit and the mounting surface. In such an embodiment, the flexible substrate with the inductor circuit is sandwiched between the two compressible buffer layers. The buffer layers on both sides of the flexible substrate with the inductor circuit advantageously protect both the flexible substrate with the inductor circuit and the adjacent battery cells by preventing excessive deformation of the flexible substrate and direct contact between inductor circuit elements and the battery cells.

[0021] The first and / or second compressible buffer layers preferably comprise a compressible foam with a magnetic filler material, such as ferrite particles / powder. Compression of the foam increases its effective permittivity. Similarly, the density of the magnetic filler increases, thus increasing the effective permittivity of the foam. This effect can be used to adjust the response / sensitivity. In other words, these measures increase the possible thickness range of the buffer layer in which the sensor exhibits good performance (high sensitivity, stable signal).

[0022] In a preferred embodiment, the at least one expansion sensor comprises a plurality of flat inductor circuits arranged at individual positions on the flexible substrate in the contact zone, each flat inductor circuit being coupled to the detection circuit by means of respective connecting leads arranged on the flexible substrate. The inductor circuits are preferably distributed more or less uniformly over the contact zone so that expansion of the battery cell or displacement of the battery's outer casing can be reliably detected, even if such displacement occurs only locally due to irregular expansion of the battery cell.

[0023] In embodiments, the flexible substrate comprises a sensor section to be arranged in the contact zone and a connecting tongue extending outwards from the sensor section to the contact, wherein the detection circuit is arranged on the connecting tongue outside the contact zone between the first battery cell and the mounting surface. The detection circuit and all other discrete electrical components, such as integrated circuits (ICs) and lumped elements like capacitors, are advantageously arranged on or coupled to the connecting tongue, so that the detection circuit and the other electrical components are located outside the contact zone between the first battery cell and the mounting surface and are therefore not compressed within the battery cell stack.This arrangement advantageously takes into account the requirement of flatness for each part of the sensor that is to be arranged between and in contact with the battery cells.

[0024] In various embodiments, the flat inductor circuits and / or connecting leads are formed on the flexible substrate using screen printing, inkjet printing, aerosol printing, or similar thick-film processes, or as laminates. Current bands, for example, can be applied using screen printing, inkjet printing, aerosol printing, or similar thick-film processes. Manufacturing the numerous electrical conductors from electrically conductive ink simplifies the application of highly precise manufacturing processes such as screen printing and inkjet printing, resulting in tight production tolerances and minimal material waste. Due to the relatively large size of the circuit, a conductor with very high conductivity is advantageous. If the conductivity is too low, ohmic losses dampen the resonance, making it more difficult to detect changes in inductance.Printing inks with a high silver particle content, preferably nanoparticles, can be used to prevent attenuation. Alternatively, etched laminates or similar processes that enable near-heavy metal conductivity can be used. To optimize costs, printed layers can be used in conjunction with laminates (e.g., etched conductive laminates or laser-cut dielectric laminates).

[0025] In embodiments of the invention, the at least one expansion sensor further comprises two conductive electrode pads arranged adjacent to one another on the flexible substrate in the contact zone between the first battery cell and the mounting surface, each electrode pad being coupled to the detection circuit by means of respective connecting leads arranged on the flexible substrate. In such an embodiment, the detection circuit is further configured to detect a change in capacitance between the electrode pads and / or a change in capacitance between one of the electrode pads and the at least one flat inductor circuit. The electrode pads are coupled to each other via the outer shell of the conductive battery cell adjacent to the pads (parallel mode). Therefore, an additional signal for detecting cell expansion can be provided by means of a capacitive measurement with the two electrode pads.Since the mutual capacitance is high when the electrodes are closer to the battery cell wall, it provides a good signal for smaller cell spacings, while inductive measurement provides a more stable signal for larger distances. Therefore, in addition to detecting different areas of the cell, the two measurement principles are complementary and (after fusion) provide a fairly stable and accurate signal.

[0026] A third type of measurement can be carried out by keeping (at least) one of the contact lines to an inductor circuit open and using the conductor surface of (at least) one inductor circuit as a capacitive electrode.

[0027] It is evident that the contact surface can be formed by an inner wall of the battery housing, in which case the expansion sensor is arranged between the outermost battery cell of the battery pack and the battery housing. In a preferred embodiment, the battery pack comprises at least one second battery cell adjacent to the first battery cell, and the contact surface is formed by the second battery cell. In this case, the at least one expansion sensor is arranged between the first battery cell and the second battery cell.

[0028] A problem with printed circuit boards is the change in DC resistance during aging. This change in DC resistance can introduce a significant change in the frequency-dependent impedance characteristics and thus lead to errors in estimating the translocation. In another embodiment, the detection circuit is configured to determine the DC resistance of the at least one inductor circuit. In this embodiment, the DC resistance can be measured and used as an additional term to evaluate the translocation. It is used, for example, as a correction term that is carefully modeled in aging experiments.

[0029] To increase the detection accuracy, it is advantageous if the detector circuit is configured to individually detect a change in the inductance of at least two flat inductor circuits of the plurality of flat inductor circuits in a first operating mode, and to detect a change in capacitance between the at least two flat inductor circuits of the plurality of flat inductor circuits in a second operating mode.

[0030] It is evident that the flexible substrate can be manufactured from a planar film of plastic material selected from a large group of plastic materials, such as polyethylene terephthalate (PET), polyimide (PI), polyetherimide (PEI), polyethylene naphthalene (PEN), polyoxymethylene (POM), polyamide (PA), polyphthalamide (PPA), polyetheretherketone (PEEK), polycarbonate (PC), poly(methyl methacrylate) (PMMA), and combinations of at least two of these plastic materials. These plastic materials allow for easy fabrication, and in this way, durable, cost-effective dielectric support elements with tight manufacturing tolerances can be provided.

[0031] The present invention also relates to a method for detecting at least one impending thermal instability state, supporting the evaluation of a safety state, and supporting the evaluation of a battery pack's health state, as disclosed above. This method comprises at least the following steps: - Detecting an expansion of a battery cell and / or a displacement of the outer shell of a battery cell, wherein the detection of an expansion of the battery cell and / or a displacement of the outer shell of the battery cell includes at least the step of detecting a change in the inductance of at least one flat inductor circuit of the at least one expansion sensor by the detection circuit; - Evaluate at least one of an impending thermal instability state, a safety state and a health state of the battery pack based on at least the expansion of the first battery cell and / or the displacement of the outer shell of the first battery cell.

[0032] It is evident that the invention is not limited to detecting the expansion of only one specific battery cell in the series of cells, but rather the method makes it possible to detect the expansion of any cell in the series. Indeed, the expansion of any cell in the set would lead to increased pressure within the battery pack and thus to compression of the buffer layer.

[0033] In embodiments, the detection of an expansion of the battery cell and / or a displacement of the outer casing of the battery cell further comprises the following steps: - Individual detection of a change in the inductance of at least two flat inductor circuits; - Detecting a change in capacitance between the at least two flat inductor circuits.

[0034] In embodiments in which the at least one expansion sensor further comprises two conductive electrode pads arranged adjacent to one another on the flexible substrate in the contact zone between the first battery cell and the mounting surface, the step of detecting an expansion of the first battery cell and / or a displacement of the outer shell of the first battery cell may further comprise the step of detecting a change in capacitance between the electrode pads and / or a change in capacitance between one of the electrode pads and the at least one flat inductor circuit.

[0035] In a further embodiment, this detection of an expansion of the first battery cell and / or a displacement of the outer casing of the first battery cell further comprises the step of determining a DC resistance of the at least one inductor circuit. This DC resistance can be used as a correction parameter for the detection of the battery cell expansion.

[0036] The present invention thus provides an expansion sensor in a battery pack with a specific design and defined geometry compatible with cost-effective manufacturing techniques (screen printing) for large-scale electronic circuits, e.g., here 300 mm in one dimension, like typical pouch cells. Details are also disclosed of how such a comparatively large circuit can be manufactured with low ohmic losses. Alternatively, the combination of high-loss printed inductors with a more specific measuring device for evaluating a complex impedance (i.e., not a commercially available eddy current sensor, but a specific oscillator) can be used. The disclosed sensor architecture provides a cost-effective solution where other methods, such as flexible or rigid PCBs, are not practical.

[0037] It is evident that in embodiments of the invention, the expansion sensor could include at least one reference inductor and / or capacitor located outside the compressible region. Measuring this reference inductor / capacitor in such an embodiment can provide additional information that may help to compensate for aging. Since the at least one reference inductor / capacitor would not change its geometry, changes in its response can be attributed to the aging of the current bands, dielectrics, and other materials in the vicinity.

[0038] These and other aspects of the invention will become apparent and be explained by the embodiments described below. Brief description of the drawings

[0039] Further details and advantages of the present invention will become apparent from the following detailed description of non-restrictive embodiments with reference to the accompanying drawing, wherein: Fig. 1 schematically shows a sectional view of a battery pack; Fig. 2 is a schematic perspective partial view of an expansion sensor located between two battery cells, Fig. 3 is a schematic illustration of an embodiment of the inductor circuits of an expansion sensor; Fig. 4 an enlarged view of a flat inductor circuit of Fig. 3 shows; Fig. 5 is a schematic illustration of a second embodiment of an expansion sensor; Fig. 6 is a schematic illustration of a third embodiment of an expansion sensor; Fig.7 illustrates the principle of eddy current displacement measurements; Fig. Figure 8 illustrates the principle of a combination of inductive and capacitive measurements.

[0040] In the various figures, the same parts are always labeled with the same reference symbols or numbers. Therefore, they are normally only described once. Description of preferred embodiments

[0041] A first embodiment of the invention is described below with reference to the Fig. 1 to 4 described.

[0042] An arrangement of battery cells 11 in a battery stack is shown schematically in Fig. Figure 1 shows that, in order to effectively use the proposed cell expansion sensor 10, the sensor 10 is integrated into a stack of several battery cells 11, all of which are mounted in a rigid housing 12. The battery cells 11 are stacked in one direction (here vertically, i.e., the y-direction).

[0043] The sensor 10 comprises a sensing layer 14 which encloses a thin, flexible dielectric substrate 31, 35 (see also Fig. 3), which is equipped with one or more layers of a structured conductive and dielectric material forming flat inductor circuits 32. The sensor 10 further comprises two compressible buffer layers 13 arranged on each side (top and bottom) of the flexible dielectric substrate 31 with the flat inductor circuits 32 applied thereon, i.e., on each side of the sensing layer.

[0044] The buffer layers 13 are made, for example, of a fine-pored foam that is compressible, such as a microcellular polyurethane foam.

[0045] In the illustrated embodiment, the sensor 10 is positioned between two of the battery cells 11, so that the buffer layers 13 are in contact with the walls of the respective battery cells 11. The remaining battery cells 11 are separated by intermediate layers 15, which may, for example, be an adhesive and / or a further compression cushion.

[0046] Fig. Figure 2 is a schematic perspective partial view of an expansion sensor 10, which is arranged between two battery cells 11. Fig. Figure 2a shows the battery cells 11 in a non-expanded state, while Fig.Figure 2b shows the deformation of part of the layer stack during cell expansion. The cells 11 deform in such a way that their sidewalls approach the sensor layer 14. The deformation of the cell 11 can result in a symmetrical cushion shape, or it can be asymmetrical, depending on the internal structure of the cell, which can lead to a non-uniform change in the distance d across the entire xz-plane. In the proposed application, it is important that the sensor layer 14 is flexible and very flat. Edges, contact bumps, and other topological irregularities could impair or damage the cell 11 when compressed. The outer shell of all battery cells 11 consists of a conductive material. For example, in most pouch cells, this is a thin layer of aluminum foil laminated onto PE or PT protective films.

[0047] Fig.Figure 3 is a schematic illustration of an embodiment of the inductor circuits 32 of the sensor layer 14 of an expansion sensor 10. A flexible substrate 31, 35 is provided and serves as a support structure for the electrical circuit. This substrate comprises a sensor section 31, which is similar in size to the battery cells 11, and a connecting tongue 35 extending from the sensor section 31. The electrical circuit includes several printed inductors 32, i.e., current bands in the form of coils or spirals 32. These inductors 32 are optimized to provide a specific inductance near the conductive outer shell of the battery cells 11.

[0048] The electrical circuit also includes contact lines or supply lines 33, which connect the inductors 32 to a control unit 37. If the sensor is arranged between two battery cells 11, the stack of cells covers the area marked by the dotted rectangle 34, i.e., the contact zone. The lower part of Fig.The sensor part shown (outside the dotted rectangle 34), i.e., among other things, the connecting tongue 35, is not compressed within the battery cell stack. Electrical components 36, such as integrated circuits (ICs) and lumped elements like capacitors, can therefore be placed on this lower part of the sensor. These components 36 are, for example, surface-mount devices (SMDs). Due to the flatness requirements of the sensor layer 14 within the contact zone 34, these components cannot be placed within the area 34. The control unit 37 includes, for example, an outer plastic housing, a printed circuit board with the necessary control electronics, a power supply, and similar components. It also includes an interconnection 39 with the rest of the flexible circuit structure and a connector 38 to a central unit (not shown).

[0049] In the embodiment of Fig. 3 The sensor 10 comprises four LC circuits. Here, four separate circuits enable the detection of thickness variations in the stack in four different areas in the plane of the sensor section 31 of the flexible substrate; these are the inner circles of the inductors 32.

[0050] Due to the occurrence of eddy currents, the inductance of the coils 32 is significantly reduced when conductive surfaces, such as the outer casing of the battery cell 11, approach the coil. Therefore, expansion and other deformations of the cell 11 cause a shift in the resonant frequency. Additionally, the real part of the impedance changes. Both of these effects can be used to estimate cell wall displacement.

[0051] Due to the relatively large size of the circuit, high ohmic losses can occur, and the LC resonance can be strongly damped. It is therefore advantageous to carefully optimize the circuit design and drastically reduce the ohmic losses by using a conductor with a conductivity close to that of a heavy metal.

[0052] The flexible substrate can be made of polyethylene (PE), polyimide, or PT film, for example. Current bands can be applied using screen printing, inkjet printing, aerosol printing, or similar thick-film processes. Due to the relatively large size of the circuit, a conductor with very high conductivity is advantageous. If the conductivity is too low, ohmic losses dampen the resonance and are more difficult to detect. Printing inks with a high silver particle content, preferably nanoparticles, can be used to prevent this damping. Alternatively, etched laminates or similar processes that achieve near-heavy metal conductivity can be used. To optimize costs, printed layers can be used in conjunction with laminates (e.g., etched conductive laminates or laser-cut dielectric laminates).

[0053] To connect a coil 32 to the feed lines 33 without creating a short circuit in the inner loops, a multi-layer printing / deposition process is required. This detail is shown in Fig.Figure 4 shows that in the illustrated embodiment, this is achieved using a small dielectric spot 44 printed on top of the current bands of the coil 32. In a subsequent step, current bands in the shape of dumbbells 43 are printed on top of both 44 and 32. Since the third printing involves only short current bands 43, conductivity is not as critical here, and most cost-effective techniques can be used. The contact area of ​​43 and 32 is preferably increased to reduce the ohmic contact resistance and to decrease geometric tolerances. To further avoid ohmic losses, the contact lines 33 are preferably considerably wider than the current bands of the coil 32, which have a specific line width required to achieve optimal inductance ( Fig. 4a).

[0054] One problem with printed current bands is the change in DC resistance during aging. This change in DC resistance can introduce a significant change in the frequency-dependent impedance characteristics and thus lead to errors in estimating the displacement. Therefore, in a preferred embodiment, the DC resistance is measured and used as an additional term to evaluate the displacement. It is used as a correction term that is carefully modeled in aging experiments.

[0055] It is evident that an important feature of the invention is that the entire sensor layer 14, including the structured current bands 32, 33, 43 and the dielectric spots 44, is flat to avoid damaging the cell when it is under pressure. When the inventors describe it as flat, they are referring to thickness variations of < 30 µm within the area defined by the dotted rectangle 34. Furthermore, the flat layered sensor enables cost-effective assembly, easy integration, and adaptation to the battery packs.

[0056] Exemplary dimensions and other parameters of the embodiment shown may be as follows: Size of cells (dx, dy, dz) (300 mm, 20 mm, 140 mm) Thickness of the flexible substrate 31 100 µm Diameter of conductive coils 32 70 mm Thickness of printed current bands 10 µm Resonance frequency 10-20 MHz Line width of supply lines 33 1 mm Line width coil 200 µm

[0057] In further embodiments, the resonant frequency can lie within the range of 100 kHz to 50 MHz. The thickness of the printed current bands can range from 1 to 30 µm. Other parameters may also vary, but would remain within the same order of magnitude.

[0058] Fig. Figure 5 illustrates another embodiment of the sensor layer 14 to be used in the present invention. Similar to the embodiment of Fig. 3 comprises the sensor layer of the embodiment of Fig.5 a flexible substrate 31, 35, several printed inductors 32, contact leads 33, a region 34 compressed between at least two battery cells 11 and two buffer layers 13. The shape of the flexible part of the sensor includes a connecting tongue or end piece 35, which is placed outside the compressed region 34. This connecting tongue 35 is shaped to simplify integration into a battery pack. In this particular case, it is comparatively long (35 cm) and forms an S-shape. The end of the end piece is connected to the control unit 37 using an interconnection structure 39 to connect the supply leads 33 to the electronic unit therein. With these features, an inductive-type measurement can be performed in a similar manner to that described in Fig. 3 illustrated sensing layer 14.

[0059] In addition to the circuits with printed inductors 32, two conductive electrode pads 71 ​​are printed onto the substrate 31. These electrode pads 71 ​​are also connected to the control unit 37 via respective contact lines. The electrode pads 71 ​​are coupled to each other via the walls of the battery cell 11, which are located in close proximity (parallel mode). Therefore, a capacitive measurement using the two electrode pads 71 ​​can provide an additional signal for detecting cell expansion. Since the mutual capacitance is high when the electrode pads 71 ​​are closer to the battery cell wall, a capacitive measurement provides a good signal for small battery cell distances d. As described above, the inductive measurement provides a more stable signal for larger distances d.Therefore, in addition to detecting different areas of cell 11, the two measurement principles are complementary and provide (after fusion) a fairly stable and accurate signal.

[0060] It is evident that another type of measurement can be carried out by keeping (at least) one of the contact lines 33 open to an inductor coil 32 and using the conductor surface of (at least) one coil 32 as a capacitive electrode.

[0061] By using specific parameterized algorithms or machine learning, different combinations of displacement measurements can help provide a signal that is robust against aging effects. This is particularly important for applications where long sensor lifetimes must be guaranteed, e.g., in the automotive sector and similar fields.

[0062] Fig.Figure 6 illustrates the advantage of the present invention with regard to sensor integration. In this case, two flexible sensor layers 81, which are provided with the same electronic components but with different contours, are connected to only one control unit 39. The two sensors could be integrated into two different battery packs, where the battery cell expansion is detected independently. By using printed electronics, such comparatively large and flexible sensors can be created at low cost. Including the longer end piece on the right side 83, this embodiment has a total length of 1 m.

[0063] It is evident that in the various embodiments disclosed above, one way to control the extent of the magnetic field around the inductor is to use a magnetic filler (such as ferrite particles / powder) in the buffer layer foam. This increases the permeability and results in a more moderate interaction with the nearby metal (more moderate eddy currents). This can thus help to control the damping of the resonance and offers greater design flexibility in selecting the foam thickness. Under compression of the foam, the effective permittivity increases. Similarly, the density of the magnetic filler increases, i.e., the effective permittivity of the foam increases. This effect can be used to adjust the response / sensitivity. For example, it can help to linearize the response.Similarly, the permittivity of the foam can be adjusted to manipulate the spacing behavior of the capacitive sensing mechanism. In other words, these measures increase the possible thickness range of the buffer layer in which the sensor performs well (high sensitivity, stable signal).

[0064] The principle of sensing the distance between an inductor coil and a metal sheet is well-known in the field and is often referred to as an eddy current proximity sensor, inductive displacement / position sensor, or by similar acronyms. Due to the requirement of low ohmic losses and high inductance, these state-of-the-art inductive sensors are typically manufactured either using printed circuit technology with highly conductive etched copper traces and coil diameters of < 10 mm, or they are manufactured in a non-planar form using copper wires (e.g., rotational speed sensors).

[0065] The measuring principle for an inductive sensor or a sensor of a combined capacitive and inductive type is described in more detail using equivalent circuit diagrams. In Fig.Figure 7 illustrates the principle of ordinary eddy current displacement sensors. Here, a parallel circuit is driven by a resonant circuit driver (labeled S). A metal object is shown as a second circuit on the right. When the distance d between the two is reduced, the inductance L in the parallel circuit is effectively reduced due to eddy currents on the metal object, leading to a change in the resonant frequency.

[0066] Fig. Figure 8 illustrates a sensing method in which ordinary eddy current displacement measurement is combined with capacitive measurements in a special way. In a first step ( Fig.10a) Two parallel-type resonant circuits are driven by the independent circuit drivers SA1 and SA2, respectively. Two contact lines, shown with dashed lines, are held open. The metallic object, in this case the battery cell walls, is represented by the circuit on the right. A fluctuation in the distances near the inductors L1 and L2 leads to a change in the resonant frequency of each circuit and can be detected by SA1 and SA2, respectively. In a second step ( Fig. 10b) SA1 and SA2 are disconnected, and the circuit is driven by SB to perform a capacitive measurement. In this second measurement, the capacitive coupling with the battery cell walls is more important and can be detected by SB.

[0067] Similar circuit configurations are possible, in which inductive and capacitive sensing are used sequentially, predominantly, using multiple sources or circuit drivers and switches. List of reference symbols 10 Expansion sensor 11 battery cell 12 cases 13 buffer layers 14 Sensing layer 31 Sensor section of the flexible dielectric substrate 32 Inductor circuit 15 Intermediate shift 35 Connecting tongue of the flexible dielectric substrate 33 Supply line 34 Contact zone 36 electrical components 37 Control unit 38 connectors 39 Interconnection 44 dielectric spots 43 electrical cords (dumbbell-shaped) 71 conductive electrode pad 81 flexible sensor layers QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0000] Koch, Sascha et al., “Fast Thermal Runaway Detection for Lithium-Ion Cells in Large Scale Traction Batteries.” (Batteries 2018, 4(2), 16; DOI: 10.3390 / batteries4020016

[0018]

Claims

[1] Battery assembly comprising a housing, at least one first battery cell arranged inside the housing and at least one expansion sensor, - wherein an outer casing of the first battery cell comprises an electrically conductive material; - wherein the at least one expansion sensor is arranged between the electrically conductive material of the outer casing of the first battery cell and a mounting surface adjacent to the first battery cell; - wherein the at least one expansion sensor comprises at least one flat inductor circuit arranged on a flexible substrate, wherein the at least one flat inductor circuit is arranged in a contact zone between the first battery cell and the mounting surface adjacent to the first battery cell, at least one first compressible buffer layer arranged between the flexible substrate with the inductor circuit and the electrically conductive material of the outer casing of the first battery cell, and a detection circuit coupled to the at least one flat inductor circuit by means of respective connecting leads arranged on the flexible substrate to detect a change in the inductance of the at least one flat inductor circuit. [2] Battery assembly according to claim 1, wherein the at least one expansion sensor comprises a second compressible buffer layer arranged between the flexible substrate with the inductor circuit and the contact surface. [3] Battery pack according to one of claims 1 or 2, wherein the at least one expansion sensor comprises a plurality of flat inductor circuits arranged at individual positions on the flexible substrate in the contact zone, wherein each flat inductor circuit is coupled to the detection circuit by means of respective connector leads arranged on the flexible substrate. [4] Battery pack according to one of the preceding claims, wherein the flexible substrate comprises a sensor section to be arranged in the contact zone and a connecting tongue extending from the sensor section outwards from the contact zone, and wherein the detection circuit is arranged on the connecting tongue outside the contact zone between the first battery cell and the mounting surface. [5] Battery pack according to any of the preceding claims, wherein the flat inductor circuits and / or the connecting leads are formed on the flexible substrate by screen printing, inkjet printing, aerosol printing or similar thick film processes, or as laminates. [6] Battery pack according to one of the preceding claims, wherein the at least one expansion sensor further comprises two conductive electrode pads arranged adjacent to each other on the flexible substrate in the contact zone between the first battery cell and the mounting surface, wherein each of the electrode pads is coupled to the detection circuit by means of respective connecting leads arranged on the flexible substrate; and wherein the detection circuit is further configured to detect a change in capacitance between the electrode pads and / or a change in capacitance between one of the electrode pads and the at least one flat inductor circuit. [7] Battery pack according to one of the preceding claims, wherein the battery pack comprises at least one second battery cell adjacent to the first battery cell and wherein the contact surface is formed by the second battery cell, such that the at least one expansion sensor is arranged between the first battery cell and the second battery cell. [8] Battery pack according to any one of the preceding claims 1 to 6, wherein the contact surface is formed by an inner wall of the housing. [9] Battery pack according to one of the preceding claims, wherein the detection circuit is configured to determine a DC resistance of the at least one inductor circuit. [10] Battery pack according to any one of the preceding claims 3 to 10, wherein the detection circuit is configured to individually detect a change in the inductance of at least two flat inductor circuits of the plurality of flat inductor circuits in a first operating mode, and to detect a change in capacitance between the at least two flat inductor circuits of the plurality of flat inductor circuits in a second operating mode. [11] Battery assembly according to one of the preceding claims, wherein the flexible substrate is largely made of a planar film of plastic material selected from a group of plastic materials consisting of polyethylene terephthalate PET, polyimide PI, polyetherimide PEI, polyethylene naphthalate PEN, polyoxymethylene POM, polyamide PA, polyphthalamide PPA, polyetheretherketone PEEK and combinations of at least two of these plastic materials. [12] Battery assembly according to any of the preceding claims, wherein the first compressible buffer layer and / or the second compressible buffer layer comprises a compressible foam with a magnetic filler material. [13] A method for at least one of the following steps: detecting an impending thermal instability state, supporting the evaluation of a safety state and supporting the evaluation of a health state of a battery according to any of the preceding claims, comprising at least the following steps: - Detecting an expansion of a battery cell and / or a displacement of the outer shell of a battery cell, wherein the detection of an expansion of the battery cell and / or a displacement of the outer shell of the battery cell includes at least the step of detecting a change in the inductance of at least one flat inductor circuit of the at least one expansion sensor by the detection circuit; - Evaluate at least one of an impending thermal instability state, a safety state and a health state of the battery pack based on at least the expansion of the first battery cell and / or the displacement of the outer shell of the first battery cell. [14] Method according to claim 13, wherein the detection of an expansion of the battery cell and / or a displacement of the outer shell of the battery cell further comprises the following steps: - Individual detection of a change in the inductance of at least two flat inductor circuits; - Detecting a change in capacitance between the at least two flat inductor circuits. [15] Method according to one of claims 13 or 14, wherein the at least one expansion sensor further comprises two conductive electrode pads arranged adjacent to each other on the flexible substrate in the contact zone between the battery cell and the mounting surface; wherein the detection of an expansion of the battery cell and / or a displacement of the outer shell of the battery cell further comprises the step of detecting a change in capacitance between the electrode pads and / or a change in capacitance between one of the electrode pads and the at least one flat inductor circuit. [16] Method according to any one of claims 13 to 15, wherein the detection of an expansion of the battery cell and / or a displacement of the outer shell of the battery cell further comprises the step of determining a DC resistance of the at least one inductor circuit.

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