Battery module and motor vehicle

Abstract

Battery module (6), comprising - a module housing (8), - at least one secondary cell (10) accommodated in the module housing (8), - a bracing device (14), by means of which the secondary cell (10) is braced with a flat side against an inner wall of the module housing (8), wherein the bracing device (14) has a spring element (16) for generating a spring force and a gear (18) with a non-linear transmission function for the spring force, which is connected between the spring element (16) and the secondary cell (10) and which is designed to generate an approximately constant bracing force (F V ) to generate.

Description

The invention relates to a battery module. The invention further relates to a motor vehicle.In the course of progressive electrification of motor vehicles, but also of building technology and the like, the use of lithium-ion technology in secondary cells has become established in the meantime. In this case, lithium ions accumulate within the (secondary) cells on an anode of the cell during charging processes and correspondingly migrate to the corresponding other electrode during discharging processes. Currently, common materials for the anode are, in particular, graphite. The subject matter of investigations as anode (host) material is also, inter alia, silicon. This is attractive because of its comparatively simple, almost unlimited availability and also a high energy density.The present invention further relates to solid secondary cells in which the liquid electrolyte usually used in lithium ion cells is replaced by a solid. The advantage lies, for example, in a comparatively low or non-existent flammability and no risk of causing damage to surrounding secondary cells as well as to damage them as a result of electrolyte leakage.Solid secondary cells-just like at least partially also conventional secondary cells-have a so-called "breathing". That is, a change in volume takes place which depends on the state of charge. Such a volume change can amount to up to or even more than 30 percent-in particular with regard to an increase in thickness of planar secondary cells. Such a "breathing stroke" can lead to various problems if it is uncontrolled. If the thickness changes, contact points can tear and / or-especially in the case of electrodes made of metallic lithium-so-called dendrimers (specific crystal structures) can grow, which may possibly lead to a short circuit between the electrodes.Therefore, in particular solid-state batteries require a bracing of the secondary cell, preferably of the cell stack which is usually used in a battery module and the contact pressure value of which remains the same over as much as possible the entire breathing stroke. This pressing force value must be so high in order to prevent the above-mentioned effects. However, the pressure force value must also not be too high in order to avoid mechanical damage to the layers between the electrodes, which layers are usually formed from plastic or contain plastic.DE 10 2019 112 327 A1 describes in this context loading such a cell stack with springs at the end. DE 10 2020 003 187 A1 describes an actively controlled application of pressure, for example via hydraulics, pneumatics, but also by means of springs, to the cell stack. U.S. Pat. No. 9,399,404 B2 describes an active pressure application to the cell stack, which is dependent on the state of charge and / or also on (de)charge rates.US 6 413 665 B1 describes a compression system for a fuel cell stack in which a spring arrangement and a mechanical connection arrangement are used in connection with tie rods and tie rods in order to exert a compressive load on a fuel cell stack. The linkage assembly includes a lever and three pins to redirect the force or motion generated by the spring assembly into a motion or force for the tie rods. The tie rods, in turn, are connected to the tie rods that span the top end plate of the fuel cell stack and transfer the load to the stack. The linkage also includes a slotted bearing that compensates for the arc of a circle formed by the lever and allows the spring unit to be mounted rigidly under the lower end plate. The spring assembly includes a plurality of parallel arranged springs configured to provide a non-linearly decreasing load when the stack is compressed due to cell consolidation. This load profile reduces the overall shrinkage of the stack and at the same time provides high pressure at the beginning of the lifetime to ensure proper contact between the cells.The object of the invention is to specify an improved battery module.This object is achieved according to the invention by a battery module having the features of claim 1.The battery module according to the invention has a module housing and at least one secondary cell. The latter is accommodated in the module housing. The battery module also has a clamping device, by means of which the secondary cell is clamped with a flat side against an inner wall of the module housing. This clamping device has a spring element for generating a spring force and a transmission with a nonlinear transfer function for the spring force. The gear is connected between the spring element and the secondary cell.A transmission with a nonlinear transmission function advantageously enables a transmission of force, in particular the provision of a force-path profile which is virtually independent of, at least different from, the force-path profile provided on the part of the spring element. The latter is recognized to be directly derived from the (function of) the spring rate of the inserted spring element. Thus, by suitable gear design, a force profile required or at least expedient for pressurizing the secondary cell over the "breathing stroke" can be set without the spring element itself having to depict this profile.In a preferred embodiment, the gear mechanism has a lever which is articulated on the secondary cell or on the module housing such that it can be displaced along a raceway and correspondingly vice versa with a lever foot on the module housing or on the secondary cell in a fixed manner, but pivotably. The lever serves to transmit a force referred to as "bracing force" to the secondary cell. The transmission is thus designed, in particular, at least partially as a coupling transmission. Preferably, a force distribution element (also "pressure piece") is connected between the lever and the secondary cell (or optionally also a stack formed from a plurality of secondary cells, in particular secondary cells of the same type), which force distribution element distributes the bracing force exerted at relatively points by the lever over the surface of the secondary cell (or the stack). In this case, the raceway is formed on the force distribution element (or, in a correspondingly reversed embodiment, on a housing inner surface of the module housing).For reducing friction, the lever advantageously has, at its free end movably supported against the raceway, a roller which is rotatably mounted on the free end and with which the lever rolls on the raceway during movement.Optionally, the transmission is configured to generate (or also: "image") a force profile, preferably nonlinear, for example progressive or degressive in partial regions of the breathing stroke of the secondary cell or a spring path of the spring element.According to the invention, however, the transmission is configured to keep the bracing force at least approximately constant over the spring travel of the spring element. "Approximately constant" is to be understood here and below in particular to the effect that the bracing force is kept within a predefined value range which is preferably determined by transmission properties and tolerances-for example. Friction between transmission elements is determined.In an expedient embodiment, the transmission has a support bearing which is arranged substantially in a fixed manner with respect to the lever foot and from which at least part of the spring force is transmitted to a force application point which is fixedly predetermined on the lever. "Substantially in a fixed manner" is to be understood in particular to the effect that a force application point on the support bearing can vary slightly spatially depending on the formation thereof. In the case of a deflection roller for a traction means, for example, the angle of wrap can vary during movement of the lever, so that the "point of departure" of the traction means from the deflection roller likewise varies.In an expedient development of the above-described embodiment of the transmission, the force application point divides the lever into a foot section and a free section. In particular, in order to keep the bracing force constant, the lengths of the foot section and the free section are selected as a function of the (in particular desired) bracing force (i.e. its "target value"), the spring rate of the spring element and the distance between the support bearing and the lever foot running in the direction of action of the bracing force. Preferably, the support bearing is arranged in the direction of action of the bracing force flush with the lever base-specifically its articulation point or bearing point. The dependence of the lengths of foot section and free section is preferably designed such that the length ratio of the length of the foot section to the total length of the lever corresponds to the ratio of bracing force to spring rate and to the distance between support bearing and lever foot, in particular according to the following formula: wherein I F represents the length of the foot section, I G represents the total length of the lever, F V represents the bracing force, c represents the spring rate, and A SF represents the distance between support bearing and lever foot.In an advantageous embodiment, the spring element is formed by a plurality of springs connected in parallel. As a result, a high spring force can be generated with a comparatively short design. In addition, in particular with a suitable two-dimensional distribution of the springs, a symmetrical introduction of force into a force-delivery plate, by means of which the springs are coupled, specifically connected in parallel, can take place. Preferably, the springs each have a constant spring rate. That is to say that the springs have a linear force-displacement behavior at least within an intended working range. The springs are preferably selected such that their respective spring rate lies within a predefined range. Additionally or alternatively, the springs are expediently integrated into the transmission under a uniform prestressing force value. That is to say that the springs are each individually prestressed, such that the installation prestressing force value lies within a predefined tolerance range.In a preferred embodiment, the gear mechanism has a traction means and a deflection roller arranged on the support bearing. The spring element is coupled to the lever-in particular at its force application point-by means of the traction means guided over the deflection roller and is thus arranged on a side of the deflection roller facing away from the lever. Preferably, the traction means is formed by a chain and the deflection roller by a sprocket (also referred to as a chain sprocket). This represents a comparatively stiff embodiment.In an expedient embodiment, the gear is configured symmetrically with respect to a longitudinal axis of the module housing. The gear is preferably constructed mirror-symmetrically. This allows a symmetrical introduction of force into the secondary cell to be achieved. For this purpose, the transmission has two levers, which are each coupled to one end of the traction means. In addition, the gear has two deflection rollers, via which the traction means is guided before coupling to the respective lever. In addition, in an expedient embodiment, the gear has at least one, in particular two further deflection rollers, which is / are coupled to the spring element. The traction means is guided over this deflection roller(s) with its central region. As a result, the spring force generated by the spring element can be transmitted uniformly to both levers.In particular in the case where, owing to mechanical tolerances, the variable wrap angle of the deflection roller(s) or the like, the bracing force is incontinuance, the raceway of the respective lever or levers is expediently designed as a curved raceway. In particular, the raceway is curved at least in sections along the longitudinal axis. As a result, it is possible to even out constancy over even only parts of the breathing stroke, or else to set a progression or degressive action of the bracing force in a targeted manner.Preferably, the clamping device is designed such that the spring element is under prestress in each state (i.e. at each value of the breathing stroke).The motor vehicle according to the invention has the battery module described here.The conjunction "and / or" is to be understood here and in the following in particular in such a way that the features linked by means of this conjunction can be formed both jointly and as alternatives to one another.Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Shown therein are: FIG. 1 shows a schematic side view of a motor vehicle, which has an electric drive and a battery comprising at least two battery modules, FIG. 2 shows a schematic plan view of such a battery module having a multiplicity of secondary cells which are in a first state, and having a clamping device, FIG. 3 shows a view according to FIG. 2 of the battery module with the secondary cells in a second state, FIG. 4 shows a schematic detail view of the clamping device, FIGS. 5, 6 show a view according to FIG. 4 of a further exemplary embodiment of the clamping device in a clamped and relaxed state, FIGS. 7, 8 show a view according to FIG. 4 of a mechanical conversion of the clamping device in the tensioned or relaxed state, and FIG. 9 shows a perspective view of the clamping device according to FIG. 7.Parts and sizes corresponding to one another are always provided with the same reference numerals in all figures.FIG. 1 schematically shows a motor vehicle, in this case specifically a passenger car 1. This includes an electric drive in the form of an electric motor 2 which acts on a drive axle of the passenger car 1 in a manner not shown. For supplying energy to the electric motor 2, the passenger car has a battery 4 which comprises a plurality of battery modules 6 (only two shown here).As can be seen in FIG. 2, the battery modules 6 have a module housing 8 which is of box-like configuration. In this cell, a plurality of secondary cells 10 are arranged in a stack-like manner. The secondary cells 10 are formed here by solid secondary cells in the form of so-called pouch cells. The secondary cells 10 have a volume difference between their different states of charge (discharged or charged) (see FIG. 3 ). The thickness of the secondary cells 10 increases or decreases substantially, i.e. in the direction transverse to the planar extent-in the present exemplary embodiment this corresponds to the longitudinal direction 12 (which runs parallel to a longitudinal axis, not shown) of the battery module 4.In order to control, at least counteract, the breathing stroke of the secondary cells 10, the battery module 6 has a clamping device 14. This serves to subject the secondary cells 10, specifically the stack formed by the secondary cells 10, to a bracing force and thus to apply a counterpressure acting counter to the breathing stroke to the secondary cells 10. This is useful in solid secondary cells which use, inter alia, metallic lithium as electrode material in order to avoid growth of so-called dendrimers. In addition, this counterpressure can also prevent contacts between the busbar and the secondary cells 10 from breaking due to the movement during the "breathing" of the secondary cells 10.The clamping device 14 has a spring element 16 which serves to generate a spring force. The spring element 16 here and in the following has linear behavior, i.e. a constant spring rate within the scope of the usual understanding. This spring element 16 acts on a gear 18 of the clamping device 14. Specifically, the transmission 18 in the present exemplary embodiment is designed such that the spring force varying (depending on the deformation of the spring element) acts in a constant bracing force F v, which acts on the stack of secondary cells 10. As a result, the same force value of the bracing force F V can be applied to the secondary cells 10 for each state of charge.In FIG. 2, the secondary cells 10 are (at least for the most part) discharged and thus comparatively flatter than in the more heavily charged state, illustrated in FIG. 3. As is schematically illustrated in FIG. 3, a force-imparting and force-distributing element (or also "pressure piece 20") of the transmission 16 (here illustrated by way of example as a type of plunger with pressure plate) is inserted and the spring element 16 is compressed.In FIG. 4, the functional principle of the transmission 18 is shown in more detail. The secondary cells 10 are not shown. The transmission 18 is designed partly as a traction drive and partly as a coupling transmission. For this purpose, the gear 18 has two levers 24, which are pivotably but fixedly articulated on a wall of the module housing 8 by a lever foot 26 in a bearing point. At the free end opposite the lever foot 26, the levers 24 are displaceably mounted on the pressure piece, specifically on a raceway 28 formed on the latter. For the purpose of minimizing friction, a roller 32 is arranged at the free end of the respective lever 24. The respective lever 24 serves for converting and transmitting the spring force applied by the spring element 16 into the bracing force F v, more precisely into in each case half the bracing force F v.Moreover, the transmission 18 has a traction means 34. This is coupled with its two ends to a fixedly predetermined force application point 36 with the respective lever 24. With a central region, the traction means 34 is guided over two deflection rollers 40 coupled to a movable force output plate 38 of the spring element 16. As a result, the traction means 34 is displaceable and can transmit the spring force generated by the spring element 16 equally to both levers 24.The gear 18 also has two further deflection rollers 42, via which the traction means 34 is guided to the lever 24. The deflecting rollers 42 each represent support bearings for the traction means 34. These deflecting rollers 42 are aligned in the longitudinal direction 12 with the respective lever foot 26, specifically its bearing point. In addition, the two deflection rollers 42 are arranged SF to the lever base 26 at a fixed predetermined distance A. The deflection rollers 40, on the other hand, are displaced in the longitudinal direction with respect to the deflection rollers 42 when the spring element 16 is loaded or unloaded.The gear 18 is designed symmetrically with respect to the longitudinal axis of the battery module 6.In the exemplary embodiments illustrated here, the spring element 16 has a plurality of helical springs 44. These are inserted under prestress between the module housing 8 and the force output plate 38 and are specifically connected in parallel.The traction means 34 divides the respective lever at the force application point 36 into a foot section 46 with a length I F and a free section 48 with a length I A. The lengths I F and I A of the two sections are selected in such a way that the force transmitted by the traction means 34 at the respective roller 32 of the two levers 24 always remains constant (within mechanically conditioned limits).FIGS. 5 and 6 each show a different (charging) state of the secondary cells 10 and thus a different position of the transmission 18. FIG. 5 shows the gear position for the state of the secondary cells 10 as in FIG. 3 The helical springs 44 (which in the exemplary embodiment according to FIGS. 5 and 6 are arranged further inwards than in the exemplary embodiment according to FIG. 4 ) are compressed and the levers 24 are arranged perpendicular to the longitudinal direction 12. Due to the force exerted by the coil springs 44 and thus the tension of the traction means 34 on the two levers 24, the bracing force F V is nevertheless applied to the secondary cells 10.FIG. 6 shows a position of the transmission 18 similar to that in FIG. 4. To clarify the movements performed by the transmission 18, an "imaginary box 50", which is of equal size for both figures, is drawn in in FIGS. 5 and 6. As shown in FIG. 6, when the thickness of the secondary cells 10 is reduced, the coil springs 44 extend and expand, thereby pushing the force output plate 38 toward the secondary cells 10 (beyond the case 50). Since the deflecting rollers 40 move together in this case, the section of the traction means 34 between the deflecting rollers 42 and the respective lever 24 is shortened, so that the latter is raised. Each of the levers 24 always applies half the bracing force F V to the raceway 28 of the pressure piece 20. Also in the "relaxed" state shown in FIG. 6, the springs 44 are under bias.FIGS. 7 to 8 show a specific exemplary embodiment of the clamping device 6. The spring element 16 here has specifically eight coil springs 44. A chain is used as the traction means 34 and corresponding chain pinions as the deflection rollers 40 and 42. The pressure piece 20 is designed as a framework component (weight-saving) in order to form an extension between the two levers 24 and the stack of secondary cells 10.The subject matter of the invention is not limited to the exemplary embodiments described above. Rather, further embodiments of the invention can be derived from the above description by the person skilled in the art. In particular, the individual features of the invention described on the basis of the various exemplary embodiments and their design variants can also be combined with one another in another manner.List of reference characters1 CAR 2 Electric motor 4 Battery 6 Battery module 8 Module housing 10 Secondary cell 12 Longitudinal direction 14 Clamping device 16 Spring element 18 Transmission 20 Pressure piece 24 Lever 26 Lever base 28 Raceway 32 Roller 34 Traction means 36 Force application point 38 Force output plate 40 Deflection roller 42 Deflection roller 44 Spring 46 Base portion 48 Free portion 50 Box F V Clamping force A SF Distance I F Length I A Length

Claims

Battery module (6), having - a module housing (8), - at least one secondary cell (10) which is accommodated in the module housing (8), - a clamping device (14) by means of which the secondary cell (10) is clamped with a flat side against an inner wall of the module housing (8), wherein the clamping device (14) has a spring element (16) for generating a spring force and a transmission (18) with a nonlinear transfer function for the spring force, which is connected between the spring element (16) and the secondary cell (10) and is configured to generate a clamping force (F V) which is approximately constant over a spring travel of the spring element (16).Battery module (6) according to Claim 1, wherein the gear mechanism (18) has a lever (24) which is articulated on the secondary cell (10) or on the module housing (8) such that it can be displaced along a raceway (28) and correspondingly vice versa with a lever foot (26) on the module housing (8) or on the secondary cell (10) in a fixed manner, but pivotably.Battery module (6) according to Claim 1 or 2, wherein the transmission (18) has a support bearing (42) which is arranged substantially in a fixed position with respect to the lever base (26) and from which at least part of the spring force is transmitted to a force application point (36) which is fixedly predetermined on the lever (24).Battery module (6) according to Claim 3, wherein the force application point (36) divides the lever (24) into a foot section (46) and a free section (48), wherein the length (I F, I A) of foot section (46) and free section (48) is selected as a function of the bracing force (F V), of the spring rate of the spring element (16) and the distance (A SF) between support bearing (42) and lever foot (26) running in the direction of action of the bracing force (F V).Battery module (6) according to Claim 4, such that the length ratio of the length (I F) of the foot section (46) to the total length of the lever (24) corresponds to the ratio of the bracing force (F V) to the spring rate and to the distance (A SF) between the support bearing (42) and the lever foot (26).The battery module (6) according to any one of claims 1 to 5, wherein the spring element (16) is formed by a plurality of springs (44) connected in parallel.Battery module (6) according to one of Claims 1 to 6, wherein the gear mechanism (18) has a traction means (34) and a deflection roller (42) arranged on the support bearing, and wherein the spring element (16) is coupled to the lever (24) by means of the traction means (34) guided over the deflection roller (42).Battery module (6) according to one of Claims 1 to 7, wherein the gear mechanism (18) is configured symmetrically with respect to a longitudinal axis of the module housing (8).Battery module (6) according to one of Claims 2 to 8, wherein the track (28) of the respective lever or levers (24) is designed as a curved track.Motor vehicle having a battery module (6) according to one of Claims 1 to 9.

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