Control device and method for determining the state of charge of an energy storage unit for a motor vehicle

Abstract

The invention relates to a method for determining the state of charge (SOC) of an energy storage unit (14) for a motor vehicle by means of a control device (16). In this process, a measured value (W) is determined that depends on an extent (Δd, ΔD) relative to a specific reference state (RZ) of the energy storage unit (14), and the state of charge (SOC) of the energy storage unit (14) is determined as a function of a provided characteristic curve (K1, K2, K3, K4) which indicates a dependence of the state of charge (SOC) of the energy storage unit (14) on the measured value (W), and as a function of the determined measured value (W).

Description

[0001] The invention relates to a method for determining a state of charge of an energy storage unit for a motor vehicle using a control device. Furthermore, the invention also relates to a control device for a motor vehicle.

[0002] Various methods are known from the prior art for determining the state of charge of an energy storage unit, for example a motor vehicle battery. For example, the state of charge, also known as SOC (State of Charge), can be determined using gradients or changes in voltage. However, with some battery chemistries, such as LFP (lithium iron phosphate) technology, there is no change in cell voltage over large sections of the charging curve, which describes, for example, the state of charge as a function of voltage, or there is hardly any change in cell voltage when the state of charge changes. Due to such a small change in cell voltage over large sections of the charging curve, determining the state of charge is difficult or only possible with great imprecision.For example, an accurate determination of the state of charge is only possible if the system or energy storage device is fully charged to 100% SOC. Accordingly, a sufficiently accurate determination of the state of charge requires that the energy storage device be fully charged regularly so that the state of charge detection algorithm can determine the SOC. However, full charges of an energy storage device can potentially have a negative impact on its service life, which is why it is generally recommended to only charge a motor vehicle's energy storage device above 80% SOC if possible.

[0003] It would therefore be desirable to be able to determine the state of charge of an energy storage device as accurately as possible without having to regularly reach a certain state of charge range of the energy storage device.

[0004] WO 2016 / 033479 A9 describes a method for determining a state of charge of a battery pack based on a pressure measured by a pressure sensor, a terminal voltage measured by a voltage sensor, and a temperature measured by a temperature sensor. To determine the state of charge, the battery cell is mathematically modulated. For this purpose, the force acting on a battery cell is modeled as a function of temperature and state of charge based on a nonlinear spring model.

[0005] The computational effort involved in using mathematical models for batteries or battery cells is very high. Furthermore, such a mathematical model only ever corresponds to a real system within certain limits, and it is difficult to design such a model in such a way that it remains valid even after a battery has aged for a certain period.

[0006] The object of the present invention is to provide a control device and a method which enable the simplest and most reliable determination of a state of charge of an energy storage unit, in particular in any state of charge ranges of the energy storage unit.

[0007] This object is achieved by a control device and a method having the features according to the respective independent patent claims. Advantageous embodiments of the invention are the subject of the dependent patent claims, the description, and the figures.

[0008] In a method according to the invention for determining a state of charge of an energy storage unit for a motor vehicle by means of a control device, a measured value dependent on an extension relative to a specific reference state of the energy storage unit is determined and the state of charge of the energy storage unit is determined as a function of a provided characteristic curve which indicates a dependence of the state of charge of the energy storage unit on the measured value, and as a function of the determined measured value.

[0009] The invention is based, on the one hand, on the realization that an energy storage unit, which may comprise, for example, one or more battery cells, exhibits a corresponding expansion depending on its current state of charge. For example, battery cells can expand during charging, i.e., as their state of charge increases, while they can contract again during discharging, i.e., as their state of charge decreases. Thus, the expansion, or more precisely, the spatial expansion, of an energy storage unit is correlated with the current state of charge of that energy storage unit.This correlation, which can be recorded in advance in the form of a characteristic curve, for example, can now be advantageously used to determine the state of charge of the energy storage unit as a function of the measured value, which in turn depends on the expansion of the energy storage unit compared to a specific reference state. This measured value can, for example, indicate the expansion of the energy storage unit itself or refer to a quantity that is directly correlated with such an expansion and which preferably allows a clear assignment to the current expansion of the energy storage unit compared to the specific reference state. Such a quantity can, for example, be the cell pressure of the battery cell or the intercell pressure between two battery cells of the energy storage unit or a tension force acting on the battery cell of the energy storage unit or an expansion of the energy storage unit, e.g.a cell expansion of one or more battery cells comprised by the energy storage unit, which can be measured, for example, directly using a strain gauge or another measuring device. Especially with cell technologies such as the LFP technology mentioned above, in which only a slight voltage change is recorded for certain state of charge ranges when the state of charge increases or decreases, the state of charge can now be determined much more accurately depending on the measured value determined, since in such state of charge ranges, a much more significant change in the expansion of the energy storage unit compared to the specific reference state is recorded when the state of charge increases or decreases. Nevertheless, the invention can also be used in the same advantageous manner with other cell chemistries, e.g., with NMC (nickel-manganese-cobalt) technology or generally with lithium-ion batteries, e.g.in a lithium cobalt dioxide accumulator and / or a lithium manganese accumulator, as an energy storage unit or part of the energy storage unit or also in one or more solid-state battery cells as an energy storage unit or part of the energy storage unit. In particular, the method according to the invention or at least one of its embodiments also allows the determination of the state of charge during operation of the energy storage unit, in particular during driving of the motor vehicle in which the energy storage unit is preferably used, as explained in more detail below. During such driving operation, the only method available to date is the generally very inaccurate determination of the state of charge via current integration.Thus, even in such a situation, a significant added value can be achieved in determining the state of charge via the measured value dependent on the size of the energy storage unit, since the accuracy of determining the state of charge can be increased for any cell chemistry of the energy storage unit.

[0010] Furthermore, the use of the expansion-dependent measured value to determine the state of charge is not limited to any specific state of charge range of the energy storage unit. This determination method, which is also referred to below as the first determination method, can advantageously be used for any state of charge range of the energy storage unit to determine the current state of charge. Consequently, it is not necessary for the energy storage unit to be charged or discharged within a specific state of charge range in order to enable an accurate determination of the state of charge. This, in turn, enables gentle operation of the energy storage unit while simultaneously enabling an accurate determination of the state of charge. The use of a characteristic curve also enables a particularly low-computational-intensive determination of the state of charge.For example, only the measured value needs to be recorded, and the associated state of charge can be determined based on the provided characteristic curve by reading the associated value. The described method can also be advantageously combined with other methods for determining the state of charge, as described in more detail below, in order to achieve even greater accuracy in determining the state of charge.

[0011] The energy storage unit is preferably an electrochemical energy storage unit, for example, a battery. The energy storage unit can comprise only one or more battery cells. The energy storage unit can, for example, be a cell stack with several battery cells arranged next to one another in a stacking direction. The energy storage unit can also comprise several such cell stacks. For example, the energy storage unit can also be designed as a high-voltage battery for a motor vehicle, or such a high-voltage battery can comprise several such energy storage units.

[0012] The state of charge is calculated by the control device. The control device can, for example, be part of the motor vehicle in which the method is applied and in which the energy storage unit is located. The control device can, for example, obtain the measured value from a corresponding measuring device. For this purpose, the control device can be communicatively connected to such a measuring device. The characteristic curve can, for example, be stored in a memory of the control device.

[0013] In general, additional parameters or measured variables can be taken into account when determining the state of charge of the energy storage unit, for example, the current temperature of the energy storage unit and / or the current operating current of the energy storage unit, such as the charging current and / or the load current of the energy storage unit. Such parameters can also be measured using the aforementioned measuring device and fed to the control device to determine the state of charge. The provided characteristic curve can be a characteristic curve that was previously determined experimentally.The experimental determination of this characteristic curve can be based on a measurement of the energy storage unit or on the measurement of an energy storage unit of similar design, for example, one with the same number of battery cells in the same installation situation with the same cell chemistry or with the same mixed cell chemistry (if the energy storage unit, for example, includes battery cells with different cell chemistry), and with the same geometry. This allows the properties of the energy storage unit to be represented very precisely by the characteristic curve.

[0014] Furthermore, not only a single characteristic curve can be provided, but also, for example, a characteristic curve field with multiple characteristic curves. Each characteristic curve can be assigned to a boundary condition parameter or to a tuple of different boundary condition parameters. Such boundary condition parameters can be the additional parameters already mentioned above, such as the current temperature of the energy storage unit and / or the current operating current. For example, a current value of the operating current and a current value of the energy storage temperature can be assigned a single such characteristic curve, which indicates the dependence of the state of charge of the energy storage unit on the measured value for these boundary condition parameters. A current aging state of the energy storage unit can also be taken into account as a further boundary condition parameter, for example.This, in turn, can be measured based on one or more aging parameters, such as the total number of charge and discharge cycles, the total charge throughput of the energy storage unit, the current capacity of the energy storage unit, and / or the current internal resistance of the energy storage unit. These boundary condition parameters advantageously allow the associated characteristic curve to be selected from a characteristic curve family for a current state of the energy storage unit, which thus accurately describes the dependence of the state of charge on the measured value for the current state of the energy storage unit. This allows the state of charge of the energy storage unit to be determined particularly accurately in every situation.

[0015] The reference state of the energy storage unit refers to a state of the energy storage unit in which it assumes a specific reference expansion. The expansion can therefore refer to a relative value related to a defined reference expansion of the energy storage unit, which the energy storage unit assumes in the reference state. The reference state can, for example, refer to a specific state of charge of the energy storage unit. If the reference state of the energy storage unit refers, for example, to a state of charge of 100%, an expansion compared to the specific reference state of the energy storage unit is to be understood as the difference between the expansion of the energy storage unit when it has a specific current state of charge, and the reference expansion of the energy storage unit when it has a state of charge of 100%.In principle, any charge level of the energy storage unit can be selected as the reference state of the energy storage unit. The expansion relative to the specific reference state of the energy storage unit can be understood, for example, as a change in length or a difference in length.

[0016] According to an advantageous embodiment of the invention, the energy storage unit comprises at least one LFP (lithium iron phosphate) battery cell. This is particularly advantageous because LFP battery cells in particular exhibit no or only a particularly strong cell voltage change over a wide state of charge range, which makes determining the state of charge via such a cell voltage change difficult. Especially for such LFP battery cells, a very precise determination of the state of charge can now be provided even within this state of charge range using the measured value, which depends on the size of the energy storage unit. Furthermore, the energy storage unit can only comprise battery cells of the same cell chemistry type. This facilitates the determination of the state of charge. The energy storage unit can therefore comprise one or more LFP battery cells, preferably exclusively LFP battery cells. However, this does not necessarily have to be the case.The energy storage unit can also comprise several battery cells of different cell chemistry types. This makes it possible to apply the method to a mixed installation of different chemistries. The reference value and the characteristic curve can then be applied accordingly to such a mixed installation.

[0017] Alternatively or additionally, the energy storage unit can comprise or be designed as a battery cell different from an LFP battery cell, e.g. an NMC battery cell and / or one of the above-mentioned lithium-ion accumulators and / or a solid-state battery cell or the like. In general, the energy storage unit can comprise or be designed as one or more lithium-ion battery cells. The energy storage unit can also comprise or be designed as a battery cell different from a lithium-ion battery cell. If the energy storage unit comprises a plurality of battery cells, these can have the same cell chemistry or different cell chemistry. The method and its embodiments can advantageously be used in principle for any cell chemistry of battery cells in order to provide an accurate and simple state of charge determination.

[0018] According to a further advantageous embodiment of the invention, a value of a linear expansion of a battery cell arrangement and / or a battery cell of the energy storage unit in a predetermined direction is determined as the measured value, and / or a value of a force and / or a pressure within a battery cell arrangement of the energy storage unit, and / or a value of an elongation and / or curvature of at least one component of the energy storage unit, in particular relating to a cell housing of a battery cell of the energy storage unit or relating to an arrangement of at least two battery cells of the energy storage unit relative to one another.

[0019] There are therefore advantageously several ways of determining or defining the measured value. For example, the linear expansion of the battery cell arrangement and / or of a battery cell of the energy storage unit in a predetermined direction can be determined as the measured value. The measured value can thus represent the expansion of the energy storage unit or of one of its battery cells compared to the determined reference state. If the battery cell arrangement is provided, for example, in the form of a cell stack with several battery cells, in particular prismatic battery cells, arranged next to one another in a stacking direction, it is preferred that the predetermined direction in which the linear expansion is determined represents the stacking direction. It is precisely in the stacking direction that prismatic battery cells experience a significant change in linear expansion during charging and discharging.This is therefore particularly well suited to determining the current state of charge of the energy storage unit. The length of the battery cell arrangement and / or the battery cell can be measured. The measurement of the length of the battery cell arrangement can also refer to the measurement of the length of an individual battery cell in the predetermined direction. This applies in particular if the battery cell arrangement comprises several battery cells. The length of an individual battery cell can, for example, be regarded as representative of the lengths of all other battery cells in the battery cell arrangement. The characteristic curve can be adapted to the precise positioning of a measuring sensor in or on the energy storage unit, which can be part of the above-mentioned measuring device. The measuring sensor, which is located at a specific position in or on the energy storage unit, e.g.on a battery cell or between two battery cells, can measure the current pressure or another measured value, such as a curvature and / or a current strain in the case of a strain gauge as a measuring sensor, at this position, and via the characteristic curve the path corresponding to this measured value can be assigned, at least implicitly, to this recorded measured value, e.g. the recorded pressure or curvature or strain measured value, i.e. the distance by which a battery cell has expanded or contracted compared to the defined reference state. This path can in turn be assigned to the assigned state of charge via the characteristic curve. The characteristic curve can therefore directly assign the state of charge to the measured value recorded by the measuring sensor, whereby the relationship between the measured value recorded by the measuring sensor at a specific position on the measuring sensor and the corresponding expansion of the battery cell orThe energy storage unit can be implicitly taken into account by creating the characteristic curve. By creating the characteristic curve, for example by using a similar energy storage unit with a similar measuring sensor at the same specific position on or in the energy storage unit, the relationship between the measured variable detected by the measuring sensor, e.g. the pressure or curvature or strain, and the expansion of the battery cell or the energy storage unit, and the relationship between this expansion and the state of charge can be taken into account in the same way.If the total length of all battery cells included in the battery cell arrangement is considered as the length of the battery cell arrangement, it is preferred that the length of the arrangement refers only to the battery cells included in the battery cell arrangement and not to additional elements, such as compressible cell separating elements between the battery cells, which partially compensate for such an extension.

[0020] Accordingly, it is very advantageous if the measured value is not the linear expansion directly, but rather indirectly via, for example, the value of a curvature and / or a pressure within the battery cell arrangement. This can be achieved in a simple manner, for example, by installing a force and / or pressure sensor in the battery cell arrangement. The relationship between the linear expansion of an individual battery cell or multiple battery cells of such a battery cell arrangement as a function of the detected force and / or the detected pressure within such a battery cell arrangement can be recorded experimentally in advance and stored in the form of a defined assignment. A force value or pressure value detected by means of the force and / or pressure sensor can be directly assigned to the expansion, in particular the linear expansion of the battery cell arrangement, via this assignment.Such a force and / or pressure sensor can, for example, be installed between two adjacent battery cells or between a battery cell and another component of the battery cell arrangement, for example, a pressure plate that delimits a cell stack in the stacking direction and / or a cell separator arranged between two battery cells. In this way, i.e., by measuring the force or pressure within the battery cell arrangement, the linear expansion can be determined particularly easily.

[0021] Furthermore, a strain or curvature value can also advantageously be recorded as the measured value. This can be easily achieved, for example, using a strain gauge. Such a strain gauge can be arranged, for example, on a cell casing of a battery cell. The battery cell, for example in the case of a prismatic battery cell, bulges increasingly with increasing state of charge and contracts accordingly as the state of charge decreases. In particular, the largest sides of such a cell casing are significantly more curved when bulged than when not bulged. This can be easily recorded using such a strain gauge. This consequently provides a strain and / or curvature value, which in turn is clearly correlated with the expansion of the energy storage unit.Such a strain gauge can, for example, also be arranged between two battery cells or have a first contact section arranged on a first battery cell and a second contact section arranged on a second battery cell. If the two battery cells increase or decrease their distance relative to each other as they expand and contract, this can also be detected by a strain gauge arranged in this way.

[0022] Thus, there are many advantageous ways to detect the current expansion of the energy storage unit relative to the defined reference state. Furthermore, to detect this expansion, for example, by detecting one or more of the aforementioned measured variables, such as linear expansion and / or force and / or pressure and / or strain and / or curvature, not only one sensor can be used, but also, for example, several sensors distributed throughout a battery cell arrangement, in particular of the same sensor type and / or different sensors.

[0023] It is particularly advantageous and preferred that the measured value relates to a change in thickness of a battery cell of the energy storage unit relative to a reference thickness of the battery cell in the reference state of the energy storage unit. If the battery cell is designed as a prismatic cell, the thickness of such a battery cell defines the distance between the two largest sides in terms of area of ​​such a prismatic battery cell. The same applies to a pouch cell. If the battery cell is a round cell, the thickness of this round cell is preferably measured perpendicular to the longitudinal axis or cylinder axis of the essentially cylindrically shaped round cell. The change in thickness can in turn be determined using the measured values ​​already mentioned above, in particular using one or more of the above-mentioned measured values.

[0024] According to a further very advantageous embodiment of the invention, the state of charge is determined during operation of the energy storage unit, in particular while a load current is being provided by the energy storage unit and / or while the energy storage unit is being charged with a charging current. This advantageously makes it possible to determine the current state of charge even during operation of the energy storage unit, and in particular during operation of the motor vehicle in which the energy storage unit is used. This advantageously provides a currently determined state of charge of the energy storage unit in every operating situation of the energy storage unit. As already described, both charging and discharging the energy storage unit lead to a change in its size or to a change in the size of the battery cells comprised by the energy storage unit.This advantageously makes it possible to reliably determine the current charge level of the energy storage unit even during operation.

[0025] According to a further advantageous embodiment of the invention, the state of charge is determined as a function of an operating current of the energy storage unit, in particular as a function of the load current and / or charging current of the energy storage unit. This is based on the realization that the dependence of the state of charge on the measured value, as described by the provided characteristic curve, also depends on the current operating current of the energy storage unit. By taking the current operating current into account, for example, its sign and / or its magnitude, the accuracy of determining the state of charge can be increased.The state of charge can also be determined depending on whether the currently present operating current is a load current or a charging current, where the charging current represents a current provided by the energy storage unit and which leads to a reduction in the state of charge of the energy storage unit, while a charging current is understood to be the current with which the energy storage unit is charged, i.e., as a result of which the state of charge of the energy storage unit increases. Taking into account whether the operating current is a load current or a charging current is only advantageous or preferred if the dependence of the state of charge on the measured value depends on the type of operating current, i.e., whether it is a load current or a charging current.If this is not the case, the type of operating current does not need to be taken into account when determining the state of charge.

[0026] The operating current, in particular its magnitude or value, can represent one of the boundary condition parameters explained above. Depending on the currently measured operating current, the provided characteristic curve can be selected from a characteristic curve family, for example.

[0027] According to a further very advantageous embodiment of the invention, it is provided that a first state of charge value of the energy storage unit is determined according to a first determination method as a function of the provided characteristic curve and as a function of the determined measured value, and a second state of charge value of the energy storage unit is determined according to a second determination method that is different from the first determination method, and the state of charge is determined as a function of the first and second state of charge values. In addition to the first determination method already described, according to which the state of charge, which is now referred to as the first state of charge value, is determined as a function of the provided characteristic curve and the determined measured value, the state of charge in the form of the second state of charge value can additionally be determined based on a second determination method.The determination of the first state of charge value and the second state of charge value preferably refers to the same determination time or the same determination period, in particular to a determination period within which no significant change in the state of charge occurs. This advantageously makes it possible to use a second determination method to determine the state of charge and to compensate for any inaccuracies by additionally using the first determination method. Ambiguities for the determined second state of charge value according to the second determination method can be resolved, for example, based on the additionally determined first state of charge value based on the first determination method.Conversely, any ambiguities regarding the first state of charge value determined according to the first determination method can also be resolved based on the additionally determined second state of charge value according to the second determination method. Overall, this further increases the accuracy of determining the state of charge of the energy storage unit. Above all, the use of the first determination method efficiently prevents jumps in the determined state of charge during operation of the energy storage unit, which could be caused by inaccuracies in determining the state of charge based on the second determination method.

[0028] The second determination method can, for example, be a current integration of the operating current, according to which, starting from a starting state of charge of the energy storage unit and knowing the capacity of the energy storage unit, the operating current is integrated over time in order to determine a corresponding final state of charge as a second state of charge value. Such a current integration can advantageously also be applied during operation of the energy storage unit in order to determine the second state of charge value of the energy storage unit, in particular repeatedly in successive time steps, e.g., quasi-permanently.Since such current integration leads to increasingly larger measurement errors over time, the second state of charge value determined on the basis of the current integration can now advantageously be corrected or calibrated by determining the first state of charge value on the basis of the first determination method.

[0029] The second determination method can also be a state of charge determination based on a voltage-state of charge characteristic curve. This can in particular also be a resting voltage-state of charge characteristic curve, which is also referred to as an OCV (open circuit voltage) characteristic curve. For this purpose, the voltage of the energy storage unit can be measured in the form of the resting voltage, particularly during operation or in an inactive state of the energy storage unit, and the corresponding second state of charge value can be determined based on the voltage-state of charge characteristic curve. Since such a characteristic curve is very flat over a large state of charge range, for example between 10% and 80% or between 10% and 90%, especially in the case of LFP battery cells, only a very inaccurate determination of the current state of charge in the form of the second state of charge value is possible in this state of charge range.It is precisely in this range that the accuracy of determining the state of charge based on the first state of charge value can now be increased using the first determination method. In other state of charge ranges, for example, less than 10% or greater than 80%, especially greater than 90%, a very precise second state of charge value can be determined based on the second determination method, which then no longer requires any correction by the first state of charge value.

[0030] It is also conceivable to combine the two above-mentioned examples for the second determination method, for example, in the form of a second determination method and an additional third determination method. Thus, further determination methods can be used to determine the second and / or a third state of charge value, or even additional state of charge values, in order to provide the most accurate calculation of the state of charge of the energy storage unit in combination with the first state of charge value determined according to the first determination method.

[0031] According to a further advantageous embodiment of the invention, the second state of charge value is corrected as a function of the first state of charge value in the event of a discrepancy between the first and second state of charge values, and the corrected second state of charge value is determined as the state of charge of the energy storage unit. Thus, if a discrepancy occurs between the first and second state of charge values, this discrepancy can advantageously be corrected, thus providing the ultimately determined state of charge of the energy storage unit more accurately. The type of correction for the discrepancy can, in turn, be performed in a variety of ways.For example, the state of charge of the energy storage unit can result from an average of the first and second state of charge values ​​or from a weighted average, wherein a weighting can be carried out depending on a current determination accuracy of the first and second state of charge values, or the first state of charge value can be taken as the state of charge if it is assumed that it has a higher accuracy than the determined second state of charge value, or the second state of charge value can also be taken as the state of charge if it is assumed in a current situation that it has a higher accuracy than the second state of charge value, or similar.

[0032] According to a further advantageous embodiment of the invention, the dependence of the state of charge on the measured value according to the characteristic curve is non-monotonic. This means that the dependence of the state of charge on the measured value described by the characteristic curve does not have to be monotonically increasing or monotonically decreasing. This can be the case in particular with the aforementioned LFP technology. This means that a currently determined measured value does not necessarily have to be clearly assigned to a specific state of charge value, but rather, under certain circumstances, several different state of charge values. However, this ambiguity can be easily resolved, as explained in more detail below.

[0033] A further highly advantageous embodiment of the invention is provided if the first state of charge value is determined as a function of the second state of charge value. In other words, the second state of charge value can be determined based on the second determination method, and any ambiguity regarding the determination of the first state of charge value can thereby be resolved. For example, according to the first determination method, several potential candidates for the first state of charge value can be provided, with the first state of charge value being selected from these potential candidates as a function of the determined second state of charge value.Even if the second state of charge value can only be determined very imprecisely, it can nevertheless be advantageously used to resolve any ambiguity in the potential first state of charge values, which does not require a particularly high level of accuracy in determining the second state of charge value.

[0034] Additionally or alternatively, it is conceivable that the first state of charge value is determined as a function of at least one previously determined measured value and / or assigned first state of charge value, in particular as a function of a temporal progression of a plurality of previously determined measured values ​​and / or assigned first state of charge values. The measured value can, for example, be recorded repeatedly over time, in particular during operation of the energy storage unit. This provides a characteristic curve on the basis of which it can be determined in which state of charge range the energy storage unit is currently located, even if the assignment to the first state of charge value is not clear for a currently recorded measured value. In other words, the ambiguity described above can also be easily resolved by observing a temporal progression of the measured value.

[0035] Additionally or alternatively, it is also conceivable, for example, to consider a temporal derivative of the temporal progression of the measured values ​​and / or a derivative of the progression of the multiple measured values, for example, also according to the state of charge, in order to resolve any ambiguity. Numerous other possibilities are also conceivable for resolving such ambiguities.

[0036] Furthermore, the invention also relates to a control device for a motor vehicle, which is designed to carry out a method according to the invention or one of its embodiments. Furthermore, the invention also relates to a measuring arrangement with a control device according to the invention or one of its embodiments. The measuring arrangement can, for example, also comprise a measuring device designed to record the measured value. Furthermore, it can also be designed to record one or more of the further parameters or measured variables mentioned in connection with the method according to the invention and its embodiments, such as, for example, the boundary condition parameters. Furthermore, the measuring arrangement can also comprise the energy storage unit.

[0037] Furthermore, the invention also relates to a motor vehicle with a control device according to the invention or one of its embodiments and / or with a measuring arrangement according to the invention or one of its embodiments.

[0038] The control device can have a data processing device or a processor device that is configured to carry out an embodiment of the method according to the invention. The processor device can for this purpose have at least one microprocessor and / or at least one microcontroller and / or at least one FPGA (Field Programmable Gate Array) and / or at least one DSP (Digital Signal Processor). In particular, a CPU (Central Processing Unit), a GPU (Graphical Processing Unit) or an NPU (Neural Processing Unit) can be used as the microprocessor. Furthermore, the processor device can have program code that is configured to carry out the embodiment of the method according to the invention when executed by the processor device. The program code can be stored in a data memory of the processor device. The processor device can, for example,based on at least one circuit board and / or on at least one SoC (System on Chip).

[0039] The invention also includes further developments of the control device according to the invention and the measuring arrangement according to the invention, which have features as already described in connection with the further developments of the method according to the invention. For this reason, the corresponding further developments of the control device according to the invention and the measuring arrangement according to the invention are not described again here.

[0040] The motor vehicle according to the invention is preferably designed as a motor vehicle, in particular as a passenger car or truck, or as a passenger bus or motorcycle.

[0041] The invention also encompasses combinations of the features of the described embodiments. The invention therefore also encompasses implementations that each comprise a combination of the features of several of the described embodiments, unless the embodiments are described as mutually exclusive.

[0042] Exemplary embodiments of the invention are described below. Shown are: Fig. 1 a graphical representation of the relationship between the open-circuit voltage and the state of charge of an LFP battery cell; Fig. 2 a schematic representation of characteristic curves showing the dependence of a change in the thickness of a battery cell on the state of charge of the battery cell for different operating currents, for use in a method for determining the state of charge according to an embodiment of the invention, Fig. 3 a schematic representation of a measuring arrangement with an energy storage unit and a control device for determining the state of charge of the energy storage unit according to an embodiment of the invention, Fig. 4 a schematic representation of the energy storage unit from Fig. 3 in a reference state according to an embodiment of the invention.

[0043] The exemplary embodiments explained below are preferred embodiments of the invention. In the exemplary embodiments, the described components of the embodiments each represent individual features of the invention that can be considered independently of one another, each of which also develops the invention independently of one another. Therefore, the disclosure is intended to encompass combinations of the features of the embodiments other than those shown. Furthermore, the described embodiments can also be supplemented by further features of the invention already described.

[0044] In the figures, the same reference symbols designate elements with the same function.

[0045] Fig. Figure 1 shows a schematic and graphical representation of the dependence of the open-circuit voltage OCV of an LFP battery cell on the state of charge SOC in percent. The open-circuit voltage is specified in volts. The depicted dependence can also be referred to as the OCV characteristic curve 10. Such an OCV characteristic curve 10 is often used to determine the state of charge SOC of such a battery cell depending on the current open-circuit voltage OCV. However, especially with LFP battery cells, the problem arises that the characteristic curve 10 is very flat over large sections of the charging curve, in this case between 10% and at least 90% state of charge SOC. Determining the state of charge in this state of charge range is therefore very inaccurate or subject to a very large measurement error. A sufficiently accurate state of charge determination based on such an OCV characteristic curve 10 can only be carried out at very high states of charge close to 100% and very low states of charge below 10%.However, to ensure battery cells are operated as gently as possible, they should not be charged to particularly high charge levels or discharged to very low charge levels. However, this is not compatible with accurate state-of-charge determination, especially with LFP technology.

[0046] The invention and its embodiments now make use of the knowledge that a spatial extension of a battery cell 12 (compare Fig. 3 and Fig. 4), in particular the thickness of such a prismatic battery cell 12, also changes depending on the state of charge SOC of the battery cell 12. This advantageously allows this thickness change ΔD (compare Fig. 2) should also be used to determine the state of charge.

[0047] Fig. Figure 2 shows a schematic representation of a characteristic curve family with four exemplary characteristic curves K1, K2, K3, and K4. Each characteristic curve K1 to K4 describes the dependence of a change in thickness ΔD of a battery cell 12 on its state of charge SOC for different operating currents. The first characteristic curve K1 relates to an operating current of C / 10, the second characteristic curve K2 to an operating current of C / 2, the third characteristic curve K3 to an operating current of 1 / C, and the fourth characteristic curve K4 to an operating current of 2C. An operating current of 1C means that a battery cell 12 with a capacity C can be charged from a fully discharged state to a fully charged state at 100% SOC within a period of one hour with a load current of 1C.At a load current of 2C, charging such a battery cell 12 with capacity C from a fully discharged state to a fully charged state takes half an hour. Accordingly, charging such a battery cell 12 from a fully discharged state at 0% to a fully charged state of 100% takes two hours at a charging current of C / 2 and ten hours at a current of C / 10.

[0048] In addition, the respective characteristic curves K1 to K4 can correspond to a specific temperature of the battery cell 12. Such a characteristic map KF can accordingly comprise any number of additional characteristic curves that relate to different temperature ranges for the temperature of the battery cell 12 or, in general, an energy storage unit 14 (cf. Fig. 4), as well as any number of other characteristic curves for other operating currents. As can be seen from the characteristic curve field KF, the thickness of a battery cell 12 tends to decrease from the fully charged state of 100% to lower states of charge SOC, although this decrease is not monotonous or strictly monotonous. The characteristic curves of the characteristic curve field KF generally have a very large gradient over large ranges of the state of charge SOC, which makes it possible to determine the state of charge SOC very precisely as a function of a current thickness change ΔD. It is also particularly advantageous if the Fig. 1 is used. This can also be related to a specific temperature of the battery cell 12 or the energy storage unit 14. An OCV characteristic field can be provided that includes numerous such OCV characteristic curves 10, for example, for different temperatures or temperature ranges of the energy storage unit 14.

[0049] Fig. 3 shows a schematic representation of a measuring arrangement 16 for determining a state of charge SOC of an energy storage unit 14 according to an exemplary embodiment of the invention. The energy storage unit 14 comprises, by way of example, a battery cell 12, for example an LFP battery cell 12. The energy storage unit 14 can also comprise numerous additional battery cells 12, wherein, for the sake of simplicity, the state of charge determination is explained here with reference to only a single battery cell 12, wherein the described procedure can, however, also be applied analogously to multiple battery cells 12. In addition to the energy storage unit 14, the measuring arrangement 16 comprises a control device 18. Furthermore, it comprises a measuring device 20 which has a sensor 22 for detecting a measured value W which is dependent on an extension Δd, ΔD of the energy storage unit 14, in particular of the battery cell 12.

[0050] The sensor 22 can be provided, for example, in the form of a strain gauge 22. Furthermore, the battery cell 12 is Fig. 3 is shown in a non-expanded or non-swollen state. In this state, the battery cell 12 has a thickness D1 in a specific direction, referred to herein as the x-direction, which can correspond to the stacking direction when the battery cell 12 is installed in a cell stack with several battery cells arranged side by side in a stacking direction.

[0051] Fig. 4 shows a schematic representation of the battery cell 12 from Fig. 3 in an expanded or swollen state, which is also referred to here as the reference state RZ of the battery cell 12 or the energy storage unit 14, and which can correspond to a state of charge SOC of 100%. In this fully charged state, the battery cell 11 has a reference thickness D0. If the battery cell 12 changes from the non-expanded state Z as in Fig. 3 shown in the extended state RZ as in Fig. 4, this leads to a corresponding expansion of the strain gauge 22, which can thus be detected by the measuring device 20 in the form of the value W. Conversely, the swelling of the battery cell 12 from the reference state RZ to the swollen state Z can also be detected via the correspondingly reduced expansion of the strain gauge 22 and accordingly by means of the measuring device 20 via the corresponding measured value W.

[0052] The expansion Δd, ΔD of the battery cell 12 is in this example compared to the reference state RZ, as shown in Fig. 4. Δd denotes half of the thickness change ΔD during the transition of cell 12 from the reference state RZ to the state Z, and thus half of the difference between the reference thickness D0 and the thickness D1 assumed in the non-swollen state Z. The total thickness change ΔD can thus be understood as twice this half thickness change Δd. Since the larger thickness D0 is assumed as the reference thickness, the thickness change ΔD results accordingly according to the characteristic curves K1 to K4 (cf. Fig. 2) a negative value. However, the reference state RZ can also be defined in any other way, for example as the non-swollen state of cell 12, as in Fig. 3 shown.

[0053] The change in thickness Δd, ΔD can now advantageously be recorded using the measured value W. Instead of recording the thickness or change in thickness Δd, ΔD using a strain gauge 22, there are also various other possibilities: For example, the pressure that builds up or decreases when charging a cell can also be recorded and can be measured using one or more additional sensors, for example force and / or pressure sensors, and used for evaluation. Such a pressure sensor or force sensor can, for example, be arranged between such a battery cell 12 and an adjacent component, for example another battery cell, a cell separator, an end plate, or the like. An optical measurement of the extension or dimension of the cell 12 with respect to the x-direction is also conceivable, for example using a camera, a light barrier, or the like.

[0054] Ultimately, the measured value W recorded by the measuring device 20 represents such an extension compared to a defined reference state RZ of the energy storage unit 14. Furthermore, for example, in a memory of the control device 18, the Fig. 2 or a similar characteristic curve field KF can be stored, which is assigned to the battery cell 12 or the energy storage unit 14. Based on a currently recorded boundary condition parameter, which can also be recorded by the measuring device 20, for example by means of a further sensor not shown here, for example the current temperature and / or the current operating current, the control device 18 can select a corresponding characteristic curve K1 to K4 from the characteristic curve field KF and determine the state of charge, in particular a first state of charge value SOC1, depending on this characteristic curve and depending on the recorded measured value W, in particular the thickness change ΔD determined therefrom. This procedure can also be referred to as the first determination method M1.In addition, the control device 18 can be designed to determine the state of charge, in particular a second state of charge value SOC2, according to a further second determination method M2, for example based on an OCV characteristic curve 10, such as this one. Fig. 1 and / or based on current integration during operation of the energy storage unit 14 or the like. The final state of charge SOC can then advantageously be determined as a function of both the first state of charge value SOC1 and the second state of charge value SOC2. For example, the two determined state of charge values ​​SOC1, SOC2 can be compared with each other, averaged, one corrected by the other, a possible ambiguity in determining the corresponding value SOC1, SOC2 can be eliminated, or the like. This enables a particularly accurate determination of the state of charge SOC.

[0055] The determination of the first state of charge value SOC1 based on the first determination method M1 is carried out repeatedly during the operation of the energy storage unit 14, for example at periodic time intervals, for example continuously or quasi-continuously, for example at intervals of milliseconds, seconds, or minutes. The measured values ​​W acquired in this process or the thickness changes ΔD determined therefrom can be stored by the control device 18, and a temporal progression of the measured values ​​W or the thickness change ΔD can be provided accordingly. Using such a temporal progression, any possible ambiguity in the determination of the first state of charge value SOC1 based on the measured value W can also be eliminated.For example, if the thickness change ΔD, or more precisely, its magnitude, decreases, and cell 12 consequently expands while cell 12 provides a load current, thus decreasing its state of charge (SOC), it can be concluded from the characteristic curve family KF that the current state of charge (SOC) must be in the range between approximately 30% and approximately 60%. Within this range, the characteristic curves K1, K2, K3, and K4 change monotonically depending on the SOC. A clear determination of the SOC in this range is therefore possible.

[0056] The ultimately determined state of charge SOC can be output, in particular displayed, and / or a function of the motor vehicle can be controlled depending on the determined state of charge S.

[0057] Overall, the examples show how the invention can be used to determine the state of charge of an HV (high-voltage) battery in a vehicle, e.g. an LFP HV battery or an HV battery with cells of any other cell chemistry or even an HV battery with cells of different cell chemistries in a mixed installation. In addition to evaluating the voltage curve over the SOC range to determine the state of charge, the pressure that builds up or decreases when a cell is charged can also be measured using one or more additional functions and used for evaluation. For this purpose, a pressure sensor can be installed and evaluated in the battery module or battery stack. This enables the state of charge to be determined precisely, regardless of whether the battery is charged to 100%. In addition to the voltage sensors already present on the cells, an additional pressure sensor can be implemented in the cell stack or cell stack and its data evaluated.The thickness, or rather the pressure, that builds up or decreases in the cell during charging changes across the entire charging range. Due to the constant change in pressure or thickness, an accurate SOC determination is possible by incorporating both characteristic curves. QUOTES CONTAINED IN THE DESCRIPTION

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

[0000] WO 2016 / 033479 A9

[0004]

Claims

[1] Method for determining a state of charge (SOC) of an energy storage unit (14) for a motor vehicle by a control device (16), characterized by , that - a measured value (W) dependent on an expansion (Δd, ΔD) relative to a specific reference state (RZ) of the energy storage unit (14) is determined, and - the state of charge (SOC) of the energy storage unit (14) is determined as a function of a provided characteristic curve (K1, K2, K3, K4) which indicates a dependence of the state of charge (SOC) of the energy storage unit (14) on the measured value (W), and is determined as a function of the determined measured value (W). [2] Method according to claim 1, characterized by that the energy storage unit (14) comprises at least one LFP (lithium iron phosphate) battery cell (12). [3] Method according to one of the preceding claims, characterized by that as the measured value (W) - a value of a linear expansion (Δd, ΔD) of a battery cell arrangement and / or a battery cell (12) of the energy storage unit (14) in a predetermined direction (x) is determined; and / or - a value of a force and / or a pressure within a battery cell arrangement of the energy storage unit (14) is determined; - a value (W) of an elongation and / or curvature of at least one component of the energy storage unit (14) is determined, in particular relating to a cell housing of a battery cell (12) of the energy storage unit (14) or relating to an arrangement of at least two battery cells (12) of the energy storage unit (14) relative to one another. [4] Method according to one of the preceding claims, characterized bythat the state of charge (SOC) is determined during operation of the energy storage unit (14), in particular while a load current is provided by the energy storage unit (14) and / or while the energy storage unit (14) is charged with a charging current. [5] Method according to one of the preceding claims, characterized by that the state of charge (SOC) is determined as a function of an operating current of the energy storage unit (14), in particular as a function of the load current and / or charging current. [6] Method according to one of the preceding claims, characterized bythat a first state of charge value (SOC1) of the energy storage unit (14) is determined according to a first determination method (M1) as a function of the provided characteristic curve (K1, K2, K3, K4) and as a function of the determined measured value (W), and a second state of charge value (SOC2) of the energy storage unit (14) is determined according to a second determination method (M2) which is different from the first determination method (M1), and the state of charge (SOC) is determined as a function of the first and second state of charge values ​​(SOC1, SOC2). [7] Method according to one of the preceding claims, characterized by that the second state of charge value (SOC2) is corrected as a function of the first state of charge value (SOC1) in the event of a deviation between the first and second state of charge values ​​(SOC1, SOC2) and the corrected second state of charge value is determined as the state of charge (SOC) of the energy storage unit (14). [8] Method according to one of the preceding claims, characterized by that a dependence of the state of charge (SOC) on the measured value (W) according to the characteristic curve (K1, K2, K3, K4) is not monotonous. [9] Method according to one of the preceding claims, characterized by , that - the first state of charge value (SOC1) is determined as a function of the second state of charge value (SOC2); and / or - the first state of charge value (SOC1) is determined as a function of at least one previously determined measured value (W) and / or assigned first state of charge value (SOC1), in particular as a function of a temporal progression of several previously determined measured values ​​(W) and / or assigned first state of charge values ​​(SOC1). [10] Control device (16) for a motor vehicle, which is designed to carry out a method according to one of the preceding claims.

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