CN114518537A

CN114518537A - Method for determining a value of a parameter of a battery cell, control device and motor vehicle

Info

Publication number: CN114518537A
Application number: CN202111367213.4A
Authority: CN
Inventors: A·巴克; M·施耐德
Original assignee: Audi AG
Current assignee: Audi AG
Priority date: 2020-11-20
Filing date: 2021-11-18
Publication date: 2022-05-20
Also published as: DE102020130732A1

Abstract

The invention relates to a method for determining a value of at least one parameter (SOC, K) of at least one cell of a battery (14) of a motor vehicle (10), wherein the value of the at least one parameter (SOC, K) is determined from a characteristic map (18) which is associated with the at least one cell and defines a relationship between a steady voltage (U) of the at least one cell and a state of charge (SOC) of the at least one cell for the at least one cell, wherein the determination of the value of the at least one parameter (SOC, K) is repeated in subsequent time steps. It is checked whether a value of at least one parameter (SOC, K), which is obtained at least at one of the time steps, meets a predetermined criterion, and, at least if the predetermined criterion is not met, a characteristic map (18) is adjusted, at least a part of the characteristic map (18) being modified as a function of the adjustment.

Description

Method for determining a value of a parameter of a battery cell, control device and motor vehicle

Technical Field

The invention relates to a method for determining a value of at least one parameter of at least one battery cell of a battery of a motor vehicle, wherein the value of the at least one parameter is determined from a characteristic map associated with the at least one battery cell, which defines a relationship between a steady voltage of the at least one battery cell and a state of charge of the battery cell for the at least one battery cell. In this case, the determination of the value of the at least one parameter is repeated in successive time steps. The invention also relates to a control device for a motor vehicle and to a motor vehicle.

Background

In order to be able to determine the capacity and thus the storable, available energy or also the remaining energy in a battery, for example a lithium-ion battery, the current State of Charge (SOC) is determined from the steady-State voltage compensation. The compensation is carried out according to a characteristic curve or a characteristic curve family, which can also take the form of a table, in particular a voltage table or an OCV (open circuit voltage) table. Such tables typically associate a respective state of charge with a corresponding regulated voltage. The capacity of the lithium ion accumulator is calculated from the difference in the state of charge before and after the charging or discharging process and from the amount of charge flowing through this time. The OCV table is the basis for determining the state of charge. As energy accumulators are developed, the OCV table is determined for the cells to be used by various measuring methods. However, the OCV table presents a problem in that it changes during the use of the accumulator. The reasons for this are different environmental influences, such as, for example, temperature, electrical load, mechanical influences or aging over time. As a result, the internal resistance of the cell changes and the resulting stable voltage also changes. However, since the stabilized voltage is a basis for determining the battery capacity, the battery capacity cannot be determined accurately any more as time elapses. However, the stable voltage does not change only due to the change in the internal resistance. Changes in the electrolyte or cathode and/or anode can also result in changes in the regulated voltage. Thus, the change in capacity cannot be clearly inferred by the change in internal resistance. Therefore, no correlation between these two parameters is possible, so that the internal resistance value is not suitable for checking the reliability of the capacity value.

Patent document EP 1702219B 1 describes a device for estimating the state of charge of a battery by means of a neural network which processes current, voltage and temperature of the battery cells and current time data as input data.

Furthermore, patent document EP 1873542B 1 describes a battery management system for estimating the state of charge of a battery using a measurement model that models the battery, the measurement model including an internal resistance, a diffusion impedance, and a no-load voltage.

Furthermore, patent application US 2017/0146608 a1 describes a method for determining the state of health and state of charge of a monomer based on the entropy of the monomer.

According to this method, the OCV table is not used to determine, for example, the state of charge or the capacity of the battery.

Furthermore, patent application DE 102019108498 a1 describes a battery state estimation based on the no-load voltage and the calibrated data. In particular, the state of charge of the battery is determined using a look-up table, which relates the no-load voltage to the state of charge. The look-up table is calibrated at vehicle design time using test records, such as test records of dynamic stress tests, that control aging of one or more additional batteries. The calibrated data is correlated with data of a battery aged in the vehicle and thus allows an accurate estimation of capacity and state of charge in the vehicle. However, this is only true when the battery cells in the vehicle are aged in the same manner as was the case in the previous test. Accordingly, for this purpose, it must first be possible to determine: in what state of aging the relevant battery cell is currently in.

Disclosure of Invention

The object of the present invention is therefore to provide a method for determining the value of at least one parameter of a battery, a control device and a motor vehicle, which allow the value of the at least one parameter to be determined in as accurate a manner as possible and nevertheless as simple a manner as possible, even as the battery cells age gradually.

This object is achieved by a method, a control device and a motor vehicle having the features of the respective independent claims. Advantageous embodiments of the invention are the subject matter of the dependent claims, the description and the figures.

In the method according to the invention for determining a value of at least one parameter of at least one battery cell of a battery of a motor vehicle, the value of the at least one parameter is determined from a characteristic map which is associated with the at least one battery cell and defines a relationship between a steady voltage of the at least one battery cell and a state of charge of the at least one battery cell for the at least one battery cell, wherein the value of the at least one parameter is determined repeatedly in successive time steps. Furthermore, it is checked whether the value of the at least one parameter obtained at least at one of the time steps meets a predetermined criterion, and at least if the predetermined criterion is not met, the characteristic map is adapted, at least a part of which is modified as a function of the adaptation.

In other words, a characteristic map that is learned over time, for example an OCV table that is learned over time, can thus advantageously be provided. For the initialization of the method, an initial characteristic map can be called, which can then advantageously be adjusted gradually over the course of time. Within the scope of the invention, characteristic curve families, characteristic curves and tables are used synonymously. In other words, if a table is mentioned, this is also to be understood as a general family of characteristics, which can also take a form different from the table form. This OCV characteristic is the basic data for the description of the monomers. The OCV characteristic curve varies over the service life of the monomer and additionally also to a varying extent, depending in most cases on the type of monomer used. By means of the self-learned OCV characteristic, it is now advantageously possible to ensure that, for example, the data base for determining the battery capacity is of the same good quality over the service life. This offers the advantage that the respective cell of the battery can always be operated in the correct voltage range. This on the one hand extends the service life of the battery, provides reproducible and reasonable capacity values and thus also improves the perceived effective distance of the vehicle from the reproducible electricity.

The battery of the motor vehicle is preferably a high-voltage battery, which in turn comprises a plurality of battery cells. The method described here can be carried out individually for each cell, so that the corresponding values for the overall battery can be obtained simply from the values of these cells. In particular, for example in the case of parallel connection of individual cells, the capacity of the individual cells can simply be added to the total capacity of the overall battery, whereas in the case of series connection, which is usually present in high-voltage batteries, the total capacity of the high-voltage battery is derived from the smallest individual capacity of the cells. The corresponding characteristic curve can then be stored for each individual battery cell accordingly, or only for the entire battery. As mentioned at the outset, the family of characteristic curves can be provided in the form of a table. However, this family of characteristics is occasionally also referred to herein as a characteristic. In this case, the characteristic curve can also relate a limited number of discrete state of charge values to corresponding steady-state voltage values. For values in between, interpolation may be performed.

At least a part of the characteristic curve family can be, for example, a value pair or value tuple or also a plurality of value pairs or value tuples or parts of a characteristic curve which relate value ranges to one another. In this case, it is furthermore preferred that the adjustment of the characteristic map is limited to the at least one part, i.e. not all adjustments are made globally, but rather locally, for example only for a specific steady voltage value and a corresponding state of charge value or for a specific steady voltage range and a corresponding state of charge range.

In addition, in the characteristic map, in addition to the steady voltage (also referred to as the no-load voltage) and the state of charge, further parameters, in particular the temperature of the at least one battery cell, can also be taken into account. In other words, the characteristic map may define a relationship between the state of charge and the stable voltage of the at least one battery cell for different temperatures or temperature ranges. In this case, the value of at least one parameter is obtained in successive time steps. It can then likewise be checked in a corresponding time step whether the value of the at least one parameter meets a predetermined criterion. The time steps do not have to be determined beforehand and have the same interval to one another, but the time steps can also be event-triggered. In this case, it is advantageous, in particular, at a time before and after the charging or discharging process, in particular, wherein the battery or at least one battery cell is to be in a fully relaxed (relax) state at this time as far as possible, i.e. the predetermined time is to be at a standstill. This has the advantage that a stable voltage can thereby be obtained particularly accurately, so that this is preferably used during the method, as will also be explained in detail below.

Furthermore, the at least one parameter is preferably a state of charge of the at least one battery cell and/or a capacity of the at least one battery cell. The state of charge can be obtained from a family of characteristic curves by: the stabilization voltage of the at least one battery cell, in particular also the current temperature of the at least one battery cell, is detected, and then the current state of charge of the at least one battery cell is read from the characteristic map for the associated values of stabilization voltage and temperature. And the capacity may be obtained by dividing an amount of charge input to or extracted from the at least one cell during a charging or discharging process by a change in state of charge occurring during the charging or discharging process. Here, the state-of-charge change is a difference between an initial state-of-charge before the charging or discharging process and a final state (hereinafter also referred to as a final state-of-charge) after the charging or discharging process. Accordingly, it is also advantageous if, between the individual time steps, a change in the state of charge of the at least one battery cell is carried out in the form of a charging or discharging of the at least one battery cell. In this case, it is not necessary for the at least one battery cell to be fully charged or fully discharged. The charging and discharging of the at least one battery cell is also to be understood as a partial charging or partial discharging process. Such a charging or discharging process, in particular in conjunction with an immediately subsequent stabilization phase, can accordingly trigger the execution of the described method and define a corresponding time step. Accordingly, there may also be relatively long time periods between the individual time steps. However, this has no adverse effect on the described method, since the change in the stable voltage-state of charge characteristic of the battery changes only very slowly over the course of time.

In a further advantageous embodiment of the invention, it is checked whether the obtained value of the at least one parameter has a predetermined minimum transition compared to a specific reference value of the at least one parameter when checking whether a predetermined criterion is fulfilled, wherein the characteristic map is adjusted at least if a predetermined minimum transition is present.

A jump is to be understood here as a change. For example, if the characteristic map, for example the OCV characteristic curve, changes during the use of the battery in such a way, more precisely the steady-state voltage/state of charge characteristic of the at least one battery cell changes in relation to the stored OCV characteristic curve in such a way that this has an effect on the determination of the state of charge, this takes the form of a state of charge jump and a capacity jump after the adaptation phase, that is to say after the stabilization phase of the battery or of the at least one battery cell. Advantageously, therefore, such a predetermined minimum transition of the at least one parameter value can conclude that the steady-state voltage/state of charge characteristic of the at least one battery cell concerned may have changed, which can then advantageously be taken into account by a corresponding adjustment of the characteristic map. As described above, the current state of charge of at least one battery cell can be determined from a characteristic map by measuring the current steady voltage. Another possible approach for obtaining the state of charge consists in integrating the current which is fed into the at least one cell during the charging process or which is drawn out of the at least one cell during the discharging process. In other words, for example for a charging process, there are:

SOC_Ende＝SOC_Anfang+∫I(t)dt/K，

therein, SOC_EndeRepresenting the state of charge, SOC, of at least one cell after a charging process_AnfangRepresenting the state of charge of the battery cell prior to the charging process, I representing the charging current, and K representing the capacity of at least one battery cell.

The state of charge of at least one cell can therefore be determined on the one hand by means of a stable voltage measurement on the basis of a characteristic map and on the other hand by means of current integration. The respective values can be compared with one another to check whether a transition in the state of charge occurs between the two time steps. That is, the state of charge value may be compared to a reference value that represents a value of state of charge obtained from current integration. The reference value for the state of charge may thus change over the time steps, since it is always retrieved for each time step. As can be seen from the above equation, the capacity of at least one battery cell can also be obtained according to the state of charge before and after the charging process, and according to the charging current. The capacity thus obtained can be compared with a reference value, for example a capacity value initially given for a new battery or at least one battery cell. Similarly, it can be checked whether the newly obtained capacity has a jump, i.e. a change, with respect to the reference value. Of course, similarly, this also applies to the discharge process. If necessary, the capacity value can also be adjusted in the course of the method, as described in more detail below. This means that the newly obtained capacity value can also be set as a new reference value for the subsequent time step. Accordingly, the reference values for capacity and state of charge change as necessary over the time step.

However, a problem that arises when optimally setting the characteristic map is that, in addition to the characteristic map, the capacity of at least one battery cell also changes over the course of time. Now, if not only the family of characteristic curves but also the capacities are unknown in the calculation (because they have changed), an equation with two unknowns is approximated (as given above). In this case, without further consideration, it is not possible to relate the change to a capacity or OCV table or, in general, a family of characteristic curves. The invention is also based on the recognition here that: evaluating the degree of the respective transition described above and the direction of the transition leads to conclusions from which measures can be defined which specify whether and in which direction the family of OCV characteristic curves must be adjusted in order to minimize or completely eliminate future transitions in state of charge and capacity. The measures to be taken can advantageously be specified when both the state of charge and the capacity are considered as the at least one parameter.

In a further advantageous embodiment of the invention, the value of the state of charge and the value of the capacity are thus obtained as the at least one parameter in each time step, and it is checked for the respective value whether it has a respective jump to a predetermined minimum extent compared to the respective reference value determined for the respective time step. At this time, the minimum degree that such a transition must have can be individually defined for each of the two parameters, i.e., state of charge and capacity. Furthermore, each of the parameters may also be assigned its own reference value, as already described and defined above. The reference value for the state of charge can thus be provided by current integration in the respective charging or discharging process, and the reference value for the capacity can be, for example, a value for the capacity in a preceding time step or initially provided. Thus, the reference value can be re-provided for the corresponding time step. Now, the conclusions that can now be drawn from the possible jumps in the obtained values are discussed next.

In this case, according to a further advantageous embodiment of the invention, for the first case, i.e., in a specific one of the time steps, if neither the value of the capacity nor the value of the state of charge obtained have a respective transition to a predetermined minimum level compared to the respective reference value for this specific time step, the characteristic map is not adjusted at least until the next time step following it. That is, if neither state of charge nor capacity has a jump, it can be assumed that these values are normal and no adjustment of the family of characteristics is required.

According to a further advantageous embodiment of the invention, for the second case, i.e., for a specific time step of the time steps, only the value of the capacity has a jump, the characteristic map is adjusted according to the direction of the jump and according to the direction of the change in the state of charge prior to the time step. The jump direction then defines the direction of deviation of the current value from the reference value. That is, if the value of the capacity provided at the determined time step is greater than the reference value, it is hereinafter referred to as an upward jump. If the value of the capacity is smaller than the reference value, it is hereinafter referred to as a downward jump. Accordingly, the upper and lower tables represent the respective transition directions of the transitions. Furthermore, the direction of the change in state of charge defines, between the specific time step and a time step directly preceding the specific time step, whether charge is supplied to the at least one battery cell (for example in the context of a charging process) or charge is extracted from the at least one battery cell, for example by discharging the at least one battery cell, during operation of the motor vehicle. For example, if at least one cell is charged, the capacity is defined by dividing the amount of charge input to the at least one cell during the charging process by the change in state of charge, that is to say by the difference between the state of charge of the at least one cell after the charging process and the state of charge of the at least one cell before the charging process. After the charging process, and in particular after waiting for a defined relaxation time, the state of charge can be determined from the characteristic map on the basis of the measurement of the steady voltage, and on the other hand also by current integration starting from the initial state of charge prior to the charging process. Accordingly, if the state of charge does not jump after charging, the final state of charge value may be considered normal. Since the capacity has a jump and the state of charge difference enters the capacity, it can be concluded that there is a problem in the initial state of charge, i.e. in the state of charge of at least one cell prior to charging. The conclusion that can be drawn using the above-described charging process is therefore that if the capacity jumps down, the low voltage, that is to say the steady voltage of the initial state of charge, is actually lower than the values given in the characteristic map. Conversely, the conclusion that can be reached when the capacity jumps up is that the low voltage is actually higher. This advantageously enables the characteristic map to be adapted accordingly. In the case of the above-described charging process, the stabilization voltage for the initial state of charge is correspondingly corrected downward during the capacity jump, in particular in such a way that this results in a new value for the initial state of charge, which correspondingly results in a new state of charge being poor and also in a new capacity value which no longer has a jump. When there is an upward capacity jump, it may be done accordingly. In this case, the stabilization voltage for the initial state of charge is corrected upward, so that a correction of the capacity value takes place as a result, so that the corrected capacity value accordingly no longer has a jump relative to the reference value. During the above-described discharge, the "high" stable voltage value associated with the initial state of charge is similarly adjusted, since in this case the initial state of charge is higher than the final state of charge. In addition, the adjustment always assumes a proportional/direct proportionality between the steady voltage and the state of charge, and assumes this, i.e. the greater the steady voltage, the higher the state of charge.

In a further advantageous embodiment of the invention, for the third case, i.e. at a specific time step of the time steps, both the value of the capacity and the value of the state of charge have a corresponding transition, and the corresponding transitions have the same transition direction, the characteristic map is adjusted in accordance with the transition direction of the transition and in accordance with the direction of the change in the state of charge prior to the time step. Thus, for example, if the state of charge and the capacity have transitions in the same direction of transition, this is not plausible first of all, since the state of charge difference and the capacity are inversely/indirectly proportional to one another. The conclusion that can be drawn from this is that there must be two superimposed effects at this time. This is explained next again by way of example for the charging process, but analogously this can also be used again for the discharging process: that is, if the final state of charge of at least one cell jumps upward, then the initial state of charge must therefore also be associated with a higher stabilization voltage, in fact, so that the state of charge difference is effectively smaller for the charging or discharging process under consideration. The steady voltage of the low state of charge, that is to say of the initial state of charge, can now be advantageously corrected upward accordingly. This applies in particular to the case where both the capacity value and the state of charge value are rising. And if it is determined that both the state of charge and the capacity are going down, the low state of charge regulated voltage is corrected down accordingly. If instead of the charging process, the discharging process is considered, the steady voltage in the low state of charge is correspondingly corrected, but rather the steady voltage in the high state of charge, which corresponds to the respective initial state of charge during the discharging process. Therefore, suitable measures can also be advantageously taken in this case to take account of the change in the characteristic map.

In a further advantageous embodiment of the invention, for the fourth case, i.e. at a specific one of the time steps, both the value of the capacity and the value of the state of charge have a corresponding transition, and the corresponding transition has the opposite transition direction, the characteristic map is not adjusted at least before the next subsequent time step. In this case, the value of the capacity is adjusted according to the direction of the jump and according to the direction of the change in the state of charge before this time step, and the plausibility is checked in particular in the next time step. If it is determined that the transition in the state of charge value is opposite to the transition in the capacity value, there are several possible conclusions. For example, both new values may be correct and the capacity may not have been learned, or both values may be incorrect. It is therefore advantageous not to adjust the characteristic map first, but only the capacity. In other words, the capacity newly obtained in this time step is set to a new reference value. This applies in particular to the charging process described above, and also to the discharging process. Subsequently, the plausibility of the measure is advantageously checked in a subsequent time step. If this measure is correct, i.e. the capacity is adjusted in the correct manner, then in the subsequent time step, no jump is recorded in the state of charge value and in the capacity value, i.e. all are normal, i.e. the first case described above occurs. Otherwise, one of the other situations discussed herein is obtained, and then the measures set forth accordingly herein can be taken again.

In the fourth case just described, it is advantageous that the newly set capacity value is not initially taken as a basis for further calculations or other functions in the motor vehicle until the plausibility check is completed. Other functional modules of the motor vehicle, which function as a function of the current capacity of the at least one battery cell and/or the current state of charge of the at least one battery cell, may, for example, use the previously and previously verified values of these parameters until a new value is also verified and a plausibility check is carried out. In other words, multiple charge and discharge cycles (which also require different measures) are often required to obtain reliable results. The fact is that, as described at the outset, an equation with two unknowns cannot be solved in one step. However, since the change process in the battery is very slow when the cell is intact, it can be continuously learned at any time. Since results which are not plausible or cannot be checked directly for plausibility may occur in a single step, it is also possible to carry out the calculation in the background. Thus, the change has no effect on the value actually calculated by the controller and is also invisible to the driver. For example, it can be provided that a result is only used by the actual calculation if it has a high degree of confidence. For example, a high degree of confidence can be defined in that the transitions in state of charge and capacity are below a predetermined value, which is preferably between 2% and 4% of the relevant value.

Furthermore, there is a fifth situation, which occurs when, at a specific one of the time steps, only the value of the state of charge has a jump and the value of the capacity has no jump. In this case, according to a further advantageous embodiment of the invention, the characteristic map is not adjusted at least before the next subsequent time step, and before this next time step, the value of the capacity is adjusted according to the direction of the transition and according to the direction of the change in the state of charge before this time step, and if this occurs again in the next time step, the characteristic map is adjusted in the next subsequent time step. That is, if it is determined in a time step that only the state of charge has a jump and the capacity has no jump, a two-stage measure is provided in this case. The knowledge that is based in this case is that the capacity is assumed to be too small or that the final state of charge stable voltage is incorrect. It is therefore particularly advantageous to first correct the capacity in a first step, specifically upward if a downward state of charge jump is determined after the charging process described above, and vice versa, that is to say downward if an upward state of charge jump is determined after the charging process described above. At this time, the amount of correction corresponds to the capacity difference obtained from the state of charge jump. This can then correct the first mentioned situation, i.e. the capacity is assumed to be too small. If this is true, the corrective action causes the first case described above, i.e., all are normal. If the problem otherwise occurs again in a subsequent time step, the characteristic map is corrected as a further measure, specifically, depending on the direction of the transition of the final state of charge, the stabilization voltage, which is associated with the final state of charge in the case of the charging process, is corrected upward or downward. In other words, if it has been determined that the state of charge jumps down, the regulated voltage is also corrected down, and in the case of a state of charge jump up, the regulated voltage is also corrected up. In the case of a discharge process, the correction direction of the stabilization voltage is exactly opposite.

In this case, it is also advantageous that the result of the correlation, i.e. in particular the adjusted capacity, is used as a basis for further calculations if it has a high degree of confidence and the assumption or measure is verified, for example, in a subsequent time step. Although the particular examples set forth above relate primarily to a charging process, these measures can also be used in a corresponding manner for a discharging process of at least one battery cell. Furthermore, the described measures apply not only to the complete charging and discharging process, but also to partial processes in the charging and discharging direction.

The invention also relates to a control device for a motor vehicle, which is designed to carry out the method according to the invention or one of its embodiments. The control device may have a data processing device or a processor device which is configured to carry out an embodiment of the method according to the invention. The processor device can 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) for this purpose. Furthermore, the processor device may have a program code which is configured to carry out an embodiment of the method according to the invention when it is run by the processor device. The program code may be stored in a data memory of the processor device.

The invention also includes a motor vehicle having such a control device. The motor vehicle according to the invention is preferably designed as a motor vehicle, in particular as a passenger car or a truck, or as a bus or a motorcycle.

The advantages described for the method according to the invention and its embodiments apply in the same way to the control device according to the invention and to the motor vehicle according to the invention.

The invention also includes a control device according to the invention and a development of a motor vehicle according to the invention, which has the features already described in connection with the development of the method according to the invention. For this reason, the control device according to the invention and corresponding modifications of the motor vehicle according to the invention are not described in detail here.

The invention also comprises combinations of features of the described embodiments. Therefore, the present invention also includes implementations having a combination of features of a plurality of the described embodiments, as long as the embodiments are not described in a mutually exclusive manner.

Drawings

Embodiments of the present invention are described next. Wherein:

fig. 1 shows a schematic representation of a motor vehicle having a battery and a control device for obtaining a value of at least one battery parameter according to an embodiment of the invention; and

fig. 2 shows a flow chart for explaining a method for obtaining a value of at least one battery parameter according to an embodiment of the invention.

Detailed Description

The examples explained below are preferred embodiments of the present invention. The components of the embodiments described in the exemplary embodiments are in each case individual features of the invention which can be considered independently of one another and which in each case independently of one another also improve the invention. Therefore, the present disclosure is intended to include other combinations of features than those of the illustrated embodiments. Furthermore, the described embodiments can also be supplemented by other features of the invention which have already been described.

In the drawings, like reference numbers indicate functionally similar elements, respectively.

Fig. 1 shows a schematic illustration of a motor vehicle 10 having a control device 12 and a battery 14, for example a high-voltage battery. The control device 12 also has a memory 16, in which a characteristic map 18, in particular an OCV table, is stored. The OCV table correlates respective stable voltage values of the battery 14 to corresponding state of charge values for respective temperature ranges. In fig. 1, a state of charge SOC is schematically shown for the battery 14 shown there, in particular figuratively in the form of a charging progress bar. Typically, the state of charge SOC is given in percentages from 0% to 100%. Here, the maximum amount of charge that can be accommodated in the battery 14 defines the capacity K of the battery 14. If the partial charge amount Δ Q is input to the battery 14, the change in state of charge Δ SOC is thereby obtained. Thereby, the capacity K can be obtained as follows: k is Δ Q/Δ SOC. The amount of charge Δ Q may in turn be derived from the charging current I, in particular in terms of Δ Q ═ Idt. For example, if the battery 14 is charged for one hour at a current of 30 amperes, this corresponds to the amount of charge Δ Q input to the battery 14 at 30 amperes. If the state of charge increases from 0% to 100% during this charging process, the 30 amps corresponds to the capacity K of the battery 14.

Starting from the initial state of charge, the final state of charge of the battery 14 can be obtained, for example, by current integration. On the other hand, the current state of charge SOC of battery 14 may also be determined from a map of characteristics 18 stored in memory 16 of control device 12. For this purpose, only a stable voltage U of the battery 14 has to be obtained. This typically takes place after a stabilization phase of the battery 14, since then a particularly accurate attainment of the stabilized voltage U can be achieved. These measured values, i.e. the regulated voltage U and the battery current I during charging and/or discharging of the battery 14, can likewise be supplied to the control device 12. Given these parameters and with the aid of characteristic map 18, control device 12 can, for example, always determine the current value of state of charge SOC and provide it to other systems or, for example, indicate it to the driver.

A problem with conventional methods for determining the state of charge using a stable voltage table or OCV table is that such an OCV table for the relevant battery is only applicable to the new state of the battery. That is, the stable voltage-state of charge characteristics vary with the use of the storage energy device, which results from different environmental influences, such as temperature, electrical load, mechanical influences and aging over time. As a result, for example, the internal resistance of the cell and thus the resulting stable voltage for the respective charge state vary. However, the stabilization voltage does not change only due to a change in internal resistance. Variations in the electrolyte or the cathode and/or anode may also result in variations in the regulated voltage. If these are not taken into account in the OCV table, the inaccuracy in the determination of the battery capacity and the acquisition of the state of charge becomes higher and higher as time passes.

Now, according to the invention, this situation can advantageously be avoided by adapting the initially based OCV table 18 over time, in particular to match the current state of the battery 14. That is, the OCV table 18 is advantageously provided that is learned over time, thereby making the OCV table advantageously optimally matched to the battery characteristics at any time. Now, this is described in detail below.

To this end, fig. 2 shows a flow chart illustrating a method for obtaining at least one value of a battery parameter according to an embodiment of the invention. In this case, the battery parameter is in particular not only the state of charge SOC but also the capacity K of the battery 14. Based on these values, which are obtained in particular repeatedly in successive time steps, the OCV table 18 is adjusted, more precisely a possible adjustment of the characteristic map 18 and of the capacity K is determined. However, this adjustment requires some consideration in the first place, since in addition to the OCV characteristic curve, the capacity K of the cell or battery 14 also changes. Now, if not only the OCV characteristic curve 18 but also the capacity K are unknown at the time of calculation (since they may both vary), an equation with two unknowns results. In this case, it is then no longer possible, at least without further consideration, to correlate the change with the capacity K or OCV table 18. To solve this problem, the method described below may be used. Now, if the OCV characteristic curve 18 changes during use in such a way that it has an effect on the determination of the state of charge SOC (which is expressed in the form of transitions in the state of charge values and the capacity values after the adaptation phase), the degree of the respective transition and its direction are evaluated, that is to say greater than, equal to, less than the state of charge values or the capacity values before the adaptation phase, and corresponding conclusions and measures are defined on the basis thereof, namely: the OCV characteristic curve 18 must be adjusted in which direction (if necessary) to minimize or completely eliminate future transitions in state of charge and capacity K.

Here, the method is started in step S10, and in step S10, a capacity value of the capacity K of the battery 14 and a current state of charge value of the state of charge SOC of the battery 14 are first determined or assumed. For the capacity K, it is assumed, for example, in a new state of the battery 14 that the capacity K initially corresponds to an initial value provided by the supplier of the battery 14. In the new state of the battery 14, the OCV table 18 initially used should also correspond to the battery characteristics, which relate to the correlation of the stabilization voltage value with the corresponding state of charge value of the state of charge SOC, in particular for the corresponding temperature range. For example, an initial value of the state of charge SOC may be provided according to the table 18 in step S10.

Next, in this example, a charging process is performed in step S12 to charge the battery 14. At this time, the battery may be fully charged or may be only partially charged. The following examples relate accordingly to the case of such a charging process as described above. However, for the aforementioned discharge process of the battery, the method can likewise be carried out by corresponding adaptation.

During this charging process, from the charging current I (the value of which is fed to the control device 12), the final state after this charging process in step S12 can be obtained knowing the initial state of charge provided in step S10. This acquisition can be carried out, for example, immediately following the charging process and likewise in step S12. In addition, in step S14, the current state of charge SOC of battery 14 is obtained from map of characteristics 18. Preferably, this is done after a stabilization phase of the battery 14, since the determination is based on a measurement of the stabilization voltage U. In step S16, the capacity K may be obtained from the thus obtained current state of charge value, which is a so-called value of the final state of charge of the battery 14 after the charging process. This capacity is derived from the current integral and the state of charge difference between the final state of charge and the initial state of charge, as already described with reference to fig. 1. Subsequently, the two values for the capacity K and the current state of charge SOC newly obtained in steps S14 and S16 may be compared with the corresponding reference values. As for the capacity K, the reference value is the capacity value K initially provided in step S10. The reference value of the current state of charge SOC is the final state of charge value calculated from the current integration obtained at the end of the charging process in step S12.

In this case, it is checked, in particular, whether the newly obtained values concerned have a transition Δ 1, Δ 2 with respect to the respective reference value. For this purpose, it can be checked, for example, in step S18 whether a state of charge jump Δ 1 is present. If not, it can furthermore be checked in step S20 whether a capacity jump Δ 2 is present. Even if it is determined in step S18 that there is a state of charge jump Δ 1, it can likewise be checked in step S22 whether there is a capacity jump Δ 2. In other words, both of these are checked anyway, i.e. whether there is a capacity jump Δ 2 and whether there is a state of charge jump Δ 1, wherein the chronological order of such checking is not critical. In particular, both examinations can also be carried out simultaneously. Therefore, if it is determined in step S18, for example, that there is no state of charge jump Δ 1 and, furthermore, in step S20, there is no capacity jump Δ 2, the process moves to step S24, in which it is determined that all are normal, that is to say that the obtained values are correct and that the OCV table 18 and the value of the capacity K are also up to date. Subsequently, the process again moves to step S10 and starts the method from the beginning, and then, in the next step S12, the charging of the battery 14 does not necessarily have to be performed again, but, for example, a discharging process may also be performed.

If it is determined in step S18 that there is no state of charge jump Δ 1, but it is determined in step S20 that there is a capacity jump Δ 2, then OCV table 18 may be adjusted in step S26. This is based on the recognition that: the steady-state voltage values associated with the initial state of charge values determined in step S10 according to table 18 are no longer up-to-date and are actually higher or lower, depending on the direction of the capacity jump Δ 2. In particular, this time can be adjusted as explained in the following example: according to the present example, an initial state of charge of the battery 14 is first obtained. This is achieved by measuring the steady voltage U (which is, for example, 3400 mV). From the OCV table 18, a state of charge value corresponding to the stable voltage U can be obtained. For example, a corresponding segment of table 18 may be as follows:

in this example, the initial state of charge value is correspondingly 20%. Further, the capacity K of the battery 14 assumed at present is 49 Ah. Subsequently, the battery 14 is charged to a final state of charge of 85%, which can be obtained from a current integration of the charging current I. After the stabilization phase of the battery 14, a state of charge value of 85% is again obtained on the basis of the acquisition of the stabilization voltage U and using the characteristic map 18, i.e., the state of charge SOC has not yet changed by Δ 1. After the stabilization phase, however, the capacity K jumps to 50Ah and thus has an upward jump Δ 2. Since the final state of charge value does not jump Δ 1, it can be considered that the final state of charge value is correct. Accordingly, it can be concluded that an incorrect initial state of charge value causes this capacity jump. That is, the capacity is actually 2% lower, that is, the initial state of charge SOC is actually 2% lower, being 18% instead of 20%. The steady-state voltage U of 3400mV measured at the beginning, that is to say before charging, is therefore not associated with a state of charge value of 20% but rather with 18%. In other words, the regulated voltage U associated with a state of charge value of 20% is actually higher than 3400 mV. Therefore, the OCV table 18 can be corrected accordingly by correcting the steady voltage value for the state of charge value of 20% upward to 3450mV, among other things.

The regulated voltage U is also corrected downward if the capacity K jumps downward, and corrected upward if the capacity K jumps upward, the regulated voltage corresponding to the initial state of charge. Immediately thereafter, the method starts again from scratch.

On the other hand, if it is determined in step S18 that the state of charge value has a transition Δ 1 relative to its reference value and, in addition, it is determined in step S22 that the capacity K also has a transition Δ 2, it is also checked in step S28 whether the capacity K and the state of charge SOC have the same transition direction or opposite transition directions. If both have the same transition direction, the process moves to step S30, and the characteristic map 18 is corrected again in step S30. This is based on the recognition that, depending on the transition direction, the steady voltage U associated with the initial state of charge value actually has a lower or higher voltage than the voltage given in table 18. If the transition direction is downward, the stabilized voltage value for the initial state of charge is correspondingly corrected downward, otherwise corrected upward. One example of a calculation may be as follows:

again, an initial capacity of 49Ah is assumed here, and the battery is charged to a state of charge value of 85%. After the stabilization phase, the capacity jump was determined to be 50Ah and the final state of charge jumps up to 88.3%. Correcting the capacity from 49Ah to 50Ah (102% of the original capacity) results in a state of charge correction of 83.3%, that is, 98% (inverse of 102%) of the original value. However, the state of charge SOC actually jumps by 5% to 90%, i.e., the total jump is then 3.3% + 1.7% + 5%. In this way, for the actual state of charge jump Δ 1:

Δ1＝SOC_neu-(K_alt/K_neu)×SOC_alt，

here, SOC_neuRepresenting a state of charge, SOC, of 88.3% after steady state_altRepresents a state of charge of 85%, K_altIs the originally assumed capacity 49Ah, K_neuIs the new capacity 50 Ah. If it is again assumed that the initial state of charge is 20%, for example according to the OCV table:

therefore, the steady voltage U for 20% now has to be corrected to a value of 25%, i.e. a total correction of 5%, with a calculated 5% jump. The voltage 3450mV is then recorded for a state of charge value of 20% accordingly.

On the other hand, if it is determined in step S28 that the transitions Δ 1, Δ 2 of the capacity K and the state of charge SOC have opposite directions, the process proceeds to step S32. This situation may have a number of reasons. In particular, this may mean that both values are normal, or that the capacity K has not been learned, or that both values are erroneous. Accordingly, provision can be made in step S32 to initially adjust the capacity K, i.e. to make the capacity value obtained in step S16 equal to the new capacity value when the method is repeated in step S10. The procedure is then checked for plausibility in a subsequent time step. If this assumption is correct, in a subsequent step, the jumps Δ 1, Δ 2 no longer occur without further changes. Otherwise, the other described situations or one of the situations which will also be described occur and corresponding measures are taken.

If it is determined in step S22 that no capacity jump has occurred, it is first checked in step S34 whether this has likewise occurred in the preceding time step, i.e. only the state of charge value has a jump Δ 1 and the capacity value has no jump. If this is not the case, the capacity K is corrected in step S36. At this time, the correction direction is correlated with the transition direction of the state of charge value. If the state of charge jumps down, the capacity K is corrected up, and vice versa. Here, the capacity K is corrected by a value corresponding to the capacity change obtained from the transition Δ 1 of the state of charge value. The method then starts from the beginning with the new capacity value in step S10. In contrast, if this situation reappears, that is, it is determined in step S34 that this situation has occurred in the previous time step, the flow shifts to step S38, and the characteristic curve 18 is corrected in step S38. In particular, the regulated voltage value associated with the final state of charge value is again corrected at this time. Here, when the state of charge value jumps up, the stabilization voltage value is also corrected upward, and when the state of charge value jumps down, the stabilization voltage value is also corrected downward. The method is then repeated in step S10 with the modified family of characteristics 18.

Here, the two adjustment schemes described for step S36 and step S38 are again based on the following recognition: the state of charge value has only one jump, again for a number of reasons. On the one hand, it may be that the assumed capacity is too small, which can be compensated for by the measures in step S36. On the other hand, however, it is also possible for the steady-state value to be abnormal, which in turn can be corrected by the measures described for step S38.

In this connection, it can be seen in particular that some corrections require a plurality of time steps to determine which values need to be changed, i.e. whether the characteristic curve 18 and/or the capacity K have to be corrected. However, this can advantageously be determined unambiguously in a plurality of time steps by the described method, if necessary, so that an OCV table 18 can ultimately be provided which is learned over time and which also reflects the correct battery behavior with regard to the steady voltage and state of charge during the aging of the battery 14. The capacity can also be matched in this way to its changes caused by aging. Since, in addition, the capacity K can be considered to slowly decrease over the service life of the battery 14, it is also conceivable, for example, that even larger transitions above a definable threshold value can be considered to be untrustworthy. In other words, for example, if an upward capacity jump higher than the threshold is determined, the capacity value can be regarded as being untrustworthy, for example. Thus, for example, it is possible to define that the measurement results are discarded and/or that, in order to take further measures, for example to adjust the capacity and/or the table 18, the plausibility must first be checked in a further time step. In particular, a narrower limit can be assumed, for example, in the case of upward correction, since the capacity K does not increase over the service life. However, depending on the measurement accuracy in the current and voltage, minor corrections in both directions should also be allowed. Furthermore, the capacity K can be determined not only for the entire battery 14, but also, for example, additionally for each cell or cell group. It is then also possible to check the calculated capacity values against one another for plausibility.

Since unreliable results may occur in a single step, it is also possible that the calculations may be performed in the background. Thus, the change has no effect on the value actually calculated by the controller 12 and is also invisible to the driver. Only if the result has a high degree of certainty and, for example, the transitions in state of charge and capacity are below certain values (for example, between 2% and 4% of the values involved) are this value used for the actual calculation.

In general, examples show how the invention provides a time-learning OCV characteristic for determining the state of charge of an electrical energy store, by means of which changes over the service life of a battery cell or a battery can be taken into account. This self-learned OCV characteristic ensures that the data base for determining the battery capacity is of the same good quality over the service life. This gives the advantage that the cell can always be operated in the correct voltage range. This, on the one hand, extends the service life of the memory, provides reproducible and reasonable capacity values, and thus also improves the perceived, reproducible electrical useful distance of the vehicle.

Claims

1. Method for obtaining a value of at least one parameter (SOC, K) of at least one cell of a battery (14) of a motor vehicle (10), wherein the value of the at least one parameter (SOC, K) is obtained from a family of characteristics (18) which are associated with the at least one cell and which define for the at least one cell a relationship between a steady voltage (U) of the at least one cell and a state of charge (SOC) of the at least one cell, wherein the value of the at least one parameter (SOC, K) is repeatedly obtained in successive time steps,

it is characterized in that the preparation method is characterized in that,

checking whether the value of the at least one parameter (SOC, K), which is obtained at least at one of the time steps, meets a predetermined criterion, and carrying out an adjustment of the characteristic map (18) at least if the predetermined criterion is not met, at least a part of the characteristic map (18) being changed as a function of the adjustment.

2. Method according to claim 1, characterized in that said at least one parameter (SOC, K) is the state of charge (SOC) of said at least one cell and/or the capacity (K) of said at least one cell.

3. The method according to any of the preceding claims, characterized in that the change in the state of charge of the at least one battery cell is carried out in the form of a charging or discharging of the at least one battery cell between the respective time steps.

4. Method according to one of the preceding claims, characterized in that, in checking whether a predetermined criterion is fulfilled, it is checked whether the obtained value of the at least one parameter (SOC, K) has a predetermined minimum degree of transition (Δ 1, Δ 2) compared to a determined reference value of the at least one parameter (SOC, K), wherein the adjustment of the family of characteristics (18) is carried out at least if a predetermined minimum degree of transition (Δ 1, Δ 2) is present.

5. Method according to any of the preceding claims, characterized in that in each of said time steps, a value of the state of charge (SOC) and a value of the capacity (K) are obtained as said at least one parameter (SOC, K) and it is checked for the respective value whether it has a respective transition (Δ 1, Δ 2) of a predetermined minimum extent compared to the respective reference value determined for the respective time step.

6. The method of claim 5,

for the first case, namely: if, at a specific one of the time steps, neither the value of the obtained capacity (K) nor the value of the obtained state of charge (SOC) has a respective transition (Δ 1, Δ 2) to a predetermined minimum extent compared to the respective reference value for the specific time step, the characteristic map (18) is not adjusted at least until the next subsequent time step, and/or

For the second case, namely: at a specific one of the time steps, only the value of the capacity (K) has a jump (Δ 2), the characteristic map (18) is adjusted as a function of the direction of the jump (Δ 2) and as a function of the direction of the change in the state of charge preceding the time step.

7. Method according to claim 5 or 6, characterized in that for the third case: at a specific one of the time steps, both the value of the capacity (K) and the value of the state of charge (SOC) have a respective transition (Δ 1, Δ 2), and the respective transitions (Δ 1, Δ 2) have the same transition direction, the characteristic map (18) is adjusted according to the transition direction of the transitions (Δ 1, Δ 2) and according to the direction of the change in the state of charge prior to the time step.

8. The method according to any of the preceding claims 5 to 7,

for the fourth case, namely: at a specific one of the time steps, both the value of the capacity (K) and the value of the state of charge (SOC) have a respective transition (Δ 1, Δ 2), and the respective transition (Δ 1, Δ 2) has the opposite transition direction, the characteristic map (18) is not adjusted at least until the next subsequent time step, and the value of the capacity (K) is adjusted as a function of the transition direction of the transition (Δ 1, Δ 2) and as a function of the direction of the change in the state of charge prior to the time step, and a plausibility check is carried out at the next time step; and/or

For the fifth case, namely: if, at a specific one of the time steps, only the value of the state of charge (SOC) has a transition (Δ 1), the characteristic map (18) is not adjusted at least until the next subsequent time step, and the value of the capacity (K) is adjusted at least until the next time step according to the transition direction of the transition (Δ 1) and according to the direction of the change in the state of charge before the time step, and if this occurs again at the next time step, the characteristic map (18) is adjusted at the next subsequent time step.

9. A control device (12) for a motor vehicle, which control device is designed to carry out the method according to one of the preceding claims.

10. A motor vehicle (10) having a control device according to claim 9.