METHOD FOR DETERMINING A STATE VALUE OF A TRACTION BATTERY

Abstract

A method for determining a state value of a traction battery of an electric vehicle characterises the ageing state, preferably an SoH value. The traction battery is charged or discharged by a test load and a respective output voltage and load current value pair is acquired. An ohmic internal resistance is established on the basis of the acquired value pair. The state value is established on the basis of the established ohmic internal resistance. At least one normalisation variable characterizing the traction battery is established. On the basis of the established ohmic internal resistance and the normalisation variable, a normalised internal resistance based on a reference value of the normalisation variable is established. The state value is established on the basis of the normalised internal resistance. A diagnostics device has an evaluation unit which is directly or indirectly couplable to the traction battery and carries out the method.

Claims

1-16. (canceled)

17. A method for determining a state value of a traction battery of an electric vehicle, which characterizes the ageing state of the traction battery, preferably an SoH value of the traction battery, comprising: loading the traction battery by a test load; at at least one point in time, acquiring a respective output voltage and load current value pair of the traction battery; establishing an ohmic internal resistance of the traction battery on the basis of the acquired output voltage and load current value pair establishing the state value of the traction battery on the basis of the established ohmic internal resistance; establishing at least one normalisation variable which characterises the traction battery; on the basis of the established ohmic internal resistance and the at least one normalisation variable, establishing a normalised internal resistance based on a reference value of the normalisation variable; wherein the state value of the traction battery is established on the basis of the normalised internal resistance; wherein the test load proceeds in a such a way that, on connection of the test load, the load current has a step change in current or a ramped profile; wherein a measurement sequence of output voltage and load current value pairs is acquired starting with the connection of the test load; wherein the measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time; wherein parameters of a compensation function, which models a profile of the measurement sequence, are established for the determination of the internal resistance by means of a mathematical adjustment calculus; wherein an optimization calculation of the compensation function is carried out in such a way that a coefficient of determination which describes a goodness of fit of the adjustment calculus, is maximised.

18. The method according to claim 17, wherein: the load current is generated during an evaluation run of the electric vehicle; and the test load is formed by a unit of the electric vehicle, preferably by a drive motor of the electric vehicle.

19. The method according to claim 17, wherein: a first normalisation variable is a temperature of the traction battery during acquisition of the output voltage and load current values; and the reference value of the first normalisation variable is a reference temperature.

20. The method according to claim 19, wherein: a second normalisation variable characterises a type of traction battery; the reference value of the second normalisation variable is a normalisation factor which relates different types of traction batteries to one another; and the normalisation factor is specified on a basis of at least one battery type parameter.

21. The method according to claim 17, wherein: the state value is established on a basis of the normalised internal resistance using a mathematical model or a table, preferably a lookup table or a performance map; and parameters or values which describe the mathematical model or the table are preferably retrieved from a database.

22. The method according to claim 17, wherein: a first normalisation variable is a temperature of the traction battery during acquisition of the output voltage and load current values; and the reference value of the first normalisation variable is a reference temperature; the temperature of the traction battery is established in that; in a first measurement step, a first ambient temperature and a first ohmic internal resistance of the traction battery are established at a first point in time; in a second measurement step after a predetermined period of time has elapsed, a second ambient temperature and a second ohmic internal resistance of the traction battery is established at a second point in time; on a basis of a difference of the first and second ohmic internal resistance and the specified period of time, a rate of change in internal resistance is established; on a basis of the rate of change in internal resistance, a differential temperature between the ambient temperature and the temperature of the traction battery is established; and the temperature of the traction battery is established by addition of a reference ambient temperature established from the first and/or second ambient temperature and the established differential temperature.

23. The method according to claim 22, wherein the predetermined period of time is between 5 and 15 minutes.

24. The method according to claim 22, wherein the state value of the traction battery is established on the basis of the second ohmic internal resistance.

25. The method according to claim 17, wherein: at least one output voltage and load current reference value pair is additionally acquired before the test load is connected, on the basis of which an open-circuit voltage and a closed-circuit current are established; the ohmic internal resistance for a respective value pair of the measurement sequence is established as a quotient of a difference of the acquired output voltage and the open-circuit voltage and a difference of the acquired load current and the closed-circuit current; parameters of a logarithmic function, which models a profile of the measurement sequence, are established for the measurement sequence by means of a mathematical adjustment calculus; and on a basis of the logarithmic function, the ohmic internal resistance is established at a desired point in time, preferably at the time of the step change in current, or at a corresponding frequency.

26. The method according to claim 25, wherein: the logarithmic function is determined by the equation
R.sub.i(t.sub.i)=a.Math.ln(t.sub.i+t.sub.offset)+b, wherein t.sub.i is the time elapsed since the load was connected, R.sub.i(t.sub.i) is an interpolated internal resistance at time t.sub.i, t.sub.offset the time between the actual activation time and the estimated activation time and a and b are parameters.

27. The method according to claim 17, wherein: an expected load current is predetermined by the test load, preferably by an ohmic resistance of the test load, and by the output voltage of the traction battery; and those value pairs in which a difference between the expected load current and the acquired load current exceeds a predetermined tolerance value are not taken into account for establishing the ohmic internal resistance of the traction battery.

28. The method according to claim 17, wherein: the at least one output voltage and load current value pair of the traction battery is acquired in a plurality of passes, wherein in each pass the test load is connected and removed again at an end of the pass; at at least one point in time of a pass a respective value pair is acquired and a respective ohmic internal resistance of the traction battery is established on a basis of the acquired value pair; an average value for the ohmic internal resistance is established from respective ohmic internal resistances established in the plurality of passes; and the state value of the traction battery is established on the basis of the average of the ohmic internal resistance.

29. The method according to claim 17, wherein establishing the ohmic internal resistance of the traction battery furthermore comprises at least one of the following steps: for at least one value of a value pair, a respective valid measurement range is defined, wherein a value pair is not taken into account if one or both values are outside the respective measurement range, wherein the measurement range is preferably defined on the basis of an absolute value or a rate of change of the associated value; a measurement sequence of output voltage and load current value pairs is acquired, wherein the test load is connected throughout the duration of the measurement sequence, wherein the measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time, wherein a value pair is not taken into account if one or both values of this pair are equal to the corresponding value of at least one value pair acquired at a previous point in time; a measurement sequence of output voltage and load current value pairs is acquired, wherein the test load is connected throughout the duration of the measurement sequence, wherein the measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time, wherein the measurement sequence is subjected to low-pass filtering; or a measurement sequence of output voltage and load current value pairs is acquired, wherein the test load is connected throughout the duration of the measurement sequence, wherein the measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time, wherein; a moving average value of the ohmic internal resistance is established from the output voltage and load current value pairs of the measurement sequence, wherein the respective ohmic internal resistance for two respective value pairs acquired in immediate succession is preferably established from the difference of the two output voltages divided by the difference of the two load currents and the moving average of the ohmic internal resistance is formed by the average of the respective ohmic resistances established in this manner, or the ohmic internal resistance is established from the respective output voltage and load current value pairs on the basis of a mathematical adjustment calculus, preferably according to the least square fit method.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Further advantages are revealed by the present description of the drawings. The drawings show developments of the invention. The drawings, and description contain numerous features in combination. A person skilled in the art will expediently also consider the features individually and combine them into meaningful further combinations.

[0075] In the drawings:

[0076] FIG. 1 is a diagram indicating the storage capacity Q of a traction battery as a function of the number of ageing cycles N;

[0077] FIG. 2 is a diagram indicating the capacity Q and the internal resistance R.sub.i of a traction battery as a function of the number of ageing cycles N;

[0078] FIG. 3 is an equivalent circuit diagram of a battery cell or a traction battery;

[0079] FIG. 4 is a block diagram which shows the establishment of a state value of a traction battery according to one development of the method according to the invention;

[0080] FIG. 5 is a diagram which shows the approximation of a temperature T of a traction battery to an ambient temperature as a function of the time t;

[0081] FIG. 6 is a diagram indicating a rate of change in internal resistance of a traction battery as a function of a temperature difference between a battery temperature and an ambient temperature by way of example for a specific battery type;

[0082] FIG. 7 is block diagram indicating the establishment of a state value of a traction battery according to a further development of the method according to the invention;

[0083] FIG. 8 is a diagram indicating the output voltage and load current of a traction battery as a function of time in the event of a step change type connection of a test load;

[0084] FIG. 9 is a diagram indicating the output voltage and load current of a traction battery as a function of time in the event of a ramped connection of a test load;

[0085] FIGS. 10 & 11 are various diagrams indicating the internal resistance of a traction battery as a function of time for various, partially processed measured values;

[0086] FIG. 12 is a diagram of a current-voltage profile I/U of a traction battery with internal resistance R.sub.i established by means of a least square regression line;

[0087] FIG. 13 is a diagram indicating the real part Re(Z) and the imaginary part Im(Z) of a complex internal resistance Z of a traction battery for various frequencies; and

[0088] FIG. 14 is a diagram indicating a coefficient of determination R.sup.2 for an established internal resistance of a traction battery as a function of variation in the point in time t.sub.offset for connection of a test load.

DETAILED DESCRIPTION OF THE INVENTION

[0089] FIG. 4 shows a block diagram illustrating the determination of a state value of a traction battery of an electric vehicle 20 by means of a diagnostics device 10 which is set up to carry out a method according to one development of the present invention. The diagnostics device 10 has an evaluation unit which comprises at least an establishing module 12 and a normalisation module 14.

[0090] The diagnostics device 10 is indirectly coupled to the traction battery (not shown) of the electric vehicle 20, such that the values of an output voltage U, a load current I and a temperature T of the traction battery can be transmitted to the evaluation unit. Coupling to the traction battery may be made either directly to corresponding measurement points or sensors or indirectly via an interface to an on-board diagnostics apparatus (OBD) of the electric vehicle 20.

[0091] The traction battery of the electric vehicle 20 can be coupled to a test load, for example a drive motor of the electric vehicle 20, in order to cause the traction battery to discharge.

[0092] The output voltage U and load current I are transmitted to the diagnostics device 10 in the form of respective value pairs, wherein both an individual value pair and a measurement sequence of a plurality of value pairs established at specific time intervals can be acquired by the diagnostics device 10.

[0093] The establishing module 12 is designed to determine an ohmic internal resistance of the traction battery. In addition to the actual calculation of the ohmic internal resistance from the output voltage U and the load current I, the establishing module 12 can carry out further data processing steps, as will be explained in greater detail below in particular with reference to further developments of the method according to the invention or of the diagnostics device according to the invention.

[0094] The established ohmic internal resistance is transmitted to a normalisation module 14. In addition, the temperature T of the traction battery is also transmitted to the normalisation module 14. The temperature T of the traction battery, hereinafter also denoted battery temperature in abbreviated form, is established in the exemplary embodiment according to FIG. 4 by means of a sensor system which may be disposed inside the traction battery, and transmitted to the normalisation module 14. An alternative way of establishing battery temperature T is explained as a variant in an exemplary embodiment which is to be described in greater detail below.

[0095] Since, as explained in detail above, the ohmic internal resistance of the traction battery to a considerable extent depends on battery temperature, a normalised internal resistance is established in the normalisation module which converts the ohmic internal resistance measured at the current battery temperature to a normalised internal resistance. The battery temperature thus constitutes a normalisation variable, wherein, on establishment of the normalised internal resistance, the currently established ohmic internal resistance is related to a reference internal resistance by means of a normalisation function or a normalisation table, which reference internal resistance was established at a reference temperature within the scope of test procedures carried out previously on traction batteries of identical or also different construction.

[0096] In order to be able to take account of different battery types, a normalisation factor can be taken into account as a further normalisation variable. The normalisation factor can be provided on the basis of at least one battery type parameter, wherein the battery type parameter may for example be a cell type (lithium-ion, lead, etc.), a number of battery cells connected in series and/or in parallel, or the like. The normalisation factor can be obtained empirically, for example by measurements on batteries in as new condition, and is generally dimensionless. The state of charge (SoC) of the traction battery can be taken into account as a further normalisation variable.

[0097] The test load can be connected, for example, by carrying out an evaluation run with the electric vehicle 20 over a relatively short distance of, for example, up to 100 m, preferably up to 50 m, wherein the highest possible acceleration is advantageously set here. The short evaluation run consists for example of a brief, powerful acceleration. As a rule, the load generally depends on various test boundary conditions (driving style of the tester, vehicle, weather conditions, road conditions such as pavement surface or inclination, vehicle load, functioning of a vehicle start control system, etc.). If necessary, relevant test boundary conditions can be taken into account in the form of further normalisation variables.

[0098] Normalisation may, for example, be carried out on the basis of a table, for example a lookup table or a performance map, or also on the basis of a mathematical model. If a plurality of normalisation variables are to be taken into account, normalisation, i.e. establishing the normalised internal resistance, can also be carried out in a plurality of sub-steps.

[0099] The parameters required for normalisation, i.e. performance map tables or performance map values or parameters of a mathematical model or a mathematical normalisation function, can be stored in the diagnostics device 10 and/or also be retrieved from an external database by the diagnostics device 10. The diagnostics device 10 may optionally also feed correction values for these parameters back into the database.

[0100] The normalisation module 14 is furthermore designed to establish a state value of the traction battery on the basis of the normalised internal resistance. In the present exemplary embodiment, the state of health (SoH) of the traction battery is output as the state value. The SoH can be calculated, for example, on the basis of an SoH allocation function or table that has been empirically established in prior testing. The SoH is output by way of example in the form of a log printout 16. It goes without saying that there are also other output options, for example by means of a display or also by means of wireless or wired transmission to appropriate display, acquisition or data processing devices. For example, the SoH value together with other acquired parameters, such as the input parameters U, I and T and optionally vehicle identification data, can be transmitted to a central server, from where transmission is made in paper or electronic form, for example to users of the electric vehicle or to a workshop.

[0101] One advantage of such central data storage is that different traction batteries or electric vehicles, even those not initially tested, can be analysed. For example, a kind of standard can be defined for several similar traction batteries or electric vehicles, which is composed of empirical values from a plurality of batteries or electric vehicles. In comparison with making reference to a single reference battery, the influence of any manufacturing tolerances can thus be reduced and, given an appropriately large database, the respective ageing state of the traction battery can also be taken into account.

[0102] A further advantageous development of the method or diagnostics device is described with reference to FIGS. 5 to 7. The measured values of output voltage U and load current I required for establishing the battery's internal resistance are generally provided by an OBD of the electric vehicle or another interface, as access to components under high voltage inside the battery is usually not possible for safety reasons. However, depending on the specific manufacturer, the battery temperature T is often not provided at all or only in encrypted form. In order to ensure that the diagnostics device or method can be used as independently of the manufacturer as possible, this advantageous development provides an alternative method for establishing battery temperature.

[0103] This exploits the fact that, when determining the state value, the battery temperature differs from the temperature of the environment in which the traction battery test is to be carried out. In an exemplary situation, which forms the basis for FIG. 5, it is assumed that the electric vehicle was parked outside, such that the traction battery only has a temperature T of about 4° C. at the start of the test. The test, on the other hand, is performed in a building where a room temperature or ambient temperature of approximately 24° C. prevails. Thus, over time t, the battery temperature will increase from 4° C. to the ambient temperature of 24° C. This process is non-linear and is represented by the solid line in FIG. 5.

[0104] Two portions of this adaptation curve are additionally marked by two respective pairs of circles in FIG. 5. It is apparent from FIG. 5 that the rate of temperature change ΔT/t, represented by the gradient of the curve, is relatively high at the beginning of the adaptation process (the portion of the curve enclosed by the left-hand pair of circles ΔT/t1), i.e. where there is a large temperature difference, and decreases with increasing adaptation, i.e. with a decreasing temperature difference between the environment and the battery (the portion of the curve enclosed by the right-hand pair of circles ΔT/t2). On the basis of the known fact that the ohmic internal resistance of the traction battery changes as a function of the battery temperature, a rate of change of the battery temperature can be established from the change in internal resistance per unit time, i.e. the rate of change in internal resistance ΔT/t.

[0105] If the temperature scale on the y-axis of FIG. 5 is now normalised such that the value which the curve approaches over time is considered to be a differential temperature with a value of 0° C., the curve directly indicates the differential temperature between the ambient temperature and the battery temperature. On the basis of the rate of change in internal resistance, it is possible to establish the rate of temperature change and from the rate of temperature change, which as explained above corresponds to the gradient T/t of the curve in FIG. 5, the associated differential temperature can in turn be derived by appropriate calculation steps. Since the ambient temperature is known or can readily be measured, the battery temperature can be directly inferred on the basis of the ambient temperature and the differential temperature.

[0106] The correlation between rate of change in internal resistance over time ΔR.sub.i/t and the differential temperature ΔT between the surroundings and the battery can also be directly represented in simplified manner by a corresponding curve ΔR.sub.i/t plotted against ΔT, see FIG. 6 with regard to battery-specific temperature behaviour. On the basis of this curve, which can be represented by a mathematical model or a table, the differential temperature can be read directly for a given rate of change in internal resistance.

[0107] The rate of change in internal resistance is established by determining at an interval of for example 5 to 15 min a first ohmic internal resistance and a second ohmic internal resistance of the traction battery. The difference of the first and the second ohmic internal resistance divided by the time interval between the two measurements then gives the rate of change in internal resistance ΔR.sub.i/t. Parallel to the measurements of ohmic internal resistance, the ambient temperature T can also be logged, from which the battery temperature can then be established by addition of the differential temperature. Should the ambient temperature T change slightly during the measurement, one of the two ambient temperatures or an average of the ambient temperatures T established at the different times can be used as the reference ambient temperature. Advantageously, more than two internal resistances R.sub.i can be established to achieve higher accuracy for determining the rate of change in resistance.

[0108] FIG. 7 shows a diagnostics device 110 designed for carrying out this variant of the method according to the invention. Since the diagnostics device 110 is a variant of the diagnostics device 10 shown in FIG. 4, only the significant differences are described below. Identical or similar elements are therefore provided with the same reference signs.

[0109] The diagnostics device 110 additionally has a buffer module 18 in which an internal resistance value R.sub.i established by the establishing module 12 can be temporarily stored. There is no provision for transmission of the battery temperature T from the electric vehicle 20 to the diagnostics device 110. Instead, the normalisation module 14 has an additional input, via which an ambient temperature T.sub.U of the surroundings in which the electric vehicle 20 is located at the time of the measurements can be acquired.

[0110] At a first point in time, an internal resistance R.sub.i is established on the basis of the output voltage U and the load current I and transmitted to the buffer module 18 and temporarily stored there as internal resistance value R.sub.i1. Once a predetermined period of time of for example between 5 and 15 min has elapsed, the output voltage U and the load current I are measured at a second point in time and converted in the establishing module 12 into a further internal resistance value R.sub.i2. This second internal resistance value is transmitted to the normalisation module 14. The temporarily stored first internal resistance value R.sub.i1 is simultaneously transmitted from the buffer module 18 to the normalisation module 14. The normalisation module now establishes the difference of the two internal resistance values R.sub.i1, R.sub.i2 and divides this difference by the period of time between the two measurement times.

[0111] On the basis of the rate of change in internal resistance established in this manner, the battery-specific curve of FIG. 6, which can be stored in the diagnostics device 110 for example in the form of a table or a mathematical function, is used as the basis for establishing an associated differential temperature. On the basis of this differential temperature and the ambient temperature T.sub.u, the normalisation module 14 now establishes the battery temperature, which can then be used as the basis of the normalisation described with reference to FIG. 4.

[0112] Further developments and variants of the method according to the invention or of the diagnostics devices 10, 110 according to the invention will now be described with reference to FIGS. 8 to 12.

[0113] FIG. 8 shows the profiles of output voltage U and load current I, which reflects a step change in charging current, of an exemplary traction battery. The diagram of FIG. 8 in each case shows the measured values for U and I at a respective point in time t. The origin of the time scale is arbitrarily chosen. At a step change time t.sub.0 at approximately t=6.5 s, a test load in the form of a charging current is connected, such that the output voltage U and the charging current as load current I change abruptly as a step change S. This period extends approximately between t=6.5 s and t=7.5 s. Thereafter, U and I change only very slowly. A description function of U and I can be identified by using a regression function, for example an exponential function. If a time offset t.sub.offset is provided in this regression function, the coefficient of determination R.sup.2 can be maximised by shifting the time base by modifying t.sub.offset and the activation time t.sub.0 of the step change S thus established, as shown for example in FIG. 14 for various values of t.sub.offset.

[0114] FIG. 9 shows a profile of output voltage U and load current of a traction battery as a function of time with ramped connection R of a test load, which in this case is a discharging current. Over the period t.sub.1=3 s to t.sub.2=6 s the discharging current I gradually declines, and after termination of the load, the current I and voltage U swing back towards closed-circuit current I.sub.0 and open-circuit voltage U.sub.0. Establishment of the internal resistance is improved by preferably taking account of the measured values within the time interval t.sub.1 to t.sub.2 of the load period of the ramped connection R, which can be approximated by means of a regression line, wherein the internal resistance R.sub.i can be exactly determined from the regression line. In order to be able to determine this time period exactly, a regression function of the load current I can be assumed which corresponds to the test load function and the time period t.sub.i to t.sub.2 can be precisely determined by consideration of a maximisation profile of the coefficient of determination R.sup.2.

[0115] Corresponding internal resistance values can be established from the measured value pairs established at a given point in time. Some of these internal resistance values R.sub.i are plotted against time t in the diagram of FIG. 10, wherein the internal resistance values R.sub.i1 to R.sub.i3 established during the step change are marked accordingly. The dotted line shown in FIG. 10 is a best fit line of the measured internal resistance values R.sub.i.

[0116] It is clear from FIG. 10 and also from FIG. 11a that in particular the measured values R.sub.i1 and R.sub.i2 deviate considerably from the best fit lines, so emphasising the need for frequent plausibility checking. This is also indicated by the minimised value of the coefficient of determination R.sup.2. To determine the internal resistance of a traction battery, the system response of the traction battery to a system excitation is evaluated in general terms. A current is imposed as the system excitation, while the system response is the voltage change across the battery terminals.

[0117] Under real-world conditions, the current and voltage difference during an acceleration process is used to calculate the internal resistance R.sub.i. With the values U.sub.0, I.sub.0 in a system resting state, i.e. an open-circuit voltage U.sub.0 and a closed-circuit current I.sub.0, and the values U.sub.i, I.sub.i at a point in time t.sub.i, the internal resistance R.sub.i is calculated according to the equation:

[00001] $R_{i} = \frac{U_{i} - U_{0}}{I_{i} - I_{0}} .$

[0118] Instead of accelerating the vehicle with a drive motor, another energy-intensive consumer can be activated. Battery charging can in principle be provided as system excitation.

[0119] It is often not possible to synchronise the time scale of the system excitation, i.e. connection of the test load or of the charging current, with the time scale of the system response, i.e. the acquisition of the measured values. However, in order to reliably determine the real part of the internal resistance, i.e. the resistance R.sub.1 in the equivalent circuit diagram of FIG. 3, it is necessary to determine the internal resistance directly after activating the test load, or at a defined point in time in the near future. Due to the large rate of change in current and voltage, the corresponding measured value pairs at this moment are often subject to major error or are not established at the correct time due to the discrete sampling. To ensure that R.sub.1 is correctly established, measurement data are therefore preferably interpolated or extrapolated. Suitable interpretation of the measurement data is therefore essential.

[0120] One approach to evaluating the measurement data is explained with reference to FIG. 11, wherein internal resistance R.sub.i is plotted against time tin each of FIGS. 11a to 11d. The dots represent the respective measured values, while the corresponding compensation function together with the associated coefficient of determination R.sup.2 is indicated in the diagram.

[0121] FIG. 11a substantially corresponds to FIG. 10, and shows a relatively low coefficient of determination R.sup.2, i.e. a relatively inaccurate determination of the internal resistance profile.

[0122] In contrast with FIG. 11a, in FIG. 11b the measured value R.sub.i1 is discarded due to its major deviation from the best fit lines or a specified expected value. A comparison with FIG. 11a reveals that the gradient of the compensation function of FIG. 11b has increased somewhat and the value of the coefficient of determination R.sup.2 has improved from 0.9249 (FIG. 11a) to 0.9665 (FIG. 11b).

[0123] The diagram shown in FIG. 11c used the same measured values as in FIG. 11b, wherein, however, a logarithmic compensation function was used instead of the linear compensation function used for FIG. 11a and. Another distinct improvement in the value of R.sup.2 to 0.9967 is obtained here.

[0124] FIG. 11d corresponds to FIG. 11c, wherein the time axis (x axis) has been logarithmically scaled.

[0125] FIG. 12 shows a diagram of a linear current and voltage profile I/U of a traction battery with a ramped load R. The internal resistance R.sub.i, is established using adjustment calculus tools to determine a regression line by means of a least squares approach, i.e. the least square fit method. The coefficient of determination R.sup.2 which can be established indicates whether the considered compensation function, in this case a curve, fits the established measured values. The method is outstandingly suitable even for establishing an open-circuit voltage U.sub.0 with a fluctuating current.

[0126] This demonstrated that the profile of the measurement sequence can be modelled very accurately by adjusting the measurement sequence by means of a mathematical adjustment calculus using a logarithmic function. The logarithmic function established in this manner can then be used to extrapolate or interpolate the ohmic internal resistance at a desired point in time, in particular a point in time close to the onset of the step change response.

[0127] A further significant improvement in data evaluation can be achieved when establishing the ohmic internal resistance by not taking account of those value pairs in which a difference between the expected load current and the acquired load current exceeds a predetermined tolerance value, as was done in the present example for the value pair on which internal resistance R.sub.i1 was based.

[0128] Further methods for improving the accuracy of internal resistance determination and thus the accuracy in determining the state value are briefly described below:

1. Internal resistance can be determined in a plurality of passes, wherein in each pass the test load is connected and removed again at the end of the pass. At least one value pair, preferably a measurement sequence, is acquired in each pass. A respective ohmic internal resistance of the traction battery is then established for each pass. Finally, an average for the ohmic internal resistance is established from the ohmic internal resistances established in a plurality of passes. This average is then used to establish the state value of the traction battery.
2. For at least one value of a value pair, a respective valid measurement range is defined, wherein a value pair is not taken into account if one or both values are outside the respective measurement range, wherein the measurement range is preferably defined on the basis of an absolute value or a rate of change of the associated value.

[0129] This can be done by defining corresponding valid measurement ranges for identifying the erroneous measured values. The measurement ranges may be defined absolutely, for example by absolute limit values for voltage, or relatively, for example by a boundary for a rate of change in voltage. Subsequently, all the measured value pairs are verified as to whether their values are within the previously defined measurement range and are screened out if they do not fall within the defined measuring ranges. This can be done, for example, by analysing the coefficient of determination R.sup.2 of the values in the step change range by establishing the change in R.sup.2 brought about by omitting the measured value.

3. A measurement sequence of output voltage and load current value pairs is acquired, wherein the test load is connected throughout the duration of the measurement sequence, wherein the measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time. A value pair is not taken into account if one or both values of this pair are equal to the corresponding value of at least one value pair acquired at a previous point in time.

[0130] This method makes it possible to eliminate “stuck” measured values which in particular arise due to transmission errors and in particular delays during data transmission from the traction battery or OBD. These data can lead to errors in the evaluation result, which are difficult or impossible to identify in retrospect. Such “stuck measured values” can, however, be screened out on the basis of firm criteria, since these erroneous data are above all characterised by the measured values not changing over a certain period of time. Due to the characteristic feature of the method according to the invention, according to which a system response to an abrupt system excitation is to be identified, any measured values which do not differ from the previous measured value can be screened out. If, for example, at least one value, i.e. voltage or current, does not change within a measurement sequence from one measured value pair to the next, the respective value pair acquired at the later point in time can be deleted from the data set to be evaluated. If, however, the error of a stuck measured value relates to just one value of a value pair, said value can also be replaced by interpolation from the other corresponding values.

4. A measurement sequence of output voltage and load current value pairs is acquired, wherein the test load is connected throughout the duration of the measurement sequence. The measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time. The measurement sequence is subjected to low-pass filtering. Surprisingly good improvements in the accuracy of the determination of the internal resistance can often be achieved with such simple filtering.

[0131] Low-pass filtering is particularly useful when it is not possible repeatedly to perform a plurality of measurement passes (see method 1.). The low-pass filter can be used as if a plurality of repeated measurements had been performed at a similar point in time, but taking account of the insufficient measurement resolution and thus forming a virtual average. This method can preferably be used for test arrangements which do not include step changes but are instead continuous.

5. Here too, a measurement sequence of output voltage and load current value pairs is acquired, wherein the test load is connected throughout the duration of the measurement sequence. Here too, the measurement sequence comprises a plurality of output voltage and load current value pairs acquired at a rapid succession of points in time.
a) A moving average value of the ohmic internal resistance is established from the output voltage and load current value pairs of the measurement sequence. This method is particularly suitable for ramped excitation, i.e. a continuous load increase or decrease. This preferably proceeds in that the respective ohmic internal resistance for two respective value pairs acquired in immediate succession is established from the difference of the two output voltages divided by the difference of the two load currents and the moving average of the ohmic internal resistance is formed by the average of the respective ohmic resistances established in this manner. Accordingly, the associated internal resistance is established in each case from two adjacent value pairs of the measurement sequence and then an average of these internal resistance values is calculated according to the following equation:

[00002] $R_{i} = \frac{{.Math.}_{m = 1}^{n - 1} R_{i m}}{n - 1},$

1. wherein n is the number of value pairs, and a respective ohmic resistance Rim is established from two value pairs acquired in immediate succession according to the equation

[00003] $R_{i m} = \frac{U_{m + 1} - U_{m}}{I_{m + 1} - I_{m}},$

2. wherein U.sub.m, U.sub.m+1 are the respective output voltages and I.sub.m, I.sub.m+1 the respective load currents of two value pairs m, m+1 acquired in immediate succession of the measurement sequence. If, instead of U.sub.m and I.sub.m, the open-circuit voltage U.sub.0 and the closed-circuit current I.sub.0 are selected, this method can also be used for step change type excitations.
3. The number n need not necessarily represent the number of all measured value pairs of the measurement sequence, but can also be a number of measured value pairs to be taken into account, e.g. if one or more measured value pairs have been removed from the measurement sequence or are not to be taken into account.
b) Alternatively to method 5a), the ohmic internal resistance can be established from the respective output voltage and load current value pairs on the basis of a mathematical adjustment calculus, preferably according to the least square fit method. This approach has already been described above with reference to FIGS. 8 to 14.

[0132] In the absence of the above-described methods for improving accuracy in determining internal resistance and determining the state value, a distinctly greater scatter of the result data could be observed. Variance of up to some 20% of the internal resistance was established over a series of various test passes. Applying the stated methods made it possible to reduce it to as low as 3% or even less.

[0133] If the time response of the battery is known as a function of battery type, age, state of charge, temperature and optionally other parameters, the switching time t.sub.0 at which current function starts can be determined relatively precisely using the comparison curve established for a reference battery.

[0134] If this is not possible, time t.sub.0 can be varied when establishing the compensation function. The switching time t.sub.0 can then be estimated by maximising the coefficient of determination R.sup.2 of the compensation function. FIG. 14 shows a diagram in which the coefficient of determination R.sup.2 is plotted against the variation or change of the switching time t.sub.0, implicitly a time offset t.sub.offset.

LIST OF REFERENCE SIGNS

[0135] 10, 110 Diagnostics device [0136] 12 Establishing module [0137] 14 Normalisation module [0138] 16 Log printout [0139] 18 Buffer module [0140] 20 Electric vehicle

METHOD FOR DETERMINING A STATE VALUE OF A TRACTION BATTERY

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G01R31/374

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G01R31/3842

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G01R31/392

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Y02T10/70

GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

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G01R31/389

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Y02E60/10

GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

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G01R31/378

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B60L58/16

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B60L50/60

PERFORMING OPERATIONS; TRANSPORTING

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G01R31/392

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G01R31/389

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G01R31/374

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G01R31/367

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G01R31/3842

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B60L50/60

PERFORMING OPERATIONS; TRANSPORTING

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