Method for estimating cell capacity and cell capacity ageing, and vehicle

Abstract

The invention relates to a method for estimating the cell capacity of a single battery cell, for which purpose the position of a peak (P1, P2, P3) on the charge axis (Q) and the value thereof on the differential voltage axis (dV / dQ) are determined by means of differential voltage analysis (DVA). The method according to the invention is characterised in that a position (FHM1, FHM2, FHM3) on the charge axis (Q) is determined which follows the maximum of the peak and at which the differential voltage value corresponds to a predefined value which depends on the differential voltage value at the maximum of the peak (P1, P2, P3) and which is specified to be at most 50% of this value. The estimated cell capacity can be used in particular to determine the capacity ageing of a single battery cell, preferably in a vehicle or in a cloud system connected to a vehicle.

Description

[0001] Mercedes-Benz Group AG

[0002] Methods for estimating cell capacity and cell capacity aging, as well as vehicle

[0003] The invention relates to a method for estimating the cell capacity of a single battery cell according to the type defined in more detail in the preamble of claim 1. The invention further relates to a method for estimating the aging of the cell capacity according to claim 3. The invention also relates to a vehicle according to claims 6 or 7.

[0004] In principle, the capacity of a single battery cell can be determined by integrating the current flowing through it and determining the state of charge at two or more time points. The mean linear regression of the charge plotted against the state of charge then yields the capacity of the single battery cell as the slope of the regression line. For this to work, it is necessary to determine the state of charge. This can be done, for example, by detecting the end of the charging or discharging process using current and voltage criteria, which are obtained by comparing open-circuit voltages with a characteristic curve or similar.

[0005] In principle, this is relatively imprecise and involves a great deal of effort. Due to the various individual steps that need to be carried out, irregularities and undesirable deviations can easily occur.

[0006] A sensible alternative is therefore the so-called differential voltage analysis, which is generally abbreviated as DVA. In their article "Calendar Aging of NCA Lithium-Ion Batteries Investigated by Differential Voltage Analysis and Coulomb Tracking," Peter Keil and Andreas Jossen, in Journal of The Electrochemical Society, 164(1) A6066-A6074 (2017), describe the basic principle of recording aging using DVA. CN 110 323 793 describes a vehicle and a method for charge balancing within a traction battery, in which DVA is used and a peak in the derived voltage curve is utilized. While the peak detection method of DVA can determine the peak positions as the maximum of the respective peak value, it does not provide a comprehensive analysis of aging.However, these values ​​depend heavily on factors such as cell aging, current, cell temperature, and hysteresis resulting from the recent current history of the individual battery cell. Typically, a very complex compensation using correction functions and / or characteristic maps is therefore necessary to determine the peak position with reasonable accuracy. In practice, this is very time-consuming and leads to unreliable results.

[0007] For further information on the state of the art, reference can also be made to CN 101 097248, which uses the shape of the voltage curve to predict battery capacity.

[0008] The object of the present invention is to improve the methods used in DVA for determining the peak position and, based on the more reliably detectable peak positions, to enable reliable conclusions to be drawn about the cell capacity and, in particular, the aging of the cell capacity.

[0009] According to the invention, this problem is solved by the method with the features in claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments and further developments are described in the dependent claim. Furthermore, a method according to claim 3 for determining the aging of the cell capacity solves the problem. Here, too, advantageous embodiments and further developments are described in the dependent claims. The method according to claim 1 and / or claim 3 can be used, in particular, in a vehicle environment.

[0010] The inventive method for estimating the cell capacity of a single battery cell uses differential voltage analysis to determine the position of a peak on the charge axis and its maximum value on the differential voltage axis, similar to the prior art. The inventors have now recognized that it is very efficient to determine a position on the charge axis that follows the peak's maximum and at which the differential voltage value corresponds to a predefined value. This predefined value depends on the differential voltage value of the peak's maximum, but is predetermined at a maximum of 50%, or, according to a particularly advantageous embodiment, at exactly 50%, of the peak value.

[0011] Such a point, which determines the peak's position not based on its maximum value but on a defined position along its descending slope, allows for improved peak position detection without the need for complex compensation using characteristic maps and / or correction functions. It has been recognized that the properties of such a point following the maximum, with a value of approximately 50% of the maximum value, are subject to significantly fewer kinetic influences and therefore require little or no compensation, as previously used. The estimation is thus improved and requires less mathematical effort for compensation using characteristic maps or correction functions; ideally, these can be dispensed with entirely.

[0012] The peak positions recorded below this point, which represent approximately half the maximum value, can now be used, in particular, to determine the aging of the capacity of a single battery cell and thus ultimately to determine the remaining amount of active lithium and active material. For this purpose, the position of the falling edge of the peak, determined using the method described above, is used by assigning it to a characteristic state of charge or a charge distance relative to another characteristic point, such as a full charge point. Based on the relative values ​​to each other, the aging of the capacity can then be inferred.

[0013] In particular, the method can be used in the context of a vehicle to estimate the capacity or aging of individual battery cells in a traction battery, and thus ultimately the capacity of the entire traction battery. The two described methods, in their various configurations, are suitable for direct integration into the vehicle's computer architecture, for example, by performing the procedure within a control unit. This allows the estimation to be carried out internally within the vehicle. Alternatively or additionally, the vehicle can also be equipped with a communication device that establishes a communication link to an external server, such as the backend of a vehicle manufacturer. The method can then be executed entirely or partially on this external server.The results can then be fed back to the vehicle to be used as a basis for further control functions and the like, either as a supplement to or alternative to the values ​​determined internally by the vehicle.

[0014] Advantageous embodiments and further developments of the method according to the invention can also be seen from the exemplary embodiment, which is described in more detail below with reference to the figures.

[0015] This shows:

[0016] Fig. 1 shows a schematic representation of a vehicle equipped to carry out the method according to the invention; and

[0017] Fig. 2 shows a representation of various peaks captured using DVA.

[0018] Figure 1 depicts a highly schematic vehicle 1, which is intended to be at least partially electrically powered and is equipped with a traction battery 2. The vehicle 1 also has a computing unit 3, which, alone or in combination with an external server 4 (represented here as a cloud), performs the procedures described below. If an external server 4, in particular the vehicle manufacturer's backend, is used to perform the procedures, the computing unit 3 is connected to a communication device 5, which can establish a communication link to the backend 4.

[0019] To estimate the cell capacity of individual cells in traction battery 2, and thus ultimately the cell capacity of the entire traction battery 2 and its calendar aging, a well-established differential voltage analysis (DVA) is used. This involves determining the position of a peak on a charge axis Q and its value on a differential voltage axis dV / dQ. Figure 2 shows three different curves recorded by the DVA in such a diagram with a differential voltage axis dV / dQ plotted against the charge Q. These curves are represented by dashed, dotted, and solid lines. The standard procedure for determining the charge involves determining the positions and differential voltage values ​​of the peaks on each curve, which are shown here outlined with a dotted line and labeled P1, P2, and P3.In practice, the measured position on the charge axis Q is corrected using complex characteristic maps and correction functions, as these peak values ​​are influenced by various factors, such as cell aging, current, cell temperature, and hysteresis resulting from recent current history. The current also significantly impacts the battery's power output and initial state of charge, and these must be compensated for accordingly. This is a complex process, which can be critical given the limited computing resources within a vehicle. Despite the compensation, the resulting peak positions (P1, P2, P3) on the charge axis Q remain relatively inaccurate.

[0020] The inventors have now discovered that an alternative method for determining the peak position offers a decisive advantage. At the point of the maximum, the falling edge of the peak is used to determine its position. For this purpose, a point is identified on the respective curve that follows the recorded maximum and exhibits a differential voltage value of up to 50% of this maximum. For example, the 50% value can always be used precisely, so these points, marked with solid circles in Figure 2, can also be referred to as Following Half Maximum or FHM points. These FHM points FHM1, FHM2, and FHM3 for determining the peak position on the charge axis have the crucial advantage of not exhibiting a significant power dependency.The influence of temperature and the initial charging point on the position of these FHM points FHM1, FHM2, FHM3 is approximately five times smaller than the dependence of the position of the maximum differential voltage values ​​of peaks P1, P2, P3. Thus, the FHM points FHM1, FHM2, FHM3 allow for a much more accurate estimation of cell capacities, either without the need for compensation or with a significantly simpler compensation than in the prior art. The range of variation in the positions of the maximum values ​​of peaks P1, P2, P3 on the charge axis Q is shown in Figure 2. It is much larger than that of the FHM points FHM1, FHM2, FHM3.

[0021] As long as only the relative positions on the charge axis Q are relevant in the following procedures, it is irrelevant whether the position of the maximum values ​​P1, P2, P3 or that of the FHM points FHM1, FHM2, FHM3 is used. Otherwise, a back-calculation would be necessary, which is typically much easier to handle than the compensation via correction functions, characteristic maps, and the like that is common in the prior art.

[0022] By comparing different recorded positions in relation to each other, it is possible to infer the aging of the cell capacity, and thus ultimately the remaining available active lithium and active material within the individual battery cells, similar to methods known from the prior art, but with less effort and less susceptibility to errors. Alternatively, instead of using the relative positions of the FHM points FHM1, FHM2, and FHM3, the relationship of the recorded values ​​to other characteristic points, such as a full charge point, could also be used to estimate the aging of the capacity of the individual battery cells.

Claims

Mercedes-Benz Group AG Patent claims 1. Method for estimating the cell capacity of a single battery cell, wherein the position of a peak (P1, P2, P3) on the charge axis (Q) and a value of its maximum on the differential voltage axis (dV / dQ) are determined using differential voltage analysis (DVA), characterized in that a position (FHM1, FHM2, FHM3) on the charge axis (Q) is determined which follows the maximum of the peak, and at which the differential voltage value corresponds to a predefined value which depends on the differential voltage value of the maximum of the peak (P1 , P2, P3), and which is predefined at a maximum of 50% of this value.

2. Method according to claim 1, characterized in that the differential stress value is specified at 50% of the differential stress value of the maximum of the peak.

3. Method for determining the aging of the cell capacity of a single battery cell using the method according to claim 1 or 2.

4. Method according to claim 3, characterized in that several positions (FHM1 , FHM2, FHM3) are determined, after which the positions of different measurements are compared in order to detect a relative change in position on the charge axis (Q) and to infer from this the aging of the cell capacity.

5. Method according to claim 3, characterized in that at least one position (FHM1, FHM2, FHM3) is determined in relation to a characteristic point on the charge axis (Q) in order to infer the aging of the capacity from the distance of the position (FHM1 , FHM2, FHM3) from this characteristic point.

6. Method according to claim 5, characterized in that the value of the full charge point of the individual battery cell on the charge axis (Q) is used as the characteristic point.

7. Vehicle (1) with at least a partially electric drive and with a computing unit (3) which is configured to perform one of the methods according to any one of claims 1 to 6.

8. Vehicle (1) with at least a partially electric drive and with a communication device (5) which is configured to establish a communication connection to a vehicle-external server (4) which is configured to carry out, alone or in cooperation with a computing unit (3) in the vehicle (1) one of the methods according to one of claims 1 to 6.

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