Method for determining aging of battery cell and method for controlling battery pack

Abstract

The invention relates to a method for determining the aging of cells of a lithium-ion battery. The method according to the invention is characterized in that the battery cells are actively charged or discharged by means of a power pulse, or the power pulse that charges or discharges the battery cells is detected during operation of the battery pack, the present current pulse and the voltage response of the battery cell are recorded in an idle phase with lower power or no power following the power pulse, and the relaxation of the battery cell is evaluated, at least the relaxation time constant (tau) being determined and stored, after which the relaxation time constant (tau) is determined and stored on the basis of the currently measured and already stored relaxation time constants (tau). During charging or discharging of a battery cell by means of a power pulse, a charging curve and a discharging curve are formed in each case with respect to the charge of the battery cell, the current capacity (K1) of the battery cell being inferred from the intersection of the charging curve and the discharging curve.

Description

[0001] The invention relates to a method for determining the aging of a battery cell of a lithium-ion battery. The invention also relates to a method for controlling a battery of the type defined in more detail in the preamble of claim 11.

[0002] Lithium-ion batteries, which are currently used as high-energy batteries for power supply in motor vehicles, are usually based on nickel-manganese-cobalt electrodes and graphite electrodes. Graphite electrodes in particular have aging effects that affect the performance of the entire battery pack or its individual cells. In order to be able to counteract these aging effects during the life cycle of the battery pack or at least prevent accelerated aging, corresponding indicators of such aging factors must be measured and correctly interpreted.

[0003] A commonly used analysis technique is the so-called differential voltage analysis (DVA). With the help of this technique, the uniformity of the lithium distribution in the battery pack can be determined. However, this technique requires full charging and full discharging with the smallest and most constant current possible. This is almost impossible in practical use, especially when used in motor vehicles, because charging is carried out as quickly as possible here, that is, with high currents, so charging is always carried out at the edge of the stability window, that is, just before the area where the anode coating appears. There is also a high dynamics in the discharge direction, which is often interrupted by short charging cycles, because very strong discharges often occur during acceleration, and when braking again, energy recovery (Rekuperieren), that is, recharging the battery pack, occurs. In this way, on the one hand, there is no constant load, and on the other hand, the current direction changes constantly. Therefore, these situations that occur in practice are not suitable for differential voltage analysis in principle. This increases the difficulty of identifying aging mechanisms during operation, and if they cannot be identified, they cannot be effectively counteracted.

[0004] In 2019, one of the inventors, Hust, Friedrich Emanuel, published a paper titled "Physico-chemically motivated parameterization and modelling of real-time capable lithium-ion battery models: a case study on the Tesla Model S battery" at Aachen University (DOI: 10.18154 / RWTH-2019-00249) presents a physicochemically driven, impedance-based, real-time lithium-ion model and parameterizes it based on a battery cell from a Tesla Model S. The relaxation behavior of the battery cell is investigated and found to be essentially caused by the negative electrode (i.e., the graphite electrode). The paper assumes that the relaxation is caused by the redistribution of lithium within the graphite electrode. Based on these findings, further studies can now be performed based on the lithium-ion battery model and further parameterized with other real cells.

[0005] Based on these basic findings, the object of the present invention is to provide a method for determining the aging of battery cells in lithium-ion batteries, which method is suitable for dynamic actual operation so that the operating strategy of the battery pack can be adjusted according to the identified aging behavior.

[0006] According to the invention, this object is achieved by a method for determining the aging of a battery cell of a lithium-ion battery having the features of claim 1. Advantageous embodiments and developments are given in the dependent claims. In addition, the object is also achieved by a method for controlling a battery pack having at least one battery cell whose aging has been determined having the features of claim 11. In this respect, advantageous embodiments also result from the associated dependent claims.

[0007] In the method according to the invention, a power pulse is used to actively charge or discharge a battery cell, or such a power pulse for charging or discharging a battery cell is detected during the operation of the battery pack. On the one hand, the method can be actively started by triggering a corresponding power pulse for charging and / or discharging at an appropriate time point (for example, in the case of a battery pack installed in a vehicle, when the battery pack is not currently needed). In practice, however, such power pulses may be more relevant when they occur during the normal operation of the battery pack (for example, when accelerating or braking suddenly). This leads to the discharge of the battery pack and its battery cells or the charging caused by energy recovery. Therefore, such power pulses during the normal dynamic operation of the battery pack can be easily detected and used for the method according to the invention. In the idle phase after such a power pulse, in addition to recording the current pulse that occurred before, the voltage response of the battery cell is also recorded. Among them, the idle phase can be a low-power phase or a power-free phase, for example, when the vehicle stops at a red light after an emergency brake.

[0008] Continuous recording of current and voltage is also particularly advantageous, wherein after a recognized power pulse and a subsequent idle phase, the recorded data can be evaluated in the sense described below. It is important here that according to the invention, the relaxation time constant is determined and stored from the voltage response. This is also known in principle from the paper on the prior art mentioned at the beginning, but is not used in the paper in the sense described here.

[0009] The determined and stored relaxation time constants can then be used to form a charging curve for the charge of the battery cell during charging or discharging, based on the relaxation time constants most recently stored and the relaxation time constants currently determined, and a discharge curve based on other data. The charging curve then forms an intersection with the discharge curve, from which the current capacity of the battery cell, in particular the graphite electrode, can be inferred. This makes it possible to create a first basis for determining the aging of the battery cell, which enables an operating method that handles aged battery cells with appropriate care, in order to prevent or at least minimize their aging process.

[0010] Preferably, this can be done based on the transition from one phase to another of the open-circuit voltage curve of lithium to graphite with respect to the capacity of the battery cell or its graphite electrode, which is determined by the intersection point, wherein the transition to the second phase can be detected particularly effectively. The current capacity of the battery cell can then be inferred from this. If the open-circuit voltage of lithium to graphite is plotted with respect to the capacity of the battery cell, a characteristic diagram is obtained, which is basically known from the prior art and is also discussed accordingly in the above-mentioned article, in particular from page 40. Therein, the different phases within the open-circuit voltage curve are characterized. The transition from one phase to another can be used to infer the current capacity of the battery cell.

[0011] The intersection point can be determined particularly effectively if, according to a very advantageous embodiment of the method according to the invention, the charge curve and the discharge curve are plotted using a logarithmic diagram of the relaxation time constant of the battery cell charge. Basically, two curves are then produced, namely the charge curve and the discharge curve, which can be well approximated as straight lines. These curves form a defined intersection point, from which the position of the transition, for example the transition to the second phase in the open-circuit voltage curve, can be inferred relatively accurately.

[0012] If these values ​​are now available, a model-based method can be used to calculate the current open-circuit voltage curve of lithium to graphite based on the current capacity of the battery cell from a given open-circuit voltage curve of the battery cell at the beginning of its life. To this end, according to a very advantageous development of the method according to the invention, the current open-circuit voltage curve is achieved in particular by compressing a given initial curve at the beginning of the life of the battery cell in the direction of the capacity. In particular, the compression can be carried out until the transition of the compressed initial curve to the second phase coincides with the corresponding transition of the current open-circuit voltage curve to the second phase. This transition can thus be used as a reference point to obtain the current open-circuit voltage curve from the essentially known initial curve of the battery cell or its graphite electrode. As a result, the current open-circuit voltage curve can be modeled with relatively high accuracy based on simple measurements that can also be carried out during operation of the battery pack (for example in a vehicle).

[0013] Furthermore, the current and stored relaxation time constants are then correlated with the simulated current voltage curve of lithium on graphite with respect to the state of charge of the battery cell. Typically, the maximum occurs in the transition region between the individual phases, i.e. at the transition from phase 2 to phase 1, and possibly at the transition from phase 3L to phase 4L.

[0014] In the method according to the invention, the relaxation time constant itself can be determined directly or indirectly from the minimum voltage, the minimum charge conversion, the shortest time required for the battery cell to reach a substantially constant voltage in time, and / or from the gradient, i.e. the slope, of the voltage response of the battery cell to a power pulse, by means of a predetermined correlation of the gradient with the relaxation time constant, which can be stored in particular in a lookup table. The actual measurement of the shortest time is of course the preferred solution here, but requires relatively long idle phases. Therefore, it is possible to use mainly the slope of the voltage response of the battery cell to the power pulse. The various voltages that have occurred can be assigned to specific relaxation time constants based on historically known correlations, so that, for example, even in the case of very short idle times, a fairly reliable statement about the relaxation time constant can be made by means of the value stored in the table, with reference to the slope.

[0015] The situation is that, according to a very advantageous development of the method according to the invention, the currently determined relaxation time constant is compared with a previously determined and stored relaxation time constant, wherein an increase in the value of the relaxation time constant indicates that the formation of a covering layer on the battery cell or its graphite electrode has become more intense. The formation of such a covering layer restricts the accessibility of the active material to a certain extent and also the equilibrium of the lithium ions, so that the relaxation time increases accordingly. Therefore, an increase in the relaxation time constant over several determined values ​​clearly indicates that a covering layer on the battery cell or its graphite electrode has formed more intensely, whereby a first specific aging mechanism is correspondingly identified.

[0016] According to another very advantageous design, it can be proposed that, in particular in the current open circuit voltage curve of lithium to graphite with respect to the cell capacity, an increased loss of anode active material can be inferred from a shift of the relaxation time constant in the direction of lower capacity. This shift, in particular in the current open circuit voltage curve diagram in the direction of lower capacity, i.e. a shift to the left in the x direction in such a diagram, indicates an increased loss of anode active material. This second specific aging mechanism is also referred to as LAAM (Loss of Active Anode Material).

[0017] Another very advantageous design of the method according to the invention also realizes that, as an alternative or supplement to the first two aging mechanisms, an increase in lithium loss can be inferred based on the reduction in the distance between the current relaxation time constant and the last stored relaxation time constant with respect to the capacity compared to the distance between the previously stored pairs of relaxation time constants, and here in particular in the current open circuit voltage curve of lithium to graphite with respect to the battery cell capacity. Therefore, when the distance between the individual relaxation time constants in the figure becomes smaller, an increase in lithium loss can be identified, and the loss of lithium is also called LLi (Loss of Lithium). This means that the relaxation time constants are closer to each other with respect to the capacity axis. The third aging mechanism can thus be specifically identified.

[0018] All three mechanisms described can now be determined together or independently of one another in the model-based evaluation of the measured relaxation time constants. Each aging mechanism has a different effect on the relaxation time constant, so that all three aging mechanisms can be read from the measured values. If the specific aging mechanism present in the battery cells or their graphite electrodes is now known, this aging mechanism can now be counteracted in a very targeted manner. For example, the power supplied to and taken from the battery pack during charging and discharging can be adjusted during operation, or the cooling or temperature control of the battery can be changed in order to operate the battery cells accordingly.

[0019] With reference to the current open circuit voltage curve of the battery cell obtained based on the model analysis, the method for controlling the power supplied to and taken from the battery pack according to the present invention can be adjusted as follows, that is, to avoid the charging and discharging of each battery cell exceeding the stage limit of the open circuit voltage curve as much as possible. Whenever possible, charging and discharging should be carried out within one of the stages, and only in special cases should the stage be exceeded accordingly. In this way, the aging process of the battery cell, especially its graphite electrode, can be correspondingly prevented or minimized.

[0020] Further advantageous embodiments of the two methods according to the invention are also given in the exemplary embodiments and are explained in more detail below with reference to the drawings.

[0021] in:

[0022] Figure 1 A schematic diagram showing a method flow diagram of a possible embodiment of the method according to the present invention;

[0023] Figure 2 A diagram showing the correlation between charge and relaxation time constant;

[0024] Figure 3 A basic open circuit voltage diagram of a lithiated graphite electrode is shown;

[0025] Figure 4 Shown according to Figure 3 An open circuit voltage diagram, wherein an open circuit voltage curve of an aged battery cell is additionally plotted and a relaxation time constant detected and stored with the formation of a cover layer is additionally plotted;

[0026] Figure 5 Shown according to Figure 3 Graph of open circuit voltage, wherein additionally plotted is the open circuit voltage curve of an aged battery cell and additionally plotted is the relaxation time constant detected and stored in the case of loss of anode active material (LAAM);

[0027] Figure 6 Shown according to Figure 3 Graph of open circuit voltage, wherein additionally plotted is the open circuit voltage curve of an aged battery cell and additionally plotted is the relaxation time constant detected and stored in the case of lithium loss (LLi);

[0028] Figure 7 shows the method for executing Figure 1 Schematic representation of the buffer concept of the method.

[0029] exist Figure 1 In the diagram, a possible method flow for determining the aging of battery cells and changes to the battery pack operating method based thereon are schematically shown. In the box marked 100, for example, a strong current consumption, i.e. a power pulse, is detected in the battery pack or at least one of its battery cells. Now, in box 200, the current pulse and the voltage response of the battery cell are recorded. In a subsequent phase with less or no power extraction or power supply, referred to here as the idle phase, the relaxation of the battery cell is evaluated. Then, in step 400, the relaxation voltage and the relaxation time constant are calculated, as is known in principle from the paper mentioned at the beginning. Now, in step 500, a correlation is established between the charge C and different relaxation time constants τ, which have been detected and stored and / or determined in real time. This will be described in detail below. Figure 2 This is discussed in more detail in the context of .

[0030] Now, in step 600, the aging can optionally be determined based on a model, wherein different aging mechanisms such as overburden formation, LAAM and / or LLi can be detected and are therefore particularly advantageous. This will be discussed in detail in the following sections. Figures 4 to 6 In step 700, which is also optional, these identified aging mechanisms can now be used to select a correspondingly adapted operating strategy for the battery pack, i.e. to avoid certain operating states and to operate the battery pack as gently as possible with respect to charging, discharging and temperature control of the battery pack by means of a cooling medium, in order to prevent gradual aging or at least slow down its progression.

[0031] In step 400, calculated relaxation time constants τ, which occur, for example, in a power pulse for charging or discharging, are calculated and stored. Figure 2 The individual relaxation time constants τ are plotted over the charge C, whereby, in the case of a logarithmic representation of the relaxation time constant T, a straight line is formed along the black points for the charging cycle and a straight line is formed along the white points for the discharging cycle. The intersection of these two straight lines then allows the capacity K present at the current time of the battery cell or its graphite electrode (anode), which essentially determines the aging mechanism, to be inferred from the charge C1 thus determined.

[0032] Figure 3 A schematic open circuit voltage curve R0 of lithium on graphite is shown for the capacity K of a battery cell or its graphite electrode. Here, the open circuit voltage V is plotted on the y-axis and the cell capacity K of the lithiated graphite electrode is shown on the x-axis, with the standard being stage 3L or stage 4L (see below). Figure 3 The open circuit voltage curve R0 in is the open circuit voltage curve of a new battery cell or graphite electrode. The initial capacity of the battery cell or its graphite electrode at the beginning of its life is denoted as K0. The open circuit voltage at the initial capacity K0 is zero, indicating a fully charged state, where the discharge direction E of the entire battery cell is shown from right to left. The open circuit voltage curve R0 has characterized stages, also called stages. These stages are marked from right to left along the capacity as stage 1, stage 2-1, stage 2L-2, stage 3L / 4L.

[0033] Now, the open-circuit voltage curve R1 with the current capacity K1 can be modeled into the open-circuit voltage curve R0 with the known initial capacity K0. The open-circuit voltage curve R1 with the current capacity K1 is generated by correspondingly compressing the initial curve with the initial capacity K0 in the direction of the capacity K. Therefore, the aged capacity K1 will be lower than the initial capacity K0, wherein different situations can now be read at different positions of these open-circuit voltage curves R0, R1.

[0034] It is scientifically known that the loss of anode active material (LAAM) causes the capacity K to shrink starting from the right side, so that a fully lithiated graphite electrode can be reached faster. On the left side, the loss of lithium (LLi) leads to a limited capacity K. Below we will discuss how these two effects affect the relaxation time constant τ, and also the impact on the formation of the capping layer.

[0035] To this end, Figures 4 to 6 In the figure, two schematic open circuit voltage curves R0 and R1 of the lithiated graphite electrode can be seen. The unaged open circuit voltage curve R0 is shown as a dotted line (similar to Figure 3), the aged open circuit voltage curve R1 is shown as a solid line. Here, the y-axis represents the relaxation time constant τ in seconds and the open circuit voltage V to lithium. The x-axis represents the capacity K again, based on the stage 3L / 4L. In the case of the aged graphite electrode, the position of the open circuit voltage curve R1 comes from the Figure 2 The current charge C1 and the resulting aging capacity are explained in the diagram.

[0036] exist Figure 4 In the graph, in addition to the open circuit voltage curves R0 and R1 of the new and old graphite electrodes, the relaxation time constant τ is also shown. The relaxation time constant τ of the new graphite electrode is represented by a dotted line extending from zero to the corresponding relaxation time constant τ, similar to the representation method of the open circuit voltage curve R0, while the relaxation time constant of the aged battery cell is represented by a solid line.

[0037] from Figure 4 As can be seen from the diagram of , the relaxation time constant τ of the aged graphite electrode exhibits higher values ​​than the relaxation time constant of the new graphite electrode at the corresponding position. This increase in the relaxation time constant τ indicates an increase in the formation of a coating as one of the aging mechanisms in the graphite electrode.

[0038] exist Figure 5 In, with Figure 4 The diagram in is similar, showing the effect of LAAM on the relaxation time constant τ. As the capacity K caused by LAAM decreases, the relaxation time constant τ shifts to a lower state of charge (relative to the initial capacity). The relaxation time constant τ is again represented by the data points. Among them, the dashed data points are the original (fresh) state, and the solid data points represent the relaxation time constant τ that has shifted due to aging.

[0039] exist Figure 6 In the previous Figure 4 and Figure 5 The diagram in Figure 1 is similar and shows the influence of LLi on the relaxation constant τ. The lithium loss increases the slope of the increase of the relaxation time constant τ. If the relaxation constant τ is now determined at multiple points during the discharge process, the extent of the lithium loss can be deduced from the slope of the change in the time constant. The relaxation time constant τ detected in this group for a new graphite electrode is again shown with a dotted line, and the situation for an aged graphite electrode is shown with a solid line. Due to aging, the distance between the values ​​within the group decreases, and the group is pushed together as if.

[0040] Therefore, the three attached Figure 4 , Figure 5 and Figure 6The mechanisms shown can be easily and automatically identified in the modeled data based on the measured values. In this case, all three mechanisms are usually superimposed on one another, but this is not a problem because the three aging mechanisms have very different effects on the relaxation time constant τ, so that even in the case of a corresponding superposition, these mechanisms can be read out easily and efficiently from the available data.

[0041] Once in Figure 1 If a high current consumption is detected in the method flow (block 100), the current value through the battery pack or battery cell and the voltage value of the battery cell are recorded 200. Advantageously, this recording 200 is carried out continuously and the recording dynamics are high. This can be Figure 7 200 is completed in a first buffer 210 with high dynamics, schematically shown in FIG. 200 . After the detection of an event suitable for evaluation based on a power pulse and a subsequent idle phase, the recorded data is transferred from the first buffer 210 to the second buffer 220 and stored there as a copy 221. The recording 200 is then continued in the second buffer 220 with lower dynamics, which is shown here by the area 222. This also allows retrospective evaluation of current pulses and a lower memory consumption for periods of lower dynamics (relaxation periods).

[0042] The recording is continued until a termination criterion is reached. This criterion can be, for example, the smallest voltage change per unit time, the smallest charge transition (only higher transitions are significant for inhomogeneities) or the shortest time.

[0043] The relaxation voltage and the relaxation time constant τ can then be calculated. The relaxation time constant τ can then be entered into a table that, for example, relates charge C, state of charge SOC or capacity K to relaxation time constant τ. This can be done for both discharge pulses and charging pulses. With the help of several of the relaxation time constants τ, the behavior of the discharge pulse and the charging pulse can then be correlated (for example, using regression analysis) to find the intersection between the maximum relaxation time constants τ in the discharge direction and in the charging direction. ( Figure 2 ) This is where the transition point is and can be used to evaluate LLi and LAAM. This can be done in Phase 3L / 4L as well as Phase 2 and Phase 1 to quantify the development of different aging effects.

[0044] With the aid of this quantification, operating limits and operating strategies can now be adjusted in step 700 of the method. For example, aging-related charging or discharging current limits can be adjusted, or the operating strategy can be adjusted to use a medium operating window. It is also conceivable that a higher requirement for cooling or heating is derived from the calculated aging variable and the thermal strategy is adjusted accordingly.

[0045] This method is particularly useful if the situation of the entire fleet is aggregated and evaluated in a back-end server. In addition to the conventional evaluation criteria (capacity, resistance), the uniformity of lithium distribution at different stages can also be evaluated.

Claims

1. A method for determining the aging of a battery cell of a lithium-ion battery pack, It is characterized in that The battery cell is actively charged or discharged using power pulses, or such power pulses for charging or discharging the battery cell are detected during operation of the battery pack, wherein the current pulses occurring and the voltage response of the battery cell are recorded in an idle phase with low power or no power after the power pulse, and the relaxation of the battery cell is evaluated, wherein at least a relaxation time constant (τ) is determined and stored, and then a charging curve and a discharging curve are formed respectively with respect to the charge of the battery cell when the battery cell is charged or discharged by the power pulse, wherein the current capacity (K1) of the battery cell is inferred from the intersection point of the charging curve and the discharging curve.

2. The method according to claim 1, It is characterized in that The current capacity (K1) of the battery cell is inferred from the transition from one phase to another phase determined by the intersection point on the open circuit voltage curve (R0, R1) of lithium to graphite in relation to the capacity (K) of the battery cell.

3. The method according to claim 1 or 2, It is characterized in that The charge curve and the discharge curve are formed by a logarithmic plot of the relaxation time constant (τ) with respect to the charge of the battery cell.

4. The method according to claim 2 or 3, It is characterized in that According to the current capacity (K1) of the battery cell, the current open circuit voltage curve (R1) of lithium to graphite with respect to the capacity (K) is calculated according to the given open circuit voltage curve (R0) of the battery cell at the beginning of its life.

5. The method according to claim 4, It is characterized in that The current open circuit voltage curve (R1) is determined by compressing the given initial curve (R0) in the direction of the capacity (K) until a specific transition between two phases thereof coincides with a corresponding transition of the current open circuit voltage curve (R0).

6. The method according to claim 4 or 5, It is characterized in that The current and stored relaxation time constants (τ) are associated with the current open circuit voltage curve (R0).

7. The method according to any one of claims 1 to 6, It is characterized in that The relaxation time constant (τ) is determined directly or indirectly based on a minimum voltage, a minimum charge conversion, a shortest time required for the battery cell to reach a voltage that is essentially constant over time, and / or based on a gradient of the voltage response of the battery cell by means of a predetermined correlation of the gradient with the relaxation time constant (τ), in particular from a table.

8. The method according to any one of claims 1 to 7, It is characterized in that The currently determined relaxation time constant (τ) is compared with a previously determined and stored relaxation time constant (τ), wherein an increase in the value of the relaxation time constant (τ) indicates that the formation of a coating in the battery cell has become more intense.

9. The method according to any one of claims 1 to 8, It is characterized in that From the shift of the relaxation time constant (τ) towards lower capacities (K), in particular in the present open circuit voltage curve (R1) of lithium versus graphite with respect to the cell capacity (K), an increased loss of anode active material (LAAM) is inferred.

10. The method according to any one of claims 1 to 9, It is characterized in that Based on the fact that the distance between the current relaxation time constant (τ) and the last stored relaxation time constant (τ) is reduced compared to the distance between the previously stored pairs of relaxation time constants (τ) with respect to the capacity (K), in particular in the current open circuit voltage curve (R1) of lithium to graphite with respect to the battery cell capacity (K), it is inferred that the lithium loss (LLi) is increased.

11. A method for controlling a battery having at least one battery cell, for which aging is determined according to the method of any one of claims 1 to 10, It is characterized in that Power supplied to and drawn from the battery pack during charging and discharging is adjusted on the fly to counteract or at least not exacerbate the identified aging mechanisms.

12. The method according to claim 11, It is characterized in that It is to be avoided as far as possible that the charging and discharging of the individual cells exceeds their step limits in the open circuit voltage curve of lithium to graphite in relation to the cell capacity.

13. The method according to claim 11 or 12, It is characterized in that During operation, thermal management of the battery pack is adjusted in order to counteract or at least not intensify the identified aging mechanisms.

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