Controlling an HPC method, and HPC method for a battery cell

Abstract

A method for controlling an HPC method for a battery cell is proposed in which the battery cell is cycled between two controllable voltage values V1, V2, wherein two target values for the open-circuit voltage Vmin, Vmax of the battery cell are ascertained for the control process. The method is characterized at least by the following steps: - (S1) switching off the current when the respective voltage value V1, V2 is reached; - (S2) determining the respective open-circuit voltage on the basis of the respective decay behavior (41) of the voltage induced by the process of switching off the current; and - (S3) controlling the voltage values V1, V2, the respective value of the voltage difference ΔV1, ΔV2 (42) between the respective determined open-circuit voltage and the corresponding target value Vmin, Vmax being used as a control difference. The invention also relates to an HPC method for a battery cell.

Description

[0001] Description

[0002] Control of an HPC process and HPC process for a battery cell

[0003] The invention relates to a method according to the preamble of patent claim 1 and a method according to the preamble of patent claim 9.

[0004] Battery cells can be characterized using various parameters. However, these parameters vary significantly depending on the cell type, operating conditions, and production quality. Important parameters include the service life and self-discharge rate of a battery cell.

[0005] The lifetime of a battery cell indicates the rate of loss of usable capacity over time and / or over operating cycles.

[0006] The self-discharge rate of a battery cell indicates the rate at which the energy stored in the cell is lost without this energy being used externally.

[0007] Typically, service life and self-discharge rate only lead to measurable changes over relatively long timescales. For practical investigations and evaluations of battery cells, however, it is important to determine these parameters within the shortest possible period.

[0008] To reduce the test duration for service life and self-discharge measurements, the state of the art uses accelerated tests conducted at higher temperatures and, if necessary, higher currents. This can provoke accelerated aging and self-discharge of the battery cell. However, these test conditions do not represent the behavior of the battery cell under typical and therefore relevant operating conditions.

[0009] Other known methods for determining the aforementioned parameters use a higher level of accuracy in order to detect small changes even on short time scales. One such method is high-precision coulometry (HPC), which can be used to very precisely record the amount of charge removed from and / or added to the battery cell. The HPC method takes advantage of the fact that a battery cell's state of charge (SOC) is related to its open-circuit voltage. The HPC method cycles between two defined states of charge, with the defined states of charge being related to corresponding fixed open-circuit voltages. The HPC method measures the amount of charge during the individual charging and discharging cycles. This allows the coulombic efficiency (CE) and / or small capacity losses to be recorded within a few cycles.This ultimately provides information about the self-discharge rate and the service life of the battery cell.

[0010] A disadvantage of known HPC methods is that the currents used in the measurement can lead to kinetic effects within the battery cell. As a result, the voltage measured at the battery cell differs from its resting voltage. This difference between the measured voltage and the resting voltage of the battery cell is referred to as overvoltage. The overvoltage can change from cycle to cycle within the HPC method, thus violating the HPC method's requirement that the test cycles be conducted between constant states of charge.

[0011] The present invention is based on the object of providing an improved HPC process, in particular by an improved control of an HPC process.

[0012] The object is achieved by a method having the features of independent patent claim 1 and by a method having the features of independent patent claim 9. Advantageous embodiments and further developments of the invention are specified in the dependent patent claims.

[0013] In the method according to the invention for controlling an HPC process for a battery cell, the battery cell is controlled between two controllable voltage values ​​V lt V2cycled, with two setpoints of the rest voltage V min , V max of the battery cell. The method according to the invention is characterized by the following steps:

[0014] - Switching off the current when the respective voltage value V is reached lt V2;

[0015] - Determining the respective rest voltage based on a respective decay behavior of the voltage induced by switching off the current; and

[0016] - Control of voltage values ​​V lt V2, where the respective amount of the voltage difference AVi, AV2between the respective determined rest voltage and its corresponding setpoint 7 min , nax is used as the control difference. Without limiting the scope of protection, V1> V2 is assumed below. In other words, according to the method, the voltage is cycled between an upper voltage value Vj and a lower voltage value V2. Here, the voltage values ​​V lt V2 adjustable, which means that these can change over time due to the regulation.

[0017] Furthermore, two target values ​​for the battery cell are specified with regard to its rest voltage V min , l{ nax The specified rest voltages can be provided for the process. The specified rest voltages V min , form the setpoints of the control. In other words, the control system according to the invention ensures that the open-circuit voltage determined from the decay behavior is adjusted to the specified open-circuit voltages V min , V max be adjusted.

[0018] In a first step of the process, the current is measured when the respective voltage value V lt V2 switched off.

[0019] In other words, the battery cell is de-energized, at least for a period of time, starting at the specified voltage values. Thus, when the voltage limits Vj and / or V2 are reached, a current-free pause is initiated, during which no current flows through the battery cell. However, the voltage is preferably still recorded or measured during these current-free periods.

[0020] According to a second step of the method, the respective rest voltage is determined based on a respective decay behavior of the voltage induced by switching off the current.

[0021] In other words, the current is reduced when the voltage limits V lt V2 is switched off, which induces a voltage decay. The respective decay behavior is related to the respective open-circuit voltage. This is the case because if the current is switched off for a sufficiently long time, i.e., if the current is not applied for a sufficiently long time, the voltage will change from the respective voltage value V ltV2 to the respective resting voltage would decay. In other words, the respective resting voltage is reached asymptotically. In this case, it is therefore not necessary to actually wait until the respective resting voltage is reached, but the respective resting voltage is already determined from the recorded decay behavior. However, a longer wait until the actual resting voltages are reached can also be provided. In other words, the overvoltage dissipates within the pause (current-free time period) so that the resting voltage of the battery cell would be established after a sufficiently long waiting period. This means that the resting voltage of the individual battery cell can be measured precisely in its current state and it is not necessary to estimate it using further measurement parameters and / or models. According to the invention, it is already sufficient to determine the respective resting voltage from the decay behavior of the respective overvoltage.According to the invention, it is therefore not necessary to wait until the overvoltage has almost completely subsided.

[0022] In a third step of the process, the voltage values ​​are regulated V2. Here, the respective amount of the voltage difference AI^, AV2between the respective determined rest voltage and its corresponding setpoint V min , V max used as control difference.

[0023] For the upper voltage value V1, this means that the amount of the voltage difference AI^ between the rest voltage determined from the decay behavior and the setpoint V max is used as a control difference for the control of the voltage value 14.

[0024] For the lower voltage value V2, this means analogously that the amount of the voltage difference AV2between the rest voltage determined from the decay behavior and the setpoint V minis used as the control difference for the control of the voltage value V2.

[0025] During control, the respective control deviation is minimized as much as possible. In other words, the optimal state to be achieved is characterized by AI = 0 or AV2 = 0. In this state, the respective determined open-circuit voltage agrees with the respective setpoints of the open-circuit voltage setpoint V min , V max essentially the same. In other words, according to the regulation, the voltage values ​​V lt V2 is controlled in such a way that the overvoltage corresponding to the respective setpoint is achieved. In this sense, the overvoltages corresponding to the setpoints of the quiescent voltages are determined and adjusted as accurately as possible by the control system.

[0026] In other words, the voltage differences AI^ , AV2 are used for adjustment or as a control variable for the HPC method in order to set more precise overvoltages in the following cycles and thus minimize the respective error AI , AV2 as much as possible. Known HPC methods assume that the internal resistance or the overvoltage can be determined sufficiently accurately and always up to date using existing models in combination with measurement data available during the test. However, measurement inaccuracies of electrical and thermal parameters can, on the one hand, falsify the determination of the internal resistance. On the other hand, fluctuations in cell quality and aging of the battery cell can change the resistance models assumed for the calculation and thus indirectly lead to incorrect resistance values.

[0027] If fluctuations in A1 and AV2 occur during the test cycles due to the aforementioned errors, the charge states at the limits of the test cycle no longer have a fixed relationship to each other. However, this violates the basic prerequisite for evaluating the charge quantities within the framework of known HPC methods. However, the method according to the invention still enables a precise and consistent method.

[0028] This is the case because the method according to the invention, which particularly uses a rest phase without power supply, determines the rest voltage at the current time and individually for each battery cell used. Therefore, this method is independent of the error influences described above, and no models of the respective battery cell type are required.

[0029] Furthermore, the method according to the invention enables control of the HPC method during ongoing test operation. Furthermore, a predefined operating point can be set more precisely and individually for the cell being tested and the operating conditions used.

[0030] In the HPC method according to the invention for a battery cell, at least one lifetime of the battery cell is determined. The method according to the invention is characterized in that the HPC method is controlled by a method according to the present invention and / or one of its embodiments.

[0031] Similar, equivalent, and equally effective advantages and / or embodiments of the HPC method according to the invention result from the control method according to the invention. According to an advantageous embodiment of the invention, the voltage values ​​according to Vj = V max + G1, V2= Knin > G 2, where G l tG2 can be used as control variables.

[0032] This advantageously allows for an asymmetry between the overvoltages during charging or discharging within the cycles to be taken into account.

[0033] Are the set values ​​of the rest voltages V max , V min the overvoltages rj1, rj2, then A1 = 0 or A72 = 0 corresponds to the case that the respective reference variable corresponds to the respective overvoltage.

[0034] In an advantageous development of the invention, the control further takes into account a temperature and / or a current intensity.

[0035] In other words, the control system takes into account that temperature and / or current can change over time. A new operating point can be set prior to the actual control process, for example, if the temperature, current, and / or the setpoints of the open-circuit voltage change. The initial voltage values ​​can be calculated in advance and used as a starting point for the control system according to the invention.

[0036] According to a particularly preferred embodiment of the invention, the current is not switched off if the control difference is below a specified threshold value.

[0037] This advantageously shortens the test time, as fewer current-free periods are required in this case. In other words, if the control deviation is below the threshold value, there is no need to measure the open-circuit voltage, as this already corresponds sufficiently accurately to its respective setpoint.

[0038] In an advantageous development of the invention, the rest voltages are each determined by means of a fit of the decay behavior.

[0039] In other words, the voltage curve after the current is switched off is recorded for at least a specified time range and fitted using a fit function. From the fit, the open-circuit voltage can be determined without actually having reached it in the measurement. In other words, the decay behavior is advantageously sufficient to determine the asymptotic value of the open-circuit voltage using the fit (curve adjustment). This advantageously eliminates the need to wait too long, so that the time range of the current-free phase can be set as small as possible. In particular, the current or current intensity is switched off for 1 to 60 seconds, 1 to 30 seconds, or 1 to 10 seconds.

[0040] According to an advantageous embodiment of the invention, F(V, A|t) = V ocv + ( - O cv)exp (-At) is used as a fitting function, where V Ocv denotes the respective rest voltage to be determined and v, A are further fitting parameters.

[0041] This advantageously enables a particularly accurate determination of the actual or the rest voltage to be determined V ocv enabled.

[0042] In an advantageous development of the invention, the current is switched off over a time range of 1 to 60 seconds, 1 to 30 seconds or 1 to 10 seconds.

[0043] The time ranges mentioned are advantageous because they allow a sufficiently accurate fit of the decay behavior and prevent excessive waiting. This avoids unnecessarily lengthening the process, especially an HPC process, but rather achieves a synergistic optimum between accuracy and time.

[0044] Furthermore, it is advantageous to define the current-free time range or its duration as a function of a decay constant of the decay behavior. For example, if the fit function F(V,A|t) = V O cv + (v - Vocv) ex If P (-At) is used, the fit parameter A corresponds to the aforementioned decay constant. The decay constant corresponds to a characteristic decay time 1 / A. It is therefore advantageous to define the current-free time range, or its duration, as a function of 1 / A.

[0045] According to an advantageous embodiment of the invention, the battery cell is designed as a lithium-ion battery cell.

[0046] The method according to the invention is advantageous for lithium-ion battery cells because they typically exhibit favorable decay behavior. In an advantageous development of the invention, self-discharge of the battery cell is further determined within the framework of the HPC method.

[0047] Advantageously, this allows the self-discharge of the battery cell to be determined more accurately. Self-discharge can be determined based on the duration of one or more cycles and the respective charge throughput determined using the HPC method.

[0048] Further advantages, features, and details of the invention will become apparent from the exemplary embodiments described below and from the drawings. The drawings schematically show:

[0049] Figure 1 is a flowchart of a method according to an embodiment of the invention; and

[0050] Figure 2 shows a voltage-time diagram;

[0051] Elements of the same type, value or effect may be provided with the same reference symbols in one or more of the figures.

[0052] Figure 1 shows a flowchart for controlling an HPC method for a battery cell according to an embodiment of the present invention.

[0053] In an HPC process, a charge throughput is determined several times in succession within time cycles. In other words, the battery cell is switched between two controllable voltage values ​​V lt V2 cycled. For charging or discharging the battery cell, corresponding voltage values ​​V lt V2 is set or regulated. For example, the battery cell is charged to the first voltage value 7-L and / or discharged to the second voltage value V2. The difference between the voltage values ​​V ltV2 associated charges form the respective recorded charge throughput AQ. From the recorded charge throughputs or charge quantities, the service life and / or the self-discharge rate of the battery cell can be determined. Furthermore, two setpoints for the rest voltage V are required for the control. min , V max of the battery cell.

[0054] According to a first step S1 of the method, the current or the current intensity is increased when the respective voltage value V ltV2 is switched off. Here, the current is switched off for a specified time range, for example, for 1 to 60 seconds, in particular for 1 to 30 seconds. These pauses in the current intensity are symbolized in Figure 2 below by the reference numeral 40. In other words, the battery cell is charged until the first voltage value V1 is reached. For this purpose, the voltage can be recorded at discrete time intervals, quasi-continuously, and / or continuously. By switching off the current or the current intensity in the time ranges 40, a decay behavior 41 of the voltage is induced. In other words, the voltage decreases asymptotically to the resting voltage of the battery cell corresponding to the state of charge.

[0055] In a second step S2 of the method, the respective rest voltage is determined based on the respective decay behavior 41 of the voltage induced by the current being switched off. For this purpose, it is not necessary to wait until the rest voltage is reached; rather, it can be determined from the decay behavior 41 by a fit. This allows the current-free time period 40 to be as short as possible or to be shortened. The difference between the voltage values ​​and the respective rest voltages forms the so-called respective overvoltage. The respective overvoltages are marked in Figure 2 with the symbol rj2 and rj2, respectively.

[0056] Furthermore, voltage differences A1 , AV2 between the fixed rest voltages V max or V minand the actual determined rest voltages Vocv,max> ^ocv,min, which were determined by means of the decay behavior 41. In other words, V O cv,min = ™ -2and V O cv,max = nax - AVi-

[0057] According to a third step S3 of the method, the voltage values ​​I , V2 are regulated. Here, the respective amount of the voltage difference AI , AV242 between the respective determined rest voltage and its associated setpoint V min , used as the control difference for the control. In other words, G lt G2 is used as a reference variable for the control. These reference variables are identified in Figure 2 with the reference numeral 43. By using the voltage differences A1, AV242 as the control deviation, these are minimized as much as possible within the control system. As a result, the reference variables G l t G2 is adjusted in such a way that it corresponds to the setpoints V mincorresponding to the respective overvoltages. In other words, the overvoltages are adjusted in such a way that the respective target values ​​for the open-circuit voltage are essentially achieved.

[0058] In Figure 2, the process already described in Figure 1 is symbolically represented by means of a voltage-time diagram.

[0059] Time is plotted in arbitrary units along the abscissa (100) of the diagram. The measured voltage is plotted in arbitrary units along the ordinate (101).

[0060] Furthermore, Figure 2 shows the detected voltage in the form of a voltage curve 10. Accordingly, the actual resting voltage of the battery cell is also shown as a temporal resting voltage curve 11 (dashed line).

[0061] The battery cell is initially charged up to a controllable voltage value or a controllable voltage limit 1. Subsequently, the current or the current intensity is switched off within the time range 40, whereby the voltage exhibits a decay behavior 41. However, due to an overvoltage, the voltage V ± does not correspond to the open-circuit voltage of the battery cell. In other words, a voltage drop occurs between the specified open-circuit voltage V max and the actual resting voltage, a voltage difference AI occurs. Similarly, when the battery cell is discharged, a voltage difference AV2 is formed between the specified resting voltage V due to an overvoltage rj2. min and the actual open-circuit voltage. The voltage differences AV1, AV2 are designated by reference numeral 42 in Figure 2. The overvoltages rj1, rj2 are designated by reference numeral 44 in Figure 2.

[0062] From Figure 2 it is clear that in the example shown the achieved overvoltage 44 is still too low, so that the first setpoint of the rest voltage l{ nax has not yet been reached. Therefore, the voltage difference AI^ 42 exists. Similarly, the overvoltage rj244 is still too low, since the second setpoint of the rest voltage V min has not yet been reached. Therefore, the voltage difference AV242 exists. According to the regulation, the amounts of the voltage differences or AF242 as the control difference. This regulates the overvoltages 43 in such a way that the respective setpoints of the rest voltages V max and V min can be achieved, which means that the rest voltage recorded by means of the respective decay behavior is essentially equal to V max or V minThis advantageously adjusts the HPC method to the setpoints of the open-circuit voltage more precisely with each cycle. Sufficient accuracy is achieved, for example, when the control deviation is below a specified threshold. If the specified threshold is reached after one cycle, a current-free time period 40 can be omitted for the subsequent cycle. This allows the test time to be shortened without reducing the accuracy of the method.

[0063] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0064] List of reference symbols

[0065] S1 first step

[0066] S2 second step S3 third step

[0067] 10 Voltage curve

[0068] 11 Rest voltage curve

[0069] 12 Open-circuit voltage characteristic 40 Switching off the current

[0070] 41 Decay behavior

[0071] 42 voltage differences

[0072] 43 Management variable

[0073] 44 Overvoltage

[0074] 100 abscissa

[0075] 101 Ordinate

Claims

Patent claims 1. Method for controlling an H PC process for a battery cell, in which the battery cell is controlled between two controllable voltage values ​​V lt V2 is cyclized, with two setpoints of the rest voltage V min , V max of the battery cell, characterized by the following steps: - (S1) Switching off the current when the respective voltage value V is reached lt V2; - (S2) determining the respective rest voltage based on a respective decay behavior (41) of the voltage induced by the switching off of the current; and - (S3) Control of voltage values ​​V lt V2, where the respective amount of the voltage difference Al^, AV2(42) between the respective determined rest voltage and its corresponding setpoint V min , V max is used as a control difference.

2. Method according to claim 1, characterized in that for the control the voltage values ​​according to Vj = V max + G lt V2= V min - G2 can be modelled, where G lt G2 can be used as reference variables (43) for the control.

3. Method according to claim 1 or 2, characterized in that the control further takes into account a temperature and / or a current intensity.

4. Method according to one of the preceding claims, characterized in that the current is not switched off if the control difference is below a predetermined threshold value.

5. Method according to one of the preceding claims, characterized in that the rest voltages are each determined by means of a fit of the decay behavior (41).

6. Method according to claim 5, characterized in that F(V,A|t) = V ocv + ( - Vocv) exP (- t) is used as a fitting function, where V O cv denotes the respective rest voltage to be determined and v, A are further fitting parameters.

7. Method according to one of the preceding claims, characterized in that the current is switched off over a time range of 1 to 60 seconds, 1 to 30 seconds or 1 to 10 seconds.

8. Method according to one of the preceding claims, characterized in that the battery cell is designed as a lithium-ion battery cell.

9. HPC method for a battery cell, in which at least one service life of the battery cell is determined, characterized in that the HPC method is controlled by a method according to one of the preceding claims.

10. HPC method according to claim 9, characterized in that a self-discharge of the battery cell is also determined.

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