Method for determining a charge throughput of a battery cell

Abstract

The invention relates to a method for determining at least one charge throughput ΔQ' of a battery cell, in particular in the context of an HPC method, in which a charge difference ΔQ associated with two ascertained voltage values V 1, V 2 is detected, wherein two target values relating to the open-circuit voltage V min, V max of the battery cell are ascertained for the battery cell, and two charge state SOCmin, SOCmax target values corresponding to the target values for the open-circuit voltage V min, V max are ascertained. The method is characterized at least by the following steps: - (S1) switching off the current (40) when the respective voltage value V 1, V 2 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; - (S3) determining a respective voltage difference ΔV 1, ΔV 2 (42) between the respective determined open-circuit voltage and the respective corresponding target value V min, V max; - (S4) determining a respective charge state difference ΔSOC1, ASOC2 (43) corresponding to the respective voltage difference ΔV 1, ΔV 2; and - (S5) determining the charge throughput ΔQ' on the basis of the detected charge difference ΔQ and the determined charge state differences ΔSOC1, ASOC2 (43). The invention also relates to an HPC method for a battery cell.

Description

[0001] Description

[0002] Method for determining a charge throughput of a battery cell

[0003] The invention relates to a method according to the preamble of patent claim 1 and to an HPC 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 these parameters use higher levels of accuracy, allowing them to detect even small changes over short timescales. One such method is high-precision coulometry (HPC), which allows the amount of charge removed from and / or added to the battery cell to be measured with great precision.

[0010] 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 corresponding to specified open-circuit voltages. The HPC method measures the charge quantities during individual charge and discharge 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 service life of the battery cell.

[0011] 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.

[0012] The present invention is based on the object of providing a method for the improved determination of charge throughputs, in particular within the framework of an HPC method.

[0013] The object is achieved by a method having the features of independent patent claim 1 and by an HPC method having the features of independent patent claim 9. Advantageous embodiments and further developments of the invention are specified in the dependent patent claims.

[0014] In the method according to the invention for determining at least one charge throughput AQ' of a battery cell, in particular within the framework of an HPC method, at least one with two fixed voltage values ​​V lt V2associated charge difference AQ is recorded, whereby two setpoints for the battery cell with respect to its rest voltage V min , and two to the set values ​​of the rest voltage V min , corresponding target values ​​of charge states SOC min , SOCmax are specified. The process is characterized by at least the following steps:

[0015] - Switching off the current when the respective voltage value V is reached lt V2;

[0016] - Determination of the respective rest voltage based on a respective decay behavior of the voltage induced by switching off the current;

[0017] - Determination of a respective voltage difference AI^, AV2 between the respective determined rest voltage and its respective corresponding setpoint V min , V max ;

[0018] - Determining a respective state of charge difference ASOCi, ASOC2 corresponding to the respective voltage difference A1, AV2; and

[0019] - Determination of the charge throughput AQ' as a function of the detected charge difference AQ and the determined charge state differences ASGC^ ASOC2. Without limiting the scope of protection, V ±> V2 is assumed. In other words, according to the method, at least one charging or discharging of the battery cell occurs within a cycle between the voltage values ​​V1, V2, so that a charge throughput AQ is associated with this.

[0020] Furthermore, two target values ​​for the battery cell are specified with regard to its rest voltage V min , and two to the set values ​​of the rest voltage V min , V max corresponding target values ​​of charge states SOC min , S0C max The above-mentioned open-circuit voltages and the associated charge states can be provided for the process. In particular, the above-mentioned target values ​​can be provided using a battery cell's open-circuit voltage characteristic.

[0021] According to the method according to the invention, the charge flow rate AQ is adjusted between the voltage values ​​V l tV2 is detected. In other words, the amount of charge supplied to or removed from the battery cell between the two voltage values ​​mentioned is detected. However, due to overvoltages induced by the finite current, the detected amount of charge or the detected charge throughput AQ does not correspond to the charge throughput that would result when using the actual open-circuit voltages. The present invention therefore provides for a correction of the charge throughput, i.e., the new, corrected charge throughput AQ' is determined from the detected charge throughput. To determine the corrected charge throughput AQ', the method according to the invention comprises at least the five steps mentioned above.

[0022] In a first step of the process, the current is measured when the respective voltage value V l t V2 switched off.

[0023] 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 V1 and / or V2 are reached, a pause is introduced, so to speak, during which no current flows through the battery cell. However, the voltage is preferably still recorded or measured during these current-free periods.

[0024] According to a second step of the method, the respective rest voltage is determined based on the respective voltage decay behavior induced by the current switching off. In other words, the current is switched off when the voltage limit values ​​V 1 :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 lt y2 would decay to the respective resting voltage. In other words, the respective resting voltage is reached asymptotically. Therefore, in this case, it is not necessary to actually wait until the respective resting voltage is reached; instead, 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.

[0025] In other words, the overvoltage dissipates during the pause (current-free period), so that after a sufficiently long waiting time, the open-circuit voltage of the battery cell would be established. This allows the open-circuit voltage of the individual battery cell to be precisely measured in its current state, and it is not necessary to estimate it using additional measurement parameters and / or models. According to the invention, it is sufficient to determine the open-circuit voltage from the decay behavior of the respective overvoltage. Waiting until the overvoltage has almost completely decayed is therefore not necessary according to the invention.

[0026] In a third step of the process, a respective voltage difference A1 , AV2 between the respective determined rest voltage and its respective corresponding setpoint 7 min, nax is determined. In other words, a voltage difference between the determined or recorded rest voltage and the predefined rest voltage is determined. In other words, a correction of the rest voltages is carried out, which is quantified by the voltage differences AI^, AV2. If the recorded rest voltages or those determined from the decay behavior are expressed as V O cv,mm> ^ocv,max, SO V0CV,min = ^min -2 and V0CV,max = nax - AV1 and thus Vocv.max = - 71 or Vocv,min = ^2 > ^2. where rj1, rj2 denotes the respective overvoltage.

[0027] According to a fourth step of the method, a respective state of charge difference ASGC^ ASOC2 corresponding to the respective voltage difference AI , AV2 is determined. This is preferably done using an open-circuit voltage characteristic of the battery cell. Furthermore, the states of charge corresponding to the actual open-circuit voltages can be determined using SOCmin - ASOC2 and SOC max - ASOCi can be calculated.

[0028] The states of charge are, by definition, proportional to the amount of stored charge. They are thus related to the measurable charge quantities during the discharge or charge of the battery cell. If the charge quantities were to be calculated when the respective voltage limits V lt V2measure, for example, the charge quantity of the discharge stage determined within the framework of a known H PC procedure, which corresponds to the discharge curve of SOC max to SOC min to be attributed to the recorded charge quantity &Q. By means of the method according to the invention, which determines the actual rest voltages via the respective decay behavior within the current-free time periods, it can be seen for the above-mentioned embodiment that it is actually a discharge curve of SOC max - ASCX^ to SOC min >ASOC2. Thus, the method allows for a correction depending on the determined state of charge differences ASGC^ ASOC2 and the detected charge difference AQ, providing a more precise method for determining the charge throughput and thus, in particular, a more precise HPC method. This correction of the detected charge difference AQ is performed in a fifth step of the method.

[0029] According to the fifth step of the method, the charge throughput AQ' is determined as a function of the detected charge difference AQ and the determined state of charge differences ASGC^ ASOC2. For example, a correction linear to the state of charge differences ASGC^ ASOC2, which is valid for sufficiently small voltage differences AI^, AV2, can be used.

[0030] The method according to the invention thus provides a method by means of which the charge quantities measurable during each charging and discharging cycle are compared to a common reference, for example the course of SOC max to SOC min , can be converted. Since known HPC methods assume constant conditions at the limits of the test cycles, the converted charge quantities or charge throughputs, i.e., corrected according to the method, can advantageously be used for an HPC method. This advantageously improves the accuracy of known HPC methods.

[0031] In other words, known HPC methods require that the internal resistance or overvoltage can be determined with sufficient accuracy and always up-to-date using existing models in combination with measurement data available during testing. However, measurement inaccuracies of electrical and thermal parameters can distort the determination of the internal resistance. Furthermore, fluctuations in cell quality and battery cell aging can alter the resistance models assumed for the calculation and thus indirectly lead to incorrect resistance values.

[0032] 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, or rather the HPC method according to the invention, still enables a precise and consistent method.

[0033] 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.

[0034] The charge states determined via the resting voltage correct the measured charge quantities or charge throughputs so that they remain comparable between individual cycles. This allows, in particular, fluctuations in the overvoltage, which can occur between individual test cycles due to external and internal influences, such as temperature fluctuations, to be calculated out or corrected. In prior art methods, this is achieved using complex models and error-prone measurements. Furthermore, the method according to the invention enables the determination of charge throughputs and thus an HPC method even when the battery cell is not in thermodynamic equilibrium with its environment.

[0035] The HPC method according to the invention for a battery cell, in which at least one service life of the battery cell is determined, is characterized in that, for determining the service life, a capacity loss of the battery cell is determined by repeatedly determining a charge throughput &Q' according to the present invention and / or one of its embodiments.

[0036] Similar, equivalent and equally effective advantages and / or embodiments of the HPC method according to the invention result from the method according to the invention.

[0037] According to an advantageous embodiment of the invention, the charge throughput &Q' is determined by means of a correction factor K according to Q' = Q / K( SOC- , ASOC2|SOC max , SOC min ) was determined.

[0038] In other words, a correction of the charge flow rate &Q' or, within the framework of an HPC process, a multiple correction of the charge flow rate recorded here is carried out by means of a correction that is proportional to the respective recorded charge flow rate AQ. Thus, the correction is carried out by means of a scaling of the recorded charge flow rate, i.e., by means of a correction factor that is determined by the determined state of charge differences ASGC^ ASOC2 and the specified target values ​​SOC max , SOC min depends on.

[0039] In a particularly preferred embodiment of the invention, the charge throughput AQ' is determined by means of determined.

[0040] In other words, so that the correction factor is linearly dependent on the determined state of charge differences ASGC^ ASOC2. In other words, a linear approximation of the correction factor was made, which is sufficiently accurate for small voltage differences AI^, AV2.

[0041] According to an advantageous embodiment of the invention, the differences in state of charge ASOC^, ASOC2 are determined by means of an open-circuit voltage characteristic of the battery cell.

[0042] The open-circuit voltage characteristic of a battery cell essentially characterizes the relationship between the battery cell's state of charge and its open-circuit voltage corresponding to that state of charge. In other words, V ocv = F(SOC), where V ocv is the open-circuit voltage, SOG the state of charge, and F the open-circuit voltage characteristic. The open-circuit voltage characteristic is typically non-linear. Using the open-circuit voltage characteristic, the inverse function F -1The corresponding state of charge differences ASGC^ ASOC2 are determined from the determined voltage differences A1 and AV2. The open-circuit voltage characteristic or the inverse open-circuit voltage characteristic can be provided in the form of a diagram, in the form of tabular values, and / or as an analytical relationship.

[0043] In a preferred development of the invention, the rest voltages are each determined by means of a fit of the decay behavior.

[0044] 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.

[0045] According to an advantageous embodiment of the invention, F(V, A|t) = V ocv + ( - y O CV) ex P (-^0 a l s Fit function is used, where V ocvdenotes the respective rest voltage to be determined and v, A are further fit parameters.

[0046] This advantageously enables a particularly accurate determination of the actual or the rest voltage to be determined V ocv enabled.

[0047] 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.

[0048] 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.

[0049] 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 ocv + (v - V ocv )exp (-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 currentless time range, or its duration, as a function of 1 / A.

[0050] According to an advantageous embodiment of the invention, the battery cell is designed as a lithium-ion battery cell.

[0051] The method according to the invention is advantageous for lithium-ion battery cells because they typically exhibit a relevant change in their open-circuit voltage with the current intensity, particularly in the context of an HPC process.

[0052] In an advantageous development of the invention, the self-discharge of the battery cell is further determined within the framework of the HPC method. Advantageously, this allows the self-discharge of the battery cell to be determined more precisely. The self-discharge can be determined using the duration of one or more cycles and the respective charge throughput determined according to the present invention.

[0053] 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:

[0054] Figure 1 is a flowchart of a method according to an embodiment of the invention;

[0055] Figure 2 shows a voltage-time diagram; and

[0056] Figure 3 shows a rest voltage characteristic.

[0057] Elements of the same type, value or effect may be provided with the same reference symbols in one or more of the figures.

[0058] Figure 1 shows a flow diagram of a method for determining at least one charge throughput &Q' of a battery cell according to an embodiment of the present invention.

[0059] According to the method, the charge throughput can also be determined several times in succession within time cycles, in particular within the framework of an H PC method.

[0060] For charging or discharging the battery cell, corresponding voltage values ​​V l t V2. For example, the battery cell is charged to the first voltage value V1 and / or discharged to the second voltage value V2. The difference between the voltage values ​​V lt V2associated charges form the detected charge difference AQ.

[0061] Furthermore, the battery cell has two setpoints regarding its rest voltage V min , V max and two to the set values ​​of the rest voltage V min , V max corresponding target values ​​of charge states SOC min , SOC max which are provided for the procedure and are thus predetermined. Preferably V1> V max and V2< V min .

[0062] According to a first step S1 of the method, the current or the

[0063] Current when the respective voltage value V is reached 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 10 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.

[0064] 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 symbols rj1 and TJ2, respectively.

[0065] Furthermore, voltage differences A1 , AV2 between the fixed rest voltages V max or V minand the actual rest voltages ^ocv,max> )cv,min, which were determined using the decay behavior 41. In other words, V0CV,min = ^min and Vocv.max = nax “ And SO with Vocv.max = ^1 “ *71 or Vocv,min = F2~ r l2-

[0066] According to a third step S3 of the method, the respective voltage differences AI^, AV2between the respective determined rest voltage and their respective associated setpoint value V min , V max The voltage differences are identified in Figure 2 by reference numeral 42.

[0067] In a fourth step S4 of the method, the respective state of charge differences ASGC^ AS0C2 associated with the respective voltage difference AI^, AV2 are determined. This is done using a resting voltage characteristic of the battery cell, which describes the dependence of the resting voltage on the state of charge of the battery cell. To determine the state of charge differences from the voltage differences, the inverse resting voltage characteristic in the respective areas is used, i.e., from the voltage differences AI^, AV2 from the voltage values ​​V max , V min The charge level differences from the specified charge levels S0C are determined using the open-circuit voltage characteristic or its inverse. min , S0C max determined (see Figure 3).

[0068] According to a fifth step S5 of the method, the charge throughput Q' is determined as a function of the detected charge difference AQ and the determined state of charge differences ASGC^ AS0C2. In other words, the detected charge throughput AQ is corrected. This correction is particularly advantageous because an overvoltage generally occurs during cycles due to the current intensity. The charge throughput AQ' corrected using the state of charge differences ASGC^ AS0C2 thus takes this behavior into account. This provides a more precise method for determining the charge throughput, or a more precise and thus improved HPC method.

[0069] Particularly preferred in the above-mentioned sense is the measured charge throughput AQ according to corrected.

[0070] In Figure 2, the process already described in Figure 1 is symbolically represented by means of a voltage-time diagram.

[0071] 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).

[0072] 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).

[0073] The battery cell is first charged up to a specified voltage value or a specified voltage limit 14. 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 1 does not correspond to the resting voltage of the battery cell. In other words, a voltage difference forms between the specified resting voltage l{ naxand the actual resting voltage, a voltage difference Al^ occurs. Similarly, when the battery cell is discharged, a voltage difference A72 is formed between the specified resting voltage V due to an overvoltage rj2. min and the actual open-circuit voltage. The voltage differences A 1 , A 72 are designated by reference numeral 42 in Figure 2.

[0074] Figure 3 shows a rest voltage characteristic 12 of the battery cell.

[0075] The state of charge of the battery cell is plotted in arbitrary units along the abscissa 100 of the diagram. The open-circuit voltage is plotted in arbitrary units along the ordinate 101 of the diagram.

[0076] The open-circuit voltage characteristic curve 12 illustrates that each battery cell's state of charge is associated with a closed-circuit voltage. Conversely, each open-circuit voltage is associated with a state of charge.

[0077] Using the determined voltage differences AI, AV2, the corresponding state of charge differences ASGC^ AS0C243 can be determined using the diagram or the open-circuit voltage characteristic. This determination is symbolized in the diagram by the dashed lines. From the determined state of charge differences ASOCi, AS0C2, the charging throughput AQ' corrected with respect to the detected charge difference AQ can be determined.

[0078] 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.

[0079] List of reference symbols

[0080] 51 first step

[0081] 52 second step

[0082] 53 third step

[0083] 54 fourth step

[0084] 55 fifth step

[0085] 10 Voltage curve

[0086] 11 Rest voltage curve

[0087] 12 Open-circuit voltage characteristic

[0088] 40 Switching off the power

[0089] 41 Decay behavior

[0090] 42 voltage differences

[0091] 43 Charge level differences

[0092] 100 abscissa

[0093] 101 Ordinate

Claims

Patent claims 1. Method for determining at least one charge throughput Q' of a battery cell, in particular in the context of an HPC method, in which a voltage V l t V2associated charge difference AQ is recorded, whereby for the battery cell two setpoints regarding its rest voltage V min , V max and two to the set values ​​of the rest voltage V min , corresponding target values ​​of charge states SOC min , SOCmax are defined, characterized by the following steps: - (S1) Switching off the current (40) when the respective voltage value V is reached l t 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; - (S3) Determination of a respective voltage difference A1 , AV2(42) between the respective determined rest voltage and its respective associated setpoint V min , V max ; - (S4) Determining a respective state of charge difference ASOC^ , ASOC2(43) corresponding to the respective voltage difference AI^, AV2; and - (S5) Determination of the charge throughput AQ' as a function of the detected charge difference AQ and the determined charge state differences ASGC^ ASOC2(43).

2. Method according to claim 1, characterized in that the charge throughput AQ' is determined by means of a correction factor K according to AQ' = A<2 / K'(ASOC1, ASOC2|SOC max , SOC min ) is determined.

3. Method according to claim 1 or 2, characterized in that the charge flow rate AQ' is determined by means of is determined.

4. Method according to one of the preceding claims, characterized in that the determination of the state of charge differences ASGC^ ASOC2(43) is carried out by means of an open-circuit voltage characteristic (13) of the battery cell.

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 + ( - V ocv )exp (-^0 a l s fitting function is used, where V ocv 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 Power is turned 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. H PC method for a battery cell, in which at least one service life of the battery cell is determined, characterized in that for determining the service life, a capacity loss of the battery cell is determined by repeatedly determining a charge throughput AQ' 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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