Method for operating an electrical energy storage system

Abstract

A method for operating an electrical energy storage system having at least two electrical energy storage cells is described, the method comprising the following steps: a) determining the voltage of the at least two electrical energy storage cells; b) determining at least one first characteristic value of the determined voltage; c) determining a charging voltage limit value and / or a discharging voltage limit value based on the determined at least one first characteristic value; d) operating the electrical energy storage system based on the determined charging voltage limit value and / or the discharging voltage limit value.

Description

Technical Field

[0001] The invention relates to a method for operating an electrical energy storage system having at least two electrical energy storage units, a corresponding device for operating an electrical energy storage system, a corresponding electrical energy storage system, a corresponding computer program and a corresponding machine-readable storage medium. Background Art

[0002] Current electrical energy storage systems, in particular electrical energy storage systems for electrically driven vehicles, have a plurality of electrical energy storage units, in particular battery units. In this case, the individual electrical energy storage units are usually interconnected in series with one another in order to be able to provide a sufficiently high voltage. During vehicle operation, these battery units are discharged to drive the vehicle and to supply power to other electrical consumers. However, when the vehicle is stationary, these battery units are also slightly discharged in the form of self-discharge or in order to supply energy to electrical consumers such as electronic control units.

[0003] The discharge of the energy storage cells does not necessarily take place uniformly. Therefore, the voltages of the energy storage cells are not all at the same level. This is due to the different self-discharges of the individual energy storage cells and to different aging. The energy storage cell with the highest voltage determines the end of the charging process, while the energy storage cell with the lowest voltage determines the end of the discharging process.

[0004] Today's electrically driven vehicles have a battery management system that contains monitoring and diagnostic functions ranging from component protection to occupant safety. In particular, the voltage of the individual electrical energy storage cells is measured and monitored. If an error occurs in the battery system, it should be recognized as quickly as possible and measures should be taken. In the event of an error, the vehicle does not necessarily have to be shut down immediately, as this would be a safety risk for subsequent traffic. Depending on the severity of the error, the vehicle is put into emergency operation and can, for example, continue to drive at reduced power for a certain time until a safe parking space is found, or in the event of a serious error (for example, in the event of a short circuit in the electrical energy storage cell) the battery protection can be opened and the vehicle simply coasts to a stop.

[0005] In autonomous electrically driven vehicles, in particular autonomous shuttles or robotaxis, there is no driver who can intervene in the event of an error in order to safely steer the vehicle out of the danger zone. The vehicle must be able to decide for itself how to handle the current error, for example whether it can continue to drive to the next service station or final stop, or whether it must stop immediately. If necessary, it must decide whether the vehicle can continue to operate at the same power or whether it should continue at reduced power and at a reduced speed.

[0006] Due to the failure of the measuring wire, it may be that the voltage of the relevant electrical energy storage unit can no longer be measured. For safety reasons, this may lead to the shutdown of the entire electrical energy storage system or to a reduced driving operation in today's vehicles. This means that the vehicle can only continue to drive at reduced power and therefore at a reduced speed. This emergency operation makes it possible to safely leave the road and park the vehicle in a parking lot or emergency parking space or even reach the next workshop. There are various safety levels for autonomous driving, which are different for driving routes of different lengths, especially after an error has occurred.

[0007] Depending on the predefined limit voltage during charging or discharging, the electrical energy storage system can also be operated for a longer or shorter time, wherein the longest possible operating duration is desirable in terms of the use of the system. However, this can only be achieved in a limited manner with strictly predefined voltage limits.

[0008] Document US 2005 / 0017684 proposes a method for charging a lead-acid battery, which method ends the charging process according to the first-order time derivative and the second-order time derivative of the charging voltage, respectively.

[0009] Document US 2008 / 0164849 proposes a method for charging a rechargeable battery which limits the charging current based on the first time derivative of the battery voltage. Summary of the invention

[0010] Advantages of the present invention

[0011] A method for operating an electrical energy storage system having at least two electrical energy storage cells is disclosed.

[0012] Here, the voltage of the at least two electrical energy storage cells is determined, for example by measurement. Subsequently, at least one first characteristic value of the detected voltage is determined. This can be done, for example, by processing and analyzing the determined voltage values ​​in a data processing unit.

[0013] Furthermore, a charging voltage limit value and / or a discharging voltage limit value is determined as a function of the determined at least one first characteristic value, and the electrical energy storage system is subsequently operated as a function of the determined charging voltage limit value and / or as a function of the determined discharging voltage limit value.

[0014] This is advantageous because by determining the limit value based on the characteristic value, more energy can be drawn from the electrical energy storage system while ensuring safety relative to the strictly predefined limit value. This increases the operating duration and expands the range of use of the electrical energy storage system.

[0015] Further advantageous embodiments of the invention are the subject matter of the dependent claims.

[0016] Suitably, the at least one first characteristic value comprises a first-order time derivative of the determined voltage and / or a second-order time derivative of the determined voltage. For example, the first-order time derivative of the recorded voltage value of the electrical energy storage unit can be determined by numerical differentiation. For the second-order time derivative, this can be done in a similar manner. In addition, the at least one first characteristic value can include the average value of the determined voltage and / or the extreme deviation from the average value of the determined voltage, i.e. the minimum deviation or the maximum deviation. This is advantageous because the mentioned variables enable the electrical energy storage system to operate particularly efficiently and safely, and therefore enable the stored energy to be used efficiently.

[0017] Suitably, an error in the voltage detection of the electrical energy storage unit of the electrical energy storage system is detected. This is advantageous in order to adapt the determination of the limit value and / or the determination of the voltage of the electrical energy storage unit in this case. For example, if there is an error in the voltage detection, for example if the sensor system for voltage detection fails, the voltage of the electrical energy storage unit can be determined from the recorded measured values ​​and the current measured values ​​of the adjacent electrical energy storage units instead. In addition, the determination can also be made with the aid of a mathematical model and the recorded current values. This advantageously allows the electrical energy storage system to continue to operate while taking into account the error. The mathematical model can include, for example, a differential equation or a difference equation or an algebraic equation. In addition, a family of characteristic curves based on data can also be a component of the mathematical model.

[0018] Advantageously, the current flowing into or out of at least one electrical energy storage unit is determined, for example by means of a measurement. At least one second characteristic value of the detected current is determined as a function of the determined current or a corresponding current value, for example a maximum or minimum current value determined within a predefined time window or its absolute value or a relative current value via the capacity of the reference battery, for example 48A at a capacity of 48Ah corresponding to 1C. Furthermore, the charging voltage limit value and / or the discharging voltage limit value are additionally determined as a function of the at least one second characteristic value determined. As a result, conservative limit values ​​can be selected for high currents or high C values, respectively, and correspondingly smaller conservative values ​​can be selected for low currents / low C values. Thus, the corresponding limit values ​​are advantageously adapted to the current operating conditions or operating state of the battery.

[0019] Advantageously, the temperature of at least two electrical energy storage cells is determined and a third characteristic value of the determined temperature is determined. In this case, the third characteristic value can also include a first-order time derivative, for example. The charging voltage limit value or the discharging voltage limit value is now additionally determined as a function of the at least one third characteristic value determined. This is advantageous because the effects of cooling or heating the electrical energy storage system are also taken into account, which allows a less conservative selection of the voltage limit.

[0020] Furthermore, the subject of the present disclosure is a device for operating an electrical energy storage system having at least two electrical energy storage units, wherein the device has at least one device, wherein the device is designed to perform the steps of the disclosed method. The device can also correspond to the at least one device. The at least one device can include, for example, a battery management control device and corresponding power electronics (e.g., an inverter), as well as current sensors and / or voltage sensors and / or temperature sensors. An electronic control unit, in particular when configured as a battery management control device, can also be such a device. An electronic control unit can be understood in particular as an electronic control device, which, for example, includes a microcontroller and / or a dedicated hardware module (e.g., an ASIC), but can also include a programmable logic controller. The above advantages are thus achieved.

[0021] Furthermore, the subject matter of the present disclosure is an electrical energy storage system having at least two electrical energy storage units, the electrical energy storage system comprising the disclosed device. Thus, the above-mentioned advantages are achieved.

[0022] Furthermore, the subject of the present disclosure is a computer program comprising instructions for causing the disclosed device to execute the disclosed method steps. The above advantages are thus achieved.

[0023] Furthermore, the subject matter of the present disclosure is a machine-readable storage medium on which the disclosed computer program is stored.

[0024] An electrical energy storage unit is to be understood as meaning, in particular, an electrochemical cell and / or a battery module having at least one electrochemical cell and / or a battery pack having at least one battery module. For example, the electrical energy storage unit may be a lithium-based cell or a lithium-based battery module or a lithium-based battery pack. The electrical energy storage unit may in particular be a lithium-ion cell or a lithium-ion battery module or a lithium-ion battery pack. Furthermore, the battery cell may be of the type of a lithium polymer battery, a nickel metal hydride battery, a lead-acid battery, a lithium-air battery or a lithium-sulfur battery, or quite generally a battery of any electrochemical composition. Capacitors may also serve as electrical energy storage units. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Advantageous embodiments of the invention are shown in the drawing and are explained in more detail in the following description.

[0026] Figure 1 shows a flow chart of the disclosed method according to a first embodiment;

[0027] Figure 2 shows a flow chart of the disclosed method according to a second embodiment;

[0028] Figure 3 shows a schematic voltage variation process, in which a discharge voltage limit value is shown; and

[0029] Figure 4 A schematic diagram of a disclosed electrical energy storage system according to one embodiment is shown.

[0030] In all the figures, the same reference numerals denote the same device components or the same method steps. DETAILED DESCRIPTION

[0031] Figure 1 A flow chart of a disclosed method for operating an electrical energy storage system according to a first embodiment is shown. The electrical energy storage system comprises at least two electrical energy storage units.

[0032] In a first step S11, the voltages of the at least two electrical energy storage cells are determined. This can be done by measuring using a corresponding voltage measuring device or, if such a voltage measuring device is defective, also by means of voltage values ​​from adjacent energy storage cells. For this purpose, the last voltage value known to be valid is used as a starting value, and the voltage values ​​of the adjacent electrical energy storage cells are used for the development of the voltage for which no current measured values ​​are available. It is assumed here that adjacent energy storage cells develop in a similar manner and that therefore at least a good estimate of the voltage can be made within a certain period of time.

[0033] In a second step S12, at least one first characteristic value of the determined voltage is determined. The at least one characteristic value preferably comprises a first-order time derivative of the determined voltage, a second-order time derivative of the determined voltage, an average value of the determined voltage, and a maximum or minimum deviation of the determined voltage from the average value. This means that for each determined voltage, the first-order time derivative, the second-order time derivative, and the extreme deviations from the average value are preferably determined. However, depending on the application, it may also be sufficient to evaluate only one or part of the determined voltages accordingly.

[0034] In a third step S13, a charging voltage limit value and / or a discharging voltage limit value is determined based on the determined at least one characteristic value. The corresponding charging voltage limit value can be obtained, for example, as a result of subtracting the extreme deviation from the predefined safety voltage limit value. For example, if the safety voltage limit value is 4.2V and the extreme deviation is 0.1V, the resulting charging voltage limit value is 4.1V. Then, charging can be performed safely up to this charging voltage limit value.

[0035] In a fourth step S14 , the electrical energy storage system is then safely operated with the charging voltage limit value determined in this way, even if the voltage measuring device fails and while providing increased power.

[0036] Figure 2 A flow chart of a method for operating an electrical energy storage system disclosed according to a second embodiment is shown. The electrical energy storage system comprises at least two electrical energy storage units. In a first step S21, the voltages of the at least two electrical energy storage units are determined. In a second step S22, an average value of the determined voltages is determined and a maximum deviation of the determined voltage from the average value is determined.

[0037] In a third step S23, it is checked whether an error occurs in the voltage detection of the electrical energy storage unit of the electrical energy storage system. This may be caused, for example, on the one hand by a defective voltage detection device or, on the other hand, for example by a defective electrical energy storage unit, the voltage of which is to be detected. If no corresponding error is detected, the first step S21 is continued. If a corresponding error is detected, the fourth step S24 is continued.

[0038] In a fourth step S24 , a first-order time derivative and a second-order time derivative are determined for the detected voltage, respectively. This can be done, for example, by means of numerical differentiation.

[0039] In a fifth step S25, it is checked whether there is a turning point in the detected voltage. For example, it can be checked whether the second-order time derivative is equal to zero, or it is better to check whether a sign change occurs in the second-order time derivative based on numerical differentiation, that is, from positive to negative or from negative to positive. If not, the first step S21 and the second step S22 are continued, and then the fifth step S25 is re-executed. If yes, the discharge voltage limit value is determined in a sixth step S26 based on the determined characteristic value. This can be done, for example, by subtracting half of the absolute value of the maximum deviation from the predefined lower voltage limit. Therefore, more available energy is available.

[0040] In a seventh step S27, it is checked whether a transition from positive to negative, i.e. a change of sign, occurs in the first-order time derivative. If so, the discharge voltage limit value is increased again to a predefined voltage limit, and the electrical energy storage system is operated with the increased voltage limit in an eighth step S28. After the eighth step S28, the fourth step S24 is continued. If not, the discharge voltage limit value determined in the sixth step S26 is maintained, and the electrical energy storage system is operated with the discharge voltage limit value determined in the sixth step S26 in a ninth step S29. The first step S21 and the second step S22 are then executed, and the seventh step S27 is then executed again.

[0041] The method can be applied in a similar manner to the charging process or the charging voltage limit value.

[0042] Figure 3A schematic voltage profile 30 of an electrical energy storage cell is shown by way of example, wherein a discharge voltage limit value profile 35 is shown and a rigid lower voltage limit profile 36 is shown. Furthermore, a lower safety voltage level 37 is shown, below which damage to the electrical energy storage cell occurs. Based on the voltage profile 30 and the variable discharge voltage limit value 35, the following explanation is given in the following example. Figure 2 For example, it can be assumed that at least two electric energy storage units present in the electric energy storage system exhibit the same voltage variation process.

[0043] The voltage of the electrical energy storage unit is determined continuously, from which, when viewed together, a voltage profile 30 is obtained. A turning point is detected at the profile point 31, and the discharge voltage limit value is determined based on the determined characteristic value and is reduced accordingly here by subtracting an absolute value (e.g. half the absolute value of the maximum deviation and / or an absolute value determined from a table based on the current intensity). This means that more energy can be safely extracted from the electrical energy storage system without having to regulate the current supply with corresponding power electronics.

[0044] This is evident from the corresponding reduction in the discharge voltage limit value profile 35. The discharge voltage limit value remains at a low level until a change from a positive time derivative value of the voltage to a negative derivative value is determined at the profile point 32. The discharge voltage limit then assumes the value of the rigid lower voltage limit again.

[0045] At the changeover point 33, a turning point in the voltage changeover 30 is detected. As a result, the discharge voltage limit value is reduced again. The reduced discharge voltage limit value is lower than the previous reduction. This may be caused, for example, by a lower current intensity, whereby a higher absolute value for reducing the discharge voltage limit value is determined from the table depending on the current intensity.

[0046] The change from a positive time derivative value of the voltage to a negative derivative value is determined at the change process point 34. The discharge voltage limit value then again assumes the value of the rigid lower voltage limit.

[0047] Figure 4 A schematic diagram of a disclosed electrical energy storage system 40 with at least two electrical energy storage cells 41 according to one embodiment is shown. In this case, a device 42 controls a power electronic component 43, such as an inverter, so that corresponding charging voltage limit values ​​or discharging voltage limit values ​​are observed. In this case, the device 42 determines the voltages of the at least two electrical energy storage cells 41, corresponding characteristic values, and the charging voltage limit values ​​or discharging voltage limit values.

Claims

1. A method for operating an electrical energy storage system (40), the electrical energy storage system comprising at least two electrical energy storage units (41), the method comprising the following steps: a) determining the voltage of the at least two electrical energy storage units (41); b) determining at least one first characteristic value of the determined voltage, wherein it is determined whether a turning point is present in the determined voltage; c) determining a charging voltage limit value and / or a discharging voltage limit value as a function of the determined at least one first characteristic value and in response to the determination of a turning point, wherein a first-order time derivative of the determined voltage is determined, and wherein a check is performed to determine whether a transition corresponding to a sign change occurs in the first-order time derivative, and in response to the transition the discharging voltage limit value is increased to a predefined voltage limit; d) operating the electrical energy storage system (40) according to the determined charging voltage limit value and / or discharging voltage limit value.

2. The method according to claim 1, wherein: The at least one first characteristic value comprises a first time derivative of the determined voltage and / or a second time derivative of the determined voltage and / or an average value of the determined voltage and / or an extreme deviation of the determined voltage from the average value of the determined voltage.

3. The method according to any one of the preceding claims, further comprising: e) Detecting errors in voltage detection of an electrical energy storage unit of the electrical energy storage system (40).

4. The method according to any one of the preceding claims, further comprising: f) determining a current flowing into or out of at least one of the electrical energy storage cells (41); g) determining at least one second characteristic value based on the determined current; In this case, the charging voltage limit value and / or the discharging voltage limit value is additionally determined as a function of the determined at least one second characteristic value.

5. The method according to any one of the preceding claims, further comprising: h) determining the temperature of the at least two electrical energy storage units (41); i) determining at least one third characteristic value of the determined temperature; In this case, the charging voltage limit value and / or the discharging voltage limit value is additionally determined as a function of the at least one determined third characteristic value.

6. A device (42) for operating an electrical energy storage system (40) having at least two electrical energy storage units (41), wherein the device (42) comprises at least one device, in particular an electronic control unit, wherein the device is designed to perform the steps of the method according to any one of claims 1 to 5.

7. An electrical energy storage system (40) comprising at least two electrical energy storage units (41) and a device (42) according to claim 6.

8. A computer program comprising commands causing a device (42) according to claim 6 to execute all the steps of the method according to any one of claims 1 to 5.

9. A machine-readable storage medium having stored thereon the computer program according to claim 8.

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