Energy storage device

Abstract

Device (20) for determining the capacity of an energy storage device (11) with a plurality of storage cells (1, 2, 3), wherein the device (20) has connecting elements for being connected to each storage cell (1, 2, 3), wherein the device (20) has a discharge / charge unit (21) and a calculation unit (22), characterized in that the discharge / charge unit (21) is configured to be connected to the storage cells (1, 2, 3) of the energy storage device (11) and to discharge and subsequently charge the storage cells (1, 2, 3), and that the calculation unit (22) is configured to monitor the voltage level of each of the storage cells (1, 2, 3) of the energy storage device (11) during the charging process following the discharge process, and the discharge / charge unit (21) is configured to charge each of the storage cells (1, 2, 3) up to a first time discharged, at which a first of the memory cells (1, 2,3) the energy storage device (11) reaches a lower predefined voltage limit and to identify and label this storage cell (1, 2, 3), and subsequently to charge each of the storage cells (1, 2, 3) until a second time point at which a first of the storage cells (1, 2, 3) of the energy storage device (11) reaches an upper predefined voltage limit and to identify and label this storage cell (1, 2, 3).

Description

The present invention relates to an energy storage device with a plurality of storage cells. Furthermore, the present invention relates to a device for determining the capacity of such an energy storage device and to a method for determining the capacity of an energy storage device. Electrical energy storage devices, such as high-voltage storage systems—especially lithium-ion batteries with a nominal voltage of 60 V or higher—or low-voltage storage systems (e.g., 12 V), lose capacity over their lifespan. This capacity loss is due, among other things, to the aging of the high-voltage storage cells. If the capacity loss is too high, the high-voltage storage system may need to be replaced. Especially when using energy storage systems in electric vehicles, it's important for users to know the actual capacity of the storage system. While it's possible to measure the capacity directly in the vehicle during operation, these online measurements are often inaccurate. A more precise capacity measurement can therefore only be performed offline, i.e., not during operation. However, current systems only measure the total capacity of the high-voltage battery when performing such offline capacity measurements. If the high-voltage battery contains cells of varying capacities or if the cells have aged differently, even in the best-case scenario, only the capacity of the cell with the smallest capacity is directly measured, as this limits the measured voltage of the high-voltage battery both upwards and downwards.In an unfavorable scenario, the voltages of the individual cells are such that even the capacity of the smallest cell cannot be fully measured. Such a measurement therefore leads to a recorded capacity loss that does not necessarily reflect reality, since the capacity of the larger cells is not fully measured. US 6,239,579 B1 discloses a device for managing battery packs by measuring and monitoring the operating capacity of individual battery modules within a battery pack. A programmable logic controller (PLC) controls the selective closing of relays to subject individual battery modules to a load test using a variable discharge load unit without affecting the usable capacity of the battery pack. A battery module whose usable capacity falls below a predefined threshold can be connected to a charger for recharging and subsequently electrically realigned with the other modules in the battery pack for continued operation. Alternatively, an alarm can be triggered to notify the user that the module needs to be replaced. This sequence of events is performed on all cells in the pack at a predetermined interval. DE 10 2009 038 663 A1 discloses a method for determining a quantity that indicates the charging capacity and / or aging state of a selected battery of a motor vehicle that has a plurality of batteries. In this method, the battery is disconnected from the vehicle's electrical system while the vehicle is in operation and connected to a diagnostic device. The diagnostic device acquires measured values ​​from which the quantity is derived. This enables continuous monitoring of the battery's condition. Disconnecting the battery under test from the vehicle's electrical system allows for battery diagnostics in a discharged state, which provides more accurate data than model-based battery diagnostic methods that continuously monitor batteries connected to the vehicle's electrical system. DE 10 2013 206 885 A1 discloses a device and a method for preparing a charging process for an energy storage device, wherein the energy storage device consists of several cells. At least one characteristic parameter of a cell is recorded. Depending on the recorded parameter, an individual starting voltage is determined for each cell, with which the charging process of the energy storage device is initiated. The starting voltage depends on the determined parameter of the respective cell. Against this background, one object of the present invention is to improve the measurement of the capacity of an electrical energy storage device. This problem is solved by a device for determining the capacity of an energy storage device according to claim 1, an energy storage device according to claim 10 and a method for determining the capacity of an energy storage device according to claim 12. According to one aspect, a device for determining the capacity of an energy storage device with a plurality of storage cells is proposed, wherein the device has connecting elements for being connected to each storage cell. The device can be arranged externally to the energy storage device or form part of the energy storage device. In either case, no online measurement of the storage cells is performed during operation, but only outside of operation, for example, in a workshop. The device includes a discharge / charge unit and a calculation unit. To determine the exact capacity and thus the capacity loss of the energy storage device, the discharge / charge unit is configured to be connected to the energy storage device's storage cells, particularly in offline mode, in order to discharge and then charge the storage cells. The discharge and charging of the storage cells preferably occur simultaneously. A storage cell can refer to a single storage cell or to a parallel connection of several storage cells that function as a single storage cell. During the charging process, and following the discharging process, the processing unit is configured to monitor the voltage level of each storage cell in the energy storage device. Unlike conventional systems, the system does not determine the capacity of the entire energy storage device, but rather the capacity of each individual storage cell by monitoring their voltage levels. This prevents the system from being assigned an insufficient capacity when the insufficient capacity is due to an unfavorable ratio of cell voltages (asymmetry) or to a single storage cell. Furthermore, the system can identify which storage cell has insufficient capacity and thus allocate the capacity loss to the individual cells. The discharge / charge unit and / or processing unit can be implemented in hardware and / or software. In a hardware implementation, the respective unit can be a device or part of a device, for example, a computer or a microprocessor. In a software implementation, the respective unit can be a computer program product, a function, a routine, part of program code, or an executable object. The discharge / charge unit is configured to discharge each of the storage cells up to a first time point at which a first of the storage cells of the energy storage device reaches a lower predefined voltage limit, and to identify and label this storage cell, and subsequently to charge each of the storage cells up to a second time point at which a first of the storage cells of the energy storage device reaches an upper predefined voltage limit, and to identify and label this storage cell. According to a further embodiment, the processing unit is configured to store the lower cell voltage of each storage cell at the end of the discharge process when the energy storage device is being discharged, and to store the upper cell voltage of each storage cell at the end of the charging process when the energy storage device is being charged. Furthermore, the processing unit is configured to compare the lower cell voltages of the storage cells with the upper cell voltages of the corresponding storage cells and, based on the result of the comparison, to calculate the available capacity of each storage cell. The cell voltages of each individual memory cell are therefore measured and compared directly. Determining the capacity of each memory cell is thus easily possible, as it can be calculated directly based on the upper and lower cell voltages of each cell. Instead of fully discharging and then fully charging each memory cell, it is also possible to start the discharge of all memory cells simultaneously and continue until a first point in time, at which one of the memory cells reaches a lower voltage limit, and then charge all memory cells simultaneously until a second point in time, at which one of the memory cells reaches an upper voltage limit. The upper voltage limit can define a target charging voltage. In this case, the cell voltages of the memory cells at the first point in time and the cell voltages of the memory cells at the second point in time are also compared, and the actual capacity is determined by upscaling based on known values ​​of the respective memory cells. If the processing unit determines that the available capacity is below a predetermined threshold, it can output a signal based on the calculated available capacity, indicating whether a memory cell needs to be replaced. Specifically, the processing unit can specify which memory cell needs to be replaced. This allows for the targeted replacement of individual cells. According to a further embodiment, the discharge / charge unit is configured to discharge the storage cells until a first time point at which a first storage cell of the energy storage device reaches a lower predefined voltage limit and identifies and marks this storage cell. Subsequently, the discharge / charge unit charges each of the storage cells until a second time point at which a first storage cell of the energy storage device reaches an upper predefined voltage limit and identifies and marks this storage cell. The marked memory cells therefore correspond to those cells whose voltage level first reaches the lower or upper voltage limit. According to a further embodiment, the processing unit is also configured to determine the voltage level of each storage cell at the second time point. Based on this information, it can then be determined whether calibration of the storage cells is necessary. This embodiment is based on the fact that asymmetries can occur between the charge states of the storage cells at different times when measuring the capacity of the energy storage device, for example, because one storage cell has a lower capacity than the other storage cells or because the charging time of the storage cells differs. Thus, the voltage of one cell can limit the capacity measurement downwards, while another cell limits it upwards, if the storage cells have different voltage profiles. Ideally, there is no asymmetry between the charge states of the storage cells. This means that the voltage of the storage cell in the energy storage device with the smallest capacity limits the measurement both upwards and downwards. In this case, if the labeled storage cells are the same storage cell, the processing unit is configured to determine the capacity of the high-voltage energy storage unit based on the difference in the voltage levels of the labeled storage cell at the first and second time points. After determining the capacity, the processing unit can ascertain whether the capacity falls below a predetermined threshold and, if so, output a signal indicating that the marked memory cell needs to be replaced. This allows the memory cell with the lowest capacity to be identified and selectively replaced. In one embodiment, the capacity determination can then be carried out again to determine whether other memory cells show insufficient capacity and then selectively replace them. If the labeled memory cells are different, i.e., possess an asymmetry, the computation unit is set up to calculate a difference in the upper voltage levels of the labeled memory cells at the second time point, and to calibrate the energy storage device based on the calculated difference. To calibrate the energy storage device, the start time of the charging process for the designated storage cells is adjusted so that they reach the upper predefined voltage limit at the same second time. This means that the start time of the charging process for one of the designated storage cells is shifted so that both reach the upper voltage limit simultaneously. Such calibration can also be referred to as balancing the storage cells. Following calibration, the discharge / charge unit can discharge and charge the energy storage device again. The discharge / charge process can be performed iteratively by the discharge / charge unit and the calculation unit until the calculated difference between the upper voltage levels of the storage cells falls below a predetermined limit. To determine whether a memory cell needs to be replaced after calibration, the processing unit is further configured to ascertain whether the capacity of the marked memory cells, as achieved within the time period from the start of charging (specifically from the adjusted time) until the upper defined voltage limit is reached, falls below a predetermined threshold. If so, the memory cell does not provide the required capacity and should therefore be replaced. Thus, if the determined capacity is below the predetermined threshold, a signal can be output indicating that the marked memory cell needs to be replaced. According to another aspect, an energy storage device with multiple storage cells is proposed, in which the capacity of each individual storage cell is recorded to improve the measurement of the energy storage device's capacity. In this way, information about the individual storage cells is available, making it possible to replace only those storage cells that actually exhibit a relevant capacity loss. To achieve this, each storage cell of the energy storage device is configured to operate offline, separately from the other storage cells, and is connected to the aforementioned device for determining the capacity of the high-voltage energy storage device. This allows the capacity of each storage cell to be recorded. If a storage cell experiences a capacity loss exceeding a predetermined limit, that specific storage cell can then be replaced without requiring the replacement of the entire energy storage device. The device can be located externally to the energy storage device or form part of it. To connect the storage cells to the device, each storage cell can be connected separately via a connecting element. Alternatively, the majority of the storage cells can be connected to the device using a single connecting element. In the latter case, the connecting element has multiple contacts, each assigned to one of the individual storage cells of the energy storage device. This allows the device to measure the capacity of each individual storage cell via these contacts. The energy storage device could, for example, be an energy storage system that can be used in a motor vehicle. For instance, the energy storage device could be a high-voltage energy storage device. The motor vehicle could be a conventional internal combustion engine vehicle, an electric vehicle, or a hybrid vehicle, with the (high-voltage) energy storage device supplying energy to an electric drive machine (electric motor or starter). According to another aspect, a motor vehicle with an energy storage device as described above is proposed. The motor vehicle can, in particular, be a passenger car. According to another aspect, a method for determining the capacity of an energy storage device with a plurality of storage cells is proposed. The method comprises the following steps: connecting each storage cell of the energy storage device to a device for determining the capacity; discharging and subsequently charging the storage cells; monitoring the voltage level of each of the storage cells of the energy storage device during the charging process following the discharge. The embodiments and features described for the proposed device apply accordingly to the proposed method. Such a procedure can be carried out, for example, as follows: • Discharging the energy storage device or each storage cell • Discharge stops when the first storage cell reaches a lower voltage limit (at a first time point) • Storing all cell voltages at the time of discharge stop • Charging the energy storage device or each storage cell • Charging stops when the first cell reaches an upper voltage limit (at a second time point) • Storing all cell voltages at the time of charging stop • Calculating the difference between the lower and upper voltages for each storage cell to determine the capacity for each storage cell. Alternatively, the unmeasured capacity fractions during discharge (lower voltage range) and charging (upper voltage range) can be calculated for each cell.Calculate the difference between the stored cell voltage and the desired voltage (which corresponds to the upper voltage limit). The cell capacities can then be determined by scaling up the measured capacity to 100% using this calculated difference. Alternatively, the capacity measurement can be performed as follows: • Discharging the energy storage device or the individual storage cells • Discharge stops when the first storage cell reaches a lower voltage limit (first time point) • Mark this storage cell • Charging the energy storage device or the individual storage cells • Starting the charge count for capacity measurement based on the lower voltage levels • Charging stops when the first storage cell reaches an upper voltage limit (second time point) • Stopping the charge count for capacity measurement • Marking this storage cell • If the marked storage cells are identical, the asymmetry potential is zero. It is assumed that the capacity measurement contains no inaccuracy due to asymmetry and that the storage cell with the smallest capacity limits the capacity of the energy storage device. This cell can then be replaced.• If the labeled memory cells are different: Calculate the voltage difference at the moment charging stops between the upper voltage limit (= voltage of the second labeled memory cell) minus the voltage of the first labeled memory cell. Based on this difference, the memory cells can be calibrated as described above. Alternatively, this voltage difference can be viewed as the proportion of the unmeasured capacity of the first memory cell and converted into information about the percentage by which the capacity measurement was too low due to asymmetry, i.e., different voltage profiles of the memory cells. This asymmetry can be corrected by the calibration described above. Furthermore, a computer program product is proposed which has program code designed to initiate the execution of the procedure described above on a computer. A computer program product, such as a computer program tool, can be provided or delivered from a server on a network, for example, as a storage medium such as a memory card, USB stick, CD-ROM, DVD, or as a downloadable file. This can be done, for example, in a wireless communication network by transmitting the corresponding file containing the computer program product or tool. Other possible implementations of the invention also include combinations of features or embodiments described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In such cases, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the invention. Further advantageous embodiments and aspects of the invention are the subject of the dependent claims and the exemplary embodiments of the invention described below. The invention is further explained below with reference to preferred embodiments and the accompanying figures. Fig. 1 shows a schematic block diagram of a motor vehicle with an energy storage device and a device for determining the energy storage device, and Figs. 2, 3, 4 to 5 show various variants of the voltage profiles of the storage cells of an energy storage device. In the figures, identical or functionally equivalent elements have been given the same reference symbols, unless otherwise indicated. Fig. 1 shows a motor vehicle 10 with an energy storage device 11. The energy storage device 11 has a plurality of storage cells 1, 2, 3 (in Fig. 1, 3 cells are shown as an example, but any other number of cells is conceivable). Um die vorhandene Kapazität der Energiespeichereinrichtung 11 zu bestimmen, oder um einen Kapazitätsverlust der Energiespeichereinrichtung 11 zu berechnen, wird jede Speicherzelle 1, 2, 3 mit einer Vorrichtung 20 zum Bestimmen der Kapazität der Energiespeichereinrichtung 11 verbunden. Obwohl die Vorrichtung 20 extern zu der Energiespeichereinrichtung 11 gezeigt ist, kann diese auch in der Energiespeichereinrichtung integriert sein. This can be achieved by directly coupling each memory cell 1, 2, 3 to the device 20, as shown in Fig. 1. Alternatively, the majority of memory cells 1, 2, 3 can be connected to the device 20 via a single connecting element, for example, a plug. The connecting element can have a plurality of contacts, which are assigned to the individual memory cells 1, 2, 3 of the energy storage device 11. The connection of the memory cells 1, 2, 3 to the device 20 is carried out in offline operation, i.e., externally, for example in a workshop, and not during the operation of the motor vehicle 10. Durch die Vorrichtung 20 kann die Kapazität jeder einzelnen Speicherzelle 1, 2, 3 der Energiespeichereinrichtung 11 erfasst werden. Basierend auf dieser Information können dann diejenigen Speicherzellen 1, 2, 3, die tatsächlich einen Kapazitätsverlust, oder einen zu hohen Kapazitätsverlust zeigen, ausgetauscht werden. To determine the capacity of the energy storage device 11, the device 20 has a discharge / charge unit 21 and a calculation unit 22. As previously explained, each storage cell 1, 2, 3 is connected to the device 20. The discharge / charge unit 21 then first discharges the storage cells 1, 2, 3 until the first of the storage cells 1, 2, 3 reaches a lower voltage limit. After discharging, the storage cells 1, 2, 3 are charged until the voltage of the first of the storage cells 1, 2, 3 reaches an upper voltage limit. During the charging process, the voltage levels of storage cells 1, 2, 3 are monitored either by the energy storage unit 11 itself or by the processing unit 22. After the charging process is complete, the processing unit 22 compares which storage cell 1, 2, 3 first reaches the lower (first time point) or upper voltage limit (second time point). If these are different memory cells 1, 2, 3, the processing unit can perform a calibration of the memory cells 1, 2, 3. During such a calibration, the voltage curves of the memory cells 1, 2, 3 are shifted, whereby the voltage curve of the cell 1, 2, 3 that first reaches the upper voltage limit is shifted until its highest value coincides with the voltage curve of the cell 1, 2, 3 that first reaches the lower voltage limit. Subsequently, the discharge and charge cycles of memory cells 1, 2, 3 can be repeated, allowing the usable capacity or capacity loss of the memory cell 1, 2, 3 that reaches both its lower and upper voltage limits to be determined. The corresponding memory cell 1, 2, 3 can then be replaced. Alternatively, the charging capacity of each individual storage cell 1, 2, 3 can be determined separately by the calculation unit 22. In this case, the cell voltage of each individual storage cell 1, 2, 3 can be directly measured and compared. If the energy storage unit 11 itself or the calculation unit 22 determines that the capacity loss of a storage cell 1, 2, 3 exceeds a predetermined limit, the corresponding storage cell 1, 2, 3 can be replaced. Thus, targeted replacement of individual cells 1, 2, 3 is possible. Different voltage profiles C1, C2, C3 of the memory cells 1, 2, 3 are shown in Fig. 2, Fig. 3, Fig. 4 to Fig. 5. In the example shown in Fig. 2, the voltage curves C1, C2, C3 of memory cells 1, 2, and 3 are parallel. Memory cell 3 reaches its lower voltage limit first, while memory cell 1 reaches its upper voltage limit first. This leads to an asymmetry, as none of the memory cells 1, 2, or 3 is fully utilized. In this case, memory cells 1 and 3 can be calibrated. The voltage curve of memory cell 1 is shifted until the upper value of curve C1 matches the upper value of curve C2. At this point, curves C1 and C3 will then overlap. The discharge / charge process can then be repeated. In the example shown in Fig. 3, the voltage curve C3 of memory cell 3 intersects the voltage curves C1 and C2 of memory cells 1 and 2. Therefore, memory cell 3 first reaches its lower and then its upper voltage limits. These limits thus restrict its usable capacity. If memory cell 3 experiences a capacity loss exceeding a predetermined limit, it can be replaced. In the example shown in Fig. 4, the voltage curves C1 and C2 of memory cells 1 and 2 are parallel, whereas the voltage curve C3 of memory cell 3 partially intersects them. Memory cell 3 reaches its lower voltage limit first, while memory cell 1 reaches its upper voltage limit first. This leads to an asymmetry, as none of the memory cells 1, 2, or 3 is measured at its full capacity. In this case, a calibration of memory cells 1 and 3 can also be performed. The voltage curve of memory cell 1 is shifted until the upper value of curve C1 corresponds to the upper value of curve C3. The discharge / charge process can then be repeated. In the example shown in Fig. 5, the voltage curves C1 and C2 of memory cells 1 and 2 are also parallel, whereas the voltage curve C3 of memory cell 3 partially intersects them. In this case, however, memory cell 2 reaches its lower voltage limit first, while memory cell 1 reaches its upper voltage limit first. This also leads to an asymmetry, as none of the memory cells 1, 2, 3 is measured at its full capacity. In this case, a calibration of memory cells 1 and 2 can therefore also be performed. The voltage curve of memory cell 1 is shifted until the upper value of curve C1 corresponds to the upper value of curve C2. Then, curves C1 and C2 would overlap. The discharge / charge process can then be repeated. The proposed energy storage device and apparatus make it possible to determine the capacity and capacity loss of individual storage cells within the device. This allows for a more precise determination of the energy storage system's capacity. Furthermore, information about the capacity of individual storage cells is available, making it possible to replace individual cells as needed, rather than replacing the entire energy storage system. Although the present invention has been described using exemplary embodiments, it can be modified in many ways. Reference sign 1, 2, 3 Storage cells 10 Motor vehicle 11 Energy storage device 20 Device 21 Discharge / charge unit 22 Calculation unit C1, C2, C3 Voltage curves

Claims

Device (20) for determining the capacity of an energy storage device (11) with a plurality of storage cells (1, 2, 3), wherein the device (20) has connecting elements for being connected to each storage cell (1, 2, 3), wherein the device (20) has a discharge / charge unit (21) and a calculation unit (22), characterized in that the discharge / charge unit (21) is configured to be connected to the storage cells (1, 2, 3) of the energy storage device (11) and to discharge and subsequently charge the storage cells (1, 2, 3), and that the calculation unit (22) is configured to monitor the voltage level of each of the storage cells (1, 2, 3) of the energy storage device (11) during the charging process following the discharge process, and the discharge / charge unit (21) is configured to charge each of the storage cells (1, 2, 3) up to a first time discharged, at which a first of the memory cells (1, 2,3) the energy storage device (11) reaches a lower predefined voltage limit and to identify and label this storage cell (1, 2, 3), and subsequently to charge each of the storage cells (1, 2, 3) until a second time point at which a first of the storage cells (1, 2, 3) of the energy storage device (11) reaches an upper predefined voltage limit and to identify and label this storage cell (1, 2, 3). Device (20) according to claim 1, characterized in that the calculation unit (22) is configured to store a lower cell voltage of each storage cell (1, 2, 3) at the end of the discharge process when discharging the energy storage device (11) and to store an upper cell voltage of each storage cell (1, 2, 3) at the end of the charging process when charging the energy storage device (11), and that the calculation unit (22) is configured to compare the lower cell voltages of the storage cells (1, 2, 3) and the upper cell voltages of the corresponding storage cells (1, 2, 3) and to calculate an available capacity of each of the storage cells (1, 2, 3) based on the result of the comparison. Device (20) according to claim 2, characterized in that the calculation unit is configured to determine whether the available capacity is below a predetermined limit and, based on the calculated available capacity, to output a signal indicating whether and / or which memory cell (1, 2, 3) needs to be replaced. Device (20) according to one of the preceding claims, characterized in that the calculation unit (22) is configured to determine the voltage level of each memory cell (1, 2, 3) at the second time. Device (20) according to one of the preceding claims, characterized in that, if the characterized storage cells (1, 2, 3) are the same storage cell (1, 2, 3), the calculation unit (22) is configured to determine a capacity of the energy storage device (11) based on the difference of the voltage levels of the characterized storage cell (1, 2, 3) at the first and the second time. Device (20) according to claim 5, characterized in that the calculation unit (22) is configured to determine whether the capacity is below a predetermined limit value and, if this is the case, to output a signal indicating that the characterized memory cell (1, 2, 3) is to be replaced. Device (20) according to one of the preceding claims, characterized in that, if the characterized storage cells (1, 2, 3) are different storage cells (1, 2, 3), the calculation unit (22) is configured to calculate a difference in the upper voltage levels of the characterized storage cells (1, 2, 3) at the second time, and to calibrate the energy storage device (11) based on the calculated difference by adjusting the time of the start of the charging process for the characterized storage cells (1, 2, 3) such that they reach the upper predefined voltage limit at the same second time, and that the discharge / charge unit (21) is configured to discharge and charge the energy storage device (11) again after calibration. Device (20) according to claim 7, characterized in that the discharge / charge unit (21) and the calculation unit (22) are configured to perform the discharge / charge process and the calibration iteratively until the calculated difference is below a predetermined limit. Device (20) according to claim 7 or 8, characterized in that the calculation unit (22) is configured to determine whether the capacity of the marked memory cells (1, 2, 3), which is reached within the time period from a start of charging, in particular from the adapted time, until reaching the upper defined voltage limit, is below a predetermined limit, and, if this is the case, to output a signal indicating that the marked memory cell (1, 2, 3) is to be replaced. Energy storage device with a plurality of storage cells (1, 2, 3), characterized in that each storage cell (1, 2, 3) is configured to be connected in offline operation separately from the other storage cells (1, 2, 3) to the device (20) according to one of the preceding claims for determining the capacity of the energy storage device (11). Energy storage device according to claim 10, characterized in that each storage cell (1, 2, 3) can be connected separately to the device (20) via a connecting element or that the plurality of storage cells (1, 2, 3) can be connected to the device (20) with a single connecting element. Method for determining the capacity of an energy storage device (11) with a plurality of storage cells (1, 2, 3), characterized by the steps: connecting the storage cells (1, 2, 3) of the energy storage device (11) to a device (20) for determining the capacity; discharging the storage cells (1, 2, 3) and subsequently charging the storage cells (1, 2, 3); monitoring the voltage level of each of the storage cells (1, 2, 3) of the energy storage device (11) during the charging process following the discharge process; discharging each storage cell (1, 2, 3) until a first time at which a first of the storage cells (1, 2, 3) of the energy storage device (11) reaches a lower predefined voltage limit, and identifying and marking this storage cell;and charging of each storage cell (1, 2, 3) until a second time point at which a first of the storage cells (1, 2, 3) of the energy storage device (11) reaches an upper predefined voltage limit, and identifying and labeling this storage cell.;

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