Device and method for controlling an interface of an electric power system

Abstract

Techniques are disclosed for performing a state-of-health (SoH) estimation of a rechargeable battery energy storage system during operation of the rechargeable battery energy storage system. The rechargeable battery energy storage system includes a plurality of individually controllable parallel strings. At least one string is selected from the plurality of parallel strings. The selected at least one string is placed in an SoH calibration mode for performing SoH calibration while at least one other string of the plurality of parallel strings remains in an operating mode. After SoH calibration has been completed for the selected at least one string, the selected at least one string is returned to the operating mode.

Description

Technical Field

[0001] The present invention relates to methods and devices for performing an estimation of the state of health (SoH) of a rechargeable battery energy storage system. The present invention particularly relates to methods and devices for performing SoH estimation on a rechargeable battery energy storage system including a parallel string of a plurality of battery cells. Background Art

[0002] Battery energy storage systems (BESSs) are considered to make an important contribution to the stability of future power grids with a significant portion of renewable energy sources, and are also of great significance for clean electric transportation. Batteries degrade over time. Therefore, an important quantity to monitor is the state of health (SoH). The SoH can be characterized by one or more key performance indicators (KPIs). The KPIs can be the maximum remaining capacity and the internal resistance. Specifically, the SoH can provide a measure of the current capacity and internal resistance of the battery relative to the same corresponding parameters of the battery when it was new. This is an important indicator for determining the ability of the battery to store and transfer energy. This measure is also generally considered to be a measure of battery degradation used in determining when the battery has reached its useful life. Therefore, the SoH has an impact on the proper control and maintenance of the battery system.

[0003] Generally, for an accurate estimation of the BESS, the SoH requires operating the battery in a specific way: starting from a specific state of charge, a specific current change curve should be applied both in the charge and discharge directions until certain cut-off voltages are met. This is called an SoH calibration cycle. Therefore, the SoC range covered should be as large as possible to maximize accuracy. Usually, performing such an SoH calibration cycle renders the entire BESS temporarily inoperable. In some critical applications, this has a significant impact on the ability of the energy storage system to provide ancillary services (e.g., frequency stabilization, load regulation, voltage support, etc.), which in turn affects the reliability of the system to which the energy storage system is connected. To estimate the SoH of a large BESS, it is taken offline for several hours today while applying one or several SoH calibration cycles. This increases the operating cost.

[0004] Methods for online estimating the SoH using operating data instead of data recorded during calibration cycles (e.g., based on least squares, artificial neural networks, and Kalman filters and their variants) are not as accurate as a full discharge cycle required for an accurate estimation. For example, full discharge cycles rarely occur, even during normal operation, and the load on the system is inconsistent during the measurement cycle.

[0005] The state-of-health (SoH) calculation of the entire BESS system also provides a large number of measurements of the health state of the entire BESS system using online data and the regular dedicated SoH of the entire BESS. As a result, the obtained result values are not accurate enough because they do not allow the evaluation of the SoH of individual strings. Since the environmental conditions in different BESS locations can be different, the degradation of different strings of battery cells is often not equal. For example, the position relative to heating, ventilation, and air conditioning (HVAC) and / or relative to the outer wall or door of the BESS container can be different for different strings. Determining the SoH only for all the strings combined limits the possibility of performing advanced uneven operations on the parallel strings of the BESS system. Furthermore, the degradation of individual strings of battery cells cannot be distinguished by using existing methods. This may make it difficult to establish the root cause of capacity reduction or failure in the BESS system, which in turn leads to low maintenance work efficiency.

[0006] "Optimizing a Battery Energy Storage System for Primary Frequency Control" by A. Oudalov et al., IEEE Transactions on Power Systems, Vol. 22, No. 3, 2007 discloses a technique for increasing battery life by avoiding high-frequency charge and discharge sequences.

[0007] DE 10 2014 210 010A1 discloses a method of operating an electrical energy storage system that is connected to an electrical power supply grid for providing a set power and includes a plurality of storage units that are electrically connected at a common coupling point and in which the set power is distributed. The total efficiency of the electrical energy storage system is matched to the set power by adjusting the individual power components to be provided by the storage units.

[0008] None of these documents address the need to perform an SoH calibration cycle.

[0009] US 2016 / 0370433 A1 describes a method that: collects information related to a physical quantity to determine the aging state of each of a plurality of batteries; selects a battery from the plurality of batteries based on the collected information and / or information regarding a battery usage plan and / or user instructions; preferentially charges the selected battery during an energy storage phase until a predetermined maximum charge level depending on the aging state is reached; preferentially discharges the selected battery during an energy release phase if the selected battery has reached the maximum charge level during the storage phase until a predetermined minimum charge level depending on the aging state is reached; or if the predefined maximum charge level has not been reached, renders the selected battery inactive and then measures the energy level generated by the selected battery; calculates the remaining capacity of the selected battery based on the measurement of the energy level generated by the battery.

[0010] US 2017 / 0033410 A1 describes a battery cabinet group that includes a plurality of battery cabinets, each of which includes a plurality of battery modules connected to each other. The battery modules are replacement units. A deterioration inspection block obtains characteristics of each of the battery modules to inspect their deterioration states. A numbering block numbers each of the battery modules in ascending order of deterioration based on the characteristics obtained by the deterioration inspection block. A rearrangement determination block determines the rearrangement position of each of the battery modules in such a way that the numbers given by the numbering block proceed in sequence within the same battery cabinet.

[0011] US 2017 / 0033410 A1 aims to provide a secondary battery system and a method of arranging battery modules that can restore system performance and maintain system performance over a long period.

[0012] This conventional technique has various drawbacks. For example, during the described information collection process, the battery system has an increased risk of not being able to deliver the required energy on demand. This can affect the availability of the BESS. This can also have a significant impact on the ability of the energy storage system to provide ancillary services (such as frequency response, load regulation, voltage support, etc.), which in turn affects the reliability of the systems to which they are connected. Summary of the Invention

[0013] There is still a need in the art for improved devices and methods that mitigate at least some of the drawbacks of conventional techniques. In the art, there is a particular need for a method and a device that allow for the execution of a dedicated state of health (SoH) calibration cycle on a string (one or more) of a rechargeable battery energy storage system (BESS) without taking the entire BESS out of operation. There is a need in the art for such a method and device that allow for the individual determination of SoH for each of a plurality of strings of a BESS.

[0014] These needs are not addressed in the conventional systems described above. According to an embodiment of the present invention, at least one of the plurality of parallel strings of the BESS exits normal operation to perform SoH calibration. The SoH calibration of the at least one selected string may include one or a number of dedicated SoH cycles, in each of which the string is substantially fully charged and discharged (e.g., charged to 80% or more and discharged to 20% or less). The remaining strings not placed in the SoH calibration mode may serve the difference in the required load profile of the remaining strings. Thus, the risk that the battery system may not be able to deliver the desired energy on demand can be mitigated or eliminated. The ability of the BESS to provide ancillary services (such as frequency response, load regulation, voltage support, etc.) is not impaired as in the traditional methods.

[0015] A method for performing SoH estimation on a rechargeable battery energy storage system during operation of the rechargeable battery energy storage system is provided. The rechargeable battery energy storage system includes a plurality of individually controllable parallel strings. The method includes selecting at least one string from the plurality of parallel strings. The method includes placing the selected at least one string in an SoH calibration mode to perform SoH calibration while keeping at least one other string of the plurality of parallel strings in an operating mode. The method includes returning the selected at least one string to the operating mode after the SoH calibration for the selected at least one string has been completed.

[0016] As used herein, the term "operating mode" is used to refer to the operating mode of a string in which the string contributes to the desired functions of the BESS, which can store energy and / or provide energy, such as for providing ancillary services (such as for stability or regulatory purposes) or other purposes.

[0017] Maintaining at least one other string in the operating mode includes using at least one other string to store energy supplied to the power grid or at least one electricity user.

[0018] The method may include using at least one other string in the operating mode to compensate for the fact that the selected at least one string is in the SoH calibration mode.

[0019] The method may include increasing the current flowing into or out of at least one other string kept in the operating mode in response to placing the selected at least one string in the SoH calibration mode to ensure that the BESS meets the required load profile. The required load profile may be a load profile that ensures the BESS provides frequency stability, load regulation, and / or voltage support to the power grid. The power grid may be a national or regional power grid or an island power grid.

[0020] The method can include increasing the current supplied to at least one other string in the operating mode in response to at least one selected string being placed in the SoH calibration mode when storing energy in the BESS.

[0021] The method can include increasing the current supplied by at least one other string in the operating mode in response to at least one selected string being placed in the SoH calibration mode when the BESS supplies energy to the grid or a consumer.

[0022] At any time during SoH estimation, at least 50% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0023] At any time during SoH estimation, at least 70% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0024] At any time during SoH estimation, at least 80% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0025] At any time during SoH estimation, at least 90% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0026] At any time during SoH estimation, only one or two of the parallel strings can be placed in the SoH calibration mode simultaneously.

[0027] The method can further include: performing one or several SoH calibration cycles on at least one selected string when the at least one selected string is in the SoH calibration mode and before causing the at least one selected string to resume the operating mode.

[0028] The method can further include sorting and / or matching the plurality of parallel strings based on performance characteristics.

[0029] The performance characteristics can include degradation metrics for each of the plurality of parallel strings.

[0030] The degradation metrics can be based on the SoH estimation results obtained in a previous SoH estimation procedure, and / or, for example, when the plurality of parallel strings are disconnected from the grid, based on the current drawn from each of the plurality of parallel strings.

[0031] Sorting the plurality of parallel strings can include establishing a sequence of the parallel strings depending on the performance characteristics.

[0032] Matching multiple parallel strings can include identifying multiple pairs of strings. The identified pairs can be those strings that have the most similar performance characteristics to each other based on a metric. The identified pairs can be strings for which the modulus of the difference in performance characteristics (e.g., the difference in the SoH quantizer or the difference in the current drawn when disconnecting the strings from the power grid) is less than a threshold or is minimized.

[0033] The steps of selecting at least one string, placing the selected at least one string in the SoH calibration mode, and returning the selected at least one string to the operating mode can be performed sequentially for at least a subset of the multiple parallel strings. The method can include repeating the following steps until all of the parallel strings have been placed in the SoH calibration mode and SoH calibration has been performed: selecting one or more strings; placing one or more strings in the SoH calibration mode; and returning one or more strings to the operating mode.

[0034] The method can include determining an order for selecting strings for SoH calibration.

[0035] The order can be determined based on sorting. For example, the strings (in a one-by-one fashion or in a set of two or more strings) can be placed in the SoH calibration mode sequentially in an order determined by the performance characteristics.

[0036] The order can be determined based on matching. For example, the strings can be placed in the SoH calibration mode sequentially such that pairs of strings identified based on the performance characteristics are placed in the SoH calibration mode simultaneously.

[0037] Selecting at least one string can include selecting a first string and selecting a second string, where, during at least a portion of the SoH calibration, the second string acts as an energy sink for the energy released from the first string.

[0038] Placing the selected at least one string in the SoH calibration mode can include controlling the power flow through a converter interconnected between the selected at least one string and the point of common coupling according to a calibration load profile curve.

[0039] The converter connected to the selected at least one string can be controlled according to the calibration load profile curve and independently of the load profile curve of the power grid or the consumer connected to the point of common coupling of the BESS.

[0040] The power flow through a converter interconnected between at least one other string remaining in the operating mode can be controlled such that the power flow depends on the load profile curve of the power grid or the consumer connected to the point of common coupling of the BESS and optionally also on the calibration load profile curve of the selected at least one string placed in the SoH calibration mode.

[0041] Placing at least one selected string in the SoH calibration mode may include: disconnecting the at least one selected string from the common coupling point; and temporarily connecting the at least one selected string to the SoH cycling energy source and / or energy sink.

[0042] At least during SoH calibration, the SoH cycling energy source and / or energy sink may be disconnected from the common coupling point of the plurality of parallel strings.

[0043] The SoH cycling energy source may be a dedicated SoH energy source used only for SoH calibration.

[0044] The SoH cycling energy source and / or energy sink may be another string in the string.

[0045] During SoH calibration, the battery cells of the at least one selected string may be charged and / or discharged according to the calibration load variation curve.

[0046] The calibration load variation curve may be pre-determined or defined by the operator.

[0047] During SoH calibration, the calibration load variation curve for charging and / or discharging the at least one selected string may be independent of the load of the remaining at least one other string that remains in the operating mode simultaneously. For example, the calibration load variation curve for charging and / or discharging the at least one selected string during SoH calibration may be independent of the rate of storing energy (e.g., from the power grid or a local energy source, such as a renewable energy source) into at least one other string operating in the operating mode simultaneously. The calibration load variation curve for charging and / or discharging the at least one selected string during SoH calibration may be independent of the rate of providing energy (e.g., providing energy to the power grid or consumers) by at least one other string operating in the operating mode simultaneously.

[0048] During each successive SoH calibration cycle performed during SoH calibration, the at least one selected string is charged to a first level above a first threshold and discharged to a second level below a second threshold.

[0049] The first threshold may be 80% or more, 90% or more, 95% or more, or 97% or more.

[0050] The second threshold may be 20% or less, 10% or less, 5% or less, or 3% or less.

[0051] The SoH calibration cycle performed during SoH calibration may involve charging and discharging the string within a range from 20% or less to 80% or more, from 10% or less to 90% or more, from 5% or less to 95% or more, or from 3% or less to 97% or more of its rated capacity.

[0052] Maintaining at least one other string in an operating mode may include maintaining all strings of a plurality of parallel strings in an operating mode in addition to the selected at least one string.

[0053] The BESS may provide at least one ancillary service to the power grid.

[0054] The at least one ancillary service may be selected from the group consisting of frequency response, load regulation, and voltage support.

[0055] The method may include automatically determining a time at which SoH estimation is to be started.

[0056] The time at which SoH estimation is to be started may be determined based on the historical load profile of the BESS. The time at which SoH estimation is to be started may be determined based on prior information regarding the load profile of the BESS.

[0057] Performing SoH calibration on the selected strings may respectively include determining a number of parameters defined by an equivalent circuit model (ECM) of the selected at least one string or each cell of the selected at least one string.

[0058] The number of parameters may be determined independently of the state of charge (SoC) estimation of the battery management system (BMS).

[0059] The number of parameters may include internal resistance.

[0060] The number of parameters may include a number of internal resistances.

[0061] The number of parameters may include information regarding the relative capacitance of two capacitors in the ECM.

[0062] The relative capacitance may include the quotient of the two capacitors.

[0063] An iterative procedure may be used to determine the number of parameters. Each iteration may include combined state and parameter estimation.

[0064] The iterative procedure may use the measured open circuit voltage (OCV) of a battery cell as an input.

[0065] The estimation of the number of parameters of the ECM in each iteration may be based on a Python optimization modeling object (Pyomo) model.

[0066] The plurality of parallel strings may form a stationary battery energy storage system BESS.

[0067] The plurality of parallel strings may be installed on a vehicle.

[0068] Each string of the plurality of parallel strings may include a plurality of batteries.

[0069] The battery may be a lithium-ion battery, but is not limited thereto.

[0070] This method can be executed by a control device.

[0071] This method can be automatically executed by a control device.

[0072] The control device can include at least one integrated semiconductor circuit that executes the method steps explained herein.

[0073] The control device can control the converter, switch, or circuit breaker of the BESS to selectively place at least one selected string in the SoH calibration mode while keeping at least one other string in the operating mode. The control converter is particularly suitable for controlling the power flow during SoH calibration.

[0074] There is also provided a control device for controlling the SoH estimation of a rechargeable battery energy storage system. The rechargeable battery energy storage system includes a plurality of individually controllable parallel strings. The control device includes an interface operable to be coupled to the plurality of parallel strings. The control device includes at least one integrated semiconductor circuit that is coupled to the interface and is operable to select at least one string from the plurality of parallel strings, control the rechargeable battery energy storage system to place the at least one selected string in the SoH calibration mode for SoH calibration while keeping at least one other string among the plurality of parallel strings in the operating mode, and control the rechargeable battery energy storage system to cause the at least one selected string to resume the operating mode after the SoH calibration for the at least one selected string is completed.

[0075] The control device can operate to control the plurality of parallel strings such that at least one other string kept in the operating mode is used to store the energy supplied to the power grid or at least one electricity user.

[0076] The control device is operable to control the plurality of parallel strings such that at least one other string in the operating mode is used to compensate for the fact that at least one selected string is in the SoH calibration mode.

[0077] The control device can operate to control the plurality of parallel strings such that in response to placing at least one selected string in the SoH calibration mode, the current flowing into or out of at least one other string kept in the operating mode is increased to ensure that the BESS meets the required load profile. The required load profile can be a load profile that ensures the BESS provides frequency stability, load regulation, and / or voltage support to the power grid. The power grid can be a national or regional power grid or an island power grid.

[0078] The control device is operable to control the plurality of parallel strings such that when storing energy in the BESS, the amount of current supplied to at least one other string in the operating mode is increased in response to placing at least one selected string in the SoH calibration mode.

[0079] The control device is operable to control a plurality of parallel strings such that when the BESS supplies energy to the grid or a user, the current supplied by at least one other string in the operating mode is increased in response to placing at least one selected string in the SoH calibration mode.

[0080] The control device is operable to control a plurality of parallel strings such that at any time during SoH estimation, at least 50% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0081] The control device is operable to control a plurality of parallel strings such that at any time during SoH estimation, at least 70% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0082] The control device is operable to control a plurality of parallel strings such that at any time during SoH estimation, at least 80% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0083] The control device is operable to control a plurality of parallel strings such that at any time during SoH estimation, at least 90% of the total number of strings in the plurality of strings can be kept in the operating mode.

[0084] The control device is operable to control a plurality of parallel strings such that at any time during SoH estimation, only one or two parallel strings in the parallel strings can be placed in the SoH calibration mode simultaneously.

[0085] The control device is operable to control a plurality of parallel strings such that one or more SoH calibration cycles are performed on at least one selected string while the at least one selected string is in the SoH calibration mode and before causing the at least one selected string to resume the operating mode.

[0086] The control device is operable to sort and / or match a plurality of parallel strings based on performance characteristics.

[0087] The control device is operable to use performance characteristics including degradation metrics for each of the plurality of parallel strings.

[0088] The degradation metric can be based on the SoH estimation results obtained in a previous SoH estimation procedure and / or, for example, based on the current drawn from each of the plurality of parallel strings when the plurality of parallel strings are disconnected from the grid.

[0089] The control device is operable such that sorting the plurality of parallel strings includes establishing a sequence of the parallel strings based on performance characteristics.

[0090] The control device is operable such that matching multiple parallel strings includes identifying multiple pairs of strings. The identified pairs can be those strings that have the most similar performance characteristics to each other based on a metric. The identified pairs can be those strings whose difference in performance characteristics (e.g., difference in SoH quantifiers or difference in current drawn when disconnecting these strings from the power grid) has a modulus less than a threshold or is minimized.

[0091] The control device is operable to control multiple parallel strings such that it sequentially performs the following steps for at least a subset of the multiple parallel strings: select at least one string; place the selected at least one string in the SoH calibration mode; and return the selected at least one string to the operating mode. The control device is operable to control multiple parallel strings such that it sequentially performs the following steps until all parallel strings have been placed in the SoH calibration mode and have had SoH calibration performed: select one or more strings; place the one or more strings in the SoH calibration mode; and return the one or more strings to the operating mode.

[0092] The control device is operable to determine the order of selecting strings for SoH calibration.

[0093] The control device is operable such that the order is determined based on sorting. For example, the control device can place the strings in the SoH calibration mode sequentially (one by one or in sets of two or more strings) in an order determined by the performance characteristics.

[0094] The control device is operable such that the order is determined based on matching. For example, the control device is operable such that pairs of strings identified based on performance characteristics are placed in the SoH calibration mode simultaneously.

[0095] The control device is operable to select a first string and a second string to be placed in the SoH calibration mode simultaneously, where during at least a portion of the SoH calibration, the second string serves as an energy sink for the energy released from the first string.

[0096] The control device is operable to control the power flow through the converter according to a calibration load variation curve to place the selected at least one string in the SoH calibration mode, where the converter is interconnected between the selected at least one string and the common coupling point.

[0097] The control device is operable to control the power flow through the converter connected to the selected at least one string according to the calibration load variation curve and independently of the load variation curve of the power grid or the user connected to the common coupling point by the BESS.

[0098] The control device is operable to control the power flow through the converter such that the power flow depends on the load profile of the BESS towards the grid or the user connected to the point of common coupling and optionally also on the calibration load profile of at least one selected string placed in the SoH calibration mode, the converter being interconnected between at least one other string remaining in the operating mode.

[0099] The control device is operable to disconnect at least one selected string from the point of common coupling and to temporarily connect the at least one selected string to the SoH cycling energy source and / or energy sink to place it in the SoH calibration mode.

[0100] At least during SoH calibration, the SoH cycling energy source and / or energy sink can be disconnected from the point of common coupling of the plurality of parallel strings.

[0101] The SoH cycling energy source can be a dedicated SoH energy source only for SoH calibration.

[0102] The SoH cycling energy source and / or energy sink can be another string in the string.

[0103] The control device is operable to control the charging and / or discharging of the cells of at least one selected string according to the calibration load profile during SoH calibration.

[0104] The calibration load profile can be pre - determined or defined by the operator.

[0105] The control device is operable such that during SoH calibration, the calibration load profile for charging and / or discharging at least one selected string is independent of the load of the remaining at least one other string remaining in the operating mode. For example, the control device is operable such that the calibration load profile for charging and / or discharging at least one selected string during SoH calibration can be independent of the rate at which energy (e.g., from the grid or a local energy source, such as a renewable energy source) is simultaneously stored in at least one other string operating in the operating mode. The control device is operable such that the calibration load profile for charging and / or discharging at least one selected string during SoH calibration is independent of the rate at which at least one other string operating in the operating mode simultaneously supplies (e.g., to the grid or the user) energy.

[0106] The control device is operable such that during each successive SoH calibration cycle performed during SoH calibration, the at least one selected string is charged to a first level above a first threshold and discharged to a second level below a second threshold.

[0107] The first threshold can be 80% or above, 90% or above, 95% or above, or 97% or above.

[0108] The second threshold may be 20% or less, 10% or less, 5% or less, or 3% or less.

[0109] The control device is operable such that the SoH calibration cycles performed during SoH calibration may involve charging and discharging the string within a range from 20% or less to 80% or more, from 10% or less to 90% or more, from 5% or less to 95% or more, or from 3% or less to 97% or more of its rated capacity.

[0110] The control device is operable to control a plurality of parallel strings such that all of the plurality of parallel strings except for at least one selected string are maintained in an operating mode.

[0111] The control device is operable to automatically determine the time at which SoH estimation is to begin.

[0112] The control device is operable to determine the time at which SoH estimation is to begin based on the historical load profile of the BESS. The control device is operable to determine the time at which SoH estimation is to begin based on prior information of the load profile for the BESS.

[0113] The control device is operable to determine a number of parameters defined by an equivalent circuit model (ECM) of at least one selected string or each cell of the at least one selected string.

[0114] The control device is operable to determine a number of parameters independent of the state of charge (SoC) estimation of the battery management system (BMS).

[0115] A number of parameters determined by the control device may include internal resistance.

[0116] A number of parameters determined by the control device may include a number of internal resistances.

[0117] A number of parameters determined by the control device may include information about the relative capacitance of two capacitors in the ECM.

[0118] The relative capacitance may include the quotient of the two capacitors.

[0119] The control device is operable to use an iterative procedure to determine a number of parameters. Each iteration may include combined state and parameter estimation.

[0120] The control device is operable to use the measured open circuit voltage (OCV) of the battery as an input to the iterative procedure.

[0121] The control device is operable to perform estimation of a number of parameters of the ECM at each iteration based on a Pyomo model.

[0122] The control device is operable to perform a method according to an embodiment.

[0123] A rechargeable battery energy storage system includes a plurality of individually controllable parallel strings and a control device according to an embodiment, the control device being coupled to the plurality of parallel strings.

[0124] The BESS is operable to provide at least one ancillary service to the power grid.

[0125] The at least one ancillary service may be selected from the group consisting of frequency response, load regulation, and voltage support.

[0126] The plurality of parallel strings may form a stationary battery energy storage system BESS.

[0127] The plurality of parallel strings may be mounted on a vehicle.

[0128] Each of the plurality of parallel strings may include a plurality of batteries.

[0129] The battery may be a lithium-ion battery, but is not limited thereto.

[0130] The device, method, and system according to an embodiment provide various advantages. A complete charge and discharge cycle can be performed on a selected at least one string while the BESS remains operable to perform its required functions. The disclosed technology is particularly applicable to BESSs that provide ancillary services such as frequency stabilization, voltage regulation, etc. For safety reasons, the size of such BESSs is typically adapted to large loads, while in fact they operate at much smaller loads for the vast majority of their service life. This enables individual strings to be taken out of the operating mode to perform a complete charge and discharge cycle while the BESS is still able to perform its required ancillary functions for the power grid.

[0131] The device, method, and system according to an embodiment also allow the SoH to be determined separately for each string. Thus, the SoH can be determined not only for the entire BESS but also separately for each string. The SoH of an individual string can be evaluated. BRIEF DESCRIPTION OF THE DRAWINGS

[0132] The subject matter of the present invention will be explained in more detail with reference to the preferred exemplary embodiments shown in the drawings, wherein:

[0133] Figure 1 is a schematic diagram of a system with a control device according to an embodiment.

[0134] Figure 2 is a schematic diagram of a system with a control device according to an embodiment.

[0135] Figure 3 is a schematic diagram of a system with a control device according to an embodiment.

[0136] Figure 4 is a flowchart of a method according to an embodiment.

[0137] Figure 5 is a flowchart of a method according to an embodiment.

[0138] Figure 6 is a flowchart of a method according to an embodiment.

[0139] Figure 7 is a flowchart of a method according to an embodiment.

[0140] Figure 8 、 9 、10 and 11 are schematic diagrams of a system for explaining the operation of an apparatus and method according to an embodiment.

[0141] Figure 12 is a circuit diagram of an equivalent circuit model (ECM) of a string or battery that can be used in an embodiment.

[0142] Figure 13 shows the open circuit voltage and hysteresis of a battery as a function of state of charge.

[0143] Figure 14 is a flowchart of a method according to an embodiment. Detailed Description

[0144] Exemplary embodiments of the present invention will be described with reference to the accompanying drawings, in which like or similar reference numerals denote like or similar elements. Although some embodiments will be described in the context of a stationary battery energy storage system (BESS) or a BESS installed in a vehicle, the present invention is not limited thereto. Although embodiments of the present invention may be applied to each energy storage system based on a lithium-ion battery, the present invention is not limited thereto, and any may be applied to other types of BESS.

[0145] The features of the embodiments may be combined with each other unless otherwise specifically stated.

[0146] Like or similar reference numerals are used to denote like or similar elements in the drawings.

[0147] Figure 1 is a schematic diagram of system 10. System 10 includes a rechargeable battery energy storage system (BESS). The rechargeable BESS includes a plurality of 20 parallel strings 21-23 that can be individually controlled. Each of the plurality of parallel strings 21-23 may include a plurality of batteries. The plurality of parallel strings 21-23 may be coupled to a common coupling point 35 via converters and / or transformers 31-34.

[0148] Each of the strings 21-23 includes a battery, for example, at least one series connection of lithium-ion batteries. AsFigure 1 As schematically shown, some or all of the strings may include a series connection of multiple batteries, and the series connections of these multiple batteries are coupled in parallel with each other.

[0149] The BESS may be coupled to the power grid at the point of common coupling 35. The power grid may be a national or regional power grid. The power grid may be an island power grid.

[0150] The BESS is operable to provide ancillary services to the power grid, such as providing frequency stability, load regulation, and / or voltage support. The size of the BESS operating to provide such ancillary services is typically designed to accommodate peak demand, but during most of its operation, the power or load it operates at is much lower than the power or load it is designed for. The techniques described in detail herein are particularly applicable to such BESSs that must be able to provide certain performance characteristics (e.g., due to contractual obligations or to accommodate worst-case scenarios), and typically operate at a load much smaller than the rated load they are designed for.

[0151] According to some embodiments, while the BESS is still fully operational, a single string (e.g., string 21) selected from among the multiple parallel strings 21-23 can be placed in the SoH calibration mode at a time, while all the other remaining strings (e.g., strings 22 and 23) provide the difference for the required load profile of the BESS. One or several SoH calibration cycles can be performed on the string selected and placed in the SoH calibration mode. During this process, the selected string can be substantially fully charged and discharged. After the SoH calibration for the string (e.g., string 21) has been completed, the string can be returned to normal operation, and the SoH calibration can be performed on the next string (e.g., any one of the remaining strings 22 and 23).

[0152] According to other embodiments, two strings can be placed in the SoH calibration mode simultaneously and can be operated in opposite directions, i.e., one of the two strings in the SoH calibration mode is charged while the other string in the SoH calibration mode is discharged. For example, if (especially in a battery with a large number of strings, such as five or more strings or ten or more strings) two strings are placed in the SoH calibration mode simultaneously, the remaining strings can be operated to provide the load required by the BESS without deviation.

[0153] For the strings placed in the SoH calibration mode, the power flow through one or two converters 31-33 connected to the strings 21-23 placed in the SoH calibration mode can be controlled according to the calibration load profile. The calibration load profile may involve substantially fully charging and discharging the selected string(s) placed in the SoH calibration mode. The converters connected to the other strings remaining in the operating mode can be controlled according to the load profile of the complete BESS towards the point of common coupling 35 and optionally also depending on the calibration load profile.

[0154] If the BESS is used in applications where the load on its load profile is typically much less than the rated load (such as frequency regulation), then the techniques disclosed herein can be easily applied throughout (if not all) the useful life of the BESS.

[0155] For other applications that regularly require the rated load (such as peak shaving), it is still possible to synchronize the SoH maintenance with the load profile. This can be easily achieved if the load profile is known in advance, but it can also be done even if the load profile is not known in advance.

[0156] As a result, an accurate SoH estimate can be made for each of the plurality of individual strings 21 - 23, enabling more advanced control and monitoring within the BESS system. This can be achieved without taking the entire BESS out of service. When one or two of the strings 21 - 23 are placed in the SoH calibration mode, all the other strings can operate to compensate for the deficit created by placing one or two strings in the SoH calibration mode. The remaining strings that remain in the operating mode (i.e., to perform the functions of the installed BESS, such as frequency regulation, etc.) can also provide the energy required to charge the string(s) placed in the SoH calibration mode.

[0157] System 10 includes a control device 40. The control device 40 is operable to control the SoH estimation process. The control device 40 can include an interface 41 connected to the plurality of strings. The interface 41 is operable to control the plurality of strings, for example, via control converters 31 - 33, switches, or by communicating with a battery management system (BMS).

[0158] The control device 40 includes one or more integrated circuits (ICs) 42. The IC(s) 42 can include one or more of a processor, a microprocessor, a controller, a microcontroller, an application specific integrated circuit (ASIC), or a combination thereof.

[0159] One or more ICs 42 are operable to execute a control module 43 for controlling the SoH estimation process. One or more ICs 42 are operable to select at least one string from among the plurality of parallel strings 21-23. The selection can be performed such that only one or only two of the strings 21-23 are selected to be placed in a dedicated SoH calibration mode at any time during the SoH estimation (in which mode they are not available to assist the other strings in performing the required functions of the BESS). One or more ICs 42 can sequentially select all of the strings 21-23 and place them in the SoH calibration mode, where only one or two of the strings 21-23 are selected at any time during the SoH estimation. As will be described below, the selection can be performed based on the sorting or matching of the strings. One or more ICs 42 can optionally use historical information about previous SoH estimations or other degradation metrics to determine in what order the strings 21-23 are placed in the SoH calibration mode. In other embodiments, one or more ICs 42 can select the strings 21-23 in a predetermined order or in a random order and place them in the SoH calibration mode.

[0160] One or more of the selected strings 21-23 can be placed in the SoH calibration mode by controlling the power flow to and from the selected at least one string during SoH calibration. Exemplary evaluation techniques that can be implemented by the control device 40 will be described below.

[0161] One or more ICs 42 are operable to place the selected at least one string in the SoH calibration mode to perform SoH calibration while maintaining at least one other string among the plurality of parallel strings and preferably all strings except the selected at least one string in the operating mode. The strings remaining in the operating mode remain coupled to the common coupling point 35 and provide energy storage to the power grid or user connected to the common coupling point 35. The strings remaining in the operating mode are operated to compensate for the fact that the selected at least one string is not available to assist in implementing normal BESS operation when placed in the SoH calibration mode. One or more ICs 42 are operable to output control signals or control commands to the converters 31-33 connected to the strings 21-23, switches, and / or BMS to place the selected at least one string in the SoH calibration mode. One or more ICs 42 are operable to disconnect the selected at least one string from the common coupling point 35 while performing the SoH calibration.

[0162] The power flow of each string of at least one other string in the operating mode can be maintained by controlling the flow direction, and the power flow from each string of the at least one other string can keep the at least one other string in the operating mode, so that the BESS meets the required load change curve towards the point of common coupling 35. For example, the converters 31-33 connected to at least one other string maintained in the operating mode can be controlled based on the load change curve of the BESS towards the point of common coupling. Optionally, when at least one other string maintained in the operating mode also serves as a SoH cycling source, the power flow through the converters 31-33 connected to at least one other string maintained in the operating mode can additionally depend on the calibrated load change curve, which is applied to the selected at least one string that exits the operating mode and is placed in the SoH calibration mode.

[0163] The IC(s) 42 can charge or discharge the selected at least one string one or more times in one or several SoH calibration cycles. The calibrated load change curve can be applied in a pre-determined or defined by the operator, for example, via the user interface of the control device 40, SoH calibration cycle. Each (one or more) SoH calibration cycle can involve charging the selected at least one string to be substantially fully charged (e.g., to at least 80%, 90%, 95% or 97% of its rated maximum state of charge) and discharging the selected at least one string to be substantially fully discharged (e.g., to less than 20%, 10%, 5% or 3% of the rated maximum state of charge).

[0164] The IC(s) 42 can control the selected at least one string to be charged and discharged one or more times according to the calibrated load change curve by controlling the converter that couples the selected at least one string to the point of common coupling 35.

[0165] During the SoH calibration of the selected string, such as string 21, one or several of the other strings 22 and 23 can serve as a SoH cycling source, so that the string is charged when it is placed in the SoH calibration mode. When the selected string (e.g., string 21) is placed in the calibration mode, the IC(s) 42 can control all the converters 31-33 so that

[0166] - the current flowing through the converter 31 connected to the selected string 21 in the SoH calibration mode conforms to the calibrated load change curve and is independent of the total current of the BESS flowing towards the power grid or the user at the point of common coupling 35; and

[0167] - The current flowing through converters 32, 33 of all other strings 22, 23 that are still in the operating mode is such that the total current required by the BESS is provided to the grid or the user and the current required for the SoH cycling of converter 31 can be selectively provided. If a dedicated SoH cycling source and / or sink separate from the multiple individually controllable strings is used, it is not necessary for the other strings 22, 23 in the operating mode to provide the current required for the SoH cycling of converter 31. Similarly, if pairs of strings are selected such that two strings exit the operating mode and one of the selected strings acts as the SoH source for the other and vice versa, it is not necessary for all the remaining strings still in the operating mode to power the SoH cycling.

[0168] After SoH calibration is completed for the selected at least one string, the IC(s) 42 can return the selected at least one string to the operating mode. This can include outputting a control signal or control command to the converter or switch connected to the selected at least one string and / or the BMS.

[0169] The IC(s) 42 can also execute the evaluation module 44 to perform SoH estimation on the selected at least one string when placing it in the SoH calibration mode. The IC(s) 42 is operable to determine a number of parameters defined by the equivalent circuit model (ECM) of the selected at least one string or each cell of the selected at least one string. The number of parameters can include a number of internal resistances and information about the relative capacitance of two capacitors in the ECM, which will be described in more detail below.

[0170] The IC(s) 42 is operable to sequentially control all the parallel-connected strings 21 - 23 to be placed in the SoH calibration mode and determine a number of parameters defined by the ECM, while all the other strings not placed in the SoH calibration mode can contribute to the normal operation of the BESS towards the grid or local users connected to the common coupling point 35. The other strings not placed in the SoH calibration mode can also selectively act as a SoH calibration cycling source that powers the selected at least one string placed in the SoH calibration mode. The SoH estimation can be implemented in such a way that at any time during the SoH estimation, at most one or two strings are in the SoH calibration mode.

[0171] Figure 2System 10 according to an embodiment is schematically shown. The system 10 may have a SoH cycle source and / or sink for providing and / or receiving power during the execution of SoH calibration when at least one selected string 21-23 is placed in the SoH calibration mode. Switches 51-53 may be provided, which may be controlled by a control device 40 for performing SoH estimation. The control device 40 may control a switch (e.g., switch 51) to disconnect at least one selected string (e.g., string 21) from the common coupling point 35 and connect the at least one selected string to the SoH cycle source / sink 47. The control device 40 may control a switch (e.g., switch 51) to reconnect the at least one selected string (e.g., string 21) to the common coupling point 35 and disconnect the at least one selected string from the SoH cycle source / sink 47, so that the at least one selected string resumes the operating mode, in which it contributes to the operation of the BESS provided to the power grid or user connected to the common coupling point 35.

[0172] When at least one selected string is placed in the SoH calibration mode and when it resumes the operating mode, the control device 40 may trigger additional activities that affect the at least one selected string placed in the calibration mode and the other strings remaining in the operating mode.

[0173] Placing the at least one selected string in the SoH calibration mode may trigger the control device 40 to cause the other strings remaining in the operating mode to compensate for the fact that the at least one selected string is not available for storing charge and / or providing charge to the power grid or user connected to the common coupling point 35 in the SoH calibration mode. The control device 40 may trigger a corresponding increase in the current provided to or from the other strings remaining in the operating mode to compensate for the fact that the at least one selected string temporarily exits normal operation. This may be achieved, for example, by corresponding commands output by the control device 40 to the converter interconnected between at least one other string remaining in the operating mode and the common coupling point. Alternatively or additionally, the control device 40 may output control commands to the BMS or another entity that controls charging and discharging in the BESS.

[0174] Restoring at least one selected string to the operating mode can trigger the control device 40 to simultaneously change the operation of other strings that were previously maintained in the operating mode. For example, those other strings are no longer needed to compensate for the fact that one string was previously not used for the normal operation of the BESS towards the power grid or the user connected to the common coupling point 35. The control device 40 can trigger a corresponding reduction in the current supplied to or provided from other strings that were previously maintained in the operating mode in consideration of the fact that at least one selected string is now restored to the operating mode. This can be achieved, for example, by corresponding commands output by the control device 40 to the converter interconnected between at least one other string maintained in the operating mode and the common coupling point. Alternatively or additionally, the control device 40 can output control commands to the BMS or another entity that controls charging and discharging in the BESS.

[0175] Figure 3 FIG. is a diagram of the system 10 according to an embodiment. Two strings, for example, the first and second strings 21, 22, can be selected simultaneously to perform SoH calibration. All other strings can still be in the operating mode. The control device 10 can operate the selected first string 21 and second string 22 such that one string acts as the SoH cycling source and the other string acts as the SoH cycling sink, and vice versa. For example, during SoH calibration, the first string 21 can first be charged with energy from the second string 22, and subsequently, the first string 21 can be discharged into the second string 22. This can be done with both the first string and the second string disconnected from the common coupling point 35.

[0176] The system 10 can have a switching matrix 45 that allows any combination of pairs of strings connected to each other. The switching matrix 45 can be integrated into the control device 40 or can be provided separately therefrom and can be controlled by the IC(s) of the control device 40.

[0177] The IC(s) 42 is operable to select multiple pairs of strings, where different pairs are sequentially placed in the SOH calibration mode in such a way that two strings are simultaneously in the SoH calibration mode. It can be automatically determined by the IC(s) 42, for example, using appropriate degradation metrics or other performance characteristics, regarding which strings are combined to form a pair, as explained below.

[0178] For Figure 1 any one of the systems of -3, the SoH estimation can operate as part of a state monitoring system, where if the health state or the rate of change of the health state of a particular string exceeds a given threshold, a notification (e.g., an alarm or other information) is sent as a signal to the end user and / or a maintenance activity is scheduled. The control device 40 can have an interface for outputting a notification (e.g., an alarm or other information) to the end user and / or for outputting information regarding the scheduled maintenance activity.

[0179] The method and control device according to the present invention allow for the accurate calculation of the current SoH of each string (e.g., each rack). Instead of providing an overall measure of the health state of the entire BESS, the method and device according to the present invention can determine the health status of each individual string (e.g., rack). This ensures that the SoH estimation can be performed more reliably and sensitively. This reduces the risk of false or missed alarms and the associated costs. In addition, since the maintenance personnel will know exactly which racks or batteries need to be replaced or maintained, the maintenance work is more efficient. When the BESS is composed of parallel strings, the principles disclosed herein can be applied in a sequential manner to each of the parallel strings, in the sense that the BESS has, for example, individual converters that act as batteries on their own. The non-uniform operation of the strings can be achieved, for example, by controlling the power flow in the respective converters and / or by disconnecting the strings from the common coupling point.

[0180] Figure 4 is a flowchart of a method 60 according to an embodiment. The method 60 can be executed by or under the control of a control device 40.

[0181] In step 61, the load of the BESS towards the common coupling point 35 is determined. This can include receiving sensor readings, using historical load profile information of the BESS, and / or using prior knowledge of the BESS load.

[0182] In step 62, it is determined whether an SoH estimation can be performed at this time during the ongoing operation of the BESS, i.e., while allowing the BESS to provide performance. The determination can be performed using the current load determined only from sensor readings, in combination with historical load profile information of the BESS and / or using prior knowledge of the BESS load. If the SoH estimation cannot be performed at this time, for example, because the current BESS load is temporarily too high such that it is not allowed to select a string to temporarily take it out of the operating mode and perform SoH calibration without violating the safety margin requirements, the method can return to step 61. Otherwise, the method can proceed to step 63.

[0183] In step 63, at least one string can be selected. The strings can be selected according to a predefined order. The order of selecting the strings can depend on performance characteristics or other knowledge about the BESS (e.g., the position of the string within the battery enclosure, which may affect degradation). The selection of the strings can depend on the performance characteristics of all the strings, e.g., the degradation index. This performance characteristic can be based on a previous SoH estimation or other information that can be provided, for example, by the BMS.

[0184] In step 64, SoH calibration is performed in a SoH calibration mode in which the selected string is in a mode different from the operating mode and in which the selected string pair does not contribute to the net power flow from the BESS to the power grid or the user connected to the common coupling point 35.

[0185] During SoH calibration, the open-circuit voltage (OCV) of the battery can be monitored based on the state of charge (SoC) and can be used to determine the SoH. Exemplary techniques for determining the SoH will be described below. Estimating the SoH can include calculating one or several internal resistances of the ECM and / or calculating the capacitance ratio of the ECM.

[0186] When at least one of the selected strings is in the SoH calibration mode, one or several SoH cycles can be performed. In each cycle, the string can be charged and discharged substantially completely. Step 63 can include charging and discharging the string in a range from 20% or less to 80% or more, from 10% or less to 90% or more, from 5% or less to 95% or more, or from 3% or less to 97% or more of its rated capacitance.

[0187] In step 65, it is determined whether the SoH estimation for all strings of the BESS has been completed. If only a part of the strings are placed in the SoH calibration mode in this round of SoH estimation, the method can return to step 63, in which one of the remaining strings is selected.

[0188] When the SoH estimation for all strings of the BESS is completed, the result can be output or the result can be used for other purposes, for example, for outputting information depending on the SoH of all strings. The output information can include an alarm, another notification, and / or information about scheduled or otherwise recommended maintenance activities.

[0189] Figure 5 is a flowchart of method 70 according to an embodiment. Method 70 can be executed by or under the control of control device 40. Method 70 can be executed to implement Figure 4 steps 63 and 64 of method 60.

[0190] In step 71, a first string and a second string among a plurality of parallel strings are selected. The first and second strings can be selected based on string matching or sorting. The matching or sorting can depend on performance characteristics or other knowledge of the BESS (for example, the position of the string within the battery housing, which may affect degradation). The matching or sorting can depend on the performance characteristics of all strings, for example, the degradation index. The performance characteristic can be based on a previous SoH estimation or other information that can be provided by the BMS, for example.

[0191] In step 72, when the first string and the second string are placed in the SoH calibration mode, the first string discharges to the second string. During this process, the battery OCVs of the first and second strings can be monitored.

[0192] In step 73, when the first and second strings are placed in the SoH calibration mode, the second string discharges to the first string. During this process, the battery OCVs of the first and second strings can be monitored.

[0193] The battery OCVs of the first and second strings can then be used to estimate the SoH. Estimating the SoH can include calculating one or several internal resistances of the ECM and / or calculating the capacitance ratio of the ECM.

[0194] Figure 6 is a flowchart of method 75 according to an embodiment. Method 75 can be executed by control device 40 or executed under the control of control device 40.

[0195] In step 76, the load change curve of the BESS can be used to determine the time when the SoH estimation can be performed. The load change curve can be based on the past historical load change curve and / or based on the prior knowledge of the predicted future load. For example, for various ancillary services, such as frequency stabilization, load regulation, voltage support, by placing all the strings in the SoH calibration mode in sequence, a sufficiently long period can be determined in advance to complete the SoH estimation, while ensuring that the remaining strings can meet the requirements that the BESS must meet for the power grid or local users (one or more local users), preferably with a safety margin.

[0196] The historical load change curve and / or the prediction about the future load change curve can be used. For isolated systems not connected to the national or regional power grid and / or systems with variable-curve energy sources (DERs) (such as solar cells / wind turbines or other renewable energy sources), information about the weather forecast is considered to determine the time when the SoH calibration can be performed.

[0197] In step 77, the SoH estimation can be performed by placing the strings in the SoH calibration mode in sequence (e.g., one by one or in pairs), while the remaining strings provide the load change curve of the BESS towards the point of common coupling. The method Figure 4 of steps 63 - 66 of method 60 can be used to implement step 77.

[0198] Figure 7 is a flowchart of method 80 according to an embodiment. Method 80 can be executed by control device 40 or executed under the control of control device 40.

[0199] In step 81, SoH estimation is performed. This can be done by placing the strings sequentially in the SoH calibration mode (e.g., one by one or in pairs), while the remaining strings provide the load change curve of the BESS towards the common coupling point. The method of Figure 4 Steps 61 - 65 of method 60 can be used to implement step 81. The SoH estimation can include determining one or several internal resistances of the ECM and / or calculating the capacitance ratio of the batteries in each string or in each of multiple independently controllable parallel strings.

[0200] In step 82, the results of the SoH estimation can be processed. Processing the results of the SoH estimation can include determining for each of the multiple independently controllable parallel strings whether information needs to be output to the user regarding the SoH of each string, dedicated control activities to be performed, and / or maintenance activities to be scheduled. The control device 40 or a separate computer can process the results of the SoH estimation by comparing the SoH estimation results with a historical BESS information database (which can include information on the operating conditions of previously observed BESSs of the same structure) or with one or several thresholds, which can be provided by the manufacturer of the BESS and which can indicate whether the state of any string is critical, whether maintenance is required, or other dedicated activities are needed.

[0201] The processing of the SoH can include determining the rate of change of the parameter(s) (one or more) indicating the SoH (e.g., the internal resistance and / or the ratio of capacitances in the ECM). As an additional or alternative to the absolute value of the parameter indicating the SoH, the rate of change can be used. For example, the absolute value of the parameter(s) (e.g., the internal resistance and / or the ratio of capacitances in the ECM) can be combined with the rate of change to predict whether a string is tending towards a critical state or towards a certain state where specific activities (e.g., alarm, control, or maintenance activities) need to be taken and which strings are tending towards such a critical state.

[0202] In step 83, alarm, control, or maintenance activities can be initiated. The alarm, control, or maintenance activities can selectively indicate which of the multiple independently controllable parallel strings are in a state where an alarm needs to be output, dedicated control activities need to be performed, or maintenance needs to be carried out by the operator.

[0203] The SoH estimation can be used as part of a condition monitoring system, where if the SoH (or the rate of change of the SoH) of a specific string exceeds a given threshold, an alarm can be sent to the end - user and / or maintenance activities can be scheduled.

[0204] Some embodiments of the present invention use sorting of strings, which determines which rack or battery string enters the SOH calibration mode first and / or in what order the strings enter the SoH calibration mode. Since, for example, they are placed in a BESS (e.g., relative to the position of the HVAC / the outer wall or door of the BESS container), each individual string of the battery may be vulnerable to different environmental conditions. This may result in different degradation rates of the strings.

[0205] Figure 8 The circuit of a BESS with three strings 21 - 23 connected in parallel in its initial state is shown. The external circuit containing the DC power supply 85 (symbolizing grid power supply) is the power supply circuit, while the internal circuit is dedicated to the SoH calibration cycle and may include the SoH cycle source / sink 86.

[0206] To determine the sorting of the strings, the BESS can be connected to the grid 85 through the main switch 50, as Figure 9 shown, until it is fully charged. The control switches 51 - 53 are controlled to connect all the strings 21 - 23 to the grid 85 and disconnect them from the SoH cycle source / sink 86.

[0207] Figure 9 A configuration is shown in which the BESS is then disconnected from the grid by opening the main switch 50. When the degradation of the different strings 21 - 23 is different (e.g., different self - discharge resistances), the current flowing through the battery circuit can continue. The most degraded batteries (e.g., strings 21 and 23) will draw current from the remaining strings (e.g., string 22) to maintain the voltage balance at the BESS terminals. In this way, string 22 performs a partial discharge cycle. In other words, the degraded string(s) or battery(ies) 21, 23 affect the remaining battery cells.

[0208] The strings 21 - 23 can be sorted based on the amount of current they draw when the main switch 50 is in the open state (where the BESS is disconnected from the grid 85). During this period, the greater the amount of current drawn by the battery, the more quickly it should enter the SoH calibration cycle.

[0209] Figure 10 The circuit when string 21 is placed in the SoH calibration mode while the remaining partial strings 21, 23 are operating normally is shown.

[0210] By using the sorting of the strings 21 - 23 based on degradation, the string with the largest degradation amplitude (e.g., string 21) can be quickly isolated in the SoH estimation. In addition, when the batteries of the strings do not suffer from the memory effect, as is the case with lithium - ion batteries, the SoH after the calibration cycle can be improved.

[0211] The implementation of SoH estimation can be done separately for each string successively placed in the SoH calibration mode in various ways. Although the implementation of SoH implementation is not closely related, SoH estimation can generally use OCV as an input to determine the parameters of the ECM of each string or each cell. Below, "battery" will be referred to. It will be understood that the battery can be, for example, a virtual battery formed by a string or a rack.

[0212] Mathematical programming, especially non - linear programming, can be used to perform SoH estimation. SoH estimation can be based on a Python Optimization Modeling Objects (Pyomo) model and an optimization routine. SoH estimation can jointly estimate all relevant parameters of the ECM of the battery (one or more) under discussion and the actual capacity of the battery (one or more).

[0213] Figure 12 is an exemplary ECM for modeling a battery or a string. The state is defined by the SoC and the voltages of two (double - layer) capacitors U1 and U2. The parameters of the ECM are the internal resistances R0, R1, and R2. These internal resistances can be calculated as fixed fractions of the total internal resistance R ch for charging and the total internal resistance R disch for discharging. The quotient of the two capacitances C1 and C2 can be another parameter Q rel .

[0214] The OCV curve (i.e., OCV versus SoC) can be used as an input to the procedure. Figure 13 Shows an exemplary OCV as a function of SoC. The hysteresis is represented by the dashed line. The hysteresis of charge and discharge can be included in the SoH estimation.

[0215] To analyze individual strings or virtual batteries, the ECM as described above can be estimated for each virtual battery (which can represent a rack or a string). This can be done by combining state and parameter estimation. The relevant states are SoC, capacitor voltage, and energy throughput. The OCV offset is caused by the hysteresis effect. The relevant parameters are internal resistance and relative capacity. The estimation may be affected by the following two:

[0216] - Over - voltage: Static and dynamic over - voltages can be observed when the sign or magnitude of the current changes. They are used to infer the internal resistance.

[0217] - Voltage difference: After removing the over - voltage, the relationship between charge throughput and voltage difference will lead to an estimation of the battery capacity. For the integration of the same charging or discharging current, a higher voltage difference indicates a lower capacity.

[0218] The SoH estimation can be based on the individual cell voltage (which can represent a virtual cell of, for example, a rack or string) and the rack current within a given time window. The SoH estimation can be performed without using the BMS. The SoC estimation is not used.

[0219] The estimation can follow a procedure of several model - based estimation steps. An exemplary embodiment will be explained with reference to Figure 14 Exemplary embodiments.

[0220] Figure 14 is a flowchart of a method 90 that can be used to estimate the SoH parameters. The method 90 can be automatically executed by the IC(s) 42 of the control device 40 or by a separate computing device.

[0221] In step 91, the OCV signal is received. The OCV signal can be received separately for each string or rack (which can be modeled by a virtual cell respectively), while performing the SoH cycle for each string.

[0222] In step 92, the OCV signal can be smoothed. This can be done through a Pyomo model.

[0223] In steps 93 - 95, the parameters and states of the ECM are estimated in an iterative search. In step 93, the ECM parameters and optional states are estimated. In step 94, the OCV offset caused by hysteresis in the model can be updated.

[0224] Individual estimations can be performed by the Pyomo model respectively. The model can be defined such that instead of modeling the current, its integral (i.e., the charge throughput so far) is modeled. This helps to better correct measurement errors. The model can be defined such that the resistance switches between the charge and discharge resistances according to the sign of the current. The over - voltage can be determined based on the current I, the internal resistance R0, and the voltages U1 and U2 of two (double - layer) capacitors (i.e., –I R0+U1+U2). The voltage can be determined as the sum of the OCV and the OCV offset. The OCV can be encoded as a linear combination of tanh terms, i.e., an artificial neural network (e.g., a 1 - layer artificial neural network).

[0225] In step 93, the estimation of the ECM parameters can be based on the minimization of an objective function respectively. The objective function can incorporate some or preferably all of the following objectives:

[0226] - Low voltage error,

[0227] - Low incremental current error,

[0228] - Low current error.

[0229] These objectives can be combined into a linear combination with different weighting factors. The voltage variations of the SoC and the capacitor can be determined from the list of constraints imposed by the ECM.

[0230] After solving the ECM to obtain the ECM parameters and the ECM state, the OCV offset can be updated (e.g., by applying a hard limit and cutting off the calculated overvoltage at the hard limit), and steps 93-95 can be repeated. The termination criterion can be checked at step 95. The termination criterion can relate to the convergence criterion.

[0231] At step 96, the ECM parameters can be used. Using the ECM parameters can include determining whether an alarm, a control, or a maintenance activity is required for each string based on the ECM parameters.

[0232] The devices and methods according to the embodiments can be applied to any BESS having two or more strings that can be independently controlled. The devices and methods according to the embodiments can be applied to, for example, a BESS having a power of 1 MW or greater, such as 2 MW or greater, such as 5 MW or greater, but not limited thereto.

[0233] The BESS can be a stationary BESS, which can be a connected or islanded power grid. Multiple strings (arrays) of the BESS can be independently controlled. If this condition is met, the devices and methods according to the embodiments can also be used for BESSs in mobile applications (e.g., BESSs installed in electric buses, trucks, cars, ships, etc.).

[0234] Although the present invention has been described in detail in the drawings and the foregoing description, such description should be considered illustrative or exemplary rather than restrictive. By studying the drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments and practice the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain elements or steps are recited in different claims does not imply that these elements or steps cannot be used in combination to obtain an advantage. In particular, any further meaningful combination of the claims, apart from the actual claim dependency, should be considered disclosed.

Claims

1. A method for performing a state of health SoH estimation of a rechargeable battery energy storage system during operation of the rechargeable battery energy storage system, wherein, The rechargeable battery energy storage system includes a plurality of parallel strings that can be controlled individually, wherein the method includes: Selecting at least one string from the plurality of parallel strings, Placing the selected at least one string in a SoH calibration mode to perform SoH calibration while keeping at least one other string among the plurality of parallel strings in an operating mode, and After completing the SoH calibration for the selected at least one string, returning the selected at least one string to the operating mode, wherein the plurality of strings are sequentially placed in the SoH calibration mode in an order determined based on performance characteristics, the performance characteristics including a degradation index of each string among the plurality of parallel strings.

2. The method according to claim 1, wherein, Keeping the at least one other string in the operating mode includes using the at least one other string to store energy supplied to the power grid or at least one power user.

3. The method according to claim 1, wherein At any time during the SoH estimation, at least 50% of the total number of strings among the plurality of parallel strings remains in the operating mode.

4. The method according to claim 1, the method further including performing one or more SoH calibration cycles on the selected at least one string while the selected at least one string is in the SoH calibration mode and before returning the selected at least one string to the operating mode.

5. The method according to claim 1, the method further comprising: Sorting and / or matching the plurality of parallel strings based on the performance characteristics, the performance characteristics including a degradation index of each string among the plurality of parallel strings.

6. The method according to claim 1, wherein, The degradation index is based on the SoH estimation result obtained in a previous SoH estimation procedure, and / or when the plurality of parallel strings are disconnected from the power grid, based on the current drawn from each string among the plurality of parallel strings.

7. The method according to claim 5, wherein Performing the following steps sequentially in an order determined based on the sorting and / or matching for at least a subset of the plurality of parallel strings: selecting the at least one string, placing the selected at least one string in the SoH calibration mode, and returning the selected at least one string to the operating mode.

8. The method according to claim 7, wherein Performing the following steps sequentially in an order determined based on the sorting and / or matching for all parallel strings: selecting the at least one string, placing the selected at least one string in the SoH calibration mode, and returning the selected at least one string to the operating mode.

9. The method according to claim 1, wherein Selecting at least one string includes: Selecting a first string and a second string, wherein during at least a portion of the SoH calibration, the second string acts as an energy sink for the energy released from the first string.

10. The method according to claim 1, wherein placing the selected at least one string in the SoH calibration mode includes: - During the SoH calibration, controlling the power flow through a converter connected between the selected at least one string and a common coupling point according to a calibration load variation curve, and / or - Disconnecting the selected at least one string from the common coupling point and temporarily connecting the selected at least one string to a SoH cycle energy source and / or energy sink.

11. The method according to claim 1, wherein, During the SoH calibration, charging and / or discharging the battery of the selected at least one string according to the calibration load variation curve.

12. The method according to claim 11, wherein, The calibration load change curve is pre-determined or defined by an operator.

13. The method according to claim 11, wherein, During each successive SoH calibration cycle performed during the SoH, at least one selected string is charged to a first level above a first threshold and discharged to a second level below a second threshold, the first threshold being equal to or greater than 80%, and the second threshold being equal to or less than 20%.

14. The method according to claim 1, wherein Maintaining the at least one other string in the operating mode includes keeping all strings of the plurality of parallel strings other than the at least one selected string in the operating mode.

15. The method according to claim 1, wherein The plurality of parallel strings form a stationary battery energy storage system BESS.

16. The method according to claim 1, wherein, The plurality of parallel strings are mounted on a vehicle.

17. A control device for controlling the state of health SoH estimation of a rechargeable battery energy storage system, wherein, The rechargeable battery energy storage system includes a plurality of individually controllable parallel strings, and the control device includes: An interface operatively coupled to the plurality of parallel strings, and At least one integrated semiconductor circuit coupled to the interface and operable to: Select at least one string from the plurality of parallel strings, Control the rechargeable battery energy storage system to place the at least one selected string in an SoH calibration mode for SoH calibration while keeping at least one other string of the plurality of parallel strings in an operating mode, and Control the rechargeable battery energy storage system to return the at least one selected string to the operating mode after completion of the SoH calibration of the at least one selected string; Wherein the control device is arranged to place the plurality of strings in the SoH calibration mode successively in an order determined based on performance characteristics, the performance characteristics including a degradation index of each string of the plurality of parallel strings.

18. A rechargeable battery energy storage system, the rechargeable battery energy storage system comprising: A plurality of individually controllable parallel strings; And The control device according to claim 17, the control device being coupled to the plurality of parallel strings.

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