Battery monitoring device

Abstract

To provide a storage battery monitoring device which can securely diagnose deterioration in a string even when an SOH of the deteriorated string is high, which can early promote a measure for suppressing the progress of the deterioration in the string, and which can easily determine the deterioration in various battery types of the strings.SOLUTION: A storage battery system 1 includes a multiple-parallel storage battery array 4, a storage battery monitoring device 5, an AC / DC conversion device 6, and a breaker 7. An MPU of a BMU 5a configurating the storage battery monitoring device 5 functions as: a calculation part for calculating a time change in a discharge current value of each string St, which is measured by a measurement part; and a determination part for determining the string St exhibiting a peculiar time change among the plurality of time changes as the deteriorated string St including a deteriorated lead storage battery 4a.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a power storage system having a function for detecting the deterioration state of strings that constitute a multi-parallel storage battery array. [Background technology]

[0002] Conventionally, in this type of power storage system, the deterioration state of the string has been detected by a charge / discharge monitoring control system disclosed in Patent Document 1, for example.

[0003] In this charge / discharge monitoring and control system, the discharge current I d N Then, measure the discharge current I d N The average discharge current I of all strings calculated from AVE and the discharge current of a particular string, I d N The difference between AVE -I d N ) is the preset current difference dI d It is determined whether the value is greater than the value of the equation (1) below. (I AVE -I d N )>dI d … (1)

[0004] Furthermore, the difference (I AVE -I d N ) is greater than the value obtained by multiplying the standard deviation σ1 calculated from the measured discharge current by the coefficient A1, and it is determined whether the following equation (2) is valid. (I AVE -I d N )>σ1·A1… (2)

[0005] The strings for which the formulas (1) and (2) are satisfied are diagnosed as including deteriorated lead-acid batteries. AVE and the discharged electricity quantity of a specific string W dN The difference between (W AVE -W d N ) also has a preset electrical difference dW d and the standard deviation σ calculated from the measured discharge quantity of electricity W Coefficient A W Similarly, the value obtained by multiplying the value by the number of the lead-acid battery is compared with the value obtained by multiplying the value by the number of the lead-acid battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5783116 Summary of the Invention [Problem to be solved by the invention]

[0007] In the charge / discharge monitoring and control system of the conventional power storage system, when the SOH (State of Health) of a deteriorated lead-acid battery, which is the ratio of the full charge capacity at the time of deterioration to the initial full charge capacity, is significantly low, such as around 40% or 30%, the average discharge current I AVE and the discharge current of a particular string, I d N The difference between AVE -I d N ) is easy to recognize, so the above formulas make it possible to diagnose string deterioration. However, when the SOH of a deteriorated lead-acid battery is relatively high, around 80% or 70%, the average discharge current I AVE and the discharge current of a particular string, I d N The difference between AVE -I d N ) becomes smaller, which can make it difficult to diagnose string degradation using the above equations.

[0008] Furthermore, even if the charge / discharge monitoring and control system provided in the conventional energy storage system described above can detect a lead-acid battery with a degraded SOH of around 30% or 40%, the energy storage system must be shut down to avoid thermal runaway caused by the degraded lead-acid battery.The battery monitoring device in the energy storage system is expected to play a role in quickly detecting a lead-acid battery with a degraded SOH of around 70% to 90% and prompting the energy storage system manager to take measures to suppress the progression of degradation of the string containing the degraded lead-acid battery (hereinafter referred to as the degraded string), such as changing the system operating conditions, such as restricting the input / output power to the multi-parallel energy storage battery array, or early replacement of the degraded lead-acid battery.

[0009] In addition, in order to calculate equation (1) in a conventional energy storage system equipped with the above-mentioned charge / discharge monitoring and control system, the current difference dI d The value of is determined by testing and the current difference dI d The value of dI must be set in advance in the charge / discharge monitoring and control system. If the lead-acid batteries that make up the string are the same type, the current difference dI d The value of the current difference dI d If the battery type is different from the lead-acid battery for which the value of d Therefore, in the conventional energy storage system equipped with the above-described charge / discharge monitoring control system, it is not easy to determine the deterioration of a string made up of various types of lead-acid batteries. [Means for solving the problem]

[0010] The present invention has been made to solve such problems, A battery monitoring device that detects the state of each string included in a multi-parallel battery array in which a plurality of strings, each string having a plurality of storage batteries connected in series, are connected in parallel, comprising: The storage battery includes a measurement unit that measures the discharge current discharged from each string, a calculation unit that calculates the current change rate of the discharge current measured by the measurement unit for each string, and a determination unit that determines that a string exhibiting an anomalous current change rate among the multiple current change rates for each string calculated by the calculation unit includes a degraded storage battery, wherein the calculation unit calculates the current change rate after a predetermined time has elapsed since the start of discharge of each string, and the determination unit detects the anomalous current change rate based on the current change rate calculated by the calculation unit.

[0011] According to this configuration, the current change rate of the discharge current measured by the measurement unit is calculated for each string by the calculation unit, and a string that exhibits a unique current change rate among the multiple current change rates calculated for each string by the calculation unit is determined by the determination unit to be a string that includes a deteriorated storage battery. Therefore, a string that includes a deteriorated storage battery is determined to be a string that includes a deteriorated storage battery by the determination unit, as in the conventional case, by comparing the average discharge current IAVE of all strings and the discharge current Id of a specific string. N Therefore, if the SOH of a deteriorated battery is relatively high, the average discharge current IAVE of all strings and the discharge current Id of a specific string are detected. N Even when the difference between the values ​​is small, it is possible to reliably diagnose the deterioration of the string.

[0012] Furthermore, by identifying strings that exhibit a unique current change rate among the multiple current change rates of the discharge current for each string, it is possible to detect degraded strings early, before their SOH significantly decreases. This makes it possible to prompt the energy storage system manager to take measures to suppress the progression of string degradation, such as changing the system operating conditions by reducing the input / output power to the multi-parallel battery array or early replacement of degraded batteries.

[0013] In addition, as in the past, the current difference dI d The value of is determined by testing and the current difference dI dSince it is not necessary to set the value of in advance in the system, it becomes possible to easily determine the deterioration of strings made up of lead-acid batteries of various battery types. Furthermore, the inventors have confirmed that the rate of change of the discharge current varies between strings for a predetermined time from the start of discharge of each string, regardless of deterioration of the storage battery. With this configuration, the anomalous rate of change of the discharge current is detected based on the rate of change of the discharge current calculated after the predetermined time has elapsed from the start of discharge of each string. Therefore, the anomalous rate of change of the discharge current between strings can be accurately detected. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a power storage system equipped with a battery monitoring device that can reliably diagnose the deterioration of a string even if the SOH of the deteriorated string is relatively high, can prompt the power storage system manager to take measures to suppress the progression of string deterioration at an early stage, and can easily determine the deterioration of strings composed of storage batteries of various battery types. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a block diagram showing a schematic configuration of a power storage system according to an embodiment of the present invention; [Figure 2] 2 is a graph showing the discharge time characteristics of the discharge current for each string obtained in a first current behavior verification experiment carried out on the power storage system shown in FIG. [Figure 3] 10 is a graph showing the discharge time characteristics of the discharge current for each string obtained in a second current behavior verification experiment carried out on the power storage system shown in FIG. [Figure 4] 10 is a graph showing the discharge voltage versus discharge time characteristics for a standard storage battery and a deteriorated storage battery in a second current behavior verification experiment. [Figure 5] 10 is a graph showing the discharge time characteristics of the discharge current for each string obtained in a third current behavior verification experiment carried out on the power storage system shown in FIG. [Figure 6] 10 is a graph showing the discharge voltage versus discharge time characteristics for a standard storage battery and a deteriorated storage battery in a third current behavior verification experiment. [Figure 7] 2 is a flowchart showing an outline of a deteriorated string identification process performed by a battery monitoring device included in the power storage system shown in FIG. [Figure 8] FIG. 10 is a table showing the measurement results of the time until a low voltage alarm is issued when two different deteriorated string detection methods are used in combination as the deteriorated string detection algorithm of the battery monitoring device that constitutes the energy storage system shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] Next, a description will be given of an embodiment of a power storage system according to the present invention.

[0017] FIG. 1 is a block diagram showing a schematic configuration of an energy storage system (ESS) 1 according to one embodiment of the present invention.

[0018] Typically, AC power from the power supply unit 2 is converted into DC power and supplied to the multi-parallel energy storage array 4 for charging, and the DC power stored in the multi-parallel energy storage array 4 is discharged or converted into AC power by an AC / DC converter 6 and supplied to the load 3. The multi-parallel energy storage array 4 is configured by connecting a plurality of strings St in parallel. Each string St is configured by connecting a plurality of lead-acid batteries 4a for cycle use in series. In this embodiment, the multi-parallel energy storage array 4 is configured by connecting 10 strings St1 to St10 in parallel, and each string St1 to St10 is configured by connecting a plurality of lead-acid batteries 4a in series.

[0019] The energy storage system 1 includes such a multi-parallel energy storage array 4, a battery monitoring device 5, a power conditioning system (PCS) 6, and a circuit protection breaker 7. The battery monitoring device 5 includes a battery monitoring unit (BMU) 5a, a cell sensor 5b provided for each lead-acid battery 4a, and a unit sensor 5c provided for each string St1 to St10, and detects the state of each string St1 to St10. Each cell sensor 5b measures the terminal voltage and temperature of the lead-acid battery 4a to which it is attached. Each unit sensor 5c measures the terminal voltage of each string St1 to St10 to which it is attached and the current flowing through each string St1 to St10. These unit sensors 5c form a measurement unit that measures the value of the discharge current discharged from each string St1 to St10.

[0020] The BMU 5a constituting the battery monitoring device 5 is configured with a microprocessor (MPU) and memory, and monitors and manages each of the multiple strings St1 to St10 connected in parallel and the lead-acid batteries 4a constituting each string St1 to St10. By operating according to the procedures of a computer program stored in the memory, the MPU functions as a calculation unit that calculates the time change in the discharge current value measured by the measurement unit, a determination unit that determines whether the string St is degraded based on the calculation results of the calculation unit, and a warning unit that outputs a degraded string detection signal when the determination unit determines that the string St is degraded. In this embodiment, the MPU also functions as an SOH calculation unit that calculates the SOH of the degraded string St before discharge begins based on the detected discharge quantity of electricity from the degraded string St. Note that the calculation unit, determination unit, warning unit, and SOH calculation unit are not limited to the operation of the MPU according to the procedures of a computer program, but can also be realized by electronic circuit hardware configurations.

[0021] The calculation unit calculates the time change of the discharge current value measured by the measurement unit as a current change rate of the discharge current for each string St1 to St10. The determination unit determines, among the multiple time changes of the discharge current for each string St1 to St10 calculated by the calculation unit, one that exhibits a unique time change as a string St including a degraded lead-acid battery 4a, i.e., a degraded string St. The lead-acid batteries 4a constituting each string St1 to St10 are configured to have the same voltage as the assembled battery. However, due to small voltage differences and differences in battery state inherent between the lead-acid batteries 4a, the terminal voltages of each string St1 to St10 are slightly different. These small voltage differences and differences in battery state affect the current behavior of the discharge current. In particular, if a string St includes a degraded lead-acid battery 4a, this has a significant effect on the current behavior of the discharge current, as described below. The warning unit outputs a degraded string detection signal to the AC / DC converter 6 when the determination unit determines that any string St is a degraded string St.

[0022] The AC / DC converter 6 normally converts AC power from the power supply unit 2 into DC power and supplies it to the multi-parallel energy storage array 4 for charging, and discharges the DC power stored in the multi-parallel energy storage array 4, or converts it into AC power by the AC / DC converter 6 and supplies it to the load 3. At this time, the multi-parallel energy storage array 4 performs a discharging operation, but when an alarm of a degraded string detection signal is input from the BMU 5a, the AC / DC converter 6 stops the discharge of the power stored in the multi-parallel energy storage array 4 to the load 3. This discharge stop operation of the AC / DC converter 6 allows the degraded string St that constitutes the multi-parallel energy storage array 4 to continue its discharging operation, thereby suppressing further deterioration of the degraded string St.

[0023] In this embodiment, the judgment unit judges whether a string St is degraded by determining, among the multiple time changes in the discharge current value for each string St1 to St10, the string St that exhibits a unique time change in which the discharge current value decreases, different from most of the other strings St, as the degraded string St.

[0024] Such a detection algorithm for the deteriorated string St was obtained from the following current behavior verification experiment carried out on the multiple parallel storage array 4.

[0025] The graph shown in Figure 2 shows the results of a first current behavior verification experiment conducted on a multi-parallel battery storage array 4 consisting of 10 parallel strings St1 to St10, each with an SOH value of 100%. This verification experiment was conducted at a discharge current of 0.11[C 10 A] and a discharge time of 6 hours. The graph shows the discharge time characteristics of the discharge current obtained for each string St1 to St10, with the horizontal axis of the graph representing the discharge time [h] and the vertical axis representing the discharge current [C 10 A]. The unit of discharge current is [C 10 A] is the ratio to the 10-hour rate capacity of each string St1 to St10. Furthermore, SOH is an index representing the state of health of the battery, and is expressed by the following formula (3). SOH= (Dischargeable capacity of string St / Rated capacity of string St) x 100 [%] ... (3)

[0026] A larger SOH value indicates less degradation of the string St, and a smaller SOH value indicates more advanced degradation of the string St. In lead-acid batteries, a state of SOH=70% where only 70% of the rated capacity can be discharged is generally considered to be a degraded state.

[0027] 2, it can be seen that there is variation in the magnitude of the discharge current at the beginning of discharge, but that around three hours after the start of discharge, the variation in the discharge current disappears and the discharge current converges. In other words, it can be seen that around three hours after the start of discharge, the change in the discharge current over time becomes almost the same for each of the strings St1 to St10.

[0028] The graph in Figure 3 shows the results of a second current behavior verification experiment conducted on a 10-parallel multi-parallel storage array 4, in which string St4 contains a storage battery with a degraded SOH value of 50% and the other strings St1 to St3 and St5 to St10 contain storage batteries with SOH values ​​of 100%. In this verification experiment, as in the first verification experiment, a discharge current of 0.11[C 10 The experiment was carried out under the discharge conditions of [A] and a discharge time of 6 hours. The horizontal and vertical axes of the graph shown in Figure 3 represent the same as those of the graph shown in Figure 2.

[0029] From the graph shown in Fig. 3, it can be seen that the behavior of the discharge current for each of the strings St1 to St10 at the beginning of the discharge varies similarly to the graph shown in Fig. 2, but that the variation disappears and the discharge current converges around the third hour. However, from around the 4.2 hour mark onwards, it can be seen that the discharge current for each of the strings St1 to 3 and St5 to 10, excluding string St4, tends to increase, while the discharge current for string St4 tends to decrease sharply.

[0030] Figure 4 shows the discharge characteristics of a monoblock lead-acid battery consisting of six cells. It also shows the discharge voltage versus discharge time characteristics of the standard battery (SOH = 100%) and the degraded battery (SOH = 50%) constituting string St4 in the second current behavior verification experiment. The horizontal axis of the graph represents discharge time [h], and the vertical axis represents the terminal voltage [V] of each battery. The graph confirms that the standard battery with an SOH value of 100% exhibits a gradual decrease in its terminal voltage. On the other hand, the degraded battery with an SOH value of 50% exhibits a terminal voltage of 11.2 [V], close to the 10.8 [V] when the SOH value is 0 [%], around the 4.2-hour mark when the discharge current drops sharply. This confirms that when the state of charge (SOC) of a degraded battery becomes a small value close to 0%, a sudden change in discharge current is observed. For example, when the terminal voltage of any of the batteries constituting string St4 becomes 11.2 V, the alarm unit constituting BMU 5a outputs a degraded string detection signal to AC / DC converter 6 and issues a low voltage alarm to prevent over-discharge of that battery.

[0031] The graph in Figure 5 shows the results of a third current behavior verification experiment conducted on a 10-parallel multi-parallel storage array 4, in which string St4 contains a storage battery with a degraded SOH value of 30% and the other strings St1 to St3 and St5 to St10 contain storage batteries with SOH values ​​of 100%. In this verification experiment, as in the first verification experiment, a discharge current of 0.11[C 10 The experiment was carried out under the discharge conditions of [A] and a discharge time of 6 hours. The horizontal and vertical axes of the graph shown in Fig. 5 represent the same as those of the graph shown in Fig. 2.

[0032] From the graph shown in Figure 5, it can be seen that the behavior of the discharge current for string St4 from the beginning of discharge differs from that of the other strings St1 to 3 and St5 to 10, remaining at a small discharge current value. It can also be seen that the discharge current for string St4 tends to decrease rapidly from around 3.2 hours onwards. It can also be seen that the behavior of the discharge current for the other strings St1 to 3 and St5 to 10 varies in the beginning of discharge, as in the graphs shown in Figures 2 and 3, but the variation disappears around 3 hours later and the discharge current converges.

[0033] FIG. 6 is a graph showing the discharge characteristics of a monoblock lead-acid battery consisting of six cells. It also shows the discharge voltage versus discharge time characteristics of a standard battery (SOH = 100%) and a degraded battery (SOH = 30%) constituting string St4 in the third current behavior verification experiment. Similar to the graph shown in FIG. 4, the horizontal axis of this graph represents discharge time [h], and the vertical axis represents the terminal voltage [V] of each battery. The graph confirms that the standard battery with an SOH value of 100% exhibits a gradual decrease in terminal voltage, similar to the graph shown in FIG. 4. On the other hand, the degraded battery with an SOH value of 30% exhibits a terminal voltage of 11.2 V, close to the terminal voltage of 10.8 V when the SOC value is 0%, around 3.2 hours after discharge, when the discharge current drops sharply. It is confirmed that the terminal voltage drops sharply from 3.2 hours onward. This also confirms that when the state of charge of a deteriorated storage battery reaches a small SOC value near SOC=0[%], a sudden change in discharge current is observed.

[0034] In the energy storage system 1 according to this embodiment, the results of the first, second, and third current behavior verification experiments show that, as described above, among the multiple time variations in the discharge current values ​​of the strings St1 to St10 constituting the storage battery array 4, a string St that exhibits a peculiar time variation in the discharge current value decreasing differently from most of the other strings St, such as string St4, is determined to be a degraded string St that includes a degraded lead-acid battery 4a. If even one degraded lead-acid battery 4a is included among the lead-acid batteries 4a constituting the degraded string St, the time variation in the discharge current value of that degraded string St will exhibit a peculiar time variation in the discharge current value decreasing differently from the other healthy strings St. Furthermore, even if multiple strings St include a degraded lead-acid battery 4a, the behavior of the discharge current of each of these strings St will be similar.

[0035] Furthermore, the determination unit in the BMU 5a detects peculiar time changes based on the time changes calculated by the calculation unit after a predetermined time has elapsed since the start of discharge in each of the strings St1 to St10, when the discharge current converges, for example, after two hours have elapsed. The determination unit also detects peculiar time changes based on the time changes in the discharge current value during a predetermined period of the order of seconds after the predetermined time has elapsed since the start of discharge in each of the strings St1 to St10. In this embodiment, the calculation unit calculates the slope of the discharge current characteristic line, for example, as shown in each of the graphs above, which represents the change in the discharge current value with respect to the discharge time, as the time change in the discharge current value, and the determination unit detects this peculiar time change based on the positive or negative value of the slope of the characteristic line calculated by the calculation unit.

[0036] Specifically, in this embodiment, the calculation unit measures the slope of the discharge current characteristic line of each string St1 to St10, with a basic measurement period of 10 seconds and a sampling time of 100 milliseconds. Therefore, in one basic measurement, the slope of 100 data points (10 seconds / 100 milliseconds) is calculated, and the average value is used as the slope of the characteristic line for that basic measurement period. As shown in the graphs above, the time-varying characteristics of the discharge current value are measured with jagged variations due to the accuracy of current measurement. Furthermore, noise may be superimposed on the discharge current when measured.

[0037] Therefore, in this embodiment, to eliminate the slope of the characteristic line not attributable to the deterioration of these strings St, the calculation unit measures the slope of the discharge current characteristic line of each string St1 to St10 for a total period of 30 seconds by repeating a 10-second basic measurement three times. An approximate line of the slope of the characteristic line is then calculated from the slopes calculated at four points: 0, 10, 20, and 30 seconds. The slope calculated at 0 seconds is zero because there is no measurement data. The determination unit compares the approximate lines of the slopes of the discharge current characteristic lines calculated for each string St1 to St10 between strings St1 to St10, and determines a string St exhibiting an unusual time change in the discharge current value as a deteriorated string St. That is, based on the positive or negative sign of the calculated slope of the characteristic line, the determination unit determines that a string St exhibiting an unusual slope (positive in this embodiment) among the slopes of the characteristic lines for each string St1 to St10 is a string including a deteriorated lead-acid battery 4a.

[0038] The SOH calculation unit calculates the discharge quantity of electricity Q (=I×T) of the degraded string St from the time when discharge of each string St1 to St10 starts until the terminal voltage of the degraded storage battery reaches 10.8 (=1.8 [V] / cell×6 cell) [V], that is, the time T until a decrease in the discharge current relative to the discharge time is detected, and the integrated value I of the discharge current value measured by the measurement unit for the degraded string St during the time T. Then, using the calculated discharge quantity of electricity Q as the capacity that the degraded string St can discharge, the SOH before discharge of the degraded string St is calculated using the above-mentioned equation (3).

[0039] FIG. 7 is a flowchart showing the process of identifying a deteriorated string St, which is performed by the MPU that constitutes the BMU 5a.

[0040] In this process of identifying a deteriorated string St, the MPU first determines whether the storage battery array 4 is in a discharging state under the control of the AC / DC converter 6 (see step 101). The determination in step 101 is repeated until the determination result is Yes. When it is determined in step 101 that the storage battery array 4 is in a discharging state and the MPU recognizes that discharging has started for each of the strings St1-St10, the calculation unit then measures the discharge current value of each of the strings St1-St10 and calculates the integrated value of the discharge current value since the start of discharge for each of the strings St1-St10 (see step 102). The process of step 102 continues until the SOH is calculated in the process of step 111, which will be described later.

[0041] Next, the MPU determines whether a predetermined time, for example, two hours, has elapsed since the start of discharge (see step 103). If the predetermined time has not elapsed since the start of discharge and the determination result in step 103 is No, the process returns to step 102, and the processes in steps 102 and 103 are repeated. On the other hand, if the predetermined time has elapsed since the start of discharge and the determination result in step 103 is Yes, the calculation unit then calculates the time change in the discharge current value, i.e., the current change rate of the discharge current, for each string St1 to St10 at a predetermined sampling time interval of 100 msec as the slope of the characteristic line (see step 104). Next, the MPU determines whether a specific period on the order of seconds, totaling 30 seconds, has elapsed (see step 105). Specifically, it is determined whether an approximation line of the slope of the characteristic line has been obtained for each string St1 to St10.

[0042] If the specific period of 30 seconds has not elapsed and the determination result in step 105 is No, the process returns to step 104 and the processes in steps 104 and 105 are repeated.

[0043] On the other hand, if the determination result in step 105 is Yes after the 30-second specific period has elapsed, the determination unit then determines whether or not there is a string St whose current change rate of the discharge current measured by the measurement unit has a positive sign, that is, whether or not there is a string St whose discharge current characteristic line has a positive upward slope and whose discharge current value is decreasing, like string St4 in the graphs shown in Figures 3 and 5 (see step 106). If there is no string St whose current change rate of the discharge current measured has a positive sign, and the characteristic lines for all strings St1 to St10 have negative downward slopes, and the determination result in step 106 is No, it is determined that there is no degraded string St among the strings St1 to St10 that make up the storage battery array 4, and the process of identifying the degraded string St ends.

[0044] On the other hand, if the measured current change rates have a positive sign and the judgment result in step 106 is Yes, then the string St with the positive current change rate is identified as a deteriorated string St by the judgment unit (see step 107).

[0045] Next, the MPU determines whether or not equal charging is being performed on each of the strings St1 to St10 (see step 108). This equal charging makes the inter-terminal voltages of each of the strings St1 to St10 uniform. The determination in step 108 is repeated until the determination result is Yes.

[0046] After a degraded string St is identified, if it is determined in step 108 that equalization charging has been performed on each of the strings St1 to St10, the MPU then determines whether or not the degraded string St was charged by the AC / DC converter 6 during discharge of the degraded string St after equalization charging (see step 109). If charging was performed during discharge of the degraded string St after equalization charging and the determination result in step 109 is No, the process of identifying the degraded string St ends.

[0047] On the other hand, if charging was not performed during the discharge of the degraded string St after the equalization charge and the determination result in step 109 is Yes, the MPU then determines whether the terminal voltage of each lead-acid battery 4a of the degraded string St measured by the measurement unit has dropped to 10.8 V (= 1.8 V / cell × 6 cells) (see step 110). If the terminal voltage of each lead-acid battery 4a of the degraded string St has not dropped to 10.8 V and the determination result in step 110 is No, the process of step 110 is repeated. If the terminal voltage of each lead-acid battery 4a of the degraded string St has dropped to 10.8 V and the determination result in step 110 is Yes, it is determined that the SOC of the degraded string St has become 0%. Next, the SOH calculation unit calculates the SOH of the degraded string St before discharge (step 111). After the process in step 111 is completed, the process of identifying the degraded string St ends.

[0048] The energy storage system 1 according to the above embodiment has been described as a case in which a degraded string St is identified and detected from the behavior of the discharge current of each of the strings St1 to St10. However, as another embodiment, in addition to the method of detecting the degradation of the string St from the behavior of the discharge current, a method of detecting the degradation of the string St from the terminal voltage of the lead storage battery 4a detected by the cell sensor 5b may be used in combination.

[0049] In the method of detecting a degraded string St using a cell sensor 5b, when the cell sensor 5b detects that the terminal voltage of a lead-acid battery 4a constituting one of the strings St reaches 10.8 [V], a degraded string detection signal is output from the alarm unit of the BMU 5a to the AC / DC conversion device 6, and a low voltage alarm is issued.

[0050] The table shown in Figure 8 shows the results of measurements taken five times, measuring the time from the start of discharge to the issuance of a low voltage alarm for each of strings St1 to St10, when these two methods for detecting deteriorated strings were used together as the deteriorated string detection algorithm for the BMU 5a in the battery monitoring device 5 shown in Figure 1 and a battery with a deteriorated SOH of 30% was mixed into string St1. The measurements were taken at a discharge current of 0.11 [C 10 A] and a discharge time of 6 hours, 10 parallel strings St1 to St10 each having 36 lead-acid batteries 4a connected in series were discharged.

[0051] The table in Figure 8 confirms that the method of detecting degraded strings based on discharge current behavior issues a low voltage warning on average 46.9 seconds later than the method of detecting degraded strings using the cell sensor 5b. With the method of detecting degraded strings using the cell sensor 5b, the cell sensor 5b directly measures the voltage between the terminals of the lead-acid battery 4a and outputs it to the BMU 5a, so the time until the low voltage warning is issued is short, at approximately 60 seconds.

[0052] On the other hand, in the method of detecting degraded strings based on the behavior of the discharge current, several verification processes by the MPU, such as removing noise components superimposed on the discharge current value and determining whether the change in the discharge current value over time is due to the degradation of the string St, require time, and a low voltage alarm is issued in an average of 108 seconds.

[0053] However, the time difference of approximately 46.9 seconds between the low voltage alarm output by the method of detecting degraded strings based on the behavior of discharge current and the low voltage alarm output by the method of detecting degraded strings based on the cell sensor 5b was not at a level that would accelerate the deterioration of string St, and it was confirmed that degraded string St was detected normally by the method of detecting degraded strings based on the behavior of discharge current.

[0054] In the method of detecting degraded strings using the cell sensor 5b, the degraded string St may not be detected correctly due to a malfunction of the cell sensor 5b that occurs when the lead-acid battery 4a malfunctions or an abnormality in communication with the BMU 5a. Also, by measuring the temperature of the lead-acid battery 4a, it is possible to detect a temperature rise during over-discharge or over-charge of the lead-acid battery 4a, but in the case of a lead-acid battery 4a with a large heat capacity, it takes time for the temperature to rise, and therefore the cell sensor 5b may not be able to detect an abnormality due to a temperature rise in the lead-acid battery 4a in a timely manner.

[0055] However, as described above, by combining the two degraded string detection methods with the degraded string detection algorithm of the BMU 5a in the battery monitoring device 5, it is possible to identify and detect the degraded string St early using the degraded string detection method based on the behavior of the discharge current, even if a failure of the cell sensor 5b or an abnormality in communication with the BMU 5a occurs. Also, when the cell sensor 5b is operating normally, the degraded string St is detected using two detection methods: degraded string detection by the cell sensor 5b and degraded string detection based on the behavior of the discharge current, so it is possible to determine with higher accuracy whether or not the degraded string St is present. This makes it possible to prevent the lead-acid battery 4a from deteriorating early due to overdischarge or polarity reversal of the lead-acid battery 4a, which would occur if the degraded string St could not be detected. [Explanation of symbols]

[0056] 1: Energy Storage System (ESS) 2: Power supply section 3: Load 4: Multi-parallel energy storage array 4a: Lead acid battery 5:Lead acid battery monitoring device 5a:BMU 5b: Cell sensor 5c: Unit sensor 6: AC / DC converter (PCS) 7: Breaker St1~St10: Strings

Claims

[Claim 1] A battery monitoring device that detects a state of each string included in a multi-parallel battery array in which a plurality of strings each including a plurality of series-connected storage batteries are connected in parallel, the device comprising: a measurement unit for measuring a discharge current discharged from each of the strings; a calculation unit that calculates a current change rate of the discharge current measured by the measurement unit for each of the strings; a determination unit that determines that a string that exhibits a unique current change rate among the multiple current change rates for each of the strings calculated by the calculation unit includes a deteriorated storage battery, the calculation unit calculates the current change rate after a predetermined time has elapsed since discharge of each of the strings started, The determination unit detects the peculiar current change rate based on the current change rate calculated by the calculation unit. A battery monitoring device characterized by:

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