GB2465469A - Power storage control system - Google Patents

Abstract

A power storage control system comprises a plurality of power storage devices 1011, 1012-101N, each comprising one or more storage elements connected in series, and a plurality of power conversion devices 1041, 1042-104N, where the plural units of power storage devices and the plural units of power conversion devices are connected in parallel to each other. A power storage monitoring device is associated with each power storage device for monitoring the temperature 1061, 1062-106N of the respective power storage device. A controller 105 receives the temperature information from the power storage monitoring devices and outputs charge/discharge current commands to the power conversion devices.

Description

POWER CIRCUIT CONTROL SYSTEM

The present invention relates to a power storage device in which a large number of power storage elements are used in a manner of being combined with each other. More particularly, it relates to a power-circuit control system in which each power storage device is configured by connecting plural units of power storage elements in series with each other, and in which the resultant plural groups of power storage devices are used in a manner of being further connected in parallel to each other.

In recent years, proposals have been made concerning effective utilization of respective types of energies through the application of power storage devices, and in particular, power storage elements such as secondary cells and electric double-layer capacitors. Moreover, a study is nowadays under way for addressing the implementation of large power not only by connecting the secondary cells in series with each other, but also by connecting the large number of in-series connected secondary cells in parallel to each other. Each secondary cell has its own internal resistance, and this internal resistance increases due to a fabrication variation and with a lapse of time.

Plso, its seemingly-observed apparent capacity decreases due to the fabrication variation and with the lapse of time similarly. On account of these undesirable changes, there is a possibility that, when the secondary cells are partially replaced by new ones after being used for some time, the remaining secondary cells will be used in a state where significant differences internally exist among their internal resistances and cell capacities.

As conventional technologies, in order to solve problems like this, the following method has been disclosed: Namely, in the in-parallel connected power storage devices, a switch and a monitoring unit are provided for each in-series connection so that excessive charge/discharge for the power storage devices is prevented from occurring. For example, in the case of discharge, if differences exist among charge amounts of the power storage devices, the power storage devices are used such that only the power storage device whose charge amount is the highest is connected to the outside. Then, when the second highest charge amount is attained, the power storage devices are used such that the power storage devices whose charge amounts are equal to each other are connected to each other. Also, the following method has been disclosed: Namely, according to this method, the excessive charge/discharge is prevented from occurring by opening a switch of the power storage device in which the excessive charge/discharge is likely to occur.

[Patent Document 1] JP-A-2005-528070 [Patent Document 2] JP-A-2005-168259 In these inventions, however, no consideration has been given to temperatures of the respective power storage devices. It is generally said that, the higher the temperature of a power storage device becomes, the more likely it becomes that the power storage device gets deteriorated. In a case where new and old power storage devices are mixed, or in a case where, although all of the power storage devices are new ones, product variations exist thereamong, differences become more likely to occur among the temperatures of the respective power storage devices. Accordingly, differences are likely to occur among the deteriorations of the respective power storage devices.

It is a preferred aim of the present invention to decrease the variations in the power storage devices'ternperatures down to the lowest possible degree, and to reduce the differences in the power storage devices deteriorations.

In the present invention, there is provided a power-circuit control system, including power storage devices, each of the power storage devices being configured by connecting one or more power storage elements in series with each other, and power conversion devices for converting input/output powers into/from the power storage devices, the plural units of power storage devices and the plural units of power conversion devices being connected in parallel to each other, wherein the power-circuit control system, further includes power-storage-device monitoring devices for monitoring temperatures of the respective power storage devices, and a controller for receiving the temperature information on the respective power storage devices from the pOwer-storage-device monitoring devices, and outputting charge/discharge current commands to the power conversion devices.

The above-described power-circuit control system makes it possible to suppress the variations in the power storage devices'temperatures, thereby making it possible to reduce the differences in the power storage devices deteriorations.

IN THE DRAWINGS

Fig. 1 is an entire schematic configuration diagram of a power-circuit control system according to a first embodiment of the present invention; Fig. 2 is a control block diagram of a controller in the first embodiment of the present invention; Fig. 3 is a processing flow in a power-storage-device charge/discharge current command calculation unit 206 in the first embodiment of the present invention; Fig. 4 is a processing example in the power-storage-device charge/discharge current command calculation unit 206 in the first embodiment of the present invention; Fig. 5A to Fig. 5C illustrate an operation example in the first embodiment of the present invention; Fig. 6 is a charge-current-vs.-raised-temperature characteristics diagram of a storage-cell device; Fig. 7 is an example of the configuration diagram in a case where the present invention is applied to railroad vehicles; Fig. 8 is a control block diagram of the controller in a second embodiment of the present invention; Fig. 9 is a control block diagram of a power-storage-device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 10 is a processing flow in the power-storage-device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 11 is a state after the termination of a step 801 in an operation example of the power-storage-device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 12 is a state after the termination of a step 802 in the operation example of the power-storage--device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 13 is a state after the termination of a step 803 in the operation example of the power-storage-device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 14 is a state after the termination of a step 804 in the operation example of the power-storage--device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 15 is a state after the termination of a step 805 in the operation example of the power-storage--device charge/discharge current judgment unit 601 in the second embodiment of the present invention; Fig. 16 is a state after the termination of a step 806 in the operation example of the power-storage-device charge/discharge current judgment unit 601 in the second embodiment of the present invention; and Fig. 17 is a state after the termination of a step 809 in the operation example of the power-storage-device charge/discharge current judgment unit 601 in the second embodiment of the present invention.

Hereinafter, referring to Fig. 1, the explanation will be given below concerning a concrete system configuration of the present invention.

This concrete system configuration is as follows: In a power-circuit control system, including power storage devices 1011, 1012, ... , lOiN, each of the power storage devices 1011, 1012, ... lOiN being configured by connecting one or more power storage elements in series with each other, power-storage-device monitoring devices 1021, 1022, ... , 102N for monitoring temperatures-&--charge-amounts 1061, 1062, 106N of the power storage devices 1011, 1012, lOiN, second power conversion devices 1041, 1042, ...

104N which are controlled so that charge/discharge current commands 1081, 1082, ... , 108N and charges/discharges into/from the power storage devices 1011, 1012, ... , lOiN coincide with each other, and a first power conversion device 103 which is used for mutually converting the power with the second power conversion devices 1041, 1042, ... , 104N, and which is connected to a driving motor, there is provided a controller 105 for outputting the charge/discharge current commands 1081, 1082, ... , 108N to the second power conversion devices 1041, 1042, ... , 104N on the basis of a charge/discharge current request 107 made to all of the power storage devices 1011, 1012, ..., lOiN and the temperatures-&-charge-amounts 1061, 1062, ... 106N.

In an embodiment of the present invention in a railroad vehicle, a driving force needed for operation service of the railroad vehicle can be determined in advance. This is because its operation-service speed and hating positions are determined in advance by its schedules such as operation-service diagram. The charge/discharge current request 107 made to all of the power storage devices is determined by its driver or vehicle operation-service control device, taking into consideration the driving force and operation-service efficiency needed for the above-described operation service.

Next, referring to Fig. 2, the explanation will be given below regarding a processing flow in the controller 105.

The use of an information division function 201 divides the temperatures-&-charge-amounts 1061, 1062, ... , 106N into respective power-storage-device temperatures 2021, 2022, ... , 202N and respective power-storage-device charge amounts 2031, 2032, ... , 203N.

Incidentally, the respective power-storage-device charge amounts 2031, 2032, ... , 203N are not used in this drawing, and accordingly are omitted.

Next, an average processing unit 204 calculates a power-storage-device average temperature 205 from the respective power-storage-device temperatures 2021, 2022, ... , 202N.

Next, a power-storage-device charge/discharge current command calculation unit 206 calculates the charge/discharge current commands 1081, 1082, ... , 1OBN on the basis of the discharge power 107 to be inputted into the first power conversion device 103, the power-storage-device average temperature 205, and the respective power-storage-device temperatures 2021, 2022, ... , 202N. Referring to Fig. 3, the explanation will be given below concerning a method for this calculation.

Fig. 3 is the processing flow in the power-storage-device charge/discharge current command calculation unit 206.

At a step 301, the power-storage-device charge/discharge current command calculation unit 206 calculates temperature differences T (i) {j = 1, 2, 15... , N} for the respective power storage devices by subtracting the respective power-storage-device temperatures 2021, 2022, ... , 202N from the power-storage-device average temperature 205. Next, the unit 206 proceeds to a step 302.

At the step 302, the unit 206 calculates a sum total X of the temperature differences iT (i) in which each T (i) > 0 holds, on the basis of T (i) {i = 1, 2, ... , N} for the respective power storage devices calculated at the step 301. Next, the unit 206 proceeds to a step 303.

At the step 303, the unit 206 judges whether or not each T (i) {i = 1, 2, ... , N} for the power storage devices > 0 holds on the basis of T (i) {i = -10 - 1, 2, ... , N} calculated at the step 301. Then, if each T (i) {i = 1, 2, ... , N) > 0 holds, the unit 206 proceeds to a step 304. Meanwhile, if each iT (1) {i = 1, 2, ... , N} > 0 does not hold, the unit 206 proceeds to a step 305.

At the step 304, based on the following (Expression 1), the unit 206 calculates temperature distribution ratios B (i) {i 1, 2, ... , N} for the respective power storage devices, on the basis of T (i) {i = 1, 2, ... , N} for the respective power storage devices calculated at the step 301 and X calculated at the step 302.

[Expression 1] B (i) iT (i)/X, where {i = 1, 2, ... , N) (Expression 1) Next, the unit 206 proceeds to a step 306.

Meanwhile, at the step 305, the unit 206 sets the temperature distribution ratios B (I) {i = 1, 2, N} for the respective power storage devices at 0, then proceeding to the step 306.

At the step 306, based on the following (Expression 2), the unit 206 determines the charge/discharge current commands 1081, 1082, ... , 108N on the basis of the temperature distribution ratios B (i) {i = 1, 2, ... , N) for the respective power storage devices acquired at the step 304 or the step 305, and a -11 -discharge current lall which is determined from the discharge power 107 inputted into the first power conversion device 103. Incidentally, in (Expression 2), the charge/discharge current commands 1081, 1082, 5... , 108N for the respective power storage devices are substituted by I (1), I (2) , ... , I (N) [Expression 2] I (i) = Iall*B (i), where {i 1, 2, ... , N} (Expression 2) The execution of the above-described processing allows the determination of the charge/discharge current commands 1081, 1082, ... , 108N for the respective power storage devices.

Based on the method as described above, the charge/discharge current commands are calculated, thereby executing the charges/discharges into/from the power storage devices. This scheme makes it possible to decrease the variations in the power storage devices'temperatures, and to reduce the differences in the power storage devices'deteriorations within a range where the charge/discharge power amount into/from all of the power storage devices is abided by up to the highest possible degree.

Fig. 4 illustrates an example of this control.

Fig. 4 illustrates a case of N 3, i.e., the -12 -case where the three units of power storage devices are connected to the first power conversion device 103 in parallel to each other. Here, the temperatures of the power storage devices 1, 2, and 3 are equal to 10, 12, and 17, respectively. Accordingly, the average temperature of the three units of power storage devices is equal to 13. Also, the discharge current lall is equal to 100, which is determined from the charge/discharge current request 107 made to all of the power storage devices inputted into the first power conversion device 103.

At the step 301, calculating the temperature differences AT (1) {i = 1, 2, 3} for the power storage devices results in answers of 3, 1, and -4, respectively. Next, at the step 302, calculating the sum total X of the AT (1) {i = 1, 2, 3} in which each AT (i) > 0 holds results in X = 3 + 1 = 4. This is because the power storage devices which satisfy AT (i) > 0 are the power storage devices 1 and 2. Next, at the step 303, the judgment processing as to whether or not each AT (i) {i = 1, 2, 3} > 0 holds causes the power storage devices 1 and 2 to proceed to the step 304. Meanwhile, this judgment processing causes the power storage device 3 to proceed to the step 305. At the step 304, the charge/discharge current command calculation unit 206 calculates the temperature distribution ratios B (i) {i 1, 2} for the power storage devices 1 and 2. Carrying out this calculation -13 -results in answers of B (1) 3/4 and B (2) = 1/4 as the temperature distribution ratios B (i) for the power storage devices 1 and 2, respectively. Meanwhile, at the step 305, the unit 206 sets the temperature distribution ratio B (3) for the power storage device 3 at 0. After performing these processings, at the step 306, the unit 206 calculates the respective discharge current I (i) {i 1, 2, 3} for the power storage devices 1, 2, and 3, thereby resulting in I (1) = 75 (= 100 x 3/4), I (2) = 25 (= 100 x 1/4), and I (3) = 0 (= x 0), respectively. Fig. 5A to Fig. 5C illustrate an operation example in a case where this processing is carried out depending on the requirements.

For convenience of explanation, the explanation will be given below regarding the case of N = 3, i.e., the case where the three units of power storage devices are connected to the first power conversion device 103 in parallel to each other.

Fig. 5A to Fig. 5C illustrate time-vs.-the discharge power inputted into the first power conversion device 103 (Fig. 5A), time-vs.-the respective power-storage-device temperatures 2021, 2022, and 2023 (Fig. 5B), and time-vs.--the respective power-storage-device discharge currents 1081, 1082, and 1083 (Fig. 5C) The lower the power-storage-device temperatures 2021, 2022, and 2023 become as compared with the power-storage-device average temperature 205, -14 -the larger the absolute values of the charge/discharge currents are caused to become. As a result, the power-storage-device temperature 2023 is distributed so that the charge/discharge current command 1083 for the power storage device becomes the largest. Also, since the power-storage--device temperature 2021 is larger than the power-storage-device average temperature 205, the charge/discharge current command 1081 becomes equal to zero. When this processing is repeated, the power-storage-device temperature 2021 falls by the cooling of the power storage device. This is because no charge/discharge is performed for the power-storage-device temperature 2021 since the charge/discharge current command 1081 is equal to zero. Meanwhile, the power-storage-device temperature 2023 rises fastest, because the power-storage-device temperature 2023 is distributed so that the charge/discharge current command 1083 becomes the largest. Also, the power-storage-device temperature 2022 rises by the condition that the charge/discharge is performed, although the power-storage-device temperature 2022 is distributed so that the charge/discharge current command 1082 become lower as compared with the charge/discharge current command 1083. On account of this operation, the power-storage-device temperatures 2021, 2022, and 2023 come nearer to the power-storage-device average temperature 205 gradually.

Incidentally, although attention is focused -15 -on only the discharge in this drawing example, the methodology is the same in the case of the charge as well.

As described above, the power-storage-device average temperature is determined by averaging the respective power-storage-device temperatures. Then, the charge/discharge currents into/from the respective power storage devices are determined so that the respective power-storage-device temperatures come nearer to the target power-storage-device average temperature. This processing makes it possible to decrease the variations in the power storage devices'temperatures, i.e., the first object of the present invention.

Also, the raised temperatures tT, which accompany the charges/discharges for the respective power storage devices, are determined by the following (Expression 3), letting the charge/discharge currents for the respective power storage devices be I, the internal resistances therefor be R, and the thermal time constants therefor be C: [Expression 3] = (Expression 3) Accordingly, I (i) {j = 1, 2, 3}, i.e., the charge/discharge current commands 1081, 1082, and 1083 for the respective power storage devices, can also be -16 -determined from the following (Expression 4) by taking advantage of T (i) {i = 1, 2, ... , N} for the respective power storage devices calculated at the step 301, the internal resistances R (i) {i = 1, 2, ... , N} for the respective power storage devices, and the thermal time constants C (i) (j 1, 2, ... , } for the respective power storage devices: [Expression 4] g0=1/ó TXO, where { i = 1, 2, ... , N (Expression 4) Moreover, as illustrated in Fig. 6, I (i) {i = 1, 2, 3} may also be determined by creating in advance the charge-current--vs. -raised-temperature characteristics in correspondence with the temperatures and the internal resistances.

Incidentally, referring to Fig. 7, the explanation will be given below concerning a case where the configuration illustrated in Fig. 1 is applied to railroad vehicles.

Fig. 7 assumes that series hybrid railroad vehicles on each of which, in addition to the power storage device, an engine and a power generator are mounted. An electric motor 702 is mounted on each of the three vehicles 701 of a train. These plurality of electric motors 702 drive the train as a whole.

Incidentally, the number of the vehicles on each of -17 -which the electric motor 702 is mounted is not limited to three. Also, the electric motor 702 may not be mounted on the front-head vehicle on which a driving panel 710 is mounted. Also, although the power storage devices 1011 and 1012 and the second power conversion devices 1041 and 1042 are respectively mounted on only the two vehicles out of the three ones, the number of the vehicles on which they are to be mounted is not limited to two. Moreover, although a power generation device 704 and a third power conversion device 703 are mounted on every vehicle, they may not necessarily be mounted on every vehicle.

The vehicles 701 are caused to drive as follows: Based on a command from the driving panel 710, a train control device 709 transmits this command to control devices 711 for controlling first power conversion devices 1031, 1032, and 1033. Then, based on the command, the power generation devices 704 and the power storage devices 1011 and 1012 are operated.

A three-phase AC electric motor (: induced electric motor or synchronized electric motor) is commonly used as each electric motor 702. In order to supply power to each electric motor 702, the second power conversion devices 1041 and 1042 carry out voltage-current mutual conversion of DC powers transmitted from the power storage devices 1011 and 1012. Moreover, the powers are converted into AC powers by the first power conversion devices 1031, -18 - 1032, and 1033, then being supplied to the respective electric motors 702. Also, not only the powers from the power storage devices 1011 and 1012 via the second power conversion devices 1041 and 1042, but also powers from the power generation devices 704 via the third power conversion devices 703 are supplied to the first power conversion devices 1031, 1032, and 1033. Each power generation device 704 is configured by combining a driving-force generation device (which, hereinafter, will be abbreviated as "engine") such as, e.g., a diesel engine, with a power generator. This engine can be caused to work as an air-exhaust brake by controlling the air exhaustion. Incidentally, each power generation device 704 is not limited to the combination of the diesel engine and the power generator. Accordingly, e.g., a fuel battery may be employed as each power generation device 704.

In the case of each railroad vehicle, the power needed for its driving is exceedingly high.

Consequently, if the voltage is made more or less lower, it cannot be avoided to increase withstand voltages for configuration elements used in the first power conversion device or the like. Meanwhile, the voltage for each power storage element is smaller as compared with the DC-portion voltage for the first power conversion device. Accordingly, the voltage needs to be ensured by employing the power storage devices each of which is configured by connecting the -19 -large number of power storage elements in series with each other. Like this, however, in the case where the in-series number of the power storage elements is increased, if one of the power storage elements fails, the other power storage elements belonging to the same in-series connection also fail to be used. This phenomenon tremendously worsens the performance of each power storage device.

At this time, the second power conversion device is set up therebetween. In this case, even if the voltage for the first power conversion device and the voltage for the power storage device configured by connecting the large number of power storage elements in series with each other differ from each other, the second power conversion device operates as a voltage ascent/decent chopper. This operation allows implementation of mutual exchange of the power between the first power conversion device and the power storage device. Then, this implementation of the mutual exchange makes it unnecessary to form the system that is dependent on the in-series number of the power storage elements and the configuration elements of the first power conversion device.

Next, the explanation will be given below concerning a second embodiment.

The explanation will be given below regarding a control about a case where the charge/discharge current commands for some of the power storage devices -20 -have exceeded the maximum charge/discharge currents.

Fig. 8 illustrates its control block diagram.

Incidentally, the same configuration components as the ones in Fig. 2 are denoted by the same reference numerals, and thus their explanation will be omitted below. What is different from the configuration in Fig. 2 in the configuration in Fig. 8 is the set-up of a power-storage-device charge/discharge current judgment unit 601. The power-storage-device charge/discharge current judgment unit 601 calculates new charge/discharge current commands 1081, 1082, ...

108N on the basis of the respective power-storage-device charge amounts 2031, 2032, .,. , 203N whose uses are unnecessary in the configuration in Fig. 2, and the charge/discharge current commands 2071, 2072, ... , 207N (which are denoted by 1081, 1082, ... , 108N in Fig. 2) calculated by the power-storage-device charge/discharge current command calculation unit 206.

Referring to Fig. 9, the explanation will be given below concerning the power-storage-device charge/discharge current judgment unit 601.

The power-storage-device charge/discharge current judgment unit 601 includes a current setting unit 901 for replacing the charge/discharge current commands 2071, 2072, ... , 207N, which are calculated by the power-storage-device charge/discharge current command calculation unit 206, by charge/discharge current commands 8011, 8012, ... , 801N which are to be -21 -used in this control flow, a maximum charge/discharge current calculation unit 902 for determining respective power-storage-device maximum charge/discharge currents 9031, 9032, ... , 903N from the respective power-storage-device charge amounts 2031, 2032, ... , 203N, and a maximum charge/discharge current judgment unit 904.

Here, the maximum charge/discharge current judgment unit 904 makes comparisons between the charge/discharge current commands 8011, 8012, ... , 801N for the respective power storage devices and the respective power-storage-device maximum charge/discharge currents 9031, 9032, ... , 903N determined by the maximum charge/discharge current calculation unit 902. Then, if the charge/discharge current commands for at least one of the power storage devices have exceeded the power-storage-device maximum charge/discharge currents, the judgment unit 904 sets the charge/discharge current commands for the condition-applicable power storage devices as the power-storage-device maximum charge/discharge currents. Moreover, the judgment unit 904 distributes the excess amounts therebetween among the power storage devices whose charge/discharge current commands have not exceeded the power-storage-device maximum charge/discharge currents. The judgment unit 904 executes these operations in a repeated manner, thereby determining the new charge/discharge current commands 1081, 1082, ... , 108N for the respective power storage devices so that the -22 -charge/discharge current commands for all of the power storage devices become equal to or smaller than the power-storage--device maximum charge/discharge currents.

Referring to Fig. 10, the explanation will be given below regarding the control flow in the power-storage-device charge/discharge current judgment unit 601.

At a step 801, the unit 601 operates the current setting unit 901, thereby setting the charge/discharge current commands 2071, 2072, ... , 207N, which are calculated by the power-storage-device charge/discharge current command calculation unit 206, as the charge/discharge current commands 8011, 8012, SO1N. Next, the unit 601 proceeds to a step 802.

At the step 802, the unit 601 operates the maximum charge/discharge current calculation unit 902 for determining the respective power-storage-device maximum charge/discharge currents 9031, 9032, ... , 903N from the respective power-storage-device charge amounts 2031, 2032, ... , 203N. As a result, the unit 902 calculates the respective power-storage-device maximum charge/discharge current commands 9031, 9032, ... , 903N on the basis of the respective power-storage-device charge amounts 2031, 2032, ... , 203N. Next, the unit 601 proceeds to a step 803. Incidentally, operations after the step 803 become the ones by the maximum charge/discharge current judgment unit 904.

At the step 803, the unit 904 sets -23 -charge/discharge redundant-capacity flags for the respective power storage devices. Each charge/discharge redundant-capacity flag is set at 1 if the redundant capacity remains for the charge/discharge; whereas each flag is set at 0 if no redundant capacity remains therefor. Here, for the initialization, the charge/discharge redundant-capacity flags for all of the power storage devices are assumed to be 1. Next, the unit 904 proceeds to a step 804.

At the step 804, the unit 904 searches for the power storage devices for which the respective power-storage-device charge/discharge current commands 8011, 8012, ... , 801N determined at the step 801 are equal to or larger than the respective power-storage-device maximum charge/discharge current commands 9031, 9032, ... , 903N determined at the step 802. Then, the unit 904 sets the flags for the condition-applicable power storage devices at 0, and determines the

condition-applicability number X of the condition-

applicable power storage devices. Furthermore, with respect to the condition-applicable power storage devices, the unit 904 calculates the differences H (i) {i = 1, 2, ... , N} between the respective power-storage--device charge/discharge current commands and the respective power-storage-device maximum charge/discharge current commands. Incidentally, the unit 904 carries out the initialization of H (i) {i = 1, 2, ... , N} before the calculation of H (1) . Also, -24 -the unit 904 replaces the respective charge/discharge current commands for the condition-applicable power storage devices by the respective power-storage-device maximum charge/discharge current commands. Next, the unit 904 proceeds to a step 805.

At the step 805, the unit 904 calculates a distribution shortage amount R, which is the sum total of H (i) {i = 1, 2, ... , N} calculated at the step 804.

Next, the unit 904 proceeds to a step 806.

The respective power storage devices whose flags are 1 still have the redundant capacity with respect to the maximum charge/discharge current commands. Accordingly, at the step 806, the unit 904 increases the charge/discharge current commands by equally distributing the distribution shortage amount R calculated at the step 805. Since the number X of the power storage devices which have no redundant capacity has been determined at the step 804, the number of the power storage devices whose flags are 1 becomes equal to N -X. On account of this, R/(N -X) becomes the value which is to be added to the charge/discharge current commands for the respective power storage devices whose flags are 1. Taking advantage of this value, the unit 904 calculates the new charge/discharge current commands 8011, 8012, ... , 801N, then proceeding to a step 807.

At the step 807, the unit 904 makes comparisons between the charge/discharge current -25 -commands 8011, 8012, ... , 801N and the maximum charge/discharge current commands 9031, 9032, ... , 903N, thereby judging whether or not the charge/discharge current commands 8011, 8012, ... , 801N are equal to or smaller than the maximum charge/discharge current commands 9031, 9032, ... , 903N. Moreover, the unit 904

counts the condition-applicability number W of the

condition-applicable power storage devices, then proceeding to a step 808.

At the step 808, in a case of W = N, or in a case where all of the charge/discharge redundant-capacity flags are 0, the unit 904 proceeds to a step 809. Meanwhile, in a case where all of the charge/discharge redundant-capacity flags are not 0, i.e., a case of W < N, the unit 904 returns to the step 804.

At the step 809, in the case of W = N, the unit 904 outputs the charge/discharge current commands 8011, 8012, ... , 801N as the charge/discharge current commands 1081, 1082, ... , 108N. Meanwhile, in the case where all of the charge/discharge redundant-capacity flags are 0, the unit 904 outputs the maximum charge/discharge current commands 9031, 9032, ... , 903N for the respective power storage devices as the charge/discharge current commands 1081, 1082, ... , 108N.

Repeating the above-described processing allows the calculation of the charge/discharge current commands 1081, 1082, ... , 108N.

-26 -As having been described above, the charge/discharge currents for the respective power storage devices are made equal to or smaller than the respective power-storage--device maximum charge/discharge currents which must be abided by. If the respective power-storage--device charge/discharge currents thus determined have exceeded the power-storage-device maximum charge/discharge currents, the charge/discharge currents for the condition-applicable power storage devices are replaced by the maximum charge/discharge currents. Moreover, the charge/discharge currents in which the excess-or--deficiency amounts occur are equally distributed among the power storage devices whose charge/discharge currents have not exceeded the maximum charge/discharge currents. This control makes it possible to prevent the charge/discharge current as all of the power storage devices from being damaged down to the lowest possible degree. Furthermore, the respective power-storage-device maximum charge/discharge currents are determined based on the respective power-storage-device charge amounts. This control allows the safety of the power storage devices to be taken into consideration.

The execution of these controls makes it possible to prevent the charge/discharge current as all of the power storage devices from being damaged down to the lowest possible degree, and to make the power-storage--device charge/discharge currents equal to or smaller -27 -than the power-storage-device maximum charge/discharge currents which must be abided by. As a consequence, it becomes possible to decrease the variations in the power storage devices'temperatures down to the lowest possible degree, and to reduce the differences in the power storage devices deteriorations.

Fig. 11 to Fig. 17 illustrate an example of the case where this control flow is used.

Referring to Fig. 11 to Fig. 17, the explanation will be given below regarding a case of N = 4, i.e., the case where the four units of power storage devices are connected to the first power conversion device 103 in parallel to each other. Incidentally, initial values of the discharge current commands for the respective power storage devices 1 to 4 are set at 125, 60, 80, and 90, respectively. Also, the maximum charge/discharge currents determined from the respective power-storage-device charge amounts are set at 85, 100, 60, and 125, respectively.

Fig. 11 illustrates a diagram after the termination of the step 801. At the step 801, the power-storage--device charge/discharge current judgment unit 601 operates the current setting unit 901, thereby inputting the discharge current commands for the respective power storage devices 1 to 4. On account of this, 125, 60, 80, and 90 are set as the discharge current commands for the power storage devices 1 to 4, respectively.

-28 -Next, Fig. 12 illustrates a diagram after the termination of the step 802. Here, the unit 601 operates the maximum charge/discharge current calculation unit 902 for determining the respective power-storage-device maximum charge/discharge currents from the respective power-storage-device charge amounts. As a result, the unit 902 performs the processing of calculating the respective power-storage-device maximum charge/discharge current commands on the basis of the respective power-storage-device charge amounts. On account of this, 85, 100, 60, and 125 are set as the maximum charge/discharge current commands for the power storage devices 1 to 4, respectively.

Next, Fig. 13 illustrates a diagram after the termination of the step 803. Here, for the initialization, the maximum charge/discharge current judgment unit 904 sets the charge/discharge redundant-capacity flags for the respective power storage devices at 1, which indicates that the redundant capacity remains for the charge/discharge.

Next, Fig. 14 illustrates a diagram after the termination of the step 804. Here, the unit 904 searches for the power storage devices for which the respective power-storage-device charge/discharge current commands determined at the step 801 are equal to or larger than the respective power-storage-device maximum charge/discharge current commands determined at the step 802. Then, the unit 904 sets the flags for -29 -the condition-applicable power storage devices at 0, and determines the condition-applicability number X of the condition-applicable power storage devices. The charge/discharge current commands for the power storage devices 1 and 3 determined at the step 801 are equal to or larger than the maximum charge/discharge current commands for the power storage devices 1 and 3 determined at the step 802. On account of this, the flags for the power storage devices 1 and 3 are set at 0. Also, the condition-applicability number X becomes equal to X = 2. Furthermore, with respect to the condition-applicable respective power storage devices, the unit 904 calculates the differences I-i (i) {i = 1, 2, 3, 4} between the respective power-storage-device charge/discharge current commands and the respective power-storage-device maximum charge/discharge current commands. This calculation results in answers of H (1) = 125 -85 = 40 and H (3) 80 -60 = 20 with respect to the power storage devices 1 and 3, respectively.

The unit 904 performs no calculation with respect to the condition-inapplicable power storage devices, thereby resulting in answers of H (2) 0 and H (4) = 0. Also, the unit 904 replaces the condition-applicable respective charge/discharge current commands by the respective maximum charge/discharge current commands. As a result, the charge/discharge current commands for the power storage devices 1 and 3 become equal to 85 and 60, respectively.

-30 -Next, Fig. 15 illustrates a diagram after the termination of the step 805. At the step 805, the unit 904 calculates the distribution shortage amount R, which is the sum total of H (1) {i = 1, 2, 3, 4} calculated at the step 804. This calculation results in an answer of R = EH (i) = 60.

Next, Fig. 16 illustrates a diagram after the termination of the step 806. The respective power storage devices whose flags are 1 still have the redundant capacity with respect to the maximum charge/discharge current commands. Accordingly, at the step 806, the unit 904 equally distributes the distribution shortage amount R calculated at the step 805. The condition-applicable power storage devices here are the power storage devices 2 and 4. Also, since the number X 2 of the power storage devices which have no redundant capacity has been determined at the step 804, the number of the power storage devices whose flags are 1 becomes equal to 4 -2 = 2. On account of this, 60/2 = 30 is added to the charge/discharge current commands 60 and 90 for the condition-applicable power storage devices 2 and 4, respectively. This addition results in answers of 90 and 120, and also allows the determination of the new charge/discharge current commands. As a result, the charge/discharge current commands for the power storage devices 1 to 4 become equal to 85, 90, 60, and 120, respectively.

-31 -Next, at the step 807, the unit 904 makes the comparisons between the charge/discharge current commands and the maximum charge/discharge current commands, thereby judging whether or not the charge/discharge current commands are equal to or smaller than the maximum charge/discharge current commands. Moreover, the unit 904 counts the condition-

applicability number W of the condition-applicable

power storage devices. This counting results in an answer of W = 4.

Next, at the step 808, because of W = N, the unit 904 proceeds to the step 809.

Fig. 17 illustrates a diagram after the termination of the step 809. At the step 809, because of W = N, the unit 904 outputs the charge/discharge current commands determined at the step 806 as the charge/discharge current commands acquired as a result of the control by the power-storage-device charge/discharge current judgment unit 601. On account of this, the charge/discharge current commands for the power storage devices 1 to 4 become equal to 85, 90, 60, and 120, respectively.

The execution of the above-described control makes it possible to prevent the charge/discharge current as all of the power storage devices from being damaged down to the lowest possible degree, and to make the power-storage-device charge/discharge currents equal to or smaller than the power-storage-device -32 -maximum charge/discharge currents which must be abided by. As a consequence, it becomes possible to decrease the differences in the power storage devices'temperatures. This allows implementation of the accomplishment of the second object.