Charge and discharge control device, charge and discharge system, charge and discharge control method, and non-transitory storage medium

A charge and discharge control device that controls charging and discharging of a battery module in which a plurality of cell blocks, each including one or more unit cells, are connected in parallel to one another. A controller of the charge and discharge control device controls a current flowing through each of the cell blocks based on at least one of a current load of each of the cell blocks or a parameter relating to the current load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-005970, filed Jan. 17, 2020; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a charge and discharge control device, a charge and discharge system, a charge and discharge control method, and a non-transitory storage medium.

BACKGROUND

As information-related apparatuses and communication apparatuses have spread, secondary batteries have widely spread as electric power supplies of the apparatuses. Secondary batteries also have been utilized in the field of electric vehicles (EV) and natural energy. In particular, lithium-ion secondary batteries are widely used, since they have a high energy density and can be downsized. In lithium-ion secondary batteries, a positive electrode active material and a negative electrode active material absorb and release lithium ions, thereby storing and releasing electric energy. When charging, the lithium ions released from the positive electrode are absorbed by the negative electrode. When discharging, the lithium ions released from the negative electrode are absorbed by the positive electrode.

In secondary batteries such as lithium-ion secondary batteries, a plurality of unit cells are electrically connected in series, so that a high voltage and a high capacity are achieved. A battery module, in which a plurality of cell blocks are electrically connected in parallel to one another, may be used as an electric power supply. In this case, each of the cell blocks includes one or more unit cells. If the cell block includes a plurality of unit cells, just a serial connection structure of a plurality of unit cells may be formed in the cell block, or both a serial connection structure and a parallel connection structure of a plurality of unit cells may be formed in the cell block.

In the battery module in which a plurality of cell blocks are connected in parallel, even if the cell blocks use the same type of unit cells and the cell blocks use the same number of unit cells and the same connection structure of the unit cells, there may be variation in the performance of the unit cells, such as in their capacity and internal resistance, between the cell blocks or there may be variation in resistance of a connecting wire between the cell blocks. Therefore, in the battery module, the cell blocks may have different performances. In addition, through repeated charging and discharging, the cell blocks may deteriorate to different degrees, and the performance may vary between the cell blocks, such as their capacity and internal resistance. In the battery module, even if the cell blocks vary in performance, it is necessary to prevent the cell blocks from excessively varying in current load and to suppress the increase in variations in deterioration between the cell blocks.

DETAILED DESCRIPTION

According to an embodiment, there is provided a charge and discharge control device that controls charging and discharging of a battery module in which a plurality of cell blocks, each including one or more unit cells, are connected in parallel to one another. A controller of the charge and discharge control device controls a current flowing through each of the cell blocks based on at least one of a current load of each of the cell blocks or a parameter relating to the current load.

According to one embodiment, there is provided a charge and discharge control method of controlling charging and discharging of a battery module in which a plurality of cell blocks, each including one or more unit cells, are connected in parallel to one another. In the charge and discharge control method, a current flowing through each of the cell blocks is controlled based on at least one of a current load of each of the cell blocks or a parameter relating to the current load.

According to one embodiment, there is provided a non-transitory storage medium storing a charge and discharge control program to be executed by a computer for charging and discharging of a battery module in which a plurality of cell blocks, each including one or more unit cells, are connected in parallel to one another. The charge and discharge control program causes the computer to control a current flowing through each of the cell blocks based on at least one of a current load of each of the cell blocks or a parameter relating to the current load.

Embodiments will be described below with reference to the accompanying drawings.

First Embodiment

FIG.1shows a charge and discharge system1according to the first embodiment. As shown inFIG.1, the charge and discharge system1includes a battery module2, a load and an electric power supply (denoted by a reference numeral3), a current measurement unit (current measurement circuit)5, a voltage measurement unit (voltage measurement circuit)6, a charge and discharge control device7, and a driving circuit8. The battery module2includes a plurality of cell blocks B1to Bn. In the battery module2, the cell blocks B1to Bnare electrically connected to one another in parallel.

Each of the cell blocks B1to Bnincludes one or more unit cells11. The unit cell11is, for example, a secondary battery such as a lithium-ion secondary battery. In the example shown inFIG.1, in each of the cell blocks B1to Bn, the unit cells11are electrically connected in series, thereby forming a serial connection structure of the unit cells11. The cell blocks B1to Bnare the same in the number of unit cells11connected in series. In one example, any of the cell blocks B1to Bnmay be formed of only one unit cell11. In another example, any of the cell blocks B1to Bnmay have a parallel connection structure in which the unit cells11are electrically connected in parallel, in addition to the serial connection structure of the unit cells11.

The battery module2can be charged and discharged. The battery module2is charged by electric power supplied from the electric power supply. The electric power discharged from the battery module2is supplied to a load. The battery module2is mounted on an electronic apparatus, a vehicle, a stationary power supply apparatus, etc. A battery independent of the battery module2, a generator, etc. may be the electric power supply that supplies electric power to charge the battery module2. An electric motor, a lighting apparatus, etc. may be the load to which the electric power discharged from the battery module is supplied. In one example, an electric motor generator may function as both the electric power supply and the load. The current measurement unit5detects and measures a current I flowing through the battery module2. The voltage measurement unit6detects and measures a voltage Vcapplied to the battery module2.

The charge and discharge control device7includes a controller12. The controller12constitutes a computer, and includes a processor and a storage medium. The processor includes one of a central processing unit (CPU), a graphics processing unit (GPU), an application specific integrated circuit (ASIC), a microcomputer, a field programmable gate array (FPGA), a digital signal processor (DSP), etc. The storage medium may include an auxiliary storage device in addition to the main storage device such as the memory. The storage medium may be a magnetic disk, an optical disk (CD-ROM, CD-R, DVD, etc.), a magneto-optical disk (MO etc.), a semiconductor memory, etc. In the controller12, each of the processor and the storage medium may be one or more. The processor of the controller12executes a program etc. stored in the storage medium, thereby performing processing. The program to be executed by the processor of the controller12may be stored in a computer (server) connected to the processor through a network such as the Internet, or a server etc. in a cloud environment. In this case, the processor downloads the program via the network. In one example, the charge and discharge control device7is formed of an integrated circuit (IC) chip or the like.

The controller12acquires a measurement value of the current I flowing through the battery module2by the current measurement unit5, and a measurement value of the voltage Vcapplied to the battery module2by the voltage measurement unit6. The measurement of the current I by the current measurement unit5and the measurement of the voltage Vcby the voltage measurement unit6are performed periodically, for example, at a predetermined timing. Thus, the controller12periodically acquires the measurement value of the current I and the measurement value of the voltage Vcat the predetermined timing. Accordingly, the change with time (time history) of the current I and the change with time (time history) of the voltage Vcare acquired by the controller12. Furthermore, the controller12controls driving of the driving circuit8, thereby controlling charging and discharging of the battery module2. As a result, in each of the charging and discharging of the battery module2, the current flowing through the battery module2is controlled.

The controller12also includes a current load determination unit13and a charge and discharge control unit15. The current load determination unit13and the charge and discharge control unit15execute some of the processing executed by the processor or the like of the controller12. The current load determination unit13performs determination about a current load of each of the cell blocks B1to Bn. The determination about the current load is periodically performed at a predetermined timing. The charge and discharge control unit15controls driving of the driving circuit8and controls charging and discharging of the battery module2based on the determination result in the current load determination unit13.

FIG.2shows a circuit model of the battery module2in which n cell blocks B1to Bnare connected in parallel to one another. In the model shown inFIG.2, it is assumed that the voltage of the entire battery module2is Vc, and the current flowing through the battery module2is I. Furthermore, a charge amount Qkof a cell block Bk(k is any one of 1 to n), an open circuit voltage Vk(Q) of the cell block Bkwhere the charge amount Qkis a variable, an internal resistance Rkincluding the wiring of the cell block Bk, and a current ikflowing through the cell block Bkare defined. In the model shown inFIG.2, the following formulas (1) and (2) are satisfied. The charge amount Q is represented relative to a state of charge (SOC) 0% as a reference (zero). The unit of the charge amount Q is, for example, (mA·h), (A·h), or the like.

In formula (1), dt represents a minute time. When formula (1) and formula (2) are arranged using a primary approximation represented by the following formula (3), the following formulas (4) and (5) are satisfied.

Thus, currents i1to inof the cell blocks B1to Bncan be calculated by using internal resistances R1to Rn, open circuit voltages V1(Q1) to Vn(Qn), and primary differential values V1′(Q) to Vn′(Qn) at the charge amount Q of the open circuit voltages V1(Q1) to Vn(Qn). Furthermore, in each of the cell blocks B1to Bn, namely, in the cell block Bk, the current load Pkis defined by the following formula (6).

The parameter Fkmay be either of a capacity (cell block capacity) such as a charge capacity (full charge capacity) or a discharge capacity of the cell block Bk, and a positive electrode capacity or a negative electrode capacity of the cell block Bk; that is, the parameter representing the internal state of the cell block Bkis used. The charge capacity (full charge capacity) is a charge amount of the cell block Bkfrom the state of the SOC 0% to the state of the SOC 100%. The discharge capacity is a discharge amount of the cell block Bkfrom the state of the SOC 100% to the state of the SOC 0%. In the cell block Bk, the state in which the voltage across a positive electrode terminal and a negative electrode terminal is Vα1is defined as the state of the SOC 0%, and the state in which the voltage across the positive electrode terminal and the negative electrode terminal is Vα2greater than Vα1is defined as the state of the SOC 100%.

The positive electrode capacity is the charge amount of the cell block Bkwhen the charge amount of the positive electrode is increased from an initial charge amount to an upper limit charge amount. The charge amount of the positive electrode in a state in which the positive electrode potential is Vβ1is defined as the initial charge amount. The charge amount of the positive electrode in a state in which the positive electrode potential is Vβ2, which is higher than Vβ1, is defined as the upper limit charge amount. The negative electrode capacity is the charge amount of the cell block Bkwhen the charge amount of the negative electrode is increased from an initial charge amount to an upper limit charge amount. The charge amount of the negative electrode in a state in which the negative electrode potential is Vγ1is defined as the initial charge amount. The charge amount of the negative electrode in a state in which the negative electrode potential is Vγ2, which is lower than Vγ1, is defined as the upper limit charge amount.

In formula (6), when the charge capacity (full charge capacity) of the cell block Bkis used as the parameter Fk, the current load Pksubstantially corresponds to a charge rate of the cell block Bkand becomes a value corresponding to the charge capacity (full charge capacity). If the aforementioned discharge capacity is used instead of the charge capacity as the parameter Fk, the current load Pksubstantially corresponds to a discharge rate of the cell block Bkand becomes a value corresponding to the discharge capacity.

In the following, explanations will be given for a case in which the battery module2includes two cell blocks B1and B2, namely, n=2. In the model of the cell blocks B1and B2, the following formula (7) is satisfied from a relationship similar to formula (1).
i1R1+V1(Q1+i1dt)=i2R2+V2(Q2+i2dt)  (7)

When formula (7) is arranged using the primary approximation represented by formula (3), the following formula (8) is satisfied.
i1(R1+V1′(Q1)dt)−i2(R2+V2′(Q2)dt)=V2(Q2)−V1(Q1)  (8)

It is assumed that dt is a minute time. Accordingly, V1′(Q1)dt is approximated to a value that is negligible relative to R1and V2′ (Q2)dt is approximated to a value that is negligible relative to R2. Therefore, the following formula (10) is satisfied.
i1(R1+R2)=V2(Q2)−V1(Q1)+IR2(10)

When i1=I−i2is substituted into formula (8) in the same manner as in the case where i2=I−i1is substituted into formula (8), the following formula (11) is satisfied.
i2(R1+R2)=−V2(Q2)+V1(Q1)+IR1(11)

By subtracting formula (11) from formula (10), a difference between the current i1flowing through the cell block B1and the current i2flowing through the cell block B2is calculated as expressed by formula (12).

The value of V2(Q2)−V1(Q1) in the numerator of formula (12) corresponds to a difference between the open circuit voltage of the cell block B1and the open circuit voltage of the cell block B2. It is assumed that the cell blocks B1and B2are cell blocks (batteries) of the same type. It is also assumed that even if the capacities of the cell blocks B1and B2differ from each other due to deterioration, the open circuit voltage characteristics (the relation of the open circuit voltage to the charge amount or the SOC) do not substantially vary between the cell blocks B1and B2. In this case, when the full charge capacity (charge capacity) FCC1of the cell block B1and the full charge capacity (charge capacity) FCC2of the cell block B2, and the open circuit voltage characteristic V of the cell blocks B1and B2represented as a function, are defined, formula (13) is satisfied. The open circuit voltage characteristic V is open circuit voltage characteristics of the cell blocks B1and B2, which are assumed not to substantially vary between the cell blocks B1and B2.

When formula (13) is substituted into formula (12), the following formula (14) is satisfied.

When the following formula (15) is assumed and formula (15) is substituted into formula (14), the following formula (16) is satisfied.

If the current I and the internal resistances R1and R2do not substantially vary, the numerator of formula (16) changes in accordance with the magnitude of the inclination of the open circuit voltage characteristic V, and changes in accordance with the magnitude of the inclination of the voltage relative to the charge amount in each of the cell blocks B1and B2. Furthermore, the numerator of formula (16) becomes greater as the inclination of the open circuit voltage characteristic V becomes greater.

If the charge current or the discharge current flowing through the battery module2is fixed and the inclination of the open circuit voltage characteristic V is fixed, the difference (i1−i2) between the currents i1and i2does not vary. Therefore, in each of the cell blocks B1and B2, a current corresponding to the capacity, such as the full charge capacity (charge capacity), flows. On the other hand, if the inclination of the open circuit voltage characteristic V varies considerably, the difference (i1−i2) between the currents i1and i2varies considerably. In other words, in a range in which the inclination of the voltage relative to the charge amount in the open circuit voltage characteristic V in each of the cell blocks B1and B2is large, the current flowing through each of the cell blocks B1and B2may vary considerably. Therefore, a large current may flow in one of the cell blocks B1and B2, and the current load of one of the cell blocks B1and B2may increase.

In a state where no current flows through the battery module2, the voltage characteristic of the battery module2(the relation of the voltage to the charge amount or the SOC) is assumed to be the same as the open circuit voltage characteristic (the relation of the open circuit voltage to the charge amount or the SOC) of each of the cell blocks B1to Bn. As described above, in the range in which the inclination of the voltage relative to the charge amount in the open circuit voltage characteristic V of each of the cell blocks B1to Bnvaries considerably, the current flowing through each of the cell blocks B1to Bnmay vary considerably. Therefore, in the range in which the inclination of the voltage relative to the charge amount in the open circuit voltage characteristic V of the battery module2varies considerably, the current flowing through each of the cell blocks B1to Bnmay vary considerably. That is, in a range in which a second derivative value at the charge amount of the open circuit voltage of the battery module2is large, the current flowing through each of the cell blocks B1to Bnmay vary considerably.

With a model of the battery module2including the two cell blocks B1and B2that are different from each other in capacity and the internal resistance, calculation was actually performed. In the model used in the calculation, the capacity, such as the charge capacity, is smaller and the internal resistance is higher in the cell block B1than in the cell block B2. Thus, the degree of deterioration in the cell block B1is higher than in the cell block B2. As a result, the relation of the open circuit voltage V1relative to the SOC (open circuit voltage characteristic) in the cell block B1is set as indicated by the solid line inFIG.3A. The relation of the open circuit voltage V2relative to the SOC (open circuit voltage characteristic) in the cell block B2is set as indicated by the broken line inFIG.3A. Furthermore, by adjusting the current I flowing through the battery module2, the relation of the voltage Vcrelative to the SOC (voltage characteristic) in the battery module2is set as indicated by the dot chain line inFIG.3A. InFIG.3A, the abscissa line represents the SOC and the ordinate line represents the voltage.

In the calculation, if the open circuit voltages V1and V2and the voltage Vcwere set as described above, the current i1flowing through the cell block B1and the current i2flowing through the cell block B2were calculated. In addition, the current load P1of the cell block B1and the current load P2of the cell block B2were calculated. Then, the relationship between the SOC and each of the currents i1and i2were calculated as shown inFIG.3B, and the relationship between the SOC and each of the current loads P1and P2was calculated as shown inFIG.3C. As the parameter Fkfor use in calculation of the current load Pk(k is either 1 or 2), the charge capacity (the charge capacity of the SOC 0% to 100%) was used. InFIG.3B, the abscissa axis represents the SOC and the ordinate axis represents the current. InFIG.3B, a change in the current i1relative to the SOC is indicated by the solid line, and a change in the current i2relative to the SOC is indicated by the broken line. InFIG.3C, the abscissa axis represents the SOC and the ordinate axis represents the current load. InFIG.3C, a change in the current load P1relative to the SOC is indicated by the solid line, and a change in the current load P2relative to the SOC is indicated by the broken line.

As shown inFIG.3AtoFIG.3C, if the SOC was at or around 70% and the SOC was at 90% or higher as a result of the calculation, the difference between the open circuit voltages V1and V2was large. If the SOC was either of at or around 70% and at 90% or higher, namely, if the SOC was within a predetermined range in which the difference between the open circuit voltages V1and V1was large, the currents i1and i2varied considerably. Therefore, if the SOC was within the predetermined range mentioned above, the current i1of the cell block B1having a smaller capacity and higher degree of deterioration became excessively large. On the other hand, if the SOC was out of the predetermined range mentioned above, namely, in most parts other than the predetermined range between the SOC 0% and the SOC 100%, the current i1of the cell block B1having a smaller capacity was smaller than the current i2of the block B2.

If the SOC was out of the predetermined range mentioned above, namely, in most parts other than the predetermined range between the SOC 0% and the SOC 100%, the current load P1of the cell block B1was smaller than the current load P2of the cell block B2, or there was substantially no difference between the current loads P1and P2. On the other hand, if the SOC was either of at or around 70% and at 90% or higher, namely, if the SOC was within the predetermined range mentioned above, the current load P1of the cell block B1having a high degree of deterioration became excessively large, and variations of the current loads P1and P2become excessively large.

In this embodiment, the controller12controls charging and discharging of the battery module2based on the relationship of the current loads P1to Pnof the cell blocks B1to Bnrelative to the SOC of the battery module2. Then, the processor of the controller12acquires information indicative of the relationship of the current loads P1to Pnof the cell blocks B1to Bnrelative to the SOC from the storage medium of the controller12, or from a server connected to the controller12through a network. The information indicative of the relationship of the current loads P1to Pnof the cell blocks B1to Bnrelative to the SOC of the battery module2includes a range of the SOC of the battery module2in which the current load (any of P1to Pn) is liable to be high in a cell block (any of B1to Bn) having a high degree of deterioration, namely, a range of the SOC of the battery module2in which the current loads P1to Pnof the cell blocks B1to Bnare liable to vary widely.

The controller12acquires the range of the SOC of the battery module2in which the current loads P1to Pnof the cell blocks B1to Bnare liable to vary widely as the predetermined range of the SOC of the battery module2. Then, in each of the charge and the discharge of the battery module2, if the SOC of the battery module2in real time is within the predetermined range mentioned above, the controller12suppresses the current I flowing through the battery module2. Since the predetermined range of the SOC of the battery module2is the range of the SOC of the battery module2in which the current loads P1to Pnof the cell blocks B1to Bnare liable to vary widely, it corresponds to a range in which the inclination of the voltage relative to the charge amount in the open circuit voltage characteristic V of each of the cell blocks B1to Bnchanges considerably. In other words, the predetermined range of the SOC of the battery module2corresponds to a range in which the second derivative value at the charge amount of the open circuit voltage in the open circuit voltage characteristic V of each of the cell blocks B1to Bnis large. Therefore, the predetermined range of the SOC of the battery module2is set on the basis of the magnitude of a change in the inclination of the voltage relative to the charge amount in each of the cell blocks B1to Bn.

FIG.4shows processing performed by the controller12(the current load determination unit13and the charge and discharge control unit15) in the charge and discharge control of the battery module2. The processing shown inFIG.4is periodically performed at predetermined timings in each of the charge and the discharge of the assembled battery2. As shown inFIG.4, in each of the charge and the discharge of the battery module2, the current load determination unit13estimates and calculates a real time SOC of the battery module2(S101). As a result, the SOC of the battery module2is acquired as a parameter relating to the current loads P1to Pnof the cell blocks B1to Bn. The current load determination unit13calculates the SOC of the battery module2using measurement results of the current I and the voltage Vc. The method of calculating the SOC of the battery module2may be a current integration method, a calculation method using the relationship between the voltage Vcand the SOC of the battery module2, an estimation method using a Kalman filter, etc.

The current load determination unit13determines whether the calculated SOC of the battery module2is within the predetermined range of the SOC (S102). As described above, the predetermined range of the SOC corresponds to the range in which the inclination of the voltage relative to the charge amount in the open circuit voltage characteristic of the battery module2changes considerably. If the SOC of the battery module2is within the predetermined range, the current loads P1to Pnof the cell blocks B1to Bnare liable to vary widely.

In this embodiment, if the SOC of the battery module2is within the predetermined range, the current load determination unit13determines that the current loads P1to Pnof the cell blocks B1to Bnvary widely, namely, determines that the current loads P1to Pnvary beyond a permissible range. On the other hand, if the SOC of the battery module2is out of the predetermined range, the current load determination unit13determines that variations of the current loads P1to Pnof the cell blocks B1to Bnare within the permissible range. In one example, if the SOC is either of at or around 70% and at 90% or higher, it is determined that the SOC of the battery module2is within the predetermined range.

If the SOC of the battery module2is within the predetermined range (S102—Yes), the charge and discharge control unit15suppresses the current I flowing through the battery module2(S103). The charge and discharge control unit15charges or discharges the battery module2under conditions in which the current I is suppressed (S104). On the other hand, if the SOC of the battery module2is out of the predetermined range (S102—No), the charge and discharge control unit15charges or discharges the battery module2without suppressing the current I (S104). Thus, based on the fact that the SOC of the battery module2is within the predetermined range, the charge and discharge control unit15suppresses the current I flowing through the battery module2as compared to the case in which the SOC of the battery module2is out of the predetermined range.

In this embodiment, the processing as described above is performed. Therefore, if the SOC of the battery module2enters the range in which the current load (any of P1to Pn) is liable to be high in the cell block (any of B1to Bn) having a high degree of deterioration, the current I flowing through the battery module2is suppressed. In other words, if the SOC of the battery module2enters the range in which the current loads P1to Pnof the cell blocks B1to Bnare liable to vary widely, the current I flowing through the battery module2is suppressed. Therefore, even if the SOC of the battery module2is within the predetermined range mentioned above, the current loads P1to Pnare prevented from excessively varying between the cell blocks B1to Bn. In addition, even if the cell blocks B1to Bnvary in performance such as in the degree of deterioration, the current load (any of P1to Pn) of the cell block (any of B1to Bn) having a high degree of deterioration cannot be excessively high. Therefore, the increase in variations of deterioration between the cell blocks B1to Bnis suppressed.

Second Embodiment

FIG.5shows a charge and discharge system1according to the second embodiment. In the following, explanations of elements similar to those of the first embodiment will be omitted. As shown inFIG.5, in the present embodiment, the battery module2includes a plurality of current measurement units (current measurement circuits) X1to Xn. The current measurement units X1to Xnare electrically parallel to one another. A current measurement unit Xk(k is any one of 1 to n) is electrically connected to a cell block Bkin series, and detects and measures a current ikflowing through the cell block Bk. The controller12periodically acquires the measurement value of the currents i1to inat predetermined timings. Accordingly, the change with time (time history) of each of the currents i1to inis acquired by the controller12.

In the present embodiment, the controller12integrates the current ikflowing through the cell block Bk, so that it can estimate the SOC of the cell block Bkand can also calculate a charge amount of the cell block Bkfrom the state of the SOC 0%. Thus, the controller12can estimate the SOC and the charge amount of each of the cell blocks B1to Bn.

Furthermore, the current load determination unit13of the controller12estimates a parameter representing the internal state of the cell block Bkbased on a measurement value and a change with time of the current ik, an estimation value of the charge amount of the cell block Bk, and a measurement value and a change with time of the voltage Vcof the battery module2. At this time, as the parameter representing the internal state of the cell block Bk, either of a capacity (cell block capacity), such as charge capacity (full charge capacity) or a discharge capacity of the cell block Bk, and a positive electrode capacity or a negative electrode capacity of the cell block Bkis estimated. In one example, in the same manner as described in Reference Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2012-251806), the parameter representing the internal state of the cell block Bkis estimated. Accordingly, in the present embodiment, the parameter representing the internal state of each of the cell blocks B1to Bkis estimated by the controller12.

Furthermore, in the present embodiment, since the parameter representing the internal state of each of the cell blocks B1to Bkis estimated as described above, the controller12can estimate a degree of deterioration of each of the cell blocks B1to Bkbased on the estimated parameter. In one example, the current load determination unit13of the controller12determines that the degree of deterioration of the cell blocks B1to Bkbecomes higher as the estimated charge capacity (full charge capacity) becomes smaller. Even by using the positive electrode capacity and the negative electrode capacity instead of the capacity such as the charge capacity, the degree of deterioration can be determined by the controller12in the same manner.

In the present embodiment, the current load determination unit13calculates a current load Pkof the cell block Bk. At this time, the measurement value of the current ikis used and the parameter representing the internal state of the cell block Bkis used as the parameter Fk. Then, the current load Pkis calculated as formula (6) described above. Thus, in the present embodiment, the current load determination unit13calculates the current loads P1to Pnof the cell blocks B1to Bn. In each of the charge and the discharge of the battery module2, the charge and discharge control unit15of the controller12controls the current I flowing through the battery module2and controls the current flowing through each of the cell blocks B1to Bnbased on the calculated current loads P1to Pn. Thus, the currents i1to inare controlled based on the calculated current loads P1to Pn.

FIG.6shows processing performed in charge and discharge control of the battery module2by the controller12(the current load determination unit13and the charge and discharge control unit15) according to the present embodiment. In this embodiment, as well as the first embodiment, the current load determination unit13performs the processing of S101and S102. However, in this embodiment, the current load determination unit13calculates the current loads P1to Pnof the cell blocks B1to Bnfrom the measurement values of the currents i1to inin the manner described above. If the SOC of the battery module2is within the predetermined range (S102—Yes), the current determination unit13determines whether there is a cell block in which the current load Pkis equal to or greater than a threshold Pth (S105).

If there is a cell block in which the current load Pkis equal to or greater than a threshold Pth, namely, if any one of the current loads P1to Pnof the cell blocks B1to Bnis equal to or greater than the threshold Pth (S105—Yes), the charge and discharge control unit15suppresses the current I flowing through the battery module2(S103). The charge and discharge control unit15charges or discharges the battery module2under conditions in which the current I is suppressed (S104). On the other hand, if all of the current loads P1to Pnof the cell blocks B1to Bnare smaller than the threshold Pth (S105—No), the charge and discharge control unit15charges or discharges the battery module2without suppressing the current I flowing through the battery module2(S104). The threshold value Pth is, for example, an upper limit of the permissible range of the current load, and stored in a storage medium of the controller12, or a storage medium of a server connected to the controller12through a network.

As described above, according to the present embodiment, based on the fact that the SOC of the battery module2is within the predetermined range and that the current load of some of the cell blocks B1to Bnis equal to or greater than the threshold value Pth, the current I flowing through the battery module2is suppressed. Thus, the current flowing through each of the cell blocks B1to Bnis controlled based on the calculated current loads P1to Pn. Furthermore, according to the present embodiment, based on the fact that the current load is equal to or greater than the threshold value Pth in some of the cell blocks B1to Bn, the current flowing through the battery module2is suppressed as compared to the case in which the current load is smaller than the threshold value Pth in all of the cell blocks B1to Bn. Thus, the current loads P1to Pnof the cell blocks B1to Bnare calculated more appropriately and the current I is controlled more appropriately based on the current loads P1to Pn.

(Modifications of Second Embodiment)

In one modification of the second embodiment, the processing of S101and S102is not performed, and determination based on the SOC of the battery module2is not performed. However, in this modification, the determination of S105based on the current loads P1to Pnof the cell blocks B1to Bnis performed by the current load determination unit13in the same manner as in the second embodiment. Also in this modification, if any one of the current loads P1to Pnof the cell blocks B1to Bnis equal to or greater than the threshold Pth (S105—Yes), the charge and discharge control unit15suppresses the current I flowing through the battery module2(S103). The charge and discharge control unit15charges or discharges the battery module2under conditions in which the current I is suppressed (S104). On the other hand, if all of the current loads P1to Pnof the cell blocks B1to Bnare smaller than the threshold Pth (S105—No), the charge and discharge control unit15charges or discharges the battery module2without suppressing the current I (S104).

In another modification of the second embodiment, the following processing may be performed instead of comparing each of the current loads P1to Pnwith the threshold value Pth in S105. In this modification, the current load determination unit13of the controller12determines a degree of deterioration of each of the cell blocks B1to Bnbased on either the full charge capacity or the positive electrode capacity and the negative electrode capacity. Here, a cell block Bεhaving the highest degree of deterioration of all cell blocks B1to Bnis defined. In this modification, instead of the determination of S105, the current load determination section13compares the current load Pεof the cell block Bεwith the current load of each of the cell blocks other than the cell block Bε.

If the current load Pεof the cell block Bεis equal to or greater than the current load of any of the cell blocks other than the cell block Bε, the charge and discharge control unit15suppresses the current I flowing through the battery module2. The charge and discharge control unit15charges or discharges the battery module2under conditions in which the current I is suppressed. On the other hand, if the current load Pεof the cell block Bεis smaller than all of the current loads of the cell blocks other than the cell block Bε, the charge and discharge control unit15charges or discharges the battery module2without suppressing the current I flowing through the battery module2.

It is assumed that the battery module2includes two cell blocks B1and B2(n=2), and the degree of deterioration of the cell block B1is higher than that of the cell block B2. In this case, according to the present modification, the current load determination unit13compared the current loads P1and P2. If the current load P1is equal to or greater than the current load P2, the charge and discharge control unit15suppresses the current I flowing through the battery module2. The charge and discharge control unit15charges or discharges the battery module2under conditions in which the current I is suppressed. On the other hand, if the current load P1is smaller than the current load P2, the charge and discharge control unit15charges or discharges the battery module2without suppressing the current I flowing through the battery module2.

Also in this modification, the current flowing through each of the cell blocks B1to Bnis controlled based on the calculated current loads P1to Pnin the same manner as in the second embodiment etc. Therefore, the present modification produces the same effects and advantages as those of the second embodiment etc.

Third Embodiment

FIG.7shows a charge and discharge system1according to the third embodiment. In the following, explanations of elements similar to those of the second embodiment will be omitted. Also in this embodiment, current measurement units (current measurement circuits) X1to Xnare provided. The controller12acquires measurement values of currents i1to inand a change with time (time history) of each of the currents i1to in. Then, the current load determination unit13calculates the current loads P1to Pnof the cell blocks B1to Bnin the same manner as in the second embodiment.

In this embodiment, variable resistors Y1to Ynare provided. The variable resistors Y1to Ynare electrically parallel to one another. The variable resistors Yk(k is any one of 1 to n) are electrically connected to the cell block Bkin series. Thus, each of the variable resistors Y1to Ynis connected in series to the corresponding one of the cell blocks B1to Bn. In this embodiment, in the same manner as in the second embodiment, the charge and discharge control unit15of the controller12controls driving of the driving circuit8, thereby controlling the current I flowing through the battery module2. Furthermore, in this embodiment, the charge and discharge control unit15is configured to adjust resistance values r1to rnof the variable resistors Y1to Yn. The charge and discharge control unit15controls currents i1to inby adjusting the resistance values r1to rn.

FIG.8shows processing performed in charge and discharge control of the battery module2by the controller12(the current load determination unit13and the charge and discharge control unit15) of the present embodiment. Also in this embodiment, in the same manner as in the second embodiment, the current load determination unit13performs the processing of S101, S102, and S105. If some of the current loads P1to Pnof the cell blocks B1to Bnis equal to or greater than the threshold value Pth (S105—Yes), the charge and discharge control unit15suppresses the current I flowing through the battery module2(S103).

If some of the current loads P1to Pnof the cell blocks B1to Bnis equal to or greater than the threshold value Pth (S105—Yes), the charge and discharge control unit15adjusts the resistance values r1to rnof the variable resistors Y1to Ynbased on the current loads P1to Pnof the cell blocks B1to Bn(S106). Then, the charge and discharge control unit15charges and discharges the battery module2under conditions in which the current I is suppressed and the resistance values r1to rnare adjusted (S104). On the other hand, if all of the current loads P1to Pnof the cell blocks B1to Bnare smaller than the threshold value Pth (S105—No), the charge and discharge control unit15charges or discharges the battery module2without either suppressing the current I flowing through the battery module2or adjusting the resistance values r1to rn(S104).

In one example, the controller12adjusts the resistance values r1to rnof the variable resistors Y1to Ynin accordance with the magnitudes of the calculated current loads P1to Pn. In this case, a variable resistor connected in series to a cell block having a large current load is set to a high resistance value, whereas a variable resistor connected in series to a cell block having a small current load is set to a low resistance value. As a result, an excessively large current is prevented from flowing through the cell block having a large current load. Thus, the resistance values r1to rnare adjusted such that the variations of the current loads P1to Pnare reduced.

In another example, the controller12calculates internal resistances R1to Rnof the cell blocks B1to Bnbased on the currents i1to in. The internal resistance Rkof the cell block Bkis expressed as formula (17) using the current ik. The charge and discharge control unit15performs a control so that the sum of the internal resistance Rkand the resistance value rkof the variable resistor Ykis equal in all cell blocks. In other words, the resistance values r1to rnare adjusted to satisfy formula (18).

By adjusting the resistance values r1to rnas described above, each of the currents i1to inis controlled such that the variations of the currents i1to inare reduced, namely, the currents i1to inare the same or substantially the same as one another. Thus, the resistance values r1to rnare adjusted such that the variations of the current loads P1and Pnare reduced. If the resistance values r1to rnare adjusted to satisfy formula (18), it is preferable that the resistance values r1to rnbe adjusted such that the sum of the resistance values r1to rnof the variable resistors Y1to Ynare as small as possible. The method of calculating the internal resistance Rkmay be an estimation method using a Kalman filter, a calculation using a sequential least squares method, a calculation using Fourier transform, etc., in addition to the method using formula (17).

The present embodiment produces the same effects and advantages as those of the second embodiment etc. Furthermore, according to the present embodiment, it is not only the current I flowing through the battery module2that is adjustable, but also the currents i1to inare adjustable by adjusting the resistance values r1to rnof the variable resistors Y1to Yn.

(Modifications of Third Embodiment)

Also in the case of providing the variable resistors Y1to Ynas in the third embodiment, the processing by the controller12may be appropriately changed as in the modifications of the second embodiment described above.

In another modification, in the configuration in which the variable resistors Y1to Ynare provided as in the third embodiment, the processing of suppressing the current I in S103may not be performed. In this modification, if some of the current loads P1to Pnof the cell blocks B1to Bnis equal to or greater than the threshold value Pth (S105—Yes), the charge and discharge control unit15only adjusts the resistance values r1to rnof the variable resistors Y1to Ynin S106. In this modification also, the resistance values r1to rnare adjusted in the same manner as in the third embodiment. Thus, the resistance values r1to rnare adjusted such that the variations of the current loads P1to Pnare reduced.

In at least one of the embodiments or examples described above, the current flowing through each of the cell blocks is controlled based on at least one of the current loads or a parameter relating to the current loads. Accordingly, in the battery module in which cell blocks are connected in parallel, the current loads are prevented from being excessively greatly varied between the cell blocks.