WO2012146963A2

WO2012146963A2 - Battery system

Info

Publication number: WO2012146963A2
Application number: PCT/IB2012/000739
Authority: WO
Inventors: Masaru Takagi; Yuji Nishi; Teruo Ishishita
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2011-04-28
Filing date: 2012-04-13
Publication date: 2012-11-01
Also published as: JP2012234700A; WO2012146963A3

Abstract

A battery system including: a first battery (10) that performs charging and discharging; and a second battery (20) that is connected in parallel to the first battery (10) and that performs charging and discharging, wherein: the first battery (10) is configured to perform charging and discharging with a higher current than the second battery (20); the second battery (20) has a higher storage capacity than the first battery (10); and a second lower limit that is the lower limit of open circuit voltage for control of the charging and discharging of the second battery (20) is higher than a reference value of open circuit voltage for control of the charging and discharging of the first battery (10).

Description

BATTERY SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a battery system including a high-power and a high-capacity battery electrically connected in parallel to each other.

2. Description of Related Art

[0002] Japanese Patent Application Publication No. 2008-041620 (JP-2008-041620 A) describes a system in which a high-power battery and a high-capacity battery are electrically connected in parallel to each other. The high-power battery can be charged and discharged with a higher current than the high-capacity battery, while the high-capacity battery has a higher storage capacity than the high-power battery.

[0003] In such a system having a high-power battery and a high-capacity battery electrically connected to each other in parallel, current (circulating current) sometimes flows from the high-power battery to the high-capacity battery.

SUMMARY OF THE INVENTION

[0004] A first aspect of the invention relates a battery system including a first battery that performs charging and discharging, and a second battery that is connected in parallel to the first battery and that performs charging and discharging. The first battery is configured to perform charging and discharging with a higher current than the second battery, while the second battery has a higher storage capacity than the first battery. A second lower limit that is the lower limit of open circuit voltage (OCV) for control of the charging and discharging of the second battery is equal to or higher than a reference value of open circuit voltage for control of the charging and discharging of the first battery. The reference value may be a value between the upper limit and the lower limit of open circuit voltage for control of the charging and discharging of the first battery. [0005] The second lower limit may be equal to or higher than a first upper limit that is the upper limit of open circuit voltage for control of the charging and discharging of the first battery.

[0006] The configuration described above makes it possible to prevent the open circuit voltage of the second battery from falling below the open circuit voltage of the first battery, and to prevent current (circulating current) from flowing from the first battery to the second battery regardless of a charging state of the second battery.

[0007] A second SOC upper limit that is the upper limit of state-of-charge (SOC) used to control the charging and discharging of the second battery may be higher than a first SOC upper limit that is the upper limit of a SOC used to control the charging and discharging of the first battery. A second SOC lower limit that is the lower limit of a SOC used to control the charging and discharging of the second battery may be lower than a first SOC lower limit that is the lower limit of a SOC used to control the charging and discharging of the first battery. This configuration makes it possible to enlarge the available range of SOC in the second battery to be greater than the available range of SOC in the first battery, and thus enables effective use of the second battery.

[0008] The battery system may further include a booster circuit that boosts an output voltage of the first battery. When the output voltage of the first battery is boosted such that the boosted voltage becomes equal to the voltage of the second battery, the flow of circulating current can be prevented between the first battery and the second battery.

[0009] The battery system may further include a first relay that switches between a state of allowing and a state of prohibiting the charging and discharging of the first battery; a second relay that switches between a state of allowing and a state of prohibiting the charging and discharging of the second battery; and a controller that controls the first relay and the second relay. In this case, the controller can control the first relay so as to allow discharging of the first battery when a first SOC that is a SOC in the first battery is higher than the reference value, and to allow charging of the first battery when the first SOC is lower than the reference value.

[0010] The first battery and the second battery may output energy used for driving a vehicle.

[0011] The first battery may include a plurality of first cells electrically connected in series, and the second battery can include a plurality of second cells electrically connected in series. In this case, the open circuit voltage of the first battery may be a product of the number of the plurality of first cells and the open circuit voltage of each of the first cells, and an open circuit voltage of the second battery may be a product of the number of the plurality of second cells and the open circuit voltage of each of the second cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a circuit diagram illustrating a configuration of a battery system according to a first embodiment;

FIG. 2 is a diagram for explaining an available range of SOC in a high-output battery pack and in a high-capacity battery pack;

FIG. 3 is a diagram illustrating total voltage in a high-output battery pack and in a high-capacity battery pack;

FIG. 4 is a circuit diagram illustrating a configuration of a battery system according to a second embodiment;

FIG. 5 is a flowchart illustrating discharge control in the battery system according to the second embodiment; and

FIG. 6 is a circuit diagram illustrating a configuration of a battery system according to a modification of the second embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0013] Exemplary embodiments of the invention will be described.

[0014] A battery system according a first embodiment of the invention will be _Λ

4 described. FIG. 1 is a circuit diagram illustrating a configuration of the battery system of the first embodiment. The battery system of the first embodiment may be mounted on a vehicle.

[0015] The battery system according to the first embodiment includes a high-output battery pack (serving as a first battery) 10 and a high-capacity battery pack (serving as a second battery) 20. The high-output battery pack 10 and the high-capacity battery pack 20 are electrically connected in parallel to each other. The high-output battery pack 10 is a battery pack configured to perform charging and discharging with a higher current than the high-capacity battery pack 20. The high-capacity battery pack 20 is a battery pack having a higher storage capacity than the high-output battery pack 10.

[0016] The high-output battery pack 10 includes a plurality of first cells 11 electrically connected in series. The high-capacity battery pack 20 includes a plurality of second cells 21 electrically connected in series. The first and second cells 11 and 21 may be secondary batteries such as nickel-metal hydride batteries or lithium ion batteries. The number of the first cells 11 and the number of second cells 21 may be determined as necessary. The high-output battery pack 10 can be formed of a single first cell 11 and the high-capacity battery pack 20 may be formed of a single second cell 21. The high-output battery pack 10 may include first cells 11 electrically connected in parallel. Likewise, the high-capacity battery pack 20 may include second cells 21 electrically connected in parallel.

[0017] Lithium ion batteries may be used as the first and second cells 11 and 21. In this case, for example, hard carbon (non-graphitizable carbon material) may be used as an anode active material for the first cells 11, while lithium-manganese composite oxide may be used as a cathode active material for the first cells 11. Alternatively, graphite may be used as an anode active material for the second cells 21 , while lithium-nickel composite oxide can be used as a cathode active material for the second cells 21.

[0018] The first cells 11 and the second cells 21 have a relationship as shown in Table 1 below.

[Table 1] Cell Characteristics Electrode C laracteristics

Output Capacity Output Capacity [W/kg] [Wh/kg] [mA/cm²] [mAh/g] [W/L] [Wh/L] [mAh/cc]

First cells 11 High Low High Low (high-output battery pack)

Second cells 21 Low High Low High (high-capacity battery pack)

[0019] The output of the first and second cells 11 and 21 may be represented, for example, by electric power per unit mass of the first and second cells 11 and 21 ([W/kg]), or by electric power per unit volume of the first and second cells 11 and 21 ([W/L]). As shown in Table 1, the output of the first cell 11 is higher than the output of the second cell 21. This means that, when the first and second cells 11 and 21 have the same mass or volume, the output (electric power) [W] of the first cell 11 is higher than the output (electric power) [W] of the second cell 21.

[0020] The capacity of the first and second cells 11 and 21 can be represented, for example, by capacity per unit mass of the first and second cells 11 and 21 ([Wh/kg]), or by capacity per unit volume of the first and second cells 11 and 21 ([Wh/L]). As shown in Table 1 , the capacity of the second cell 21 is higher than the capacity of the first cell 11. This means that when the first and second cells 11 and 21 have the same mass or volume, the capacity [Wh] of the second cell 21 is higher than the capacity [Wh] of the first cell 11.

[0021] Output of the electrodes of the first and second cells 11 and 21 may be represented, for example, by a current value per unit area of the electrodes ([mA/cm ]). As shown in Table 1, the output of the electrode of the first cell 11 is higher than the output of the electrode of the second cell 21. This means that, when the electrodes have the same area, the output of the electrode of the first cell 11 (current value flowing through the electrode of the first cell 1 1 ) is greater than the output of the electrode of the second cell 21 (current value flowing through the electrode of the second cell 21).

[0022] Capacity of the electrodes of the first and second cells 11 and 21 can be represented, for example, by capacity per unit mass of the electrodes ([mAh/g]) or by capacity per unit volume of the electrodes ([mAh/cc]). As shown in Table 1, the capacity of the electrodes of the second cells 21 is higher than the capacity of the electrodes of the first cells 11. This means that, when the electrodes have the same mass or volume, the capacity of the electrode of the second cell 21 is higher than the capacity of the electrode of the first cell 11.

[0023] A first voltage sensor 31 detects terminal-to-terminal voltage (total voltage) of the high-output battery pack 10 and outputs a detection result to a controller 53. A second voltage sensor 32 detects terminal-to-terminal voltage (total voltage) of the high-capacity battery pack 20 and outputs a detection result to the controller 53.

[0024] A first relay 41 allows charging and discharging of the high-output battery pack 10 in its ON state, and prohibits charging and discharging of the high-output battery pack 10 in its OFF state. The first relay 41 is switched between ON state and OFF state in response to a control signal from the controller 53. The controller 53 connects the high-output battery pack 10 to an inverter 51 by switching first relay 41 from OFF state to ON state.

[0025] A second relay 42 allows charging and discharging of the high-capacity battery pack 20 in its ON state, and prohibits charging and discharging of the high-capacity battery pack 20 in its OFF state. The second relay 42 is switched between ON state and OFF state in response to a control signal from the controller 53. The controller 53 connects the high-capacity battery pack 20 to the inverter 51 by switching the second relay 42 from OFF state to ON state.

[0026] The inverter 51 converts direct current (DC) electric power from the high-output battery pack 10 and the high-capacity battery pack 20 into alternating current (AC) electric power, and outputs the converted AC electric power to a motor-generator 52. A three-phase motor may be used as the motor-generator 52. The motor-generator 52 is supplied with AC electric power from the inverter 51 to generate kinetic energy for driving a vehicle. The kinetic energy generated by the motor-generator 52 is transmitted to wheels.

[0027] When the vehicle is decelerated or halted, the motor-generator 52 converts kinetic energy generated during braking of the vehicle into electric energy. The AC electric power generated by the motor-generator 52 is converted into DC electric power by the inverter 51 and then supplied to the high-output battery pack 10 and the high-capacity battery pack 20. As a result, regenerative energy is stored in the high-output battery pack 10 and the high-capacity battery pack 20.

[0028] A charging circuit 54 is connected to the high-output battery pack 10 and the high-capacity battery pack 20. The charging circuit 54 is supplied with electric power from an external power supply to charge the high-output battery pack 10 and the high-capacity battery pack 20. A household power supply, for example, may be used as the external power supply. When using a household power supply, the charging circuit 54 converts AC electric power supplied from the household power supply into DC electric power and supplies the converted DC electric power to the high-output battery pack 10 and the high-capacity battery pack 20.

[0029] Although, in the first embodiment, the high-output battery pack 10 and the high-capacity battery pack 20 are connected to the inverter 51, the embodiment of the invention is not limited to this. Specifically, the high-output battery pack 10 and the high-capacity battery pack 20 may be connected to a booster circuit (not shown), and the booster circuit may be connected to the inverter 51. By using the booster circuit, for example, output voltage of the high-output battery pack 10 may be boosted so that the boosted voltage is supplied to the inverter 51, or output voltage of the inverter 51 may be stepped down so that the stepped down voltage is supplied to the high-output battery pack 10.

[0030] FIG. 2 is a diagram for explaining available ranges of state-of-charge (SOC) in the high-output battery pack 10 and the high-capacity battery pack 20. In the high-output battery pack 10, the charging and discharging thereof is controlled such that the SOC is contained within the range between a first SOC lower limit SOCminl and a first SOC upper limit SOCmax l . In the high-capacity battery pack 20, the charging and discharging thereof is controlled such that the SOC is contained in a range between a second SOC lower limit SOCmin2 and a second SOC upper limit SOCmax2. [0031] The second SOC lower limit SOCmin2 is a lower value than the first SOC lower limit SOCminl. The second SOC upper limit SOCmax2 is a higher value than the first SOC upper limit SOCmaxl . In the high-capacity battery pack 20, the available range of SOC for the charge and discharge control is wider than that in the high-output battery pack 10. Therefore, the vehicle is enabled to travel longer distance by using output energy of the high-capacity battery pack 20 if the high-capacity battery pack 20 is discharged from the second SOC upper limit SOCmax2 to the second SOC lower limit SOCmin2.

[0032] FIG. 3 is a diagram illustrating total voltage (open circuit voltage) (OCV) in the high-output battery pack 10 and the high-capacity battery pack 20. The total voltage of the high-output battery pack 10 in its fully charged state is lower than the total voltage of the high-capacity battery pack 20 in its fully charged state. The total voltage of the high-output battery pack 10 and the total voltage of the high-capacity battery pack 20 have relationships represented by the following equations (1) and (2).

Vltotal=VlcellxNl ...(1)

V2total=V2cellxN2 ...(2)

[0033] In the equation (1), Vltotal denotes a total voltage of the high-output battery pack 10, and VI cell denotes a voltage of each of the first cells 11 which are electrically connected in series in the high-output battery pack 10. It is assumed here that the first cells 11 which are electrically connected in series have the same voltage. Nl denotes the number of first cells 11 electrically connected in series in the high-output battery pack 10.

[0034] In the equation (2), V2total denotes a total voltage of the high-capacity battery pack 20, and V2cell denotes a voltage of each of- the second cells 21 which are electrically connected in series in the high-capacity battery pack 20. It is assumed here that the second cells 21 which are electrically connected in series have the same voltage. N2 denotes the number of second cells 21 electrically connected in series in the high-capacity battery pack 20.

[0035] As described with reference to FIG. 2, the charging and discharging of the high-output battery pack 10 is controlled within the range between the first SOC lower limit SOCminl and the first SOC upper limit SOCmaxl . Therefore, as shown in FIG. 3, the total voltage of the high-output battery pack 10 varies within a range between a first lower limit voltage Vminl and a first upper limit voltage Vmaxl. The first lower limit voltage Vminl corresponds to the first SOC lower limit SOCmin l, and the first upper limit voltage Vma l corresponds to the first SOC upper limit SOCma l.

[0036] The charging and discharging of the high-capacity battery pack 20 is controlled within the range between the second SOC lower limit SOCmin2 and the second SOC upper limit SOCmax2. Therefore, the total voltage of the high-capacity battery pack 20 varies within a range between a second lower limit voltage Vmin2 and a second upper limit voltage Vmax2. The second lower limit voltage Vmin2 corresponds to the second SOC lower limit SOCmin2, and the second upper limit voltage Vmax2 corresponds to the second SOC upper limit SOCmax2. As shown in FIG. 3, the second lower limit voltage Vmin2 of the high-capacity battery pack 20 is higher than the first upper limit voltage Vmax l of the high-output battery pack 10.

[0037] In the battery system according to the first embodiment, the high-capacity battery pack 20 is discharged from the second SOC upper limit SOCmax2 to the second SOC lower limit SOCmin2, so that the vehicle can be driven with use of this discharge energy. When high current is required in response to an accelerator operation by the driver, the output of the high-output battery pack 10 may be used in addition to the output of the high-capacity battery pack 20.

[0038] After the high-capacity battery pack 20 has been discharged to the second SOC lower limit SOCmin2, the vehicle may be driven with use of the output of the high-output battery pack 10. This makes it possible to increase the travel distance of the vehicle using the output energy of the high-capacity battery pack 20 and the output energy of the high-output battery pack 10. When the vehicle according to the first embodiment is provided with an internal combustion engine or a fuel battery, energy generated by the internal combustion engine or fuel battery may be used together with the energy output from the high-output battery pack 10 to drive the vehicle. [0039] When the high-capacity battery pack 20 is discharged until the SOC in the high-capacity battery pack 20 reaches the second SOC lower limit SOCmin2, the high-capacity battery pack 20 need be charged. When the SOC in the high-output battery pack 10 comes close to the first SOC lower limit SOCminl, the high-output battery pack 10 need be charged. For example, the high-output battery pack 10 may be charged up to an SOC reference value SOCref in the charge and discharge control.

[0040] The SOC reference value SOCref is a reference value used in the control of the charging and discharging of the high-output battery pack 10. In the charge and discharge control using the SOC reference value SOCref, discharging of the high-output battery pack 10 is allowed when the SOC in the high-output battery pack 10 becomes higher than the SOC reference value SOCref, whereas charging of the high-output battery pack 10 is allowed when the SOC in the high-output battery pack 10 becomes lower than the SOC reference value SOCref.

[0041] The high-output battery pack 10 and the high-capacity battery pack 20 are charged with use of the charging circuit 54. Although, in the first embodiment, charging current from the charging circuit 54 is supplied to the high-output battery pack 10 and high-capacity battery pack 20, the embodiment of the' invention is not limited to this. Another system instead of the charging circuit 54 may be used for supplying electric power to the high-output battery pack 10 and the high-capacity battery pack 20. For example, kinetic energy generated by the internal combustion engine may be converted into electric energy, and this electric energy may be used to charge the high-output battery pack 10 and the high-capacity battery pack 20. Further, electric energy generated by the fuel battery may be used to charge the high-output battery pack 10 and the high-capacity battery pack 20.

[0042] According to the first embodiment, as shown FIG. 3, the second lower limit voltage Vmin2 of the high-capacity battery pack 20 is higher than the first upper limit voltage Vmaxl of the high-output battery pack 10. Therefore, when the high-output battery pack 10 and the high-capacity battery pack 20 are electrically connected in parallel to each other, electric current can be prevented from flowing from the high-output battery pack 10 to the high-capacity battery pack 20. The current value during charging and discharging of the high-output battery pack 10 is higher than the current value during charging and discharging of the high-capacity battery pack 20. Accordingly, the current value of the electric current flowing from the high-output battery pack 10 to the high-capacity battery pack 20 may exceeds an allowable current value of the high-capacity battery pack 20. However, according to the first embodiment, the electric current is prevented from flowing from the high-output battery pack 10 to the high-capacity battery pack 20, and hence the flow of electric current having a current value exceeding the allowable current value of the high-capacity battery pack 20 can be prevented.

[0043] When a voltage of the high-capacity battery pack 20 is higher than a voltage of the high-output battery pack 10, electric current sometimes flows from the high-capacity battery pack 20 to the high-output battery pack 10. However, the allowable current value of the high-output battery pack 10 is higher than the current value of the electric current flowing from the high-capacity battery pack 20 to the high-output battery pack 10. Therefore, the electric current can be allowed to flow from the high-capacity battery pack 20 to the high-output battery pack 10. ¹

[0044] According to the first embodiment, after the high-capacity battery pack 20 has been discharged until the SOC in the high-capacity battery pack 20 reaches the second SOC lower limit SOCmin2, the high-capacity battery pack 20 may be charged until the SOC in the high-capacity battery pack 20 reaches the second SOC upper limit SOCmax2. This makes it possible to utilize the high-capacity battery pack 20 within the range between the second SOC upper limit SOCmax2 and the second SOC lower limit SOCmin2.

[0045] Although, in the first embodiment, the second lower limit voltage Vmin2 of the high-capacity battery pack 20 is higher than the first upper limit voltage Vmax 1 of the high-output battery pack 10, the embodiment of the invention is not limited to this. Specifically, the second lower limit voltage Vmin2 may be equal to the first upper limit voltage Vmaxl. Alternatively, the second lower limit voltage Vmin2 of the high-capacity battery pack 20 may be higher than a reference voltage corresponding to the aforementioned SOC reference value SOCref. Even in these cases, it is possible to prevent the flow of electric current from the high-output battery pack 10 to the high-capacity battery pack 20.

[0046] A battery system according a second embodiment of the invention will be described. FIG. 4 is a circuit diagram illustrating a configuration of the battery system according to the second embodiment. In the description of the second embodiment, components having the same functions as those described in the first embodiment are assigned with the same reference numerals, and detailed description thereof will be omitted. The description of the second embodiment will be mainly focused on different features from the first embodiment.

[0047] A booster circuit 60 is arranged between the high-output battery pack 10 and the high-capacity battery pack 20. The booster circuit 60 boosts an output voltage of the high-output battery pack 10 and outputs the boosted voltage to the high-capacity battery pack 20. In addition, the booster circuit 60 steps down a voltage output by the high-capacity battery pack 20 and outputs the stepped-down voltage to the high-output battery pack 10.

[0048] The booster circuit 60 includes a reactor 61, diodes 62 and 63, and transistors (npn transistors) 64 and 65 as switching elements. The reactor 61 is connected to a first relay 41 at one end, and to a connection point of the transistor 64 and 65 at the other end.

[0049] The transistors 64 and 65 are connected in series. A control signal from the controller 53 is input to the bases of the transistors 64 and 65. The diodes 62 and 63 are connected between the collectors and emitters of the transistors 64 and 65, respectively, so that current flows from the emitters to the collectors.

[0050] Insulated Gate Bipolar Transistors (IGBTs), for example, may be used as the transistor 64 and 65. Electric power switching elements such as Power Metal Oxide Semiconductor Field-Effect Transistors (MOSFETs) may be used instead of the npn transistors. [0051] A smoothing capacitor (not shown) may be connected in parallel to the high-output battery pack 10 so as to smooth voltage variation between a cathode line PLl and an anode line NL of the high-output battery pack 10. Also, a smoothing capacitor (not shown) may be connected in parallel to the high-capacity battery pack 20 so as to smooth voltage variation between a cathode line PL2 and an anode line NL of the high-capacity battery pack 20.

[0052] The booster circuit 60 boosts a DC voltage supplied through the cathode line PLl of the high-output battery pack 10, and outputs the boosted voltage to the cathode line PL2 of the high-capacity battery pack 20. Specifically, the controller 53 switches the transistor 65 to ON state, while switching the transistor 64 to OFF state. This causes current to flow from the high-output battery pack 10 to the reactor 61, whereby magnetic field energy according to an amount of the current is accumulated in the reactor 61.

[0053] The controller 53 then switches the transistor 65 from ON state to OFF state, whereby current is caused to flow from the reactor 61 to the cathode line PL2 of the high-capacity battery pack 20 via the diode 62. This causes the energy accumulated in the reactor 61 to be discharged and boosting is performed. The output voltage of the high-output battery pack 10 may be boosted such that the boosted voltage becomes equal to the output voltage of the high-capacity battery pack 20.

[0054] On the other hand, the booster circuit 60 steps down the DC voltage supplied from the inverter 51 to the voltage level of the high-output battery pack 10. Specifically, the controller 53 switches the transistor 64 to ON state, while switching the transistor 65 to OFF state. This causes electric power to be supplied from the inverter 51 to the high-output battery pack 10 to charge the high-output battery pack 10.

[0055] An example of discharge control in the battery system according to the second embodiment will be described with reference to the flowchart shown in FIG. 5. It is assumed here that the SOC in the high-capacity battery pack 20 has been charged to the second SOC upper limit SOCmax2.

[0056] In step S lOl, the controller 53 switches the second relay 42 from OFF , ,

14 state to ON state. This causes the high-capacity battery pack 20 to discharge. During this discharge, the controller 53 controls the first relay 41 to be in OFF state, whereby the charging and discharging of the high-output battery pack 10 are prohibited.

[0057] In step S 102, the controller 53 calculates the SOC in the high-capacity battery pack 20. A conventional appropriate method may be used as the method of calculating the SOC in the high-capacity battery pack 20.

[0058] For example, the SOC in the high-capacity battery pack 20 may be calculated by integrating the charging and discharging current of the high-capacity battery pack 20. In this case, a current sensor is required for detecting charging and discharging current of the high-capacity battery pack 20. Alternatively, the SOC in the high-capacity battery pack 20 may be specified by measuring an OCV of the high-capacity battery pack 20. Since OCV and SOC are in correspondence relationship, the SOC may be specified from the OCV by using a map showing the correspondence relationship obtained beforehand.

[0059] In step S 103, the controller 53 determines whether or not the SOC in the high-capacity battery pack 20 calculated in step S102 has reached the second SOC lower limit SOCmin2. When the SOC in the high-capacity battery pack 20 has not reached the second SOC lower limit SOCmin2, the controller returns to step S 101 to continue the discharge of the high-capacity battery pack 20. In contrast, when the SOC in the high-capacity battery pack 20 has reached the second SOC lower limit SOCmin2, the controller 53 proceeds to processing of step S 104.

[0060] In step S 104, the controller 53 switches the second relay 42 from ON state to OFF state, whereby discharging of the high-capacity battery pack 20 is prohibited (stopped).

[0061] In step S 105, the controller 53 switches the first relay 41 from OFF state to ON state, whereby charging and discharging of the high-output battery pack 10 is started. The charge and discharge control of the high-output battery pack 10 may be performed on the basis of the aforementioned SOC reference value SOCref, for example.

[0062] Although in the discharge control shown in FIG. 5, the charging and discharging of the high-output battery pack 10 are performed after the SOC in the high-capacity battery pack 20 has reached the second SOC lower limit SOCmin2, the embodiment of the invention is not limited to this. For example, the high-output battery pack 10 may be discharged while the high-capacity battery pack 20 is discharged.

[0063] According to the second embodiment, the booster circuit 60 can be used to boost the output voltage of the high-output battery pack 10 so that the boosted voltage becomes equal to the output voltage of the high-capacity battery pack 20. This makes it possible to prevent current from flowing from the high-output battery pack 10 to the high-capacity battery pack 20, or from the high-capacity battery pack 20 to the high-output battery pack 10.

[0064] When current (circulating current) exceeding the allowable current value of the high-output battery pack 10 may flow to the high-output battery pack 10, the voltage input to the high-output battery pack 10 can be stepped down by the booster circuit 60. This makes it possible to prevent current exceeding the allowable current value of the high-output battery pack 10 from flowing to the high-output battery pack 10.

[0065] A battery system as shown in FIG. 6 can be used as a modification example of the second embodiment. In the configuration shown in FIG. 6, a transistor 43 and a diode 44 are used in place of the second relay 42 in FIG. 4. A control signal from the controller 53 is input to the base of the transistor 43. The controller 53 switches the transistor 43 to OFF state so that the high-capacity battery pack 20 is discharged. When the controller 53 switches the transistor 43 to ON state, the high-capacity battery pack 20 is charged.

Claims

1. A battery system comprising:

a first battery (10) that performs charging and discharging; and

a second battery (20) that is connected in parallel to the first battery (10) and that performs charging and discharging, wherein:

the first battery (10) is configured to perform charging and discharging with a higher current than the second battery (20);

the second battery (20) has a higher storage capacity than the first battery (10); and a second lower limit that is the lower limit of open circuit voltage for control of the charging and discharging of the second battery (20) is equal to or higher than a reference value of open circuit voltage for control of the charging and discharging of the first battery (10).

2. The battery system according to claim 1, wherein the second lower limit is equal to or higher than a first upper limit that is the upper limit of open circuit voltage for control of the charging and discharging of the first battery (10).

3. The battery system according to claim 1 or 2, wherein:

a second SOC upper limit that is the upper limit of state-of-charge used to control the charging and discharging of the second battery (20) is higher than a first SOC upper limit that is the upper limit of state-of-charge used to control the charging and discharging of the first battery (10); and

a second SOC lower limit that is the lower limit of state-of-charge used to control the charging and discharging of the second battery is lower than a first SOC lower limit that is the lower limit of state-of-charge used to control the charging and discharging of the first battery (10).

4. The battery system according to any one of claims 1 to 3, further comprising a booster circuit (60) that boosts an output voltage of the first battery (10).

5. The battery system according to claim 4, wherein the booster circuit (60) boosts the output voltage of the first battery (10) such that the boosted output voltage of the first battery (10) becomes equal to an output voltage of the second battery (20).

6. The battery system according to any one of claims 1 to 5, further comprising:

a first relay (41) that switches between a state of allowing and a state of prohibiting charging and discharging of the first battery (10);

a second relay (20) that switches between a state of allowing and a state of prohibiting charging and discharging of the second battery (20); and

a controller (53) that controls the first relay (41) and the second relay (42).

7. The battery system according to claim 6, wherein the controller controls the first relay (41) so as to allow discharging of the first battery (10) when a first SOC that is the state-of-charge of the first battery (10) is higher than a SOC reference value, and to allow charging of the first battery (10) when the first SOC is lower than the SOC reference value.

8. The battery system according to any one of claims 1 to 7, wherein the first battery (10) and the second battery (20) output energy used for driving a vehicle.

9. The battery system according to any one of claims 1 to 8, wherein the first battery (10) includes a plurality of first cells (11) electrically connected in series, and the second battery (20) includes a plurality of second cells (21) electrically connected in series.

10. The battery system according to claim 9, wherein:

the open circuit voltage of the first battery (10) is a product of the number of the plurality of first cells (11) and the open circuit voltage of each of the first cells (11); and the open circuit voltage of the second battery (20) is a product of the number of the plurality of second cells (21) and the open circuit voltage of each of the second cells (21).