Battery pack and battery pack system

Abstract

To provide a battery pack and a battery pack system in which control accuracy of output voltage is improved and a switching loss is reduced.SOLUTION: A battery pack 1 comprises a series circuit 2 including a plurality of unit modules EMs connected in series and a controller 3 for controlling the series circuit. Each unit module EM includes a first terminal T1 and a second terminal T2; a power storage unit BT for storing electric charges; and a switching circuit 2 for switching a connection state including a subscription state in which the power storage unit BT is connected between the first terminal T1 and the second terminal T2 and a withdrawal state of short-circuit between the first terminal T1 and the second terminal T2. The controller 3 includes a switching control unit 31 for controlling connection state switching by respective switching circuits 2 and a voltage adjustment unit 32 for performing PWM control on a unit module EM other than a unit module EM switched into the subscription state by the switching control unit 31.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a battery pack and a battery pack system that control an output voltage. [Background technology]

[0002] In recent years, modular multilevel converters (MMCs), which combine modules in which multiple submodules are connected in series, have been used as inverters for DC-AC conversion (see, for example, Patent Document 1). Modular multilevel converters are capable of outputting any output voltage by integrating the terminal voltages of energy storage elements such as capacitors included in the submodules. Inverters using modular multilevel converters are suitable for use in electrically powered vehicles such as electric vehicles and hybrid vehicles. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-012769 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been an increasing demand for higher voltage power conversion devices, and power conversion devices such as modular multilevel converters have been outputting higher output voltages by increasing the number of sub-modules connected in series and increasing the terminal voltage. However, battery packs using conventional power conversion devices could only control the terminal voltage by gradually integrating it using switching elements included in the sub-modules, which limited the accuracy of output voltage control.

[0005] An object of the present invention is to provide a battery pack and a battery pack system that improves the control accuracy of the output voltage and suppresses an increase in switching loss. [Means for solving the problem]

[0006] The battery pack of the present invention is a battery pack comprising a series circuit in which a plurality of unit modules are connected in series, and a control device that controls the series circuit, wherein each of the unit modules includes a first terminal and a second terminal, a storage unit that stores an electric charge, and a switching circuit that switches connection states including a connected state in which the storage unit is connected between the first terminal and the second terminal, and a disconnected state in which the first terminal and the second terminal are short-circuited, and the control device includes a switching control unit that controls the switching of connection states by each of the switching circuits, and a voltage adjustment unit that performs PWM control on the unit modules other than the unit module that has been switched to the connected state by the switching control unit.

[0007] With this type of battery pack configuration, the output voltage is controlled by switching the connection state and using PWM control, which allows for continuous control of the output voltage rather than stepwise control, improving the control accuracy of the output voltage and minimizing the increase in switching loss using only the minimum number of unit modules.

[0008] Preferably, the control device further includes a phase modulation unit that modulates the phase of a carrier wave in the PWM control performed by the voltage adjustment unit.

[0009] With such a battery pack configuration, the phase of the carrier wave is modulated, thereby increasing the switching frequency and improving the control accuracy of the output voltage.

[0010] Furthermore, it is preferable that the phase modulation section modulates the phase of the carrier wave with a phase difference based on the number of unit modules that have been PWM controlled by the voltage adjustment section.

[0011] According to such a battery pack configuration, the phase of the carrier wave is modulated based on the number of unit modules that have undergone PWM control, thereby preventing overlapping of waveforms in modulated waves generated by a plurality of carrier waves.

[0012] Preferably, the control device further includes a priority setting unit that sets the priority of the switching of the connection state by the switching control unit and the priority of the PWM control performed by the voltage adjustment unit.

[0013] With such a battery pack configuration, the connection state is switched and PWM control is performed efficiently based on the order set by the order setting unit.

[0014] It is also preferable that the voltage adjusting section changes the number of the unit modules that perform PWM control based on the number of the unit modules that have been switched to the joining state by the switching control section.

[0015] With this battery pack configuration, even if the number of unit modules that have switched to the joining state changes, the voltage adjustment unit adjusts the number of unit modules that perform PWM control, thereby maintaining the output voltage of the battery pack.

[0016] The battery pack system according to the present invention is configured with a plurality of battery packs.

[0017] With this type of battery pack system configuration, the output voltage is controlled by switching the connection state and using PWM control, which allows for continuous control of the output voltage rather than stepwise control, improving the control accuracy of the output voltage and reducing switching losses. [Effects of the Invention]

[0018] According to the present invention, the control accuracy of the output voltage is improved, and it becomes easier to suppress an increase in switching loss. [Brief explanation of the drawings]

[0019] [Figure 1]1 is a block diagram showing an example of the configuration of a battery pack included in a battery pack system according to an embodiment of the present invention. [Figure 2] FIG. 2 is a conceptual circuit diagram showing an example of the configuration of the unit module shown in FIG. [Figure 3] FIG. 10 is a conceptual circuit diagram showing an example of a unit module using a full bridge as a switching circuit. [Figure 4] A conceptual circuit diagram showing priority in a series circuit. [Figure 5] 5 is a graph showing output voltages obtained from a unit module in a connected state and a unit module subjected to PWM control in the battery pack shown in FIG. 4. [Figure 6] FIG. 4 is an explanatory diagram showing the switching frequencies of a plurality of unit modules that are PWM controlled in the battery pack according to the embodiment of the present invention. [Figure 7] FIG. 4 is an explanatory diagram showing an intermediate voltage under PWM control in the battery pack according to the embodiment of the present invention. [Figure 8] FIG. 10 is an explanatory diagram showing a change in the target of PWM control in accordance with a unit module switching to a supplementary state in the battery pack according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, a battery pack system 10 and a battery pack 1 according to an embodiment of the present invention will be described with reference to the drawings. Note that components with the same reference numerals in each drawing are the same components, and their description will be omitted.

[0021] The battery pack system 10 is composed of a plurality of battery packs 1. As shown in Fig. 1, the battery pack 1 includes a series circuit 2 and a control device 3, and a high-potential side power line WH and a low-potential side power line WL are respectively extended to generate an output voltage V OUTis output. A plurality of unit modules EM are connected in series to the series circuit 2. Although the series circuit 2 shown in FIG. 1 only shows a configuration in which a plurality of unit modules EM are connected in series, the series circuit 2 may include other components as long as a plurality of unit modules EM are connected in series.

[0022] As shown in Fig. 2, the unit module EM includes a first terminal T1, a second terminal T2, a power storage unit BT that stores electric charge, and a switching circuit SW. Various secondary batteries can be suitably used as the power storage unit BT, and are not limited to single cells. A battery pack formed by combining multiple secondary batteries may also be used as the power storage unit BT. The switching circuit SW is a circuit that switches the electrical connection state between the power storage unit BT and the first terminal T1 and second terminal T2, and is configured, for example, by a half-bridge circuit including switching elements SW1 and SW2 as shown in Fig. 2.

[0023] Specifically, the power storage unit BT and switching element SW1 are connected in series, and the circuit in which the power storage unit BT and switching element SW1 are connected in series is connected in parallel to switching element SW2. The connection point of switching element SW1 and switching element SW2 is connected to a first terminal T1, and the connection point of switching element SW2 and power storage unit BT is connected to a second terminal T2.

[0024] The first terminal T1 is connected to the second terminal T2 of the unit module EM on the higher potential side than the unit module EM, and the second terminal T2 is connected to the first terminal T1 of the unit module EM on the lower potential side than the unit module EM. As a result, the series circuit 2 is formed by connecting a plurality of unit modules EM in series, and the first terminal T1 of the unit module EM on the highest potential side in the series circuit 2 extends from the battery pack 1 as a power line WH, and the second terminal T2 of the unit module EM on the lowest potential side in the series circuit 2 extends from the battery pack 1 as a power line WL.

[0025] Various switching elements can be used as the switching elements SW1 and SW2, and for example, semiconductor switching elements such as transistors can be suitably used. The switching elements SW1 and SW2 are turned on and off in response to a control signal from the control device 3.

[0026] When the unit module EM receives an on / off control signal from the control device 3, the connection state is switched between a joined state and a separated state by the switching circuit SW, as shown in Fig. 2. The connection state of the unit module EM indicated by symbol A is the joined state in which the switching element SW1 is on and the switching element SW2 is off. The connection state of the unit module EM indicated by symbol B is the separated state in which the switching element SW1 is off and the switching element SW2 is on.

[0027] The power storage units BT of the joined unit modules EM are connected in series, and the unit modules EM of the unjoined unit modules are short-circuited. As a result, the output voltage V output between the power lines WL and WH of the battery pack 1 OUT is the voltage obtained by integrating the terminal voltages Vc of the unit modules EM in the joined state, that is, the total voltage of the power storage units BT of the unit modules EM in the joined state.

[0028] Furthermore, the unit module EM may use a full-bridge circuit as the switching circuit SW, as shown in Fig. 3. The unit module EM of the full-bridge circuit shown in Fig. 3 further includes switching elements SW3 and SW4 in addition to the switching elements SW1 and SW2 shown in Fig. 2. The switching elements SW3 and SW4 may be the same as the switching elements SW1 and SW2.

[0029] Specifically, a circuit in which switching elements SW3 and SW4 are connected in series is connected in parallel to a circuit in which switching elements SW1 and SW2 are connected in series. The connection point of switching elements SW1 and SW2 is connected to a first terminal T1, and the connection point of switching elements SW3 and SW4 is connected to a second terminal T2. Similar to the switching circuit SW of the half-bridge circuit, switching elements SW1, SW2, SW3, and SW4 are turned on and off in response to control signals from the control device 3.

[0030] When the unit module EM of the full-bridge circuit receives an on / off control signal from the control device 3, the connection state is switched by the switching circuit SW among a connected state, a disconnected state, and an inverted state, as shown in Fig. 3. In addition to the connected state indicated by symbol A and the disconnected state indicated by symbol B, the connection state of the unit module EM indicated by symbol C is an inverted state in which the polarity of the power storage unit BT is inverted.

[0031] In the add-on state, the switching elements SW1 and SW4 are on and the switching elements SW2 and SW3 are off, in the remove-on state the switching elements SW1 and SW3 are off and the switching elements SW2 and SW4 are on, and in the reverse state the switching elements SW1 and SW4 are off and the switching elements SW2 and SW3 are on. Note that in the remove-on state, the switching elements SW1 and SW3 may be on and the switching elements SW2 and SW4 may be off.

[0032] In the case of a battery pack 1 using the unit module EM of the full-bridge circuit shown in Figure 3, the output voltage V output between the power lines WL and WH is OUT is the voltage obtained by subtracting the total voltage of the power storage units BT of the unit modules EM in the inverted state from the total voltage of the power storage units BT of the unit modules EM in the joined state in the series circuit 2.

[0033] Hereinafter, the control device 3 controlling the connection states including the join state and the detach state by controlling the switching elements SW1 and SW2, or controlling the connection states including the join state, the detach state, and the inverted state by controlling the switching elements SW1 to SW4, will be simply referred to as controlling the connection states.

[0034] The control device 3 is configured with, for example, a CPU such as a microprocessor that executes predetermined logical operations, a memory such as RAM that temporarily stores data, and peripheral circuits thereof, and operates by executing a predetermined program. The control device 3 includes a switching control unit 31, a voltage adjustment unit 32, a phase modulation unit 33, and a priority setting unit 34.

[0035] The switching control unit 31 controls the switching of the connection state by the switching circuit SW of each unit module EM. The switching control unit 31 controls the switching of the connection state by the switching elements SW1, SW2 (SW3, SW4), thereby switching the connection state of each unit module EM in the series circuit 2. If the switching circuit SW is a half-bridge circuit, the switching control unit 31 switches each unit module EM in the series circuit 2 to either a join state or a disconnect state. If the switching circuit SW is a full-bridge circuit, the switching control unit 31 switches each unit module EM in the series circuit 2 to either a join state, a disconnect state, or an inverted state. The switching control unit 31 controls the switching circuit SW to switch the connection state to the join state, and the terminal voltage Vc output from each unit module EM, i.e., the voltage of the power storage unit BT, increases or decreases. The battery pack 1 integrates the voltage of the power storage unit BT in the unit module EM in the join state to calculate the output voltage V OUT It is output as.

[0036] The voltage adjustment unit 32 performs PWM control on the unit modules EM other than the unit module EM switched to the join state by the switching control unit 31, and it is particularly preferable that the voltage adjustment unit 32 performs PWM control on the unit modules EM in the unjoined state. In the battery pack 1, a difference in instantaneous value occurs due to switching when the unit modules EM are switched to the join state by the switching control unit 31. The voltage adjustment unit 32 performs PWM control on some unit modules EM other than the unit modules EM switched to the join state by the switching control unit 31, thereby outputting an intermediate voltage Va that reduces the difference between the terminal voltages Vc of the joined unit modules EM. The battery pack 1 reduces the difference between the terminal voltages Vc using the intermediate voltage Va, thereby reducing the output voltage V OUT The control can be performed continuously, not stepwise.

[0037] The phase modulation unit 33 modulates the phase of the carrier wave in the PWM control performed by the voltage adjustment unit 32 on the unit modules EM. When the carrier wave of each unit module EM is modulated by the phase modulation unit 33, the switching frequency of the modulated wave increases, thereby improving the control accuracy of the output voltage. The phase modulation unit 33 also modulates the phase of the carrier wave with a predetermined phase difference. The predetermined phase difference is preferably calculated by dividing one cycle of the control signal in the PWM control by the number of control targets, which is the number of unit modules on which the voltage adjustment unit 32 performed PWM control. It is preferable that the number of control targets is an odd number. For example, if one cycle (360 degrees) is divided by three control targets on which the voltage adjustment unit 32 performed PWM control, the phase of the carrier wave is modulated with a phase difference of 120 degrees. By modulating the carrier wave of each unit module at equal intervals with a predetermined phase difference, the phase modulation unit 33 can prevent overlapping of modulated waves generated by the carrier wave.

[0038] The priority setting unit 34 sets the priority of the switching of the connection state by the switching control unit 31 and the priority of the PWM control performed by the voltage adjustment unit 32. The priority setting unit 34 sets each priority according to the usage status of the battery pack 1, such as discharging from the battery pack 1 or charging to the battery pack 1, and the status of the power storage unit BT of each unit module EM, such as SOC (State Of Charge), temperature, continuous current-carrying time, continuous rest time, etc. For example, when discharging from the battery pack 1, the priority setting unit 34 gives a higher priority to the unit module EM having a power storage unit BT with a high SOC value. Also, when charging the battery pack 1, the priority setting unit 34 gives a higher priority to the unit module EM having a power storage unit BT with a low SOC value. The priority of the switching of the connection state and the priority of the PWM control set by the priority setting unit 34 may be set in advance or may be set by an application or the like that uses the battery pack 1.

[0039] Each operation of the battery pack 1 will be explained in more detail with reference to Figs. 4 to 8. To facilitate the explanation, the following assumptions are made as a premise: the battery pack 1 is in a supply state where it supplies power, the unit modules EM use half bridges, and the series circuit 2 included in the battery pack 1 includes eight unit modules EM. The unit modules EM of the battery pack 1 shown in Fig. 4 have a priority set in advance by the priority setting unit 34 based on the amount of power stored in each power storage unit BT, and each <1> ~ <8> It is expressed as: <1> The unit module EM represented by has the highest priority to switch to the joining state, <8> The unit module EM represented by has the lowest priority for switching to the joining state. In addition, the voltage of each unit module EM is the same, the switching frequency of the PWM control for each unit module EM is 100 Hz, and the duty ratio of the carrier wave at the switching frequency is 50%.

[0040] The above assumed conditions are set for the convenience of explanation, and the battery pack 1 is not limited to these assumed conditions.

[0041] Output voltage V OUTThe control of the above will be explained below. In the battery pack 1, the switching control unit 31 controls the switching circuit SW of each unit module EM, and switches each unit module EM to the joining state in stages according to the order set by the order setting unit 34. As shown in Fig. 5, the battery pack 1 outputs the terminal voltage Vc of each unit module EM while sequentially accumulating it.

[0042] The voltage adjusting unit 32 performs PWM control on any one or more of the unit modules EM that have not been switched to the joining state by the switching control unit 31. For example, the unit module EM with the highest priority <1> outputs a terminal voltage Vc, the voltage adjustment unit 32 <2> PWM control is performed on the unit module EM <2> As a result, the intermediate voltage Va, which can be adjusted by PWM control, is integrated between the terminal voltages Vc of each unit module EM in the joined state, and the difference between the terminal voltages Vc of the battery pack 1 is reduced, resulting in the output voltage V of the battery pack 1. OUT is continuously controlled.

[0043] However, the unit module EM <2> If only one unit module EM outputs the intermediate voltage Va, <1> Next, the unit module EM <2> When the power supply switches to the on-state, the output voltage V OUT For this reason, the voltage adjustment unit 32 controls one unit module, in this case, the unit module EM <2> Without outputting the intermediate voltage Va from one module, <1> Unit modules other than EM <2> ~ <8> It is preferable that the intermediate voltage Va is output from any two or more of the unit modules EM <2> ~ <8> Each unit module EM <1> ~ <8> The difference between the terminal voltages Vc is reduced, and the output voltage V of the battery pack 1 is increased. OUT is continuously controlled.

[0044] Each unit module EM <1> ~ <8> The PWM control in the battery pack 1 is explained. <1> ~ <8> Among the plurality of unit modules EM that have not been switched to the joining state by the switching control unit 31, <1> ~ <8> The voltage adjusting unit 32 performs PWM control on each of these.

[0045] For example, the switching control unit 31 is a unit module EM <1> , <2> is switched to the on-state, and the voltage adjustment unit 32 switches the unit module EM <3> , <4> , <5> When PWM control is performed on each unit module EM <3> , <4> , <5> In each of the waveforms, a rectangular modulated wave Wa is formed as shown in FIG. 6 by comparing a triangular carrier wave Wc with a voltage command value Vr, which is a reference value in PWM control.

[0046] At this time, the phase modulation unit 33 modulates each unit module EM <3> , <4> , <5> The phase difference when the phase modulation unit 33 modulates each carrier wave Wc is a value obtained by dividing one period by the number of control objects that have undergone PWM control. In this case, since the number of control objects, which is the number of unit modules EM that have undergone PWM control, is three, the phase difference between each unit module EM is 120°. <3> , <4> , <5> The voltage adjusting unit 32 modulates the carrier wave Wc of each unit module EM <3> , <4> , <5> The modulated wave Wa is synthesized and the output voltage V as shown in Figure 6 is generated. OUT The output waveform Wp is generated.

[0047] As a result, each unit module EM <3> , <4> , <5> Although the switching frequency of the output waveform Wp is 100Hz, the apparent switching frequency of the output waveform Wp is three times higher. OUTThe control accuracy of each unit module EM <3> , <4> , <5> Since the switching frequency of each unit module EM <3> , <4> , <5> Since the switching frequency does not increase, there is no need to provide a filter to remove high-frequency components that cause EMI (electromagnetic interference), which reduces costs and simplifies the configuration.

[0048] In the above, PWM control is performed on three unit modules EM, but PWM control may be performed on one or more unit modules EM instead of three. However, if the number of control targets, which is the number of unit modules EM subjected to PWM control, is an even number, at least some waveforms of the output waveforms Wp will overlap, and the control accuracy of the output voltage will not be effectively improved. For this reason, it is preferable that the number of control targets is an odd number.

[0049] The average voltage in PWM control will now be described. The voltage adjustment unit 32 controls the intermediate voltage Va, which is the average voltage of the composite voltage obtained by combining the unit modules EM, by adjusting the duty ratio D of each modulated wave Wa in, for example, three unit modules EM modulated by the phase modulation unit 33, as shown in FIG.

[0050] For example, when the voltage adjusting unit 32 performs PWM control on n unit modules, the mth stage of the n unit modules sets the phase difference of the carrier wave shown in the following equation 1.

[0051] (Number 1) 360° / n×m (degrees)

[0052] The composite voltage has a voltage adjustment range shown in the following formula 3, centered around the voltage value shown in the following formula 2. The frequency of the composite voltage at this time is as shown in the following formula 4. Note that fc in the following formula 4 is, for example, 100 Hz, which is the switching frequency of the unit module.

[0053] (Number 2) n×Vc / 2(V)

[0054] (Number 3) n×Vc(V)

[0055] (Number 4) n×fc(Hz)

[0056] When the duty ratio of the mth stage is Dm and the voltage of the power storage unit in the unit module is Vcm, the intermediate voltage Va, which is the average voltage of the composite voltage, is expressed as shown in the following equation 5. In this way, the voltage adjustment unit 32 adjusts the intermediate voltage Va in the battery pack 1.

[0057] (Number 5) TIFF2025121600000002.tif3043

[0058] In the battery pack 1, each unit module EM is sequentially switched to the join state by the switching control unit 31 in the order shown in Fig. 4. The voltage adjustment unit 32 also changes the number of control targets for PWM control as each unit module EM is switched to the join state. For example, as shown in Fig. 8, when the join state of the unit module EM <1> , <2> Two units of the module EM <1> , <2> , <3> When the voltage regulator 32 switches to the three units, <3> Instead, the unit module EM <6> That is, the switching control unit 31 performs PWM control on the unit module EM <1> , <2> In addition, the unit module EM <3> When the voltage regulator 32 switches the unit module EM <4> , <5> , <6> As a result, even if the number of unit modules switched to the joining state by the switching control unit 31 changes, the output voltage V of the battery pack 1 is controlled by adjusting the duty ratio by the voltage adjustment unit 32. OUT is maintained.

[0059] Furthermore, as the number of unit modules EM switching to the join state increases, the number of control targets for which the voltage adjustment unit 32 performs PWM control may become insufficient for the unit modules EM. Using the above-mentioned assumptions, if six or more of the eight unit modules switch to the join state, the number of control targets for which the voltage adjustment unit 32 performs PWM control may fall below three. In this case, the voltage adjustment unit 32 can change the number of control targets according to the number of unit modules EM switching to the join state. That is, the voltage adjustment unit 32 changes the number of control targets for which PWM control is performed from three to one, an odd number also suitable for PWM control, and performs PWM control. As a result, even if the number of unit modules EM switching to the join state increases, the battery pack 1 can improve the control accuracy of the output voltage and suppress an increase in switching loss.

[0060] Therefore, in the battery pack system 10 configured with a plurality of battery packs 1, the output voltage V output from each battery pack 1 OUT By combining these, multiple battery packs can cooperate to supply the necessary power during peak power consumption periods, allowing the battery pack system 10 to cope with temporary high loads.

[0061] Furthermore, in the battery pack system 10, each battery pack 1 is used evenly. The battery pack system 10 is efficiently charged and discharged, thereby optimizing the life of the power storage unit BT of the unit module EM. This improves the life of the entire battery pack system 10 and reduces the frequency of replacement of the battery pack 1, unit module EM, or power storage unit BT, as well as the maintenance of the battery pack system 10. [Explanation of symbols]

[0062] 1 battery pack 2 Series Circuit 3. Control device 10 Battery Pack System 31 Switching control section 32 Voltage adjustment unit 33 Phase modulation section 34 Ranking Setting Section BT power storage unit D Duty ratio EM Unit Module SW switching circuit SW1 to SW4 switching elements T1 first terminal T2 second terminal V OUT Output Voltage Va intermediate voltage Vc terminal voltage Vr Voltage command value WL,WH Power line Wa modulated wave Wc carrier Wp output waveform

Claims

1. A battery pack comprising a series circuit in which a plurality of unit modules are connected in series, and a control device that controls the series circuit, Each of the unit modules includes: a first terminal and a second terminal; a storage unit that stores electric charges; a switching circuit that switches a connection state between the power storage unit and the power storage unit, the connection state including an on-state in which the power storage unit is connected between the first terminal and the second terminal and a off-state in which the power storage unit is short-circuited between the first terminal and the second terminal; The control device includes: a switching control unit that controls switching of the connection state by each of the switching circuits; a voltage adjusting section that performs PWM control on the unit modules other than the unit module that has been switched to the joining state by the switching control section.

2. The battery pack according to claim 1 , wherein the control device further includes a phase modulation unit that modulates the phase of a carrier wave in the PWM control performed by the voltage adjustment unit.

3. 3. The battery pack according to claim 2, wherein the phase modulation unit modulates the phase of the carrier wave with a phase difference based on the number of the unit modules that have been PWM-controlled by the voltage adjustment unit.

4. 3. The battery pack according to claim 1, wherein the control device further includes a priority setting unit that sets a priority of the switching of the connection state by the switching control unit and a priority of the PWM control performed by the voltage adjustment unit.

5. 5. The battery pack according to claim 4, wherein the voltage adjusting section changes the number of the unit modules that perform PWM control based on the number of the unit modules that have been switched to the joining state by the switching control section.

6. A battery pack system comprising a plurality of battery packs according to claim 5.

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