Battery pack and battery pack system

Abstract

Provided are a battery pack and a battery pack system in which the control accuracy of an output voltage is improved and switching loss is reduced.　The present invention pertains to a battery pack (1) that is provided with: a series circuit (2) in which a plurality of unit modules (EM) are connected in series; and a control device (3) that controls the series circuit. Each unit module (EM) includes: a first terminal (T1) and a second terminal (T2); a power storage unit (BT) that stores charge; and a switching circuit (2) that switches between connection states including a connected state in which the power storage unit (BT) is connected between the first terminal (T1) and the second terminal (T2), and a disconnected state in which the first terminal (T1) and the second terminal (T2) are short-circuited. The control device (3) includes: a switching control unit (31) that controls the switching of the connection state by each switching circuit (2); and a voltage adjustment unit (32) that performs PWM control on the unit modules (EM) other than unit modules (EM) switched to the connected state by the switching control unit (31).　

Description

Battery pack and battery pack system

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

[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.

[0003] JP 2015-012769 A

[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.

[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 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] It is also 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 this configuration of the battery pack, 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 add-on 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 by 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.

[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.

[0019] 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. FIG. 2 is a conceptual circuit diagram showing an example of the configuration of a unit module shown in FIG. 1. FIG. 3 is a conceptual circuit diagram showing an example of a unit module using a full bridge as a switching circuit. FIG. 4 is a conceptual circuit diagram showing priority in a series circuit. FIG. 5 is a graph showing output voltages obtained from a unit module in a joined state and a unit module that has undergone PWM control in the battery pack shown in FIG. 6. FIG. 7 is an explanatory diagram showing switching frequencies of a plurality of unit modules that have undergone PWM control in a battery pack according to an embodiment of the present invention. FIG. 8 is an explanatory diagram showing intermediate voltages under PWM control in a battery pack according to an embodiment of the present invention. FIG. 9 is an explanatory diagram showing a change in the target for PWM control as a unit module switches to a joined state in a battery pack according to an embodiment of the present 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 the various drawings 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. A high-potential side power line WH and a low-potential side power line WL extend to generate an output voltage V ＯＵＴ is output. A plurality of unit modules EM are connected in series to the series circuit 2. Note that 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 for storing electric charge, and a switching circuit SW. The power storage unit BT can be formed of various secondary batteries, and is not limited to a single battery. A battery pack formed by combining a plurality of secondary batteries may also be used as the power storage unit BT. The switching circuit SW 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 the switching element SW1 are connected in series, and the circuit in which the power storage unit BT and the switching element SW1 are connected in series is connected in parallel to the switching element SW2. The connection point between the switching elements SW1 and SW2 is connected to a first terminal T1, and the connection point between the switching element SW2 and the 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 that is 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 that is on the lower potential side than the unit module EM. In this way, the series circuit 2 has a plurality of unit modules EM connected in series, and the first terminal T1 of the unit module EM that is 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 that is 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 in the connected unit modules EM are connected in series, and the disconnected unit modules EM are short-circuited. As a result, the output voltage V ＯＵＴ is a voltage obtained by integrating the terminal voltages Vc of the unit modules EM in the joining state, that is, the total voltage of the power storage units BT of the unit modules EM in the joining 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 join state, the switching elements SW1 and SW4 are on and the switching elements SW2 and SW3 are off, in the detach state the switching elements SW1 and SW3 are off and the switching elements SW2 and SW4 are on, and in the inverted state the switching elements SW1 and SW4 are off and the switching elements SW2 and SW3 are on. Note that in the detach 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 the battery pack 1 using the unit module EM of the full-bridge circuit shown in FIG. 3, the output voltage V output between the power lines WL and WH is ＯＵＴis a 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, for example, with 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 calculates the output voltage V by integrating the voltage of the power storage unit BT in the join state unit module EM. 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 disconnected 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 unit modules EM in the join state. 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. An odd number of control targets is preferable. 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 connection state switching 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 the SOC (State of Charge), temperature, continuous current-carrying time, continuous rest time, etc. For example, when the battery pack 1 is being discharged, the priority setting unit 34 assigns a higher priority to the unit module EM having a power storage unit BT with a higher SOC value. Also, when the battery pack 1 is being charged, the priority setting unit 34 assigns a higher priority to the unit module EM having a power storage unit BT with a lower SOC value. The priority of the connection state switching 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 described in more detail using Figures 4 to 8. For ease of explanation, the following assumptions are made: the battery pack 1 is in a power supply state, supplying 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 Figure 4 are designated by EM1 to EM8 in order of switching to the join state, which is determined in advance by the priority setting unit 34 based on the amount of power stored in each power storage unit BT. The unit module EM designated by EM1 has the highest priority for switching to the join state, and the unit module EM designated by EM8 has the lowest priority for switching to the join state. Furthermore, the voltages of the unit modules EM are all 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-mentioned 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 ＯＵＴIn 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 two or more of the unit modules EM that have not been switched to the add-on state by the switching control unit 31. For example, if the unit module EM1 with the highest priority outputs a terminal voltage Vc, the voltage adjusting unit 32 performs PWM control on the unit module EM2 with the next highest priority, and outputs an intermediate voltage Va from the unit module EM2. As a result, the intermediate voltage Va that can be adjusted by PWM control is integrated between the terminal voltages Vc of the unit modules EM in the add-on state, and the difference between the terminal voltages Vc in the battery pack 1 is reduced, and the output voltage V of the battery pack 1 is reduced. ＯＵＴ is continuously controlled.

[0043] However, if only the unit module EM2, that is, only one unit module EM, outputs the intermediate voltage Va, when the unit module EM1 switches to the joining state after the unit module EM1, the output voltage V ＯＵＴ Therefore, it is preferable that the voltage adjustment unit 32 does not output the intermediate voltage Va from one unit module, in this case, from one unit module EM2, by PWM control, but outputs the intermediate voltage Va from any of the unit modules EM2 to EM8 other than the unit module EM1. The battery pack 1 uses the adjustable intermediate voltage Va output from any of the unit modules EM2 to EM8 to suppress instantaneous fluctuations that occur when switching to the add-on state in each of the unit modules EM1 to EM8, while reducing the difference between the terminal voltages Vc, and thereby the output voltage V of the battery pack 1. ＯＵＴ is continuously controlled.

[0044] The PWM control in each of the unit modules EM1 to EM8 will now be described. In the battery pack 1, the voltage adjustment unit 32 performs PWM control on each of the plurality of unit modules EM1 to EM8 that have not been switched to the joining state by the switching control unit 31 among the unit modules EM1 to EM8.

[0045] For example, when the switching control unit 31 switches the unit modules EM1 and EM2 to the joining state and the voltage adjustment unit 32 performs PWM control on the unit modules EM3, EM4, and EM5, each of the unit modules EM3, EM4, and EM5 forms a rectangular wave modulated wave Wa as shown in Figure 6 by comparing the carrier wave Wc, which is a triangular wave, with the voltage command value Vr, which is the reference value in PWM control.

[0046] At this time, the phase modulation unit 33 modulates the phase of the carrier wave Wc of each unit module EM3, EM4, EM5. The phase difference when the phase modulation unit 33 modulates each carrier wave Wc is a value obtained by dividing one cycle 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 carrier waves Wc of each unit module EM3, EM4, EM5 are modulated with a phase difference of 120°. The voltage adjustment unit 32 combines the modulated waves Wa of each unit module EM3, EM4, EM5, each modulated by 120° with the carrier wave Wc, and adjusts the output voltage V as shown in FIG. ＯＵＴ The output waveform Wp is generated.

[0047] As a result, although the switching frequency of each of the unit modules EM3, EM4, and EM5 in the battery pack 1 is 100 Hz, the apparent switching frequency of the output waveform Wp is tripled. ＯＵＴ This improves the control accuracy. Also, because the switching frequency of each of the unit modules EM3, EM4, and EM5 is not increased, an increase in switching loss is suppressed. Furthermore, because the switching frequency of each of the unit modules EM3, EM4, and EM5 is not increased, there is no need to provide a filter that removes high-frequency components that cause EMI (electromagnetic interference), which makes it possible to reduce costs and simplify 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 modulation 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] (Math. 2) n×Vc / 2(V)

[0054] (Math. 3) n×Vc (V)

[0055] (Math. 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] (Equation 5) ​

[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 which PWM control is performed as each unit module EM is switched to the join state. For example, as shown in FIG. 8 , when the join state is switched from two unit modules EM1 and EM2 to three unit modules EM1, EM2, and EM3, the voltage adjustment unit 32 performs PWM control on the unit module EM6 instead of the unit module EM3. In other words, when the switching control unit 31 switches the unit module EM3 to the join state in addition to the unit modules EM1 and EM2, the voltage adjustment unit 32 performs PWM control on the unit modules EM4, EM5, and EM6. As a result, even if the number of unit modules switched to the join state by the switching control unit 31 changes, the output voltage V of the battery pack 1 remains constant together with the adjustment of 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 ＯＵＴBy combining these, multiple battery packs can cooperate to supply the necessary power during peak power consumption periods. This allows 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.

[0062] REFERENCE SIGNS LIST 1 Battery pack 2 Series circuit 3 Control device 10 Battery pack system 31 Switching control section 32 Voltage adjustment section 33 Phase modulation section 34 Priority setting section BT Power storage section D Duty ratio EM, EM1 to EM8 Unit module SW Switching circuit SW1 to SW4 Switching element 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 Modulation wave Wc Carrier wave 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, 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 an on-state in which the storage unit is connected between the first terminal and the second terminal, and a off-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 on-state by the switching control unit.

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

3. The battery pack according to claim 2, wherein 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.

4. The battery pack according to claim 1 or 2, wherein 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.

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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