Battery pack and control method thereof

Abstract

To provide a battery pack capable of blocking a charge / discharge path in various states in which the battery pack should not be used.SOLUTION: A battery pack according to one embodiment of the present disclosure comprises: a storage battery provided on a path connecting a first terminal and a second terminal, and having a plurality of battery cells; a monitoring circuit capable of detecting a cell voltage of the respective battery cells; a first connection terminal led to the first terminal via a first path; a second connection terminal led to the second terminal via a second path; a fusible element provided on the first path; a heat generating body provided on a third path connecting the first path and the second path, and capable of fusing the fusible element; a switching element provided on the third path; and a control circuit capable of controlling a switching operation of the switching element by PFM controlling so as to keep an on-time of the switching element in a predetermined time and vary an off-time of the switching element according to a storage battery voltage on the basis of a detection result of the monitoring circuit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a battery pack including a storage battery and a method for controlling the battery pack used in such a battery pack. [Background technology]

[0002] In a battery pack equipped with a storage battery, for example, if the storage battery becomes overcharged, the charge / discharge path is cut off. For example, Patent Document 1 discloses a technology in which a fusible element provided in the charge / discharge path is melted when the storage battery becomes overcharged. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-53780 Summary of the Invention [Problem to be solved by the invention]

[0004] In a battery pack, it is desirable to cut off the charge / discharge path not only in an overcharged state, but also in various other states in which the battery pack should not be used.

[0005] It is desirable to provide a battery pack and a method for controlling the battery pack that can cut off the charge / discharge path in various states in which the battery pack should not be used. [Means for solving the problem]

[0006] A battery pack according to an embodiment of the present disclosure includes a storage battery, a monitoring circuit, a first connection terminal, a second connection terminal, a fusible element, a heating element, a switch element, and a control circuit. The storage battery is provided in a path connecting the first terminal and the second terminal and has a plurality of battery cells connected in series. The monitoring circuit is capable of detecting the cell voltage of each of the plurality of battery cells. The first connection terminal is connected to the first terminal via the first path. The second connection terminal is connected to the second terminal via the second path. The fusible element is provided in the first path and is capable of being melted by heat. The heating element is provided in a third path connecting the first path and the second path and is capable of melting the fusible element by generating heat. The switch element is provided in the third path. The control circuit is capable of controlling the switching operation of the switch element by maintaining the on time of the switch element at a predetermined time based on the detection results of the monitoring circuit, and by performing PFM control to change the off time of the switch element in accordance with the battery voltage, which is the voltage across the battery. The control circuit can set the off time to a first time when the storage battery voltage is a first voltage, and can set the off time to a second time longer than the first time when the storage battery voltage is a second voltage higher than the first voltage.

[0007] A control method for a battery pack according to an embodiment of the present disclosure includes a battery pack including: a storage battery having a plurality of battery cells connected in series and provided in a path connecting a first terminal and a second terminal; a first connection terminal led to the first terminal via the first path; a second connection terminal led to the second terminal via the second path; a fusible element provided in the first path and capable of being blown by heat; a heating element provided in a third path connecting the first path and the second path and capable of melting the fusible element by generating heat; and a switch element provided in the third path, the control method includes detecting cell voltages of each of the plurality of battery cells; and controlling operation of the switch element by performing PFM control based on the detected cell voltages so as to maintain an on-time of the switch element for a predetermined time and to vary an off-time according to the storage battery voltage, which is the voltage across the storage battery. The PFM control includes setting the off time to a first time when the storage battery voltage is a first voltage, and setting the off time to a second time that is longer than the first time when the storage battery voltage is a second voltage that is higher than the first voltage.

[0008] According to the battery pack and the battery pack control method in one embodiment of the present disclosure, the charge / discharge path can be cut off in various states in which the battery pack should not be used. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a block diagram illustrating an example configuration of a battery pack according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is an explanatory diagram showing an example of the characteristics of the battery cell shown in FIG. [Figure 3] FIG. 3 is a waveform diagram showing an example of the waveform of the control signal shown in FIG. [Figure 4] FIG. 4 is a flowchart illustrating an example of the operation of the control circuit shown in FIG. [Figure 5] FIG. 5 is an explanatory diagram showing an example of the voltage range of the storage battery voltage. [Figure 6] FIG. 6 is a characteristic table showing an example of the characteristics of the protection circuit shown in FIG. [Figure 7] FIG. 7 is a characteristic diagram illustrating an example of the characteristics of the protection circuit shown in FIG. [Figure 8] FIG. 8 is another waveform diagram illustrating an example of the waveform of the control signal shown in FIG. [Figure 9] FIG. 9 is another characteristic table showing an example of the characteristics of the protection circuit shown in FIG. [Figure 10] FIG. 10 is another characteristic diagram illustrating an example of the characteristics of the protection circuit shown in FIG. [Figure 11] FIG. 11 is another characteristic table showing an example of the characteristics of the protection circuit shown in FIG. [Figure 12] FIG. 12 is a flowchart showing a procedure for determining parameters for PFM control. [Figure 13] FIG. 13 is a waveform diagram showing an example of the waveform of a control signal when PWM control is performed. [Figure 14] FIG. 14 is a characteristics table showing an example of characteristics of a protection circuit when PWM control is performed. [Figure 15A]FIG. 15A is a circuit diagram illustrating an example of the protection circuit illustrated in FIG. [Figure 15B] FIG. 15B is a circuit diagram illustrating an example of a protection circuit according to a modified example. [Figure 15C] FIG. 15C is a circuit diagram illustrating an example of a protection circuit according to another modified example. [Figure 16] FIG. 16 is a block diagram showing an example of the configuration of a battery pack according to another modified example. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings.

[0011] <Embodiment> [Configuration example] 1 shows an example of the configuration of a battery pack (battery pack 1) according to one embodiment. The battery pack 1 includes a positive terminal TP, a negative terminal TN, a storage battery 11, a protection circuit 20, a monitoring circuit 30, a control circuit 12, and a transistor 13.

[0012] The positive terminal TP and the negative terminal TN are configured to electrically connect the battery pack 1 to a device to which the battery pack 1 is attached. The device may be, for example, a charger that supplies power to the battery pack 1, or a load device that operates based on the power from the battery pack 1. The positive terminal TP is connected to the positive electrode EP of the storage battery 11 via a power line PL1. Here, "connected" does not only include the case where the positive terminal TP is connected to the positive electrode EP of the storage battery 11 via a protection circuit 20 as shown in FIG. 1, but also includes the case where the positive terminal TP is connected to the positive electrode EP of the storage battery 11 via a switch or a resistive element (not shown). The negative terminal TN is connected to the negative electrode EN of the storage battery 11 via a power line PL2. Here, "connected" does not only include the case where the negative terminal TN is directly connected to the negative electrode EN of the storage battery 11 as shown in FIG. 1, but also includes the case where the negative terminal TN is connected to the negative electrode EN of the storage battery 11 via a switch or a resistive element (not shown).

[0013] The storage battery 11 is configured to store power. For example, when the battery pack 1 is connected to a charger, a charging current flows in the order of the positive terminal TP, the protection circuit 20, the storage battery 11, and the negative terminal TN, thereby charging the storage battery 11. When the battery pack 1 is connected to a load device, a discharging current flows in the order of the negative terminal TN, the storage battery 11, the protection circuit 20, and the positive terminal TP, thereby discharging the storage battery 11.

[0014] The storage battery 11 has a plurality of battery cells BC (five battery cells BC1 to BC5 in this example). In this example, each of the battery cells BC1 to BC5 is constructed using a lithium-ion secondary battery. The battery cells BC1 to BC5 are connected in series. Specifically, the positive electrode of battery cell BC1 is connected to the negative electrode of battery cell BC2, and the negative electrode is connected to the negative electrode EN of the storage battery 11. The positive electrode of battery cell BC2 is connected to the negative electrode of battery cell BC3, and the negative electrode is connected to the positive electrode of battery cell BC1. The positive electrode of battery cell BC3 is connected to the negative electrode of battery cell BC4, and the negative electrode is connected to the positive electrode of battery cell BC2. The positive electrode of battery cell BC4 is connected to the negative electrode of battery cell BC5, and the negative electrode is connected to the positive electrode of battery cell BC3. The positive electrode of battery cell BC5 is connected to the positive electrode EP of the storage battery 11, and the negative electrode is connected to the positive electrode of battery cell BC4.

[0015] The cell voltage VBC of a battery cell BC is the voltage between the positive and negative electrodes of that battery cell BC. Specifically, the cell voltage VBC1 of battery cell BC1 is the voltage between the positive and negative electrodes of battery cell BC1. The cell voltage VBC2 of battery cell BC2 is the voltage between the positive and negative electrodes of battery cell BC2. The cell voltage VBC3 of battery cell BC3 is the voltage between the positive and negative electrodes of battery cell BC3. The cell voltage VBC4 of battery cell BC4 is the voltage between the positive and negative electrodes of battery cell BC4. The cell voltage VBC5 of battery cell BC5 is the voltage between the positive and negative electrodes of battery cell BC5. The cell voltage VBC may change depending on the amount of charge stored in that battery cell BC.

[0016] 2 shows an example of a cell voltage VBC. When the cell voltage VBC is equal to or higher than voltage V11 (1.5 [V] in this example) and equal to or lower than voltage V12 (4.3 [V] in this example), the battery cell BC is in a normal state S2 and is usable. For example, when the cell voltage VBC is higher than voltage V12, the battery cell BC is in an overcharged state S3 and cannot be used. Also, when the cell voltage VBC is lower than voltage V11, the battery cell BC is in an overdischarged state S1 and cannot be used. In the battery pack 1, the five cell voltages VBC1 to VBC5 of the five battery cells BC1 to BC5 are monitored by a monitoring circuit 30.

[0017] The protection circuit 20 is provided on the power line PL1 and is configured to be able to interrupt the charge / discharge path of the storage battery 11. The protection circuit 20 has a fusible element 21 and a heating element 22. In this example, the protection circuit 20 is housed in a single package. However, this is not limited to this, and the protection circuit 20 may be a combination of individual components. The fusible element 21 is provided on the power line PL1 and is configured to be able to melt due to the heat of the heating element 22. One end of the fusible element 21 is connected to the positive terminal EP of the storage battery 11, and the other end of the fusible element 21 is connected to the positive terminal TP of the battery pack 1. The heating element 22 is provided in the path connecting the power line PL1 and the power line PL2, and is configured to generate heat in response to the current flowing through the heating element 22. One end of the heating element 22 is connected to the positive terminal TP, and the other end of the heating element 22 is connected to the drain of the transistor 13.

[0018] The monitoring circuit 30 is configured to monitor the cell voltages VBC1 to VBC5 of the battery cells BC1 to BC5 in the storage battery 11, and the temperature of the storage battery 11. The monitoring circuit 30 has a temperature sensor 31 and an analog front-end circuit 32.

[0019] The temperature sensor 31 includes, for example, a thermistor, and is configured to detect the temperature of the storage battery 11. The temperature sensor 31 is provided, for example, near the storage battery 11, and detects the temperature of the storage battery 11. The temperature sensor 31 then supplies the detection result to the analog front-end circuit 32.

[0020] The analog front-end circuit 32 is configured to detect the cell voltages VBC1 to VBC5 of the battery cells BC1 to BC5 by detecting the voltages V1 to V5 of the storage battery 11 based on the voltage of the power supply line PL2, and to supply this detection result, together with the detection result of the temperature sensor 31, to the control circuit 12.

[0021] The control circuit 12 is configured using, for example, a microcontroller, and is configured to determine whether or not to stop use of the battery pack 1 based on the detection result of the monitoring circuit 30. Specifically, the control circuit 12 determines that use of the battery pack 1 should be stopped when, for example, any one or more of the battery cells BC1 to BC5 of the storage battery 11 are in an overcharged state S3, any one or more of the battery cells BC1 to BC5 of the storage battery 11 are in an overdischarged state S1, the temperature of the storage battery 11 is outside a predetermined temperature range, or the monitoring circuit 30 is not operating normally. When use of the battery pack 1 should be stopped, the control circuit 12 generates a control signal CTL by performing PFM (Pulse Frequency Modulation) control based on the voltage between the positive electrode EP and the negative electrode EN of the storage battery 11 (storage battery voltage VB), and controls the switching operation of the transistor 13 using this control signal CTL. In this PFM control, the control circuit 12 maintains the on-time Ton of the transistor 13 at a predetermined time, and changes the off-time Toff of the transistor 13 according to the voltage between the positive electrode EP and the negative electrode EN of the storage battery 11 (storage battery voltage VB).

[0022] The transistor 13 is configured to perform a switching operation based on a control signal CTL. In this example, the transistor 13 is an N-type FET (Field Effect Transistor), and the control signal CTL is supplied to the gate of the transistor 13, the drain of the transistor 13 is connected to the other end of the heating element 22, and the source of the transistor 13 is connected to the power supply line PL2.

[0023] With this configuration, in the battery pack 1, when the control circuit 12 performs PFM control to generate the control signal CTL, the heating element 22 consumes power according to the duty ratio of the control signal CTL and generates heat. The heat from the heating element 22 then melts the fusible element 21. In this way, in the battery pack 1, the charge / discharge path is irreversibly cut off when the use of the battery pack 1 should be stopped.

[0024] Here, the storage battery 11 corresponds to a specific example of a "storage battery" in an embodiment of the present disclosure. The battery cells BC1 to BC5 correspond to a specific example of a "plurality of battery cells" in an embodiment of the present disclosure. The positive electrode EP corresponds to a specific example of a "first terminal" in an embodiment of the present disclosure. The negative electrode EN corresponds to a specific example of a "second terminal" in an embodiment of the present disclosure. The monitoring circuit 30 corresponds to a specific example of a "monitoring circuit" in an embodiment of the present disclosure. The positive terminal TP corresponds to a specific example of a "first connection terminal" in an embodiment of the present disclosure. The power line PL1 corresponds to a specific example of a "first path" in an embodiment of the present disclosure. The negative terminal TN corresponds to a specific example of a "second connection terminal" in an embodiment of the present disclosure. The power line PL2 corresponds to a specific example of a "second path" in an embodiment of the present disclosure. The fusible element 21 corresponds to a specific example of a "fusible element" in an embodiment of the present disclosure. The heating element 22 corresponds to a specific example of a "heating element" in an embodiment of the present disclosure. The transistor 13 corresponds to a specific example of a "switching element" in an embodiment of the present disclosure. The control circuit 12 corresponds to a specific example of a "control circuit" in an embodiment of the present disclosure. The voltage V12 corresponds to a specific example of a "first threshold voltage" in an embodiment of the present disclosure. The voltage V11 corresponds to a specific example of a "second threshold voltage" in an embodiment of the present disclosure.

[0025] [Actions and Actions] Next, the operation and function of the battery pack 1 of this embodiment will be described.

[0026] (Overview of overall operation) First, an overview of the overall operation of the battery pack 1 will be described with reference to FIG. 1. The storage battery 11 stores power. The monitoring circuit 30 monitors the cell voltages VBC1 to VBC5 of the battery cells BC1 to BC5 and the temperature of the storage battery 11. The control circuit 12 determines whether or not to stop using the battery pack 1 based on the detection results of the monitoring circuit 30. If the use of the battery pack 1 should be stopped, the control circuit 12 performs PFM control based on the storage battery voltage VB to generate a control signal CTL, and uses this control signal CTL to control the switching operation of the transistor 13. The transistor 13 performs switching operation based on the control signal CTL. The heating element 22 generates heat by consuming power according to the duty ratio of the control signal CTL, and the heat from the heating element 22 melts the fusible element 21.

[0027] (Detailed operation) The control circuit 12 determines that use of the battery pack 1 should be stopped when, for example, one or more of the battery cells BC1 to BC5 of the storage battery 11 are in the overcharge state S3, one or more of the battery cells BC1 to BC5 of the storage battery 11 are in the overdischarge state S1, the temperature of the storage battery 11 is outside a predetermined temperature range, or the monitoring circuit 30 is not operating normally. When use of the battery pack 1 should be stopped, the control circuit 12 generates a control signal CTL by performing PFM control based on the storage battery voltage VB. The transistor 13 performs a switching operation based on this control signal CTL.

[0028] FIG. 3 shows an example of the control signal CTL. In this PFM control, the control circuit 12 maintains the on-time Ton of the transistor 13 at a predetermined time and varies the off-time Toff of the transistor 13 in response to the battery voltage VB. Specifically, for example, when the battery voltage VB is high, the control circuit 12 lengthens the off-time Toff while maintaining the on-time Ton, thereby lengthening the switching period T and lowering the duty ratio. Here, the duty ratio is the ratio of the on-time Ton to the switching period T. Also, for example, when the battery voltage VB is low, the control circuit 12 shortens the off-time Toff while maintaining the on-time Ton, thereby shortening the switching period T and increasing the duty ratio. In this way, the control circuit 12 adjusts the duty ratio based on the battery voltage VB to adjust the power consumed by the heating element 22 and the amount of heat generated by the heating element 22. The fusible element 21 then melts due to the heat from the heating element 22, irreversibly interrupting the charge / discharge path. When the fusible element 21 blows, the control circuit 12 ends the PFM control. The time from the timing t1 when the PFM control starts to the timing t2 when the PFM control ends is the blow time Tf required for the fusible element 21 to blow.

[0029] FIG. 4 shows an example of the operation of the control circuit 12.

[0030] First, the control circuit 12 checks whether the monitoring circuit 30 is operating normally based on the cell voltages VBC1 to VBC5 and the temperature of the storage battery 11 (step S101). Specifically, the control circuit 12 determines that the monitoring circuit 30 is not operating normally if, for example, each of the cell voltages VBC1 to VBC5 is a voltage outside the expected voltage range or if the temperature of the storage battery 11 is a voltage outside the expected temperature range. If the monitoring circuit 30 is not operating normally ("N" in step S102), the process proceeds to step S106.

[0031] In step S101, if the monitoring circuit 30 is operating normally ("Y" in step S102), the control circuit 12 checks whether any one or more of the cell voltages VBC1 to VBC5 is higher than the voltage V12 (e.g., 4.3 V) (step S103). In other words, the control circuit 12 checks whether any one or more of the battery cells BC1 to BC5 is in the overcharge state S3 based on the cell voltages VBC1 to VBC5. If any one or more of the cell voltages VBC1 to VBC5 is higher than the voltage V12 ("Y" in step S103), the process proceeds to step S106.

[0032] In step S103, if all of the cell voltages VBC1 to VBC5 are equal to or lower than voltage V12 ("N" in step S103), the control circuit 12 checks whether any one or more of the cell voltages VBC1 to VBC5 are lower than voltage V11 (e.g., 1.5 V) (step S104). In other words, the control circuit 12 checks whether any one or more of the battery cells BC1 to BC5 are in the over-discharge state S1 based on the cell voltages VBC1 to VBC5. If any one or more of the cell voltages VBC1 to VBC5 are lower than voltage V11 ("Y" in step S104), the process proceeds to step S106.

[0033] In step S104, if all of the cell voltages VBC1 to VBC5 are equal to or higher than the voltage V11 ("N" in step S103), the control circuit 12 checks whether the temperature of the storage battery 11 is outside a predetermined temperature range (step S105). This predetermined temperature range is the normal operating temperature range of the battery pack 1. For example, if the battery pack 1 is generating heat due to an operational abnormality, the temperature of the storage battery 11 may be higher than the predetermined temperature range. If the temperature of the storage battery 11 is outside the predetermined temperature range ("Y" in step S105), the process proceeds to step S106.

[0034] In step S105, if the temperature of the storage battery 11 is within the predetermined temperature range ("N" in step S105), this process ends.

[0035] If the monitoring circuit 30 is not operating normally ("N" in step S102), if one or more of the cell voltages VBC1 to VBC5 is higher than voltage V12 ("Y" in step S103), if one or more of the cell voltages VBC1 to VBC5 is lower than voltage V11 ("Y" in step S104), or if the temperature of the storage battery 11 is outside the predetermined temperature range ("Y" in step S105), the control circuit 12 calculates the switching period T based on the storage battery voltage VB (step S106). Specifically, for example, when the storage battery voltage VB is high, the control circuit 12 lengthens the switching period T by lengthening the off time Toff while maintaining the on time Ton. On the other hand, if the storage battery voltage VB is low, the control circuit 12 shortens the switching period T by shortening the off time Toff while maintaining the on time Ton.

[0036] Then, the control circuit 12 starts PFM control using the switching period T calculated in step S106 (step S107). In this way, the control circuit 12 generates the control signal CTL as shown in FIG.

[0037] This completes the process.

[0038] (PFM control parameter settings) Next, the settings of the following parameters in PFM control will be described in detail. (1) Voltage range W of storage battery voltage VB (2) On time Ton (3) Switching period T and fusing time Tf

[0039] (1) Voltage range W of storage battery voltage VB The control circuit 12 performs PFM control based on the storage battery voltage VB. First, a voltage range W of the storage battery voltage VB is set.

[0040] FIG. 5 shows an example of the storage battery voltage VB. The voltage range W of the storage battery voltage VB is set by considering the potential voltage of the storage battery voltage VB based on, for example, the characteristics of the battery cells BC shown in FIG. 2. In this example, the lower limit voltage V21 of the voltage range W of the storage battery voltage VB is set to 7.5 [V], and the upper limit voltage V22 is set to 21.5 [V]. This lower limit voltage V21 (7.5 [V]) is estimated on the assumption that the cell voltages VBC of all five battery cells BC are voltage V11 (1.5 [V]), and the upper limit voltage V22 (21.5 [V]) is estimated on the assumption that the cell voltages VBC of all five battery cells BC are voltage V12 (4.3 [V]). However, the present invention is not limited to this example, and the lower limit voltage V21 and the upper limit voltage V22 can be set as appropriate. The control circuit 12 generates the control signal CTL when the storage battery voltage VB is equal to or greater than the lower limit voltage V21 and equal to or less than the upper limit voltage V22.

[0041] For example, when any one or more of the battery cells BC1 to BC5 in the storage battery 11 are in the overcharge state S3, the storage battery voltage VB may be a high voltage within the voltage range W. When any one or more of the battery cells BC1 to BC5 in the storage battery 11 are in the overdischarge state S1, the storage battery voltage VB may be a low voltage within the voltage range W. When the temperature of the storage battery 11 is outside a predetermined temperature range or when the monitoring circuit 30 is not operating normally, the storage battery voltage VB may be any voltage within the voltage range W. The voltage range W of the storage battery voltage VB is set to a voltage range that keeps the storage battery voltage VB within the voltage range W under such various circumstances.

[0042] (2) On-time Ton During the on-time Ton, power is applied to the heating element 22, causing the heating element 22 to generate heat. If this on-time Ton is too long, stress may increase the resistance of the heating element 22, potentially resulting in failure of the heating element 22. Therefore, assuming a voltage range W (FIG. 5) of the storage battery voltage VB, various DC voltages from 13.3 V to 21.5 V were applied to the heating element 22, and an experiment was conducted to confirm the time until the heating element 22 failed (failure time).

[0043] 6 and 7 show an example of the experimental results of failure time. Here, the applied power is the power applied to the heating element 22, and is calculated based on the voltage applied to the heating element 22 and the resistance value of the heating element 22 before the voltage was applied. For example, the applied power is 530 W when a voltage of 21.5 V is applied to the heating element 22. In this example, the heating element 22 is determined to have failed if the resistance value of the heating element 22 increases and the fusible element 21 does not melt even after applying a voltage to the heating element 22 for a predetermined time.

[0044] As shown in Figures 6 and 7, the greater the applied power, the more stress is placed on the heating element 22, shortening the time to failure and causing the heating element 22 to fail sooner. For example, the time to failure is 6 msec when a voltage of 21.5 V is applied to the heating element 22. This result shows that the on-time Ton when performing PFM control should be at least 6 msec. In this example, taking a margin into consideration, the on-time Ton is set using the following equation EQ1. On time Ton = Failure time × Rated power / Applied power (EQ1) Here, the rated power is the rated power of the heating element 22, which is 100 [W] in this example. For example, if the applied power is 530 [W], the rated power is 100 [W], and the failure time is 6 [msec.], the on-time Ton is 1 [sec.]. Therefore, in this example, the on-time Ton is set to 1 [sec.].

[0045] (3) Switching period T and fusing time Tf 3, in PFM control, power is applied to the heating element 22 during the on time Ton, and no power is applied during the off time Toff. Therefore, the power applied to the heating element 22 is adjusted by the duty ratio of the control signal CTL.

[0046] Fig. 8 shows an example of the control signal CTL when a voltage of 21.5 [V] is applied to the heating element 22. As shown in Fig. 6, when a voltage of 21.5 [V] is applied to the heating element 22, the applied power is 530 [W]. Fig. 8(A) shows an example of a waveform when the power applied to the heating element 22 during the switching period T (hereinafter also referred to as the in-period average power Pave) is 100 [W], and Fig. 8(B) shows an example of a waveform when the in-period average power Pave is 400 [W].

[0047] When the in-cycle average power Pave is set to 100 [W], the duty ratio is set to 18.9% (= 1 / 5.3) as shown in FIG. 8(A). That is, since the applied power is 530 [W], the in-cycle average power Pave can be set to 100 [W] (= 530 [W] × 1 / 5.3). Since the on-time Ton is fixed at 1 [msec], the off-time Toff is 4.3 [msec], and the switching period T is 5.3 [msec]. In this way, when the duty ratio is 18.9%, the in-cycle average power Pave can be made equal to the rated power (100 [W]) of the heating element 22.

[0048] When the average power within a period Pave is set to 400 [W], the duty ratio is set to 75.2% (= 1 / 1.33) as shown in Figure 8(B). In other words, since the applied power is 530 [W], the average power within a period Pave can be set to 400 [W] (= 530 [W] x 1 / 1.33). Since the on-time Ton is fixed at 1 [msec], the off-time Toff is 0.33 [msec] and the switching period T is 1.33 [msec].

[0049] FIG. 9 shows an example of experimental results of the melting time Tf of the fusible element 21 when the in-cycle average power Pave is changed from 100 [W] to 450 [W]. For example, when the in-cycle average power Pave is 100 [W], the switching period T is 5.3 [msec] and the melting time Tf is 430 [msec]. For example, when the in-cycle average power Pave is 400 [W], the switching period T is 1.33 [msec] and the melting time Tf is 110 [msec]. Thus, the larger the in-cycle average power Pave, the shorter the melting time Tf. In other words, the shorter the switching period T, the shorter the melting time Tf.

[0050] FIG. 10 shows an example of experimental results for the melting time Tf when the average power Pave within a cycle is changed from 30 [W] to 450 [W]. For example, as the average power Pave within a cycle decreases, the melting time Tf increases more rapidly. The heat from the heating element 22 is transferred not only to the fusible element 21 but also to the surrounding circuit board patterns, cell tabs, bus bars, etc. of the protection circuit 20. For example, if the melting time Tf exceeds 1 [sec.], the melting time Tf becomes longer due to the influence of the thermal capacity of the circuit board patterns, cell tabs, and bus bars. Therefore, in this example, the melting time Tf was set to be within 1 [sec.].

[0051] FIG. 11 shows an example of setting the switching period T according to the storage battery voltage VB. This example shows the switching period T in the voltage range W (FIG. 5) of the storage battery voltage VB. Note that when the storage battery voltage VB is 8.9 [V] or less, even if a voltage is applied continuously, the applied power is smaller than the rated power of the heating element 22, so PFM control is not performed. Therefore, when the storage battery voltage VB is 8 [V], the switching period T and switching frequency are indicated by "-". When PFM control is performed, the switching period T is calculated using the following equation EQ2. Switching period T = (battery voltage VB) 2 / resistance value of heating element 22 / Average power within a cycle Pave × On time Ton (EQ2)

[0052] When the battery voltage VB is 8 V, the blowout time Tf is 420 msec. Furthermore, in the voltage range of 10 V to 21.5 V where PFM control is performed and the battery voltage VB is higher, the blowout time Tf becomes longer. The blowout time Tf is kept within 1 sec. throughout the entire voltage range W of the battery voltage VB. Therefore, in this example, the control circuit 12 can perform PFM control using the switching period T shown in FIG. 11.

[0053] (PFM control parameter setting procedure) 12 shows an example of a procedure for setting parameters for PFM control. For example, an engineer developing the battery pack 1 can set parameters for PFM control according to this procedure.

[0054] First, the engineer determines the lower limit voltage V21 and the upper limit voltage V22 of the storage battery voltage VB as shown in (1) above (step S201).

[0055] Next, the engineer selects a protection circuit 20 that can blow out the fusible element 21 when the storage battery voltage VB is the lower limit voltage V21 (step S202). That is, when the storage battery voltage VB is low, the power applied to the heating element 22 is small, and the heating element 22 may not be able to supply the fusible element 21 with enough heat to blow out the fusible element 21. Therefore, the engineer selects a protection circuit 20 that can blow out the fusible element 21 when the storage battery voltage VB is the lower limit voltage V21.

[0056] Next, as shown in (2) above, the engineer applies an upper limit voltage V22 to the protection circuit 20 and measures the failure time until the heating element 22 fails (step S203), and determines the on-time Ton using equation EQ1 and the measurement results at the upper limit voltage V22 (step S204).

[0057] Next, as shown in (3) above, the engineer sets the average power Pave within the period to a value equal to the rated power of the heating element 22 (step S205), and calculates the switching period T using equation EQ2 for various storage battery voltages VB that are to be PFM controlled, and measures the melting time Tf (step S206).

[0058] Next, the engineer checks whether all of the melting times Tf measured in step S206 are within 1 [sec.] (step S207). If there is data showing that the melting time Tf is longer than 1 [sec.] ("N" in step S207), the engineer sets the average power Pave within the cycle to a higher value (step S208). The process returns to step S206. The engineer repeats the processes of steps S206 to S208 until all of the melting times Tf are within 1 [sec.].

[0059] If all of the blowout times Tf are within 1 [sec.] ("Y" in step S207), this procedure ends.

[0060] Using this procedure, the engineer determines the various parameters used in PFM control. Then, for example, the engineer implements Equation EQ2 in the control circuit 12 so that the control circuit 12 uses Equation EQ2 to calculate the switching period T based on the storage battery voltage VB. In Equation EQ2, the resistance value of the heating element 22, the average power within the period Pave, and the on-time Ton are fixed values. Therefore, the switching period T can be expressed by the following Equation EQ3 using a constant C. Switching period T = C × (battery voltage VB) 2 ···(EQ3) Here, the constant C is expressed by the following equation EQ4. C = On time Ton / Resistance of heating element 22 / Average power within the cycle Pave ···(EQ4)

[0061] In this example, the control circuit 12 calculates the switching period T based on the storage battery voltage VB using the equation EQ2, but this is not limiting. Alternatively, for example, the control circuit 12 may store a lookup table showing the relationship between the storage battery voltage VB calculated based on the equation EQ2 and the switching period T, and use this lookup table to calculate the switching period T based on the storage battery voltage VB.

[0062] (Comparison of PFM control and PWM control) When the use of the battery pack 1 should be stopped, the control circuit 12 generates a control signal CTL by performing PFM control based on the storage battery voltage VB, and uses this control signal CTL to control the switching operation of the transistor 13. This reduces the possibility of failure of the heating element 22 and more reliably melts down the fusible element 21 compared to, for example, when PWM (Pulse Width Modulation) control is performed. A comparison between PFM control and PWM control will be described below.

[0063] Fig. 13 shows an example of the control signal CTL when PWM control is performed. In this example, as with the case of PFM control (Fig. 8), a voltage of 21.5 [V] is applied to the heating element 22. Fig. 13(A) shows an example of the waveform when the in-cycle average power Pave is set to 100 [W], and Fig. 13(B) shows an example of the waveform when the in-cycle average power Pave is set to 400 [W].

[0064] To set the average power Pave within a cycle to 100 [W], the duty ratio is set to 18.9% (= 1 / 5.3), as shown in Figure 13(A). In other words, since the applied power is 530 [W], the average power Pave within a cycle can be set to 100 [W] (= 530 [W] x 1 / 5.3). As with PFM control (Figure 8), if the switching cycle is 5.3 [msec], the on time Ton is 1 [msec] and the off time Toff is 4.3 [msec]. The waveform in Figure 13(A) is the same as the waveform in Figure 8(A).

[0065] To set the average power Pave within a period to 400 [W], the duty ratio is set to 75.2% (= 1 / 1.33) as shown in FIG. 13(B). Since the applied power is 530 [W], the average power Pave within a period can be set to 400 [W] (= 530 [W] × 1 / 1.33). Since the switching period is fixed at 5.3 [msec], the on-time Ton is 4 [msec] and the off-time Toff is 1.3 [msec]. The waveform in FIG. 13(B) resembles a waveform in which the four pulses in the waveform in FIG. 8(B) are combined into one. In other words, in the case of PFM control (FIG. 8(B)), four pulses are used to apply power to the heating element 22, whereas in the case of PWM control (FIG. 13(B)), a single pulse is used to apply all of the power to the heating element 22.

[0066] Fig. 14 shows an example of experimental results of the melting time Tf of the fusible element 21 when the average power within the cycle Pave is changed from 100 [W] to 450 [W]. This Fig. 14 corresponds to Fig. 9, which shows the case of PFM control. For example, when the average power within the cycle Pave is 100 [W], the on-time Ton is 1 [msec.] and the melting time Tf is 448 [msec.]. As with PFM control, the larger the average power within the cycle Pave, the shorter the melting time Tf.

[0067] However, with this PWM control, when the average power Pave within a cycle exceeds 350 [W], for example, the resistance value of the heating element 22 increases, causing the heating element 22 to fail, and the fusible element 21 does not melt. For example, as shown in Figure 6, when a power of 400 [W] is continuously applied to the heating element 22, the failure time is 43 [msec.], but in the example of Figure 13 (B), the heating element 22 fails even though the on-time Ton is 4 [msec.]. This is thought to be because, with PWM control, damage to the heating element 22 accumulates due to the repeated pulses.

[0068] Thus, in PWM control, as shown in FIG. 14, when the intra-cycle average power Pave exceeds 350 [W], the heating element 22 fails. On the other hand, in PFM control, as shown in FIG. 9, even if the intra-cycle average power Pave exceeds 350 [W], the heating element 22 does not fail and the fusible element 21 can be fused. For example, when the intra-cycle average power Pave is 400 [W], comparing the waveform of the control signal CTL in PFM control (FIG. 8(B)) with the waveform of the control signal CTL in PWM control (FIG. 13(B)), the intra-cycle average power Pave is the same, but the on-time Ton is different. In other words, in the case of PFM control (FIG. 8(B)), on-off is repeated more frequently than in the case of PWM control (FIG. 13(B)), so it is thought that the accumulation of damage is smaller.

[0069] As described above, when PFM control is performed, the possibility of failure of the heating element 22 can be reduced compared to when PWM control is performed, and the fusible element 21 can be blown out more reliably.

[0070] As described above, the battery pack 1 includes the storage battery 11 provided in a path connecting the positive electrode EP and the negative electrode EN and having five battery cells BC1 to BC5 connected in series, the monitoring circuit 30 capable of detecting the cell voltage VBC of each of the five battery cells BC1 to BC5, the positive terminal TP connected to the positive electrode EP via a first path (power line PL1), the negative terminal TN connected to the negative electrode EN via a second path (power line PL2), the fusible element 21 provided in the first path (power line PL1) and capable of being blown out by heat, and the power supply 22 connected between the first path (power line PL1) and the second path (power line PL2). The battery pack further includes a heating element 22 provided in a third path connecting the first path (power line PL2) and the second path (power line PL3) and capable of generating heat to blow a fusible element 21, a transistor 13 provided in the third path, and a control circuit 12 capable of controlling the switching operation of the transistor 13 by performing PFM control to maintain the on-time Ton of the transistor 13 at a predetermined time based on the detection results of a monitoring circuit 30 and to vary the off-time Toff of the transistor 13 in accordance with a battery voltage VB, which is the voltage across the storage battery 11. This allows the power applied to the heating element 22 to be adjusted in accordance with, for example, the battery voltage VB, thereby allowing the fusible element 21 to blow in accordance with various battery voltages VB. As a result, the charge / discharge path can be blocked in various states in which the battery pack should not be used.

[0071] For example, the technology described in Patent Document 1 can blow the fusible element when the battery voltage VB is high, but has difficulty blowing the fusible element when the battery voltage VB is low. On the other hand, the battery pack 1 varies the off-time Toff of the transistor 13 depending on the battery voltage VB. For example, the control circuit 12 can set the off-time Toff to a first time when the battery voltage VB is a first voltage, and set the off-time Toff to a second time longer than the first time when the battery voltage VB is a second voltage higher than the first voltage. This allows the battery pack 1 to blow the fusible element 21 even when the battery voltage VB is low. As a result, the battery pack 1 can cut off the charge / discharge path in various states in which the battery pack should not be used.

[0072] Furthermore, since the battery pack 1 is designed to perform PFM control, the possibility of failure of the heating element 22 can be reduced compared to, for example, when PWM control is used, and the fusible element 21 can be more reliably melted.

[0073] Furthermore, in the battery pack 1, the control circuit 12 is capable of performing PFM control when any one or more of the five cell voltages VBC1 to VBC5 is higher than a first threshold voltage (voltage V12). Furthermore, the control circuit 12 is capable of performing PFM control when any one or more of the five cell voltages VBC1 to VBC5 is lower than a second threshold voltage (voltage V11). Furthermore, the monitoring circuit 30 detects the temperature of the storage battery 11, and the control circuit 12 is capable of performing PFM control when the temperature of the storage battery 11 is outside a predetermined temperature range. Furthermore, the control circuit 12 is capable of determining whether the monitoring circuit 30 is operating normally based on the detection result of the monitoring circuit 30, and is capable of performing PFM control when the monitoring circuit 30 is not operating normally. In this way, the battery pack 1 can cut off the charge / discharge path in various states in which the battery pack should not be used.

[0074] Furthermore, in the battery pack 1, the fuse time Tf, which is the time from when the control circuit 12 starts PFM control until the fusible element 21 fuses, is set to within one second. This makes it possible to reduce the influence of the board pattern, cell tabs, and bus bars around the protection circuit 20 on the fuse time Tf, thereby shortening the fuse time Tf.

[0075] [effect] As described above, this embodiment includes a storage battery having five battery cells connected in series and provided in a path connecting the positive and negative electrodes; a monitoring circuit capable of detecting the cell voltages of each of the five battery cells; a positive terminal connected to the positive electrode via a first path; a negative terminal connected to the negative electrode via a second path; a fusible element provided in the first path and capable of melting due to heat; a heating element provided in a third path connecting the first path and the second path and capable of melting the fusible element by generating heat; a transistor provided in the third path; and a control circuit capable of controlling the switching operation of the transistor by performing PFM control to keep the on time of the transistor at a predetermined time based on the detection result of the monitoring circuit and to change the off time of the transistor in accordance with the battery voltage, which is the voltage across the battery.Therefore, the charge / discharge path can be blocked in various states in which the battery pack should not be used.

[0076] In this embodiment, PFM control is performed, so that the fusible element can be blown out more reliably than in the case of, for example, PWM control.

[0077] [Variation 1] In the above embodiment, as shown in FIG. 15A, a protection circuit 20 including a fusible element 21 and a heating element 22 is provided, but this is not limited thereto. Alternatively, for example, as shown in FIG. 15B, a protection circuit 20A including a fusible element 23 and a heating element 22 may be provided. One end of the fusible element 23 is connected to the positive electrode EP of the storage battery 11 and one end of the heating element 22, and the other end of the fusible element 23 is connected to the positive terminal TP of the battery pack 1. Alternatively, for example, as shown in FIG. 15C, a protection circuit 20B including fusible elements 21, 23, and a heating element 22 may be provided. One end of the fusible element 21 is connected to the positive electrode EP of the storage battery 11, and the other end of the fusible element 21 is connected to one end of the heating element 22 and one end of the fusible element 23. One end of the fusible element 23 is connected to the other end of the fusible element 21 and one end of the heating element 22, and the other end is connected to the positive terminal TP of the battery pack 1.

[0078] [Variation 2] Although the above embodiment has been described with reference to a single temperature sensor 31, this is not limiting. Alternatively, multiple temperature sensors may be provided, as in the battery pack 1C shown in FIG. 16. This battery pack 1C includes a monitoring circuit 30C and a control circuit 12C. The monitoring circuit 30C includes temperature sensors 31 and 33 and an analog front-end circuit 32C. In this example, the temperature sensor 33 is provided on a circuit board (not shown) and is configured to detect the temperature of the circuit board. The temperature sensor 33 supplies the detection result to the analog front-end circuit 32C. The analog front-end circuit 32C is configured to supply the detection results of the cell voltages VBC1 to VBC5 and the detection results of the temperature sensors 31 and 33 to the control circuit 12C. For example, the control circuit 12C determines that use of the battery pack 1C should be stopped if the temperature of the circuit board is outside a predetermined temperature range.

[0079] [Variation 3] In the above embodiment, the control circuit 12 determines that use of the battery pack 1 should be stopped when any one or more of the battery cells BC1 to BC5 in the storage battery 11 is in the overcharge state S3, when any one or more of the battery cells BC1 to BC5 in the storage battery 11 is in the overdischarge state S1, when the temperature of the storage battery 11 is outside a predetermined temperature range, or when the monitoring circuit 30 is not operating normally, but this is not limited to these. The cases in which use of the battery pack 1 should be stopped are not limited to these four, and some of these four cases may be omitted, or other cases may be present.

[0080] [Other variations] Two or more of these variations may also be combined.

[0081] Although the present technology has been described above by giving embodiments, the present technology is not limited to these embodiments and can be modified in various ways.

[0082] For example, in the above-described embodiments, as shown in FIG. 1, five battery cells BC are provided, but this is not limited to this, and four or fewer battery cells BC or six or more battery cells BC may be provided.

[0083] The effects described in this specification are merely examples, and the effects of the present disclosure are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present disclosure.

[0084] Furthermore, the present disclosure may take the following aspects. <1> a storage battery provided in a path connecting the first terminal and the second terminal and having a plurality of battery cells connected in series; a monitoring circuit capable of detecting the cell voltage of each of the plurality of battery cells; a first connection terminal connected to the first terminal via a first path; a second connection terminal led to the second terminal via a second path; a fusible element provided in the first path and capable of being blown out by heat; a heating element that is provided in a third path connecting the first path and the second path and that generates heat to melt the fusible element; a switch element provided in the third path; A control circuit capable of controlling the switching operation of the switch element by performing PFM control to maintain the on-time of the switch element at a predetermined time based on the detection result of the monitoring circuit and to vary the off-time of the switch element in accordance with the storage battery voltage, which is the voltage between both ends of the storage battery. A battery pack equipped with <2> The control circuit When the storage battery voltage is a first voltage, the off time can be set to a first time; When the storage battery voltage is a second voltage higher than the first voltage, the off time can be set to a second time longer than the first time. <1> The battery pack described. <3> The control circuit is capable of performing the PFM control when any one or more of the plurality of cell voltages is higher than a first threshold voltage. <1> or <2> The battery pack described. <4> the control circuit is further capable of performing the PFM control when any one or more of the plurality of cell voltages is lower than a second threshold voltage; The second threshold voltage is lower than the first threshold voltage. <3> The battery pack described. <5> The control circuit is capable of performing the PFM control when any one or more of the plurality of cell voltages is lower than a second threshold voltage. <1> or <2> The battery pack described. <6> the monitoring circuit is further capable of detecting a temperature of the storage battery; The control circuit is further capable of performing the PFM control when the temperature of the storage battery is outside a predetermined temperature range. <1> from <4> 10. The battery pack according to claim 9, wherein: <7> The control circuit is further capable of determining whether the monitoring circuit is operating normally based on the detection result of the monitoring circuit, and is capable of performing the PFM control when the monitoring circuit is not operating normally. <1> from <4> 10. The battery pack according to claim 9, wherein: <8> The fuse-opening time from when the control circuit starts the PFM control to when the fuse element blows out is within 1 second. <1> from <7> 10. The battery pack according to claim 9, wherein: <9> The control circuit is capable of performing the PFM control by calculating the switching period T of the switch element based on the voltage value V of the storage battery voltage using equation (1). <1> from <8> 10. The battery pack according to claim 9, wherein: T = C × V 2 ···(1) where C is a predetermined constant <10> The control circuit is capable of performing the PFM control by calculating the switching period T based on the voltage value V of the storage battery voltage using table data indicating the relationship between the voltage value V of the storage battery voltage and the switching period T of the switch element, the relationship being calculated using the following equation (1): <1> from <8> 10. The battery pack according to claim 9, wherein: T = C × V 2 ···(1) <11> a battery pack including: a storage battery having a plurality of battery cells connected in series and provided in a path connecting a first terminal and a second terminal; a first connection terminal led to the first terminal via a first path; a second connection terminal led to the second terminal via a second path; a fusible element provided in the first path and capable of being melted by heat; a heating element provided in a third path connecting the first path and the second path and capable of melting the fusible element by generating heat; and a switch element provided in the third path; detecting a cell voltage of each of the plurality of battery cells; Based on the detected cell voltages, the on-time of the switch element is maintained at a predetermined time, and PFM control is performed so that the off-time is changed according to the battery voltage, which is the voltage across the battery, thereby controlling the operation of the switch element. Contains How to control the battery pack. [Explanation of symbols]

[0085] 1,1C...battery pack, 11...storage battery, 12,12C...control circuit, 13...transistor, 20,20A,20B...protection circuit, 21,23...fusible element, 22...heating element, 30,30C...monitoring circuit, 31,33...temperature sensor, 32,32C...analog front-end circuit, BC,BC1,BC2,BC3,BC4,BC5...battery cell, CTL...control signal, EN...negative electrode, EP...positive electrode, Pave...average power within a cycle, PL1,PL2...power line, S1...overdischarge state, S2...normal state, S3...overcharge state, T...switching cycle, Tf...fusing time, TN...negative terminal, Ton...on time, Toff...off time, TP...positive terminal, VB...storage battery voltage, V1 to V5, V11, V12...voltage, V21...lower limit voltage, V22...upper limit voltage, W...voltage range.

Claims

1. a storage battery having a plurality of battery cells connected in series and provided in a path connecting the first terminal and the second terminal; a monitoring circuit capable of detecting the cell voltage of each of the plurality of battery cells; a first connection terminal led to the first terminal via a first path; a second connection terminal led to the second terminal via a second path; a fusible element provided in the first path and capable of being blown out by heat; a heating element that is provided in a third path connecting the first path and the second path and that generates heat to melt the fusible element; a switch element provided in the third path; a control circuit that can control the switching operation of the switch element by performing PFM control to keep the on time of the switch element at a predetermined time based on the detection result of the monitoring circuit and change the off time of the switch element in accordance with the storage battery voltage, which is the voltage between both ends of the storage battery; Equipped with The control circuit When the storage battery voltage is a first voltage, the off time can be set to a first time; When the storage battery voltage is a second voltage higher than a first voltage, the off time can be set to a second time longer than the first time. Battery pack.

2. The control circuit is capable of performing the PFM control when any one or more of the plurality of cell voltages is higher than a first threshold voltage. The battery pack according to claim 1 .

3. the control circuit is further capable of performing the PFM control when any one or more of the plurality of cell voltages is lower than a second threshold voltage; The second threshold voltage is lower than the first threshold voltage. The battery pack according to claim 2 .

4. The control circuit is capable of performing the PFM control when any one or more of the plurality of cell voltages is lower than a second threshold voltage. The battery pack according to claim 1 .

5. the monitoring circuit is further capable of detecting a temperature of the storage battery; The control circuit is further capable of performing the PFM control when the temperature of the storage battery is outside a predetermined temperature range. The battery pack according to claim 1 .

6. The control circuit can further determine whether the monitoring circuit is operating normally by checking whether the detection result of the monitoring circuit is within a predetermined range, and can perform the PFM control when the monitoring circuit is not operating normally. The battery pack according to claim 1 .

7. The fuse-opening time from when the control circuit starts the PFM control to when the fuse element blows out is within 1 second. The battery pack according to claim 1 .

8. The control circuit is capable of performing the PFM control by calculating the switching period T of the switch element based on the voltage value V of the storage battery voltage using the following equation (1): The battery pack according to claim 1 . T = C × V ２ ・・・(1) where C is a predetermined constant

9. The control circuit is capable of performing the PFM control by calculating the switching period T based on the voltage value V of the storage battery voltage using table data indicating the relationship between the voltage value V of the storage battery voltage and the switching period T of the switch element, the relationship being calculated using the following equation (1): The battery pack according to claim 1 . T = C × V ２ ・・・(1)

10. a battery pack including: a storage battery having a plurality of battery cells connected in series and provided in a path connecting a first terminal and a second terminal; a first connection terminal led to the first terminal via a first path; a second connection terminal led to the second terminal via a second path; a fusible element provided in the first path and capable of being blown out by heat; a heating element provided in a third path connecting the first path and the second path and capable of blowing out the fusible element by generating heat; and a switch element provided in the third path; detecting a cell voltage of each of the plurality of battery cells; Based on the detected cell voltages, the on-time of the switch element is maintained at a predetermined time, and PFM control is performed so that the off-time is changed in accordance with the storage battery voltage, which is the voltage across the storage battery, thereby controlling the operation of the switch element. Including, The PFM control includes setting the off time to a first time when the storage battery voltage is a first voltage, and setting the off time to a second time longer than the first time when the storage battery voltage is a second voltage higher than the first voltage. How to control the battery pack.

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