Battery output blocking circuit and charge / discharge control circuit having same

Abstract

Proposed is a battery output blocking circuit for blocking battery output and a charge / discharge control circuit having same, the battery output blocking circuit comprising: a three-terminal fuse provided on an output path between a battery and a battery output terminal; and an NTC thermistor having one end connected to the three-terminal fuse and the other end connected to a ground terminal.

Description

Battery output cut-off circuit and charge / discharge control circuit including the same

[0001] The present invention relates to a battery output cutoff circuit, and more particularly, to a battery output cutoff circuit that cuts off the output of a battery when the battery overheats, and a charge / discharge control circuit having the same.

[0002] Rechargeable secondary cells, or batteries, are widely used as energy sources for mobile devices such as smartphones. Furthermore, batteries are also being used as energy sources for eco-friendly vehicles, such as electric and hybrid electric vehicles, which are being proposed as a solution to air pollution caused by fossil fuel-powered gasoline and diesel vehicles. The types of applications utilizing batteries are becoming increasingly diverse, and batteries are expected to be applied to a wider range of fields and products in the future.

[0003] Currently commercially available batteries include nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, and lithium-ion batteries. Among these, lithium-ion batteries are gaining attention for their advantages, including virtually no memory effect compared to nickel-based batteries, allowing for easy charging and discharging, a very low self-discharge rate, and high energy density. Furthermore, lithium-ion batteries can be manufactured in small, lightweight designs, making them ideal power sources for mobile devices and expanding their use as power sources for electric vehicles, drawing attention as a next-generation energy storage medium. However, lithium-ion batteries are more active than other batteries, which makes them somewhat less safe.

[0004] These batteries are typically used in the form of battery packs rather than as individual cells. A battery pack comprises at least one battery module, which may be comprised of multiple battery cells. Furthermore, a battery management system (BMS) is provided to manage the overall health of the battery cells, modules, or pack. The BMS monitors battery performance, status, and performs diagnostics to ensure stable battery operation.

[0005] Meanwhile, as battery applications expand, battery safety is emerging as a critical issue. For example, the number of users of laptops, mobile phones, and electric vehicles is rapidly increasing. Battery explosions not only cause product damage but can also lead to personal injury or fire, making battery safety urgent. In particular, products utilizing highly active lithium-ion batteries require a protective circuit to protect the battery or electrical devices connected to it in the event of a battery malfunction.

[0006] These protection circuits are cutoff circuits that sever the electrical connection between the battery and the electrical device, thereby shutting off the battery's output. For example, the cutoff circuit can utilize a fuse connected between the battery's output terminal and the electrical device. The fuse can blow when the battery overheats, such as when the battery cells are overcharged or overdischarged. To blow the fuse, a temperature sensor can be used to detect the battery's temperature, or a current sensor or voltage sensor can be used to detect overcharge or overdischarge of the battery cells.

[0007] However, if the component that detects the battery's temperature (i.e., the temperature sensor) or the component that detects overcharge or overdischarge of the battery cells (i.e., the voltage sensor or current sensor) malfunctions or fails to function properly, the fuse may not blow. Consequently, even if the battery overheats, the connection between the battery and the electrical device may not be sever, preventing overheating of the electrical device.

[0008] Related prior art inventions include the following:

[0009] Patent Document 1: Korean Patent No. 10-1709540 (February 17, 2017)

[0010] Patent Document 2: Japanese Patent Publication No. JP 2003-297206 A (October 17, 2003)

[0011]

[0012] The present invention proposes a battery output cut-off circuit capable of blowing a fuse when a battery overheats even in the event of a failure of a component related to temperature detection, and a charge / discharge control circuit including the same.

[0013] The present invention proposes a battery output cut-off circuit capable of melting and cutting a fuse at high temperatures using a three-terminal fuse and a thermistor, and a charge / discharge control circuit including the same.

[0014]

[0015] A battery output cutoff circuit according to one embodiment of the present invention is a battery output cutoff circuit that cuts off battery output, and includes a three-terminal fuse provided in an output path between a battery and a battery output terminal, and an NTC thermistor having one end connected to the three-terminal fuse and the other end connected to a ground terminal.

[0016] The three-terminal fuse includes a first fuse having one end connected to the battery side, a second fuse having one end connected to the other end of the first fuse and the other end connected to the battery output terminal, and a fuse resistor having one end connected between the first and second fuses.

[0017] The above NTC thermistor has one end connected to the fuse resistor and the other end connected to a ground terminal.

[0018] The resistance value of the above fuse resistor is set so that the combined resistance value with the resistance value of the NTC thermistor at the set output cut-off temperature is not less than the fuse rupture current flowing through the first fuse or the second fuse.

[0019]

[0020] According to another embodiment of the present invention, a charge / discharge control circuit includes: a battery having a plurality of battery cells; a battery output cutoff circuit connected to the battery and including a three-terminal fuse and an NTC thermistor to cut off the output of the battery depending on temperature; a switching unit having one end connected to the battery output cutoff circuit and the other end connected to a battery output terminal to set a charge / discharge path; a first control unit measuring a state of the battery; and a second control unit controlling the switching unit depending on charge / discharge of the battery.

[0021] The above three-terminal fuse includes a first fuse having one end connected to the battery side, a second fuse having one end connected to the other end of the first fuse and the other end connected to the switching unit, and a fuse resistor having one end connected between the first and second fuses, and the NTC thermistor has one end connected to the fuse resistor and the other end connected to a ground terminal.

[0022] The NTC thermistor has a lower resistance at a temperature higher than a set output cut-off temperature, so that the current flowing through the fuse resistor increases, and the fuse resistor is heated according to the increase in current, thereby melting and cutting at least one of the first and second fuses.

[0023] The resistance value of the above fuse resistor is set so that the combined resistance value with the resistance value of the NTC thermistor at the set output cut-off temperature is not less than the fuse rupture current flowing through the first fuse or the second fuse.

[0024]

[0025] A battery output cut-off circuit according to embodiments of the present invention may include a three-terminal fuse provided between a battery and a battery output terminal, and an NTC thermistor provided between the three-terminal fuse and a ground terminal. The three-terminal fuse may include first and second fuses connected between the battery and the battery output terminal, and a fuse resistor having one end connected to a connection point of the first and second fuses, and the NTC thermistor is connected between the other end of the fuse resistor and the ground terminal.

[0026] In the battery output cutoff circuit according to embodiments of the present invention, when the NTC thermistor is exposed to high temperatures, the resistance of the NTC thermistor decreases, and as the resistance of the NTC thermistor decreases, the fuse resistance, i.e., the current flow in the heater section increases. Accordingly, the fuse resistance generates heat, thereby blowing at least one of the first and second fuses, thereby cutting off the battery output.

[0027] Therefore, the present invention can protect the system by permanently shutting off the fuse in high-temperature conditions even if a problem occurs in a component or circuit related to temperature sensing. Furthermore, the present invention can be used as a redundant circuit to address pin shortages or MCU signal errors (single faults).

[0028]

[0029] FIG. 1 is a circuit diagram of a battery output cut-off circuit according to one embodiment of the present invention.

[0030] FIG. 2 is a schematic diagram for explaining the operation of a battery output cut-off circuit according to one embodiment of the present invention.

[0031] Figure 3 is a configuration diagram of a battery charge / discharge control circuit according to another embodiment of the present invention.

[0032]

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms. These embodiments are provided solely to ensure complete disclosure of the present invention and to fully inform those of ordinary skill in the art of the scope of the invention.

[0034] FIG. 1 is a circuit diagram of a battery output cutoff circuit according to an embodiment of the present invention, and FIG. 2 is a schematic diagram for explaining the operation of a battery output cutoff circuit according to an embodiment of the present invention.

[0035] Referring to FIG. 1, a battery output cut-off circuit according to one embodiment of the present invention may include a three-terminal fuse (10) connected on an output path between a battery and a battery output terminal, and a thermistor (20) having one end connected to a control terminal of the three-terminal fuse (10) and the other end connected to a ground terminal.

[0036] A three-terminal fuse (10) is connected to the output path between the battery and the output terminal to block the flow of current when an abnormal situation occurs. That is, the three-terminal fuse (10) is cut off when the battery overheats, that is, the temperature of the battery is higher than the set temperature, thereby blocking the output path between the battery and the output terminal. This three-terminal fuse (10) may include a first fuse (11) having one end connected to the battery, a second fuse (12) having one end connected to the other end of the first fuse (11) and the other end connected to the output terminal, and a fuse resistor (13) having one end connected between the first fuse (11) and the second fuse (12).

[0037] In this three-terminal fuse (10), an output path is formed between the battery and the battery output terminal through the first and second fuses (11, 12) in a normal state where the temperature of the battery is lower than the set temperature. However, in a high-temperature state where the temperature of the battery is higher than the set temperature, at least one of the first and second fuses (11, 12) is blown by the fuse resistor (13) to cut off the output path. That is, when the battery overheats, the fuse resistor (13) of the three-terminal fuse (10) heats, and when the fuse resistor (13) is heated above a predetermined temperature, at least one of the first and second fuses (11, 12) is blown to cut off the connection. At this time, the fuse resistor (13) is not heated when a current below the set value flows from the battery or the temperature of the battery is lower than the set temperature, and the output of the battery is transmitted to the output terminal through the first and second fuses (11, 12). However, when a current exceeding a set value flows from the battery or the temperature of the battery is higher than the set temperature, the fuse resistor (13) is heated, and when the fuse resistor (13) is heated above the set temperature, at least one of the first and second fuses (11, 12) is melted to block the output path between the battery and the output terminal. Meanwhile, the fuse resistor (13) is set so that the combined resistance value with the resistance value of the thermistor (20) at the set predetermined output blocking temperature is not smaller than the fuse rupture current of the current value flowing through the first fuse (11) or the second fuse (12).

[0038] The thermistor (20) has one end connected to the fuse resistor (13) and the other end connected to the ground terminal. This thermistor (20) is a device that has a characteristic in which the resistance value changes greatly according to a change in temperature by adding impurities such as cobalt, copper, manganese, nickel, and titanium to a ceramic material. The thermistor (20) of the present invention may be an NTC (Negative Temperature Coefficient) thermistor that has a characteristic of a negative temperature coefficient in which the resistance decreases as the temperature increases. That is, the NTC thermistor has a high initial resistance value, and as current flows, the thermistor temperature increases and the resistance value decreases accordingly.

[0039] According to the present invention, a battery output cut-off circuit including a three-terminal fuse (10) and an NTC thermistor (20) has a structure in which, as illustrated in FIG. 2, when an NTC thermistor (20) having an initial high resistance value is exposed to high temperature, the resistance of the NTC thermistor (20) decreases. When the resistance of the NTC thermistor (20) decreases, the current flow to the fuse resistor (13) increases, and accordingly, the fuse resistor (13) generates heat. When the fuse resistor (13) generates heat and the temperature of the fuse resistor (13) rises above a predetermined temperature, at least one of the first and second fuses (11, 12) is blown, and accordingly, the output path between the battery and the output terminal is cut off.

[0040]

[0041] As described above, a battery output cut-off circuit according to an embodiment of the present invention may include a three-terminal fuse (10) provided between a battery and a battery output terminal, and an NTC thermistor (20) provided between the three-terminal fuse (10) and a ground terminal. The three-terminal fuse (10) may include first and second fuses (11, 12) connected between the battery and the battery output terminal, and a fuse resistor (13) having one end connected to a connection point of the first and second fuses (11, 12), and the NTC thermistor (20) is connected between the other end of the fuse resistor (13) and the ground terminal.

[0042] In the battery output cut-off circuit according to embodiments of the present invention, when the NTC thermistor (20) that initially maintains high resistance is exposed to high temperature, the resistance of the NTC thermistor (20) decreases, and as the resistance of the NTC thermistor (20) decreases, the current flow of the fuse resistor (13), i.e., the heater part, increases. Accordingly, the fuse resistor (13) generates heat and, accordingly, melts at least one of the first and second fuses (11, 12), thereby cutting off the output of the battery.

[0043] According to one embodiment of the present invention, a battery output cutoff circuit can permanently cut off the power source by melting and cutting the fuse at high temperatures using the characteristics of a three-terminal fuse (10) and an NTC thermistor (20) without a separate signal for diagnosing the battery. Therefore, even if a problem occurs in a component or circuit related to temperature detection, the fuse can be permanently cut off in high-temperature conditions, thereby protecting the system. In addition, the circuit can be used as a redundant circuit for cases of pin shortage of a temperature sensor or MCU signal error (single fault).

[0044]

[0045] FIG. 3 is a drawing for explaining a battery charge / discharge control circuit according to another embodiment of the present invention having a battery output cut-off circuit according to the present invention.

[0046] Referring to FIG. 3, a charge / discharge control circuit according to another embodiment of the present invention may include a battery output cutoff circuit (200) provided in an output path between a battery (100) and a battery output terminal, i.e., a load, a switching unit (300) provided between the battery output cutoff circuit (200) and the battery output terminal, a first control unit (400) connected to the battery (100) to measure the state of the battery, and a second control unit (500) that controls the switching unit (300).

[0047] 1. Battery

[0048] A battery (100) is an electrical energy source that can be charged and discharged and provides energy to an electric device to drive the electric device. Here, the battery (100) may include at least one battery pack, and each of the at least one battery packs may include a plurality of battery modules, and the battery modules may include a plurality of rechargeable and dischargeable battery cells. That is, the battery (100) includes a plurality of battery cells, and the plurality of battery cells may be grouped into predetermined units to form a battery module, and the plurality of battery modules may form a single battery pack. Here, an embodiment of the present invention may include at least one battery pack. Meanwhile, the plurality of battery cells may be connected in series and / or in parallel in various ways to meet the specifications of the electric device. Of course, a plurality of battery packs each including a plurality of battery cells may also be connected in series and / or in parallel. Here, the battery cell may include a lithium ion battery. However, the battery cell may also be configured as not only a lithium ion battery, but also a lithium polymer battery, a nickel cadmium battery, a nickel metal hydride battery, a nickel zinc battery, etc.

[0049] 2. Battery output cut-off circuit

[0050] The battery output cut-off circuit (200) may include a three-terminal fuse (10) connected on an output path between a battery (100) and a battery output terminal, and a thermistor (20) having one end connected to a control terminal of the three-terminal fuse (10) and the other end connected to a ground terminal. The three-terminal fuse (10) is connected on an output path between the battery and the output terminal so as to cut off the flow of current when an abnormal situation occurs. That is, the three-terminal fuse (10) is cut off when the battery overheats, i.e., the temperature of the battery is higher than a set temperature, thereby cutting off the output path between the battery and the output terminal. This three-terminal fuse (10) may include a first fuse (11) whose end is connected to a battery, a second fuse (12) whose end is connected to the other end of the first fuse (11) and whose other end is connected to an output terminal so that the output of the battery passing through the first fuse (11) is output to the other end, and a fuse resistor (13) whose end is connected to a fuse control terminal connected between the first fuse (11) and the second fuse (12). A thermistor (20) has one end connected to the fuse resistor (13) and the other end connected to a ground terminal. The thermistor (20) of the present invention may be an NTC (Negative Temperature Coefficient) thermistor having a characteristic of a negative temperature coefficient in which resistance decreases as temperature increases. That is, the NTC thermistor has an initial high resistance value, and as current flows, the thermistor temperature increases and the resistance value decreases accordingly.

[0051] In this battery output cut-off circuit (200), when a current below a set value flows from the battery or the temperature of the battery is below a set temperature, the fuse resistor (13) is not heated and the output of the battery is transmitted to the output terminal through the first and second fuses (11, 12). However, when a current above a set value flows from the battery or the temperature of the battery is above a set temperature, the NTC thermistor (20) is exposed to high temperature and the resistance of the NTC thermistor (20) decreases. When the resistance of the NTC thermistor (20) decreases, the current flow to the fuse resistor (13) increases and the fuse resistor (13) heats up accordingly. When the fuse resistor (13) heats up and the temperature of the fuse resistor (13) rises above a predetermined temperature, at least one of the first and second fuses (11, 12) is blown, thereby cutting off the output path between the battery and the output terminal. At this time, the fuse resistance (13) is set so that the combined resistance value with the resistance value of the thermistor (20) at the set predetermined output cut-off temperature is not less than the fuse rupture current of the current flowing through the first fuse (11) or the second fuse (12).

[0052] 3. Switching section

[0053] The switching unit (300) may include a charge switch (310) and a discharge switch (320). The charge switch (310) and the discharge switch (320) are installed on the charge / discharge path of the battery (100) to selectively block the flow of charge current and discharge current. Accordingly, a charge / discharge path is formed from the battery (100) to the load via the fuses (11, 12) of the battery output cutoff circuit (200), the charge switch (310), and the discharge switch (320).

[0054] A switching unit (300) is provided between a current path between a battery (100) and a load, and controls charging and discharging of the battery (100) by a second control unit (500). This switching unit (300) is provided between the battery (100) and the load, and a charging switch (310) may be provided on the battery (100) side, and a discharging switch (320) may be provided on the load side. The charging and discharging switches (310, 320) are driven according to a control signal generated by the second control unit (500), and may be driven simultaneously during charging and discharging of the battery (100), or one of them may be driven. For example, the charging switch (310) may be driven during charging of the battery (100), and the discharging switch (320) may be driven during discharging of the battery (100). Here, the load may include an external power source for charging the battery (100) and an electronic device equipped with the battery (100) that is driven according to the discharge voltage of the battery (100). That is, the battery (100) may be connected to an external power source when charging and to an electronic device when discharging.

[0055] The charging switch (310) may include a first FET (310a) and a first parasitic diode (310b). The first FET (310a) has a source terminal and a drain terminal provided between the battery (100) and the second FET (320a), and a gate terminal connected to the second control unit (500). Therefore, the first FET (310a) is driven according to a control signal output from the second control unit (500) and serves to supply current to the battery (100) during charging. The first parasitic diode (310b) is connected in parallel to the first FET (310a). That is, the first parasitic diode (310b) is connected in the forward direction between the battery (100) and the second FET (320a). This first parasitic diode (310b) sets a discharge path for the battery (100) when the first FET (310a) is turned off. That is, the battery (100) can be charged through the first FET (310a), and the battery (100) can be discharged through the first parasitic diode (310b).

[0056] The discharge switch (320) may include a second FET (320a) and a second parasitic diode (320b). The second FET (320a) has a source terminal and a drain terminal provided between the first FET (310a) and a load, and a gate terminal connected to the second control unit (500). Therefore, the second FET (320a) is driven according to a control signal output from the second control unit (500), and serves to apply the discharge current of the battery (100) to an electronic device connected thereto during discharge. The second parasitic diode (320b) is connected in parallel to the second FET (320a). That is, the second parasitic diode (320b) is connected in the reverse direction between the first FET (310a) and the load. This second parasitic diode (320b) sets a path for the charging current when charging the battery (100). That is, the battery (100) can be discharged through the second FET (320a), and the battery (100) can be charged through the second parasitic diode (320b).

[0057] In this switching unit (300), a second control unit (500) is connected to the gate terminal of the first FET (310a) and the gate terminal of the second FET (320a), so that the first and second FETs (310a, 320a) are driven respectively according to a control signal output from the second control unit (500).

[0058] 4. First Control Unit

[0059] The first control unit (400) is connected to both ends of the battery (100) and / or both ends of the battery cells included in the battery (100) and can measure a state such as a voltage across the battery (100) or the battery cells. At this time, the first control unit (400) can measure the temperature of the battery (100) and the current of the battery (100) or the battery cells. Meanwhile, the first control unit (400) is electrically connected to a three-terminal fuse (10) and can control the melting of the three-terminal fuse (10). To this end, the first control unit (400) is equipped with a fuse control switch (not shown), and the first control unit (400) can melt the three-terminal fuse (10) through the fuse control switch. At this time, the fuse control switch may be formed of a FET having a parasitic diode included therein. However, the present invention can blow the three-terminal fuse (10) at high temperatures by utilizing the change in resistance according to the temperature change of the three-terminal fuse (10) and the NTC thermistor (20) without providing a separate fuse control switch in the first control unit (400). Of course, the present invention can also be provided with the fuse control switch and the battery output cut-off circuit (200) provided in the first control unit (400) at the same time. In this case, even if the fuse control switch does not function properly due to a breakdown or the like, the battery output cut-off circuit (200) can cut off the battery output according to the temperature change.

[0060] 5. Second Control Unit

[0061] The second control unit (500) controls the charging and discharging of the battery (100) according to the state of the battery cell. For example, the second control unit (500) can control the charging and discharging of the battery (100) according to the voltage of the battery (100) measured by the first control unit (400). That is, the second control unit (500) can control the charging and discharging of the battery (100) by controlling the charging switch (310) and the discharging switch (320) of the switching unit (300).

[0062] The second control unit (500) is connected to the gate terminal of the first FET (310a) and the gate terminal of the second FET (320a) and can drive the first and second FETs (310a, 320a) respectively according to a control signal. The second control unit (500) turns on the first FET (310a) and turns off the second FET (320b) when charging the battery (100). Therefore, the battery (100) is charged from a load, i.e., an external power source, through the second parasitic diode (320b) and the first FET (310a). In addition, the second control unit (500) turns on the second FET (320a) and turns off the first FET (310a) when discharging the battery (100). Accordingly, the battery (100) is discharged from the battery (100) through the first parasitic diode (310b) and the second FET (320a). At this time, the control signal for turning on the first and second FETs (310a, 320a), respectively, may be a logic high signal, and the control signal for turning off the first and second FETs (310a, 320a), respectively, may be a logic low signal.

[0063]

[0064] As described above, another embodiment of the present invention may include a battery output cutoff circuit (200) that is provided in a charge / discharge control circuit including a charge / discharge switch (310, 320). Accordingly, charging and discharging of the battery (100) may be performed through the first and second fuses (11, 12) of the battery output cutoff circuit (200) in a normal temperature range, and at a high temperature higher than a set temperature, at least one of the first and second fuses (11, 12) of the battery output cutoff circuit (200) may be melted by the characteristics of the NTC thermistor (20) and the fuse resistor (13), thereby cutting off the battery (100) and the output path.

[0065]

[0066] While the technical concepts of the present invention have been specifically described through the above-described embodiments, it should be noted that the embodiments are intended for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, those skilled in the art will appreciate that various embodiments are possible within the scope of the technical concepts of the present invention.

[0067] The names of each drawing symbol used in the description and drawings of the present invention are as follows.

[0068] 11: 1st fuse 12: 2nd fuse

[0069] 13: Fuse resistor 10: 3-terminal fuse

[0070] 20: Thermistor 100: Battery

[0071] 200: Battery output cut-off circuit

[0072] 300: Switching unit 400: First control unit

[0073] 500: Second Control Unit

Claims

1. A battery output cutoff circuit that cuts off the battery output. A three-terminal fuse provided in the output path between the battery and the battery output terminal, A battery output cut-off circuit including an NTC thermistor, one end of which is connected to the three-terminal fuse and the other end is connected to a ground terminal.

2. In claim 1, the three-terminal fuse, First, the first fuse is connected to the battery side, A second fuse, one end of which is connected to the other end of the first fuse and the other end is connected to the battery output terminal, A battery output cut-off circuit including a fuse resistor having one end connected between the first and second fuses.

3. In claim 2, the NTC thermistor, A battery output cut-off circuit in which one end is connected to the above fuse resistor and the other end is connected to the ground terminal.

4. In claim 3, the resistance value of the fuse resistor is A battery output cut-off circuit in which the combined resistance value of the resistance value of the NTC thermistor at the set output cut-off temperature is set to be not less than the fuse rupture current flowing through the first fuse or the second fuse.

5. A battery having multiple battery cells; A battery output cut-off circuit connected to the above battery and including a three-terminal fuse and an NTC thermistor to cut off the output of the battery depending on the temperature; A switching unit having one side connected to the battery output cut-off circuit and the other side connected to the battery output terminal to establish a charge / discharge path; A first control unit for measuring the state of the above battery; and A charge / discharge control circuit including a second control unit that controls the switching unit according to charging / discharging of the battery.

6. In claim 5, the three-terminal fuse includes a first fuse having one end connected to the battery side, a second fuse having one end connected to the other end of the first fuse and the other end connected to the switching unit, and a fuse resistor having one end connected between the first and second fuses. The above NTC thermistor is a charge / discharge control circuit in which one end is connected to the above fuse resistor and the other end is connected to the ground terminal.

7. A charge / discharge control circuit according to claim 6, wherein the NTC thermistor has a lower resistance at a temperature higher than a set output cut-off temperature, so that the current flowing through the fuse resistor increases, and the fuse resistor is heated according to the increase in current to melt and cut at least one of the first and second fuses.

8. In claim 7, the resistance value of the fuse resistor is A charge / discharge control circuit in which the sum of the resistance values ​​of the NTC thermistor at the set output cut-off temperature is set to be not less than the fuse rupture current flowing through the first fuse or the second fuse.

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