Safety unit, battery pack and electrical device

Abstract

Safety unit (10, 20) provided in a battery pack containing a battery (12, 22) comprising at least one cell (11, 21) of a secondary battery, a first external connection (13, 23) and a second external connection (14, 24), wherein the safety unit (10, 20) comprises: a safety element (18, 28) comprising a fuse element (16, 26) provided between the battery (12, 22) and the first external connection (13, 23) and a heat-generating resistor (17, 27) which heats the fuse element (16, 26), a first switching element (19, 29) whose one terminal is connected to the battery (12, 22) and the second external terminal (14, 24) and whose other terminal is connected to the heat-generating resistor (17, 27), wherein the heat-generating resistor (17, 27) is arranged inside the safety element (18, 28) and is configured to blow the fuse element (16, 26), a second switching element (200, 300) and a resistance element (201, 301) which are provided in parallel to the first switching element (19, 29), wherein the resistance element (201, 301) is arranged outside the safety element (18, 28), and a control device 15, 25), which controls operations of the first switching element (19, 29) and the second switching element (200, 300) in accordance with a voltage of the cell (11, 21) or the battery (12, 22), wherein the control device (15, 25) switches the safety unit (10, 20) to a first state in which the first switching element (19, 29) is switched on to allow current to flow through the heat-generating resistor (17, 27) and to a second state in which the second switching element (200, 300) is switched on to allow current to flow through the heat-generating resistor (17, 27) via the resistance element (201, 301).

Description

BACKGROUND OF THE INVENTION Field of invention

[0001] The present invention relates to a safety unit, a battery pack and an electrical device, each containing a safety element comprising a heat-generating resistor and a fusible link. State of the art

[0002] When a secondary battery, such as a lithium-ion battery, is used as a single cell, for example, in a mobile phone, it is supplied in the form of a battery pack with a safety unit integrated into a housing. This is because it is difficult to provide sufficient protection when the secondary battery cell, such as the lithium-ion battery, is used alone and independently. Lithium-ion batteries are characterized by their susceptibility to overcharging and overheating.

[0003] Therefore, the battery pack contains a safety unit, which includes the safety element, and the secondary battery, all housed in an outer casing, such as a plastic enclosure. A charging and discharging control system (a battery management system or BMS) controls the charging and discharging of the secondary battery, monitoring voltage, current, temperature, or similar parameters.

[0004] The battery pack mounted on a notebook or similar device contains a battery in which a large number of cells, such as lithium-ion batteries, are connected. In recent years, a second layer of protection has generally been mounted on the lithium-ion battery pack to provide double protection against overcharging, overheating, and over-discharging.

[0005] In the second protection circuit, a charging and discharging path is irreversibly interrupted by a safety element (including a fuse element) known as an SCP (Self-Control Protector). Specifically, a second protection IC controls and activates the safety element connected in series with the battery, thus interrupting the current if the battery is charged and discharged under abnormal conditions.

[0006] The safety element contains a heat-generating resistor. If, for example, the battery is charged beyond a reference value, the secondary protection IC makes the heat-generating resistor conductive, causing it to heat up and blow a low-melting-point metal fuse element within a short time. The safety element is also activated by an abnormal battery current. When the abnormal current is generated, the fuse element blows due to Joule heating caused by the overcurrent (see published Japanese patent no. 2017-228379).

[0007] The safety element is contained within the battery pack's safety unit. The safety unit includes a secondary protection IC that monitors the voltage of the battery or each cell, and a switching element such as a FET. The secondary protection IC controls the operation of the fuse element.

[0008] For example, in a safety element of a battery pack charging and discharging control circuit described in published Japanese patent no. 2012-231649, a fuse is blown to disconnect an electrical circuit in response to an overcurrent in the battery pack and an increase in ambient temperature. Alternatively, the fuse is blown by a heat-generating resistor that is made conductive under a prescribed condition, thereby disconnecting the electrical circuit.

[0009] In other words, this safety element can disconnect the circuit by means of a fuse element that blows due to overcurrent. When the safety unit detects an abnormal condition caused in a device, it sends a signal current that causes a resistive element to generate heat, so that the fuse element, made of a fusible alloy, blows due to the heat, thus disconnecting the circuit.

[0010] A conventional safety device is designed such that in a battery pack where the charging voltage is regulated within a range of 8.4 to 19.1 V, a secondary protection IC detects the occurrence of an overcharge when the charging voltage exceeds 19.1 V. The secondary protection IC makes the heat-generating resistor in a safety element conductive via a FET to generate heat and thereby blow the fuse. When the charging voltage drops below 8.4 V, the secondary protection IC detects an overdischarge, causing the fuse to blow.

[0011] Therefore, the conventional safety unit protects a battery and an electrical device by disconnecting a circuit when a voltage is outside a relatively narrow voltage range (see the disclosed Japanese patent no. 2018-060659).

[0012] Publication US 2013 / 0044402A1 describes a protection circuit for a battery pack and a corresponding control device. In an example with two switching elements (FETs), the protection circuit includes a pair of fuses and two resistors in series, through which current can be supplied to the fuses. One of the resistors can be bypassed by one of the two switching elements. SUMMARY OF THE INVENTION Problem Statement

[0013] It is an object of the present invention to provide a safety unit, a battery pack and an electrical device that can provide protection against overcharging, overheating and over-discharging in the safety unit, the battery pack and the electrical device, which are used in a wide voltage range. Problem solving

[0014] A security unit according to the present invention is a security unit according to claim 1.

[0015] If the voltage of a cell or battery in the safety unit falls below a prescribed voltage range, the control unit can switch the safety unit to the first state. Conversely, if the voltage of a cell or battery rises above the prescribed voltage range, the control unit can switch the safety unit to the second state.

[0016] In an example where the specified voltage range is set to 8.4 to 43.3 V, if the discharge voltage falls below 8.4 V, the control unit activates the first switching element to switch the safety unit to the first state, allowing the heat-generating resistor to generate heat and blow the fuse. If the charging voltage exceeds 43.3 V, the control unit activates the second switching element to switch the safety unit to the second state, thereby making the heat-generating resistor conductive through the resistance element to generate heat and blow the fuse. Even if the charging voltage reaches a high voltage of approximately 43.3 V, the heat-generating resistor can be made conductive through the resistance element to generate heat without being destroyed.Protection against overcharging, overheating and over-discharging can also be achieved in a battery pack used in a wide voltage range of approximately 8.4 to 43.3 V.

[0017] In the safety unit, R2 can satisfy the relationship R2 = {R1×(V2-V1)} / V1, where R1 represents a resistance value of the heat-generating resistor, V1 represents a nominal voltage value of the safety element, V2 represents a voltage of the cell or battery, and R2 represents a resistance value of the resistor element.

[0018] In an example where the resistance value R1 of the heat-generating resistor is set to 31 Ω and the specified voltage range is set to 33.6 to 51.0 V, the resistance value R2 of the resistor connected to the second switching element is set to 10 Ω. If the battery voltage then drops below 33.6 V, the control unit can cause the heat-generating resistor to generate heat via the first switching element. And if the battery voltage rises above 51.0 V, the control unit can cause the heat-generating resistor to generate heat via the second switching element.

[0019] The switching element according to the present invention is implemented by a switching transistor such as a field-effect transistor (FET) or a MOSFET or a safety unit module with the switching element contained therein.

[0020] The safety unit can contain a large number of cells in the battery, and the control unit can control the operations of the first switching element and the second switching element in accordance with a voltage of at least one of the cells.

[0021] A battery pack containing the safety unit further comprises a battery that includes at least one secondary battery cell, a first external connection and a second external connection.

[0022] An electrical device containing the battery pack further comprises a charging and discharging device that charges and discharges the battery pack, and a device body to which the battery pack is removably attached.

[0023] If the voltage of the cell or battery in the safety unit described above falls below the specified voltage range, the heat-generating resistor can be made conductive via the first switching element, generating heat and blowing the fuse element of the safety device. If the voltage of the cell or battery rises above the specified voltage range, the heat-generating resistor can be made conductive via the second switching element and the resistor element, generating heat and blowing the fuse element of the safety device. Because the second switching element and the resistor element are arranged in parallel to the first switching element, a corresponding voltage can be applied to the heat-generating resistor regardless of whether the battery voltage is high or low.The use of a higher voltage value is therefore permitted without exceeding the withstanding voltage of the heat-generating resistor within the safety element. Consequently, the safety unit can be used in a wider voltage range. Effect of the invention

[0024] A safety unit, a battery pack, and an electrical device can be provided that can achieve protection against overcharging, overheating, and over-discharging in the safety unit, the battery pack, and the electrical device, which are used in a wide voltage range.

[0025] The aforementioned and other tasks, features, aspects and advantages of the present invention are clarified by the following detailed description of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 and Fig. Figure 2 shows schematic views of a security unit according to an embodiment of the present invention. DESCRIPTION OF A PREFERRED VERSION

[0026] The following describes a safety unit, a battery pack and an electrical device according to one embodiment.

[0027] A safety unit according to this embodiment is a safety unit provided in a battery pack which contains a battery (a battery arrangement) comprising at least one cell of a secondary battery, a first external connection and a second external connection.The safety unit comprises a safety element, which includes a fuse element provided between the battery and the first external connection and a heat-generating resistor that heats the fuse element; a first switching element, one terminal of which is connected to the battery and the second external connection and the other terminal of which is connected to the heat-generating resistor; a second switching element and a resistor element provided in parallel to the first switching element; and a control device that controls the operations of the first switching element and the second switching element in accordance with a voltage of the cell or the battery.The control device switches the safety unit to a first state in which the first switching element is turned on to make the heat-generating resistor conductive, and to a second state in which the second switching element is turned on to make the heat-generating resistor conductive via the resistance element.

[0028] If the voltage of a cell or battery in the safety unit falls below a prescribed voltage range, the control unit switches the safety unit to the first state. If the voltage of the cell or battery rises above the prescribed voltage range, the control unit switches the safety unit to the second state.

[0029] The control unit provides a control signal corresponding to the voltage of the cell or battery. The first switching element is connected to the control unit, and the second switching element is also connected to the control unit. The first and second switching elements are switched on and off in response to a control signal provided by the control unit, corresponding to the battery voltage.

[0030] In an example where the specified voltage range is set to 8.4 to 43.3 V, if the battery discharge voltage falls below 8.4 V, the control unit activates the first switching element to switch the safety unit to the first state, causing the heat-generating resistor to generate heat and the fuse to blow. If the charging voltage exceeds 43.3 V, the control unit activates the second switching element to switch the safety unit to the second state, causing the heat-generating resistor to conduct across the resistor element, generating heat and blowing the fuse.

[0031] Even when the charging voltage reaches a high level, such as 43.3 V, the heat-generating resistor can be made conductive via the resistive element to generate heat without damage. Protection against overcharging, overheating, and over-discharging can also be achieved in a battery pack used across a wide voltage range, such as 8.4 V to 43.3 V.

[0032] In the example described above, the specified voltage range of 8.4 V to 43.3 V can be divided into a first voltage range of 8.4 V to 19.1 V and a second voltage range of 19.2 V to 43.3 V. When the battery discharge voltage is within the first voltage range of 8.4 V to 19.1 V, the control unit regulates the battery voltage to maintain this range. When the battery discharge voltage drops below 8.4 V (lower than the first voltage range), the secondary protection IC in the control unit activates the first switching element to make the heat-generating resistor conductive, generating heat and blowing the fuse element (activating the safety element). When the battery charging voltage exceeds 19.1 V (exceeding the first voltage range), the control unit regulates the battery voltage within the second voltage range of 19.2 V to 43.3 V.When the battery voltage exceeds 43.3 V (exceeds the second voltage range), the secondary protection IC in the control unit activates the second switching element. This makes the heat-generating resistor conductive via the resistive element, generating heat and blowing the fuse (activating the safety element). Even at relatively high voltages, the heat-generating resistor can still generate heat without being damaged due to the conduction through the resistive element.

[0033] Because the first switching element is provided and the second switching element and the resistance element are provided in parallel to the first switching element, a safety unit can be implemented that is designed for a wide voltage range, which includes the first voltage range and the second voltage range in combination.

[0034] In the safety unit, R2 satisfies the relationship R2 = {R1×(V2-V1)} / V1, where R1 represents a resistance value of the heat-generating resistor, V1 represents a nominal voltage value of the safety element, V2 represents a voltage of the cell or battery, and R2 represents a resistance value of the resistor element.

[0035] In an example where the resistance value R1 of the heat-generating resistor is set to 31 Ω and the specified voltage range is set to 33.6 to 51.0 V, the resistance value R2 of the resistor connected to the second switching element is set to 10 Ω. If the battery voltage then drops below 33.6 V, the control unit (the secondary protection IC) can cause the heat-generating resistor to generate heat via the first switching element. And if the battery voltage rises above 51.0 V, the control unit can cause the heat-generating resistor to generate heat via the second switching element.

[0036] The specified voltage range of 33.6 to 51.0 V can be divided into a first voltage range of 33.6 to 46.9 V and a second voltage range of 47.0 to 51.0 V. If the battery voltage falls below the first voltage range of 33.6 to 46.9 V, the control device activates a heat-generating resistor via the first switching element. If the battery voltage rises above the second voltage range of 47.0 to 51.0 V, the control device can activate the heat-generating resistor via the second switching element to generate heat.By selecting the resistance value R2 of the resistor element to satisfy the above-mentioned relationship, protection can be achieved without destroying the heat-generating resistor in the battery pack, which is used in a wide voltage range that includes the first voltage range and the second voltage range in combination.

[0037] If the voltage of the cell or battery in the safety unit described above falls below the specified voltage range, the heat-generating resistor can be made conductive via the first switching element, generating heat and blowing the fuse element of the safety device. If the voltage of the cell or battery rises above the specified voltage range, the heat-generating resistor can be made conductive via the second switching element and the resistor element, generating heat and blowing the fuse element of the safety device. Because the second switching element and the resistor element are arranged in parallel with the first switching element, a suitable voltage can be applied to the heat-generating resistor regardless of whether the battery voltage is high or low.The use of a higher voltage value is therefore permitted without exceeding the withstanding voltage of the heat-generating resistor within the safety element. Furthermore, the heat-generating resistor can be effectively activated even at low voltages. Consequently, the safety unit can be used across a wider voltage range.

[0038] The safety unit of this embodiment can provide protection against overcharging, overheating and over-discharging in a battery pack used in a wide voltage range.

[0039] It is known to short-circuit the electrodes of a cell in an overcharge or over-discharge state to generate an excessively high current, thereby increasing the temperature in the cell. By activating the safety element in the safety unit of the present invention in the event of an overcharge or over-discharge, protection can be achieved.

[0040] The switching element of this embodiment is implemented by a switching transistor such as a field-effect transistor (FET) or a MOSFET, or a safety unit module containing the switching element.

[0041] The safety unit of this embodiment can be mounted on the battery pack to protect a charging and discharging device.

[0042] The safety unit of this embodiment is applied to the battery pack. The battery pack containing the safety unit further includes a battery comprising at least one cell of a secondary battery, a first external connection, and a second external connection.

[0043] The battery pack of this embodiment is used in a wireless electrical device such as a mobile device. The electrical device further comprises a charging and discharging device that charges and discharges the battery pack, and a device body to which the battery pack is removably attached.

[0044] In the safety unit, the battery pack can contain a large number of cells, and the control unit can control the operations of the first switching element and the second switching element in accordance with a voltage of at least one of the cells.

[0045] The cells of the second battery are connected in series. The control unit provides a control signal corresponding to a cell or battery voltage (battery array). VR1 is assumed to represent the first voltage range of at least one cell of the battery. When the voltage drops below VR1, the heat-generating resistor is made conductive across the first switching element, causing the fuse element of the safety device to blow. VR2 is assumed to represent the second voltage range. When the voltage drops above VR2, the heat-generating resistor is made conductive across the second switching element and the resistor element, causing the fuse element of the safety device to blow.

[0046] The battery pack, to which the battery with its numerous interconnected secondary battery cells is mounted, can be implemented. The electrical device can be implemented by mounting the battery pack, to which the battery with its numerous secondary battery cells is mounted.

[0047] As in Fig. As shown in Figure 1, a safety unit 10 in a first example is a safety unit that protects a battery pack of a secondary battery, such as a lithium-ion battery, against overcharging, overheating, and over-discharging. The safety unit 10 comprises a safety element 18, a first switching element 19, a second switching element 200, a control device 15, and a resistor element 201.

[0048] In the first example, at least one cell 11 of the secondary battery is connected in series to form a battery 12. The battery 12 is mounted on the battery pack. The battery pack includes a first external connection 13 and a second external connection 14.

[0049] The control unit 15 has a first terminal 15a, which is connected to a first electrode 12a of the battery 12 and a safety element 18. The control unit 15 has a second terminal 15b, which is connected to a second electrode 12b of the battery 12 and the second external terminal 14. When the first terminal 15a and the second terminal 15b of the control unit 15 are connected to the first electrode 12a and the second electrode 12b of the battery 12, respectively, the control unit 15 monitors the voltage of the battery 12. The control unit 15 has a third terminal 15c, which is connected to the first switching element 19. The control unit 15 has a fourth terminal 15d, which is connected to the second switching element 200. The control device 15 optionally provides a control signal to the first switching element 19 and the second switching element 200 in accordance with the voltage of the battery 12.

[0050] A fuse element 16 of the safety element 18 is connected in series between the first electrode 12a of the battery 12 and the first external terminal 13. The safety element 18 comprises the fuse element 16 and the heat-generating resistor 17, which heats the fuse element 16. The heat-generating resistor 17 can be positioned such that it overlaps the fuse element 16 on the same side face of a substrate, or it can be positioned on an opposite side of the substrate to heat the fuse element 16 through the substrate.

[0051] The first switching element 19 has one terminal connected to the second electrode 12b of the battery 12 and the second external terminal 14. The first switching element 19 has another terminal connected to the heat-generating resistor 17 of the safety element 18. The first switching element 19 is connected to the control unit 15. The control unit 15 switches between a conductive state (on) and a non-conductive state (off) between one terminal and the other terminal of the first switching element 19 by providing a control signal.

[0052] The second switching element 200 and the resistor element 201 are arranged in parallel to the first switching element 19. The second switching element 200 is connected to the control device 15. The control device 15 switches between the conductive state (on) and the non-conductive state (off) between one terminal and the other terminal of the second switching element 200 by providing a control signal.

[0053] The second switching element 200 and the resistor element 201 are connected in series. The first switching element 19 and, in parallel to it, the second switching element 200 and the resistor element 201 are connected to the heat-generating resistor 17. The heat-generating resistor 17 is electrically connected to a central point of the fuse element 16.

[0054] The control unit 15 controls the operation of the safety element 18 by switching the first switching element 19 and the second switching element 200. For example, if the discharge voltage of the battery 12 falls below 8.4 V, the control unit 15 (secondary protection IC) switches on the first switching element 19 (first state) to make the heat-generating resistor 17 conductive, thus generating heat and activating the safety element (blowing the fuse element 16 of the safety element 18). If the charging voltage exceeds 43.3 V, the control unit 15 (secondary protection IC) can switch on the second switching element 200 (second state) to make the heat-generating resistor 17 conductive via the resistor element 201, so that the heat-generating resistor 17 generates heat and activates the safety element.

[0055] As in Fig.As shown in Figure 2, a safety unit 20 in a second example is a safety unit that protects a battery pack of a secondary battery, such as a lithium-ion battery, against overcharging, overheating, and over-discharging. The safety unit 20 comprises a safety element 28, a first switching element 29, a second switching element 300, a control device 25, and a resistor element 301.

[0056] In the second example, at least one cell 21 of the secondary battery is connected in series to form a battery 22. The battery 22 is mounted on the battery pack. The battery pack includes a first external connection 23 and a second external connection 24.

[0057] The control device 25 has a first terminal 25a, which is connected to a first electrode 22a of the battery 22 and a safety element 28. The control device 25 has a second terminal 25b, which is connected to a second electrode 22b of the battery 22 and a second external terminal 24. The control device 25 has a plurality of fifth terminals 25e, each of which is connected to an electrode 21a between cells 21. The control device 25 has a first terminal 25a, a second terminal 25b, and fifth terminals 25e, each of which is connected to a first electrode 22a, a second electrode 22b, and electrodes 21a between cells 21 of the battery 22, such that the control device 25 monitors a voltage of the battery 22 and a voltage of each cell 21.

[0058] The control unit 25 has a third terminal 25c, which is connected to the first switching element 29. The control unit 25 has a fourth terminal 25d, which is connected to the second switching element 300. The control unit 25 selectively provides a control signal to the first switching element 29 and the second switching element 300 corresponding to a voltage of the battery 22 or the cell 21.

[0059] A fuse element 26 of the safety element 28 is connected between the first electrode 22a of the battery 22 and the first external terminal 23. The safety element 28 comprises the fuse element 26 and a heat-generating resistor 27, which heats the fuse element 26.

[0060] The first switching element 29 has one terminal connected to the second electrode 22b of the battery 22 and the second external terminal 24. The first switching element 29 has another terminal connected to the heat-generating resistor 27 of the safety element 28. The first switching element 29 is connected to the control device 25. The control device 25 switches between the conductive state (on) and the non-conductive state (off) between one terminal and the other terminal of the first switching element 29 by providing a control signal.

[0061] The second switching element 300 and the resistor element 301 are arranged in parallel to the first switching element 29. The second switching element 300 is connected to the control device 25. The control device 25 switches between the conductive state (on) and the non-conductive state (off) between one terminal and the other terminal of the second switching element 300 by providing a control signal.

[0062] The second switching element 300 and the resistor element 301 are connected in series. The first switching element 29, and in parallel the second switching element 300 and the resistor element 301, are connected to the heat-generating resistor 27. The heat-generating resistor 27 is electrically connected to a central point of the fuse element 26.

[0063] The control unit 25 controls an operation of the safety element 28 by switching the first switching element 29 and the second switching element 300. For example, if the discharge voltage of the battery 22 or the cell 21 falls below 8.4 V, the control unit 25 (secondary protection IC) switches on the first switching element 29 to make the heat-generating resistor 27 conductive, generating heat and activating the safety element (blowing the fuse element 26 of the safety element 28). If a charging voltage exceeds 43.3 V, the control unit 25 (secondary protection IC) switches on the second switching element 300 to make the heat-generating resistor 27 conductive via the resistor element 301, generating heat and activating the safety element.

[0064] In a third example, a battery pack is a battery pack that contains the safety unit 10 from the first example. In other words, the battery pack containing a single cell of a secondary battery, such as a lithium-ion battery, contains the safety unit 10.

[0065] In a fourth example, a battery pack is a battery pack that contains the safety unit 20 from the second example. In other words, the battery pack containing a single cell of a secondary battery, such as a lithium-ion battery, contains the safety unit 20.

[0066] In a fifth example, an electrical device is an electrical device that includes the battery pack from the third example or the battery pack from the fourth example. In other words, the electrical device includes the battery pack, which contains a cell of a secondary battery, such as a lithium-ion battery. The battery pack is detachably connected to the electrical device. The electrical device includes a main body that charges the battery pack or is powered by receiving a current from the battery pack. The electrical device includes a charging and discharging device connected to the battery pack. Industrial applicability

[0067] The safety unit, battery pack and electrical device according to the invention can be used for an electrical or electronic device that includes a safety element with a heat-generating resistor and a fusible link, and in particular for a safety device for a battery pack containing a secondary battery.

[0068] An embodiment of the present invention has been described above; however, it should be noted that the embodiment described here is exemplary and does not limit the scope of the invention in any way. The scope of the invention is defined by the following claims and may include various modifications to the embodiment described here. List of reference symbols

[0069] 10, 20: Safety unit; 11, 21: Cell; 12, 22: Battery; 12a, 22a: First electrode; 12b, 22b: Second electrode; 13, 23: First external connection; 14, 24: Second external connection; 15, 25: Control unit; 15a, 25a: First connection; 15b, 25b: Second connection; 15c, 25c: Third connection; 15d, 25d: Fourth connection; 16, 26: Fusible element; 17, 27: Heat-generating resistor; 18, 28: Safety element; 19, 29: First switching element; 21a: Electrode; 25e: Fifth connection; 200, 300: Second switching element; 201, 301: Resistor element.

Claims

[1] Safety unit (10, 20) provided in a battery pack containing a battery (12, 22) comprising at least one cell (11, 21) of a secondary battery, a first external connection (13, 23) and a second external connection (14, 24), wherein the safety unit (10, 20) comprises: a safety element (18, 28) comprising a fuse element (16, 26) provided between the battery (12, 22) and the first external connection (13, 23) and a heat-generating resistor (17, 27) which heats the fuse element (16, 26), a first switching element (19, 29) whose one terminal is connected to the battery (12, 22) and the second external terminal (14, 24) and whose other terminal is connected to the heat-generating resistor (17, 27), wherein the heat-generating resistor (17, 27) is arranged inside the safety element (18, 28) and is configured to blow the fuse element (16, 26), a second switching element (200, 300) and a resistance element (201, 301) which are provided in parallel to the first switching element (19, 29), wherein the resistance element (201, 301) is arranged outside the safety element (18, 28), and a control device 15, 25), which controls operations of the first switching element (19, 29) and the second switching element (200, 300) in accordance with a voltage of the cell (11, 21) or the battery (12, 22), wherein the control device (15, 25) switches the safety unit (10, 20) to a first state in which the first switching element (19, 29) is switched on to allow current to flow through the heat-generating resistor (17, 27) and to a second state in which the second switching element (200, 300) is switched on to allow current to flow through the heat-generating resistor (17, 27) via the resistance element (201, 301). [2] Security unit (10, 20) according to claim 1, wherein: The control unit (15, 25) switches the safety unit (10, 20) to the first state when a voltage value of the cell (11, 21) or the battery (12, 22) becomes lower than a prescribed voltage range, and the safety unit (10, 20) switches to the second state when the voltage value of the cell (11, 21) or the battery (12, 22) becomes higher than the prescribed voltage range. [3] Security unit (10, 20) according to claim 1 or 2, wherein: R2 satisfies the relationship R2 = {R1×(V2-V1)} / V1, where R1 represents a resistance value of the heat-generating resistor (17, 27), V1 represents a nominal voltage value of the safety element (18, 28), V2 represents a voltage of the cell (11, 21) or the battery (12, 22), and R2 represents a resistance value of the resistance element (201, 301). [4] Security unit (10, 20) according to one of claims 1 to 3, wherein: the battery (12, 22) contains a plurality of cells (11, 21), and the control device (15, 25) controls the operations of the first switching element (19, 29) and the second switching element (200, 300) in accordance with a voltage of at least one of the cells (11, 21). [5] Battery pack, comprising: the safety unit (10, 20) according to one of claims 1 to 4, a battery (12, 22) containing at least one cell (11, 21) of a secondary battery, a first external port (13, 23), and a second external port (14, 24). [6] Electrical appliance, comprising: the battery pack according to claim 5, a charging and discharging device that charges and discharges the battery pack, and a main device body to which the battery pack is removablely attached.

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