JP2001286067A - Protection circuit for secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protection circuit for a secondary cell that detects temperature increase by overvoltage charging for effectively decreasing a charge current, and at the same time keep the charge current being decreased for safely protecting the secondary cell even if the temperature of the cell falls. SOLUTION: A resistance heating element 5 is connected to the series circuit of a secondary cell 1 and a PTC element 3 in parallel, at the same time, the secondary cell 1 thermally comes into contact with the PTC element 3, and, furthermore, the PTC element 3 thermally comes into contact with the resistance heating element 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection circuit for a secondary battery which suppresses overcharging or the like of a secondary battery for charging and discharging by using a PTC element.

[0002]

2. Description of the Related Art Up to now, rechargeable secondary batteries such as nickel-cadmium and nickel-metal hydride batteries have been used in portable electronic devices (portable radios, portable tape recorders, portable optical disk players, portable video recorders, portable telephones). , Notebook personal computers, etc.). These electronic devices are becoming smaller and lighter year by year,
Since a long continuous driving time is required, demand for a lithium ion secondary battery having a large discharge capacity per volume is growing.

[0003] The lithium ion secondary battery has the above advantages, but has the disadvantage that the characteristics are greatly deteriorated due to an extreme rise or fall of the voltage. Further, in order to prevent destruction at the time of short-circuit, it is usual to have a protection circuit for precisely managing the battery voltage.

[0004] That is, the charging of the secondary battery is performed by monitoring the battery voltage and, after reaching the set voltage, continuing charging at a constant voltage so that the charging voltage does not increase beyond the set voltage. However, in the case of a lithium ion secondary battery, if the charging voltage rises above the set value for some reason, irreversible reactions such as thermal decomposition of the electrolytic solution and decomposition of the electrode plate occur, resulting in a thermal runaway state and the worst battery Dangerous state of smoking and ignition.

Therefore, in charging a lithium ion secondary battery, a protection circuit is usually provided which monitors the charging voltage and stops the charging if the charging voltage rises above a set value. In the event that the protection circuit does not function or is damaged, a double or triple protection means is provided to prevent a thermal runaway state of the lithium ion secondary battery. .

As one of the protection means, a PTC (Positive) having a characteristic that the resistance value increases as the temperature rises.
(Temperature Coefficient)
An element is used. The PTC element usually has a characteristic (PTC characteristic) in which the resistance value rises rapidly at a certain temperature, and the resistance change rate reaches 3 digits or more, and some materials reach 6 digits or more. The PTC element is used as a heating protection element or an overcurrent protection element by utilizing the fact that the resistance value increases when heated in an external atmosphere or when heated due to generation of Joule heat due to the flow of a large current.

[0007] The protection circuit of the secondary battery has a PTC
The elements are connected in series, and a load (at the time of discharging) and a charger (at the time of charging) are connected to both ends of the series circuit. With this configuration, when an overcurrent due to a short circuit or the like at the time of discharge or an overcharge current due to a failure of the protection circuit at the time of charging flows, the P connected in series
The resistance value of the TC element rapidly rises due to Joule heat, and acts to suppress overcurrent. In order to connect the PTC element in series with the secondary battery, it is usually desirable that the resistance value of the PTC element be low at normal times. This is because, during discharging, the continuous power supply time is shortened by consuming unnecessary power, and during charging, the charging time is extended.

[0008] In addition, since the lithium ion secondary battery originally has low thermal stability, it is desirable to limit charging and discharging when the ambient temperature rises. Therefore, the PTC element is electrically connected in series with the lithium ion secondary battery and is arranged in thermal contact. As a result, the PTC element itself detects the temperature rise of the lithium ion secondary battery, increases the resistance value, and suppresses the charging current.

[0009]

As described above, since the heat generated by overcharging of the lithium ion secondary battery involves the danger of causing thermal runaway, the protection circuit using the PTC element has a desired function in an extremely short time. It is necessary to demonstrate. Furthermore, the performance and stability of a lithium-ion secondary battery, etc., deteriorates once abnormal heat is generated due to overcharging or the like. Therefore, it is dangerous to recharge the battery even after the battery temperature is lowered by the protection circuit. Is accompanied. Therefore, it is necessary to maintain a state in which the charging current is reduced even when the battery temperature drops.

SUMMARY OF THE INVENTION An object of the present invention is to detect a rise in battery temperature due to overvoltage charging to effectively reduce the charging current, and to maintain a state in which the charging current is reduced even when the battery temperature drops, to recharge the secondary battery. An object of the present invention is to provide a protection circuit for a secondary battery that protects safely.

[0011]

The object of the present invention is to provide a PT which is electrically connected in series to a secondary battery and is in thermal contact with the secondary battery.
A protection circuit for a secondary battery, comprising: a C element; and a resistance heating element electrically connected in parallel to the secondary battery and the PTC element and in thermal contact with the PTC element. Achieved.

According to the present invention, when the secondary battery abnormally generates heat during charging, the PTC element first increases the resistance value to suppress the charging current. Then, most of the current flows through the heating resistance element, the heating resistance element generates heat, and the PTC element is heated. Thereby, the operation of suppressing the charging current can be continued. This current suppression operation is maintained even when the battery temperature drops.

Further, the protection circuit for a secondary battery according to the present invention includes:
Switching means for connecting the resistance heating element to a circuit when charging the secondary battery and disconnecting the resistance heating element when discharging is provided. ADVANTAGE OF THE INVENTION According to this invention, it becomes possible not only to achieve the effect | action of the secondary battery protection at the time of charge, but to suppress the power consumption at the time of discharge.

Further, the protection circuit for a secondary battery according to the present invention includes:
A common electrode formed between the PTC element and the resistance heating element; a PTC-side electrode disposed to face the common electrode via the PTC element; and a common electrode opposed to the common electrode via the resistance heating element. Having a heating element side electrode,
The PTC-side electrode, the PTC element, the common electrode, the resistance heating element, and the heating element-side electrode are sequentially laminated and integrally formed. According to the present invention, thermal contact between the PTC element and the resistance heating element can be ensured, and the size of the circuit can be reduced.

Further, in the above protection circuit for a secondary battery according to the present invention, the operating temperature of the PTC element is 100 ° C. or less. According to the present invention, it is possible to reliably start the protection operation before the secondary battery reaches a dangerous temperature state.

Further, in the above protection circuit for a secondary battery according to the present invention, the resistance heating element has a PTC characteristic. According to the present invention, the heat generation characteristics of the resistance heating element can be matched with those of the PTC element.

In the above protection circuit for a secondary battery, the operating temperature of the resistance heating element is higher than the operating temperature of the PTC element. According to the present invention,
After the PTC element starts the current suppressing operation, the resistance heating element can start the heating operation.

In the above protection circuit for a secondary battery according to the present invention, the PTC element includes a conductive polymer material in which conductive particles are dispersed in a thermoplastic polymer. By doing so, the resistance value at room temperature can be reduced, and the protection circuit can be easily formed.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A protection circuit for a secondary battery according to one embodiment of the present invention will be described with reference to FIGS. FIG.
1 is a configuration diagram of a protection circuit for a secondary battery according to the present embodiment. As shown in FIG. 1, the protection circuit for a secondary battery includes:
The resistance heating element 5 is connected in parallel to the PTC element 3 connected in series with the secondary battery 1. This series-parallel circuit is connected between the input / output terminals 7 and 9. Secondary battery 1 and P
The TC element 3 is arranged in a state of being in thermal contact.
Similarly, the PTC element 3 and the resistance heating element 5 are also arranged in a state of being in thermal contact. A load is connected between the input / output terminals 7 and 9 during discharging, and a charger is connected during charging.

As shown in FIG. 1, the positive electrode of the secondary battery 1 is connected to the input / output terminal 9 and one end of the resistance heating element 5.
The negative electrode of the secondary battery 1 is connected to the input / output terminal 7 via the PTC element 3.
And the other end of the resistance heating element 5. In this embodiment, a lithium ion secondary battery is used as the secondary battery 1, but the present embodiment can be applied to other secondary batteries.

Materials having PTC characteristics are roughly classified into inorganic ceramics and organic polymers. The former is doped barium titanate, and the latter is mainly a conductive polymer material in which conductive particles are dispersed in a thermoplastic polymer. For organic polymer systems see US Pat.
No. 53, No. 33,1882 and the like. In the present embodiment, the latter conductive polymer material is preferably used because the room temperature (at steady state) resistance value can be reduced and molding is easy.

The room temperature resistance of the PTC element 3 is
It is preferably equal to or less than the internal resistance value of
Specifically, it is 100 mΩ or less, more preferably 50 mΩ or less, and further preferably 20 mΩ or less. Secondary battery 1
With the downsizing of PTCs, small PTC elements are desired.
In order to lower the resistance value of the PTC element 3, it is necessary to increase the surface area. Therefore, the lower the volume resistance of the material, the better.

In order to ensure the safety of the secondary battery 1,
The operating temperature of the PTC element 3 is set at 100 ° C. at the highest, preferably around 70 to 90 ° C. When the operating temperature is too high, when the resistance value of the PTC element 3 becomes high and the trip state in which the current suppressing operation is performed is performed, the secondary battery 1 may have already shifted to the dangerous state of the thermal runaway state. Because there is a nature.

Such a PTC element 3 is disclosed in, for example, JP-A-10-214705, JP-A-11-168005, and JP-A-11-1 proposed by the present inventors.
95506 and the like.

On the other hand, when the resistance heating element 5 has no abnormality in the secondary battery 1, it is necessary that the charging current mainly flows to the secondary battery 1 to the PTC element 3 having a low resistance value. . In addition, it is necessary not to consume useless power in a normal state. Therefore, the resistance value of the resistance heating element 5 is set higher than the sum of the internal resistance value of the secondary battery 1 and the resistance value of the PTC element 3, and is set at least one digit, preferably at least two digits. As the resistance heating element 5, a PTC heater having self-temperature control can be used in addition to a normal resistance heater. At this time, it is desirable that the operating temperature be set slightly higher than that of the PTC element 3.

As a preferred example of this embodiment, PTC
The element 3 and the resistance heating element 5 may be integrated. Thus, stable thermal contact between the two can be reliably obtained.
FIG. 2 is a configuration example of a protection circuit for a secondary battery using a protection element 21 in which a PTC element 3 and a resistance heating element 5 are integrated. As shown in FIG. 2, the protection element 21 includes a common electrode 23.
, A PTC layer 25 and a resistance heating layer 27 facing each other. An electrode 29 is attached to the surface of the PTC layer 25 facing the contact surface with the common electrode 23. An electrode 31 is attached to the surface of the resistance heating layer 27 facing the contact surface with the common electrode 23. As described above, the protection element 21 includes the PTC side electrode 29, the PTC layer 25, and the common electrode 2.
3, the resistance heating layer 27, and the heating element side electrode 31 are laminated in this order and integrally formed, and the PTC layer 25 and the resistance heating layer 27 can be thermally connected well.

The common electrode 23 of the protection element 21 is connected to the input / output terminal 7. The electrode 31 on the resistance heating layer 27 side is connected to the input / output terminal 9 and the positive electrode of the secondary battery 1. Also, PTC
The electrode 29 on the layer 25 side is connected to the negative electrode of the secondary battery 1.
Thereby, the same electrical connection relationship as the circuit configuration shown in FIG. 1 is obtained.

PTC element and resistance heating element, or PT
A protection element in which a C element and a resistance heating element having PTC characteristics are laminated and integrated is disclosed in, for example, Japanese Patent Publication No. 60-4446.
No. 789 (Document 1). This publication discloses an electric device in which a PTC element is provided with a heat supply device. Japanese Patent Publication No. Sho 62-45673 (Reference 2) discloses that a PTC sheet having a high resistance value is closely adhered to a low resistance sheet having a resistance value of 10 ^-3 times Ω · cm or less, and electrodes are provided on the sheet. A self-temperature controlled heating element is disclosed. Japanese Patent Application Laid-Open No. 55-95203 (Reference 3) discloses an electrode in which a current flows along an electric path running sequentially through a CW element containing a carbon black having a defined particle system and surface area and a PTC element. Is disclosed.

However, in all of the documents described in these documents 1 to 3, the PTC element and the resistance heating element are electrically connected in series, and wasteful power consumption occurs when the secondary battery is discharged in a normal state. It has the disadvantage that it cannot be avoided.

The operation of the secondary battery protection circuit according to the present embodiment during charging will be described with reference to FIG. 2
When the secondary battery 1 has no heat generation abnormality, the charging current mainly flows to the PTC element 3 having a low resistance value and the secondary battery 1 side. The amount of heat generated by the resistance heating element 5 is negligible.

If the secondary battery 1 is abnormally heated due to an increase in the charging voltage due to an abnormality in the secondary battery 1 or a failure in the charger, the temperature of the thermally contacted PTC element 3 rises and its resistance increases (tripping). Status). Thereby, the protection operation for suppressing the charging current flowing through the secondary batteries 1 connected in series is started. Further, when the sum of the internal resistance value of the secondary battery 1 and the resistance value of the PTC element 3 exceeds the resistance value of the resistance heating element 5, a current mainly flows through the resistance heating element 5,
The value of the charging current flowing to the next battery 1 is further reduced.

When a current flows through the resistance heating element 5 to generate heat and the temperature rises, the temperature of the PTC element 3 in thermal contact therewith further rises and the resistance value is further increased, so that the current flowing through the secondary battery 1 further increases. The value decreases. As a result, the temperature of the secondary battery 1 drops without reaching a dangerous temperature state. Thus, by providing the resistance heating element 5, the PTC element 3 can quickly perform the protection operation for suppressing the current.

Since the performance and stability of a lithium ion secondary battery or the like deteriorates once abnormal heat is generated due to overcharging or the like, it is necessary to recharge the battery even after the battery temperature is lowered by a protection circuit. There is danger. On the other hand, in the protection circuit according to the present embodiment, even if the battery temperature is reduced by reducing the charging current, the PTC element 3 is continuously heated by the resistance heating element 5 electrically connected in parallel, and the PTC element 3 is switched from the trip state. Does not return. Therefore, even if the battery temperature drops, the state where the charging current is reduced can be maintained, and dangerous charging can be prevented from being restarted.

If the resistance heating element 5 is connected in series with the PTC element 3 as described in the above-mentioned documents 1 to 3, if the current decreases due to the operation of the PTC element 3, the heating by the resistance heating element 5 is not performed. Becomes insufficient. As a result PT
When the temperature of the C element 3 decreases and the resistance value decreases, recharging may occur depending on the degree of decrease in the battery temperature. On the other hand, in the present embodiment, the resistance heating element 5 is set to P
Since the resistance heating element 5 is connected in parallel to the TC element 3, the resistance heating element 5 can continue to heat the PTC element 3 and prevent recharge of the secondary battery.

When the current flowing through the resistance heating element 5 decreases for some reason and heating by the resistance heating element 5 becomes insufficient for the tripped PTC element 3, the resistance value of the PTC element 3 decreases. Become. However,
Even if the resistance value of the PTC element 3 becomes lower than the resistance value of the resistance heating element 5, the charging current flows to the PTC element 3 at that point, so that the PTC element 3 generates heat by Joule heat. Therefore, the current suppression by the PTC element 3 is continued, and the charging current does not increase again.

As described above, according to the present embodiment, dangerous recharging can be prevented. In order to restart charging, the power supply from the charger is cut off once, and the temperature of the PTC element 3 is completely lowered. By doing so, it is possible to prevent unintentional return from the protection operation of the PTC element 3.

The protection circuit for the secondary battery shown in FIGS. 1 and 2 has a configuration in which the resistance heating element 5 (the resistance heating layer 27 in FIG. 2) remains connected to the circuit during both charging and discharging of the secondary battery. It has become. However, in this configuration, at the time of discharging, the load and the resistance heating element 5 are connected in parallel, and almost no current may flow through the load depending on the resistance value of the load. Therefore, the resistance heating element 5 may be connected only when charging, and may be provided with switching means for disconnecting from the circuit when discharging. 3 to 5 show an example of a protection circuit for a secondary battery including a switching unit.

In FIG. 3, a load 11 is connected between the input / output terminals 7 and 9. For example, a switch 13 is provided between the input / output terminal 9 and one end of the resistance heating element 5.
As the switch 13, a switch having a mechanical contact or a circuit switch for electrically disconnecting the switch can be used.

In FIG. 4, a third terminal (charging terminal) 41 is provided on the battery pack containing the protection element 21.
The connection between the electrode 31 on the resistance heating layer 27 side and the input / output terminal 9 (see FIG. 2) is cut off, and the electrode 31 is connected to the third terminal (charging terminal) 41. At the time of charging, the charging terminal 41 and the input / output terminal 9 are short-circuited on the charger side, and at the time of discharging (when connected to a load), the resistance heating element 5 is disconnected without connecting to the charging terminal 41.

In FIG. 5, a push switch 15 is provided on the battery pack containing the protection element 21, and the connection between the electrode 31 on the resistance heating layer 27 side and the input / output terminal 9 (see FIG. 2) is cut off. 31 is connected to one end of the push switch 15, and the input / output terminal 9 is connected to the other end of the push switch 15. Push switch 15 when discharging
Is turned off, and the electrode 31 and the input / output terminal 9 are electrically disconnected. When the battery pack is inserted into the charger, the push switch 15 is pressed, and the electrode 31 and the input / output terminal 9 are pressed.
And are connected.

FIG. 6 is a diagram for explaining a mounted state of the secondary battery protection circuit according to the present embodiment. FIG. 6 (a)
FIG. 6B is a perspective view illustrating an appearance of the secondary battery 1, and FIG.
FIG. 6A shows a side view of the secondary battery 1 viewed from the mounting direction side of the secondary battery 1.
It is a perspective view showing the appearance of a protection circuit for the following batteries. FIG. 6C shows a configuration of the protection element 21.

As shown in FIG. 6A, the secondary battery 1 has a thin rectangular parallelepiped shape. Positive and negative electrodes of the battery are provided on both sides of a mounting position 52 to which the protection element 21 is attached in close contact. In the figure, only the positive electrode 50 is shown.

As shown in FIGS. 6B and 6C, a lead wire 44 extends from the common electrode 23 of the protection element 21. The lead wire 4 is connected from the PTC element side electrode 29.
0 is drawn out, and the lead wire 42 is drawn out from the resistance heating element side electrode 31. As shown in FIG.
The lead wire 40 is connected to a wiring board 54 connected to a negative electrode (not shown) of the secondary battery 1. The lead wire 42 is connected to a wiring board 58 connected to the input / output terminal 9 (not shown). Wiring board 58
Is connected to the positive electrode 50 of the secondary battery. The lead wire 44 is connected to the input / output terminal 7 (not shown).

With such a configuration, the PTC of the protection element 21
The element 25 side is mounted in close contact with the mounting position 52 of the secondary battery 1 so that good thermal contact can be obtained. Hereinafter, the present embodiment will be described using specific examples and comparative examples.

(Example) [PTC layer] A high-density polyethylene (manufactured by Nippon Polychem, product name HY540) and an equivalent amount of paraffin wax (manufactured by Nippon Seiwa Co., Ltd., product name: HNP-10), 4 times the total weight of both Filamentous Ni powder (manufactured by INCO, trade name Type
255 nickel powder) in a mill at 150 ° C.
1.0% by weight of the total weight of polyethylene and wax of a silane coupling agent (KBE1 manufactured by Shin-Etsu Chemical Co., Ltd.)
003) 20% by weight of an organic peroxide (trade name: Trigonox DT50, manufactured by Kayaku Akzo) was dropped into the kneaded material and kneaded for 60 minutes. The kneaded product was formed into a sheet having a thickness of about 0.6 mm at 150 ° C., immersed in a 20 wt% dibutyltin dilaurate emulsion water solution, and subjected to a crosslinking treatment at 65 ° C. for 8 hours.

The sheet was sandwiched between two Ni foils having a thickness of 25 μm and pressed at 150 ° C. to a total thickness of 0.4 mm, and punched into a disk having a diameter of 10 mm. The temperature-resistance curve was measured. Resistance 5mΩ, operating temperature 75 ℃
Met.

[Resistance heating layer] 66% by weight of low density polyethylene (manufactured by Nippon Polychem, product name LC500), carbon black (manufactured by Tokai Carbon, trade name: Toka Black #)
4500) in a mill at 120 ° C., and the kneaded product was added with 1.0% by weight of polyethylene as a silane coupling agent (KBE1003, manufactured by Shin-Etsu Chemical Co., Ltd.), and 20% by weight thereof.
Was added dropwise to the kneaded product and kneaded for 20 minutes. The kneaded material is formed into a sheet having a thickness of about 0.6 mm at 120 ° C., and immersed in a 20 wt% dibutyltin dilaurate emulsion water solution.
Crosslinking treatment was performed at 65 ° C. for 8 hours.

This sheet was sandwiched between two Ni foils having a thickness of 25 μm and pressed at 150 ° C. to a total thickness of 0.4 mm, punched out into a disk having a diameter of 10 mm, and the temperature-resistance curve was measured. The resistance value was 1.2Ω and the operating temperature was 90 ° C.

[Protective element] Ni foil 25 μm thick / crosslinked PTC sheet / Ni foil / crosslinked resistance heating sheet /
Ni foil in the order, 15 mm so that the whole is 0.8 mm
It was thermocompression-bonded at 0 ° C. and punched into a disk having a diameter of 10 mm.

[Protection Circuit] Each electrode of protection element and 0.55
The Ahr lithium ion secondary battery 1 was connected as shown in FIG. 2, and the input / output terminals 7 and 9 were connected to a power supply for charging.

[Charging Test] Charging was performed under the condition of a constant current of 1 A up to a charging voltage of 5 V. When the battery voltage reached about 4.3 V, the battery temperature started to rise, the PTC element operated at about 75 ° C., and the charging current was limited to 50 mA. After that, the charging current did not increase even if left for 3 hours.

(Comparative Example) A disc-shaped PT having a thickness of 0.4 mm and a diameter of 10 mm with only the PTC body of the example interposed between electrodes.
A C element was prepared and connected in series between a battery and a power supply. The element and the battery were brought into thermal contact, and the same charging and wiping test as in the example was performed.

The residual current after the operation of the PTC element is 50
At 0 mA, it was 10 times higher than that of the example.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above embodiment, the description has been given of the protection element in which the PTC element and the resistance heating element 5 are integrally formed. However, the PTC element and the resistance heating element 5 may be configured as individual components.

[0055]

As described above, according to the present invention, a rise in battery temperature due to overvoltage charging is detected, thereby effectively reducing the charge current, and maintaining the charge current in a reduced state even when the battery temperature falls. Thus, the secondary battery can be safely protected.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a protection circuit for a secondary battery according to an embodiment of the present invention.

FIG. 2 is a configuration example of a protection circuit for a secondary battery using a protection element in which a PTC element and a resistance heating element are integrated.

FIG. 3 is a configuration diagram of a protection circuit for a secondary battery including a switching unit that disconnects a resistance heating element from a circuit when discharging.

FIG. 4 is a configuration diagram of a protection circuit for a secondary battery including a switching unit that disconnects a resistance heating element from a circuit when discharging.

FIG. 5 is a configuration diagram of a protection circuit for a secondary battery including a switching unit that disconnects a resistance heating element from a circuit when discharging.

FIG. 6 is a diagram illustrating a mounted state of a protection circuit for a secondary battery according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Secondary battery 3 PTC element 5 Resistance heating element 7, 9 I / O terminal 11 Load 13 Switch 15 Push switch 21 Protection element 23 Common electrode 25 PTC layer 27 Resistance heating layer 29, 31 Electrode 40, 42, 44 Lead wire 41 Terminal 3 (Charging terminal) 50 Positive electrode 52 Mounting position 54, 56, 58 Wiring board

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tetsuya Takahashi 1-13-1, Nihonbashi, Chuo-ku, Tokyo TDK Corporation F-term (reference) 5G003 AA01 BA01 FA04 GA07 5H030 AA04 AS11 BB01

Claims (7)

[Claims] 1. A PTC element electrically connected in series and thermally in contact with a secondary battery, and said PTC element electrically connected in parallel with said secondary battery and said PTC element. And a resistance heating element that is in thermal contact with the protection circuit. 2. The protection circuit for a secondary battery according to claim 1, further comprising switching means for connecting the resistance heating element to the circuit when charging the secondary battery and disconnecting the resistance heating element when discharging the battery. Protection circuit for secondary battery. 3. The protection circuit for a secondary battery according to claim 1, wherein the common electrode is formed between the PTC element and the resistance heating element, and the common electrode is provided via the PTC element. A PTC-side electrode disposed opposite to the PTC-side electrode, and a heating element-side electrode disposed opposite to the common electrode via the resistance heating element. The PTC-side electrode, PTC element, common electrode, resistance heating element, heating A protection circuit for a secondary battery, wherein the protection circuit is integrally formed by being stacked in the order of element-side electrodes. 4. The method according to claim 1, wherein
A protection circuit for a secondary battery, wherein the operating temperature of the PTC element is 100 ° C. or less. 5. The method according to claim 1, wherein
A protection circuit for a secondary battery, wherein the resistance heating element has a PTC characteristic. 6. The method according to claim 1, wherein
A protection circuit for a secondary battery, wherein an operating temperature of the resistance heating element is higher than an operating temperature of the PTC element. 7. The method according to claim 1, wherein
A protection circuit for a secondary battery, wherein the PTC element includes a conductive polymer material in which conductive particles are dispersed in a thermoplastic polymer.