JPH0919053A - Protective device of secondary battery - Google Patents

Abstract

PURPOSE: To provide a protective device, of a secondary battery, which has a protective function with reference to the degradation or the like of a battery cell block. CONSTITUTION: In a secondary battery, a plurality of battery cell blocks 1A, 1B are connected in series, and a charging FET 4 and a charging FET 5 which are composed of FETs with parasitic diodes as switching elements which can be on-off-controlled by a control circuit 6 are interposed so as to be in series with a charging and discharge route. A means which detetcs voltages of the respective battery cell blocks 1A, 1B in a charging and discharge operation and a means which controls the FETs 4, 5 so as to inhibit the charging and discharge operation when a relative voltage across the battery cell blocks 1A, 1B or its time-differentiated value exceeds a definite value are installed at the control circuit 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery protection device, and more particularly to a secondary battery protection device effective in a lithium ion secondary battery.

[0002]

2. Description of the Related Art In a secondary battery, when an overcharge, an overdischarge, or an overcurrent flows over an appropriate charge / discharge condition, a gas is generated along with the decomposition of the electrolytic solution. However, problems such as short circuit and overheating may occur inside the battery.

Therefore, various types of secondary battery protection devices have been proposed in the past, which are provided with means for preventing overcharge, overdischarge, and overcurrent. For example, in Japanese Unexamined Patent Publication (Kokai) No. 4-75430, switching means for overcharge protection and overdischarge protection, which is composed of a MOS FET with a parasitic diode or the like, is provided in the charging / discharging path of a secondary battery in series with the battery for control. It is disclosed that a circuit detects the voltage of a secondary battery and the switching means is turned on / off by the detected voltage.

A specific description will be given with reference to FIGS.
In FIG. 5, a plurality of battery cell blocks 1A and 1B are +
A MOSFET with a parasitic diode (hereinafter, abbreviated as a charge FET) 4 for overcharge protection, which is connected in series between the terminals 2 and −3 and is connected in series with these battery cell blocks 1A and 1B, and for overdischarge protection. MO with parasitic diode
S FET (hereinafter, abbreviated as discharge FET) 5 is provided. A control circuit 6 for controlling on / off of the charge FET 4 and the discharge FET 5 is provided.
The voltage across each of the battery cell blocks 1A and 1B is input to. Further, the discharge FET 5 on the charge / discharge path and the battery cell block 1B are grounded, and the charge FET 4 and the-terminal 3 are connected.
Is input to the comparator 9 of the control circuit 6, and the discharge current I _{DCHG is generated by the} voltage across the charge FET 4 and the discharge FET 5.
Is configured to be detected.

At the time of overcharge protection, the control circuit 6
As shown in (a), the charging FET 4 is turned off and the charging current is cut off. However, even in that case, the discharge current I _DCHG
Is the parasitic diode 4a of the charging FET 4 as shown by the arrow.
Flow through. On the contrary, at the time of over discharge protection, FIG.
As shown in, the discharge FET 5 is turned off and the discharge current is interrupted, and the charge current I _CHG also flows through the parasitic diode 5a of the charge FET 5 as indicated by the arrow.

The control circuit 6 includes battery cell blocks 1A, 1
The voltages V _BA and V _BB of B are detected, and during charging, as shown in FIG.
As shown in FIG. 5, the voltage V _BA of the battery cell block 1A is the first voltage value (eg, 4.30) in the illustrated example.
V) or more, the charge FET 4 is turned off and the charge current I
_Cut off _CHG . When the voltage V _BA of the battery cell block 1A becomes equal to or lower than the second voltage value (4.00 V, for example), the charge FET 4 is turned on to release the overcharge protection function.

At the time of discharging, as shown in FIG. 8, one of the voltages, in the illustrated example, the voltage V _BB of the battery cell block 1B is the fourth voltage V _BB .
When the voltage value becomes less than (for example, 2.60V), the discharge F
ET5 is turned off to cut off the discharge current I _DCHG . When the voltage V _BB of the battery cell block 1B becomes equal to or higher than the third voltage value (for example, 3.20V), the discharge FET 5 is turned on and the overdischarge protection function is released.

Further, in order to prevent an overcurrent from flowing at the time of discharging, the discharging current is detected by the potential difference generated across the FETs 4 and 5 connected in series, and as shown in FIG. 9, a predetermined current value (for example, When a current of 5.0 A or more continuously flows for a specified time (for example, 1.0 ms) or more, the discharge FE
T5 is off. Further, when the detected potential difference becomes a predetermined value or less, the overcurrent protection function is released.

[0009]

By the way, it is unavoidable that each battery cell block 1A, 1B is deteriorated by repeated use, and the deterioration etc. is uniformly progressed in each battery cell block 1A, 1B. Is rather rare. When the battery cells block 1A, 1B are unbalanced and the capacities of the battery cell blocks 1A, 1B become uneven, the voltage of the battery cell block having a smaller capacity is smaller than that of the other battery cell blocks during charging. During the discharge, the voltage of the battery cell block having the smallest capacity decreases at the time of discharging, and decreases earlier than the other battery cell blocks.

Therefore, such a battery cell block 1
A protection function against deterioration of A and 1B is provided in combination with the above-mentioned overcharge / discharge prevention function to provide an early protection function for overcharge / discharge due to deterioration, and protection in the case of overcharge / discharge due to deterioration etc. It is desirable not to cancel the function.

Further, in the above-mentioned conventional example, the release of the overcharge protection function is not performed until the voltage of the battery cell block becomes equal to or lower than a predetermined value. Therefore, the current flowing through the parasitic diode before the release (the discharge current during the overcharge protection or (Charging current at over discharge protection)
There is a risk that the FET itself may be destroyed due to the power loss (heat generation) due to. For example, the V _F voltage of the parasitic diode is 0.7
When the discharge current is V and the current is 4 A, there is a power loss of 2.8 W during charge protection, and the allowable loss of the charge FET 4 (1.4 W).
Therefore, if the continuous discharge is continued in this state, the charge FET 4 will be thermally destroyed.

Conventionally, the overcurrent protection is performed only for the discharge current, but especially in the lithium ion secondary battery, it is important to protect the charge FET 4 and the discharge FET 5 from thermal damage due to overcurrent with a large current. It is desirable to provide overcurrent protection for the charging current as well. Further, conventionally, if the current is a certain value or more, the current is cut off when the current is continued for a certain time regardless of the magnitude.
In order to protect the ET from thermal damage, it is desirable to change the delay time from the overcurrent detection to the confirmation according to the magnitude of the current value.

From the above viewpoint, the present invention has a protection function against deterioration of the battery cell block and the like, and at the time of overcharge and overdischarge protection, there is no risk of the switching element being destroyed by the power loss in the parasitic diode, and at the time of charging and Overcurrent protection is performed at any time during discharge to prevent damage to the switching element, and the detection delay time in overcurrent protection is varied according to the magnitude of the current value to prevent damage to the switching element. It is intended to provide a protection device for a secondary battery.

[0014]

SUMMARY OF THE INVENTION A secondary battery protection device of the present invention comprises a plurality of battery cell blocks connected in series and a charging / discharging path capable of on / off control by a control circuit for switching between charging and discharging. In a secondary battery in which an element is interposed, a means for detecting the voltage of each battery cell block during charging / discharging, and a switching element to prohibit charging / discharging when the relative voltage between the battery cell blocks exceeds a certain value. The control circuit is provided with a means for doing so.

Further, instead of the relative voltage, a time differential value of the relative voltage is used, and the control circuit is provided with means for controlling the switching element so as to prohibit charging / discharging when the differential value exceeds a certain value. Good.

Preferably, a means for controlling the switching element is also used so as to prohibit charging or discharging when the voltage of any of the battery blocks exceeds a predetermined voltage range, and at the time of charging, a discharging current of a specified value or more is discharged. Sometimes, the control circuit is provided with means for controlling the switching element so as to release the prohibition of charging or discharging when the charging current of the specified value or more continues for the specified time or more.

Further, a charging / discharging current detecting means is provided,
The control circuit is provided with means for controlling the switching element so as to prohibit charging / discharging when detecting an overcurrent in which the charging current or the discharging current exceeds a predetermined value.

Preferably, the switching element is controlled so that the detection operation is started when the charging / discharging current exceeds a predetermined value, the overcurrent is confirmed after a predetermined delay time, and the charging / discharging is prohibited. The delay time is set in inverse proportion to the detected current value. The delay time may be set in inverse proportion to the square of the detected current value, or may be set to be exponentially shorter than the detected current value.

[0019]

According to the protective device for a secondary battery of the present invention, charging / discharging is prohibited when the relative voltage between the battery cell blocks exceeds a certain value. When the cell balance becomes unbalanced and the cell balance is compromised, it is reliably detected and charging / discharging is prohibited, so it is possible to prevent overcharging and overdischarging due to deterioration etc. It is possible to improve.

Further, if the time differential value of the relative voltage is used instead of the relative voltage, even if the magnitude of the relative voltage varies due to factors other than deterioration or the like, the deterioration of each battery cell block can be more accurately caused. It is possible to detect the collapse of the cell balance, and it is possible to prevent overcharge and overdischarge due to deterioration or the like with high reliability.

Further, during the overcharge protection operation, the prohibition of charging or discharging is released when the discharge current of the specified value or more at the time of charging and the charge current of the specified value or more at the time of discharging continue for a specified time or more, respectively. By controlling
It is possible to prevent the switching element from generating heat and breaking due to the power loss due to the passage of the current.

Further, when an overcurrent in which the charging current or the discharging current exceeds a predetermined value is detected, the switching element is controlled so that the charging or the discharging is prohibited, and the overcurrent protection is performed not only during the discharging but also during the charging. In this case, it is possible to prevent thermal destruction of the switching element due to passage of a large current, and especially in a lithium-ion battery, the destruction of these switching elements is important for ensuring safety, so that it is very effective.

In addition, the charging / discharging is prohibited after a predetermined delay time after the charging / discharging current exceeds a predetermined value.
When the delay time is set in inverse proportion to the detected current value, the power loss due to the switching element varies depending on the magnitude of the current value.Therefore, compared with the case where a constant delay time is set with a constant detected current value, the switching element It is possible to reliably prevent thermal destruction.

Further, if the delay time is set in inverse proportion to the square of the detected current value, the current is cut off when the amount of heat generated by the current of the switching element becomes constant, so that a reasonable delay time can be set. As a result, the breakdown of the switching element can be reliably prevented while widening the allowable range of the large current.

Further, even if the delay time is set to be exponentially shorter than the detected current value, the current can be cut off on the safer side with respect to the switching element, and the switching element with higher reliability can be provided. Can prevent destruction.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the secondary battery protection device of the present invention will be described below with reference to FIGS.

In FIG. 1, a plurality of battery cell blocks 1A, 1B made of lithium ion batteries are connected in series between the + terminal 2 and the -terminal 3, and the battery cell blocks 1A, 1B are overcharged in series. MOS FET with parasitic diode for protection (hereinafter, abbreviated as charge FET)
4 and MOS FET with parasitic diode for over discharge protection
(Hereinafter, abbreviated as discharge FET) 5 is provided. These FETs 4 and 5 have a resistance of about 100 mΩ when turned on. The V _F of the parasitic diodes 4a and 5a is about 0.7V. A control circuit 6 for controlling on / off of the charge FET 4 and the discharge FET 5 is provided,
The voltage across the battery cell blocks 1A and 1B is input to the control circuit 6. Further, in order to detect the charge / discharge current, a current detection resistor 7 of about 20 to 50 mΩ is interposed between the discharge FET 5 and the battery cell block 1B in the charge / discharge path. The control circuit 6 is provided with means 8 for amplifying and detecting the minute potential difference generated in the current detecting resistor 7 by the charging current or the discharging current, and is configured to detect the charging current I _CHG and the discharging current I _DCHG. Has been done. By interposing the current detecting resistor 7 in this manner, the charging F
Highly accurate current detection can be performed regardless of the operating states of ET4 and discharge FET5.

Next, the overcharge protection operation by the above configuration,
The overdischarge protection operation and the overcurrent protection operation will be sequentially described.

First, the overcharge protection operation will be described with reference to FIG. The control circuit 6 detects both-end voltages V _BA and V _BB of the battery cell blocks 1A and 1B, and their relative voltage values | V
_BA -V _BB | is the specified voltage value (for example, 0.3~0.5V,
2 (a) when the voltage is smaller than 0.3 V in the illustrated example.
As shown in FIG. 5, when one of the voltages, that is, the voltage V _BA of the battery cell block 1A in the illustrated example becomes equal to or higher than a first voltage value (eg, 4.30 V) in the illustrated example, the charge FET 4 is turned off to charge the battery. The current I _CHG is cut off, and the voltage V _BA of the battery cell block 1A is set to the second voltage value (for example, 4.00).
When it becomes V or less, the charge FET 4 is turned on to release the overcharge protection function.

Regarding the operation of releasing the overcharge protection function, the discharge current I _{DCHG larger} than the specified current value (for example, 0.2 A) is further supplied while the charge FET 4 is turned off and the charge current I _CHG is cut off. When the flow continues for a predetermined time (for example, 10 ms) or more, as shown by a virtual line in FIG. 2A, the voltage of the battery cell block is the second voltage value (for example,
Even if the voltage does not drop below 4.00 V, the charge FET 4 is turned on to release the overcharge protection function. In this way, by turning on the charge FET 4 when the discharge current of the specified value or more continuously flows during the overcharge protection operation, heat is generated by the power loss in the parasitic diode 4a of the charge FET 4 due to the discharge current, and the charge FET 4 is discharged. It prevents it from being destroyed.

On the other hand, each of the battery cell blocks 1A, 1B described above
When the relative voltage value | V _BA −V _BB | of the both-end voltages V _BA and V _BB becomes 0.3 V or more, as shown in FIG. 2B, even if both ends of the battery cell blocks 1A and 1B are Even if the voltages V _BA and V _BB have not reached the first voltage value, the charge F
Both the ET4 and the discharge FET5 are turned off, and thereafter the charging and discharging are permanently prohibited, that is, the use is prohibited without performing the releasing operation.

By detecting the relative voltage value | V _BA -V _BB | when the relative voltage value exceeds the specified value, the capacity becomes non-uniform due to deterioration of the battery cell blocks 1A and 1B. It is possible to detect the occurrence of the cell balance failure, and based on that, prohibit the use of the secondary battery to prevent overcharging.

Next, the over-discharge protection operation will be described with reference to FIG. At this time also, as in the overcharge protection operation, the control circuit 6
Is the voltage V _BA , V _BB across each battery cell block 1A, 1B
3 is detected and their relative voltage value | V _BA −V _BB | is smaller than the specified voltage value (for example, 0.3 V), FIG.
As shown in (a), when one of the voltages is the same as the conventional example, that is, the voltage V _BB of the battery cell block 1B in the illustrated example becomes a fourth voltage value (for example, 2.60 V) or less, the discharge FET 5
_Is turned off to interrupt the discharge current I _DCHG, and the voltage V _BB of the battery cell block 1B is set to a third voltage value (for example, 3.
20 V) or more, the discharge FET 5 is turned on to release the overdischarge protection function.

Regarding the release operation of the over-discharge protection function, as in the case of the over-charge protection operation, the specified current value (for example, 0.2 A) with the discharge FET 5 turned off and the discharge current I _DCHG cut off. When the above charging current I _CHG continuously flows for a specified time (for example, 10 ms) or more, FIG.
, The voltage of the battery cell block is the third
The discharge FET 5 is turned on to cancel the over-discharge protection function even if the voltage value of (3.20 V or more) is not exceeded. As described above, when the charging current of the specified value or more continuously flows during the over-discharge protection operation, the discharging FET 5 is turned on, so that heat is generated due to the power loss in the parasitic diode 5a of the discharging FET 5 due to the charging current and the discharging FET 5 is discharged.
Prevents 5 from breaking.

On the other hand, the battery cell blocks 1A, 1B described above.
When the relative voltage value | V _BA −V _BB | of the both-end voltages V _BA and V _BB becomes 0.3 V or more, as shown in FIG. 3B, as in the overcharge protection operation, Even if the voltages V _BA and V _BB across the battery cell blocks 1A and 1B have not dropped to the fourth voltage value, both the charge FET 4 and the discharge FET 5 are turned off, and thereafter, the charging operation is not performed and the charging operation is performed permanently. Discharge is prohibited, that is, use is prohibited.

In the above description, in detecting the deterioration of each battery cell block 1A, 1B, the relative voltage value | V _BA of the both-end voltages V _BA , V _BB of each battery cell block 1A, 1B.
Although −V _BB │ was used, the time derivative of the relative voltage │V _BA -V _BB │ of the voltages V _BA and V _BB across the battery cell blocks 1A, 1B, that is, d│V _BA -V _BB │ / dt. When the differential value, for example, the change of the relative voltage per 10 minutes, is calculated, the charge FET 4 and the discharge FET 5 are both turned off, and thereafter the use may be prohibited without performing the releasing operation. Generally, when deterioration occurs in each of the battery cell blocks 1A and 1B, the deterioration progresses at an accelerated rate. Therefore, when the differential value of the relative voltage is used as described above, the magnitude of the relative voltage varies due to factors other than the deterioration or the like. While easy, it is possible to more accurately detect the cell balance failure due to deterioration of the battery cell blocks 1A and 1B.

Next, the overcurrent protection operation will be described with reference to FIG. With respect to both the charging current and the discharging current, the minute potential difference generated in the current detecting resistor 7 is detected by the amplification detecting means 8 of the control device 6, and the detecting operation is started when the charging current or the discharging current exceeds a predetermined value. As shown by the solid line in FIG. 4A, the charging FET 4 and the discharging FET 5 are connected so as to check the overcurrent after a predetermined delay time set so as to be inversely proportional to the detected current value and prohibit charging or discharging. Cut off.
In the illustrated example, when the detected current value is 5 A, the delay time is 1.
It is set to 0 ms and 500 μs at 10 A. In addition,
No matter how large the detected value is, a delay time is set to 100 μs to prevent malfunction due to peak current during charging or device use. The overcurrent protection operation is automatically restored after the specified time.

As described above, by performing overcurrent protection not only at the time of discharging but also at the time of charging, the charging F
It is possible to prevent the ET4 and the discharge FET5 from being destroyed by heat, and especially in a lithium ion battery, these charging FEs are used.
The destruction of T4 and the discharge FET 5 is important because it is important to ensure safety. Further, in order to prevent thermal destruction of the FETs 4 and 5, it is important to change the delay time according to the current value. That is, the FET can be regarded as a resistor (R) when it is on, and the power loss when the current I flows is I.
² R, and assuming that the power loss is allowed up to a certain delay time (Δt), I ² R · Δt = Const. Holds. Therefore, Δt = Const. / I ² R, and the delay time should be inversely proportional to I ² . Therefore,
By setting the delay time in inverse proportion to the detected current value as described above, thermal destruction of the FETs 4 and 5 can be surely prevented as compared with the case where the constant delay time is set with a constant detected current value. it can.

In the above description, the delay time is set in inverse proportion to the detected current value, but if the delay time is set in inverse proportion to the square of the detected current value as shown by the phantom line in FIG. 4 (a). As is clear from the above description, the current is cut off when the amount of heat generated by the current becomes constant, so that a reasonable delay time is set and the breakdown of the FET is ensured while widening the allowable range of large current. Can be prevented. Further, FIG.
As shown in (b), the delay time may be set to be exponentially shorter than the detected current value.

[0040]

According to the protective device for a secondary battery of the present invention,
As is clear from the above description, since the charging / discharging is prohibited when the relative voltage between the battery cell blocks exceeds a certain value, the capacity becomes uneven due to deterioration of each battery cell block, etc. When an imbalance occurs, the secondary battery is definitely detected and the use of the secondary battery is prohibited, so it is possible to prevent overcharging and overdischarging due to deterioration etc. and improve safety. it can.

Further, if the time differential value of the relative voltage is used instead of the above relative voltage, even if the magnitude of the relative voltage varies due to factors other than deterioration or the like, the deterioration of each battery cell block can be more accurately caused. It is possible to detect the collapse of the cell balance, and it is possible to prevent overcharge and overdischarge due to deterioration or the like with high reliability.

Further, during the overcharge protection operation, the prohibition of charging or discharging is released when the discharge current of the specified value or more during the charging and the charging current of the specified value or more during the discharging continue for the specified time or more, respectively. By controlling
It is possible to prevent the switching element from generating heat and breaking due to the power loss due to the passage of the current.

Furthermore, when an overcurrent in which the charging current or the discharging current exceeds a predetermined value is detected, the switching element is controlled so that the charging or the discharging is prohibited, and the overcurrent protection is performed not only during the discharging but also during the charging. In this case, it is possible to prevent thermal destruction of the switching element due to passage of a large current, and especially in a lithium-ion battery, the destruction of these switching elements is important for ensuring safety, so that it is very effective.

Further, the charging / discharging is prohibited after a predetermined delay time after the charging / discharging current exceeds a predetermined value, and
If the delay time is set in inverse proportion to the detected current value, the power loss due to the switching element varies depending on the current value, so thermal damage to the switching element will occur compared to the case where a fixed delay time is set with a constant detected current value. It can be surely prevented.

Further, if the delay time is set in inverse proportion to the square of the detected current value, the current is cut off when the amount of heat generated by the current of the switching element becomes constant, so that a reasonable delay time can be set. As a result, the breakdown of the switching element can be reliably prevented while widening the allowable range of the large current.

Further, even if the delay time is set to be exponentially shorter than the detected current value, the current can be cut off on the safer side of the switching element, and the switching element can be operated with higher reliability. Can prevent destruction.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a secondary battery protection device of the present invention.

FIG. 2 is an explanatory diagram of an overcharge protection operation in the embodiment.

FIG. 3 is an explanatory diagram of an overdischarge protection operation in the same embodiment.

FIG. 4 is an explanatory diagram of an overcurrent protection operation in the embodiment.

FIG. 5 is a circuit diagram of a conventional secondary battery protection device.

FIG. 6 is a diagram for explaining the action of the FET with a parasitic diode during charge protection and discharge protection.

FIG. 7 is an explanatory diagram of an overcharge protection operation in the embodiment.

FIG. 8 is an explanatory diagram of an overdischarge protection operation in the same embodiment.

FIG. 9 is an explanatory diagram of an overcurrent protection operation in the example.

[Explanation of symbols]

 1A Battery cell block 1B Battery cell block 4 Charge FET 5 Discharge FET 6 Control circuit

Claims (7)

[Claims] 1. A secondary battery in which a plurality of battery cell blocks are connected in series and a charging / discharging switching element that can be on / off controlled by a control circuit is interposed in a charging / discharging path, The control circuit is provided with means for detecting the voltage of the battery cell block and means for controlling the switching element so as to prohibit charging / discharging when the relative voltage between the battery cell blocks exceeds a certain value. Battery protector. 2. A secondary battery in which a plurality of battery cell blocks are connected in series and a charging / discharging switching element, which can be on / off controlled by a control circuit, is interposed in a charging / discharging path. Controls the means for detecting the voltage of the battery cell block and the means for calculating the time differential value of the relative voltage between the battery cell blocks and controlling the switching element so as to prohibit charging / discharging when the differential value exceeds a certain value. A protective device for a secondary battery, which is provided in a circuit. 3. A means for controlling the switching element so as to prohibit charging or discharging when the voltage of any one of the battery blocks exceeds a predetermined voltage range, and a discharge current of a specified value or more at the time of charging and a specified value or more at the time of discharging. 3. The rechargeable battery according to claim 1, wherein the control circuit is provided with means for controlling the switching element so as to release the prohibition of the charging or discharging when the charging currents of the respective are continuous for a predetermined time or more. Protective device. 4. A secondary battery in which a plurality of battery cell blocks are connected in series and a charging / discharging switching element capable of on / off control by a control circuit is interposed in a charging / discharging path is used. A protection device for a secondary battery, characterized in that the control circuit is provided with a detection means, and means for controlling the switching element so as to prohibit charging / discharging when an overcurrent in which the charging current or the discharging current exceeds a predetermined value is detected. . 5. The switching device is controlled so as to start a detection operation when the charging / discharging current exceeds a predetermined value, check an overcurrent after a predetermined delay time, and prohibit the charging / discharging, and to delay the delay time. 5. The secondary battery protection device according to claim 4, wherein is set in inverse proportion to the detected current value. 6. The switching device is controlled so that a detection operation is started when the charging / discharging current exceeds a predetermined value, an overcurrent is confirmed after a predetermined delay time, and the charging / discharging is prohibited. The protection device for the secondary battery according to claim 4, wherein is set in inverse proportion to the square of the detected current value. 7. A switching element is controlled so as to start a detection operation when the charging / discharging current exceeds a predetermined value, check an overcurrent after a predetermined delay time, and prohibit the charging / discharging, and to delay the delay time. 5. The protection device for a secondary battery according to claim 4, wherein is set to be exponentially shorter than the detected current value.