JP4999004B2 - Secondary battery protection circuit and battery pack provided with the secondary battery protection circuit - Google Patents

Description

  The present invention relates to a secondary battery protection circuit that prevents an excessive discharge current or an excessive charging current from flowing through a secondary battery, and a battery pack including the secondary battery protection circuit.

  In recent years, demand for lithium ion secondary batteries having a large discharge capacity per volume has increased with the miniaturization and high functionality of portable terminal devices. In such a secondary battery, when an excessive discharge current or an excessive charging current flows, the electrolytic solution is decomposed to generate heat and gas, resulting in a disadvantage that the battery performance is deteriorated or the battery is damaged.

As a countermeasure, for example, it is conceivable to connect a protection circuit made of a PTC element to the secondary battery (see FIG. 2 of Patent Document 1). According to the protection circuit, when an excessive current flows through the secondary battery, the PTC element itself generates heat, thereby interrupting the current and avoiding deterioration or breakage of the secondary battery.
In the protection circuit shown in FIG. 2 of Patent Document 1, the discharge current and the charge current flow through the same PTC element. In normal operation, the discharge current is generally larger than the charge current. For this reason, a PTC element having a large allowable current value (discharge current limit value) that can correspond to a discharge current in a normal state is selected. However, when a PTC element having a large allowable current value is selected, even if an excessive charging current flows through the secondary battery due to a failure of the charging device or the like, the allowable current value of the PTC element is not reached. The PTC element may not operate.

  In the protection circuit shown in FIG. 1 of Patent Document 1, a first diode and a PTC element are connected in series to a secondary battery, and a second diode other than the first diode is connected to the PTC element and the second diode. The diodes are arranged in parallel to the series circuit of one diode so as to exert a rectifying action in the direction opposite to that of the first diode. According to this mode, the discharge current flows through the second diode in the reverse direction, while the charging current flows through the series circuit. Therefore, by selecting a PTC element with a small allowable current value (charging current limit value) according to the charging current, when an excessive charging current flows through the secondary battery, the PTC element is operated to appropriately adjust the charging current. It becomes possible to shut off with certainty. Thereby, the protection circuit excellent in the safety at the time of overcharge can be obtained.

JP 2004-152580 A (FIGS. 1 and 2)

  The charging device for charging the secondary battery outputs a voltage of about 4.2 V for proper charging, for example, when the battery voltage of the secondary battery is set to 3.7 V. Yes. However, in the form of Patent Document 1, since the charging current flows to the secondary battery via the diode, the charging voltage applied to the secondary battery is equal to the forward voltage drop (about 0.3 V) of the diode. There is a risk that the secondary battery will not be properly charged. Further, in Patent Document 1, since the discharge current flows into the secondary battery without flowing through the PTC element, the discharge current cannot be properly interrupted when an excessive discharge current flows, and the secondary battery The battery may be deteriorated or damaged.

  The present invention has been made to solve the problems of the secondary battery protection circuit as described above, and its purpose is to ensure that an excessive discharge current or an excessive charge current flows through the secondary battery. An object of the present invention is to provide a secondary battery protection circuit that can be prevented and that can apply an appropriate charging voltage to the secondary battery, and a battery pack including the secondary battery protection circuit.

  As shown in FIG. 1, the present invention includes a discharge protection element 3 that cuts off a discharge current when the discharge current flowing through the secondary battery 2 exceeds a preset discharge current limit value, and the charge flowing through the secondary battery 2. A charging protection element 5 is provided for cutting off the charging current when the current becomes larger than a preset charging current limit value. The discharge protection element 3 and the charge protection element 5 are connected in series to the secondary battery 2, and the charge protection element 5 is in parallel with a rectifying element 6 whose forward direction is the discharge current flow direction. It is connected to the. The charging current limit value is set to be smaller than the discharge current limit value.

  Specifically, the rectifying element 6 can be a Schottky barrier diode.

  The discharge protection element 3 and the charge protection element 5 can be PTC elements.

  When the secondary battery 2 is discharged, the discharge current is cut off at a current value smaller than the discharge current limit value. At the time of charge, the charge current is cut off at a current value smaller than the charge current limit value. A current-voltage control circuit 7 that cuts off the discharging current or the charging current can be connected.

  The secondary battery 2 can be configured by connecting a plurality of batteries in series and / or in parallel.

  And this invention shall be a battery pack provided with the protection circuit 1 for secondary batteries comprised as mentioned above.

  In the secondary battery protection circuit according to the present invention, during charging, a reverse voltage is applied to the rectifying element 6 by the charging voltage, so the rectifying element 6 is turned off. For this reason, the charging current does not flow to the rectifying element 6 but flows to the discharge protection element 3 and the charge protection element 5. Therefore, as in the conventional mode (see FIG. 1 of Patent Document 1), the charging voltage is not applied to the secondary battery 2 in a state where the charging voltage is lowered by the forward voltage of the rectifying element 6, and the secondary battery 2 Therefore, the secondary battery 2 can be charged under predetermined appropriate charging conditions. Further, when a charging current larger than the charging current limit value of the charging protection element 5 flows due to a failure of the charging device or the like, the charging protection element 5 cuts off the charging current. There is no possibility that the battery 2 is deteriorated or damaged.

  At the time of discharging, the battery voltage of the secondary battery 2 is applied as a forward voltage to the rectifying element 6 via a load or the like, and the rectifying element 6 is turned on. For this reason, even if a discharge current flows through the rectifying element 6 and hardly flows through the protection element 5 for charging, and a discharge current larger than the charging current limit value of the protection element 5 for charging flows through the protection circuit 1, the protection for charging is performed. The element 5 is not activated. Therefore, although the discharge is normal, even when the discharge current is larger than the charging current limit value of the charging protection element 5, the charging protection element 5 does not operate, and a large discharge current from the secondary battery 2 to the load. Can be supplied properly. When a discharge current larger than the discharge current limit value of the discharge protection element 3 flows, the discharge protection element 3 operates, so that an excessive discharge current is prevented from flowing, and the secondary battery 2 is damaged. Can be prevented.

  If the rectifier element 6 is a Schottky barrier diode, the discharge voltage is reduced in the forward direction of the rectifier element 6 because the forward voltage of the Schottky barrier diode is smaller than the forward voltage of a general PN junction diode. A drop in voltage can be suppressed.

  When the discharge protection element 3 and the charge protection element 5 are PTC elements, it is easier to reduce the size than when the discharge protection element 3 and the charge protection element 5 are formed of a fuse or the like. Therefore, the discharge protection element 3 and the charge protection element 5 can be arranged even in a narrow installation space. This means that the overall configuration of the protection circuit can be made compact. As a result, the degree of freedom in designing the arrangement of the protection circuit and the degree of freedom in designing the equipment (battery pack, etc.) having the protection circuit are reduced. Contribute to improvement.

  When the current / voltage control circuit 7 is connected to the secondary battery 2, the current / voltage control circuit 7 and the protection circuit 1 can prevent the excessive discharge current and the excessive charge current from flowing through the secondary battery 2. Further, it is possible to more reliably prevent the secondary battery 2 from being damaged due to an excessive charging current or an excessive charging current flowing. Further, the current / voltage control circuit 7 cuts off the discharge current or the charging current when the voltage of the secondary battery 2 is abnormal, and can prevent the secondary battery 2 from being damaged due to the voltage abnormality of the secondary battery 2.

  When the secondary battery 2 is configured by connecting a plurality of batteries in series and / or in parallel, an excessive charging current or an excessive charging current flows even in the plurality of batteries. It is possible to reliably prevent the battery from being damaged.

  By providing the secondary battery protection circuit 1 configured as described above in the battery pack, it is possible to obtain a battery pack in which the secondary battery 2 is not damaged due to an excessive charging current or an excessive charging current. . That is, a highly reliable battery pack can be obtained.

  FIG. 1 shows an embodiment of a secondary battery protection circuit according to the present invention. A secondary battery protection circuit (hereinafter simply referred to as a protection circuit) 1 is a discharge PTC element (for discharge) that cuts off the discharge current when the discharge current flowing through the secondary battery 2 exceeds a preset discharge current limit value. Protection element) 3 and a charging PTC element (charging protection element) 5 that cuts off the charging current when the charging current flowing through the secondary battery 2 becomes larger than a preset charging current limit value. The discharging PTC element 3 and the charging PTC element 5 are connected in series to the secondary battery 2, and the charging PTC element 5 includes a Schottky barrier diode (a forward direction in which the discharge current flows). Rectifying element) 6 is connected in parallel. A current / voltage control circuit 7 is connected to a series circuit including the protection circuit 1 and the secondary battery 2. The secondary battery 2 corresponds to a lithium ion secondary battery or the like, and the battery voltage (nominal voltage) of the secondary battery 2 is 3.7V.

  The discharging PTC element 3 and the charging PTC element 5 generate heat in accordance with the flowing current, and the discharging PTC element 3 resists by generating heat in accordance with the current when a current larger than the discharge current limit value flows. The value suddenly increases and the current is cut off (tripped). Similarly, when a current larger than the charging current limit value flows, the charging PTC element 5 suddenly increases in resistance value due to heat generation according to the current, and interrupts (trips) the current.

  The charging current limit value (trip current value) of the charging PTC element 5 is set to 1 A, for example, and is set to be smaller than the discharging current limit value (for example, 6 A) of the discharging PTC element 3. The discharging PTC element 3 has a very low resistance value of 0.03Ω or less in a normal state where the trip is not performed, and the charging PTC element 5 has a low resistance value of 0.3Ω or less in the normal state. As a result, the voltage drop at the PTC elements 3 and 5 in the normal state becomes extremely small. The Schottky barrier diode 6 has a forward voltage drop, that is, a forward voltage (0.2 to 0.4 V) smaller than a forward voltage (0.5 to 0.7 V) of a general PN junction diode. It has become.

  The current / voltage control circuit 7 cuts off the discharge current at a set discharge current value smaller than the discharge current limit value at the time of discharge, and cuts off the charge current at a set charge current value smaller than the charge current limit value at the time of charge. That is, the current / voltage control circuit 7 determines whether the secondary battery 2 is in a discharged state or a charged state based on the battery voltage of the secondary battery 2, and when the discharge current value exceeds the set discharge current value, or It operates when the charge current value exceeds the set charge current value and cuts off the current. Thus, the current / voltage control circuit 7 prevents the discharge current or the charging current flowing through the secondary battery 2 from becoming excessive prior to the protection circuit 1. The protection circuit 1 is activated to cut off the current when the current / voltage control circuit 7 is not activated due to a failure of the current / voltage control circuit 7 or the like.

  In addition, the current voltage control circuit 7 discharges or discharges the secondary battery 2 when an abnormal charging voltage is applied, for example, when an excessive charging voltage is applied during charging or when the battery voltage decreases excessively during discharging. Cut off current. Note that the resistance value with respect to the discharge current and the charge current is extremely low when the current-voltage control circuit 7 is not in operation.

  The protection circuit 1 and the current / voltage control circuit 7 are molded with resin or covered with an exterior cover in a state of being arranged on the outer surface of the secondary battery 2, whereby the secondary battery 2, the protection circuit 1, and A battery pack in which the current / voltage control circuit 7 is integrated is formed. In the battery pack, a positive terminal 10 and a negative terminal 11 for connecting a load such as a charging device (not shown) or a portable terminal (not shown) are exposed through a window formed in the resin mold or the outer cover. The positive electrode terminal 10 is connected to the positive electrode of the secondary battery 2, and the negative electrode terminal 11 is connected to the negative electrode of the secondary battery 2 via the current / voltage control circuit 7 and the protection circuit 1.

Next, the operation of the protection circuit 1 will be described. For convenience of explanation, it is assumed that the current / voltage control circuit 7 does not operate.
First, the case where the secondary battery 2 is charged will be described. When a charging device is connected to the positive terminal 10 and the negative terminal 11, a charging voltage is applied between the terminals 10 and 11. A voltage is applied in the reverse direction to the Schottky barrier diode 6 of the protection circuit 1 by the charging voltage, and the Schottky barrier diode 6 is turned off. Thus, the charging current flows in the order of the secondary battery 2, the charging PTC element 5, and the discharging PTC element 3. For example, as shown in FIG. 2, the charging device starts charging with a constant current (charging current I) of 190 mA, and after a charging time of 4 hours has elapsed from the start of charging, a charging voltage of 4.2 V (charging voltage) Charge with V).

  However, for example, an excessive voltage is applied between the terminals 10 and 11 due to the failure of the charging device, and as shown in FIG. 3, the charging is larger than the charging current limit value I0 of the charging PTC element 5 at the charging start time t0, for example. When the current I flows, the charging PTC element 5 operates and the charging current I is interrupted. This prevents an excessive charging current I from flowing into the secondary battery 2 and damaging the secondary battery 2. Since the discharge current limit value of the discharge PTC element 3 is larger than the charge current limit value I0 of the charge PTC element 5, the discharge PTC element 3 does not operate.

  Next, a case where the secondary battery 2 is discharged will be described. When a load is connected to the positive terminal 10 and the negative terminal 11, a voltage is applied to the Schottky barrier diode 6 in the forward direction, and the Schottky barrier diode is discharged. 6 turns on. Thus, the discharge current flows in the order of the secondary battery 2, the load, the discharge PTC element 3 and the Schottky barrier diode 6, and hardly flows through the charge PTC element 5. That is, a voltage corresponding to the forward voltage of the Schottky barrier diode 6 is applied to the charging PTC element 5 and a current corresponding to the voltage flows, but the forward voltage of the Schottky barrier diode 6 is small. In addition, almost no current flows through the charging PTC element 5.

  For example, if a short circuit accident occurs on the load side and an excessive discharge current flows to the discharge PTC element 3, and the discharge current is larger than the discharge current limit value of the discharge PTC element 3, the discharge PTC element 3 In operation, the discharge current is interrupted. This prevents an excessive discharge current from flowing into the secondary battery 2 and damaging the secondary battery 2. As described above, since the discharge current hardly flows through the charging PTC element 5, the charging PTC element 5 does not operate.

  As described above, according to the protection circuit 1 according to the present invention, at the time of charging, the Schottky barrier diode 6 is turned off, and the charging current flows to the charging PTC element 5 and the discharging PTC element 3 to cause the shot. It does not flow through the key barrier diode 6. That is, the charging voltage is prevented from being applied to the secondary battery 2 in a state where the charging voltage is lowered by the forward voltage of the Schottky barrier diode 6. Therefore, an appropriate charging voltage is applied to the secondary battery 2, and the secondary battery 2 is appropriately charged. Even when the current / voltage control circuit 7 does not operate due to a failure or the like, when a charging current larger than the charging current limit value of the charging PTC element 5 flows, the charging PTC element 5 operates reliably and is excessive. The secondary battery 2 can be reliably prevented from being damaged by the charging current.

  At the time of discharging, the Schottky barrier diode 6 is turned on and the discharging current hardly flows through the charging PTC element 5, so that a discharging current larger than the charging current limit value of the charging PTC element 5 flows into the protection circuit 1. However, the charging PTC element 5 does not operate. Therefore, although the discharge is normal, the charging PTC element 5 does not operate even when the discharge current is larger than the trip current value of the charging PTC element 5. For this reason, a large discharge current can be appropriately supplied from the secondary battery 2 to the load. Even if the current / voltage control circuit 7 does not operate due to a failure or the like, when a discharge current larger than the discharge current limit value of the discharge PTC element 3 flows, the discharge PTC element 3 operates reliably and is excessive. It is possible to prevent the secondary battery 2 from being damaged by the flow of a proper discharge current.

  The rectifying element 6 connected in parallel to the charging PTC element 5 is preferably as the forward voltage is low, and may be a point contact type germanium diode or the like instead of the Schottky barrier diode. The discharge PTC element 3 and the charge PTC element 5 can self-recover and flow the discharge current and the charge current again when the temperature decreases after the discharge current and the charge current are cut off. Instead of the PTC element 3 and the charging PTC element 5, a fuse or the like may be used that permanently cuts off the current when a current equal to or higher than the set current flows.

  The secondary battery 2 may be configured by connecting a plurality of batteries in series and / or in parallel.

It is a circuit diagram of the protection circuit for secondary batteries which concerns on this invention. It is a chart for demonstrating an appropriate charge state. It is a graph for demonstrating the charging current at the time of abnormality.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Secondary battery protection circuit 2 Secondary battery 3 Discharge PTC element 5 Charging PTC element 6 Schottky barrier diode 7 Current voltage control circuit

Claims (6)

When the discharge current flowing through the secondary battery becomes larger than a preset discharge current limit value, the discharge protection element that cuts off the discharge current, and the charge current flowing through the secondary battery is larger than the preset charge current limit value And having a protective element for charging that cuts off the charging current,
The discharge protection element and the charge protection element are connected in series to the secondary battery,
The charging protection element is connected in parallel with a rectifying element whose forward direction is the direction in which the discharge current flows,
A secondary battery protection circuit, wherein the charging current limit value is set to be smaller than the discharge current limit value.   The secondary battery protection circuit according to claim 1, wherein the rectifying element is a Schottky barrier diode.   The secondary battery protection circuit according to claim 2, wherein the discharge protection element and the charge protection element are PTC elements.   When the secondary battery is discharged, the discharge current is cut off at a current value smaller than the discharge current limit value, and at the time of charging, the charge current is cut off at a current value smaller than the charge current limit value. 4. The secondary battery protection circuit according to claim 1, wherein a current-voltage control circuit that cuts off the discharging current or the charging current is connected when an abnormality occurs. 5.   The secondary battery protection circuit according to claim 1, wherein the secondary battery is configured by connecting a plurality of batteries in series and / or in parallel.   A battery pack comprising the secondary battery protection circuit according to claim 1.

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