Power storage device

Abstract

To provide a power storage device capable of achieving a low voltage protection function and suppressing thermal destruction of a power FET due to electric conduction in a high resistance state.SOLUTION: A power storage device 20 includes: a battery pack 30; a voltage fluctuation detection circuit 170 which detects voltage fluctuations of the battery pack 30; an internal power supply circuit 120 powered by the battery pack; an N-channel power FET 71 which interrupts discharge of the battery pack 30; a main switch 150 provided on a line connecting the internal power supply circuit 120 and a gate of the power FET 71; and a control circuit 130. When the voltage of the battery pack 30 falls below a threshold voltage, the voltage fluctuation detection circuit 170 interrupts the power FET 71 by turning OFF the main switch 150 via the control circuit 130. The threshold voltage is higher than the gate voltage at which the power FET 71 enters a high-resistance state.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a technique for suppressing current flow in a high resistance state of an FET. [Background technology]

[0002] One type of protection device for an electricity storage device is a current interruption device. A relay or a power FET can be used for the current interruption device. Patent Document 1 is an example of a document that uses a power FET for a current interruption device. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-166454 Summary of the Invention [Problem to be solved by the invention]

[0004] One of the protection measures for a power storage device is protection against low voltage abnormalities, which is generally performed from the viewpoint of battery protection.

[0005] However, when the voltage of the storage device drops, an incomplete voltage may be applied to the gate at the same time, which may cause the power FET to conduct electricity in a high resistance state, leading to thermal breakdown. An object of the present invention is to suppress thermal destruction of a power FET caused by current flow in a high resistance state while realizing a low voltage protection function for a power storage device. [Means for solving the problem]

[0006] The power storage device includes a battery pack, a voltage fluctuation detection circuit that detects voltage fluctuations in the battery pack, an internal power supply circuit powered by the battery pack, an N-channel power FET that cuts off discharge of the battery pack, a main switch provided on a gate line connecting the internal power supply circuit and the gate of the power FET, and a control circuit. When the voltage of the battery pack falls below a threshold voltage, the voltage fluctuation detection circuit turns off the main switch via the control circuit, thereby opening the power FET. The threshold voltage is a voltage higher than the gate voltage at which the power FET enters a high-resistance state. [Effects of the Invention]

[0007] The present invention not only realizes a low voltage protection function for a power storage device, but also makes it possible to suppress thermal destruction of a power FET caused by current flow in a high resistance state. [Brief explanation of the drawings]

[0008] [Figure 1] Side view of a motorcycle [Figure 2] Block diagram of a motorcycle battery [Figure 3] Exploded perspective view of the battery [Figure 4] Battery Schematic [Figure 5] Isolation circuit diagram [Figure 6] Diagram showing the discharge path of the charge accumulated in the gate (comparison example) [Figure 7] Power FET D-S resistance vs. G-S voltage characteristics [Figure 8] ASO characteristics [Figure 9] Diagram showing the discharge path of the charge accumulated in the gate DETAILED DESCRIPTION OF THE INVENTION

[0009] (Outline of this embodiment) (1) A power storage device includes a battery pack, a voltage fluctuation detection circuit that detects voltage fluctuations in the battery pack, an internal power supply circuit powered by the battery pack, an N-channel power FET that cuts off discharge of the battery pack, a main switch provided on a gate line connecting the internal power supply circuit and the gate of the power FET, and a control circuit. When the voltage of the battery pack falls below a threshold voltage, the voltage fluctuation detection circuit turns off the main switch via the control circuit to open the power FET. The threshold voltage is a voltage higher than the gate voltage at which the power FET enters a high resistance state. In the power storage device of (1), any configuration other than the above is optional and may be used.

[0010] According to the energy storage device described in (1), when the battery pack voltage falls below the threshold voltage, the voltage fluctuation detection circuit switches the main switch from ON to OFF via the control circuit. This causes the gate voltage to drop, opening the N-channel power FET and cutting off discharge. This protects the energy storage device from low-voltage abnormalities such as over-discharge.

[0011] Because the threshold voltage is higher than the gate voltage at which the N-channel power FET enters a high resistance state, it is possible to prevent an incomplete voltage from being applied to the gate when the voltage of the energy storage device drops, causing the power FET to enter a high resistance state and conduct electricity, thereby preventing thermal breakdown of the N-channel power FET.

[0012] (2) The energy storage device described in (1) further includes a gate-source resistor disposed between the gate and source of the power FET, and a discharge circuit having a resistance smaller than that of the gate-source resistor, which discharges parasitic capacitance between G and S of the power FET. The voltage fluctuation detection circuit may be configured to conduct the discharge circuit to discharge the parasitic capacitance between G and S when the voltage of the battery pack falls below a threshold voltage. According to (2), the parasitic capacitance C between G and S is discharged using a discharge circuit having a resistance smaller than that of the gate-source resistor, thereby improving the responsiveness of the power FET when it is turned off and shortening the switching time. This prevents current from passing through in a high-resistance state when it is turned off, thereby preventing the power FET from being thermally damaged.

[0013] (3) In the energy storage device described in (1) or (2), the internal power supply circuit may include a backup circuit that maintains the power supply voltage when the voltage of the battery pack drops. According to (3), it is possible to prevent the voltage fluctuation detection circuit from stopping when the voltage of the battery pack drops.

[0014] (4) In the power storage device according to any one of (1) to (3), the voltage fluctuation detection circuit may turn off the power FET by turning off the main switch via the control circuit when the voltage of the battery pack falls below a threshold voltage and this state continues for a time required for engine cranking or longer. According to (4), it is possible to prevent the power FET from being turned off due to a temporary voltage drop caused by cranking.

[0015] <Embodiment 1> 1. Battery structure explanation 1, a battery 20 (an example of a power storage device) according to this embodiment is a battery for a motorcycle mounted on a motorcycle 10. The battery 20 has a rated voltage of 12 volts (V) and can be used to replace (for example, as a retrofit) conventional lead-acid batteries.

[0016] As shown in Fig. 2, a starter 10A, an alternator 10B, and accessories 10C (headlights, a car navigation system, etc.) mounted on a motorcycle 10 are connected to a battery 20. The battery 20 supplies 12V power to the starter 10A to start the engine. The battery 20 is charged by the alternator 10B while the engine is running.

[0017] 3, the battery 20 includes a management unit 53, a plurality of storage cells 3 (an example of a storage element), and a rectangular parallelepiped storage case 40 that houses them. The storage cells 3 may be battery cells such as lithium ion secondary batteries, or may be electrochemical cells such as capacitors.

[0018] Four storage cells 3 are connected in series to form the assembled battery 30. Alternatively, some of the storage cells 3 may be connected in parallel. For example, the assembled battery 30 may have eight storage cells 3 connected in two parallel connections and four in series, or twelve storage cells 3 connected in three parallel connections and four in series.

[0019] The storage case 40 is made of synthetic resin. The storage case 40 includes a case body 41, a lid 42 that closes the opening of the case body 41, a storage section 43 provided in the lid 42, a cover 44 that covers the storage section 43, an inner lid (bus bar frame) 45, and a partition plate 46. The inner lid 45 and the partition plate 46 do not necessarily have to be provided. The energy storage cells 3 are inserted between the partition plates 46 of the case body 41.

[0020] A plurality of metal bus bars 47 (conductive members) are placed on the inner lid 45. The inner lid 45 is placed near the terminal surface on which the cell terminals 32 of the storage cells 3 are provided, and the adjacent cell terminals 32 of adjacent storage cells 3 are connected by the bus bars 47, so that the storage cells 3 are connected in series.

[0021] The storage section 43 is box-shaped and has a protruding portion 43a that protrudes outward from the center of one long side in a plan view. A positive terminal 51 and a negative terminal 52 made of metal such as a lead alloy are provided on both sides of the protruding portion 43a on the lid section 42. A management unit 53 is stored in the storage section 43. The management unit 53 is connected to the energy storage cells 3 via wiring members and bus bars 47 (not shown). Instead of being stored in the storage section 43, the management unit 53 may be disposed adjacent to the battery pack 30, for example, above or to the side. The management unit 53 may have multiple circuit boards.

[0022] The energy storage cell 3 includes a hollow rectangular parallelepiped case 31 and a pair of cell terminals 32, 32 with opposite polarities provided on one side (terminal surface, top surface) of the case 31. The case 31 accommodates an electrode assembly 33 formed by stacking a positive electrode, a separator, and a negative electrode, and an electrolyte (electrolytic solution) not shown.

[0023] Although not shown in detail, the electrode assembly 33 is constructed by stacking a sheet-shaped positive electrode and a sheet-shaped negative electrode with two sheet-shaped separators in between and winding them (vertical or horizontal). The separators are formed from a porous resin film. Examples of the porous resin film that can be used include porous resin films made of resins such as polyethylene (PE) and polypropylene (PP).

[0024] The positive electrode is an electrode plate in which a positive electrode active material layer is formed on the surface of a long strip-shaped positive electrode substrate made of, for example, aluminum, an aluminum alloy, or the like. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material used in the positive electrode active material layer can be a material capable of absorbing and releasing lithium ions. The positive electrode active material is, for example, LiFePO4, but is not limited thereto, and so-called ternary positive electrode active materials may also be used. The positive electrode active material layer may further contain a conductive additive, a binder, etc.

[0025] The negative electrode is an electrode plate in which a negative electrode active material layer is formed on the surface of a long strip-shaped negative electrode substrate made of, for example, copper or a copper alloy. The negative electrode active material layer contains a negative electrode active material. The negative electrode active material can be a material capable of absorbing and releasing lithium ions. Examples of the negative electrode active material include graphite, hard carbon, and soft carbon. The negative electrode active material layer may further contain a binder, a thickener, and the like.

[0026] The electrolyte housed in the housing case 40 together with the electrode assembly 33 can be the same as that used in conventional lithium-ion secondary batteries. For example, an electrolyte containing a supporting salt in an organic solvent can be used. As the organic solvent, for example, an aprotic solvent such as carbonates, esters, or ethers can be used. As the supporting salt, for example, a lithium salt such as LiPF6, LiBF4, or LiClO4 can be suitably used. The electrolyte may contain various additives such as a gas generating agent, a film-forming agent, a dispersant, or a thickener.

[0027] FIG. 3 shows a prismatic lithium ion battery including a wound electrode assembly 33 as an example of the storage cell 3. Alternatively, the storage cell 3 may be a cylindrical lithium ion battery or a laminated (pouch) lithium ion battery. The storage cell 3 may also be a lithium ion battery including a laminated electrode assembly. The storage cell 3 may also be an all-solid-state lithium ion battery using a solid electrolyte.

[0028] 2. Description of the Electrical Configuration of Battery 20 4 is a circuit diagram of the battery 20. The battery 20 is composed of a battery pack 30, a circuit breaker 70, a protection IC 110, an internal power supply circuit 120, a control circuit 130, a main switch 150, and a recovery circuit 160. Each of these circuits 110 to 160 is provided on the management unit 53, for example.

[0029] The battery pack 30 is composed of four storage cells 3 connected in series. The positive electrode of the battery pack 30 is connected to a positive terminal 51 via a power line 53a. The negative electrode of the battery pack 30 is connected to a negative terminal 52 via a power line 53b.

[0030] The breaker device 70 is disposed on the power line 53b on the negative electrode side of the battery pack 30. The breaker device 70 is composed of a power FET 71 and a power FET 72.

[0031] The power FETs 71 and 72 are N-channel. FET stands for Field Effect Transistor. The source of the power FET 71 is connected to the negative electrode of the battery pack 30, and the drain is connected to the drain of the power FET 72. The source of the power FET 72 is connected to the negative terminal 52 of the battery 20.

[0032] The power FET 71 is a discharge cutoff switch that cuts off discharging, and the power FET 72 is a charge cutoff switch that cuts off charging.

[0033] A gate-source resistor R1 is connected between the gate and source of the power FET 71. A gate-source resistor R2 is connected between the gate and source of the power FET 72. The gate-source resistors are provided to prevent the power FETs 71 and 72 from floating and causing the circuit to malfunction.

[0034] The protection IC 110 monitors the state of the battery 20. Items monitored include the cell voltage of each storage cell 3, the total voltage, current, and temperature of the battery pack 30, etc. If the protection IC 110 detects an abnormality in the battery 20, it outputs a HIGH level shutdown signal S1 to the control circuit 130. The protection IC 110 is an optional component and may be omitted.

[0035] The internal power supply circuit 120 is a power supply circuit for the protection IC 110 and a voltage fluctuation detection circuit 170 (described later). The internal power supply circuit 120 includes a first power supply line L1, a second power supply line L2, an isolation circuit 121, and a backup circuit 125.

[0036] The first power supply line L1 is connected to the positive electrode of the battery pack 30. The second power supply line L2 is connected to the first power supply line L1 via a separation circuit 121.

[0037] The separation circuit 121 is a circuit that separates the two power supply lines L1 and L2 when the voltage of the battery pack 30 drops. As shown in FIG. 5, the separation circuit 121 can be configured with a voltage holding circuit 122, a switch 123, a resistor 124, etc.

[0038] The backup circuit 125 is a circuit that maintains the voltage of the second power supply line L2 for a predetermined time when the voltage of the battery pack 30 drops. In this example, the backup circuit 125 is made up of a plurality of capacitors.

[0039] The diode D1 has an anode connected to the second power supply line L2 via the main switch 150, and a cathode connected to the gate G of the power FET 71 via a resistor R3.

[0040] The diode D2 has an anode connected to the second power supply line L2 via the main switch 150, and a cathode connected to the gate G of the power FET 72. L3 shown in FIG.

[0041] The control circuit 130 switches the main switch 150 from CLOSE to OPEN when a HIGH-level shutoff signal S1 or S2 is output from the protection IC 110 or a voltage fluctuation detection circuit 170 (described later). The recovery circuit 160 is a circuit that returns the main switch 150 from OPEN to CLOSE upon detection of external charging.

[0042] 2. Issues with undervoltage protection operation <Assignment 1> When the power FET 71 is opened to cut off the discharge of the battery 20, the parasitic capacitance C between G and S must be discharged and the gate voltage must be lowered to a low level (0 V), as shown in Figure 6. A gate-source resistor R1 can be used for this discharge. To instantly lower the gate voltage and improve responsiveness, it is desirable to reduce the resistance value of the gate-source resistor R1.

[0043] However, because the gate-source resistor R1 constantly consumes current, if the resistance value is reduced too much, the current consumption increases, and there is room for improvement in terms of the toughness (long-term storage performance) of the battery 20. In other words, when the gate-source resistor R1 is used for gate discharge, there is a trade-off between responsiveness and current consumption.

[0044] 7, the threshold voltage of the power FET 71 is approximately 5V, and when a voltage of approximately 5V or more is applied to the gate via the gate line L3, the power FET 71 closes (conducts). The D-S section of the power FET 71 enters a high resistance state when the gate voltage is 4V or less, and opens when the gate voltage drops to a low level (0V). If the gate-source resistance R1 is high, the gate voltage of the power FET 71 drops gradually when the power FET 71 opens, and current flows in a high resistance state when the gate voltage is around 3.8V to 4V.

[0045] As a result, as shown in part A of Figure 8, the area of ​​safe operation (ASO) of the power FET 71 may be exceeded, potentially causing thermal breakdown. One way to prevent thermal breakdown is to increase the number of power FETs 71 connected in parallel and reduce the current per FET, but this increases costs. Therefore, it is desirable to instantly transition the gate voltage of the power FET 71 to a low level to avoid current flow in a high resistance state.

[0046] <Assignment 2> Conventionally, battery 20 has been equipped with a low voltage protection function from the viewpoint of battery protection, but has not been equipped with a function to shut off power FET 71 before power FET 71 reaches a high resistance state, and no measures have been taken to prevent thermal breakdown of power FET 71. In particular, in the case of specifications without a current detection circuit, it is not possible to detect abnormal current and provide protection, so there is a high need for measures.

[0047] One use case that can cause thermal breakdown is a lock current. In particular, if the motor of the motorcycle 10 locks and current continues to flow in a low-temperature environment and the battery 10 is in a low SOC state, the voltage of the battery 20 drops, and the gate voltage of the power FET also drops. As a result, an incomplete voltage, for example, less than 4V, is applied to the gate G of the power FET 71, allowing current to flow in a high-resistance state. This raises concerns about thermal breakdown.

[0048] To solve the above problems 1 and 2, the battery 20 includes a voltage fluctuation detection circuit 170 and a discharge circuit 190 in addition to the assembled battery 30, the circuit breaker 70, the protection IC 110, the internal power supply circuit 120, the control circuit 130, the main switch 150, the recovery circuit 160, etc., as shown in FIG.

[0049] The voltage fluctuation detection circuit 170 is connected to the positive electrode of the battery pack 30 via a signal line L4, and is connected to the second power supply line L2 via a power supply line L5. The voltage fluctuation detection circuit 170 detects voltage fluctuations in the positive electrode voltage Vg of the battery pack 30 and issues a low voltage protection command, outputting a HIGH level shutdown signal S2 if the following low voltage protection conditions are met, and outputting a LOW level otherwise.

[0050] <Low voltage protection conditions> The positive electrode voltage Vg of the battery pack 30 is less than the threshold voltage V2, and this state continues for at least the time required for cranking. The threshold voltage V2 is higher than the gate voltage V1 at which the power FET 71 enters a high resistance state (V2>V1). For example, V2=5V and V1=4V (see FIG. 7). V2 and V1 may be values ​​other than 5V and 4V.

[0051] The output of the voltage fluctuation detection circuit 170 is branched into two, one of which is connected to the control circuit 130 via a diode 181 and the other of which is connected to the gate G of the discharge circuit 190 via a diode 191 .

[0052] The discharge circuit 190 is a circuit that improves the responsiveness of the power FET 71 when it is turned off by discharging the parasitic capacitance C between the G and S terminals of the power FET 71. The resistance value of the discharge circuit 190 is smaller than the resistance value of the gate-source resistor R1 of the power FET 71. The discharge circuit 190 is, for example, an N-channel FET, and has a source connected to ground and a drain connected to the gate G of the power FET 71.

[0053] An RC circuit consisting of a resistor 183 and a capacitor 185 is disposed at the input of the control circuit 130, and an RC circuit consisting of a resistor 193 and a capacitor 195 is disposed at the gate G of the discharge circuit 190.

[0054] The two RC circuits are provided for the purpose of adjusting the timing so that the main switch 150 switches from ON to OFF before the discharge circuit 190 becomes conductive after a HIGH-level shutdown signal S2 is output from the voltage fluctuation detection circuit 170. In other words, the circuit constants of the resistor 193 and the capacitor 195 are determined so that the transient response of the resistor 183 and the capacitor 185 is faster than the transient response of the resistor 193 and the capacitor 195.

[0055] By turning off the main switch 150 first, the discharge circuit 190 operates to instantly drop the voltage of the second power line L2, preventing the protection IC 100 and the voltage fluctuation detection circuit 170 from stopping operation.

[0056] 3. Operation explanation The low voltage protection operation of the battery pack 30 will now be described. The positive electrode voltage Vg of the battery pack 30 under normal conditions is assumed to be approximately 12V. Under normal conditions, the main switch 150 is ON, a voltage of 12V is applied to the gate of the power FET 71 via the gate line L3, and the power FET 71 is closed. The discharge circuit 190 is non-conductive. The threshold voltage V2 is assumed to be 5V.

[0057] <When the positive electrode voltage Vg of the battery pack 30 drops to 5 V to 10 V due to discharge, etc.> The first power supply line L1 drops to 5 to 10 V, but the second power supply line L2 maintains 12 V for a certain period of time due to the influence of the backup circuit 125.

[0058] In this case, Vg≧5V, which does not satisfy the low voltage protection condition, and therefore the voltage fluctuation detection circuit 170 outputs a LOW level (GND level) signal.

[0059] When the voltage fluctuation detection circuit 170 outputs a LOW level signal, the discharge circuit 190 remains non-conductive. In addition, the control circuit 130 keeps the main switch 150 ON. As a result, the gate G remains HIGH level (12 V), and the power FET 71 remains CLOSE. Similarly, the power FET 72 also remains CLOSE.

[0060] <When the positive electrode voltage Vg of the battery pack 30 drops to 4.5 V> The first power supply line L1 drops to 4.5V, but the second power supply line L2 maintains 12V for a certain period of time due to the influence of the backup circuit 125.

[0061] In this case, since Vg<5V, if the voltage drop continues for a period of time required for cranking or longer, the low voltage protection condition is met, and the voltage fluctuation detection circuit 170 outputs a HIGH level shutoff signal S2.

[0062] When the interruption signal S2 is output from the voltage fluctuation detection circuit 170, the control circuit 130 first turns off the main switch 150. After that, the discharge circuit 190 becomes conductive (ON).

[0063] 9, the parasitic capacitance C between G and S of the power FET 71 is discharged through a path that passes through the discharge circuit 190. As a result, the gate voltage drops instantaneously to a low level, and the power FET 71 switches from CLOSE to OPEN.

[0064] This cuts off the discharge of the battery pack 30 (low voltage protection). Furthermore, since the power FET 72 discharges the parasitic capacitance C through the gate-source resistor R2, it responds slowly but opens when the gate voltage drops to a low level.

[0065] Thereafter, when the low voltage protection condition is no longer satisfied, for example, when the positive electrode voltage Vg of the battery pack 30 returns to 5 V or higher, the voltage fluctuation detection circuit 170 again outputs a LOW level signal.

[0066] When a LOW level signal is output from the voltage fluctuation detection circuit 170, the control circuit 130 switches the main switch 150 from OFF to ON. Also, the discharge circuit 190 switches from conductive (ON) to non-conductive (OFF) and is cut off.

[0067] As a result, when a threshold voltage (5V or more in this example) is applied to the gate via the gate line L3, the power FET 71 switches from OPEN to CLOSE. Similarly, the power FET 71 also switches from OPEN to CLOSE.

[0068] 4.Effects (1) The low voltage protection condition is that the positive electrode voltage Vg of the battery pack 30 is less than the threshold voltage V2 and that this state continues for at least the time required for cranking, so that the battery 20 can be protected from low voltage abnormalities such as over-discharge while avoiding shutdown due to a voltage drop caused by cranking (within the normal usage range). Possible use cases for shutdown include a voltage drop due to an abnormally large current (such as a short circuit), a voltage drop in a short period of time due to battery deterioration, and a voltage drop due to insufficient capacity, all of which are necessary measures for the safe use of the battery 20.

[0069] In this embodiment, the threshold voltage V2 is set to 5 V, but in many cases the motorcycle itself is generally designed to stop control (to stop operating) when the voltage drops below 8 to 9 V. Therefore, even if the power FET 71 is opened at a voltage below 8 V (in the above example, the threshold voltage V2 is less than 5 V), the impact on convenience is considered to be small.

[0070] (2) As shown in Fig. 7, the threshold voltage V2 at which the power FET 71 is opened is higher than the gate voltage V1 at which the power FET 71 enters a high resistance state. This prevents an intermediate voltage of less than 4V from being applied to the gate G when the voltage of the battery pack 30 drops, causing the power FET 71 to enter a high resistance state and conduct electricity. As a result, it is possible to prevent the power FET 71 from being thermally destroyed.

[0071] (3) The battery 20 discharges the parasitic capacitance C between G and S using the discharge circuit 190, which has a resistance smaller than the gate-source resistance R1, thereby improving the responsiveness of the power FET 71 when it is turned off and shortening the switching time. This makes it possible to avoid current flow in a high resistance state when it is turned off, and to prevent the power FET 71 from being thermally destroyed without increasing the number of parallel connections.

[0072] (4) The discharge circuit 190 is conductive only during low voltage protection operation (the period during which the voltage fluctuation detection circuit 170 outputs the shutdown signal S2), and is otherwise shut off. Therefore, compared to when the gate-source resistor R1 is used for gate discharge, current consumption can be reduced, improving the toughness (long-term storage performance) of the battery 20. Note that the gate-source resistor R1 can have a high resistance because discharge characteristics do not need to be taken into consideration.

[0073] In short, the battery 20 can eliminate the trade-off between responsiveness at the time of cutoff and current consumption, which has been an issue when the gate-source resistor R1 is used for gate discharge.

[0074] <Other embodiments> The present invention is not limited to the embodiments described above and illustrated in the drawings, and the following embodiments, for example, are also included within the technical scope of the present invention.

[0075] (1) In the above embodiment, the battery 20 is for a motorcycle. However, the use of the battery 20 is not limited to motorcycles. The battery 20 may be for a four-wheeled vehicle or for use in an automobile or vehicle other than a motor vehicle.

[0076] (2) In the above embodiment, the low voltage protection condition is that the positive electrode voltage Vg of the battery pack 30 is less than the threshold voltage V2 and that this state continues for at least the time required for cranking. The low voltage protection condition may simply be that the positive electrode voltage Vg of the battery pack 30 is less than the threshold voltage V2, provided that V2 > V1. The upper limit of V2 may also be the voltage at which control of the motorcycle (host device) is stopped.

[0077] (3) In the above embodiment, the parasitic capacitance C between the gate and source of the power FET 71 is discharged by the discharge circuit 190. The discharge circuit 190 may be omitted, and the parasitic capacitance C may be discharged through the gate-source resistor R1.

[0078] (4) In the above embodiment, the interrupter 70 is configured with the power FET 71 that interrupts discharging and the power FET 72 that interrupts charging. The interrupter 70 may be configured with the power FET 71 that interrupts discharging, and the power FET 72 that interrupts charging may be omitted. [Explanation of symbols]

[0079] 20 Battery (electricity storage device) 30 battery packs 70 Circuit Breaker 71, 72 Power FET 120 Internal power supply circuit 130 control circuit 170 Voltage fluctuation detection circuit 190 Discharge circuit C parasitic capacitance R1, R2 Gate-source resistors

Claims

1. An electricity storage device, A battery pack; a voltage fluctuation detection circuit for detecting voltage fluctuations of the battery pack; an internal power supply circuit powered by the battery pack; an N-channel power FET that cuts off discharge of the battery pack; a main switch provided on a gate line connecting the internal power supply circuit and the gate of the power FET; a control circuit; When the voltage of the battery pack falls below a threshold voltage, the voltage fluctuation detection circuit turns off the main switch via the control circuit, thereby opening the power FET; The power storage device, wherein the threshold voltage is a voltage higher than a gate voltage at which the power FET is in a high resistance state.

2. The power storage device according to claim 1, a gate-source resistor disposed between the gate and source of the power FET; a discharge circuit having a resistance smaller than that of the gate-source resistor, and discharging a parasitic capacitance between G and S of the power FET; When the voltage of the battery pack falls below a threshold voltage, the voltage fluctuation detection circuit makes the discharge circuit conductive to discharge the parasitic capacitance between G and S.

3. The power storage device according to claim 1 or 2, The internal power supply circuit includes a backup circuit that maintains a power supply voltage when the voltage of the battery pack drops.

4. 3. The engine starting power storage device according to claim 1, When the voltage of the battery pack is less than a threshold voltage and this state continues for a time required for cranking the engine or longer, the voltage fluctuation detection circuit turns off the main switch via the control circuit, thereby opening the power FET.

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