Patent Publication Number: US-2022221520-A1

Title: Energy storage apparatus, capacity estimation method for energy storage device, and capacity estimation program for energy storage device

Description:
TECHNICAL FIELD 
     The present invention relates to a technique for estimating a capacity of an energy storage device. 
     BACKGROUND ART 
     Some energy storage apparatuses include a measurement unit that measures a current and a voltage of an energy storage device, and a management unit that manages the energy storage device. Patent Document 1 below describes that power is supplied from an energy storage device to a measurement unit and a management unit. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-A-2017-184534 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In a case where the energy storage device supplies power to the measurement unit, when the voltage of the energy storage device becomes equal to or lower than the minimum operating voltage of the measurement unit, the measurement unit stops to be in an unmeasurable state. When the measurement unit is in an unmeasurable state, the current and the voltage of the energy storage device cannot be measured, and the capacity of the energy storage device becomes unstable. 
     An object of the present invention is to estimate a capacity of an energy storage device even when a voltage of the energy storage device becomes equal to or lower than a minimum operation of a measurement unit. 
     Means for Solving the Problems 
     According to one aspect of the present invention, there is provided an energy storage apparatus including: an external terminal; an energy storage device; a switch located in a current path from the energy storage device to the external terminal; a measurement unit that measures a current and a voltage of the energy storage device; and a management unit. The measurement unit and the management unit receive power supply from the energy storage device. A minimum operating voltage of the management unit is lower than a minimum operating voltage of the measurement unit. The management unit cuts off discharge to an outside through the external terminal by controlling the switch when the voltage of the energy storage device falls below a threshold voltage equal to or higher than the minimum operating voltage of the measurement unit, and the management unit estimates a capacity of the energy storage device after an arbitrary time point in a first period in which the voltage of the energy storage device decreases from the threshold voltage to the minimum operating voltage of the measurement unit based on an elapsed time from the arbitrary time point. 
     The present technology can be applied to a capacity estimation method for an energy storage apparatus, a capacity estimation program, and a recording medium in which the capacity estimation program is recorded. 
     Advantages of the Invention 
     According to the present technology, it is possible to estimate the capacity of the energy storage device even when the voltage of the energy storage device becomes equal to or lower than the minimum operation of the measurement unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a vehicle according to an embodiment. 
         FIG. 2  is an exploded perspective view of a battery. 
         FIG. 3  is a plan view of a secondary battery. 
         FIG. 4  is a sectional view taken along the line A-A of  FIG. 3 . 
         FIG. 5  is a block diagram illustrating an electrical configuration of the vehicle. 
         FIG. 6  is a block diagram illustrating an electrical configuration of the battery. 
         FIG. 7  is a block diagram of a charge detection circuit. 
         FIG. 8  is a flowchart of a monitoring process. 
         FIG. 9  is a flowchart of a capacity estimation process. 
         FIG. 10  is a graph showing a voltage transition of an assembled battery after parking. 
         FIG. 11  is a flowchart of charge control. 
         FIG. 12  is a diagram illustrating a reusable area and a reuse prohibition area of the secondary battery. 
         FIG. 13  is a data table of an elapsed time T and a capacity C of the secondary battery. 
         FIG. 14  is a circuit diagram of a switch. 
         FIG. 15  is a graph showing the voltage transition of the assembled battery after parking. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An energy storage apparatus includes: an external terminal; an energy storage device; a switch located in a current path from the energy storage device to the external terminal; a measurement unit that measures a current and a voltage of the energy storage device; and a management unit. The measurement unit and the management unit receive power supply from the energy storage device. A minimum operating voltage of the management unit is lower than a minimum operating voltage of the measurement unit. The management unit cuts off discharge to an outside through the external terminal by controlling the switch when the voltage of the energy storage device falls below a threshold voltage equal to or higher than the minimum operating voltage of the measurement unit, and the management unit estimates a capacity of the energy storage device after an arbitrary time point in a first period in which the voltage of the energy storage device decreases from the threshold voltage to the minimum operating voltage of the measurement unit based on an elapsed time from the arbitrary time point. 
     When the voltage of the energy storage device falls below the threshold voltage equal to or higher than the minimum operating voltage of the measurement unit, the management unit cuts off the discharge to the outside. When the discharge to the outside is cut off, the current is not taken out from the energy storage device to the outside. Since the energy storage device is in a state of discharge a constant current such as a current consumed inside the energy storage apparatus and a self-discharge current, a capacity decrease amount is substantially proportional to the elapsed time. By estimating the capacity of the energy storage device based on the elapsed time, the capacity can be estimated even after the measurement unit is stopped. 
     The management unit may calculate a capacity decrease amount of the energy storage device after an arbitrary time point based on the elapsed time, and the management unit may estimate a capacity of the energy storage device after the arbitrary time point by subtracting the capacity decrease amount from the capacity of the energy storage device at the arbitrary time point. When the voltage of the assembled battery is equal to or higher than the minimum operating voltage of the measurement unit, the measurement unit can measure the voltage and the current. Therefore, the capacity of the energy storage device at an arbitrary time point can be estimated with less error based on the measurement value of the measurement unit. Since the capacity of the energy storage device after the arbitrary time point is estimated by subtracting the capacity decrease amount according to the elapsed time from the capacity at the arbitrary time point with less estimation error, the capacity estimation accuracy is high. 
     The management unit may permit charge to the energy storage device by controlling the switch when the charge is detected after the arbitrary time point if the capacity of the energy storage device is within a reusable area. 
     When the capacity is within the reusable area, the energy storage device can be reused by receiving charge. Accordingly, the usability of the energy storage device is enhanced. 
     The management unit may prohibit charge to the energy storage device by controlling the switch when the charge is detected after the arbitrary time point if the capacity of the energy storage device is within a reuse prohibition area. 
     When the capacity is within the reuse prohibition area, the energy storage device can be prevented from being reused by prohibiting the receiving of charge. By prohibiting reuse, safety can be ensured. 
     The energy storage apparatus may be for a vehicle, the management unit may cut off discharge to an outside through the external terminal by controlling the switch when the voltage of the energy storage device falls below a threshold voltage equal to or higher than the minimum operating voltage of the measurement unit while the vehicle is parked, and the management unit may estimate a capacity of the energy storage device after an arbitrary time point in a first period in which the voltage of the energy storage device decreases from the threshold voltage to the minimum operating voltage of the measurement unit based on an elapsed time from the arbitrary time point. 
     During parking, a vehicle generator does not generate power because an engine stops. Since the energy storage device is not charged and continues to be discharged, the voltage of the energy storage device falls below the minimum operating voltage of the measurement unit, and the measurement tends to be impossible. By applying the present technology during parking in which measurement tends to be impossible, it is possible to estimate the capacity of the energy storage device even after the voltage of the energy storage device falls below the minimum operating voltage of the measurement unit. 
     First Embodiment 
     1. Description of Battery  50   
       FIG. 1  is a side view of a vehicle, and  FIG. 2  is an exploded perspective view of a battery. A vehicle  10  is an engine-driven vehicle, and includes an engine  20  and a battery  50 . In  FIG. 1 , only the engine  20  and the battery  50  are illustrated, and other components constituting the vehicle  10  are omitted. The battery  50  is an example of an “energy storage apparatus”. 
     As illustrated in  FIG. 2 , the battery  50  includes an assembled battery  60 , a circuit board unit  65 , and a housing  71 . 
     The housing  71  includes a main body  73  made of a synthetic resin material and a lid body  74 . The main body  73  has a bottomed cylindrical shape. The main body  73  includes a bottom surface portion  75  and four side surface portions  76 . An upper opening  77  is formed at the upper end portion by the four side surface portions  76 . 
     The housing  71  houses the assembled battery  60  and the circuit board unit  65 . The assembled battery  60  includes twelve secondary batteries  62 . The twelve secondary batteries  62  are connected in three parallel and four series. The circuit board unit  65  is disposed above the assembled battery  60 . In the block diagram of  FIG. 6 , three secondary batteries  62  connected in parallel are represented by one battery symbol. The secondary battery  62  is an example of an “energy storage device”. 
     The lid body  74  closes the upper opening  77  of the main body  73 . An outer peripheral wall  78  is provided around the lid body  74 . The lid body  74  has a protrusion  79  having a substantially T-shape in plan view. An external terminal  51  of the positive electrode is fixed to one corner portion of the front portion of the lid body  74 , and an external terminal  52  of the negative electrode is fixed to the other corner portion. 
     As illustrated in  FIGS. 3 and 4 , in the secondary battery  62 , an electrode assembly  83  is housed in a rectangular parallelepiped case  82  together with a nonaqueous electrolyte. The secondary battery  62  is, for example, a lithium ion secondary battery. The case  82  includes a case body  84  and a lid  85  that closes an opening portion above the case body. 
     Although not illustrated in detail, the electrode assembly  83  is formed by disposing a separator formed of a porous resin film between a negative electrode element formed by applying an active material to a substrate formed of a copper foil and a positive electrode element formed by applying an active material to a substrate formed of an aluminum foil. The negative electrode element, the positive electrode element, and the separator all have a band shape and are wound in a flat shape so as to be housed in the case body  84  in a state where the negative electrode element and the positive electrode element are displaced to opposite sides in the width direction with respect to the separator. 
     A positive electrode terminal  87  is connected to the positive electrode element via a positive electrode current collector  86 , and a negative electrode terminal  89  is connected to the negative electrode element via a negative electrode current collector  88 . Each of the positive electrode current collector  86  and the negative electrode current collector  88  includes a flat plate-shaped pedestal portion  90  and a leg portion  91  extending from the pedestal portion  90 . A through hole is formed in the pedestal portion  90 . The leg portion  91  is connected to the positive electrode element or the negative electrode element. Each of the positive electrode terminal  87  and the negative electrode terminal  89  includes a terminal body portion  92  and a shaft portion  93  protruding downward from a center portion of a lower surface of the terminal body portion  92 . The terminal body portion  92  and the shaft portion  93  of the positive electrode terminal  87  are integrally formed of aluminum (single material). In the negative electrode terminal  89 , the terminal body portion  92  is made of aluminum, and the shaft portion  93  is made of copper, and these are assembled. The terminal body portions  92  of the positive electrode terminal  87  and the negative electrode terminal  89  are disposed at both end portions of the lid  85  via gaskets  94  made of an insulating material, and are exposed outward from the gaskets  94 . 
     The lid  85  includes a pressure release valve  95 . As illustrated in  FIG. 3 , the pressure release valve  95  is located between the positive electrode terminal  87  and the negative electrode terminal  89 . When the internal pressure of the case  82  exceeds the limit value, the pressure release valve  95  is released to lower the internal pressure of the case  82 . 
       FIG. 5  is a block diagram illustrating an electrical configuration of the vehicle  10 . 
     The vehicle  10  includes the engine  20  which is a driving device, an engine control unit  21 , an engine starting device  23 , an alternator  25  which is a vehicle generator, electrical equipment  27 , a vehicle electronic control unit (ECU)  30 , the battery  50 , and the like. 
     The battery  50  is connected to a power line  37 . The engine starting device  23 , the alternator  25 , and the electrical equipment  27  are connected to the battery  50  via the power line  37 . 
     The engine starting device  23  is a cell motor. When an ignition switch  24  is turned on, a cranking current flows from the battery  50 , and the engine starting device  23  is driven. The driving of the engine starting device  23  rotates a crankshaft, and the engine  20  can be started. 
     The electrical equipment  27  is rated at 12 V and examples thereof include an air conditioner, an audio system, a car navigation system, and auxiliary equipment. The engine starting device  23  and the electrical equipment  27  are examples of an “electric load”. 
     The alternator  25  is a vehicle generator that generates power by the power of the engine  20 . When the power generation amount of the alternator  25  exceeds the electric load amount of the vehicle  10 , the battery  50  is charged by the alternator  25 . When the power generation amount of the alternator  25  is smaller than the electric load amount of the vehicle  10 , the battery  50  is discharged to compensate for the shortage of the power generation amount. 
     The vehicle ECU  30  is communicably connected to the battery  50  via a communication line L 1 , and is communicably connected to the alternator  25  via a communication line L 2 . The vehicle ECU  30  receives information on SOC and a capacity C from the battery  50 , and controls the SOC and the capacity C of the battery  50  by controlling the power generation amount of the alternator  25 . 
     The vehicle ECU  30  is communicably connected to the engine control unit  21  via a communication line L 3 . The engine control unit  21  is mounted on the vehicle  10  and monitors the operating state of the engine  20 . The engine control unit  21  monitors the traveling state of the vehicle  10  from measurement values of meters such as a speed measuring instrument. The vehicle ECU  30  can obtain information on whether the ignition switch  24  is turned on or off, information on an operating state of the engine  20 , and information on a traveling state (in the middle of traveling, traveling stop, idling stop, etc.) of the vehicle  10  from the engine control unit  21 . 
       FIG. 6  is a block diagram illustrating an electrical configuration of the battery  50 . The battery  50  includes the assembled battery  60 , a resistor  54 , a switch  53 , a management unit  130 , a measurement unit  150 , a temperature sensor, and a charge detection circuit  200 . The assembled battery  60  includes the plurality of secondary batteries  62  connected in series. The battery  50  is rated at 12 V. 
     The assembled battery  60 , the switch  53 , and the resistor  54  are connected in series via power lines  55 P and  55 N. 
     The power line  55 P is a power line that connects the external terminal  51  of the positive electrode and the positive electrode of the assembled battery  60 . The power line  55 N is a power line that connects the external terminal  52  of the negative electrode and the negative electrode of the assembled battery  60 . The power line  55 P and the power line  55 N are current paths. 
     The switch  53  is located on the positive electrode side of the assembled battery  60 , and is provided on the power line  55 P on the positive electrode side. The switch  53  is a semiconductor switch such as an FET or a relay. By opening the switch  53 , the current of the battery  50  can be cut off. The switch  53  is controlled to be closed in a normal state. 
     The resistor  54  is located at the negative electrode of the assembled battery  60  and is provided in the power line  55 N on the negative electrode side. 
     The measurement unit  150  is mounted on the circuit board  100 , and includes a current measurement unit  160  and a voltage measurement unit  170 . The measurement unit  150  is connected to the power line  55 P on the positive electrode side via a branch line  57 . The branch line  57  is connected to a point C on the power line  55 P. The point C is located closer than the switch  53  as viewed from the assembled battery  60 , and the measurement unit  150  receives power supply from the assembled battery  60  through the branch line  57  regardless of turning on and off of the switch  53 . The measurement unit  150  includes a step-down circuit  155 . The step-down circuit  155  steps down a voltage Vab of the assembled battery  60  and supplies power to the current measurement unit  160  and the voltage measurement unit  170 . 
     The current measurement unit  160  includes an amplifier  161  and an AD converter  163 . The amplifier  161  amplifies a voltage Vr between both ends of the resistor  54 . The AD converter  163  converts a power value of the amplifier  161  from an analog signal into a digital signal and outputs the converted signal. The current measurement unit  160  detects an current I of the battery  50  from a voltage difference Vr between two input terminals  161 A and  161 B. 
     The voltage measurement unit  170  includes a multiplexer  171  and an AD converter  173 . The multiplexer  171  includes five input terminals  171 A to  171 E. The five input terminals  171 A to  171 E are electrically connected to electrodes of the secondary batteries  62 , respectively. 
     The multiplexer  171  sequentially detects and outputs the voltage V of each secondary battery  62  while switching the secondary battery  62  to be measured. The AD converter  173  converts a power value of the multiplexer  171  from an analog signal to a digital signal and outputs the converted signal. 
     The current measurement unit  160  and the voltage measurement unit  170  are connected to the management unit  130  via a bus  180 , and power (measurement values) of both the measurement units  160  and  170  are input to the management unit  130 . 
     As illustrated in  FIG. 6 , the management unit  130  is mounted on the circuit board  100 . The management unit  130  includes a CPU  131 , a memory  133 , and a timer  135 . The management unit  130  is connected to the power line  55 P on the positive electrode side via a branch line  58 . The branch line  58  is connected to the point C on the power line  55 P. The point C is located closer than the switch  53  as viewed from the assembled battery  60 , and the management unit  130  receives power supply from the assembled battery  60  through the branch line  58  regardless of turning on and off of the switch  53 . The branch line  58  is provided with a regulator  59 . The regulator  59  steps down the voltage Vab of the assembled battery  60  and supplies power to the management unit  130 . 
     The management unit  130  can obtain information regarding the state of the vehicle  10 , such as whether the vehicle  10  is parked or traveling, from the vehicle ECU  30 . 
     The management unit  130  measures the current I of the assembled battery  60 , the voltage V of each secondary battery  62 , the voltage Vab of the assembled battery  60 , and the temperature of the assembled battery  60  at a predetermined measurement cycle N by the current measurement unit  160 , the voltage measurement unit  170 , and the temperature sensor, and monitors the state of the battery  50 . The memory  133  stores a monitoring program for executing a monitoring process of the battery  50  (flowchart of  FIG. 8 ) and a capacity estimation program for executing a capacity estimation process of the battery  50  (flowchart of  FIG. 9 ). 
     As illustrated in  FIG. 7 , the charge detection circuit  200  includes two detection resistors R 1  and R 2  and a comparator  210 . The two detection resistors R 1  and R 2  are connected in series between the two external terminals  51  and  52 . 
     The comparator  210  compares a voltage at a connection point Pr of the two detection resistors R 1  and R 2  with a reference voltage. When the voltage at the connection point Pr is higher than the reference voltage (during charge), the comparator  210  outputs a charge detection signal Sc. The management unit  130  can detect whether or not the battery  50  is charged based on the charge detection signal Sc. 
     2. Monitoring Process of Battery  50   
       FIG. 8  is a flowchart of a monitoring process of the battery  50 . The monitoring process of the battery  50  is always executed at the predetermined measurement cycle N during the activation of the management unit  130  regardless of the state of the vehicle  10 . 
     The monitoring process of the battery  50  includes S 10  to S 60 . When the monitoring process is started, the management unit  130  measures the current I of the assembled battery  60  using the current measurement unit  160  (S 10 ). The management unit  130  measures the voltage V of each secondary battery  62  and the voltage Vab of the assembled battery  60  using the voltage measurement unit  170 , and measures a temperature T of the assembled battery  60  using the temperature sensor (S 20 , S 30 ). The voltage Vab of the assembled battery  60  is a voltage between points A and B in  FIG. 6 , and is a total of the voltages (total voltage) of the four secondary batteries  62 . 
     Thereafter, the management unit  130  temporarily stores each measurement value in the memory  133 , and performs a process of determining whether there is an abnormality in the measurement value (S 40 ). 
     When there is no abnormality in the measurement value (S 40 : YES), the management unit  130  calculates the capacity C of each secondary battery  62  based on the integral value with respect to time of the current I measured by the current measurement unit  160  as expressed by Expression (1) below (S 50 ). + is charge, and − is discharge. 
         C=Cf ±(∫ Idt )   (1)
 
     Cf is an initial value (full charge capacity) of the capacity C of each secondary battery  62 , and I is a current. 
     The capacity C is not limited to be calculated from the integrated value of the current I, and may be calculated from the correlation with the voltage Vab. That is, the capacity C may be calculated from the measurement value of the voltage Vab using the correlation between the voltage Vab and the capacity C. 
     When there is no abnormality in the measurement value (S 40 : YES), the processes of S 10  to S 50  is repeatedly performed at the predetermined measurement cycle N, and the capacity C of each secondary battery  62  is calculated for each measurement cycle N. 
     When there is an abnormality in the measurement value (S 40 : NO), the management unit  130  performs a process of notifying the abnormality to the vehicle ECU  30  (S 60 ). 
     3. Capacity Estimation in Unmeasurable State 
     The management unit  130  and the measurement unit  150  use the assembled battery  60  as a power source, and receive power supply from the assembled battery  60 . The minimum operating voltage is the minimum voltage at which the management unit  130  and the measurement unit  150  can continuously operate without stopping the operation. 
     The measurement unit  150  can operate only when the minimum operating voltage is V 1  and the voltage Vab of the assembled battery  60  is equal to or higher than the minimum operating voltage V 1 . 
     The management unit  130  can operate only when the minimum operating voltage is V 2  and the voltage Vab of the assembled battery  60  is equal to or higher than the minimum operating voltage V 2 . 
     The minimum operating voltage V 2  of the management unit  130  is lower than the minimum operating voltage V 1  of the measurement unit  150 . That is, V 1 &gt;V 2 . As an example, V 1 =5 [V], and V 2 =3.3 [V]. 
     In a case where the minimum operating voltage V 2  is smaller than the minimum operating voltage V 1  (V 1 &gt;V 2 ), when the voltage Vab of the assembled battery  60  gradually decreases along with discharge, the measurement unit  150  first stops at the time point when the voltage Vab decreases to the minimum operating voltage V 1 , and then the management unit  130  stops at the time point when the voltage Vab decreases to the minimum operating voltage V 2 . That is, during a period W 12  in which the voltage Vab of the assembled battery  60  decreases from the minimum operating voltage V 1  to the minimum operating voltage V 2 , the measurement unit  150  is stopped, but the management unit  130  continues the operation (see  FIG. 10 ). 
     By executing the following capacity estimation process, the management unit  130  estimates the capacity C of each secondary battery  62  for the period W 12  in which the measurement unit  150  is stopped to be in an unmeasurable state. 
       FIG. 9  is a flowchart of the capacity estimation process. The capacity estimation process includes eight steps of S 100  to S 170 , and is executed after the vehicle  10  is parked. 
     When the management unit  130  receives information indicating that the vehicle  10  is parked from the vehicle ECU  30  through communication, the management unit  130  performs a process of comparing the voltage Vab of the assembled battery  60  measured by the measurement unit  150  with a threshold voltage V 0  (S 100 ). The threshold voltage V 0  is a voltage equal to or higher than the minimum operating voltage V 1  of the measurement unit  150 , and is 6 V as an example. The threshold voltage V 0  may be equal to the minimum operating voltage V 1  (V 0 =V 1 ). 
       FIG. 10  is a graph showing a voltage transition of the assembled battery  60  with respect to time after parking. When the assembled battery  60  is normal, the switch  53  is closed, and since a dark current flows from the battery  50  to the vehicle  10  after parking, the voltage Vab of the assembled battery  60  decreases after a parking start time point tp. The dark current is a current consumed by the vehicle  10  during parking. 
     When the voltage Vab of the assembled battery  60  becomes equal to or lower than the threshold voltage V 0 , the management unit  130  switches the switch  53  from the closed state to the opened state (S 110 ). In the graph of  FIG. 10 , the switch  53  is opened at time t 0  when the voltage Vab of the assembled battery  60  becomes equal to or lower than the threshold voltage V 0 . 
     By opening the switch  53 , it is possible to cut off discharge from the battery  50  to the vehicle  10 , that is, discharge to the electrical equipment  27  connected to the external terminals  51  and  52 . 
     After the discharge is cut off, the management unit  130  performs a process of comparing the voltage Vab of the assembled battery  60  measured by the measurement unit  150  with the minimum operating voltage V 1  of the measurement unit  150  (S 120 ). 
     When the voltage Vab of the assembled battery  60  decreases to the minimum operating voltage V 1  or lower, the management unit  130  stores the capacity C 1  of each secondary battery  62  immediately before the voltage Vab of the assembled battery  60  reaches the minimum operating voltage V 1  in the memory  133  (S 130 ). 
     In the graph of  FIG. 10 , the voltage Vab of the assembled battery  60  decreases to the minimum operating voltage V 1  at first time t 1 , and the capacity C 1  of each secondary battery  62  which is calculated in the monitoring process (S 50 ) immediately before the first time t 1  is stored in the memory  133 . 
     After the first time t 1 , since the voltage Vab of the assembled battery  60  is equal to or lower than the minimum operating voltage V 1 , the measurement unit  150  stops (S 140 ). Therefore, the capacity C of each secondary battery  62  cannot be calculated using the measurement values of the current and the voltage of the assembled battery  60 . 
     After the first time t 1 , the management unit  130  counts an elapsed time T from the first time t 1  using the timer  135 . The management unit  130  obtains a capacity decrease amount ΔC from the elapsed time T. For example, as expressed in Expression (2), the capacity decrease amount ΔC is obtained by the product of a current value Ia and the elapsed time T. 
     The current value Ia is a current discharged by each secondary battery  62  after the first time t 1 . The current value Ia is a total value of the consumption current of the management unit  130 , the consumption current of the measurement unit  150 , the consumption current of the charge detection circuit  200 , and the self-discharge current of each secondary battery  62 . The current value Ia can be a theoretical value or an empirical value, and is a fixed value. 
     As expressed in Expression (3), the management unit  130  estimates the capacity C of each secondary battery  62  after the first time t 1  by subtracting the capacity decrease amount ΔC from the capacity C 1  immediately before the first time t 1  (S 150 ). 
       Δ C=Ia×T    (2)
 
         C=C 1·Δ C    (3)
 
     The estimation process of the capacity C is executed at predetermined intervals during the period W 12  in which the voltage Vab of the assembled battery  60  decreases from the minimum operating voltage V 1  to the minimum operating voltage V 2 . Then, when the voltage Vab of the assembled battery  60  decreases to V 2  (S 160 : YES), the management unit  130  stops (S 170 ). 
       FIG. 11  is a charge control flowchart, and is performed when the charge detection circuit  200  detects charge in the period W 12 . The charge may be charge from the alternator or charge from an external charger outside the vehicle. 
     When detecting the charge from the power of the charge detection circuit  200 , the management unit  130  compares the capacity C of each secondary battery  62  estimated in S 150  with a reuse prohibition area H 2 . 
     As illustrated in  FIG. 12 , the secondary battery  62  is provided with a reusable area H 1  and the reuse prohibition area H 2  depending on the size of the capacity C. The reusable area H 1  is a region from Cb to Ca that can be reused by charge. The reuse prohibition area H 2  is a region from Cc to Cb in which safety cannot be secured at the time of reuse due to overdischarge or the like, and reuse by charge is prohibited. A relationship of Cc&lt;Cb&lt;Ca is satisfied. 
     When any one of the four secondary batteries  62  constituting the assembled battery  60  is included in the reuse prohibition area H 2 , the management unit  130  keeps the switch  53  open (S 220 ). By keeping the switch  53  open, the assembled battery  60  is kept disconnected from the external terminal  51 . Therefore, it is possible to prohibit charge and suppress reuse of the battery  50 . 
     When all the secondary batteries  62  are not included in the reuse prohibition area H 2  (included in the reusable area), the management unit  130  switches the switch  53  from the opened state to the closed state (S 230 ). By switching the switch  53  from the opened state to the closed state, the assembled battery  60  is conducted to the external terminal  51 . Therefore, it is possible to receive charge and reuse the battery  50 . 
     4. Effects 
     When the voltage Vab of the assembled battery  60  decreases to the threshold voltage V 0  while the vehicle is parked, the management unit  130  opens the switch  53  to cut off the discharge from the battery  50  to the vehicle  10 . Since there is no discharge to the vehicle  10  after the discharge is cut off, the secondary battery  62  thereafter discharges only the current consumed in the battery and the self-discharge current, and the capacity decrease amount ΔC is substantially proportional to the elapsed time T. 
     Therefore, the capacity C of each secondary battery  62  after the measurement unit is stopped can be estimated based on the elapsed time T. From the capacity C after the measurement unit is stopped, it is possible to determine whether or not the battery  50  can be reused after the measurement unit is stopped. 
     When the capacity of the secondary battery  62  cannot be estimated after the measurement unit  150  is stopped, it is conceivable that the state of the battery  50  is indeterminate and reuse is uniformly prohibited. However, in this case, reuse of the battery  50  that can actually still be used may be prohibited. By using the present technology, the battery  50  can be used up to the use limit, and the usability of the battery  50  is high. 
     Other Embodiments 
     The present invention is not limited to the embodiments described above referring to the drawings, and, for example, the following embodiments are also included in the technical scope of the present invention. 
     (1) In the embodiment, the secondary battery  62  is described as an example of an energy storage device. The energy storage device is not limited to the secondary battery  62 , and may be a capacitor. The secondary battery  62  is not limited to a lithium ion secondary battery, and may be another nonaqueous electrolyte secondary battery. A lead-acid battery or the like can also be used. The energy storage device is not limited to a plurality of energy storage devices, and may have a single (single cell) configuration. 
     (2) In the embodiment, the battery  50  is used for starting the engine. The use of the battery  50  is not limited to a specific use. The battery  50  may be used in various applications such as a mobile object (vehicle, ship, AGV, etc.) and an industrial application (an energy storage apparatus of an uninterruptible power supply system or a solar power generating system). 
     (3) In the embodiment, in the embodiment, the capacity C 1  immediately before the first time t 1  is calculated from the current measurement value. The capacity C has a correlation with the voltage V. The capacity C 1  at the time t 1  is a correlation value with the minimum operating voltage V 1 . Therefore, the capacity C 1  at the time t 1  is not limited to the case of being obtained from the current measurement value, and may be a fixed value (correlation value of the minimum operating voltage V 1 ). That is, the capacity C of the secondary battery  62  after the first time t 1  may be estimated by subtracting the capacity decrease amount ΔC from the fixed value (correlation value of the minimum operating voltage V 1 ). 
     In addition, since the capacity C of the secondary battery  62  after the first time t 1  decreases substantially in proportion to the elapsed time T, the capacity C of the secondary battery  62  after the first time t 1  may be estimated using a data table in which the elapsed time T and the capacity C are associated with each other as illustrated in  FIG. 13 . 
     (4) In the embodiment, the capacity estimation process of  FIG. 9  is performed while the vehicle is parked, but may be performed at any time when the voltage Vab of the assembled battery  60  is lower than the threshold voltage V 0 . It may be performed at a timing other than during parking. 
     (5) As illustrated in  FIG. 14 , the switch  53  may include two FETs  310  and  320  connected back-to-back. The FET  310  is a P-channel, and has a source connected to the external terminal  51  of the positive electrode. The FET  320  is a P-channel, and has a source connected to the positive electrode of the assembled battery  60 . A drain of the FET  310  is connected to a drain of the FET  320 . 
     By turning on the two FETs  310  and  320 , both charge and discharge can be performed. By turning off the two FETs  310  and  320 , both charge and discharge can be cut off. By turning on the FET  310  and turning off the FET  320 , the discharge is cut off and only the charge is possible, and by turning off the FET  310  and turning on the FET  320 , the charge is cut off and only the discharge is possible. 
     When the voltage of the assembled battery  60  falls below the threshold voltage V 0  after parking, the management unit  130  may turn on the FET  310  and turn off the FET  320  to cut off only the discharge. In a case where charge is detected after the first time t 1 , when any of the secondary batteries is included in the reuse prohibition area, the two FETs  310  and  320  may be turned off to prohibit reuse of the battery  50 . When all the secondary batteries are not included in the reuse prohibition area, the two FETs  310  and  320  may be turned on to enable reuse of the battery  50 . 
     (6) In the embodiment, after the discharge is cut off, the management unit  130  measures the elapsed time T from the first time t 1  using the timer  135 , and estimates the capacity C of each secondary battery  62  after the first time t 1  based on the elapsed time T (S 150 ). The estimation process of the capacity C (S 150 ) may be started at any point from the time t 0  to the time t 1 . For example, the estimation processing may be started at time ta in  FIG. 15 . 
     That is, the management unit  130  may measure the elapsed time T from the arbitrary time point ta in a first period W 01 , and estimate the capacity C of the secondary battery  62  after the arbitrary time point ta on the basis of the elapsed time T from the arbitrary time point ta. The capacity C may be estimated until time t 2  when the voltage Vab of the assembled battery  60  decreases to the minimum operating voltage V 2  of the management unit  130 . In a case where charge is detected after the arbitrary time point ta, the management unit  130  may determine whether to keep the switch  53  open or close by performing the charge control flow of  FIG. 11 . 
     (8) The present technology can be applied to a capacity estimation program for an energy storage device. The capacity estimation program for an energy storage device is a program for causing a computer to execute: a process (S 110 ) of cutting off discharge to an outside through an external terminal by controlling a switch when a voltage of the energy storage device falls below a threshold voltage equal to or higher than a minimum operating voltage of a measurement unit, and a process (S 150 ) of estimating a capacity of the energy storage device after an arbitrary time point in a first period in which the voltage of the energy storage device decreases from the threshold voltage to the minimum operating voltage of the measurement unit based on an elapsed time from the arbitrary time point after the discharge is cut off. The present technology can be applied to a recording medium in which a capacity estimation program for an energy storage device is recorded. The computer is, for example, the management unit  130 . The energy storage device is, for example, the secondary battery  62 . The capacity estimation program can be recorded in a recording medium such as a ROM. 
     DESCRIPTION OF REFERENCE SIGNS 
       10 : vehicle 
       50 : battery (energy storage apparatus) 
       53 : switch 
       60 : assembled battery 
       62 : secondary battery (energy storage device) 
       130 : management unit 
       150 : measurement unit 
       160 : current measurement unit 
       170 : voltage measurement unit 
     V 1 , V 2 : minimum operating voltage 
     t 1 : first time 
     T: elapsed time