Patent Publication Number: US-2023152386-A1

Title: Deterioration diagnosis apparatus of battery and deterioration diagnosis method of battery

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-185714 filed on Nov. 15, 2021, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a deterioration diagnosis apparatus of a battery and a deterioration diagnosis method of a battery. 
     2. Description of Related Art 
     An assembled battery includes a plurality of secondary batteries electrically connected to each other. By combining the secondary batteries, a large-capacity assembled battery can be obtained. However, a full charge capacity (an electricity amount accumulated in the secondary battery at a time of full charging) of a secondary battery decreases as the secondary battery deteriorates. For example, Japanese Unexamined Patent Application Publication No. 2013-110906 (JP 2013-110906 A) discloses a deterioration diagnosis method of a battery, which discharges an assembled battery until its voltage (a voltage between terminals) reaches a predetermined discharging end voltage, and estimates a deterioration degree of the assembled battery using data (a discharging curve) indicating a transition of the voltage of the assembled battery from a discharging start voltage to the discharging end voltage. 
     SUMMARY 
     In the deterioration diagnosis method of the battery described in JP 2013-110906 A, discharging is ended at a timing at which the voltage between the terminals of the assembled battery reaches the discharging end voltage during the discharging. In such a method, since voltages of some of the secondary batteries included in the assembled battery do not reach the discharging end voltage at the end of the discharging, it is difficult to estimate the deterioration degree of each secondary battery included in the assembled battery. Hereinafter, each secondary battery included in the assembled battery is referred to as a “cell”. 
     Therefore, a deterioration diagnosis method of a battery, which ends the discharging at the timing at which voltages of all the cells included in the assembled battery reach the discharging end voltage, can be considered. However, in such a method, it is required that the discharging end voltage is set to an appropriate magnitude. When the discharging end voltage is too low, some cells included in the assembled battery may be over-discharged during the discharging. The over-discharging of a cell accelerates the deterioration of the cell. On the other hand, when the discharging end voltage is too high, sufficient data cannot be obtained during the discharging, and accuracy of estimating a deterioration degree (for example, a full charge capacity) of a cell may decrease. 
     The present disclosure provides a deterioration diagnosis apparatus of a battery and a deterioration diagnosis method of a battery that set a discharging end voltage to an appropriate magnitude, and estimate, with sufficient accuracy, a deterioration degree of each cell included in an assembled battery while restricting deterioration of a cell during discharging. 
     A deterioration diagnosis apparatus of a battery according to a first aspect of the present disclosure includes one or more processors. The one or more processors are configured to execute discharging of each of a plurality of cells included in an assembled battery while measuring a voltage of each of the cells, estimate a deterioration degree of each of the cells included in the assembled battery using voltage data indicating a transition of the voltage of each of the cells included in the assembled battery from a discharging start voltage to a predetermined discharging end voltage, and end the discharging when voltages of all the cells included in the assembled battery reach the predetermined discharging end voltage. The predetermined discharging end voltage is a cell voltage at which a change degree of the cell voltage per unit discharging amount starts to sharply rise while the cell voltage is dropping due to the discharging. 
     In the deterioration diagnosis apparatus of the assembled battery, since the discharging is continued until the voltages of all the cells included in the assembled battery reach the discharging end voltage, it is possible to estimate the deterioration degree of each cell included in the assembled battery using the voltage data indicating the transition of the voltage of each cell included in the assembled battery from the discharging start voltage to the discharging end voltage. 
     A deterioration diagnosis apparatus of a battery according to a second aspect of the present disclosure includes a storage device and one or more processors. The storage device is configured to manage a discharging end voltage indicating a timing of ending discharging of the battery. The discharging of the battery is executed to acquire data for estimating a deterioration degree of the battery. The one or more processors are configured to store, in the storage device, as the discharging end voltage, a voltage of the battery when a change degree of a voltage per unit discharging amount of the battery after a start of the discharging of the battery becomes a predetermined value or higher. 
     With the above configuration, it is possible to manage an appropriate discharging end voltage. The deterioration diagnosis apparatus of the battery can provide an appropriate discharging end voltage to a device (a discharging device) that executes the discharging of the battery to acquire the data for estimating the deterioration degree of the battery. Alternatively, the deterioration diagnosis apparatus of the battery may execute the discharging of the battery by itself to acquire the data for estimating the deterioration degree of the battery. By discharging the battery based on the appropriate discharging end voltage, it is possible to estimate, with sufficient accuracy, the deterioration degree of the battery while restricting the deterioration degree of the battery during the discharging. More specifically, when the voltages of all the cells included in the assembled battery reach the discharging end voltage, by ending the discharging of the assembled battery, it is possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cells during the discharging. The storage device may manage the discharging end voltage in association with a type of the battery. The type of the battery may be distinguished using at least one of a battery manufacturer, a model number, and a serial number. 
     A deterioration diagnosis method of a battery according to a third aspect of the present disclosure is executed by one or more processors. The deterioration diagnosis method includes executing discharging of each of a plurality of cells included in an assembled battery while measuring a voltage of each of the cells, ending the discharging when the voltages of all the cells included in the assembled battery reach a predetermined discharging end voltage, and estimating a deterioration degree of each of the cells included in the assembled battery using voltage data indicating a transition of the voltage of each of the cells included in the assembled battery from a discharging start voltage to the predetermined discharging end voltage. The predetermined discharging end voltage is a cell voltage at which a change degree of the cell voltage per unit discharging amount starts to sharply rise while the cell voltage is dropping due to the discharging. 
     With the above deterioration diagnosis method of the assembled battery, in the same manner as the deterioration diagnosis apparatus described above, it is also possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG.  1    is a diagram illustrating a configuration of a vehicle according to an embodiment of the present disclosure; 
         FIG.  2    is a diagram illustrating a configuration of a deterioration diagnosis apparatus of a battery according to the embodiment of the present disclosure; 
         FIG.  3    is a flowchart illustrating a deterioration diagnosis method of the battery according to the embodiment of the present disclosure; 
         FIG.  4    is a graph illustrating an example of a discharging characteristic of a cell included in an assembled battery illustrated in  FIG.  2   ; 
         FIG.  5    is a graph for describing an issue that may arise when a discharging lower limit voltage is set as a discharging end voltage; 
         FIG.  6    is a graph for describing a merit of raising the discharging end voltage; 
         FIG.  7    is a graph for describing a demerit of raising the discharging end voltage; 
         FIG.  8    is a graph for describing a method of deciding the discharging end voltage in the embodiment of the present disclosure; 
         FIG.  9    is a graph illustrating a discharging characteristic of a lithium-ion secondary battery (a ternary LIB); 
         FIG.  10    is a graph illustrating a relationship between the discharging end voltage and a coverage ratio; 
         FIG.  11    is a flowchart illustrating a method of setting the discharging end voltage according to a modified example of the embodiment of the present disclosure; 
         FIG.  12    is a flowchart illustrating a modified example of processes illustrated in  FIG.  3   ; 
         FIG.  13    is a graph for describing a method of setting a threshold value used in processes illustrated in  FIG.  12   ; 
         FIG.  14    is a diagram illustrating a modified example of a service tool illustrated in  FIG.  2   ; 
         FIG.  15    is a flowchart illustrating processes related to data acquisition executed by the service tool illustrated in  FIG.  14   ; 
         FIG.  16    is a flowchart for describing a method of setting the discharging end voltage based on the data that is acquired using the processes illustrated in  FIG.  15   ; 
         FIG.  17    is a diagram illustrating a modified example of a vehicle control device illustrated in  FIG.  2   ; and 
         FIG.  18    is a diagram illustrating a modified example of the assembled battery illustrated in  FIG.  2   . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In a deterioration diagnosis apparatus of an assembled battery, as described below, a discharging end voltage is set to an appropriate magnitude. Hereinafter, a change degree (an absolute value) of a cell voltage per unit discharging amount is expressed by “|ΔV/ΔQ|”. A discharging amount corresponds to a time integral value of a discharging current. A cell is a secondary battery composing the assembled battery. The assembled battery is composed of a plurality of cells electrically connected to each other. The cell voltage drops during discharging of the cell. At an initial stage of the discharging, |ΔV/ΔQ| is substantially constant. At a final stage of the discharging, |ΔV/ΔQ| sharply rises due to an increase in reaction resistance. Specifically, |ΔV/ΔQ| starts to sharply rise immediately after the cell voltage reaches a predetermined voltage (hereinafter, also referred to as “a voltage at a change point”) during the discharging of the cell. Basically, accuracy of the deterioration diagnosis becomes higher and the deterioration of the cell becomes easier as a period during which the discharging of the cell is continued is longer. Before |ΔV/ΔQ| starts to sharply rise, by continuing the discharging of the cell, a merit of improving the accuracy of the diagnosis is larger than a demerit of easy deterioration of the cell. On the other hand, after |ΔV/ΔQ| starts to sharply rise, by continuing the discharging of the cell, the demerit of easy deterioration of the cell is larger than the merit of improving the accuracy of the diagnosis. Therefore, by setting, as the discharging end voltage, the cell voltage at which the change degree of the cell voltage per unit discharging amount starts to sharply rise while the cell voltage is dropping due to the discharging, it is possible to promote both ensuring sufficient diagnostic accuracy and restricting deterioration of the cell. Accordingly, it is possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. 
     The voltage at the change point may be a cell voltage at which the change degree of the cell voltage per unit discharging amount is the lowest during the discharging. At the final stage of the discharging of the cell, |ΔV/ΔQ| tends to lower once and then starts to sharply rise. The discharging end voltage may be at or near the voltage at the change point (for example, a voltage slightly lower than the voltage at the change point). 
     In aspects of the present disclosure, each of the cells included in the assembled battery may be a lithium-ion secondary battery. The predetermined discharging end voltage may be higher than a voltage at which an entire lithium site in a positive electrode active material of the lithium-ion secondary battery is occupied. 
     In the lithium-ion secondary battery, a lithium site exists in the positive electrode active material. The site is a crystallographically equivalent grid position. The presence of an atom at the grid position is expressed as occupying the site. The lithium site is a site occupied by lithium. Hereinafter, the voltage at which the entire the lithium site in the positive electrode active material of the lithium-ion secondary battery is occupied is also referred to as a “lithium occupying voltage”. In a lithium-ion secondary battery (hereinafter, also referred to as a “ternary LIB”) having a positive electrode of a layer-shaped crystal structure expressed by a general formula of Li x Ni y Co z Mn (1-x-z) , the lithium occupying voltage is 3.0 V. 
     When a deterioration degree of only one ternary LIB is estimated in an assembled battery including a plurality of ternary LIBs, the discharging of the assembled battery is ended at a timing at which a voltage of at least one ternary LIB included in the assembled battery reaches the discharging end voltage. In such a deterioration diagnosis apparatus of a battery, by setting the discharging end voltage to 3.0 V, it is possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. 
     On the other hand, in the deterioration diagnosis apparatus of the battery, when an assembled battery including a plurality of ternary LIBs is diagnosed, since the deterioration degree of each ternary LIB is estimated in the assembled battery including the ternary LIBs, the discharging of the assembled battery is continued until voltages of all the ternary LIBs included in the assembled battery reach the discharging end voltage. In such a deterioration diagnosis apparatus of a battery, 3.0 V is too low as the discharging end voltage. Therefore, in the above configuration, the discharging end voltage is set to be higher than the lithium occupying voltage. 
     In the aspects of the present disclosure, each of the cells included in the assembled battery may be a lithium-ion secondary battery having a positive electrode of a layer-shaped crystal structure expressed by a general formula of Li x Ni y Co z Mn (1-x-z) . The predetermined discharging end voltage may be 3.1 V or higher and 3.5 V or lower. With the above configuration, it is possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. In an embodiment in which each of the cells included in the assembled battery is a ternary LIB, the discharging end voltage may be 3.3 V or higher and 3.4 V or lower. 
     In the aspects of the present disclosure, each of the cells included in the assembled battery may be a lithium-ion secondary battery having a positive electrode of an olivine-shaped crystal structure expressed by a general formula of Li x FePO 4 . The predetermined discharging end voltage may be 2.0 V or higher and 3.2 V or lower. With the above configuration, it is also possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. 
     In the aspects of the present disclosure, the predetermined discharging end voltage may be a cell voltage at which the change degree of the cell voltage per unit discharging amount becomes a predetermined value or higher while the cell voltage is dropping due to the discharging. The one or more processors may be configured to determine whether the change degree of the cell voltage per unit discharging amount becomes the predetermined value or higher while the cell voltage is dropping due to the discharging of each cell included in the assembled battery, and, upon determining that the change degrees of the cell voltages per unit discharging amount of all the cells included in the assembled battery become the predetermined value or higher, end the discharging. Therefore, the one or more processors may be configured to, upon determining that the change degrees of the cell voltages per unit discharging amount of all the cells included in the assembled battery are the predetermined value or higher, end the discharging. 
     The one or more processors can determine, based on whether |ΔV/ΔQ| becomes the predetermined value or higher while the cell voltage is dropping due to the discharging, whether |ΔV/ΔQ| has started to sharply rise. With the above configuration, it is easy to end the discharging of the cell when |ΔV/ΔQ| starts to sharply rise while the cell voltage is dropping due to the discharging. The predetermined value is set such that the cell voltage at which |ΔV/ΔQ| becomes the predetermined value or higher while the cell voltage is dropping due to the discharging becomes the cell voltage at which |ΔV/ΔQ| starts to sharply rise while the cell voltage is dropping due to the discharging. 
     In the aspects of the present disclosure, the assembled battery may be mounted on a vehicle. The one or more processors may be configured to estimate the deterioration degree of the assembled battery using a usage history of the vehicle, and change the predetermined discharging end voltage such that the predetermined discharging end voltage becomes higher as the estimated deterioration degree of the assembled battery is higher. 
     The cell voltage at which |ΔV/ΔQ| starts to sharply rise while the cell voltage is dropping due to the discharging tends to gradually rise as the battery deteriorates. With the above configuration, it is possible to finely adjust the discharging end voltage according to a change in the cell voltage at which |ΔV/ΔQ| starts to sharply rise. The one or more processors may finely adjust the discharging end voltage within a change width of, for example, 0.1 V. Examples of parameters indicating the usage history of the vehicle can include an integrated mileage of the vehicle, an input/output power amount (an integrated value) of the assembled battery, the number of times that the assembled battery is charged and discharged, and a period of use of the assembled battery (for example, an elapsed time from the start of use). The one or more processors can roughly determine the deterioration degree of the assembled battery (for example, large deterioration/medium deterioration/small deterioration) using usage history information of the vehicle. 
     In the aspects of the present disclosure, all the cells included in the assembled battery may be connected in series. The one or more processors may be configured to maintain a current value during the discharging of each of the cells. 
     With the above configuration, it is easy to match the current value during the discharging of each cell included in the assembled battery. As such, it is easy to estimate, with high accuracy, the deterioration degree of the assembled battery. 
     In the aspects of the present disclosure, the assembled battery may be configured to supply power to an in-vehicle power load. The one or more processors may be configured to execute the discharging by controlling the in-vehicle power load. 
     With the above configuration, it is possible to easily and appropriately execute the deterioration diagnosis of the assembled battery for the vehicle. The in-vehicle power load that is controlled by the one or more processors during the discharging may include at least one of air conditioning equipment, a seat heater, and a lighting device. 
     The deterioration of the assembled battery may be diagnosed while the assembled battery is mounted on the vehicle. The vehicle may include an internal combustion engine and a motor (hereinafter, also referred to as a “first motor”) that executes start processing of the internal combustion engine. The assembled battery of which the deterioration is diagnosed may be mounted on the vehicle and configured to supply power to the first motor. Hereinafter, the internal combustion engine mounted on the vehicle may be referred to as an “engine”. The engine may be configured to generate traveling driving power. In addition to the first motor, the vehicle may further include a second motor that receives supply of power from the assembled battery and generates traveling driving power. The first motor may be configured to generate power using drive power output from the engine and supply the generated power to the assembled battery. The start processing of the internal combustion engine is processing for starting the internal combustion engine. The internal combustion engine can be started by receiving assistance from the motor. The start processing may be cranking. The cranking is to start the internal combustion engine by rotating a crankshaft of the internal combustion engine. 
     A full charge capacity of the assembled battery in an initial state may be 5 kWh or smaller. When a capacity of the assembled battery is 5 kWh or smaller, a diagnosis with sufficient throughput can be executed by the above-described deterioration diagnosis method using discharging. The full charge capacity of the assembled battery to be diagnosed in the initial state may be 0.1 kWh or larger and 5 kWh or smaller, or 0.3 kWh or larger or 3 kWh or smaller. The assembled battery to be diagnosed may be a drive battery mounted on a hybrid electric vehicle (HEV). 
     Embodiments of the present disclosure will be described in detail with reference to drawings. In the drawings, the same or corresponding parts are denoted by the same reference signs and the description thereof will not be repeated. Hereinafter, an electronic control unit is also referred to as an “ECU”. 
       FIG.  1    is a diagram illustrating a configuration of a vehicle according to an embodiment. With reference to  FIG.  1   , a vehicle  100  is a hybrid electric vehicle (HEV). In this embodiment, it is assumed that the vehicle is a front-wheel drive four-wheel vehicle (more specifically, an HEV), but the number of wheels and a drive system can be appropriately changed. For example, the drive system may be four-wheel drive. 
     The vehicle  100  includes a drive battery  11 , a voltage sensor  12   a , a current sensor  12   b , a temperature sensor  12   c , a system main relay (SMR)  14 , a first motor generator  21   a  (hereinafter, referred to as an “MG  21   a ”), a second motor generator  21   b  (hereinafter, referred to as an “MG  21   b ”), a power control unit (PCU)  24 , and an engine  31 . 
     The drive battery  11  includes a rechargeable secondary battery. The drive battery  11  is configured to supply power to the PCU  24  (and thus the MGs  21   a ,  21   b ). In this embodiment, an assembled battery including a plurality of secondary batteries that are electrically connected to each other is employed as the drive battery  11 . A full charge capacity of the drive battery  11  in an initial state may be, for example, approximately 1.5 kWh. The secondary batteries included in the drive battery  11  may be modularized by a predetermined number. The assembled battery may be composed by combining a plurality of modules. The number of secondary batteries included in the drive battery  11  may be 10 or more and less than 100, or may be 100 or more. In this embodiment, the number of secondary batteries included in the drive battery  11  is approximately 50. The drive battery  11  is assembled in a form of a battery pack on, for example, a floor panel of the vehicle  100 . In this embodiment, the battery pack is formed by installing accessories (a voltage sensor  12   a , a current sensor  12   b , a temperature sensor  12   c , a battery ECU  13 , an SMR 14 , and the like) in a battery case that accommodates the drive battery  11 . 
     Each secondary battery included in the assembled battery is referred to as a “cell”. In this embodiment, all the cells included in the assembled battery are connected in series (see, for example,  FIG.  2    described below). In this embodiment, a lithium-ion secondary battery (a ternary LIB) having a positive electrode (a ternary positive electrode) of a layer-shaped crystal structure expressed by a general formula of Li x Ni y Co z Mn (1-x-z)  is employed as a cell. However, examples of the cell are not limited to the lithium-ion secondary battery, and may include other secondary batteries (for example, a nickel-hydrogen battery). Further, an all-solid-state secondary battery may be employed as a cell. Examples of the form of assembling the drive battery  11  in the vehicle  100  are not limited to the battery pack, and may include a packless form. 
     The voltage sensor  12   a  detects a voltage of each cell of the drive battery  11 . The current sensor  12   b  detects current flowing through the drive battery  11 . The temperature sensor  12   c  detects a temperature of each cell of the drive battery  11 . Each sensor outputs a detection result to the battery ECU  13 . The battery ECU  13  calculates a State of Charge (SOC) of each cell and an SOC of the drive battery  11  using the detection result of each sensor. The SOC indicates a remaining accumulated power amount and represents a ratio of, for example, a current accumulated power amount to an accumulated power amount in a fully charged state from 0% to 100%. The current sensor  12   b  is provided in a current path of the drive battery  11 . In this embodiment, one voltage sensor  12   a  and one temperature sensor  12   c  are provided to each cell. 
     The SMR  14  is configured to switch between connection/disconnection of the current path that connects the PCU  24  to the drive battery  11 . As the SMR  14 , for example, an electromagnetic mechanical relay can be employed. When the SMR  14  is in a closed state (a connected state), power can be sent and received between the drive battery  11  and the PCU  24 . On the other hand, when the SMR  14  is in an open state (a disconnected state), the current path that connects the drive battery  11  to the PCU  24  is disconnected. The SMR  14  is controlled by an HVECU  50 . The SMR  14  is switched to the closed state when, for example, the vehicle  100  is traveling. 
     Each of the MGs  21   a  and  21   b  is a motor generator that has both a function as a motor that outputs torque by receiving supply of driving power and a function as a generator that generates generated power by receiving torque. As each of the MGs  21   a  and  21   b , an alternating current motor (for example, a permanent magnet type synchronous motor or an induction motor) is used. Each of the MGs  21   a  and  21   b  is electrically connected to the drive battery  11  via the PCU  24 . The MGs  21   a ,  21   b  have rotor shafts  43   a ,  43   b , respectively. The rotor shafts  43   a ,  43   b  correspond to rotation shafts of the MGs  21   a ,  21   b , respectively. 
     The vehicle  100  further includes a single pinion type planetary gear  431 . An output shaft  41  of the engine  31  is connected to the planetary gear  431 . As the engine  31 , any internal combustion engine can be employed, but in this embodiment, as the engine  31 , a spark-ignition type internal combustion engine including a plurality of cylinders (for example, four cylinders) is employed. The engine  31  generates drive power by combusting fuel (for example, gasoline) in each cylinder, and rotates a crankshaft (not shown) common to all cylinders using the generated drive power. The crankshaft of the engine  31  is connected to the output shaft  41  via a torsional damper (not shown). As the crankshaft rotates, the output shaft  41  also rotates. An example of the engine  31  is not limited to a gasoline engine, and may include a diesel engine or a hydrogen engine. 
     The output shaft  41  of the engine  31  is connected to an input shaft  42  of the planetary gear  431 . The planetary gear  431  has three rotating elements, that is, an input element, an output element, and a reaction force element. More specifically, the planetary gear  431  has a sun gear, a ring gear coaxially arranged with the sun gear, a pinion gear that meshes with the sun gear and the ring gear, and a carrier that holds the pinion gear such that it can rotate and revolve. The carrier corresponds to the input element, the ring gear corresponds to the output element, and the sun gear corresponds to the reaction force element. The input shaft  42  of the planetary gear  431  is connected to the carrier. 
     The rotor shaft  43   a  of the MG  21   a  is connected to the sun gear of the planetary gear  431 . Torque is input from the engine  31  to the carrier of the planetary gear  431 . The planetary gear  431  is configured to divide and transfer the torque output by the engine  31  to the sun gear (and thus the MG  21   a ) and the ring gear. When the torque output by the engine  31  is output to the ring gear, a reaction force torque output by the MG  21   a  acts on the sun gear. 
     The planetary gear  431  and MG  21   b  are configured to combine drive power output from the planetary gear  431  (that is, drive power output to the ring gear) and drive power output from the MG  21   b  (that is, drive power output to the rotor shaft  43   b ) and transfer the combined power to drive wheels  45   a ,  45   b . More specifically, an output gear (not shown) that meshes with the driven gear  432  is installed at the ring gear of the planetary gear  431 . Further, a drive gear (not shown) installed at the rotor shaft  43   b  of the MG  21   b  also meshes with the driven gear  432 . The driven gear  432  acts to combine torque output by the MG  21   b  to the rotor shaft  43   b  with torque output from the ring gear of the planetary gear  431 . The drive torque combined as above is transferred to a differential gear  44 , and further transferred to the drive wheels  45   a ,  45   b  via the drive shafts  44   a ,  44   b  extending to the right and left from the differential gear  44 . 
     A transmission mechanism (not shown) may be provided on a downstream side of the planetary gear  431  (for example, between the driven gear  432  and the differential gear  44 ). The transmission mechanism includes a clutch and a brake, and is configured to change a gear ratio (that is, a ratio of rotation speed of an input shaft of the transmission mechanism to rotation speed of an output shaft of the transmission mechanism) depending on a state (engagement/disengagement) of each of the clutch and the brake. The vehicle  100  may further include a hydraulic pressure circuit (not shown) that supplies hydraulic pressure to each of the clutch and the brake included in the transmission mechanism. The HVECU  50  may switch the state (engagement/disengagement) of each of the clutch and the brake included in the transmission mechanism by controlling the hydraulic pressure circuit. The transmission mechanism may be positioned on the upstream side (for example, between the engine  31  and the planetary gear  431 ) of a power split device (the planetary gear  431 ). 
     The vehicle  100  further includes a shift lever  101  and a P position switch  102 . Each of the shift lever  101  and the P position switch  102  is configured to be able to switch between shift ranges according to a shift operation of a user. The user can select any of a neutral (N) range, a reverse (R) range, a drive (D) range, and a brake (B) range by moving the shift lever  101  to a predetermined position. Further, the user can select a parking (P) range by stopping the vehicle  100  and pressing the P position switch  102 . The HVECU  50  switches the shift range of the vehicle  100  to a range selected by the user. The HVECU  50  controls the hydraulic pressure circuit according to, for example, the shift range. 
     The vehicle  100  further includes a battery ECU  13 , a motor ECU  23 , an engine ECU  33 , and an HVECU  50 . In this embodiment, a computer (for example, a microcomputer) is employed as each of the battery ECU  13 , the motor ECU  23 , the engine ECU  33 , and the HVECU  50 . The ECUs are connected to each other in a manner capable of executing CAN communication therebetween. 
     The HVECU  50  includes a processor  51 , a random access memory (RAM)  52 , and a storage device  53 . As the processor  51 , for example, a central processing unit (CPU) can be employed. The RAM  52  functions as a working memory that temporarily stores data processed by the processor  51 . The storage device  53  is configured to be able to retain the stored information. In addition to the program, the storage device  53  stores information (for example, a map, a mathematical formula, and various parameters) used in the program. When the processor  51  executes the program stored in the storage device  53 , various processes are executed in the HVECU  50 . 
     Although  FIG.  1    illustrates a detailed configuration of only the HVECU  50 , each of the other ECUs also includes a processor, a RAM, and a storage device. The number of processors included in each ECU is arbitrary, and any ECU may include a plurality of processors. Further, various processes in each ECU are not limited to execution by software, and may be executed by dedicated hardware (an electronic circuit). 
     Motor sensors  22   a ,  22   b  that detect states (for example, current, a voltage, a temperature, and rotation speed) of the MGs  21   a ,  21   b  are provided in the MGs  21   a ,  21   b , respectively. Each of the motor sensors  22   a ,  22   b  outputs a detection result to the motor ECU  23 . An engine sensor  32  that detects a state (for example, an air intake amount, air intake pressure, an air intake temperature, exhaust pressure, an exhaust temperature, a catalyst temperature, an engine coolant temperature, and rotation speed) of the engine  31  is provided in the engine  31 . The engine sensor  32  outputs a detection result to the engine ECU  33 . The HVECU  50  receives detection values of the motor sensors  22   a ,  22   b  and the engine sensor  32  from the motor ECU  23  and the engine ECU  33 , as necessary. Further, the HVECU  50  receives a state (for example, a cell voltage, current, a temperature, and an SOC) of the drive battery  11  from the battery ECU  13 , as necessary. 
     The vehicle  100  includes a monitoring unit  80   a  that detects a state of an auxiliary battery  80  described below. The monitoring unit  80   a  includes various sensors that detect a state (for example, a temperature, current, and a voltage) of the auxiliary battery  80 , and outputs a detection result to the HVECU  50 . The HVECU  50  can acquire the state (for example, the temperature, current, the voltage, and an SOC) of the auxiliary battery  80  based on an output of the monitoring unit  80   a . Further, although not shown, other sensors (for example, a vehicle speed sensor, a fuel gauge, an odometer, an accelerator operation amount sensor, and an atmospheric pressure sensor) that indicate a situation of the vehicle  100  are also mounted on the vehicle  100 . The HVECU  50  can grasp information of the vehicle  100  based on outputs of various sensors (in-vehicle sensors) mounted on the vehicle  100 . 
     The HVECU  50  is configured to output a command (a control command) for controlling the engine  31  to the engine ECU  33 . The engine ECU  33  is configured to control various actuators (for example, a throttle valve, an ignition device, and an injector) (none of them shown) of the engine  31  according to the command from the HVECU  50 . The HVECU  50  can control the engine through the engine ECU  33 . 
     The HVECU  50  is configured to output a command (a control command) for controlling each of the MG  21   a  and the MG  21   b  to the motor ECU  23 . The motor ECU  23  is configured to generate a current signal (for example, a signal indicating a magnitude and frequency of current) corresponding to a target torque of each of the MG  21   a  and the MG  21   b  according to the command from the HVECU  50 , and to output the generated current signal to the PCU  24 . The HVECU  50  can control the motors through the motor ECU  23 . 
     The PCU  24  includes, for example, two inverters (not shown) provided to correspond to the MGs  21   a ,  21   b , and a converter (not shown) arranged between each inverter and the drive battery  11 . The PCU  24  is configured to supply power accumulated in the drive battery  11  to each of the MG  21   a  and the MG  21   b , and to supply power generated by each of the MG  21   a  and the MG  21   b  to the drive battery  11 . The PCU  24  is configured to be able to separately control the states of the MGs  21   a ,  21   b , that is, for example, it can turn the MG  21   b  to a power running state while turning the MG  21   a  to a power generation state. 
     The MG  21   a  is configured to execute start processing of the engine  31 . Specifically, when the engine  31  is started, the MG  21   a  that receives supply of power from the drive battery  11  executes cranking of the engine  31 . 
     The MG  21   a  is configured to generate power (that is, engine power generation) using drive power output from the engine  31 . The HVECU  50  charges the drive battery  11  with power generated by the engine power generation such that the SOC of the drive battery  11  does not become excessively low while the vehicle  100  is traveling. Further, the drive battery  11  is also charged with power generated by regenerative braking by the MG  21   b.    
     The vehicle  100  is configured to execute HV traveling and EV traveling. The HV traveling is executed by the engine  31  and the MG  21   b  while the engine  31  is generating traveling driving power. The EV traveling is executed by the MG  21   b  when the engine  31  is in a stopped state. When the engine  31  is in the stopped state, combustion in each cylinder is not executed. When the combustion in each cylinder is stopped, combustion energy (and thus traveling driving power) is not generated in the engine  31 . 
     The vehicle  100  further includes an auxiliary battery  80 , DC/DC converters  81 ,  82 , an auxiliary relay  83 , a high voltage load  91 , and a low voltage load  92 . The full charge capacity of the auxiliary battery  80  is smaller than that of the drive battery  11 . A full charge capacity of a battery is an electricity amount accumulated in the battery in a fully charged state and decreases as the battery deteriorates. As the auxiliary battery  80 , for example, a lead battery can be employed. However, as the auxiliary battery  80 , a secondary battery (for example, a nickel-hydrogen battery) other than the lead battery may be employed. The DC/DC converters  81 ,  82 , the auxiliary relay  83 , the high voltage load  91 , and the low voltage load  92  are controlled by the HVECU  50 . The HVECU  50  may control these through the battery ECU  13 . 
     The high voltage load  91  is an auxiliary machine of a high voltage system. The low voltage load  92  is an auxiliary machine of a low voltage system. A drive voltage of the low voltage load  92  is lower than a drive voltage of the high voltage load  91 . The auxiliary battery  80  is an in-vehicle battery of the low voltage system (for example, a 12 V system), and is configured to supply power to the low voltage load  92 . In this embodiment, the high voltage load  91  includes air conditioning equipment and the low voltage load  92  includes a lighting device. The air conditioning equipment is configured to heat and cool a cabin of the vehicle  100 . The lighting device includes a lighting device that illuminates the inside of the vehicle and a lighting device (for example, a headlight) that illuminates the outside of the vehicle. At least one of the high voltage load  91  and the low voltage load  92  may further include a seat heater that heats a seat of the vehicle  100 . 
     The DC/DC converter  81  is provided between the drive battery  11  and the high voltage load  91 , steps down power supplied from the drive battery  11  and outputs it to the high voltage load  91 . The DC/DC converter  82  steps down power supplied from the drive battery  11  and outputs it to each of the auxiliary battery  80  and the low voltage load  92 . When the SMR  14  is in the open state (the disconnected state), power of the drive battery  11  is not supplied to any of the high voltage load  91 , the low voltage load  92 , and the auxiliary battery  80 . An auxiliary relay  83  is arranged in a current path that connects the DC/DC converter  82  to the low voltage load  92 . When the auxiliary relay  83  is in the open state (the disconnected state), power is not supplied to the low voltage load  92 . 
     When the SMR  14  is in the closed state (the connected state), power can be supplied from the drive battery  11  to the auxiliary battery  80  through the DC/DC converter  82 . For example, when the SOC of the auxiliary battery  80  is lower than a predetermined value, the HVECU  50  charges the auxiliary battery  80  with power of the drive battery  11 . Further, the HVECU  50  drives the high voltage load  91  and the low voltage load  92  using power of the drive battery  11  according to an instruction from a service tool  200  (see  FIG.  2   ) in the deterioration diagnosis of the assembled battery (see S 16  in  FIG.  3   ) described below. At this time, the HVECU  50  controls the SMR  14 , the DC/DC converters  81 ,  82 , and the auxiliary relay  83  such that power of the drive battery  11  is supplied to each of the high voltage load  91  and the low voltage load  92 . 
     The HVECU  50  is configured to execute an SOC limit control to the drive battery  11 . The SOC limit control is a control for limiting the SOC of the drive battery  11  to within a predetermined SOC range. The HVECU  50  limits an input/output of the drive battery  11  such that the SOC of the drive battery  11  does not leave the SOC range. Specifically, the HVECU  50  controls the MGs  21   a ,  21   b , the engine  31 , and the DC/DC converters  81 ,  82  such that the SOC of the drive battery  11  is within the SOC range. The SOC range is variably set depending on a state of the vehicle  100 . The HVECU  50  may set an SOC range for protecting the drive battery  11  and its peripheral parts using, for example, a map stored in the storage device  53 . 
     The vehicle  100  further includes a power switch  103 . The power switch  103  is used for switching between start/stop of a vehicle system (the HVECU  50  and the like). The power switch  103  is operated by the user. 
     The vehicle  100  further includes a notification device  104 . The notification device  104  is configured to send a notification to the user of the vehicle  100  in response to a request from the HVECU  50 . Examples of the notification device  104  can include a meter panel, a head-up display, a navigation display, a warning light, or a speaker. The notification device  104  may function as an input device that receives an input from the user. The notification device  104  may include a touch panel display or a smart speaker that receives a voice input. The notification device  104  may be mounted on a portable device (that is, an electronic device that can be carried by the user), such as a tablet terminal, a smartphone, or a wearable device. 
       FIG.  2    is a diagram illustrating a configuration of a deterioration diagnosis apparatus of a battery according to this embodiment. With reference to  FIG.  2    together with  FIG.  1   , in this embodiment, the service tool  200  functions as a deterioration diagnosis apparatus of a battery. The service tool  200  includes a computer including a processor  201 , a RAM  202 , and a storage device  203 . The storage device  203  stores a diagnosis program. The deterioration diagnosis method (see  FIG.  3    described below) of the battery according to this embodiment is executed when the processor  201  executes the diagnosis program stored in the storage device  203 . 
     The service tool  200  further includes a human machine interface (HMI)  204 . The HMI  204  includes an input device and a display device. The HMI  204  may be a touch panel display. The HMI  204  may include a smart speaker that receives a voice input. 
     The HVECU  50  further includes a data link connector (DLC)  55   a  and an interface  55   b  of the DLC  55   a . The DLC  55   a  is a connector that can be connected to a connector  250  of the service tool  200 , and is arranged in, for example, the vicinity of a driver seat of the vehicle  100 . The service tool  200  is an external diagnosis machine used by, for example, a worker (such as a mechanic) in a maintenance shop to grasp a state of a vehicle. Examples of the service tool  200  can include a general scan tool (GST). By connecting the connector  250  of the service tool  200  to the DLC  55   a , the service tool  200  can read vehicle data stored in the storage device  53 . 
     In the deterioration diagnosis method of the battery according to this embodiment, the service tool  200  discharges each cell while measuring a voltage of each cell included in the drive battery  11  (the assembled battery). Then, when the voltages of all the cells included in the drive battery  11  reach a predetermined discharging end voltage (hereinafter, referred to as “V end ”), the service tool  200  ends the discharging. After ending the discharging, the service tool  200  estimates a deterioration degree of each cell included in the drive battery  11 , using voltage data indicating a transition of the voltage of each cell included in the drive battery  11  from a discharging start voltage to V end . 
     However, when V end  is too low, some cells included in the assembled battery may be over-discharged during the discharging of the assembled battery. On the other hand, when the V end  is too high, sufficient data cannot be obtained during the discharging of the assembled battery, and accuracy of estimating a deterioration degree (for example, a full charge capacity) of a cell may decrease. Therefore, in the deterioration diagnosis method of the battery according to this embodiment, the cell voltage, at which |ΔV/ΔQ| (that is, the change degree of the cell voltage per unit discharging amount) starts to sharply rise while the cell voltage is dropping due to the discharging, is set as V end . A method of deciding V end  and a technical significance thereof will be described below (see  FIGS.  5  to  10    described below). 
     The service tool  200  according to this embodiment includes a discharging unit  211  and an estimation unit  212 . The discharging unit  211  is configured to discharge each cell while measuring the voltage of each cell included in the drive battery  11  mounted on the vehicle  100 , and to end the discharging when the voltages of all the cells included in the assembled battery reach V end . The estimation unit  212  is configured to estimate a deterioration degree of each cell included in the assembled battery using the voltage data indicating the transition of the voltage of each cell included in the assembled battery from the discharging start voltage to V end . 
       FIG.  3    is a flowchart illustrating the deterioration diagnosis method of the battery according to this embodiment. Processes illustrated in this flowchart are executed when, for example, a predetermined instruction is input from the user to the HMI  204  after the connector  250  of the service tool  200  is connected to the DLC  55   a  of the vehicle  100  in a parked state. However, a condition of starting the process illustrated in  FIG.  3    is not limited thereto, and can be arbitrarily set. Hereinafter, each step in the flowchart is simply referred to as “S”. The discharging unit  211  of the service tool  200  transmits a control command to the HVECU  50 , whereby S 10  to S 18  of  FIG.  3    are executed. 
     With reference to  FIG.  3    together with  FIGS.  1  and  2   , in S 10 , the service tool  200  releases an SOC range related to the SOC limit control. As such, the SOC limit (the SOC limit control) of the drive battery  11  becomes invalid. 
     Subsequently in S 11 , the service tool  200  drives the engine  31  and charges the drive battery  11  with power generated by the engine power generation. By the process of S 11 , power generated by the MG  21   a  using drive power output from the engine  31  is input to the drive battery  11  via the PCU  24  and the SMR  14 . 
     In S 12 , the service tool  200  determines whether the voltages of all the cells included in the drive battery  11  have become a predetermined start voltage (hereinafter, referred to as “V start ”) or higher. The voltage of each cell included in the drive battery  11  is measured by the voltage sensor  12   a . V start  may be a cell voltage indicating that the cell has turned to the fully charged state, or may be a charging upper limit voltage of the cell. The charging upper limit voltage corresponds to an upper limit of a recommended voltage range. When the charging of the cell is continued until the voltage exceeds the charging upper limit voltage, the cell may be over-charged. The over-charging accelerates the deterioration of the cell. V start  may be 3.6 V or higher and 3.9 V or lower, or may be approximately 3.6 V. Further, the service tool  200  may determine whether the voltages of all the cells included in the drive battery  11  have become V start  or higher based on the SOC of the drive battery  11 . For example, when the SOC of the drive battery  11  has become a predetermined SOC value (for example, 70%) or higher, the service tool  200  may determine that the voltages of all the cells included in the drive battery  11  have become V start  or higher. 
     The processes of S 11  and S 12  are repeated until the voltages of all the cells included in the drive battery  11  become V start  or higher (NO in S 12 ). When the voltages of all the cells become V start  or higher (YES in S 12 ), in S 13 , the service tool  200  stops the engine  31 . Thereafter, in S 14 , the service tool  200  determines whether the voltages of all the cells included in the drive battery  11  have become stable. The process stands by in S 14  until the voltage of each cell included in the drive battery  11  becomes stable, and, when the voltage of each cell included in the drive battery  11  becomes stable (YES in S 14 ), the process proceeds to S 15 . 
     In S 15 , the service tool  200  measures a state (a voltage, current, and a temperature) of each cell included in the drive battery  11 , and records a measurement result in the storage device  203 . Subsequently in S 16 , the service tool  200  discharges the drive battery  11  by controlling a power load of the vehicle  100 . The drive battery  11  is configured to supply power to the power load mounted on the vehicle  100 . 
     Specifically, in S 16 , the service tool  200  controls the power load (for example, at least one of the high voltage load  91  and the low voltage load  92 ) of the vehicle  100  such that discharging current of each cell included in the drive battery  11  becomes a predetermined value (hereinafter, referred to as “Vd”). In this embodiment, the air conditioning equipment (the high voltage load  91 ) and the lighting device (the low voltage load  92 ) are driven by power supplied from the drive battery  11 . The service tool  200  adjusts power supplied from the drive battery  11  to the high voltage load  91  and the low voltage load  92 , using the DC/DC converters  81 ,  82 , respectively. Then, the service tool  200  maintains a current value during the discharging of each cell included in the drive battery  11 . Vd may be 1 A or higher and 10 A or lower, or may be approximately 5 A. In this embodiment, the current value during the discharging of each cell is maintained at Vd. In this embodiment, Vd is set to a fixed value (for example, 5 A), but Vd may be variable depending on the situation. 
     In S 17 , the service tool  200  determines whether the voltages of all the cells included in the drive battery  11  have reached a predetermined discharging end voltage (V end ). 
       FIG.  4    is a graph illustrating an example of a discharging characteristic of the cell included in the drive battery  11 . Each of the lines L 1  to L 3  in  FIG.  4    illustrates an example of a transition of current and a voltage of the drive battery  11  (the assembled battery) when S 10  to S 16  of  FIG.  3    are executed, and the discharging (S 16 ) is continued for a predetermined time. The line L 1  illustrates a transition of current of the drive battery  11 . The lines L 2 , L 3  respectively illustrate the discharging characteristics (more specifically, the transitions of the cell voltages during the discharging) of a first cell and a second cell included in the drive battery  11 . A full charge capacity of the first cell is larger than that of the second cell. 
     With reference to  FIG.  4   , in a comparison between the transition of a voltage of the first cell (the line L 2 ) and the transition of a voltage of the second cell (the line L 3 ), the voltage of the second cell starts to decrease earlier than that of the first cell and decreases to a voltage lower than a voltage to which the voltage of the first cell decreases. As such, the cell voltage tends to easily decrease during the discharging as a full charge capacity is smaller. When the voltage of the cell decreases too much due to the discharging of the cell, the deterioration of the cell is accelerated. The fact that the discharging of the voltage of the cell is continued until it decreases too much is referred to as “over-discharging”. 
     Hereinafter, with reference to  FIGS.  5  to  9   , the method of deciding the discharging end voltage (V end ) will be described.  FIG.  5    is a graph for describing an issue that may arise when a discharging lower limit voltage is set as V end . In  FIG.  5   , a line L 11  illustrates a voltage distribution of all cells included in a first assembled battery (hereinafter, referred to as a “first cell voltage distribution”). A line L 12  illustrates a voltage distribution of all cells included in a second assembled battery (hereinafter, referred to as a “second cell voltage distribution”). The second cell voltage distribution (the line L 12 ) has wider variations on cell voltages than the first cell voltage distribution (the line L 11 ). The first cell voltage distribution and the second cell voltage distribution are distributions when the discharging is continued until the voltages of all the cells included in the first assembled battery and all the cells included in the second assembled battery reach the discharging lower limit voltage (V end ), respectively. 
     In  FIG.  5   , the discharging lower limit voltage corresponds to a lower limit value of the recommended voltage range. When the discharging of the cell is continued until the voltage falls below the discharging lower limit voltage, the deterioration of the cell may be accelerated. The fact that the discharging of the cell is continued until the deterioration of the cell may be accelerated corresponds to the above-described “over-discharging”. A discharging prohibition voltage corresponds to a dischargeable limit value. When the discharging of the cell is continued until the voltage falls below the discharging prohibition voltage, an abnormality (for example, malfunction or failure) may occur in the cell. In the vehicle  100  according to this embodiment, the HVECU  50  has a warning flag for self-diagnosis (OBD) in the storage device  53 , and when a voltage of any of the cells included in the drive battery  11  falls below the discharging prohibition voltage, the HVECU  50  is configured to raise the warning flag (for example, a value of the flag is changed from “0” to “1”). The discharging lower limit voltage and the discharging prohibition voltage of each cell included in the drive battery  11  according to this embodiment are 3.0 V and 1.6 V, respectively. 
     With reference to  FIG.  5   , in the deterioration diagnosis apparatus of the assembly battery in which the discharging lower limit voltage is set as V end , when the variations on cell voltages are wide (see, for example, the line L 12 ), the deterioration of some cells included in the assembly battery may excessively progress and the life of the assembly battery may be shortened. Further, when the engine cranking is executed by power supplied by the assembled battery, the voltages of the entire assembled battery decrease as illustrated by a line L 13 . As such, the voltages of some cells included in the assembled battery may fall below the discharging prohibition voltage, and the warning flag may be raised. 
       FIG.  6    is a graph for describing a merit of raising V end . A line L 12  in  FIG.  6    is the same as the line L 12  in  FIG.  5   . A line L 14  in  FIG.  6    illustrates the second cell voltage distribution at the end of the discharging in the embodiment where V end  is set to a voltage higher than the discharging lower limit voltage. 
     With reference to  FIG.  6   , by raising V end  to a voltage higher than the discharging lower limit voltage (see, for example, the line L 14 ), a margin from the discharging prohibition voltage becomes large, and the warning flag is less likely to be raised. Further, the number of cells in an over-discharged state is reduced. For this reason, the shortening of the life of the assembled battery is restricted, and a voltage shortage of the assembled battery at the time of engine cranking is restricted. Further, a variation width of cell voltages tends to become wider as a discharging period is longer. Thus, by raising V end , the variation width of the cell voltages becomes narrow. 
       FIG.  7    is a graph for describing a demerit of raising V end . A line L 21  in  FIG.  7    illustrates an example of a transition of the voltages of the cells included in the drive battery  11  during the discharging. The transition of the voltage during the discharging is slightly different for each cell, but the tendency is substantially the same. 
     With reference to  FIG.  7   , when the discharging of the cell is started, the cell voltage gradually decreases. ΔV/ΔQ (corresponding to a slope of the graph), which is a value obtained by differentiating the cell voltage (the vertical axis) with respect to the discharging amount (the horizontal axis), is substantially constant immediately after the start of the discharging, but becomes high on the negative side when the discharging is continued for a while. In the deterioration diagnosis method of the assembled battery according to this embodiment, voltage data (that is, data indicating the transition of the cell voltage) of each cell included in the assembled battery during a discharging period (a period from the start of the discharging to the end of the discharging) is acquired and the deterioration degree of each cell is estimated based on the voltage data. “Q end ” in  FIG.  7    illustrates the discharging amount (the discharging amount at the end of the discharging) corresponding to V end . When V end  is raised, a discharging end timing becomes earlier and the discharging period becomes shorter. When the discharging period becomes shorter, the number of pieces of the voltage data for the deterioration diagnosis of the assembled battery is reduced, and the accuracy of estimating the deterioration degree of the cell decreases. 
       FIG.  8    is a graph for describing the method of deciding V end . The vertical axis of the graph represents −ΔV/ΔQ, and the horizontal axis represents the discharging amount. Since the cell voltage decreases as the discharging amount is increased, ΔV/ΔQ becomes a negative value and −ΔV/ΔQ becomes a positive value. −ΔV/ΔQ indicates the change degree of the cell voltage per unit discharging amount. −ΔV/ΔQ has the same value as |ΔV/ΔQ|. A line L 22  in  FIG.  8    illustrates an example of a transition of −ΔV/ΔQ of the cells included in the drive battery  11  during the discharging. The transition of −ΔV/ΔQ during the discharging is slightly different for each cell, but the tendency is substantially the same. 
     With reference to  FIG.  8   , at an initial stage of the discharging of the cell, −ΔV/ΔQ is substantially constant. Thereafter, at a final stage of the discharging, −ΔV/ΔQ sharply rises due to the increase in reaction resistance. At the final stage of the discharging of the cell, −ΔV/ΔQ lowers once, passes through the cell voltage (the voltage at the change point) at which −ΔV/ΔQ becomes the lowest during the discharging, and then starts to sharply rise. A narrowed width (more specifically, a narrowed width of the discharging period per the raised voltage) of the discharging period due to the raising of V end  becomes wider as −ΔV/ΔQ becomes higher. Before |ΔV/ΔQ| starts to sharply rise, a merit of raising V end  is larger than a demerit of raising V end . After |ΔV/ΔQ| starts to sharply rise, the demerit of raising V end  is larger than the merit of raising V end . In the deterioration diagnosis method of the battery according to this embodiment, the cell voltage at which −ΔV/ΔQ (that is, the change degree of the cell voltage per unit discharging amount) starts to sharply rise while the cell voltage is dropping due to the discharging is set as V end . Such V end  may be obtained in advance by experiments or simulation. In the examples illustrated in  FIGS.  7  and  8   , the length of the discharging period is expressed by the discharging amount, but the length of the discharging period may be expressed by time. 
       FIG.  9    is a graph illustrating a discharging characteristic of a lithium-ion secondary battery. Lines L 31 , L 32 , and L 33  in  FIG.  9    respectively illustrate transitions of the voltages during the discharging of a first lithium-ion secondary battery (hereinafter, referred to as a “first LIB”), a second lithium-ion secondary battery (hereinafter, referred to as a “second LIB”), and a third lithium-ion secondary battery (hereinafter, referred to as a “third LIB”). The first to third LIBs are lithium-ion secondary batteries (more specifically, ternary LIBs) having different deterioration degrees from each other, in order of the third LIB (deterioration degree: high), the second LIB (deterioration degree: intermediate), and the first LIB (deterioration degree: low) from the battery having the highest deterioration degree. 
     With reference to  FIG.  9   , in a case where the lithium-ion secondary battery employed as each cell of the drive battery  11  in this embodiment has been discharged, |ΔV/ΔQ| starts to sharply rise when the voltage of the lithium-ion secondary battery has become 3.4 V. For this reason, in the deterioration diagnosis method of the battery according to this embodiment, 3.4 V is set as V end . In the lithium-ion secondary battery, the voltage at which the entire lithium site in the positive electrode active material is occupied is 3.0 V. In other words, V end  is set to a voltage higher than the voltage at which the entire lithium site in the positive electrode active material in the lithium-ion secondary battery (the cell) is occupied. As such, the discharging is ended before the entire lithium site in the positive electrode active material is occupied. 
     The discharging characteristic of the lithium-ion secondary battery changes (see lines L 31  to L 33 ) as the lithium-ion secondary battery deteriorates. However, the battery voltage (the voltage of the lithium-ion secondary battery) at which |ΔV/ΔQ| starts to sharply rise during the discharging of the lithium-ion secondary battery does not significantly change even when the deterioration of the lithium-ion secondary battery progresses. In the example illustrated in  FIG.  9   , a change width is within 0.1 V. 
     As described above, in this embodiment, 3.4 V is set as V end . With reference to  FIG.  3    again, together with  FIGS.  1  and  2   , while a voltage of any of the cells included in the drive battery  11  is higher than V end  (NO in S 17 ), the processes of S 15  to S 17  are repeated, and the discharging of the drive battery  11  is continued. Then, when the voltages of all the cells included in the drive battery  11  become V end  or lower (YES in S 17 ), in S 18 , the service tool  200  ends the discharging of the drive battery  11 . 
     After ending the discharging of the drive battery  11  in S 18 , the service tool  200  restarts the SOC limit control. As such, the SOC of the drive battery  11  is limited to within the predetermined SOC range, again. 
     By repeating the above-described processes of S 15  to S 17 , data indicating the state (particularly, the deterioration degree) of the drive battery  11  is recorded in the storage device  203  of the service tool  200 . After the discharging is ended in S 18 , subsequently in S 19 , the estimation unit  212  of the service tool  200  estimates the deterioration degree of each cell included in the drive battery  11  using the recorded data of the drive battery  11 . 
     Specifically, the service tool  200  acquires a section discharging amount (Ah) from the discharging start voltage to V end  (the discharging end voltage) of each cell using the data (including the voltage data indicating the transition of the cell voltage from the discharging start voltage to the discharging end voltage) acquired in S 15 . The discharging amount corresponds to a time integral value of a discharging current (A). When the discharging current fluctuates within a section, the section discharging amount can be obtained by integrating the discharging current for each unit time with respect to time. When the discharging current is constant within the section, a value obtained by multiplying the discharging current (A) by the discharging period (h) corresponds to the discharging amount. 
     As described above, the service tool  200  calculates the section discharging amount (that is, the section discharging amount from the discharging start voltage to V end ) of the cell and converts the section discharging amount into the full charge capacity using a predetermined map. A map illustrating a relationship between a temperature of the cell, the section discharging amount, and the full charge capacity may be used to obtain the full charge capacity of the cell. When the temperature of the cell and the section discharging amount are given to the map, the full charge capacity of the cell is output from the map. The temperature of the cell that is used may be the average temperature during the discharging or the temperature at the start of the discharging. The map may be stored in the storage device  203  in advance. The map may be a map common to all the cells included in the drive battery  11 . The service tool  200  may acquire the map from an external server (for example, a server that manages information on various batteries), or may acquire the map from the vehicle  100 . 
     As described above, in S 19 , the estimation unit  212  of the service tool  200  estimates the full charge capacity of each cell included in the drive battery  11 . The full charge capacity (the electricity amount accumulated in the cell at a time of full charging) of the cell indicates the deterioration degree of the cell. The deterioration degree of the cell becomes higher as the full charge capacity of the cell is smaller. When the process of S 19  is executed, a series of processes illustrated in  FIG.  3    ends. 
     After the processes illustrated in  FIG.  3   , the service tool  200  may transmit a diagnosis result (that is, information indicating the full charge capacity of each cell included in the drive battery  11 ) to the vehicle  100 . The diagnosis result received by the vehicle  100  may be stored in the storage device  53  of the HVECU  50 . The notification device  104  may send a notification of the diagnosis result in response to a request from the user. 
     After the processes illustrated in  FIG.  3   , the HVECU  50  may start the engine  31  by the above-described cranking and charge the drive battery  11  with power generated by the engine power generation. The HVECU  50  may return the SOC of the drive battery  11  to the SOC value before the diagnosis. Alternatively, the HVECU  50  may charge the drive battery  11  until the vehicle  100  turns to a state of being capable of EV traveling. 
     As described above, the deterioration diagnosis method of the battery according to this embodiment includes the series of processes illustrated in  FIG.  3   . 
     In the processes illustrated in  FIG.  3   , each cell is discharged while the voltage of each of the cells included in the assembled battery (the drive battery  11 ) is measured, and the discharging is ended when the voltages of all the cells included in the assembled battery reach the predetermined discharging end voltage (V end ) (S 15  to S 18 ). The discharging end voltage (V end ) is the cell voltage (for example, 3.4 V) at which |ΔV/ΔQ| (that is, the change degree of the cell voltage per unit discharging amount) starts to sharply rise while the cell voltage is dropping due to the discharging (see  FIGS.  7  and  8   ). In the processes illustrated in  FIG.  3   , the deterioration degree of each cell included in the assembled battery (the drive battery  11 ) is estimated using the voltage data indicating the transition of the voltage of each cell included in the assembled battery from the discharging start voltage to the discharging end voltage (V end ) (S 19 ). 
       FIG.  10    is a graph illustrating a relationship between the discharging end voltage (V end ) and a coverage ratio. Lines L 41 , L 42 , and L 43  in  FIG.  10    respectively illustrate the coverage ratio when the deterioration diagnosis of the above-described first LIB, second LIB, and third LIB has been executed by the processes illustrated in  FIG.  3   . Specifically, the coverage ratio is measured while the discharging end voltage (V end ) is changed in the range from 3.0 V to 3.5 V. The coverage ratio corresponds to a ratio of the section discharging amount to the full charge capacity. The coverage ratio is obtained by dividing the section discharging amount calculated in S 19  of  FIG.  3    by the full charge capacity. The coverage ratio can be expressed as a percentage. The deterioration degrees of the batteries are in order of the third LIB (deterioration degree: high), the second LIB (deterioration degree: intermediate), and the first LIB (deterioration degree: low) from the highest. 
     With reference to  FIG.  10   , when V end  is raised above 3.4 V, the coverage ratio sharply decreases (see lines L 41  to L 43 ). In the deterioration diagnosis method where V end  is 3.4 V, the coverage ratio is approximately 70%, and the deterioration degree of the cell (the ternary LIB) can be estimated with sufficient accuracy. Further, by raising V end  to 3.4 V, the deterioration of the cell (the ternary LIB) during the discharging is restricted (see  FIG.  6   ). As such, with the deterioration diagnosis method of the battery according to the embodiment, it is possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. 
     With the deterioration diagnosis method of the battery, the full charge capacity of each cell included in the assembled battery (the drive battery  11 ) is acquired. For this reason, it is possible to rebuild the assembled battery by replacing only the cell having a high deterioration degree from among the cells in the assembled battery. However, the present disclosure is not limited thereto, and a battery pack including the drive battery  11  may be replaced. Whether the assembled battery needs replacing may be determined based on the full charge capacity of a cell having the minimum capacity (a cell having the smallest full charge capacity in the assembled battery). 
     In the embodiment, 3.4 V is set as the discharging end voltage (V end ). However, the discharging end voltage is not limited thereto, and can be appropriately changed. For example, in an embodiment where each of the cells included in the assembled battery (the drive battery  11 ) is a lithium-ion secondary battery (an LFP cell) having a positive electrode (a cobalt-free iron phosphate positive electrode) of an olivine-shaped crystal structure expressed by a general formula of Li x FePO 4 , a voltage selected from a range from 2.0 V or higher and 3.2 V or lower may be set as the discharging end voltage. With the above discharging end voltage, it is also possible to estimate, with sufficient accuracy, the deterioration degree of each cell included in the assembled battery while restricting the deterioration of the cell during the discharging. Further, in the deterioration diagnosis method of the battery according to the embodiment, the discharging end voltage (V end ) is a fixed value (3.4 V). However, the present disclosure is not limited thereto, and the discharging unit  211  may be configured to estimate the deterioration degree of the drive battery  11  using the usage history of the vehicle  100 , and change V end  such that V end  becomes higher as the estimated deterioration degree of the drive battery  11  is higher. 
       FIG.  11    is a flowchart illustrating a method of setting the discharging end voltage according to a modified example. The processes illustrated in this flowchart are executed before the process illustrated in  FIG.  3    is started. For example, when a predetermined instruction is input to the HMI  204  from the user after the connector  250  of the service tool  200  is connected to the DLC  55   a  of the vehicle  100  in the parked state, the processes illustrated in  FIG.  11    are executed. Then, the process illustrated in  FIG.  3    is started by a process of S 38  described below. 
     With reference to  FIG.  11    together with  FIGS.  1  and  2   , in S 31 , the HVECU  50  transmits vehicle history information (that is, information indicating the usage history of the vehicle  100 ) to the service tool  200 . The vehicle history information is sequentially acquired by various sensors mounted on the vehicle  100  when the user is using the vehicle  100 , and is stored in the storage device  53 . In one example, the vehicle history information transmitted in S 31  includes the integrated mileage of the vehicle  100 . 
     In S 32 , the service tool  200  receives the vehicle history information. Thereafter, the discharging unit  211  of the service tool  200  executes the processes of S 33  to S 38  described below. 
     In S 33 , the service tool  200  estimates the deterioration degree of the drive battery  11  using the vehicle history information. Specifically, it is estimated that the deterioration degree of the drive battery  11  becomes higher as the integrated mileage of the vehicle  100  is longer. Subsequently, in S 34 , the service tool  200  determines whether the deterioration degree of the drive battery  11  is high, intermediate, or low. 
     When the estimated deterioration degree of the drive battery  11  is low (“low” in S 34 ), in S 35 , the service tool  200  sets 3.40 V as the discharging end voltage (V end ). When the estimated deterioration degree of the drive battery  11  is approximately intermediate (“intermediate” in S 34 ), in S 36 , the service tool  200  sets 3.42 V as the discharging end voltage (V end ). When the estimated deterioration degree of the drive battery  11  is high (“high” in S 34 ), in S 37 , the service tool  200  sets 3.44 V as the discharging end voltage (V end ). 
     When any of the processes S 35  to S 37  is executed, in S 38 , the service tool  200  generates a discharging control start trigger used for the deterioration diagnosis of the assembled battery. As such, the process illustrated in  FIG.  3    is started. 
     As such, according to the processes illustrated in  FIG.  11   , V end  is changed such that V end  becomes higher as the estimated deterioration degree of the drive battery  11  is higher. With such an embodiment, it is possible to finely adjust the discharging end voltage (V end ) in accordance with the cell voltage (see  FIGS.  9  and  10   ) at which |ΔV/ΔQ| starts to sharply rise, the cell voltage at which |ΔV/ΔQ| starts to sharply rise being changed depending on the deterioration degree of the drive battery  11 . 
     The processes illustrated in  FIG.  3    may be appropriately changed. For example, the discharging end voltage (V end ) may be the cell voltage at which the change degree of the cell voltage per unit discharging amount while the cell voltage is dropping due to the discharging becomes a predetermined value or higher. Then, the discharging unit  211  may be configured to determine whether |ΔV/ΔQ| has become the predetermined value or higher while the cell voltage is dropping due to the discharging of each cell included in the assembled battery, and end the discharging upon determining that |ΔV/ΔQ| of all the cells included in the assembled battery has become the predetermined value or higher. 
       FIG.  12    is a flowchart illustrating a modified example of the processes illustrated in  FIG.  3   . The processes illustrated in  FIG.  12    are equivalent to the processes illustrated in  FIG.  3   , except that S 17 A and S 17 B are employed instead of S 17  ( FIG.  3   ). Hereinafter, the processes illustrated in  FIG.  12    will be described, focusing on differences from those illustrated in  FIG.  3   . 
     With reference to  FIG.  12   , together with  FIGS.  1  and  2   , in S 17 A, the service tool  200  calculates |ΔV/ΔQ| of each cell by differentiating the cell voltage with respect to the discharging amount in the transition of the voltage mapped (plotted) during the discharging of each cell. In S 17 B, the service tool  200  determines whether |ΔV/ΔQ| of all the cells included in the drive battery  11  has become a predetermined value (hereinafter, referred to as “Th”) or higher. 
       FIG.  13    is a graph for describing a method of setting Th. A line L 22  in  FIG.  13    is the same as the line L 22  in  FIG.  8   . As illustrated in  FIG.  13   , Th is set such that the cell voltage at which |ΔV/ΔQ| becomes Th or higher while the cell voltage is dropping due to the discharging becomes the cell voltage at which |ΔV/ΔQ| starts to sharply rise while the cell voltage is dropping due to the discharging. 
     While |ΔV/ΔQ| of any of the cells included in the drive battery  11  is lower than Th, the service tool  200  makes a negative determination in S 17 B of  FIG.  12   , repeats the processes of S 15  to S 17 B, and continues the discharging of the drive battery  11 . Then, when |ΔV/ΔQ| of all the cells included in the drive battery  11  becomes Th or higher (YES in S 17 B), in S 18 , the service tool  200  ends the discharging of the drive battery  11 . 
     As such, the discharging unit  211  can determine whether |ΔV/ΔQ| starts to sharply rise, based on whether |ΔV/ΔQ| has become the predetermined value or higher while the cell voltage is dropping due to the discharging. With the processes illustrated in  FIG.  12   , it is possible to easily determine whether |ΔV/ΔQ| starts to sharply rise while the cell voltage is dropping due to the discharging. Then, when |ΔV/ΔQ| starts to sharply rise, the discharging unit  211  can end the discharging of the cell. 
       FIG.  14    is a diagram illustrating a modified example of the service tool  200  illustrated in  FIG.  2   . With reference to  FIG.  14   , a service tool  200 A further includes a management unit  213  and a data acquisition unit  214  in addition to the discharging unit  211  and the estimation unit  212 . The management unit  213  is configured to manage V end . V end  corresponds to the discharging end voltage indicating the timing of ending the discharging of the battery, which is executed to acquire the data for estimating the deterioration degree of the battery. The data acquisition unit  214  stores, in the management unit  213 , as the discharging end voltage, the voltage of the battery when |ΔV/ΔQ| (the change degree of the voltage per unit discharging amount of the battery) has become a predetermined value (Th) or higher after the start of the discharging of the battery. The management unit  213  in the service tool  200 A is embodied by a storage device  203 A. The data acquisition unit  214  in the service tool  200 A is embodied by the processor  201  and a diagnosis program stored in the storage device  203 A. 
       FIG.  15    is a flowchart illustrating processes related to data acquisition executed by the service tool  200 A illustrated in  FIG.  14   . The data acquisition unit  214  of the service tool  200 A transmits a control command to the HVECU  50  in a state where the vehicle  100  in an unused state (for example, before shipment) is connected to the service tool  200 A, whereby S 41  to S 47  of  FIG.  15    are executed. 
     With reference to  FIG.  15    together with  FIG.  14   , in S 41 , the service tool  200 A prepares for the discharging of the drive battery  11 . In S 41 , the service tool  200 A may execute the above-described processes of S 10  to S 14  of  FIG.  3   . 
     Subsequently in S 42 , the service tool  200 A discharges the drive battery  11 . The process of S 42  may be equivalent to that of S 16  of  FIG.  3   . 
     Subsequently in S 43 , the service tool  200 A measures a state (a voltage, current, and a temperature) of each cell included in the drive battery  11 , and records a measurement result in the storage device  203 A. The process of S 43  may be equivalent to that of S 15  of  FIG.  3   . 
     Subsequently in S 44 , the service tool  200 A calculates |ΔV/ΔQ| of each cell by differentiating the cell voltage with respect to the discharging amount in the transition of the voltage during the discharging of each cell included in the drive battery  11 . Then, in S 45 , the service tool  200 A determines whether |ΔV/ΔQ| of all the cells included in the drive battery  11  has become Th or higher. The processes of S 44  and S 45  may be equivalent to those of S 17 A and S 17 B of  FIG.  12   , respectively. 
     When |ΔV/ΔQ| of all the cells included in the drive battery  11  becomes Th or higher (YES in S 45 ), in S 46 , the service tool  200 A specifies a battery voltage at which |ΔV/ΔQ| starts to sharply rise while the battery voltage is dropping due to the discharging from the discharging amount (Q end ) when |ΔV/ΔQ| of all the cells included in the drive battery  11  has become Th or higher. Then, the service tool  200 A decides the specified battery voltage to be V end . V end  may be the voltage (for example, the average cell voltage) of the drive battery  11  when |ΔV/ΔQ| of all the cells included in the drive battery  11  has become Th or higher. 
     Subsequently in S 47 , the service tool  200 A ends the discharging of the drive battery  11 . The process of S 47  may be equivalent to that of S 18  of  FIG.  3   . 
     Subsequently in S 48 , the data acquisition unit  214  of the service tool  200 A stores, in the management unit  213 , the discharging end voltage (V end ) in association with information indicating the type of the battery composing the drive battery  11 . The service tool  200 A may acquire the information indicating the type of the battery from the vehicle  100 . The information indicating the type of the battery may be stored in the storage device  53  in advance. In one example, the information indicating the type of the battery includes a battery manufacturer and a model number. The service tool  200 A can discriminate, based on the battery manufacturer and the model number, the type of the battery (for example, a lithium-ion secondary battery/a nickel-hydrogen secondary battery) composing the drive battery  11 . The discharging end voltage (V end ) may be stored in the management unit  213  by executing a series of processes illustrated in  FIG.  15    to the batteries mounted on vehicles other than the vehicle  100 . The management unit  213  manages a discharging end voltage (V end ) of a plurality of types of batteries. The management unit  213  manages the discharging end voltage (V end ) in association with information indicating the type of the battery in each battery. When the process of S 48  is executed, the series of processes illustrated in  FIG.  15    ends. 
       FIG.  16    is a flowchart for describing a method of setting the discharging end voltage based on the data that is acquired using the processes illustrated in  FIG.  15   . The processes illustrated in this flowchart are executed before the process illustrated in  FIG.  3    is started. For example, when a predetermined instruction is input from the user to the HMI  204  after the service tool  200 A is connected to the vehicle  100  that has been used, the processes illustrated in  FIG.  16    are executed. Then, the process illustrated in  FIG.  3    is started by a process of S 54  described below. 
     With reference to  FIG.  16    together with  FIG.  14   , in S 51 , the HVECU  50  transmits, to the service tool  200 A, the information (for example, the battery manufacturer and the model number of the battery composing the drive battery  11 ) indicating the type of the battery composing the drive battery  11 . 
     In S 52 , the service tool  200 A receives the information indicating the type of the battery. Thereafter, the discharging unit  211  of the service tool  200 A executes the processes of S 53  and S 54  described below. 
     In S 53 , the service tool  200 A acquires, from the management unit  213 , the discharging end voltage (V end ) corresponding to the type of the battery composing the drive battery  11  using the information indicating the type of the battery, and sets it. Thereafter, in S 54 , the service tool  200 A generates a discharging control start trigger used for deterioration diagnosis of the assembled battery. As such, the process illustrated in  FIG.  3    is started. In the processes illustrated in  FIG.  3   , the battery is discharged based on an appropriate discharging end voltage (that is, V end  set in S 53 ) corresponding to the type of the battery mounted on the vehicle  100 . As such, it is possible to estimate, with sufficient accuracy, the deterioration degree of the battery while restricting the deterioration of the cell during the discharging. 
     The functions of the discharging unit  211  and the estimation unit  212  may be implemented in the vehicle  100 .  FIG.  17    is a diagram illustrating a modified example of the HVECU  50  illustrated in  FIG.  2   . With reference to  FIG.  17   , an HVECU  50 A mounted on the vehicle  100  may include the discharging unit  211  and the estimation unit  212 . In such an embodiment, the HVECU  50 A mounted on the vehicle  100  executes the processes illustrated in  FIG.  3   . The discharging unit  211  and the estimation unit  212  in the HVECU  50 A may be embodied by the processor  51  and a program (for example, a diagnosis program stored in a storage device  53 A) executed by the processor  51 . However, each of the discharging unit  211 , the estimation unit  212 , and the data acquisition unit  214  that are described above may be embodied by dedicated hardware (an electronic circuit). 
     In the above embodiment, the assembled battery mounted on the HEV which does not include an inlet for a plug-in is a target of the deterioration diagnosis. However, the present disclosure is not limited thereto, and an assembled battery mounted on a plug-in hybrid electric vehicle (PHEV) which includes an inlet for a plug-in may be a target of the deterioration diagnosis. Further, an assembled battery mounted on other xEVs (for example, a BEV, an FCEV, a range extender EV) may be a target of the deterioration diagnosis. The xEV is a vehicle that uses power as all or part of a drive power source. 
     In some embodiments, all the cells may be or may not be connected in series in the assembled battery of which the deterioration is diagnosed by any of the above-described methods (see  FIG.  2   ). The structure of the assembled battery of which the deterioration is diagnosed is arbitrary.  FIG.  18    is a diagram illustrating a modified example of the assembled battery illustrated in  FIG.  2   . For example, an assembled battery  500  illustrated in  FIG.  18    may be a target of the deterioration diagnosis. The assembled battery  500  includes N parallel cell blocks (that is, parallel cell blocks CB- 1  to CB-N). Each of the parallel cell blocks CB- 1  to CB-N includes a plurality of cells connected in parallel. The number of cells connected in parallel in each parallel cell block is arbitrary, but in the example illustrated in  FIG.  18   , it is three. The parallel cell blocks CB- 1  to CB-N are connected in series via a power line. 
     The embodiment disclosed herein needs to be considered as illustrative in all points and not restrictive. The scope of the present disclosure is shown not by the above description of the embodiments but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope thereof.