Patent Publication Number: US-2023137917-A1

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

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-178695 filed on Nov. 1, 2021, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a deterioration diagnosis apparatus of an assembled battery and a deterioration diagnosis method of an assembled 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. A life of the assembled battery is often determined by a deterioration degree of a secondary battery (hereinafter, also referred to as a “minimum capacity cell”) having the smallest full charge capacity from among secondary batteries (hereinafter, also referred to as “cells”) composing the assembled battery. For this reason, a deterioration degree of the assembled battery can be estimated from data of the minimum capacity cell. For example, Japanese Unexamined Patent Application Publication No. 2020-38812 (JP 2020-38812 A) discloses a method of diagnosing a deterioration degree of an assembled battery using a transition of a voltage of a minimum capacity cell during discharging. In this method, the discharging of each of cells composing the assembled battery is executed, and a cell of which a voltage reaches a discharging lower limit voltage (a lower limit voltage indicating that a cell reaches over-discharging when the discharging is continued longer than the lower limit voltage) earliest during the discharging is regarded as the minimum capacity cell. 
     SUMMARY 
     In the technique described in JP 2020-38812 A, the cell of which the voltage reaches the discharging lower limit voltage earliest during the discharging is regarded as the minimum capacity cell, and discharging data (a transition of the voltage during the discharging) of only one cell is measured. However, the cell of which the voltage reaches the discharging lower limit voltage earliest during the discharging is not always the minimum capacity cell. For example, when voltage variations exist between cells at a start of the discharging, the lower a voltage of a cell at the start of the discharging, the easier the cell voltage early reaches the discharging lower limit voltage. As such, a voltage of a cell other than the minimum capacity cell may reach the discharging lower limit voltage earlier than that of the minimum capacity cell. When the deterioration degree of the assembled battery is estimated from data of the cell other than the minimum capacity cell, the deterioration degree (and thus the life) of the assembled battery may not be correctly diagnosed. 
     On the other hand, when the discharging is continued until voltages of all the cells included in the assembled battery reach the discharging lower limit voltage, at least one cell may be over-discharged. The over-discharging accelerates deterioration of a cell. 
     The present disclosure enhances accuracy of deterioration diagnosis of an assembled battery while restricting deterioration of a cell during the diagnosis of the assembled battery. 
     A deterioration diagnosis apparatus of an assembled battery according to a first aspect of the present disclosure includes at least one processor. The at least one processor is configured to execute discharging of each of a plurality of cells included in the assembled battery, measure a voltage of each of the cells during the discharging, end the discharging at a predetermined discharging end timing, and estimate a deterioration degree of the assembled battery using voltage data indicating a transition of a voltage of at least one of the cells from a discharging start voltage to a predetermined discharging end voltage. The at least one processor is configured to, when the voltage of at least one of the cells reaches the discharging end voltage, decide the discharging end timing using the voltage data of the cell of which the voltage reaches the discharging end voltage. 
     In the deterioration diagnosis apparatus of the assembled battery, when a voltage of any cell reaches the discharging end voltage of the assembled battery during the discharging, the at least one processor decides the discharging end timing based on the voltage data (a transition of the voltage from the discharging start voltage to the discharging end voltage) of the cell. Hereinafter, a cell of which a voltage reaches the discharging end voltage earliest during the discharging from among the cells included in the assembled battery is referred to as a “target cell”. With the above configuration, even when a voltage of a target cell reaches the discharging end voltage, the discharging of the assembled battery is continued until the discharging end timing, which is decided based on the voltage data of the target cell, without stopping the discharging of the assembled battery. It is easy to appropriately decide the discharging end timing using the voltage data of the target cell. 
     For example, the lower the voltage of the target cell at the start of the discharging, the lower a possibility that the target cell is the minimum capacity cell (the cell having the smallest full charge capacity in the assembled battery). The lower the voltage of the target cell at the start of the discharging, the further the at least one processor may delay the discharging end timing. By doing so, it is easy for the at least one processor to acquire the voltage data of the minimum capacity cell. However, when the discharging is continued for too long, the target cell may deteriorate due to over-discharging. In order to prevent the target cell from excessively deteriorating due to the over-discharging, the at least one processor may decide that the discharging end timing is within a period from the time at which the voltage of the target cell reaches the discharging end voltage to a time when a predetermined time (an allowable time that is set to prevent the over-discharging) elapses. As such, it is easy for the at least one processor to appropriately decide the discharging end timing using the voltage data of the target cell. As such, it is easy to raise accuracy of deterioration diagnosis of an assembled battery while restricting deterioration of a cell during the diagnosis of the assembled battery. A setting mode of the discharging end timing is not limited thereto. Other modes will be described below. 
     In the first aspect, the at least one processor may be configured to decide the discharging end timing, further using a predetermined voltage of at least one of the cells. The predetermined voltage may correlate with a discharging period of the assembled battery. 
     It is easy for the at least one processor to determine whether the voltage of the target cell at the start of the discharging is low, using a voltage of each of the cells at the start of the discharging. For example, when the voltage of the target cell is low at the start of the discharging, the possibility that the target cell is the minimum capacity cell is low. For this reason, the at least one processor may continue the discharging within a range in which the target cell does not significantly deteriorate due to the over-discharging. By doing so, it is easy to acquire the voltage data of the minimum capacity cell. On the other hand, when the voltage of the target cell is high at the start of the discharging, the possibility that the target cell is the minimum capacity cell is high. For this reason, the at least one processor may end the discharging immediately after the voltage of the target cell reaches the discharging end voltage. By doing so, it is easy to restrict the deterioration of the cell during the diagnosis of the assembled battery. 
     In the first aspect, the at least one processor may be configured to determine whether a voltage of a target cell at a start of the discharging corresponds to a maximum cell voltage that is a largest voltage from among voltages of the cells at the start of the discharging. The target cell is the cell of which a voltage reaches the discharging end voltage earliest during the discharging from among the cells. Upon determining that the voltage of the target cell at the start of the discharging does not correspond to the maximum cell voltage, the at least one processor may decide that a timing at which the voltage of the target cell reaches the discharging end voltage when assuming that the discharging of the target cell is started from the maximum cell voltage is the discharging end timing. Upon determining that the voltage of the target cell at the start of the discharging corresponds to the maximum cell voltage, the at least one processor may end the discharging immediately after the voltage of the target cell reaches the discharging end voltage. 
     When the voltage of the target cell at the start of the discharging corresponds to the maximum cell voltage, the target cell is considered to be the minimum capacity cell. In such a case, it is easy to restrict the deterioration of the cell during the diagnosis of the assembled battery by ending the discharging immediately after the voltage of the target cell reaches the discharging end voltage. On the other hand, when the voltage of the target cell at the start of the discharging does not correspond to the maximum cell voltage, a possibility that the target cell would not be the minimum capacity cell exists. However, when the discharging is continued until the timing at which the voltage of the target cell reaches the discharging end voltage when assuming that the discharging of the target cell is started from the maximum cell voltage, the voltage of the minimum capacity cell is considered to reach the discharging end voltage. With the above configuration, it is easy to estimate, with high accuracy, the deterioration degree of the assembled battery using the voltage data of the minimum capacity cell. 
     In the first aspect, the at least one processor may be configured to decide that a timing at which a voltage of a target cell reaches the discharging end voltage when assuming that the discharging of the target cell is started from the predetermined voltage is the discharging end timing. The target cell is a cell of which a voltage reaches the discharging end voltage earliest during the discharging from among the cells. 
     It may be possible to predict a voltage corresponding to the maximum cell voltage by experiments or simulation. The voltage that is predicted as above may be set in the at least one processor. Then, the at least one processor may decide the discharging end timing as above using the set voltage. With the above configuration, it is also possible to enhance the accuracy of the deterioration diagnosis of the assembled battery while restricting deterioration of the cell during the diagnosis of the assembled battery. 
     In the first aspect, the discharging end voltage may be set based on a discharging lower limit voltage common to the cells. As above, it is easy to restrict the deterioration of the cell during the diagnosis of the assembled battery by setting the discharging end voltage based on the discharging end limit voltage (a lower limit voltage indicating that a cell reaches the over-discharging when the discharging is continued longer than the lower limit voltage). The discharging end voltage may be set such that it matches the discharging lower limit voltage, or may be set such that it is slightly higher than the discharging lower limit voltage. 
     In the first aspect, all the cells included in the assembled battery may be connected in series. The at least one processor may be configured to maintain a current value of each of the cells during the discharging. 
     With the above configuration, it is easy to match a current value of each cell included in the assembled battery during the discharging. As such, it is easy to estimate, with the high accuracy, the deterioration degree of the assembled battery. 
     In the first aspect, the assembled battery may be configured to supply power to a power load mounted on a vehicle. The at least one processor may be configured to execute the discharging by controlling the 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 power load controlled by the at least one processor during the discharging may include at least one of air conditioning equipment, a seat heater, and a lighting device. 
     A deterioration diagnosis apparatus of an assembled battery according to a second aspect of the present disclosure includes at least one processor. At least one processor is configured to execute charging of each of a plurality of cells included in the assembled battery, measure a voltage of each of the cells during the charging, end the charging at a predetermined charging end timing, and estimate a deterioration degree of the assembled battery using voltage data indicating a transition of a voltage of at least one of the cells from a charging start voltage to a predetermined charging end voltage. The at least one processor is configured to, when the voltage of at least one of the cells reaches the charging end voltage, decide the charging end timing using the voltage data of the cell of which the voltage reaches the charging end voltage. 
     With the deterioration diagnosis apparatus of the assembled battery, it is possible to estimate the deterioration degree of the assembled battery from the transition of the voltage during the charging basically in a mode equivalent to that of the diagnosis of deterioration due to the discharging described above. As such, it is possible to enhance accuracy of the deterioration diagnosis of the assembled battery while restricting the deterioration of the cell during the diagnosis of the assembled battery. 
     However, the direction of current in the diagnosis of the deterioration due to the discharging is opposite to that in the diagnosis of the deterioration due to the charging. Further, the voltage of each cell decreases during the discharging, and the voltage of each cell increases during the charging. In the diagnosis of the deterioration due to the charging, a cell of which a voltage reaches the charging end voltage earliest during the charging from among the cells included in the assembled battery is referred to as a “target cell”. The charging end voltage may be set based on a charging upper limit voltage (an upper limit voltage indicating that a cell reaches over-charging when the charging is continued longer than the upper limit voltage) common to the cells included in the assembled battery. The charging end voltage may be set such that it matches the charging upper limit voltage, or may be set such that it is slightly lower than the charging upper limit voltage. 
     A deterioration diagnosis method of an assembled battery according to a third aspect of the present disclosure includes executing discharging of each of a plurality of cells while measuring a voltage of each of the cells included in the assembled battery, deciding, when a voltage of any cell reaches a predetermined discharging end voltage during the discharging, a discharging end timing based on voltage data indicating a transition of a voltage of the cell from a discharging start voltage to the discharging end voltage, ending the discharging at the discharging end timing, and estimating a deterioration degree of the assembled battery using voltage data of at least one of the cells that is acquired during the discharging. 
     In the same manner as in the above-described deterioration diagnosis apparatus due to the discharging, with the deterioration diagnosis method of the assembled battery, it is also possible to enhance accuracy of the deterioration diagnosis of the assembled battery while restricting deterioration of the cell during the diagnosis of the assembled battery. 
     In the third aspect, the discharging may be executed while the assembled battery is mounted on a vehicle. A full charge capacity of the assembled battery in an initial state may be 5 kWh or smaller. 
     When the discharging of the assembled battery is executed while it is mounted on the vehicle, rapid discharging is rarely easy. However, when a size of a capacity of the assembled battery to be diagnosed is appropriate, diagnosis with sufficient throughput can be executed using the above-described deterioration diagnosis method due to the discharging. Specifically, when the capacity of the assembled battery is 5 kWh or smaller, the diagnosis with the sufficient throughput can be executed using the above-described deterioration diagnosis method due to the discharging. A 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 may be 0.3 kWh or larger and 3 kWh or smaller. The assembled battery to be diagnosed using the above-described method may be a drive battery mounted on a hybrid electric vehicle (HEV). 
     A deterioration diagnosis method of an assembled battery according to a fourth aspect of the present disclosure includes executing charging of each of a plurality of cells while measuring a voltage of each of the cells included in the assembled battery, deciding, when a voltage of any cell reaches a predetermined charging end voltage during the charging, a charging end timing based on voltage data indicating a transition of a voltage of the cell from a charging start voltage to the charging end voltage, ending the charging at the charging end timing, and estimating a deterioration degree of the assembled battery using voltage data of at least one of the cells that is acquired during the charging. 
     In the same manner as in the above-described deterioration diagnosis apparatus due to the charging, with the deterioration diagnosis method of the assembled battery, it is also possible to enhance accuracy of deterioration diagnosis of the assembled battery while restricting deterioration of the cell during the diagnosis of the assembled battery. 
     In the fourth aspect, the charging may be executed while the assembled battery is mounted on a vehicle. A full charge capacity of the assembled battery in an initial state may be 10 kWh or larger. 
     When the charging is executed while the assembled battery is mounted on the vehicle, it is relatively easy to raise a charging speed. For example, rapid charging can be executed using electric vehicle supply equipment (EVSE) having a high output. With the above-described deterioration diagnosis method due to the charging, it is possible to diagnose a large-capacity assembled battery (that is, an assembled battery having a capacity of 10 kWh or larger) with sufficient throughput. A full charge capacity of the assembled battery to be diagnosed in an initial state may be 10 kWh or larger and 500 kWh or smaller, or may be 50 kWh or larger and 150 kWh or smaller. The assembled battery to be diagnosed using the above-described method may be a drive battery mounted on a battery electric vehicle (BEV) or a plug-in hybrid electric vehicle (PHEV). 
     With each aspect of the present disclosure, it is possible to enhance accuracy of deterioration diagnosis of an assembled battery while restricting deterioration of a cell during the diagnosis of the assembled battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments 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 an assembled battery according to the embodiment of the present disclosure; 
         FIG.  3    is a flowchart illustrating a discharging control in a deterioration diagnosis method of the assembled battery according to the embodiment of the present disclosure; 
         FIG.  4    is a graph for describing a discharging characteristic of the assembled battery illustrated in  FIG.  2   ; 
         FIG.  5    is a first graph for describing a method of deciding a discharging end timing in the deterioration diagnosis method of the assembled battery according to the embodiment of the present disclosure; 
         FIG.  6    is a second graph for describing the method of deciding the discharging end timing in the deterioration diagnosis method of the assembled battery according to the embodiment of the present disclosure; 
         FIG.  7    is a flowchart illustrating processes regarding a determination of a battery life in the deterioration diagnosis method of the assembled battery according to the embodiment of the present disclosure; 
         FIG.  8    is a flowchart illustrating a modified example 1 of processes illustrated in  FIG.  3   ; 
         FIG.  9    is a diagram illustrating a modified example of an HVECU illustrated in  FIG.  2   ; 
         FIG.  10    is a diagram illustrating a modified example of a service tool illustrated in  FIG.  2   ; 
         FIG.  11    is a flowchart illustrating a modified example 2 of the processes illustrated in  FIG.  3   ; 
         FIG.  12    is a graph for describing a method of deciding a charging end timing; 
         FIG.  13    is a flowchart illustrating a modified example of the processes illustrated in  FIG.  7   ; 
         FIG.  14    is a diagram illustrating a vehicle control device having a charging unit and an estimation unit, illustrated in  FIG.  10   , mounted thereon; and 
         FIG.  15    is a diagram illustrating a modified example of the assembled battery illustrated in  FIG.  2   . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     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 , an engine  31 , a transmission mechanism  421 , and a hydraulic pressure circuit  422 . 
     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 , which are motors for traveling). 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 liquid system lithium-ion secondary battery is employed as a cell. However, examples of the cell are not limited thereto, and an all-solid-state secondary battery may be employed as a cell. Further, 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). 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 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  through the transmission mechanism  421 . 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  corresponds to an input shaft of the transmission mechanism  421 . The transmission mechanism  421  includes a clutch and a brake (neither shown), and is configured to change a gear ratio (that is, a ratio of rotation speed of the input shaft of the transmission mechanism  421  to rotation speed of an output shaft  42  of the transmission mechanism  421 ) depending on states (engagement/disengagement) of the clutch and the brake. The hydraulic pressure circuit  422  is configured to adjust hydraulic pressure supplied to each of the clutch and the brake included in the transmission mechanism  421  according to a command from the HVECU  50 . The HVECU  50  may switch the state (engagement/disengagement) of each of the clutch and the brake included in the transmission mechanism  421  by controlling the hydraulic pressure circuit  422 . In the configuration illustrated in  FIG.  1   , the transmission mechanism  421  is positioned on the upstream side (the planetary gear  431 ) of a power split device, but the transmission mechanism  421  may be positioned on the downstream side (the side close to drive wheels  45   a ,  45   b ). 
     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 a plurality of 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  422  according to, for example, the shift range. 
     Each of the output shaft  42  of the transmission mechanism  421  and the rotor shaft  43   a  of the MG  21   a  is connected to 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 output shaft  42  of the transmission mechanism  421  is connected to the carrier of the planetary gear  431 . 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 output shaft  42  of the transmission mechanism  421  to the carrier of the planetary gear  431 . The planetary gear  431  is configured to, when the transmission mechanism  421  is in a non-neutral state (that is, a state of transferring the drive power), 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 the 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 a 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 . 
     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 (that is, a detection value of each of the voltage sensor  12   a , the current sensor  12   b , and a temperature sensor  12   c ) 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, 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 power generation (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 (see S 16  and S 22  of  FIG.  3   ) of the assembled battery 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 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 an assembled 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 an assembled 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. A deterioration diagnosis method (see  FIGS.  3  and  7    described below) of an assembled 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 assembled battery according to this embodiment, the service tool  200  acquires voltage data indicating a transition of a voltage of a cell inferred to be a minimum capacity cell (a cell having the smallest full charge capacity in the assembled battery) from a discharging start voltage to a predetermined discharging end voltage (hereinafter, referred to as “V end2 ”), and estimates a deterioration degree of the drive battery  11  (the assembled battery) using the acquired voltage data of the minimum capacity cell. 
     As a method of estimating the minimum capacity cell, a method where a cell (hereinafter, referred to as a “target cell”) of which a voltage has reached V end1  earliest during the discharging of all the cells included in the drive battery  11  is inferred to be the minimum capacity cell is considered. However, this method has low estimation accuracy. On the other hand, when the discharging is continued until the voltages of all the cells included in the assembled battery reach V end1  in order to accurately specify the minimum capacity cell, a possibility that one or more cells may be over-discharged is high. Over-discharging accelerates deterioration of a cell. 
     Therefore, in the deterioration diagnosis method of the assembled battery according to this embodiment, even when the voltage of the target cell reaches the discharging end voltage during the discharging of all the cells included in the drive battery  11 , the discharging of the assembled battery is continued until the discharging end timing, which is decided based on the voltage data of the target cell, without stopping the discharging of the assembled battery. Using the voltage data of the target cell, it is easy to appropriately decide the discharging end timing. As such, it is easy to raise accuracy of the deterioration diagnosis of the assembled battery while restricting deterioration of the cell during the diagnosis of the assembled battery. 
     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 all the cells included in the drive battery  11 , measure the voltage of each cell during the discharging, and end the discharging at the discharging end timing. The estimation unit  212  is configured to estimate the deterioration degree of the drive battery  11  using the acquired voltage data indicating the transition of the voltage of at least one cell included in the drive battery  11  from the discharging start voltage to V end1 . The discharging unit  211  is configured to, when a voltage of any cell (a target cell) included in the drive battery  11  reaches V end1 , decide the discharging end timing using voltage data of the cell (the target cell). 
       FIG.  3    is a flowchart illustrating discharging control in the deterioration diagnosis method of the assembled 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. The discharging unit  211  of the service tool  200  transmits a control command to the HVECU  50 , whereby each step of  FIG.  3    is executed. Hereinafter, each step in the flowchart is simply referred to as “S”. 
     With reference to  FIG.  3    together with  FIGS.  1  and  2   , 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 voltage slightly lower than the cell voltage in the fully charged state. V start  may be 3.7 V or higher and 3.9 V or lower. 
     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. 
       FIG.  4    is a graph for describing a discharging characteristic of the drive battery  11  (the assembled battery). Each of 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 11  to S 16  of  FIG.  3    are executed. The line L 1  illustrates a transition of current of the drive battery  11 . Lines L 2  and 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 . Specifically, 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 the “over-discharging”. In the example illustrated by the line L 3 , the discharging is continued until the cell reaches the over-charging. However, in the deterioration diagnosis method of the assembled battery according to this embodiment, the discharging end timing is set such that the cell does not reach the over-discharging. 
     With reference to  FIG.  3    together with  FIGS.  1  and  2    again, in S 17 , the service tool  200  determines whether a voltage of any cell (the target cell) included in the drive battery  11  has reached a predetermined V end1 . V end1  is set based on a discharging lower limit voltage common to all the cells included in the drive battery  11 . The discharging lower limit voltage corresponds to a lower limit voltage regarding the discharging. When the discharging of a cell is continued until its voltage falls below the discharging lower limit voltage, the cell may be over-discharged. In this embodiment, V end1  is set to a voltage slightly higher than the discharging lower limit voltage (for example, a value obtained by adding an extra voltage to the discharging lower limit voltage). V end1  may be 2.8 V or higher and 3.2 V or lower. In this embodiment, V end1  is set to 3.0 V. Until a negative determination is made in S 17 , the processes of S 15  to S 17  are repeated. When a voltage of any cell included in the drive battery  11  reaches V end1  (YES in S 17 ), the process proceeds to S 18 . A cell of which a voltage has reached V end1  earliest from among all the cells included in the drive battery  11  corresponds to the target cell. 
     In S 18 , the service tool  200  acquires a voltage (hereinafter, referred to as “V A ”) of the target cell at a start of the discharging, a maximum cell voltage (hereinafter, referred to as “V max ”), and a function (hereinafter, referred to as “Q A ”) indicating a discharging characteristic of the target cell, using the data acquired in S 15 . From among voltages (measured in S 15 ) of the cells included in the drive battery  11  at the start of the discharging, the highest voltage is V max  and the voltage of the target cell is V A . In other words, V A  is V max  or lower. 
     The service tool  200  can derive Q A  using the voltage data indicating the transition (measured in S 15 ) of the voltage of the target cell from V A  (the discharging start voltage) to V end1  (the discharging end voltage). Q A  may be specified using the least squares method. Q A  is a function in which a section discharging amount (Ah) of the target cell for a designated section is returned. For example, when V A  and V end1  are input to Q A , Q A  outputs a discharging amount of the target cell in a voltage section from V A  to V end1 . 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. 
     Subsequently in S 19 , the service tool  200  determines whether V A  corresponds to V max . Then, when V A  corresponds to V max  (YES in S 19 ), in S 24 , the service tool  200  ends the discharging of the drive battery  11 . On the other hand, when V A  does not correspond to V max  (NO in S 19 ), in S 20 , the service tool  200  decides the discharging end timing using Q A , V A , and V max . Hereinafter, with reference to  FIGS.  5  and  6   , a method of deciding the discharging end timing in the deterioration diagnosis method of the assembled battery according to this embodiment will be described. 
       FIG.  5    is a first graph for describing the method of deciding the discharging end timing in the deterioration diagnosis method of the assembled battery according to this embodiment. Lines L 11  and L 12  in  FIG.  5    respectively illustrate the discharging characteristics of a third cell and a fourth cell included in the drive battery  11 . Specifically, a full charge capacity of the third cell is larger than that of the fourth cell. With reference to  FIG.  5   , in a comparison between a transition (the line L 11 ) of a voltage of the third cell and a transition (the line L 12 ) of a voltage of the fourth cell, the voltage of the third cell at the start of the discharging is lower than the voltage of the fourth cell at the start of the discharging, and the voltage of the third cell reaches V end1  earlier than the voltage of the fourth cell. As such, when voltage variations exist between the cells at the start of the discharging, the lower the voltage of the cell at the start of the discharging, the easier the voltage early reaches V end1 . In the discharging of the drive battery  11 , basically, the smaller the full charge capacity of the cell, the easier the cell voltage tends to decrease (see  FIG.  4   ). However, as illustrated in  FIG.  5   , a case where a voltage of a cell having a large full charge capacity reaches V end1  earlier than a voltage of a cell having a small full charge capacity can also occur. Therefore, in the deterioration diagnosis method of the assembled battery according to this embodiment, even when the voltage of the target cell reaches V end1 , the discharging of the drive battery  11  is continued until the discharging end timing without stopping the discharging of the drive battery  11 . 
       FIG.  6    is a second graph for describing the method of deciding the discharging end timing in the deterioration diagnosis method of the assembled battery according to this embodiment. A line L 21  in  FIG.  6    illustrates a transition of the discharging amount of the target cell in the voltage section from V A  to V end1 . A line L 22  in  FIG.  6    illustrates a transition of the voltage of the target cell when assuming that the discharging of the target cell is started from V max . 
     With reference to  FIG.  6   , the service tool  200  decides that the timing at which the discharging amount of the target cell reaches ΔQ A  after the voltage (the line L 21 ) of the target cell has reached V end1  is the discharging end timing. 
     Specifically, in S 19  of  FIG.  3   , the service tool  200  determines whether V A  (the voltage of the target cell at the start of the discharging) corresponds to V max  (the maximum cell voltage). When it is determined that V A  corresponds to V max  (V A =V max ) (YES in S 19 ), the line L 21  matches the line L 22 , and ΔQ A  becomes zero. For this reason, the service tool  200  decides that the timing at which the voltage of the target cell has reached V end1  is the discharging end timing. As such, the discharging of the drive battery  11  is ended immediately after the voltage of the target cell has reached V end1  (S 24  of  FIG.  3   ). 
     On the other hand, upon determining that V A  does not correspond to V max  (NO in S 19 ), in S 20  of  FIG.  3   , the service tool  200  decides that the timing at which the voltage of the target cell reaches V end1  when assuming that the discharging of the target cell is started from V max  is the discharging end timing. Specifically, the service tool  200  sets, as ΔQ A , a value obtained by subtracting the section discharging amount (the discharging amount from V A  to V end1 ) of the target cell, obtained by inputting V A  and V end1  to Q A , from the section discharging amount (the discharging amount from V max  to V end1 ) of the target cell, obtained by inputting V max  and V end1  to Q A . When the discharging of the drive battery  11  is continued until the timing at which the voltage of the target cell reaches V end1  when assuming that the discharging of the target cell is started from V max , it is considered that the voltage of the minimum capacity cell included in the drive battery  11  reaches V end1 . 
     With reference to  FIG.  3    together with  FIGS.  1  and  2    again, when the discharging end timing is decided in S 20 , processes of S 21  and S 22  are executed. In S 21  and S 22 , the same processes as those of S 15  and S 16  described above are executed, respectively. By the process of S 22 , the discharging of the drive battery  11  is continued. In S 23 , the service tool  200  determines whether the discharging end timing has arrived. Specifically, the service tool  200  starts accumulation of the discharging amount from the timing at which the voltage of the target cell has reached V end1 , and determines that the discharging end timing has arrived when the accumulated discharging amount has reached ΔQ A  (see  FIG.  6   ). Until a negative determination is made in S 23 , the processes of S 21  to S 23  are repeated. When the discharging end timing arrives (YES in S 23 ), in S 24 , the service tool  200  ends the discharging of the drive battery  11 . 
     By the above-described processes illustrated in  FIG.  3   , the data indicating a state (particularly, the deterioration degree) of the drive battery  11  is recorded in the storage device  203  of the service tool  200 . The service tool  200  estimates the deterioration degree of the drive battery  11  using the recorded data of the drive battery  11 . Then, the service tool  200  determines whether a life of the drive battery  11  has run out (that is, whether the use of drive battery  11  can be continued). 
       FIG.  7    is a flowchart illustrating processes regarding a determination of a battery life in the deterioration diagnosis method of the assembled battery according to this embodiment. The processes illustrated in this flowchart are executed when a predetermined instruction is input from the user to the HMI  204  after the service tool  200  acquires the data of the drive battery  11  by, for example, the processes illustrated in  FIG.  3   . However, a condition for starting the process illustrated in  FIG.  7    is not limited thereto and can be arbitrarily set. For example, the process illustrated in  FIG.  7    may be automatically started after the process illustrated in  FIG.  3    end. Each step illustrated in  FIG.  7    is executed by the estimation unit  212  of the service tool  200 . The processes illustrated in  FIG.  7    may be executed in a state where the vehicle  100  and the service tool  200  are connected to each other, or may be executed in a state where the service tool  200  is removed from the vehicle  100 . 
     With reference to  FIG.  7    together with  FIGS.  1  and  2   , in S 31 , the service tool  200  obtains a full charge capacity of each cell of which a voltage has reached V end1  until the discharging end timing from among all the cells included in the drive battery  11 . In a case where only the voltage of the target cell reaches V end1  even when the discharging is continued until the discharging end timing, only the full charge capacity of the target cell is obtained. 
     In order to obtain the full charge capacity of the cell in S 31 , the service tool  200  uses the data (that is, the data recorded in the storage device  203  by the processes of S 15  and S 21  of  FIG.  3    during the discharging of the drive battery  11 ) of the drive battery  11  acquired by the processes illustrated in  FIG.  3   . In this embodiment, the service tool  200  calculates the section discharging amount (for example, the section discharging amount from the discharging start voltage to V end1 ) 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 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 . 
     Subsequently in S 32 , the service tool  200  acquires the full charge capacity (hereinafter, referred to as “Qc 1 ”) of the minimum capacity cell in the drive battery  11 . Specifically, the service tool  200  estimates the smallest full charge capacity from among the full charge capacities of the cells obtained in S 31  to be Qc 1 . By continuing the discharging of the drive battery  11  until the discharging end timing decided in S 18  to S 20  of  FIG.  3   , estimation accuracy of Qc 1  in S 32  becomes high. 
     Subsequently in S 33 , the service tool  200  determines whether Qc 1  is lower than a predetermined reference value (hereinafter, referred to as “Th 1 ”). Th 1  is a threshold value indicating the life of the drive battery  11  in the vehicle  100 . The fact that Qc 1  is smaller than Th 1  means that the life of the drive battery  11  for a current purpose has run out. Th 1  may be stored in the storage device  203  in advance. The service tool  200  may acquire Th 1  from an external server (for example, a server that manages information on various batteries) or may acquire Th 1  from the vehicle  100 . 
     When Qc 1  is lower than Th 1  (YES in S 33 ), in S 331 , the service tool  200  prompts a replacement of the drive battery  11 . For example, the HMI  204  or the notification device  104  displays a message indicating that a time to replace the drive battery  11  has arrived. However, the present disclosure is not limited thereto, and the HMI  204  may prompt the replacement of the drive battery  11  by sound (including voice). Alternatively, the service tool  200  may send a predetermined notification (for example, a notification indicating that a replacement of the drive battery  11  is required) to a terminal (for example, a smartphone or a wearable device) carried by the user of the vehicle  100 . After the process of S 331 , the drive battery  11  is replaced. In this embodiment, a battery pack including the drive battery  11  is replaced. However, the present disclosure is not limited thereto, and the assembled battery may be replaced (rebuilt) by a unit of a cell. The drive battery  11  removed from the vehicle  100  may be used for other purposes. 
     On the other hand, when Qc 1  is Th 1  or higher (NO in S 33 ), in S 332 , the service tool  200  sends a notification indicating that a replacement of the drive battery  11  is not required. For example, the HMI  204  or the notification device  104  displays a message indicating that the use of the drive battery  11  can be continued. In this case, the use of the drive battery  11  in the vehicle  100  is continued. 
     When the process of S 331  or S 332  is executed, a series of processes illustrated in  FIG.  7    ends. The service tool  200  may store a diagnosis result of S 331  or S 332  in the vehicle  100  (for example, the storage device  53  of the HVECU  50 ). 
     As described above, the deterioration diagnosis method of the assembled battery according to this embodiment includes a series of processes illustrated in  FIG.  3    and a series of processes illustrated in  FIG.  7   . In the processes illustrated in  FIG.  3   , the service tool  200  discharges each cell while measuring the voltage of each cell included in the drive battery  11  (S 15  and S 16 ). When a voltage of any cell reaches a predetermined discharging end voltage during the discharging (YES in S 17 ), the service tool  200  decides the discharging end timing based on the voltage data indicating the transition of the voltage of the cell from the discharging start voltage to the discharging end voltage (S 18  to S 20 ). Then, when the discharging end timing arrives (YES in S 19  or S 23 ), the service tool  200  ends the discharging of the drive battery  11  (S 24 ). In the processes illustrated in  FIG.  7   , the deterioration degree (for example, Qc 1 ) of the drive battery  11  is estimated using the voltage data of one or more cells acquired during the discharging. With such a deterioration diagnosis method of the assembled battery, the discharging end timing is appropriately decided based on the voltage data of the target cell. For this reason, it is possible to enhance the accuracy of the deterioration diagnosis of the assembled battery while restricting the deterioration of the cell during the diagnosis of the assembled battery. Further, in the above method, the drive battery  11  is discharged while it is mounted on the vehicle  100 . Since the capacity of the drive battery  11  to be diagnosed is 5 kWh or smaller, it is possible to execute the diagnosis with sufficient throughput. 
     The processes illustrated in  FIG.  3    may be appropriately changed.  FIG.  8    is a flowchart illustrating a modified example 1 of the processes illustrated in  FIG.  3   . The processes illustrated in  FIG.  8    are equivalent to those illustrated in  FIG.  3    except that the discharging end timing is decided in S 18 A and S 20 A instead of S 18  to S 20  (see  FIG.  3   ). 
     With reference to  FIG.  8   , in S 18 A, the service tool  200  acquires V A  (a voltage of the target cell at the start of the discharging) and Q A  (a function indicating a discharging characteristic of the target cell) using the data acquired in S 15 . In this modified example, the service tool  200  further acquires a predetermined voltage (hereinafter, referred to as “V x ”) in S 18 A. V x  correlates with a discharging period of the drive battery  11 . In this modified example, V x  is decided by experiments or simulation in advance such that the discharging of the drive battery  11  is continued by an appropriate discharging amount (or time) after the voltage of the target cell has reached V end1  (the discharging end voltage). V x  is stored in, for example, the storage device  203 . 
     In S 20 A, the service tool  200  decides the discharging end timing using Q A , V A , and V x . For example, the service tool  200  decides that the timing at which the voltage of the target cell reaches V end1  when assuming that the discharging of the target cell is started from V x  is the discharging end timing. Specifically, the service tool  200  sets, as ΔQ A , a value obtained by subtracting the section discharging amount (the discharging amount from V A  to V end1 ) of the target cell, obtained by inputting V A  and V end1  to Q A , from the section discharging amount (the discharging amount from V x  to V end1 ) of the target cell, obtained by inputting V x  and V end1  to Q A . Then, the service tool  200  decides that the timing at which the discharging amount of the target cell reaches ΔQ A  after the voltage of the target cell has reached V end1  is the discharging end timing. 
     In the service tool  200  according to the modified example, the discharging unit  211  is configured to decide that the timing at which the voltage of the target cell reaches V end1  when assuming that the discharging of the target cell is started from V x  is the discharging end timing. V x  may be a fixed value or may be variable depending on the situation. When V x  is a fixed value, the lower V A  is, the higher ΔQ A  becomes. An upper limit value (a guard value) may be set for ΔQ A  such that the target cell does not excessively deteriorate due to the over-discharging. Further, the discharging unit  211  may decide V x  using the voltage of each cell included in the drive battery  11  at the start of the discharging. For example, the discharging unit  211  may calculate the average value (hereinafter, referred to as “V ave ”) of voltages of the cells in the drive battery  11  at the start of the discharging, and determine a threshold value based on V ave . The threshold value may be the same value as V ave , or may be a value lower than V ave . Then, when V A  is higher than the threshold value, the discharging unit  211  may set V max  (the maximum cell voltage) as V x , and when V A  is lower than the threshold value, it may set a value lower than V max  as V x . 
     In the above embodiment, the discharging unit  211  and the estimation unit  212  in the service tool  200  are embodied by the processor  201  and a program executed by the processor  201 . However, the present disclosure is not limited thereto, and the discharging unit  211  and the estimation unit  212  may be embodied by dedicated hardware (an electronic circuit). 
     Further, functions of the discharging unit  211  and the estimation unit  212  may be implemented in the vehicle  100 .  FIG.  9    is a flowchart illustrating a modified example of the HVECU  50  illustrated in  FIG.  2   . With reference to  FIG.  9   , an HVECU  50 A mounted on the vehicle  100  may include the discharging unit  211  and the estimation unit  212 . The discharging unit  211  and the estimation unit  212  in the HVECU  50 A may be embodied by the processor  51  and a program executed by the processor  51  (for example, a diagnosis program stored in a storage device  53 A). 
     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 the above embodiment, the diagnosis of the deterioration due to the discharging is described. However, the present disclosure is not limited thereto, and it is also possible to execute the charging instead of the discharging and estimate the deterioration degree of the assembled battery from the transition of the voltage during the charging. Hereinafter, with reference to  FIG.  10    to  FIG.  13   , a diagnosis of a deterioration due to the charging will be described focusing on differences from the diagnosis of the deterioration due to the discharging. In the diagnosis of the deterioration due to the charging described below, an assembled battery (a drive battery) mounted on a battery electric vehicle (BEV) having a function of vehicle-to-home (V2H) is set as a target of the deterioration diagnosis. A full charge capacity of the assembled battery of which the deterioration is diagnosed in an initial state may be, for example, approximately 100 kWh. 
       FIG.  10    is a flowchart illustrating a modified example of the service tool  200  illustrated in  FIG.  2   . With reference to  FIG.  10   , a service tool  200 A includes a charging unit  213  and an estimation unit  214 . The charging unit  213  and the estimation unit  214  in the service tool  200 A are embodied by a processor  201  and a diagnosis program stored in a storage device  203 A. The charging unit  213  is configured to charge all the cells included in the assembled battery, measure the voltage of each cell during the charging, and end the charging at a predetermined charging end timing (hereinafter, referred to as “V end2 ”). The estimation unit  214  is configured to estimate the deterioration degree of the assembled battery using voltage data indicating a transition of a voltage of at least one cell included in the assembled battery from the charging start voltage to V end2 . The charging unit  213  is configured to, when a voltage of any cell (the target cell) included in the assembled battery reaches V end2 , decide the charging end timing using voltage data of the cell (the target cell). 
       FIG.  11    is a flowchart illustrating a modified example 2 of the processes illustrated in  FIG.  3   . Each step illustrated in  FIG.  11    is executed by the charging unit  213  of the service tool  200 A. With reference to  FIG.  11   , in S 51 , the service tool  200 A discharges the assembled battery by a function of V2H of the BEV. In other words, the service tool  200 A discharges the assembled battery mounted on the BEV by removing power therefrom to the outside of the vehicle. The power of the assembled battery may be consumed by a power load outside the vehicle, or may be accumulated in a power accumulation device outside the vehicle. However, the discharging method is not limited to V2H, and the power of the assembled battery may be consumed by a power load mounted on the BEV. 
     In S 52 , the service tool  200 A determines whether the voltages of all the cells included in the assembled battery are a predetermined charging start voltage or lower. The charging start voltage may be the discharging lower limit voltage or may be a voltage slightly higher than the discharging lower limit voltage. When the voltages of all the cells fall below the charging start voltage due to the discharging of S 51  (YES in S 52 ), in S 53 , the service tool  200 A stops the discharging. Thereafter, the BEV is connected to electric vehicle supply equipment (EVSE). Then, in S 54 , the service tool  200 A determines whether the voltages of all the cells included in the assembled battery are stable. When the voltage of each cell included in the assembled battery is stable (YES in S 54 ), the process proceeds to S 55 . 
     In S 55 , the service tool  200 A measures a state (a voltage, current, and a temperature) of each cell included in the assembled battery, and records a measurement result in the storage device  203 A. Subsequently in S 56 , the service tool  200 A charges the assembled battery by the EVSE (an external power source). The assembled battery is charged (in more detail, charged by the external power source from the outside) while it is mounted on the BEV. The service tool  200 A may execute a rapid charging of the assembled battery using a rapid charger as the EVSE. In S 57 , the service tool  200 A determines whether a voltage of any cell (the target cell) included in the assembled battery has reached a predetermined V end2 . V end2  may be a cell voltage indicating that the cell is in a fully charged state, or may be a voltage slightly lower than the cell voltage in the fully charged state. 
     When a voltage of any cell included in the assembled battery reaches V end2  (YES in S 57 ), in S 58 , the service tool  200 A acquires a voltage (hereinafter, referred to as “V B ”) of the target cell at the start of the charging, a minimum cell voltage (hereinafter, referred to as “V min ”), and a function (hereinafter, referred to as “Q B ”) indicating a charging characteristic of the target cell, using the data acquired in S 55 . From among the voltages (measured in S 55 ) of the cells included in the assembled battery at the start of the charging, the lowest voltage is V min , and the voltage of the target cell is V B . In other words, V B  is V min  or higher. The service tool  200 A can derive Q B  using the voltage data (measured in S 55 ) indicating the transition of the voltage of the target cell from V B  to V end2 . Q B  may be specified using the least squares method. 
     Subsequently in S 59 , the service tool  200 A determines whether V B  corresponds to V min . Then, when V B  corresponds to V min  (YES in S 59 ), in S 64 , the service tool  200 A ends the charging of the assembled battery. On the other hand, when V B  does not correspond to V min  (NO in S 59 ), in S 60 , the service tool  200 A decides the charging end timing using Q B , V B , and V min . 
       FIG.  12    is a graph for describing a method of deciding the charging end timing. A line L 31  in  FIG.  12    illustrates a transition of a charging amount of the target cell in a voltage section from V B  to V end2 . A line L 32  in  FIG.  12    illustrates a transition of the voltage of the target cell when assuming that the charging of the target cell is started from V min . 
     With reference to  FIG.  12   , in S 59  of  FIG.  11   , when it is determined that V B  corresponds to V min , the line L 31  matches the line L 32  and ΔQ B  becomes zero. For this reason, the service tool  200 A decides that the timing at which the voltage of the target cell has reached V end2  is the charging end timing. As such, the charging of the assembled battery is ended immediately after the voltage of the target cell has reached V end2  (S 64  of  FIG.  11   ). 
     On the other hand, in S 59  of  FIG.  11   , upon determining that V B  does not correspond to V min , subsequently in S 60 , the service tool  200 A sets, as ΔQ B , a value obtained by subtracting the section charging amount (the charging amount from V B  to V end2 ) of the target cell, obtained by inputting V B  and V end2  to Q B , from the section charging amount (the charging amount from V min  to V end2 ) of the target cell, obtained by inputting V min  and V end2  to Q B . 
     With reference to  FIG.  11    again, when the charging end timing is decided in S 60 , processes of S 61  and S 62  are executed. In S 61  and S 62 , the same processes as those of S 55  and S 56  described above are executed, respectively. By the process of S 62 , the charging of the assembled battery is continued. In S 63 , the service tool  200 A determines whether the charging end timing has arrived. When the charging end timing arrives (YES in S 63 ), in S 64 , the service tool  200 A ends the charging of the assembled battery. 
     By the above-described processes illustrated in  FIG.  11   , the data indicating the state (particularly, the deterioration degree) of the assembled battery is recorded in the storage device  203 A of the service tool  200 A. The service tool  200 A estimates the deterioration degree of the assembled battery using the data of the assembled battery recorded in S 55  and S 61  of  FIG.  11   . 
       FIG.  13    is a flowchart illustrating a modified example of the processes illustrated in  FIG.  7   . Each step illustrated in  FIG.  13    is executed by the estimation unit  214  of the service tool  200 A. In the processes illustrated in  FIG.  13   , the deterioration degree of the assembled battery is estimated basically in a mode equivalent to that of the diagnosis of the deterioration (see  FIG.  7   ) due to the discharging described above. In other words, by the processes of S 71  and S 72 , the full charge capacity (Qc 2 ) of the minimum capacity cell in the assembled battery is acquired. In S 73 , the service tool  200 A determines whether Qc 2  is lower than a predetermined reference value (Th 2 ). Th 2  is a reference value corresponding to Th 1  (see  FIG.  7   ). When Qc 2  is lower than Th 2  (YES in S 73 ), the drive battery (the assembled battery) of the BEV is replaced (S 731 ), and when Qc 2  is Th 2  or higher (NO in S 73 ), the use of the assembled battery in the BEV is continued (S 732 ). As such, with the deterioration diagnosis method of the assembled battery according to the modified example, it is possible to estimate the deterioration degree of the assembled battery from the transition of the voltage during the charging. With the above-described deterioration diagnosis method due to the charging, it is possible to diagnose a large-capacity assembled battery (that is, an assembled battery having a capacity of 10 kWh or larger) with sufficient throughput. 
     Various modifications made for the diagnosis of the deterioration due to the discharging (for example, the modifications illustrated in  FIGS.  8  and  9   ) may be executed for the diagnosis of the deterioration due to the charging. For example, functions of the above-described charging unit  213  and estimation unit  214  (see  FIG.  10   ) may be implemented in the vehicle (for example, an xEV).  FIG.  14    is a diagram illustrating an ECU  50 B having the charging unit  213  and the estimation unit  214 , illustrated in  FIG.  10   , mounted thereon. With reference to  FIG.  14   , the ECU  50 B mounted on the vehicle (for example, a BEV or a PHEV) includes a processor  51 B, a RAM  52 B, and a storage device  53 B. In the ECU  50 B, the charging unit  213  and the estimation unit  214  are embodied by the processor  51 B and a program executed by the processor  51 B. The service tool or the ECU of the vehicle may be configured to selectively execute the diagnosis of the deterioration due to the discharging (for example, the processes illustrated in  FIGS.  3  and  7   ) and the diagnosis of the deterioration due to the charging (for example, the processes illustrated in  FIGS.  11  and  13   ). For example, when the full charge capacity of the assembled battery to be diagnosed in the initial state is a predetermined value or higher, the diagnosis of the deterioration due to the charging may be executed, and when the full charge capacity of the assembled battery to be diagnosed in the initial state is lower than the predetermined value, the diagnosis of the deterioration due to the discharging may be executed. 
     It is not essential that all the cells are 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.  15    is a diagram illustrating a modified example of the assembled battery illustrated in  FIG.  2   . For example, an assembled battery  500  illustrated in  FIG.  15    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.  15   , 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.