Abstract:
Provided is a simple secondary battery tester that obtains, without applying an AC signal for determination based on capacitance, the capacitance of a secondary battery from time characteristics of a current flowing to the secondary battery and a terminal voltage and determines whether the secondary battery is degraded based on the capacitance thus obtained.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japanese Patent Application No. 2012-228844 (filed on Oct. 16, 2012), the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure relates to a tester for determining degradation of a secondary battery. 
     BACKGROUND 
     For example, a secondary battery that is used by repeatedly charging has become essential in recent social life due to the popularization of hybrid vehicles, electric vehicles, mobile phones, and the like. 
     Although a secondary battery has evolved to have a large battery capacity, charging/discharging in a repeating manner gradually increases an internal change and inhibits sufficient charging/discharging, thereby reducing the life of some secondary batteries. 
     Therefore, especially when the secondary battery is used in a vehicle, there is a risk that the vehicle suddenly becomes undrivable due to degradation of the secondary battery. It is thus desired to use the secondary battery after determining whether the secondary battery is degraded. 
     As such, methods of determining degradation of the secondary battery have conventionally been developed. 
     For example, NPL 1 set forth the below discloses degradation determination by measuring internal resistance corresponding to degradation of a lithium-ion battery. 
     Also, PLT 1 set forth below discloses detection of a state of a lithium-ion secondary battery by applying an AC voltage and/or an alternating current at a particular frequency to the lithium-ion secondary battery. PLT 2 set forth below discloses a method of measuring an AC impedance of a non-aqueous electrolyte secondary battery at a predetermined frequency and estimating reversible capacity of the battery from a relational expression of the AC impedance and the reversible capacity allowing charging/discharging (a battery capacity that allows charging/discharging). 
     Also, PLT 3 set forth below discloses: deriving voltage-current characteristics of the lithium-ion battery; deriving an open-circuit voltage (Open-Circuit Voltage: OCV) of the lithium-ion battery based on the voltage-current characteristics thus obtained; estimating a charging capacity (State Of Charge: SOC) of the lithium-ion battery by employing current integration or the like; and determining deposition degradation based on changes in the OCV and the SOC. 
     PLT 4 set forth below discloses determination on degradation based on information about a change in a voltage obtained in a diagnostic mode for continuously discharging and charging the lithium-ion battery at a constant electric power. 
     Further, PLT 5 set forth below discloses: in charging the lithium-ion battery by employing a constant current and constant voltage scheme, setting a charging current at C 0 /(20 hours) or less, provided that the C 0  represents a nominal capacity of the battery; obtaining time t from when a charging voltage during charging with the constant current reaches a predetermined voltage Vs to when the charging voltage reaches an upper limit Vc of the charging voltage; and estimating the capacity (Ce) of the lithium-ion battery from the estimated capacity ratio Ce/C o  by using a relational expression Ce/C 0 =At+B (A, B=const); and determining degradation. 
     A degradation determination method described in PLT 6 set forth below is as follows. That is, when the secondary battery such as the lithium-ion battery, a nickel-cadmium battery, or a nickel-metal hydride battery is connected to a charging device, a type of the secondary battery is detected and, based on a voltage of the battery, constant-current charging processing commences. During the constant-current charging processing, when the voltage of the battery reaches a reference voltage corresponding to the type of the battery, a controller starts measuring constant-current charging time. Depending on a charging control method corresponding to the type of the secondary battery being used, when the constant-current charging is switched over to constant-voltage charging, or when −ΔV is detected, the measurement of the time ends. The control unit compares the constant-current charging time obtained by the measurement and a constant-current charging time of a battery having a state of charging capacity of a brand new battery. 
     CITATION LIST 
     Patent Literatures 
     
         
         PTL 1: Japanese Patent Application Laid-Open Publication No. 2009-244088 
         PTL 2: Japanese Patent Application Laid-Open Publication No. 2012-122817 
         PTL 3: Japanese Patent Application Laid-Open Publication No. 2010-66232 
         PTL 4: Japanese Patent Application Laid-Open Publication No. 2010-60408 
         PTL 5: Japanese Patent Application Laid-Open Publication No. 2001-332310 
         PTL 6: Japanese Patent Application Laid-Open Publication No. H11-329512 
       
    
     Non-Patent Literature 
     
         
         NPL1: “Lithium-ion battery” by Hideaki Horie, published by Baifukan, August 2010 
       
    
     The PLT 1 and the PLT 2 set forth above determine degradation of the secondary battery of a degradation determination target by applying the AC voltage and/or the alternating current to the secondary battery of the degradation determination target. 
     However, a determination device that determines degradation by applying the AC voltage and/or the alternating current requires an AC power source and an impedance measuring device. Therefore, the determination device is large in size, causing inconvenience in handling. Especially when a general user using the secondary battery performs the degradation determination, the determination apparatus is desired to be simple, compact, and lightweight. 
     The methods described in the PLT 3 to PLT 6 and the NPL 1 determine degradation of the secondary battery by using parameters such as the internal resistance, the voltage, the charge amount, and the discharge amount, alone or in combination. 
     On the other hand, as a result of diligent research on determining degradation of the secondary battery by detecting capacitance of the secondary battery, the inventors have conceived a determination device that is simple, compact, and lightweight as well as being capable of determining degradation in an excellent manner. 
     As such, it could be helpful to provide a secondary battery tester that is simple, compact, and lightweight. 
     SUMMARY 
     A secondary battery tester includes: a voltage measuring unit with voltage terminals for measuring a voltage between a positive electrode and a negative electrode of a secondary battery without applying an AC voltage; a current measuring unit for measuring a current flowing between the positive electrode and the negative electrode of the secondary battery without applying an alternating current; a capacitance deriving means for estimating the capacitance of the secondary battery at a predetermined time from the start of measurement, using the voltage measured by the voltage measuring unit and the current measured by the current measuring unit; a storage unit for preliminarily storing temporal variation of capacitance of a normal secondary battery; and a determination means for comparing the capacitance at the predetermined time derived by the capacitance deriving means and the capacitance of the normal secondary battery at the predetermined time stored in the storage unit, and determining whether the secondary battery being measured is degraded. 
     Since this configuration determining degradation based on the capacitance does not require an AC power source or the like, a determination device may be simple, compact, and lightweight, allowing a general user using the secondary battery to easily evaluate the degradation determination. 
     The capacitance deriving means described above may derive the capacitance of the secondary battery by dividing the current measured by the current measuring unit by time differential of the voltage measured by the voltage measuring unit. 
     In this configuration, by dividing a current amount i(t) at a desired time by a time differential dv/dt of a voltage v(t), the capacitance at the desired time may be derived from C=i(t)·dt/dv. 
     Also, the capacitance deriving means described above may derive the capacitance of the secondary battery by using an amount ΔQ of charge flowing in a predetermined period of time Δt from the current measured by the current measuring unit and a changing amount Δv of the voltage in the predetermined period of time Δt measured by the voltage measuring unit and dividing the amount ΔQ of charge flowing in the predetermined period of time Δt by the changing amount Δv of the voltage in the predetermined time. 
     In this configuration, the amount ΔQ of charge flowing in the predetermined period of time Δt may be derived from a current I (a mean current in the period of time)·Δt, and capacitance C may be derived from C=ΔQ/Δv. 
     Also, the capacitance deriving means may derive the capacitance of the secondary battery by dividing a value obtained by carrying out time integration on the current flowing in a desired period of time Δt by the changing amount Δv of the voltage in the desired period of time Δt measured by the voltage measuring unit, or by dividing a value obtained by sampling the current amount i(t) in the desired period of time Δt once or a plurality of times, followed by summing the sampling values, dividing the summed values thus obtained by the number of sampling times and then multiplying the value thus obtained by the desired period of time Δt, by the changing amount Δv of the voltage in the desired period of time Δt measured by the voltage measuring unit. 
     In this configuration, as the amount ΔQ of charge flowing in the desired period of time Δt, a value obtained by the following method may be used; a time integral ∫i(t)dt of the current amount i(t) in the desired period of time Δt, or a value obtained by sampling the current amount i(t) in the desired period of time Δt once or a plurality of times, followed by summing the sampling values, dividing the summed values thus obtained by the number of sampling times and then multiplying the value thus obtained by the desired period of time Δt. The capacitance C may be derived from C=ΔQ/Δv. 
     The secondary battery tester may include a DC power source for applying a direct current to the secondary battery of a degradation determination target, and the capacitance deriving means, during constant current charging by the DC power source, or during charging with varied voltages, may derive the capacitance based on the changing amount Δv of the voltage in the predetermined period of time Δt measured by the voltage measuring unit and the charge amount ΔQ in the predetermined period of time obtained from the current measured by the current measuring unit. 
     The storage unit preliminarily stores a plurality of combinations of the voltages and the capacitances of normal secondary batteries. The determination means, based on the voltage measured by the voltage measuring unit and the capacitance derived by the capacitance deriving means, selects one combination of the voltage and the capacitance from the plurality of combinations stored in the storage unit and compares the measured voltage and the derived capacitance with the selected voltage and capacitance. 
     Also, the storage unit may preliminarily store a relation between the terminal voltage between a positive electrode and a negative electrode and the capacitance of a normal secondary battery, and the determination means may compare a relation between the voltage measured by the voltage measuring unit and the capacitance derived by the capacitance deriving means and a relation between the terminal voltage between the positive electrode and the negative electrode and the capacitance of the normal secondary battery stored in the storage unit. 
     Further, the secondary batteries of the degradation determination target may have a connection state in which a plurality of secondary batteries are connected in parallel, in series, or in parallel and in series. 
     Accordingly, the secondary battery tester that is simple, compact, and lightweight may be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, 
         FIG. 1  is an explanatory view illustrating a secondary battery tester and a connection configuration thereof according to the first embodiment; 
         FIG. 2  is a graph illustrating temporal variation of the derived capacitances; 
         FIG. 3  is an explanatory view illustrating a secondary battery tester and a connection configuration thereof according to the second embodiment; 
         FIG. 4  is a graph illustrating the temporal variation of the capacitance with a horizontal axis representing a time longer than that of the graph in  FIG. 2 ; and 
         FIG. 5  is an explanatory view illustrating a storage unit storing capacitances of lithium-ion batteries of a plurality of types. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a secondary battery tester will be described with reference to the accompanying drawings. 
     Note that although the embodiments herein use a lithium-ion battery as an example of the secondary battery, the secondary battery is not limited thereto. 
     First Embodiment of Secondary Battery Tester 
       FIG. 1  illustrates the secondary battery tester and a connection configuration thereof according to the first embodiment. 
     A secondary battery tester  30  according to the present embodiment is represented by a range surrounded by bold lines in  FIG. 1  and includes a voltage sensor  32 , a current sensor  34 , a storage unit  40 , a capacitance deriving means  36 , and a degradation determination means  38 . 
     The secondary battery tester  30  measures a terminal voltage and a current of a lithium-ion battery  10  of a degradation determination target by means of the voltage sensor  32  and the current sensor  34 , respectively, and calculates the capacitance of the lithium-ion battery  10  from these values thus obtained. The secondary battery tester  30 , based on the capacitance and the terminal voltage stored in the storage unit  40 , may determine whether the lithium-ion battery  10  is degraded. 
     Although the secondary battery tester  30  according to the present embodiment includes two voltage terminals connected to the voltage sensor  32  and two current terminals connected to the current sensor  34 , the secondary battery tester  30  may include three terminals sharing one voltage terminal and one current terminal. A positive electrode and a negative electrode of the voltage terminal may be specified optionally. 
     The lithium-ion battery  10  of the degradation determination target is connected to a load  20  or a power charging source  22 . As illustrated in  FIG. 1 , in particular, the lithium-ion battery  10  is connected to either the load  20  or the power charging source  22  via a changeover switch  24 . The secondary battery tester  30  according to the present embodiment, either in a charging state or in a discharging state switched by the changeover switch  24 , may determine whether the lithium-ion battery  10  is degraded based on the terminal voltage and the current thereof. 
     Also, the load  20  may be of any kind and either a device actually using the lithium-ion battery  10  or a dummy having internal impedance similar to the device. 
     That is, the degradation determination of the lithium-ion battery  10  only requires that the terminal voltage and the current of the lithium-ion battery  10  be measured by the voltage sensor  32  and the current sensor  34 , respectively. Regardless of a type of the load, and also regardless of whether in the charging state or the discharging state, whether the lithium-ion battery  10  is degraded is determined by the secondary battery tester  30 . 
     The secondary battery tester  30  according to the present embodiment will be described in detail. The secondary battery tester  30  includes the voltage sensor  32  for measuring a voltage between the positive electrode and the negative electrode (hereinafter, also simply referred to as a terminal voltage) of the lithium-ion battery  10  of the degradation determination target and the current sensor  34  for measuring current flowing to the lithium-ion battery  10 . The voltage sensor  32  and the current sensor  34  correspond to the voltage measuring unit and the current measuring unit, respectively, those being referred to in the appended claims. 
     The secondary battery tester  30  measures the terminal voltage and the current of the lithium-ion battery  10 , derives the capacitance based thereon, and then compares the capacitance with the known capacitance, thereby carrying out degradation determination. 
     To that end, the secondary battery tester  30  includes a capacitance deriving means  36  connected to the voltage sensor  32  and the current sensor  34 , and also includes the degradation determination means  38  for carrying out the degradation determination based on the derived capacitance. The secondary battery tester  30  further includes the storage unit  40  for preliminarily storing the normal capacitance to be used for the comparison with the derived capacitance. 
     In particular, the capacitance deriving means  36  and the degradation determination means  38  may be substantialized by a microprocessor and a memory storing a program to operate the microprocessor. Also, the storage unit  40  may be substantialized by this memory. 
     The storage unit  40  stores temporal variation of the capacitance and time characteristics of the terminal voltage of a non-degraded lithium-ion battery (here, the term “non-degraded” includes slight degradation but is used for convenience). The time characteristics of the terminal voltage of the lithium-ion battery  10  is necessary for the derivation of the capacitance, determination on whether the battery is in the discharging state or in the charging state at the time of measurement of the capacitance, the determination whether the lithium-ion battery  10  is degraded based on the capacitance, and determination on end of discharging or charging. 
     A graph illustrated in  FIG. 2 , for example, illustrates two capacitance, and one larger than the other overall indicates the temporal variation of the capacitance of the non-degraded lithium-ion battery.  FIG. 2  illustrates the temporal variation of the capacitance from the start of discharge to 600 seconds thereafter. 
     The capacitance of the non-degraded lithium-ion battery as described above needs to be preliminary measured for a predetermined continuous period of time and stored as the temporal variation in the storage unit  40 . Also, since the capacitance varies depending on types of the lithium-ion batteries, when a user uses lithium-ion batteries of a plurality of types as the degradation determination targets, it is necessary to preliminarily measure the capacitance of each of the lithium-ion batteries of the plurality of types for the predetermined continuous period of time and store the capacitance of each of the lithium-ion batteries as the temporal variation in the storage unit  40 . 
     Second Embodiment of a Secondary Battery Tester 
       FIG. 3  illustrates a secondary battery tester according to the second embodiment. 
     A secondary battery tester  31  according to the present embodiment is configured to be able to determine, even when the load and the charging power source are not connected to the lithium-ion battery  10  of the degradation determination target, whether the lithium-ion battery  10  of the degradation determination target is degraded. 
     As illustrated in  FIG. 3 , in particular, the secondary battery tester  31  according to the present embodiment is different from the secondary battery tester  30  according to the first embodiment, in terms of including a dummy load  50  and a switch  55 . 
     Also, the secondary battery tester  31  includes voltage terminals only as external connection terminals. Each of the positive electrode and the negative electrode of the lithium-ion battery  10  is connected to the voltage terminals of the secondary battery tester  31 . 
     Each of the voltage terminals connected to the positive electrode or the negative electrode of the lithium-ion battery  10  is connected to the voltage sensor  32  within the secondary battery tester  31 , and one of the voltage terminals branches off to connect to one terminal of the current sensor  34 , while the other branches off to connect to one end of the dummy load  50 . Also, the other end of the dummy load  50  is connected to one end of the switch  55 , and the other end of the switch  55  is connected to the other terminal of the current sensor  34 . When the switch  55  is turned on, the dummy load  50  and the lithium-ion battery  10  are connected in series, whereby the current flows to the dummy load  50 . 
     In the secondary battery tester  31 , the voltage sensor  32 , the current sensor  34 , the storage unit  40 , the capacitance deriving means  36 , and the degradation determination means  38  may be configured in the same manner as those of the first embodiment described above. 
     Note that a positive electrode and a negative electrode of the terminals of the secondary battery tester may be specified optionally. 
     (Capacitance Deriving Method 1) 
     There may be considered several methods of deriving the capacitance of the lithium-ion battery employed by the capacitance deriving means  36 . First, a deriving method 1 will be described. 
     When a charge amount stored upon application of a voltage v is represented by Q, the capacitance C may be derived from a fundamental equation of a steady state: C=Q (charge amount)/v (voltage). According to the present embodiment, with respect to the fundamental equation: C=Q/v, the capacitance C is derived from the time characteristics of the current and the voltage those being measured. 
     The current sensor  34  measures a current i (t) using the time t as a variable, and the current i(t) is input to the capacitance deriving means  36 . Also, a voltage v(t) measured by the voltage sensor  32  is also input to the capacitance deriving means  36 . The capacitance deriving means  36  obtains dv/dt by carrying out time differential of the voltage v(t). Further, the capacitance deriving means  36  divides the current i(t) by the time differential of the voltage dv/dt. The value thus obtained satisfies i(t)/(dv/dt)=(i(t)·dt)/dv, and (i(t)·dt) represents a change dQ in the charge amount by the current i(t) flowing in a time dt. Thereby, the capacitance in the time t in accordance with the differential corresponding to C=Q/v, i.e., i(t)/(dv/dt)=(i(t)·dt)/dv=dQ/dv=C may be derived. 
     (Capacitance Deriving Method 2) 
     Next, a capacitance deriving method 2 of the lithium-ion battery employed by the capacitance deriving means  36  will be described. In this method, with respect to the fundamental equation of the capacitance C=Q/v, the capacitance C=ΔQ/Δv is derived from a changing amount ΔQ of the charge amount and a changing amount Δv of the voltage in a predetermined period of time Δt. 
     The current sensor  34  inputs the current measured in the predetermined period of time Δt to the capacitance deriving means  36 . The capacitance deriving means  36  derives a mean value I of the applied currents. The capacitance deriving means  36 , assuming that the charge amount flowing in the predetermined period of time Δt is represented by ΔQ, derives the ΔQ from ΔQ=I·Δt. 
     Also, the voltage measured by the voltage sensor  32  is input to the capacitance deriving means  36 . The capacitance deriving means  36  derives the changing amount Δv of the voltage in the predetermined period of time Δt, the same as those used for the derivation of the charge amount ΔQ. With respect to the fundamental equation of the capacitance C=Q/v, from the changing amount ΔQ of the charge amount and the changing amount Δv of the voltage in a predetermined period of time Δt, the capacitance C may be derived from the equation C=ΔQ/Δv. That is, the capacitance deriving means  36  divides the changing amount ΔQ of the charge amount (i.e., I·Δt) by the changing amount Δv of the voltage, (ΔQ/Δv=I·Δt/Δv=C), and thus derives the capacitance C. 
     (Capacitance Deriving Method 3) 
     Next, a capacitance deriving method 3 of the lithium-ion battery employed by the capacitance deriving means  36  will be described. 
     The current sensor  34  inputs the measured current to the capacity deriving means  36  at any time. The capacitance deriving means  36  derives the value ∫i(t)dt obtained by carrying out time integration on the current i(t) flowing at the time t in a desired period of time Δt from the time t to (t+Δt), that is, the changing amount ΔQ of the charge amount flowing in the period of time Δt. Alternately, the capacitance deriving means  36  samples the current amount i(t) flowing in the desired period of time At from the time t to (t+Δt) once or a plurality of times, divides the sample value thus obtained or a sum of the sample values thus obtained by the number of sampling times, and then multiples the value thus obtained by the desired period of time Δt. That is, the capacitance deriving means  36  derives the changing amount ΔQ of the charge amount by carrying out the sampling. 
     The voltages sensor  32  inputs the measured voltage to the capacitance deriving means  36  at any time. 
     The capacitance deriving means  36  divides the changing amount ΔQ of the charge amount, which is either the value ∫i(t)dt obtained by carrying out the time integration on the current i(t) flowing in a desired period of time Δt, or the value obtained by sampling the current amount flowing in the desired period of time Δt, dividing a sum of the sample values thus obtained by the number of sampling times, and multiplying a value thus obtained by the desired period of time Δt, by the changing amount Δv of the voltage in the desired period of time Δt, and thus derives the capacitance C. 
     (Capacitance Deriving Method 4) 
     Next, a capacitance deriving method 4 of the lithium-ion battery employed by the capacitance deriving means  36  will be described. 
     This deriving method may derive the capacitance in a charging state in which the current flows to the lithium-ion battery  10  in a direction opposite to the current flowing in the discharging state. In particular, the changeover switch  24  in the circuit in  FIG. 1  is switched to connect to the power charging source  22  such that the power charging source  22  is connected to the lithium-ion battery  10  of the degradation determination target, and the capacitance is derived from the terminal voltage and the current when the lithium-ion battery  10  is being charged by the power charging source  22 . 
     Here, a case in which the power charging source  22  carries out a constant current charge to the lithium-ion battery  10  will be described. 
     The voltage sensor  32  measures a charging voltage that varies during charging, and inputs the charging voltage to the capacitance deriving means  36  at any time. 
     Also, the current sensor  34  measures a charging current during the charging and inputs the charging current to the capacitance deriving means  36 . Since there should be no change in the current during the constant current charge in principle, the current I being input has no temporal variation and is an approximately constant value. 
     The capacitance deriving means  36  derives the capacitance C based on the changing amount Δv of the voltage v(t) in the predetermined period of time Δt from the desired time t to (t+Δt) that has been input, the current I having being input, and the predetermined period of time Δt. That is, as described in the capacitance deriving method 2, the charge amount ΔQ flowing into the lithium-ion battery  10  in the predetermined period of time Δt is derived from I·Δt, and the capacitance C is derived from C=ΔQ/Δv. Therefore, the capacitance C may be derived from ΔQ/Δv=I·Δt/Δv. 
     Although the constant current charge is carried out for charging the lithium-ion battery  10  and the capacitance is derived from the time characteristics of the voltage and the current at that time in the above method, the charging current may be varied during the charging. 
     In employing such a charging method varying the charging current, the voltage sensor  32  measures the changing amount Δv of the voltage during the period of time from the desired time t to (t+Δt) and inputs the voltage thus measured to the capacitance deriving means  36  at any time. Also, the current sensor  34  measures the charging current during the charging and inputs the current thus measured to the capacitance deriving means  36 . 
     The capacitance deriving means  36 , based on the changing amount Δv of the voltage in the predetermined period of time Δt from the desired time t to (t+Δt) being input and the mean value I of the current in the time t+Δt from the desired time t being input as well as the predetermined period of time Δt, obtains the charge amount ΔQ flowing in the period of time t+Δt from the desired time t, and thus derives the capacitance C. That is, as described in the capacitance deriving method 2, the charge amount ΔQ is derived from I·Δt, and the capacitance C is derived from QΔ/Δv. Therefore, the capacitance C may be derived from ΔQ/Δv=I·Δt/Δv. 
     Note that, the mean value I of the current in the period of time Δt from the desired time t to (t+Δt) being input may be the current obtained by sampling the current i(t) flowing in the period of time Δt from the desired time t to (t+Δt) once or a plurality of times in the period of time Δt and dividing the sample value thus obtained or a sum of the sample values thus obtained by the number of sampling times. 
     (Degradation Determination Method 1) 
     The capacitance C of the lithium-ion battery in the desired time t derived by the capacitance deriving means  36  is compared, by the degradation determination means  38 , with the temporal variation of the capacitance of the non-degraded normal lithium-ion battery preliminarily stored in the storage unit  40  for degradation determination. 
     The following is a description about the degradation determination method, with reference to the graph illustrating the time characteristics of the capacitance in  FIG. 2 . In  FIG. 2 , a horizontal axis represents an elapsed time (unit: sec) from the start of discharge, and a vertical axis represents the capacitance (unit: F). 
     The capacitance larger than the other overall in  FIG. 2  indicates the capacitance of the non-degraded lithium-ion battery. The other capacitance smaller overall indicates capacitance of a degraded lithium-ion battery. 
     The degradation determination means  38  compares the capacitance C of the lithium-ion battery at a certain time elapsed from the start of discharge derived by the capacitance deriving means  36  with the capacitance of the non-degraded lithium-ion battery at the certain time. For example, it is assumed that the capacitance of the lithium-ion battery of the degradation determination target is 16000 F, while the capacitance of the non-degraded lithium-ion battery at 120 seconds from the start of discharge stored in the storage unit  40  is 30000 F. 
     The degradation determining means  38  compares those capacitance at the same elapsed time. The degradation determining means  38  determines that the lithium-ion battery is not degraded when the capacitance C derived is the same as the capacitance of the non-degraded lithium-ion battery stored in the storage unit  40 , and determines that the lithium-ion battery is degraded when the capacitance C derived is smaller than the capacitance stored in the storage unit  40 . Here, as the lithium-ion battery is degraded more, the capacitance C derived becomes further smaller than the capacitance of the non-degraded lithium-ion battery. 
       FIG. 4  illustrates the temporal variation of the capacitance when the elapsed time is longer than that in  FIG. 2 . 
     In  FIG. 4 , the capacitance of the non-degraded lithium-ion battery is measured from the start of discharge to 16000 seconds, and the temporal variation of the capacitance is stored in the storage unit  40 . Maximum capacitance of the non-degraded lithium-ion battery is approximately 300000 F when 4000 seconds elapsed. The capacitance of the non-degraded lithium-ion battery becomes small at 16000 seconds from the start of discharge and, simultaneously, the terminal voltage thereof also becomes small, and the discharge stops. On the other hand, the degraded lithium-ion battery has a peak capacity, which is as small as 90000 F, between 1000 sec to 4000 sec from the start of discharge. The capacitance of the degraded lithium-ion battery becomes small at 7000 seconds from the start of discharge and, simultaneously, the terminal voltage thereof also becomes small, and the discharge stops. 
     As can be seen in  FIG. 4 , when the capacitance of the lithium-ion battery of the degradation determination target is measured for a time longer than that in  FIG. 2 , the capacitance of the non-degraded lithium-ion battery is larger and maintained for a longer time, while the capacitance of the degraded lithium-ion battery is smaller and reduces in a short time. Therefore, by utilizing a large difference in the time characteristics of the capacitance between the non-degraded lithium-ion battery and the degraded lithium-ion battery, the degradation determining means  38  may carry out the degradation determination in an excellent manner. 
     (Degradation Determination Method 2) 
     The following describes an embodiment to determine the degradation of the lithium-ion batteries of a plurality of types. 
     As illustrated in  FIG. 5 , first, the storage unit  40  preliminarily stores each of the temporal variations of capacitance of the non-degraded lithium-ion batteries of the plurality of types and, in relation thereto, each of voltages between the positive electrode and the negative electrode of the lithium-ion batteries of the plurality of types. Although in  FIG. 5  the storage unit  40  stores the changing amount of the voltage between the terminals and the temporal variation of the capacitance of each lithium-ion batteries A and B of different types, the temporal variations of more than two lithium-ion batteries may be stored. 
     Subsequently, the voltage sensor  32  inputs the voltage measured between the terminals to the capacitance deriving means  36  and, simultaneously, the current sensor  34  inputs the measured current to the capacitance deriving means  36 . The degradation determination means  38 , based on the voltage between the terminals and the capacitance those input by the capacitance deriving means  36 , selects a lithium-ion battery having corresponding voltage and capacitance stored in the storage unit  40 . 
     Then, the degradation determination means  38  compares the temporal variation of the capacitance of the lithium-ion battery being selected and the capacitance C of the lithium-ion battery derived by the capacitance deriving means  36  for the degradation determination. In particular, in a manner similar to that described above, the degradation determination unit  38  compares the capacitance of the selected lithium-ion battery and the capacitance C derived at the same point of time. When the capacitance C derived is the same as the capacitance of the lithium-ion battery selected from the storage unit  40 , the degradation determination unit  38  determines that the lithium-ion battery of the degradation determination target is not degraded. When the capacitance C derived is smaller than the capacitance of the lithium-ion battery selected from the storage unit  40 , the degradation determination unit  38  determines that the lithium-ion battery of the degradation determination target is degraded. 
     (Degradation Determination Method 3) 
     Note that the degradation determination means  38  may carry out the degradation determination based on, instead of the temporal variation of the capacitance, a relation between a change in the voltage between the positive electrode and the negative electrode of the non-degraded lithium-ion battery and a change in the capacitance. 
     The storage unit  40  preliminarily stores the changing amount of the voltage between the positive terminal and the negative terminal of the non-degraded lithium ion battery  10  and, in relation thereto, the capacitance of the non-degraded lithium ion battery  10 . In general, the voltage between the terminals gradually becomes lower with the discharge time. Therefore, the present embodiment, based on the measured voltage between the terminals (that reduces over time), compares the capacitance corresponding to the voltage between the terminals. 
     In particular, the degradation determination means  38  compares the capacitance of the non-degraded lithium-ion battery and the capacitance C derived at the same voltage between the terminals. When the capacitance C derived is the same as the capacitance stored in the storage unit  40 , the degradation determination means  38  determines that the lithium-ion battery of the degradation determination target is not degraded. When the capacitance C derived is smaller than the capacitance stored in the storage unit  40 , the degradation determination means  38  determines that the lithium-ion battery of the degradation determination target is degraded. 
     Other Embodiments 
     In each of the embodiments described above, the degradation determination for one lithium-ion battery has been described. 
     However, the degradation determination target may be a plurality of lithium-ion batteries connected in parallel, in series, or in parallel and in series. In this case, the storage unit needs to preliminarily store temporal variations of capacitance of the plurality of lithium-ion batteries connected in parallel, in series, or in parallel and in series. 
     Note that the secondary battery targeted by the tester is not limited to the lithium-ion battery but may be a nickel-cadmium battery or a nickel-hydrogen battery. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  lithium-ion battery 
               20  load 
               22  power charging source 
               24  changeover switch 
               30 ,  31  secondary battery tester 
               32  voltage sensor 
               34  current sensor 
               36  capacitance deriving means 
               38  degradation determination means 
               40  storage unit 
               50  dummy load 
               55  switch