Patent Publication Number: US-2013249565-A1

Title: Power storage apparatus, mobile device, and electric-powered vehicle

Description:
TECHNICAL FIELD 
     The technology disclosed herein relates to a power storage apparatus, and a mobile device and an electric-powered vehicle operating after being supplied with power from the power storage apparatus; and particularly relates to an improvement in a power storage apparatus having a function of measuring impedance of a power storage element inside the power storage apparatus. 
     BACKGROUND ART 
     For measuring impedance of a power storage apparatus represented by a conventional power storage element or a conventional battery pack (assembled battery) obtained by assembling a plurality of power storage elements, a large-sized apparatus represented by products from Solartron Corp (Registered Trademark) are used. 
       FIG. 11  shows a method for measuring impedance of a power storage element, and shows a schematic diagram of electrochemical measurement using such large-sized system. In ( a ) of  FIG. 11 , “ 1 ” represents a power storage element, “ 10 ” represents a unit that includes a frequency-sweep oscillator  10 A and an impedance measuring equipment  10 B, and “ 20 ” represents a unit that includes an amplifier  20 A and a voltage-current monitor  20 B. Voltage and current terminals for 4-terminal measurement are mounted on the power storage element  1 . In addition, the amplifier  20 A is supplied with power from an external power supply such as an AC power supply  15 . As a measuring procedure, the frequency-sweep oscillator  10 A, while changing frequencies step-by-step at an interval of, for example, 10 points/decade, produces a single period of sine wave in each frequency (cf. ( b ) of  FIG. 11 ). After receiving this sine wave signal, the amplifier  20 A provides the power storage element with amplitude of a sine-wave minute current or minute voltage; and the voltage-current monitor  20 B monitors voltage or current of the power storage element  1 . Based on a response of the monitored voltage/current of the power storage element, the impedance measuring equipment  10 B measures impedance of the power storage element  1  (e.g., cf. Patent Literature 1). 
       FIG. 12  shows impedance characteristic diagrams of a power storage element. ( a ) of  FIG. 12  is a characteristic diagram in which the vertical axis represents absolute value of impedance Z and the horizontal axis represents frequency f, and ( b ) of  FIG. 12  is a characteristic diagram in which the vertical axis represents phase angle θ and the horizontal axis represents frequency f. ( c ) of  FIG. 12  shows a vector locus (so-called cole-cole plot) on a complex plane. It has been general practice to, based on the method for measuring impedance of the power storage element, produce the frequency characteristics of impedance shown in ( a ) and ( b ) in  FIG. 12  or produce a vector locus (cole-cole plot) on a complex plane shown in ( c ) of  FIG. 12 , and evaluate characteristics, deterioration, and reliability of an electrochemical element. 
     In addition, Patent Literature 2 discloses a method for measuring impedance of each power storage element by charging and discharging among power storage elements forming an assembled battery. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Specification of U.S. Pat. No. 4,743,855 
     [PTL 2] Japanese Laid-Open Patent Publication No 2007-294322 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the context of diversification of needs to utilize power storage apparatuses in electric-powered vehicles etc., there is a demand to increase capacity and voltage of assembled batteries. In order to deal with this demand, a larger number of power storage elements are being connected in series, and an efficient measurement method that can handle such assembled batteries that are connected in series is strongly demanded. 
     The technology disclosed herein is derived in view of such point, and an objective is to provide a power storage apparatus that has a function of measuring impedance and that can detect a deteriorated power storage element among power storage elements in the power storage apparatus in a short period of time, and a mobile device and an electric-powered vehicle. 
     SOLUTION TO THE PROBLEMS 
     One mode of the technology disclosed herein is a power storage apparatus having a plurality of power storage elements. The apparatus includes: first to fourth power storage elements connected in series; voltage measuring means and current measuring means for respectively measuring voltage and current of each of the first to fourth power storage elements; a first power storage unit including a first switch and a second switch connected in series at both ends of the first power storage element and the second power storage element, and a first inductor on which an inter-terminal voltage of either one of the first power storage element or the second power storage element selected through ON/OFF actions of the first switch and the second switch is applied; a second power storage unit, connected in series with the first power storage unit, including a third switch and a fourth switch connected in series at both ends of the third power storage element and the fourth power storage element, and a second inductor on which an inter-terminal voltage of either one of the third power storage element or the fourth power storage element selected through ON/OFF actions of the third switch and the fourth switch is applied; a fifth switch and a sixth switch connected in series at both ends of the first power storage unit and the second power storage unit; a third inductor on which an inter-terminal voltage of either one of the first power storage unit or the second power storage unit selected through ON/OFF actions of the fifth switch and the sixth switch is applied; and a control section configured to switch ON/OFF of the first to sixth switches at a predetermined timing. The control section switches the fifth switch and the sixth switch to sequentially form a closed circuit including the third inductor and either one of the first power storage unit and the second power storage unit and a closed circuit including the third inductor and the other storage unit, and measures and compares magnitudes of impedances of the first and second power storage units using the voltage measuring means and the current measuring means. When impedance of the first power storage unit is larger, the control section switches the first switch and the second switch to sequentially form a closed circuit including the first inductor and either one of the first power storage element and the second power storage element included in the first power storage unit and a closed circuit including the first inductor and the other storage element, and measures and compares magnitude of impedances of the first power storage element and the second power storage element using the voltage measuring means and the current measuring means to identify the power storage element having a larger impedance. Whereas, when impedance of the second power storage unit is larger, the control section switches the third switch and the fourth switch to sequentially form a closed circuit including the second inductor and either one of the third power storage element and the fourth power storage element included in the second power storage unit and a closed circuit including the first inductor and the other storage element, measures and compares magnitude of impedances of third power storage element and the fourth power storage element using the voltage measuring means and the current measuring means to identify the power storage element having a larger impedance. 
     Furthermore, in the power storage element described above, preferably, a notification is generated when impedance of the identified power storage element is larger than a predetermined reference value. 
     Another mode of the technology disclosed herein is also directed toward a power storage apparatus including not less than four power storage elements connected in series. Another mode of the technology disclosed herein is also directed toward a mobile device, an electric-powered vehicle, or the like including the power storage apparatus. 
     ADVANTAGEOUS EFFECTS OF THE INVENTION 
     With the technology disclosed herein, a deteriorated power storage element can be detected among power storage elements in a power storage apparatus in a short period of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a configuration of a power storage apparatus according to a first embodiment. 
         FIG. 2  shows state transition of the power storage apparatus according to the first embodiment. 
         FIG. 3  shows the manner how voltage and current are controlled when measuring impedance in the first embodiment. 
         FIG. 4  is a flowchart showing a flow in a normal mode in the first embodiment. 
         FIG. 5  is a flowchart showing a flow in a deterioration mode in the first embodiment. 
         FIG. 6  is a circuit diagram showing a configuration of a power storage apparatus according to a second embodiment. 
         FIG. 7  is a flowchart showing a flow in a normal mode in the second embodiment. 
         FIG. 8  is a flowchart showing a flow in a deterioration mode in the second embodiment. 
         FIG. 9  is a circuit diagram showing basic configuration of a power storage apparatus according to a third embodiment. 
         FIG. 10  is a circuit diagram showing a configuration of a conventional power storage apparatus. 
         FIG. 11  shows a method for measuring impedance of a power storage element of a conventional power storage apparatus. 
         FIG. 12  is an impedance characteristic diagram of a conventional power storage element. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With conventional technologies described above, there has been a problem of needing a long period of time for measuring impedance of power storage elements one by one. In order to measure impedance of power storage elements, since it is necessary to measure mainly in the range of kHz to mHz or μHz, considerable period of time is required for conducting the measurement for each of the power storage elements. During this period of time, use efficiency is reduced since the power storage apparatus practically cannot be used. In the following, with reference to the drawings, embodiments will be described for a power storage apparatus capable of detecting a deteriorated power storage element in a short period of time. 
     First Embodiment 
       FIG. 1  shows a configuration of a power storage apparatus according to a first embodiment. A power storage apparatus  100  according to the present embodiment has, in one example, four power storage elements. In the power storage apparatus  100 , a first power storage element B 1 , a second power storage element B 2 , a third power storage element B 3 , and a fourth power storage element B 4  are connected in series. At both ends of the first power storage element B 1  and the second power storage element B 2 , a first switch SW 1  and a second switch SW 2  are connected in series to form a first switch pair. In addition, at both ends of the third power storage element B 3  and the fourth power storage element B 4 , a third switch SW 3  and a fourth switch SW 4  are connected in series to form a second switch pair. 
     Furthermore, a first inductor L 1  is connected between a point connecting the first power storage element B 1  and the second power storage element B 2 , and a point connecting the first switch SW 1  and the second switch SW 2 . Furthermore, a second inductor L 2  is connected between a point connecting the third power storage element B 3  and the fourth power storage element B 4 , and a point connecting the third switch SW 3  and the fourth switch SW 4 . 
     The first power storage element B 1  and the second power storage element B 2  form a first power storage unit BU 1 . Furthermore, the third power storage element B 3  and the fourth power storage element B 4  form a second power storage unit BU 2 . 
     The first power storage unit BU 1  and the second power storage unit BU 2  are connected in series, and a fifth switch SW 5  and a sixth switch SW 6  are connected in series to form a third switch pair. Furthermore, a third inductor L 3  is connected between a point connecting the first power storage unit BU 1  and the second power storage unit BU 2 , and a point connecting the fifth switch SW 5  and the sixth switch SW 6 . 
     The fifth and sixth switches SW 5 , SW 6  and the third inductor L 3  form a first judgment circuit; and the first to fourth switches SW 1  to SW 4  and the first and second inductors L 1 , L 2  form a second judgment circuit. 
     Furthermore, a control section C 4  outputs control signals VG 1  to VG 6  respectively to the first to sixth switches (SW 1  to SW 6 ) to control ON/OFF switching of each of the switches. 
     The first to sixth switches SW 1  to SW 6  are switch elements including, for example, a MOSFET or a transistor. The control section C 4 , while switching these switches, detects current flowing through the first to fourth power storage elements B 1  to B 4  using ampere meters IB 1  to IB 4 , and detects voltage applied on each of those using volt meters VB 1  to VB 4 . The control section C 4  switches ON/OFF the first and second switches SW 1 , SW 2  such that alternating current or voltage, required for measuring impedance of one of the power storage elements between the first and second power storage elements B 1 , B 2 , is used for charging the other power storage element or is derived by discharging the other power storage element. Here, the control section C 4  switches ON/OFF the first and second switches SW 1 , SW 2  such that there is at least, in a single period of alternating current or voltage, a period of time in which current is supplied from the first power storage element B 1  to the first inductor L 1 , a period of time in which current is supplied from the first inductor L 1  to the second power storage element B 2 , a period of time in which current is supplied from the second power storage element B 2  to the first inductor L 1 , and a period of time in which current is supplied from the first inductor L 1  to the first power storage element B 1 . 
     In addition, the third and fourth switches SW 3 , SW 4  are switched ON/OFF such that alternating current or voltage, required for measuring impedance of one of the power storage element between the third and fourth power storage elements B 3 , B 4 , is used for charging the other power storage element or is derived by discharging the other power storage element. Here, the control section C 4  switches ON/OFF the third and fourth switches SW 3 , SW 4  such that there is, in a single period of alternating current or voltage, at least, a period of time in which current is supplied from the third power storage element B 3  to the second inductor L 2 , a period of time in which current is supplied from the second inductor L 2  to the fourth power storage element B 4 , a period of time in which current is supplied from the fourth power storage element B 4  to the second inductor L 2 , and a period of time in which current is supplied from the second inductor L 2  to the third power storage element B 3 . Since impedances of the first to fourth power storage elements B 1  to B 4  are measured using the second judgment circuit in such manner, this measurement is referred to as a second judgment. 
     Measuring impedance is conducted with a method described in, for example, Patent Literature 2. In ( a ) of  FIG. 2 , a case in which impedance of the fourth power storage element B 4  is measured is used as an example, and switch control by the control section C 4  and the state of current resulting therefrom is shown. It should be noted that components that do not contribute to the measurement here are shown with dashed lines. First, the third switch SW 3  is switched ON by the control section C 4 . At this moment, as shown with a thick line, current flows from the third power storage element B 3  to the second inductor L 2  ( a - 1 ). Next, the third switch SW 3  is switched OFF and the fourth switch SW 4  is switched ON. At this moment, current flows from the second inductor L 2  to the fourth power storage element B 4  ( a - 2 ). With these, discharging from the third power storage element B 3  to the fourth power storage element B 4  is conducted. After repeating this operation once or multiple times, SW 4  is switched ON so as to reverse direction of the current of L 2 , and current flows from the fourth power storage element B 4  to the second inductor L 2  ( a - 3 ). Next, when SW 4  is switched OFF, the third switch SW 3  is switched ON together. At this moment, current flows from the second inductor L 2  to the third power storage element B 3  ( a - 4 ). With these, charging from the fourth power storage element  4  to the third power storage element B 3  is conducted. With the above described cycle, alternating current or voltage can be applied to the fourth power storage element B 4 , and impedance of the fourth power storage element B 4  can be measured. In addition, in a similar manner, impedance of the third power storage element B 3  can be measured. Furthermore, impedances of the first and second power storage elements B 1 , B 2  can be measured through similar ON/OFF control of the first and second switches SW 1 , SW 2 . 
     It should be noted that, in practice, the third and fourth switches SW 3 , SW 4  are controlled ON/OFF through, for example, PWM modulation such that sine-wave shaped current or voltage is inputted to each of the power storage elements.  FIG. 3  shows an example of changing a current Ibatt and a voltage Vbatt inputted to the fourth power storage element B 4  into sine-wave shapes through PWM modulation. As shown in ( a ) of  FIG. 3 , when ON/OFF control of the third and fourth switches SW 3 , SW 4  is conducted, a pulse-expressed sine wave of the voltage Vbatt inputted to the fourth power storage element B 4  is obtained as shown in ( b ) of  FIG. 3 . As a result, as shown in ( c ) of  FIG. 3 , the current Ibatt corresponding to the voltage Vbatt is supplied to the fourth power storage element B 4 . 
     Furthermore, the control section C 4  can also measure impedance at the level of each of the power storage units. The control section C 4  switches ON/OFF the fifth and sixth switches SW 5 , SW 6  such that alternating current or voltage, required for measuring impedance of one of the power storage units of the first and second power storage units BU 1 , BU 2 , is used for charging the other power storage unit or is derived by discharging the other power storage unit. Here, the control section C 4  switches ON/OFF the fifth and sixth switches SW 5 , SW 6  such that there is at least, in a single period of alternating current or voltage, a period of time in which current is supplied from the first power storage unit BU 1  to the third inductor L 3 , a period of time in which current is supplied from the third inductor L 3  to the second power storage unit BU 2 , a period of time in which current is supplied from the second power storage unit BU 2  to the third inductor L 3 , and a period of time in which current is supplied from the third inductor L 3  to the first power storage unit BU 1 . Since impedances of the first and second power storage units BU 1 , BU 2  are measured using the first judgment circuit in such manner, this measurement is referred to as a first judgment. 
     In ( b ) of  FIG. 2 , a case in which impedance of the second power storage unit BU 2  is measured is used as an example, and switch control by the control section C 4  and the state of current resulting therefrom are shown. First, only the fifth switch SW 5  is switched ON by the control section C 4 . At this moment, as shown with a thick line, current flows from the first power storage unit BU 1  to the third inductor L 3  ( b - 1 ). Next, the fifth switch SW 5  is switched OFF and the sixth switch SW 6  is switched ON. At this moment, current flows from the third inductor L 3  to the second power storage unit BU 2  ( b - 2 ). With these, discharging from the second power storage unit BU 2  to the first power storage unit BU 1  is conducted. After repeating this operation once or multiple times, SW 6  is switched ON so as to reverse direction of the current of L 3 , and current flows from the second power storage unit BU 2  to the third inductor L 3  ( b - 3 ). Next, when SW 6  is switched OFF, the fifth switch SW 5  is switched ON together. At this moment, current flows from the third inductor L 3  to the first power storage unit BU 1  ( b - 4 ). With these, charging from the second power storage unit BU 2  to the first power storage unit BU 1  is conducted. With the above described cycle, alternating current or voltage can be applied to the first power storage unit BU 1 , and impedance of the first power storage unit BU 1  can be measured. In addition, in a similar manner, impedance of the first power storage unit BU 1  can be measured. It should be noted that, in practice, the fifth and sixth switches SW 5 , SW 6  are controlled ON/OFF similarly to the third and fourth switches SW 3 , SW 4  in  FIG. 3  through, for example, PWM modulation such that sine-wave shaped current or voltage is inputted to each of the power storage units. 
     Furthermore, for such measurement of impedance, a method described in, for example, Japanese Patent No. 4138502 may be used. 
     In the following, a flow for measuring impedance by the power storage apparatus  100  will be described.  FIG. 4  shows a flowchart of a basic operation of an impedance-measurement process by the power storage apparatus  100 . The basic operation consists of a normal mode operation. 
     (1) After receiving an instruction to start measuring impedance in the normal mode, the control section C 4  repeats a cyclic operation of charging and discharging the first power storage unit BU 1  with the above described method, measures current flowing through the first power storage unit BU 1  using the ampere meter IB 1  or IB 2 , and measures electrical potential difference of both ends of the first power storage unit BU 1  using the volt meters VB 1  and VB 2 , to measure an impedance Z 5  of the first power storage unit BU 1  in a charge-and-discharge cycle. Similarly, the control section C 4  repeats a cyclic operation of charging and discharging the second power storage unit BU 2 , measures current flowing through the second power storage unit BU 2  using the ampere meter IB 3  or IB 4 , and measures electrical potential difference of both ends of the second power storage unit BU 2  using the volt meters VB 3  and VB 4 , to measure an impedance Z 6  of the second power storage unit BU 2  in a charge-and-discharge cycle (step S 101 ). 
     (2) The control section C 4  judges whether or not the impedance Z 5  of the first power storage unit BU 1  is larger than the impedance Z 6  of the second power storage unit BU 2  (step S 102 ). 
     (3) As a result, the control section C 4  selects a power storage unit having a larger impedance, and measures impedances of every power storage elements included in the power storage unit. In the following, description will be provided supposing that Z 5  is smaller than Z 6  (NO at step S  105 ) and that the second power storage unit BU 2  is selected. 
     (4) The control section C 4  measures impedances of the power storage elements included in the selected power storage unit using the above described method (steps S 103 , S 104 ). Here, step S 104  is executed. The control section C 4  repeats a cyclic operation of charging and discharging the third power storage element B 3 , measures current flowing through the third power storage element B 3  using the ampere meter IB 3 , and measures electrical potential difference of both ends of the third power storage element B 3  using the volt meter VB 3 , to measure an impedance Z 3  of the third power storage element B 3  in a charge-and-discharge cycle. Furthermore, similarly, the control section C 4  repeats a cyclic operation of charging and discharging the fourth power storage element B 4 , measures current flowing through the fourth power storage element B 4  using the ampere meter IB 4 , and measures electrical potential difference of both ends of the fourth power storage element B 4  using the volt meter VB 4 , to measure an impedance Z 4  of the fourth power storage element B 4  in a charge-and-discharge cycle (step S 104 ). 
     (5) The control section C 4  judges the magnitude of the measured impedances of each of the power storage elements (step S 105 , S 106 ). Here, step S 106  is executed, and it is judged whether or not the impedance Z 3  of the third power storage element B 3  is larger than the impedance Z 4  of the fourth power storage element B 4  (step S 106 ). 
     (6) As a result, the control section C 4  selects a power storage element Bk (k=1, 2, . . . , 4) having a large impedance. In the following, description will be provided supposing that Z 3  is smaller than Z 4  (NO at step S 106 ) and that the fourth power storage element B 4  is selected. The control section C 4  compares the impedance of the selected power storage element Bk and a first reference value Zak (k=1, 2, . . . , 4) that corresponds to the power storage element Bk and is pre-stored or calculated each time from parameters such as temperature and SOC (charging state) (step S 107  to S 110 ). Here, step S 110  is executed, and Z 4  and Za 4  are compared. As a result, when the impedance is larger than the first reference value (in this case, when Z 4  is larger than Za 4  (NO at step S 110 )), it is judged that the power storage element (the fourth power storage element B 4 ) has deteriorated, and the judgment is displayed on a display section (not shown) or transmitted to an external apparatus (step S 111 ). Then, the control section C 4  records and saves deterioration information including, for example, an identifier (B 4 ), the impedance (Z 4 ), and the like of the power storage element as an execution result (step S 112 ). Furthermore, when the impedance is smaller than the first reference value (in this case, when Z 4  is smaller than Za 4  (YES at step S 110 )), step S 111  is not executed and the flow shifts to step S 112 , and information or the like indicating that there is no deterioration in each of the power storage elements is recorded and saved as an execution result. 
     Since replacement of a power storage element based on this deterioration information is prompted, maintainability of the power storage apparatus increases. 
     (7) The control section C 4  further compares an impedance Zk of the power storage element Bk judged to have the highest impedance among those that have been measured (here, the impedance Z 4  of the fourth power storage element B 4 ) and a second reference value Zbk that corresponds to the power storage element Bk and is pre-stored in the control section C 4  or calculated each time from parameters such as temperature and SOC (here, compares Z 4  and Zb 4 ) (step S 113 ). When the impedance Zk is larger than the second reference value Zbk (in this case, when Z 4  is larger than Zb 4 ) (NO at step S 113 ), it is judged that the power storage element (in this case, the fourth power storage element B 4 ) is malfunctioning, and the judgment is displayed on a display section or transmitted to an external apparatus (step S 114 ). Furthermore, when the impedance is smaller than the second reference value (in this case, when Z 4  is smaller than Zb 4 ) (YES at step S 113 ), step S 114  is not executed. 
     Here, the first reference value and the second reference value can be suitably determined. For example, they may be determined respectively as an impedance value when slight performance deterioration has occurred in a power storage element, and an impedance value when serious performance deterioration has occurred in a power storage element. 
     By doing so, a user or an administrator of the power storage apparatus  100  who have been notified to replace the fourth power storage element B 4  as a warning can recognized that a replacement is necessary. 
     (8) Then, the control section C 4  returns the flow to step S 101  again at an appropriate time such as an unused time slot learnt in advance as a time slot during which the power storage apparatus  100  is not charged or discharged, or after elapsing of a period of time determined in advance. With this, the function as a power storage apparatus can be exerted until then. 
     By repeating the above described steps, impedance of a deteriorated power storage element, which becomes a bottleneck for the performance of the power storage apparatus  100 , can be accurately obtained in a short period of time, and the user or administrator can be provided with information required for replacement. In addition, measurement can be conducted further accurately by following the method described in Japanese Patent No. 4138502. 
     Here, for simplicity, although a power storage apparatus having four in-series connections has been used as an example, 
     in a conventional power storage apparatus having a large number of in-series connections, if the method of forming switches simply at both ends of a power storage element is adopted, the number of switches and the number of wiring and circuits for controlling those become enormous. For example, when there are eight in-series connections, as can be figured out from a comparison between  FIG. 6  and  FIG. 10 , the number of switches can be reduced from  22  to  14 . With this, ON/OFF signals for the switches can be reduced, and components and cost thereof can also be reduced. 
     The switch formed inside the power storage apparatus is envisioned to be a MOSFET or a transistor, and in a case with a MOSFET, it is necessary to have a voltage source having a voltage of about several volts to 10 volts with respect to a source potential, and apply a gate potential in accordance with a control signal to conduct the ON/OFF control. In a case with a transistor, it is necessary to obtain a base current source corresponding to collector current, in order to apply a voltage equal to or higher than 0.7 volts with respect to an emitter potential for supplying current from the base to an emitter. For this, it is necessary to prepare charge pump circuits and isolated DC/DC converters by a quantity corresponding to each electric potential. 
     On the other hand, since each of the switches fixes a respective power storage element to a source potential or an emitter potential, it is necessary to insulate a signal VG 1  or the like from the control section C 4  or supply a base current or a gate voltage using a level shift circuit having necessary voltage withstandability. For that, signal transmission circuit components utilizing magnetic coupling or optical isolation represented by photo couplers, photo MOS, pulse transformers, and i-couplers are also needed by a quantity of the switches. 
     Since the cost of such components is reflected in the cost of the power storage apparatus, it is overwhelmingly advantageous to have a small number of switches for providing the apparatus cheaply. 
     Next, an applicational operation of the impedance-measurement process of the power storage apparatus  100  will be described. With reference to  FIG. 5 , processes in the applicational operation will be described. In the present applicational operation, the processes in the normal mode are conducted similarly to the basic operation described above, and then, in accordance with a processing result, processes in a deterioration mode are further executed. 
     (1) The control section C 4  first executes steps S 201  to S 214 . When these steps are executed for the first time, they are similar to steps S 101  to S 114  in the normal mode in the basic operation. However, when step S 211  is executed, the results are saved (step S 212 ) and then the flow shifts to processes in a deterioration mode. 
     (2) After shifting to the deterioration mode, the control section C 4  repeats execution of steps S 201  to S 214 . In this case, in steps S 201  to step S 210 , with regard to an impedance Zm of a power storage unit Bm including the power storage element Bk (k=1, 2, . . . , 4) that has already been judged to have deterioration occurred therein, the processes are conducted using, as Zm, a value obtained by subtracting (Zk−Zrefk) from an actually measured Zm. Here, Zrefk is a predetermined reference value of the impedance of the power storage element Bk, and is determined, for example, by the value of the impedance of the power storage element Bk when there is no deterioration. Therefore, (Zk−Zrek) can be considered as an amount of increase (deterioration amount) of the impedance of the power storage element Bk. Thus, Zm−(Zk−Zrefk) which is used in the processes instead of Zm is an estimated value of the impedance of the power storage unit Bm when it is assumed that there is no deterioration in the power storage element Bk. 
     For example, when it is judged that the fourth power storage element B 4  is deteriorated, a measured value Z 6  of the impedance of the second power storage unit BU 2  (B 6 ) including the fourth power storage element B 4  contains a deterioration amount of the impedance of the fourth power storage apparatus B 4 . However, the value of Z 6 −(Z 4 −Zref 4 ) becomes an estimated value of the impedance of the second power storage unit BU 2  (B 6 ) when there is no deterioration of the fourth power storage unit B 4 , since the deterioration amount is subtracted from Z 6 . 
     Furthermore, in steps S 201  to step S 210 , with regard to the impedance Zk of the power storage element Bk that has already been judged to have deterioration occurred therein, the control section C 4  conducts the processes using Zrefk as Zk. Thus, these processes are conducted as there is no deterioration in the power storage element Bk. Therefore, at step S 203  or step S 204 , with regard to the power storage element Bk, it is not necessary to measure its impedance Zk. For example, when it is judged that the fourth power storage element B 4  is deteriorated, its impedance Z 4  does not have to be measured at step S 204 . 
     By substituting the value of the impedance for the power storage element Bk and the power storage unit Bm including thereof, the power storage element Bk is excluded as a subject for a deterioration judgment. By repeating this, judgment of deterioration can be made for other power storage elements whose performances are deteriorated the second most or less. 
     It should be noted that, at step S 213 , the control section C 4  does not conduct such substitution of impedance values, and conducts the judgment based on the impedance value Zk that has been actually measured most recently. Therefore, when the impedance Zk of the power storage element Bk is not measured at step S 203  or step S 204 , the impedance Zk is preferably measured, for example, between step S 212  and step S 213 . 
     It should be noted that, when comparison of magnitude of impedances in the basic operation and the applicational operation resulted in equal, the flow may be advanced to either YES or NO. In either case, it is possible to give a deterioration judgment or a malfunction judgment to one among multiple power storage elements that have been deteriorated to the same degree. In addition, by repeatedly executing the deterioration mode in the applicational operation, deterioration judgment can be sequentially given to all of the multiple power storage elements whose performances have deteriorated. Execution of the deterioration mode is preferably repeated in the above described unused time slot, or after elapsing of a period of time determined in advance. 
     Even when there are multiple deterioration judgment, if the procedure of the deterioration judgment is changed to that in  FIG. 5 , until a malfunction judgment is been made, the user can use the power storage apparatus within the range of the performance of the power storage element while understanding that there are multiple deteriorations, until it is judged to have malfunctioned. 
     In this case, although there is one additional step at the end for measuring the power storage element that has first judged to be deteriorated, and the time required for measurements increases accordingly, when compared to inspecting all, the number of inspections can be reduced as the number of in-series connected power storage elements increases, and the advantageous effect of the present embodiment becomes significant. 
     With this, for example, assuming a case where deteriorations of power storage elements have progressed almost equivalently, the user or administrator can continue using the power storage apparatus within a range of its performance while understanding the state of the deteriorated power storage element until a malfunctioning power storage element that becomes a bottleneck of its performance emerges. 
     This information is extremely important in use applications in which a sudden halt of operation due to battery malfunction becomes a problem. Such applications are cases in which maintenance is extremely troublesome unless a certain degree of deterioration is accepted while it is too late to deal with that once a critical defect emerges, and examples of such cases include, needless to say, movable bodies such as vehicles, backup power storage apparatuses for communication stations set at mountainous areas and islandy areas, and power storage apparatuses for natural energy sources such as solar batteries. 
     The advantageous effect of the present embodiment is summarized as follows. 
     (1) By being able to accurately measure and understand judgment of deteriorating cell that becomes a bottleneck for a power storage apparatus in a short period of time, it is possible to conduct a detailed battery test and judge deteriorating cells. In addition, a prediction of effective battery life can be achieved. 
     (2) It is possible to have fine portability and simplicity that had been conventionally available, retain a characteristic of being able to measure impedance of a power storage element using unused time such as nighttime, dramatically reduce the number of switches necessary for measuring impedance associated with the increase in the number of in-series connections in the power storage apparatus, and, as a result, largely reduce the circuit scale of drive circuits. In addition, the cost will also become low. 
     (3) When a deteriorating power storage element having increased impedance emerges in a power storage apparatus, it is possible to show the deteriorating power storage element to the user or administrator, or transmit the information to prompt a replacement, and also continue its usage as a power storage apparatus. 
     (4) When it is determined that the deterioration state has advanced further, it is possible to give a warning to replace the one determined as malfunctioning, restrict operation as a power storage apparatus for the purpose of ensuring safety, prepare a replacement power storage element quickly, and improve maintainability of the power storage apparatus since the target to be replaced can be figured out. 
     (5) By conducting the replacement at a power storage element level, unnecessary cost can be reduced when compared to replacing the whole power storage apparatus. 
     Second Embodiment 
       FIG. 6  shows a configuration of a power storage apparatus according to a second embodiment. A power storage apparatus  200  according to the present embodiment has, in one example, eight power storage elements. In the power storage apparatus  200 , first to eighth power storage elements B 1  to B 8  are connected in series. At both ends of the first power storage element B 1  and the second power storage element B 2 , the first switch SW 1  and the second switch SW 2  are connected in series to form the first switch pair. In addition, at both ends of the third power storage element B 3  and the fourth power storage element B 4 , the third switch SW 3  and the fourth switch SW 4  are connected in series to form the second switch pair. At both ends of a fifth power storage element B 5  and a sixth power storage element B 6 , the fifth switch SW 5  and the sixth switch SW 6  are connected in series to form the third switch pair. In addition, at both ends of a seventh power storage element B 7  and an eighth power storage element B 8 , a seventh switch SW 7  and an eighth switch SW 8  are connected in series to form the second switch pair. 
     Furthermore, the first inductor L 1  is connected between a point connecting the first power storage element B 1  and the second power storage element B 2 , and a point connecting the first switch SW 1  and the second switch SW 2 . Further, the second inductor L 2  is connected between a point connecting the third power storage element B 3  and the fourth power storage element B 4 , and a point connecting the third switch SW 3  and the fourth switch SW 4 . Still further, the third inductor L 3  is connected between a point connecting the fifth power storage element B 5  and the sixth power storage element B 6 , and a point connecting the fifth switch SW 5  and the sixth switch SW 6 . In addition, a fourth inductor L 4  is connected between a point connecting the seventh power storage element B 7  and the eighth power storage element B 8 , and a point connecting the seventh switch SW 7  and the eighth switch SW 8 . 
     The first power storage element B 1  and the second power storage element B 2  form the first power storage unit BU 1 . Furthermore, the third power storage element B 3  and the fourth power storage element B 4  form the second power storage unit BU 2 . The fifth power storage element B 5  and the sixth power storage element B 6  form a third power storage unit BU 3 . Furthermore, the seventh power storage element B 7  and the eighth power storage element B 8  form a second power storage unit BU 4 . 
     The first power storage unit BU 1  and the second power storage unit BU 2  are connected in series, and a ninth switch SW 9  and a tenth switch SW 10  are connected in series to form a fifth switch pair. Furthermore, a fifth inductor L 5  is connected between a point connecting the first power storage unit BU 1  and the second power storage unit BU 2 , and a point connecting the ninth switch SW 9  and the tenth switch SW 10 . The third power storage unit BU 3  and the fourth power storage unit BU 4  are connected in series, and an eleventh switch SW 11  and a twelfth switch SW 12  are connected in series to form a sixth switch pair. Furthermore, a sixth inductor L 6  is connected between a point connecting the third power storage unit BU 3  and the fourth power storage unit BU 4 , and a point connecting the eleventh switch SW 11  and the twelfth switch SW 12 . 
     The first power storage unit BU 1  and the second power storage unit BU 2  form a fifth power storage unit BUS. Furthermore, the third power storage unit BU 3  and the fourth power storage unit BU 4  form a sixth power storage unit BU 6 . It should be noted that, for convenience, the fifth and sixth power storage units BU 5 , BU 6  are each regarded as one power storage element, and are referred also with reference characters B 13  and B 14 . 
     The fifth power storage unit BU 5  and the sixth power storage unit BU 6  are connected in series, and a thirteenth switch SW 13  and a fourteenth switch SW 14  are connected in series to form a seventh switch pair. Furthermore, a seventh inductor L 7  is connected between a point connecting the fifth power storage unit BU 5  and the sixth power storage unit BU 6 , and a point connecting the thirteenth switch SW 13  and the fourteenth switch SW 14 . 
     The thirteenth and fourteenth switches SW 13 , SW 14  and the seventh inductor L 7  form the first judgment circuit; the ninth to twelfth switches SW 9  to SW 12  and the fifth and sixth inductors L 5 , L 6  form the second judgment circuit; and the first to eighth switches SW 1  to SW 8  and the first to fourth inductors L 1  to L 4  form a third judgment circuit. 
     Furthermore, a control section C 8  outputs control signals VG 1  to VG 14  respectively to the first to fourteenth switches (SW 1  to SW 14 ) to control ON/OFF switching of each of the switches. 
     The first to fourteenth switches SW 1  to SW 14  are switch elements including, for example, a MOSFET or a transistor. The control section C 8 , while switching these switches, detects current flowing through the first to eighth power storage elements B 1  to B 8  using ampere meters IB 1  to IB 8 , and detects voltage applied on each of those using volt meters VB 1  to VB 8 . 
     Similarly to the control section C 4  of the first embodiment, the control section C 8  can measure impedances of every power storage elements and every power storage units. For example, with regard to the fifth power storage unit and the sixth power storage unit, impedances thereof can be measured by controlling ON/OFF of the thirteenth and fourteenth switches SW 13 , SW 14 . Since impedances of the fifth and sixth power storage units BU 5 , BU 6  are measured using the first judgment circuit in such manner, this measurement is referred to as a first judgment. Similarly, since impedances of the first to fourth power storage unit BU 1  to BU 4  are measured using the second judgment circuit in such manner, this measurement is referred to as a second judgment. Furthermore, since impedances of the first to eighth power storage elements B 1  to B 8  are measured using the third judgment circuit in such manner, this measurement is referred to as a third judgment. 
     In the following, a flow for measuring impedance by the power storage apparatus  200  will be described.  FIG. 7  shows a flowchart of a basic operation of an impedance-measurement process by the power storage apparatus  200 . The basic operation consists of a normal mode operation. It should be noted that, when compared to the flow of processes in the first embodiment, the flow of processes here is different only regarding a point that there is one more process for measuring and judging impedance. In order to simplify the description, a case in which the fourth power storage element B 4  is deteriorated the most and its impedance Z 4  is the largest is described as an example. Furthermore, reference characters, except for one portion thereof, showing steps in the drawing are omitted. 
     (1) After receiving an instruction to start measuring impedance in the normal mode, the control section C 8  measures an impedance Z 13  of the fifth power storage unit BUS and an impedance Z 14  of the sixth power storage unit BU 6  (step S 301 ). 
     (2) The control section C 8  judges whether or not the impedance Z 13  of the fifth power storage unit BUS is larger than impedance Z 14  of the sixth power storage unit BU 6  (step S 302 ). 
     (3) As a result, the control section C 8  selects the fifth power storage unit BUS (B 13 ) having a larger impedance (YES at step S 302 ). 
     (4) The control section C 8  measures impedances Z 9 , Z 10  of the first and second power storage units BU 1 , BU 2  included in the selected fifth power storage unit BUS (step S 303 ). 
     (5) The control section C 8  judges whether or not the impedance Z 9  of the first power storage unit BU 1  is larger than impedance Z 10  of the second power storage unit BU 2  (step S 304 ). 
     (6) As a result, the control section C 8  selects the second power storage unit BU 2  (B 10 ) having a larger impedance (NO at step S 304 ). 
     (7) The control section C 8  measures impedances Z 3 , Z 4  of the third and fourth power storage elements B 3 , B 4  included in the selected second power storage unit BU 2  (step S 305 ). 
     (8) The control section C 8  judges whether or not the impedance Z 3  of the third power storage element B 3  is larger than the impedance Z 4  of the fourth power storage element B 4  (step S 306 ). 
     (9) As a result, the control section C 8  selects the fourth power storage element B 4  having a larger impedance (NO at step S 306 ). 
     The control section C 8  compares the impedance of the selected fourth power storage element B 4  and a first reference value Za 4  that corresponds to the fourth power storage element B 4  and is pre-stored or calculated each time from parameters such as temperature and SOC (charging state) (step S 307 ). As a result, when the impedance Z 4  is larger than the first reference value Za 4  (NO at step S 307 ), it is judged that the fourth power storage element B 4  has deteriorated, and the judgment is displayed on a display section (not shown) or is transmitted to an external apparatus (step S 308 ). Then, the control section C 8  records and saves deterioration information including, for example, an identifier, the impedance (Z 4 ), and the like of the fourth power storage element B 4  as an execution result (step S 309 ). Furthermore, when the impedance Z 4  is smaller than the first reference value Za 4  (YES at step S 307 ), step S 308  is not executed and the flow shifts to step S 309 , and information or the like indicating that, for example, there is no deterioration in each of the power storage element is recorded and saved as an execution result. 
     (10) The control section C 8  compares the impedance Z 4  of the fourth power storage element B 4  and a second reference value Zb 4  that corresponds to the power storage element B 4  and is pre-stored by the control section C 8  or calculated each time from parameters such as temperature and SOC (step S 310 ). When the impedance Z 4  is larger than the second reference value Zb 4  (NO at step S 310 ), it is judged that the fourth power storage element B 4  is malfunctioning, and the judgment is displayed on a display section or is transmitted to an external apparatus (step S 311 ). Furthermore, when the impedance Z 4  is smaller than the second reference value Zb 4  (YES at step S 310 ), step S 311  is not executed. 
     Here, the first reference value and the second reference value can be suitably determined. For example, they may be determined respectively as an impedance value when slight performance deterioration has occurred in a power storage element, and an impedance value when serious performance deterioration has occurred in a power storage element. 
     (11) Then, the control section C 8  returns the flow to step S 301  again at an appropriate time such as an unused time slot learnt in advance as a time slot during which the power storage apparatus  200  is not charged or discharged, or after elapsing of a period of time determined in advance. With this, the function as a power storage apparatus can be exerted until then. 
     By repeating the above described steps, impedance of a deteriorated power storage element, which becomes a bottleneck for the performance of the power storage apparatus  200 , can be accurately obtained in a short period of time, and the user or administrator can be provided with information required for replacement. In addition, measurement can be conducted further accurately by following the method described in Japanese Patent No. 4138502. 
     Next, an applicational operation of the impedance-measurement process of the power storage apparatus  200  will be described. With reference to  FIG. 8 , processes in the applicational operation will be described. In the present applicational operation, the processes in the normal mode are conducted similarly to the basic operation described above, and then, in accordance with a processing result, processes in a deterioration mode are further executed. Also in the following, a case in which the fourth power storage element B 4  is deteriorated the most and its impedance Z 4  is the largest is described as an example. Furthermore, reference characters, except for one portion thereof, showing steps in the drawing are omitted. 
     (1) The control section C 8  first executes steps S 401  to S 411 . When these steps are executed for the first time, they are similar to steps S 301  to S 314  in the normal mode in the basic operation. However, when step S 408  is executed, the results are saved (step S 409 ) and then the flow shifts to processes in a deterioration mode. 
     (2) After shifting to the deterioration mode, the control section C 8  repeats execution of steps S 401  to S 411 . In this case, with regard to the impedance Zm of the power storage unit Bm (k=1, 2, . . . , 6) including the power storage element Bk (k=1, 2, . . . , 8) that has already been judged to have deterioration occurred therein, the processes in steps S 401  to S 411  are conducted using, as Zm, a value obtained by subtracting (Zk−Zrefk) from an actually measured Zm. Here, Zrefk is a predetermined reference value of the impedance of the power storage element Bk, and is determined, for example, by the value of the impedance of the power storage element Bk when there is no deterioration. Therefore, (Zk−Zrek) can be considered as an amount of increase (deterioration amount) of the impedance of the power storage element Bk. Thus, Zm−(Zk−Zrefk) which is used in the processes instead of Zm is an estimated value of the impedance of the power storage unit Bm when it is assumed that there is no deterioration in the power storage element Bk. 
     For example, when it is judged that the fourth power storage element B 4  is deteriorated, a measured value Z 10  of the impedance of the second power storage unit BU 2  (B 10 ) including the fourth power storage element B 4  contains a deterioration amount of the impedance of the fourth power storage apparatus B 10 . However, the value of Z 10 −(Z 4 −Zref 4 ) becomes an estimated value of the impedance Z 10  of the second power storage unit BU 2  (B 10 ) when there is no deterioration of the fourth power storage unit B 4 , since the deterioration amount is subtracted from Z 10 . 
     Furthermore, with regard to the impedance Zk of the power storage element Bk that has already been judged to have deterioration occurred therein, the control section C 8  conducts the processes using Zrefk as Zk. Thus, these processes are conducted as there is no deterioration in the power storage element Bk. Therefore, with regard to the power storage element Bk, it is not necessary to measure its impedance Zk. For example, when it is judged that the fourth power storage element B 4  is deteriorated, its impedance Z 4  does not have to be measured at step S 405 . 
     By substituting the value of the impedance for the power storage element Bk and the power storage unit Bm including thereof, the power storage element Bk is excluded as a subject for a deterioration judgment. By repeating this, judgment of deterioration can be made for other power storage elements whose performances are deteriorated the second most or less. 
     It should be noted that, at step S 410 , the control section C 8  does not conduct such substitution of impedance values, and conducts the judgment based on the impedance value Zk that has been actually measured most recently. Therefore, for example, when the impedance Z 4  of power storage element B 4  is not measured at step S 405 , the impedance Z 4  is preferably measured, for example, between step S 409  and step S 410 . 
     It should be noted that, when comparison of magnitude of impedances in the basic operation and the applicational operation resulted in equal, the flow may be advanced to either YES or NO. In either case, it is possible to give a deterioration judgment or a malfunction judgment to one among multiple power storage elements that have been deteriorated to the same degree. In addition, by repeatedly executing the deterioration mode in the applicational operation, deterioration judgment can be sequentially given to all of the multiple power storage elements whose performances have deteriorated. 
     In addition, the present embodiment is an embodiment that partially includes the power storage apparatus  100  according to the first embodiment, and is an extended configuration having eight power storage elements, and thereby has the same advantageous effect as that of the first embodiment. 
     Third Embodiment 
       FIG. 9  shows a basic configuration example of a power storage apparatus according to the third embodiment. It should be noted that the assignment rule of the numbers in the reference characters of each component described in the following is different from that of the first and second embodiments for convenience of description. Therefore, the reference characters are denoted with an “n” before the numbers. A power storage apparatus  300 A according to the present configuration example has, in one example, 2 n  (n is 3 or larger) power storage elements B 1  to B( 2   2 ) connected in series in an order of the numbers in the reference characters. At both ends of a group of continuous 2 n-1  power storage elements B 1  to B( 2   n-1 ), n3-th and n4-th switches SWn 3 , SWn 4  are connected in series. Furthermore, at both ends of a group of continuous 2 n-1  power storage elements B( 2   n-1 +1) to B( 2   n ), n5-th and n6-th switches SWn 5 , SWn 6  are connected in series. 
     Furthermore, an n2-th inductor Ln 2  is connected between a point connecting a power storage element B( 2   n-2 ) and a power storage element B( 2   n-2 +1), and a point connecting the n3-th switch SWn 3  and the n4-th switch SWn 4 . Furthermore, an n3-th inductor Ln 3  is connected between a point connecting a power storage element B( 2   n-1 + 2   n-2 ) and a power storage element B( 2   n-1 + 2   n-2 +1), and a point connecting the n5-th switch SWn 5  and the n6-th switch SWn 6 . 
     The group of power storage elements B 1  to B( 2   n-1 ) forms an n1-th power storage unit BUn 1 . The group of power storage elements B( 2   n-1 +1) to B( 2   n ) forms an n2-th power storage unit BUn 2 . 
     The n1-th power storage unit BUn 1  and the n2-th power storage unit BUn 2  are connected in series, and an n1-th switch SWn 1  and an n2-th switch SWn 2  are connected in series. Furthermore, an n1-th inductor Ln 1  is connected between a point connecting the n1-th power storage unit BUn 1  and the n2-th power storage unit BUn 2 , and a point connecting the n1-th switch SWn 1  and the n2-th switch SWn 2 . 
     The n1-th and n2-th switches SWn 1 , SWn 2  and the n1-th inductor Ln 3  form an n1 judgment circuit, and the n3 to n6-th switches SWn 3  to SWn 6  and the n2-th and n3-th inductors Ln 2 , Ln 3  form the second judgment circuit. 
     Furthermore, a control section C 2   n  outputs control signals VGn 1  to VGn 6  respectively to the n1-th to n6-th switches (SWn 1  to SWn 6 ) to control ON/OFF switching of each of the switches. Similarly to the first and second embodiments, although an ampere meter and a volt meter are connected to each of the power storage elements B 1  to B 2   n , diagrammatic representations thereof are omitted. 
     When the four groups of power storage elements B 1  to B( 2   n-2 ), B ( 2   n-2 +1) to B ( 2   n-1 ), B ( 2   n-1 +1) to B( 2   n-1 + 2   n-2 +1) are respectively regarded as power storage elements B′ 1  to B′ 4 ; the power storage apparatus  300 A will have a configuration similar to that of the power storage apparatus  100  according to the first embodiment. Therefore, similarly to the first embodiment, by conducting the processes in the normal mode and the deterioration mode in an order of the first judgment and the second judgment, it is possible to identify an element having the largest impedance among the power storage elements B′ 1  to B′ 4  with a small number of measurements, efficiently detect a group of power storage elements whose performances have deteriorated, and provide a display indicating deterioration or a display indicating malfunction. 
     It should be noted that, for example, even with a power storage apparatus  900  disclosed in Patent Literature 2 shown in  FIG. 10 , it is possible to reduce the number of measurements. The power storage apparatus  900  has 2 n  power storage elements (in  FIG. 10 , a case in which n=3 is shown), and impedance of each of the power storage elements can be measured through turning ON/OFF each switch, and conducting charging and discharging with another power storage element. However, the numbers of switches of the power storage apparatus  900  and a power storage apparatus  300 B are, respectively, 11 and 6 when n=2, 22 and 14 when n=3, 47 and 30 when n=4, and 95 and 62 when n=5. Furthermore, the numbers of measurements are, respectively, 2 and 2 when n=2, 4 and 3 when n=3, 8 and 4 when n=4, and 16 and 5 when n=5. Therefore, the power storage apparatuses according to each mode of the technology disclosed herein can further reduce the number of switches and cost, increase efficiency by further reducing the number of measurements, and obtain a further advantageous effect. For example, if it requires 10 minutes to measure impedance of a single power storage element (power storage unit) while changing frequency (from 10 kHz to 10 mHz at 10 step/decade) of current and voltage during the measurement, when n=5, the measuring time can be shortened from 160 minutes to 50 minutes. 
     Since impedance of a power storage element (power storage unit) is a function of frequency of current and voltage during a measurement, it is possible to appropriately select, calculate, or make corrections depending on the configuration of a power storage element and steps that are taken, regarding at which frequency should the comparison is to be conducted, whether to conduct the comparison by suitably assigning weight to impedance data at each frequency in a specific frequency range, how much of the assigned weight should be changed by SOC, or how much weight should be assigned by temperature. For example, instead of measuring frequency of current and voltage during measurement while changing those at an equal interval in a logarithmic axis, the measurement frequency can be selected to further shorten the measuring time. 
     In each Example, although a power storage apparatus having 2 n  power storage elements connected in series has been used as a representative example, even with other number of power storage elements, there are cases where impedances of each of the power storage elements can be specified by combining comparison circuits and making additions and subtractions to measurement results, and each mode of the technology disclosed herein can be incorporated in a part thereof. 
     Furthermore, the power storage element which is a minimum unit for measuring impedance may be an electrochemical minimum unit referred to as “cell”, or may be a combination of a plurality of cells. In any of such cases, measurement and replacement can be conducted at the power storage element level. 
     In the power storage apparatus according to the technology disclosed herein, it is conceivable that deterioration will not generally progress when the power storage element is brand-new. Therefore, by quickly figuring out a deteriorated power storage element using a simple circuit, it is possible to minimally suppress effect of measuring time relative to usage time, and dramatically improve maintainability by quickly figuring out deterioration. 
     In addition, the power storage apparatus  100  according to the first embodiment is obtained when n=2 in the power storage apparatus  300 B, and the power storage apparatus  200  according to the second embodiment is obtained when n=3 in the power storage apparatus  300 B. Thus, the present embodiment has the similar advantageous effect as that of the first and second embodiments. 
     Furthermore, in each of the examples shown in the first to third embodiments, since the power storage apparatus cannot be used as a power storage element while impedance is measured, the measurement is preferably conducted in an appropriately selected time slot, using one or more methods among a plurality of methods shown below. 
     As a first method, the control section C 2   n  (n=1, 2, . . . ) may be formed so as to conduct the impedance-measurement process based on schedule information. 
     The schedule information is, for example, information specifying a time slot in which impedance is measured and including start time, and end time or process continuation time. The control section C 2   n  may conduct the impedance-measurement process in the time slot specified by the schedule information. 
     As a second method, the control section C 2   n  preferably repetitively measures impedance at an appropriate interval to monitor deterioration of the power storage apparatus. 
     As a third method, the above described schedule information may be configured as information indicating a plurality of time slots, and the control section C 2   n  may appropriately select each of the time slots to conduct the impedance-measurement process. 
     For example, the control section C 2   n  may set a priority level on the time slots indicated by the schedule information, and may select a time slot having a high priority level in accordance with a usage status of the power storage apparatus to conduct the impedance-measurement process. More specifically, the control section C 2   n  may, for example, select a time slot having a high priority level among time slots that are not used by the power storage apparatus as a power storage element to conduct the impedance-measurement process. 
     Alternatively, as a fourth method, the control section C 2   n  may predict the period of time required for the impedance-measurement process in advance, and prioritize an executable time slot for measuring impedance. More specifically, the control section C 2   n  may, for example, predict a time slot that will not be used by the power storage apparatus as a power storage element, and conduct the impedance-measurement process when it is predicted that the impedance-measurement process will end in the time slot. 
     For example, the schedule information may be received by the power storage apparatus from an external server, may be accepted as an input from the user through a user interface included in the power storage apparatus, or may be kept in advance in information storage means such as a memory or the like included inside the power storage apparatus. 
     Furthermore, the schedule information may be generated by the user or the control section C 2   n  etc., based on unused time slots learnt in advance as a time slot in which the power storage apparatus does not conduct charging or discharging. 
     It should be noted that a processing section for conducting a process for determining execution timing of such impedance-measurement process may be, for example, formed separately in the power storage apparatus as a schedule management section, or may be incorporated with any processing section such as the control section C 2   n  or the like. 
     Alternatively, the processing section for conducting the process for determining execution timing of the impedance-measurement process may be formed on an external server, and the power storage apparatus may remotely accept control from the server and conduct start/end control of the impedance-measurement process. 
     With this, it is possible to measure impedance while reducing effect on a user of not being able to use the power storage apparatus as a power storage apparatus. 
     INDUSTRIAL APPLICABILITY 
     The power storage apparatus disclosed here is useful in mobile devices and electric-powered vehicles as a power storage apparatus with a function of measuring impedance. In addition, it is also applicable for use application such as backup power supplies and the like. Furthermore, it is widely applicable for power storage apparatuses in electronic equipment other than mobile devices and electric-powered vehicles. 
     DESCRIPTION OF THE REFERENCE CHARACTERS 
       1  power storage element 
       10 A frequency-sweep oscillator 
       10 B impedance measuring equipment 
       15  AC power supply 
       20 A amplifier 
       20 B voltage-current monitor 
       100 ,  200 ,  300 A,  300 B,  900  power storage apparatus 
     B 1 , B 2 , . . . power storage element 
     BU 1 , BU 2 , . . . power storage unit 
     SW 1 , SW 2 , . . . switch 
     L 1 , L 2 , . . . inductor 
     C 4 , C 8 , C 2   n  control section