Patent Publication Number: US-2018038917-A1

Title: Sealed secondary battery deterioration diagnosis method and deterioration diagnosis system

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a system for diagnosing deterioration of a sealed secondary battery. 
     BACKGROUND ART 
     In recent years, a sealed secondary battery (which may be hereinafter simply referred to as “secondary battery”) represented by a lithium ion secondary battery is used as a power source not only for a mobile apparatus such as a portable phone or a notebook personal computer but also for an electrically driven vehicle such as an electric automobile or a hybrid car. By repetition of charging and discharging, the secondary battery becomes deteriorated, and also it becomes difficult to grasp the residual capacity accurately according as the deterioration proceeds. 
     Patent Document 1 discloses a method in which change in the battery voltage per predetermined period of time is successively measured while performing constant-current charging or constant-current discharging of a secondary battery, and a deterioration rate of the secondary battery is calculated on the basis of the period of time during which the change in the battery voltage is below or equal to a predetermined value. However, such battery voltage per predetermined period of time changes depending on a charge/discharge history up to the time point of measurement, so that the method is not suitable for use while the charging and discharging are repeated, for example, for use in an environment where the battery is mounted on an electrically driven vehicle in which the charging and discharging are frequently repeated. 
     Patent Document 2 discloses a method of calculating a charged capacity by fully charging a battery after the battery is once completely discharged, and accumulating the electric current amount of the charging current during that period. However, in actual use, it is a rare case that the battery is completely discharged, so that the method is not suitable for use while the charging and discharging are repeated, for example, for use in an environment where the battery is mounted on an electrically driven vehicle in which the charging and discharging are frequently repeated. 
     Patent Document 3 discloses a method of detecting the voltage and the internal pressure of a secondary battery and calculating a deterioration of the secondary battery by using a relationship data between the voltage and the internal pressure of a secondary battery that is not yet deteriorated and a relationship data between the internal pressure and the battery capacity of the secondary battery. However, there are cases in Which the deterioration mechanism differs depending on the environment or conditions in which the secondary battery is used, so that relationship data of various patterns must be provided, and moreover, it is difficult to determine which pattern the case corresponds to. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP-A-2002-340997 
     Patent Document 2: JP-A-2008-278624 
     Patent Document 3: JP-A-2013-92398 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention has been made in view of the aforementioned circumstances, and an object thereof is to provide a method and a system capable of diagnosing deterioration of a sealed secondary battery in a convenient manner and at a high precision even in use while the charging and discharging are repeated. 
     Means for Solving the Problems 
     The present invention provides a sealed secondary battery deterioration diagnosis method comprising: a step of detecting deformation of the sealed secondary battery and determining a first curved line representing a relationship between a discharged capacity from a fully charged state or a charged capacity up to the fully charged state and a detected deformation amount of the sealed secondary battery; a step of determining a second curved line representing a relationship between the charged/discharged capacity and a slope of the first curved line; a step of calculating a charged/discharged capacity Qc between stage change points appearing as extremal values on the second curved line; and a step of calculating a retention rate of an active substance on the basis of a ratio Qc/Qs of the charged/discharged capacity Qc relative to a charged/discharged capacity Qs between stage change points in a predetermined reference state. 
     The extremal values appearing on the second curved line are derived from stage change of electrodes. When the active substance is deactivated, the charged/discharged capacity Qc between the stage change points decreases because of decrease in the amount of the active substance contributing to charging and discharging. For this reason, by using the ratio Qc/Qs of the charged/discharged capacity Qc to the charged/discharged capacity Qs in the reference state as an index, it is possible to determine to what degree the active substance contributing to charging and discharging is retained, that is, a retention rate of the active substance. Also, the first curved line from which the second curved line is derived represents the relationship between the discharged capacity from the fully charged state or the charged capacity up to the fully charged state and the deformation amount of the secondary battery. There are a lot of opportunities in which the battery comes into a fully charged state even in an environment in which charging and discharging are frequently repeated, such as in an electrically driven vehicle. Therefore, by this method, deterioration of a sealed secondary battery can be diagnosed in a convenient manner and at a high precision even in use while the charging and discharging are repeated. 
     In the above deterioration diagnosis method, it is preferable to include a step of correcting the second curved line by using the retention rate so that the charged/discharged capacity Qc comes to have the same value as the charged/discharged capacity Qs; and a step of calculating a capacity balance shift amount brought about by side reaction on the basis of a charged/discharged capacity difference between the stage change point on the corrected second curved line and the corresponding stage change point in the predetermined reference state. 
     By correcting the second curved line in this manner, a second curved line is obtained in which it is assumed that there is no deactivation of the active substance grasped by the retention rate. Further, the aforementioned charged/discharged capacity difference that is seen between the corrected second curved line and the reference state is a result of generation of shift in the capacity balance between the positive electrode and the negative electrode, and this shift is generated by the difference between a side reaction amount at the positive electrode and a side reaction amount at the negative electrode. Therefore, on the basis of this charged/discharged capacity difference, the capacity balance shift amount brought about by the side reaction can be calculated, and the deterioration of the sealed secondary battery can be diagnosed at a higher precision. Furthermore, this capacity balance shift amount brought about by the side reaction, combined with the retention rate of the active substance, can help to predict the residual capacity of the sealed secondary battery. 
     The above deterioration diagnosis method may include a step of determining that the battery is in a deterioration mode brought about by enlargement of reaction distribution when the charged/discharged capacity calculated on the basis of a width of a peak appearing on the second curved line is larger than the charged/discharged capacity calculated on the basis of a width of a corresponding peak in the predetermined reference state. This allows that the enlargement of reaction distribution can be easily detected. 
     It is preferable to include a step of performing constant-current charging within a range that does not exceed a double of a value obtained by adding a residual capacity at the time of start of charging to the charged capacity at the time of rise of the peak appearing on the second curved line when it is determined that the battery is in the deterioration mode brought about by enlargement of reaction distribution. This allows that an inconvenience such as generation of deposition of lithium metal can be avoided, and moreover, there is no need to bring the battery into a completely discharged state for that purpose. 
     In the sealed secondary battery deterioration diagnosis method according to the present invention, it is preferable that a polymer matrix layer is attached to the sealed secondary battery, and the polymer matrix layer contains a filler that is dispersed therein and that changes an external field in accordance with deformation of the polymer matrix layer; and the deformation of the sealed secondary battery is detected by detecting change in the external field accompanying the deformation of the polymer matrix layer. This allows that deformation of the sealed secondary battery can be detected with a high degree of sensitivity, and deterioration of the sealed secondary battery can be diagnosed with a good precision. 
     In the above method, it is preferable that the polymer matrix layer contains a magnetic filler as the filler, and the deformation of the sealed secondary battery is detected by detecting change in a magnetic field as the external field. This allows that change in the magnetic field accompanying the deformation of the polymer matrix layer can be wirelessly detected. Also, because a Hall element having a wide sensitivity region can be used, detection with a high degree of sensitivity can be made in a wider range. 
     Further, the present invention provides a sealed secondary battery deterioration diagnosis system comprising: a detection sensor for detecting deformation of the sealed secondary battery; and a controlling device for determining a first curved line representing a relationship between a discharged capacity from a fully charged state or a charged capacity up to the fully charged state and a deformation amount of the sealed secondary battery detected by the detection sensor, determining a second curved line representing a relationship between the charged/discharged capacity and a slope of the first curved line, calculating a charged/discharged capacity Qc between stage change points appearing as extremal values on the second curved line, and calculating a retention rate of an active substance on the basis of a ratio Qc/Qs of the charged/discharged capacity Qc relative to a charged/discharged capacity Qs between stage change points in a predetermined reference state. 
     The extremal values appearing on the second curved line are derived from stage change of electrodes. When the active substance is deactivated, the charged/discharged capacity Qc between the stage change points decreases because of decrease in the amount of the active substance contributing to charging and discharging. For this reason, by using the ratio Qc Qs of the charged/discharged capacity Qc to the charged/discharged capacity Qs in the reference state as an index, it is possible to determine to what degree the active substance contributing to charging and discharging is retained, that is, a retention rate of the active substance. Also, the first curved line from which the second curved line is derived represents the relationship between the discharged capacity from the fully charged state or the charged capacity up to the fully charged state and the deformation amount of the secondary battery. There are a lot of opportunities in which the battery comes into a fully charged state even in an environment in which charging and discharging are frequently repeated, such as in an electrically driven vehicle. Therefore, by this system, deterioration of a sealed secondary battery can be diagnosed in a convenient manner and at a high precision even in use while the charging and discharging are repeated. 
     In the above deterioration diagnosis system, it is preferable that the controlling device corrects the second curved line by using the retention rate so that the charged/discharged capacity Qc comes to have the same value as the charged/discharged capacity Qs, and calculates a capacity balance shift amount brought about by side reaction on the basis of a charged/discharged capacity difference between the stage change point on the corrected second curved line and the corresponding stage change point in the predetermined reference state. 
     By correcting the second curved line in this manner, a second curved line is obtained in which it is assumed that there is no deactivation of the active substance grasped by the retention rate. Further, the aforementioned charged/discharged capacity difference that is seen between the corrected second curved line and the reference state is a result of generation of shift in the capacity balance between the positive electrode and the negative electrode, and this shift is generated by the difference between a side reaction amount at the positive electrode and a side reaction amount at the negative electrode. Therefore, on the basis of this charged/discharged capacity difference, the capacity balance shift amount brought about by the side reaction can be calculated, and the deterioration of the sealed secondary battery can be diagnosed at a higher precision. Furthermore, this capacity balance shift amount brought about by the side reaction, combined with the retention rate of the active substance, can help to predict the residual capacity of the sealed secondary battery. 
     The above deterioration diagnosis system may determine that the battery is in a deterioration mode brought about by enlargement of reaction distribution when the charged/discharged capacity calculated on the basis of a width of a peak appearing on the second curved line is larger than the charged/discharged capacity calculated on the basis of a width of a corresponding peak in the predetermined reference state. This allows that the enlargement of reaction distribution can be easily detected. 
     It is preferable that constant-current charging is performed within a range that does not exceed a double of a value obtained by adding a residual capacity at the time of start of charging to the charged capacity at the time of rise of the peak appearing on the second curved line when it is determined that the battery is in the deterioration mode brought about by enlargement of reaction distribution. This allows that an inconvenience such as generation of deposition of lithium metal can be avoided, and moreover, there is no need to bring the battery into a completely discharged state for that purpose. 
     In the sealed secondary battery deterioration diagnosis system according to the present invention, it is preferable that the detection sensor comprises a polymer matrix layer attached to the sealed secondary battery and a detection unit; the polymer matrix layer contains a filler that is dispersed therein and that changes an external field in accordance with deformation of the polymer matrix layer; and the detection unit detects change in the external field. This allows that deformation of the sealed secondary battery can be detected with a high degree of sensitivity, and deterioration of the sealed secondary battery can be diagnosed with a good precision. 
     In the above system, it is preferable that the polymer matrix layer contains a magnetic filler as the filler, and the detection unit detects change in a magnetic field as the external field. This allows that change in the magnetic field accompanying the deformation of the polymer matrix layer can be wirelessly detected. Also, because a Hall element having a wide sensitivity region can be used, detection with a high degree of sensitivity can be made in a wider range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing one example of a system for executing a deterioration diagnosis method according to the present invention. 
         FIG. 2  is a view schematically illustrating a sealed secondary battery, where  FIG. 2( a )  is a perspective view, and  FIG. 2( b )  is an A-A cross-sectional view. 
         FIG. 3  is a view schematically illustrating a sealed secondary battery, where  FIG. 3( a )  is a perspective view, and  FIG. 3( b )  is an B-B cross-sectional view. 
         FIG. 4  is a graph showing a relationship between a discharged capacity from a fully charged state and a detected deformation amount of the secondary battery. 
         FIG. 5  is a graph showing a relationship between a discharged capacity from a fully charged state and a slope of a first curved line. 
         FIG. 6  is a graph showing a relationship between a discharged capacity from a fully charged state and a slope of a first curved line. 
         FIG. 7  is a graph showing a relationship between a charged capacity up to the fully charged state and a slope of a first curved line. 
         FIG. 8  is a graph showing a relationship between a charged capacity up to the fully charged state and a slope of a first curved line. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be explained. 
       FIG. 1  shows a system that is mounted on an electrically driven vehicle such as an electric automobile or a hybrid car. This system includes a battery module  1  in which an assembled battery constructed with a plurality of sealed secondary batteries  2  is accommodated in a case. In the present embodiment, four secondary batteries  2  are connected in an arrangement of two in parallel by two in series; however, the number of batteries or the connection mode is not limited to this alone. In  FIG. 1 , only one battery module  1  is shown. Actually, however, the batteries are mounted as a battery pack including a plurality of battery modules I. In the battery pack, a plurality of battery modules  1  are connected in series, and the battery modules  1  are accommodated in a case together with various apparatus such as a controller. The case of the battery pack is formed in a shape suitable for mounting on a vehicle, for example, a shape that conforms to the under-floor shape of the vehicle. 
     The secondary battery  2  shown in  FIG. 2  is constructed as a cell (unit cell) in which an electrode group  22  is accommodated in the inside of a sealed outer casino  21 . The electrode group  22  has a structure in which a positive electrode  23  and a negative electrode  24  are stacked or wound via a separator  25  interposed therebetween, and an electrolytic solution is held in the separator  25 . The secondary battery  2  of the present embodiment is a laminate battery using a laminate film such as an aluminum laminate foil as the outer casing  21 , and is specifically a laminate-type lithium ion secondary battery having a capacity of 1.44 Ah. The secondary battery  2  is formed to have a thin-type rectangular parallelepiped shape as a whole. The X-, Y-, and Z-directions correspond to the length direction, the width direction, and the thickness direction, respectively, of The secondary battery  2 . Also, the Z-direction is the thickness direction of the positive electrode  23  and the negative electrode  24 . 
     A detection sensor  5  that detects deformation of the secondary battery  2  is mounted on the secondary battery  2 . The detection sensor  5  includes a polymer matrix layer  3  attached to the secondary battery  2  and a detection unit  4 . The polymer matrix layer  3  contains a filler that is dispersed therein and that changes an external field in response to deformation of the polymer matrix layer  3 . The polymer matrix layer  3  of the present embodiment is in a sheet form and is formed of an elastomer material that is capable of flexible deformation. The detection unit  4  detects the change in the external field accompanying the deformation of the polymer matrix layer  3 . When the secondary battery  2  swells to be deformed, the polymer matrix layer  3  is deformed in accordance therewith, and change in the external field accompanying the deformation of the polymer matrix layer  3  is detected by the detection unit  4 . In this manner, deformation of the secondary battery  1  can be detected with a high degree of sensitivity. 
     In the example shown in  FIG. 2 , the polymer matrix layer  3  is attached to the outer casing  21  of the secondary battery  2 , so that the polymer matrix layer  3  can be deformed in accordance with the deformation (mainly the swelling) of the outer casing  21 . Alternatively, the polymer matrix layer  3  may be attached to the electrode group  22  of the secondary battery  2  as shown in  FIG. 3 . According to such a construction, the polymer matrix layer  3  can be deformed in accordance with the deformation (mainly the swelling) of the electrode group  22 . The deformation of the secondary battery  1  to be detected may be a deformation of either the outer casing  21  or the electrode group  22 . 
     Signals obtained by detection with the detection sensor  5  are transmitted to a controlling device  6 , whereby information on the deformation of the secondary battery  2  is supplied to the controlling device  6 . The controlling device  6  performs deterioration diagnosis of the secondary battery  2  by using the information, specifically, on the basis of processing including the following steps  1  to  4 , more preferably including the steps  5  and  6 . In the following example, a behavior of the battery discharged from a fully charged state town arbitrary discharged state will be described; however, the present invention is not limited to this alone. 
     First, deformation of the secondary battery  2  is detected, and a first curved line representing a relationship between a discharged capacity from a fully charged state and a detected deformation amount of the secondary battery  2  is determined (Step  1 ). The graph of  FIG. 4  shows the first curved line L 1  that was determined in the secondary battery  2  after 500 cycles of a charging/discharging step were repeated. In the charging/discharging step, the secondary battery  2  was put into a thermostat kept at 25° C. and, after being left to stand quietly for 120 minutes, the secondary battery  2  was subjected to constant-current charging up to 4.3 V with a charging current of 1.44 A. After the voltage reached 4.3 V, the secondary battery  2  was subjected to constant-voltage charging until the electric current value decayed down to 0.07 A. Thereafter, an open-circuit state was maintained for 10 minutes, and the secondary battery  2  was subjected to constant-current discharging down to 3.0 V with an electric current of 1.44 A. In a secondary battery  2  that had not been used yet and had not been deteriorated, the discharged capacity from the fully charged state thereof to the completely discharged state was 1440 mAh. 
     In the graph of  FIG. 4 , the lateral axis represents a discharged capacity Q with the point of origin representing a fully charged state, and the longitudinal axis represents a detected deformation amount T of the secondary battery  2 . According as the discharged capacity Q from the fully charged state increases, the deformation amount T of the secondary battery  2  decreases. This is due to the following reason. In the charged secondary battery  2 , swelling of the electrode group  22  (which may be hereafter referred to as “electrode swelling”) is generated due to change in volume of an active substance. According as the discharging proceeds, the swelling of the electrode group  22  decreases. The curved line Ls 1  refers to a secondary battery  2  in a reference state and represents a relationship between a discharged capacity from a fully charged state and a deformation amount of the secondary battery  2 . This curved line Ls 1  is determined in the same manner as the first curved line L 1  by using a secondary battery  2  that is in an initial stage and is not deteriorated vet as a reference state, for example, by using a secondary battery  2  just after being produced or before being shipped as an object. 
     In the secondary battery  2 , when the electrolytic solution is decomposed due to overcharging or the like, swelling due to rise in the internal pressure caused by the decomposed gas (which may be hereafter referred to as “gas swelling”) may be generated. The detection sensor  5  detects deformation of the secondary battery  2  due to this gas swelling as well; however, this is reflected merely as an overall magnitude of the deformation amount T and does not appear as a change accompanying the increase in the discharged capacity Q. Therefore, in  FIG. 4 , the fact that the deformation amount T decreases in accordance with increase in the discharged capacity Q is due to the influence of the electrode swelling, while the fact that the first curved line L 1  shows a larger deformation amount T than the curved line Ls 1  with the same discharged capacity Q is due to the influence of the gas swelling. 
     The first curved line L 1  has a shape including concavity and convexity of some degree, as shown in  FIG. 4 , due to stage changes of the electrode. For example, in the case of a lithium ion secondary battery using graphite (graphite) in the negative electrode, it is known that the crystal state of the graphite successively undergoes stage change according as the discharging proceeds from the fully charged state. This is because the crystal state of graphite changes stepwise in accordance with the amount of intercalation of lithium ions, and the active substance of the negative electrode expands due to stepwise increase of an average distance between the graphene layers. To sum up, the volume of the active substance changes stepwise by the stage change, and this is reflected on the first curved line L 1  and the curved line Ls 1 . In determining the first curved line L 1  such as this, the detection sensor  5  that detects deformation of the secondary battery  2  with a high degree of sensitivity is suitable. 
     Next, a second curved line is determined, representing a relationship between the charged/discharged capacity thereof (which is a collective term for discharged capacity and charged capacity and, in the present embodiment, refers to a discharged capacity from the fully charged state) and a slope of the first curved line (Step  2 ). The graph of  FIG. 5  shows the second curved line L 2  as determined from the first curved line L 1 . This slope dT/dQ of the first curved line L 1  is obtained as a differential value when the deformation amount T is differentiated with respect to the discharged capacity Q. The second curved line L 2  has two stage change points P 1 , P 2  that appear as extremal values, and these are due to stage changes described above. The curved line Ls 2  represents a relationship between the charged/discharged capacity (discharged capacity in the present embodiment) and a slope of the first curved line with respect to the secondary battery  2  that is in the reference state. This curved line Ls 2  can be determined from the curved line Ls 1  according to the same procedure as that in determining the second curved line L 2  from the first curved line L 1 . The curved line Ls 2  also has two stage change points Ps 1 , Ps 2 . 
     As an active substance for use in the negative electrode of a lithium ion secondary battery, those capable of electrochemically intercalating and deintercalating lithium ions are used. In obtaining the second curved line having a plurality of stage change points such as described above, a negative electrode containing graphite is preferably used. Also, examples of the active substance used in the positive electrode include LiCoO 2 , LiMn 2 O 4 , LiNiO 2 , Li(MnAl) 2 O 4 , Li(NiCoAl)O 2 , LiFePO 4 , and Li(NiMnCo)O 2 . 
     Subsequently, a charged/discharged capacity Qc between the stage change points appearing as extremal values on the second curved line is calculated (Step  3 ). The extremal value is a collective term for a local minimal value and a local maximal value, and the stage change points appear as local minimal values during the discharging and appear as local maximal values during the charging. In the present embodiment, the discharged capacity from the fully charged state is observed, so that the stage change points P 1 , P 2  appear as local minimal values.  FIG. 5  shows the charged/discharged capacity Qc between the stage change points P 1 , P 2  and the charged/discharged capacity Qs between the stage change points Ps 1 , Ps 2 . For example, in the case of a lithium ion secondary battery in which graphite is used in the negative electrode, the stage change thereof occurs when one or more lithium ions are intercalated with respect to 24 carbons and when one or more lithium ions are intercalated with respect to 12 carbons. Therefore, decrease in the charged/discharged capacity Qc suggests decrease in the amount of carbon capable of intercalating and deintercalating lithium ions such as this. Based on this, deactivation of the active substance can be inferred. 
     Further, a retention rate of the active substance is calculated on the basis of a ratio Qc/Qs of the charged/discharged capacity Qc relative to the charged/discharged capacity Qs between stage change points in a predetermined reference state (Step  4 ). For example, when the charged/discharged capacity Qc is 411 mAh and the charged/discharged capacity Qs is 514 mAh, the retention rate R of the active substance can be calculated to be 0.8 (≈411/514) on the basis of the ratio Qc/Qs thereof. This means that, in the secondary battery  2  that has been subjected to repetition of the charging and discharging step for 500 cycles, about 80% of the active substance contributing to the charging and discharging is retained, in other words, the active substance contributing to the charging and discharging has decreased down to be about 80%. In this manner, deterioration of the secondary battery  2  can be diagnosed. 
     As described above, the charged/discharged capacity Qc is determined from the second curved line L 2 , and the first curved line L 1  from which the second curved line L 2  is derived is determined as a relationship between the discharged capacity Q from the fully charged state and the deformation amount T of the secondary battery  2 . Further, there are a lot of opportunities in which the battery comes into a fully charged state even in an environment in which charging and discharging are frequently repeated, such as in an electrically driven vehicle. Therefore, by the method of the present embodiment, deterioration of the secondary battery  2  can be diagnosed in a convenient manner and at a high precision even in such usage while the charging and discharging are repeated. 
     The curved line Ls 1  is obtained in advance by using a secondary battery  2  that is in an initial stage and is not deteriorated yet as a reference state, for example, by using a secondary battery  2  just after being produced or before being shipped as an object. Therefore, the curved line Ls 2 , the stage change points Ps 1 , Ps 2 , and the charged/discharged capacity Qs can be determined in advance as well. These data can be stored in a storage unit which is provided in the controlling device  6  and not illustrated in the drawings; however, for the above-described deterioration diagnosis, it is sufficient that at least the charged/discharged capacity Qs is known among these. 
     In the present embodiment, by further executing the following steps  5  and  6 , the deterioration of the secondary battery  2  can be diagnosed at a higher precision by calculating a capacity balance shift amount brought about by side reaction. In addition, the capacity balance shift amount brought about by the side reaction, when considered in combination with the retention rate of the active substance, can help to predict the residual capacity of the secondary battery  2 . 
     After the charged/discharged capacity Qc is calculated, the second curved line is corrected by using the retention rate of the active substance so that the charged/discharged capacity Qc comes to have the same value as the charged/discharged capacity Qs (Step  5 ). Specifically, the value of the discharged capacity of the second curved line is divided by the retention rate of the active substance. It is sufficient that this step is carried out after the charged/discharged capacity Qc is calculated, and may be carried out before the retention rate is calculated. The graph of  FIG. 6  shows a second curved line L 2 ′ obtained by correcting the second curved line L 2 . When the charged/discharged capacity Qc is 411 mAh and the charged/discharged capacity Qs is 514 mAh, the value of the discharged capacity Q of the second curved line L 2  may be divided by 0.8 which is the retention rate R. This allows that the charged/discharged capacity Qc′ of the corrected second curved line L 2 ′ will be 514 mAh which is the same as the charged/discharged capacity Qs. 
     The corrected second curved line L 2 ′ is determined as a curved line representing a relationship between the discharged capacity Q from the fully charged state and the slope dT/dQ of the first curved line L 1  supposing that there is no deactivation of the active substance grasped by the retention rate R. However, as shown in  FIG. 6 , the second curved line L 2 ′ does not coincide with the curved line Ls 2 , and a charged/discharged capacity difference Qd is seen between the stage change point P 1 ′ (or P 2 ′) thereof and the corresponding stage change point Ps 1  (or Ps 2 ). This charged/discharged capacity difference Qd is a result of generation of a shift in the capacity balance between the positive electrode  23  and the negative electrode  24 , and this shift is generated by the difference between a side reaction amount at the positive electrode  23  and a side reaction amount at the negative electrode  24 . 
     After the second curved line is corrected in the above-described manner, the capacity balance shift amount brought about by side reaction is calculated on the basis of the charged/discharged capacity difference between the stage change point on the corrected second curved line and the corresponding stage change point in a predetermined reference state (Step  6 ). For example, when the charged/discharged capacity difference Qd is 116 mAh, it can be determined that a superfluous side reaction corresponding to that amount has been generated in the negative electrode  24 , and the capacity balance shift amount brought about by the side reaction can be calculated to be an amount corresponding to 116 mAh. This is considered so because, in order to correct the deactivation amount of the active substance (that is, to make the second curved line L 2 ′ coincide with the curved line Ls 2 ), the second curved line L 2 ′ must be shifted for an amount of 116 mAh in the positive direction (rightward direction in the graph) of the discharged capacity Q. 
     In this manner, by not only calculating the retention rate of the active substance but also calculating the capacity balance shift amount brought about by the side reaction, the deterioration of the secondary battery  2  can be diagnosed at a higher precision. Furthermore, by considering this capacity balance shift amount brought about by the side reaction in combination with the retention rate of the active substance, it is possible to infer the end point of the first curved line L 1 , whereby the residual capacity of the secondary battery  2  can be predicted. For example, when it is diagnosed that the retention rate R of the active substance is 0.8 and the capacity balance shift amount brought about by the side reaction corresponds to an amount of 116 mAh after 500 cycles of charging and discharging are repeated in a secondary battery in which the capacity before deterioration is 1440 mAh, the value (mAh) of the discharged capacity Q at the end point of the first curved line L 1  is inferred from 1440×0.8+116, and the residual capacity can be predicted by subtracting therefrom the discharged capacity at the time of diagnosis. 
     As described above, the deterioration diagnosis method and the deterioration diagnosis system of the present embodiment do not simply detect capacity decrease of the secondary battery but can grasp what kind of capacity deterioration is occurring in the secondary battery, and specifically can obtain deterioration information in detail regarding in what ratio the active substance is retained (conversely speaking, in what ratio the active substance has been deactivated) and in what amount the side reaction (electrochemical reaction that does not contribute to charging and discharging) has occurred. Further, the residual capacity can be predicted by inferring the end point of the discharged capacity in the deteriorated secondary battery. The deterioration diagnosis such as this, including the prediction of the residual capacity, is executed by the controlling device  6 . 
     In the above-described embodiment, the behavior when the battery is discharged from the fully charged state to an arbitrary discharged state has been described. Conversely, with respect to a behavior when the battery is charged from an arbitrary discharged state to the fully charged state, deterioration diagnosis can be carried out by the same procedure as described above. In that case, a first curved line representing a relationship between a charged capacity up to the fully charged state and a detected deformation amount of the secondary battery is determined in step  1 . The lateral axis of the graph of  FIG. 4  is reversed and, according as the charged capacity up to the fully charged state increases (that is, according as the battery approaches the fully charged state), the deformation amount of the secondary battery increases. Further, in step  2 , a second curved line is determined, representing a relationship between the charged/discharged capacity thereof (charged capacity up to the fully charged state) and a slope of the first curved line. In the graphs of  FIGS. 5 and 6 , not only the lateral axis but also the longitudinal axis are reversed, whereby the second curved line having a peak directed upwards is obtained, and stage change points appear as local maximal values on the second curved line (for example, see  FIG. 7 ). 
     Next, an embodiment of determining a deterioration mode by enlargement of reaction distribution will be described with reference to  FIGS. 7 and 8 . This determination of the deterioration mode can be used in combination with the above-described calculation of the retention rate of the active substance. In this case, either the calculation of the retention rate of the active substance or the determination of the deterioration mode may be carried out first, and also it is possible to carry these out simultaneously. 
       FIG. 7  is a graph showing a relationship between the charged/discharged capacity (charged capacity up to the fully charged state) and the slope of the first curved line. The second curved line L 3  is determined by steps  1  and  2  such as described above. Depiction of the first curved line from which the second curved line L 3  is derived is omitted. The second curved line L 3  has two stage change points P 3 , P 4  that appear as extremal values (as local maximal values, because the battery is being charged). Also, on the second curved line L 3 , there appear two upward-directed peaks having the stage change points P 3 , P 4 , and these are due to stage changes described above. 
     The two peaks that appear on the second curved line L 3  each show an amount of individual active substances at the time when the stage change with that capacity (charged capacity in  FIG. 7 ) is completed. The capacity Qp 31 , Qp 41  of the start point of the peak (the point at which the curved line starts to go away from a base line BL) each is a capacity at which the reaction proceeds earliest among the numerous active substances in the electrode, that is, a capacity at which the stage change starts earliest. The Capacity Qp 32 , Qp 42  of the end point of the peak (the point at which the curved line starts to be in contact with the base line BL) each is a capacity at which the stage change of all of the active substances in the electrode is completed. The base line BL is determined by a straight line connecting the inflection points before and after each peak. A charged/discharged capacity Qw 3  is calculated on the basis of a base width extending from the start point of the peak to the end point of the peak. Specifically, the charged/discharged capacity Qw 3  is determined by subtracting the capacity Qp 31  from the capacity Qp 32 . A charged/discharged capacity Qw 4  is determined in the same manner. The charged discharged capacity Qw 3 , Qw 4  each shows a distribution of the reaction speed of individual active substances in the electrode. Therefore, by comparing the width of the peak such as the base width before and after deterioration of the secondary battery  2 , the reaction distribution of the active substances in the electrode can be grasped. 
     Generally, enlargement of the reaction distribution is due to increase in the ion resistance or electric resistance within the electrode. By enlargement of the reaction distribution, the charged capacity up to the deposition of lithium metal decreases at the time of charging, whereby lithium metal becomes liable to be deposited. The deposited lithium metal grows in a dendrite form, thereby possibly generating an inconvenience such as provoking a short-circuit between the positive electrode and the negative electrode. Also, at the time of discharging, by enlargement of the reaction distribution, overcharging occurs in the active substance in which the reaction is most liable to proceed, thereby generating an inconvenience of promoting the deterioration of the battery. 
     The graph in  FIG. 8  shows a curved line Ls 3  representing a relationship between the charged/discharged capacity (charged capacity in the present embodiment) and the slope of the first curved line in a secondary battery  2  that is in a reference state. In the same manner as the curved line Ls 1  in  FIG. 4 , the first curved line from which the curved line Ls 3  is derived can be obtained in advance by using a secondary battery  2  that is in an initial stage and is not deteriorated yet as a reference state, for example, by using a secondary battery  2  just after being produced or before being shipped as an object. Therefore, not only the curved line Ls 3  but also the stage change points Ps 3 , Ps 4 , the base line BLs, the capacities Qps 31 , Qps 32 , Qps 41 , Qps 42 , and the charged/discharged capacities Qws 3 , Qws 4  can be determined in advance as well. The charged/discharged capacity Qws 3  is calculated on the basis of a base width of the peak. Specifically, the charged/discharged capacity Qws 3  is determined by subtracting the capacity Qps 31  from the capacity Qps 32 . The charged/discharged capacity Qws 4  is determined in the same manner. 
     As described above, the width of each peak reflects the distribution of reaction speed of the active substances in the electrode. Therefore, when the width of the peak appearing on the second curved line L 3  is larger than the width of the corresponding peak in a predetermined reference state, it can be determined that the battery is in a deterioration mode due to enlargement of the reaction distribution. In the present embodiment, as shown in  FIGS. 7 and 8 , the charged/discharged capacity Qw 3  of the second curved line L 3  is larger than the corresponding charged/discharged capacity Qws 3  in the reference state, so that it is determined that the reaction distribution has enlarged as compared with that before deterioration, that is, the battery is in a deterioration mode due to enlargement of the reaction distribution. The same applies to comparison between the charged/discharged capacity Qw 4  and the charged/discharged capacity Qws 4 . In determining the deterioration mode, the comparison may be made with use of either one of the peaks. 
     In this manner, by comparison between the charged/discharged capacity Qw 3  and the charged/discharged capacity Qws 3  or by comparison between the charged/discharged capacity Qw 4  and the charged/discharged capacity Qws 4 , enlargement of the reaction distribution can be easily detected. In the present embodiment, the charged/discharged capacities that are compared for determination are calculated on the basis of a base width of the peak. However, the present invention is not limited to this alone, so that the charged/discharged capacities may be calculated on the basis of another width of the peak. For example, the charged discharged capacities may be calculated on the basis of a half-value width of the peak (width at a position of half the height of the peak), and determination of the deterioration mode may be made by comparison of these. By such a method as well, enlargement of the reaction distribution before and after the deterioration can be determined. 
     Further, by obtaining in advance the relationship between the width of the peak and the discharge rate or the capacity for each temperature, the discharge rate of the currently standing battery and the residual capacity dependent on the temperature can also be predicted by making a reference to the results thereof. 
     Typically, in a lithium ion secondary battery using graphite in the negative electrode, stage changes of a stage  2  and a stage  3  are observed during the charging and discharging. The present embodiment is an example in which graphite is used in the negative electrode. In the graphs of  FIGS. 7 and 8 , change to the stage  3  is observed as a left peak, and change to the stage  2  is observed as a right peak. Among these, change to the stage  2  is a state in which one lithium ion is intercalated with respect to 12 carbons (carbon atoms). Deposition of lithium metal that invites various inconveniences occurs in a state in which it is attempted to intercalate two or more lithium ions with respect to 12 carbon atoms (that is, in a state in which it is attempted to intercalate one or more lithium ions with respect to 6 carbon atoms). Therefore, deposition of lithium metal occurs by intercalation of a charged capacity of double or more of the charged capacity from the completely discharged state to the stage  2 . 
     As already described, the charged capacity at the rise of the peak (the charged capacity at the start point) is a capacity at which the reaction proceeds earliest among the numerous active substances in the electrode, that is, a capacity at which the stage change starts earliest in the battery. Therefore, in the peak showing change to the stage  2 , the capacity at which lithium metal is deposited can be predicted on the basis of a value of double of the charged capacity at the rise of the peak. From this, it can be determined that, in the present embodiment, the value of double of the charged capacity Qp 41  is a capacity at which lithium metal is deposited. 
     In an initial design of the battery, the battery reaches a set upper-limit voltage before the capacity of double of the charged capacity at the rise of the peak is reached, and operation is switched from the constant-current charging to constant-voltage charging, so that the deposition of lithium metal does not occur. However, according as the deterioration of the battery proceeds, the ion resistance or electric resistance increases by sediment onto the active substance surface or loosening of the binding agent, whereby the reaction distribution enlarges, and the charged capacity at the rise of the peak makes a shift towards the lower-capacity side. In that case, it is not possible to avoid the deposition of lithium metal by a controlling method in which the operation is switched to constant-voltage charging after the battery reaches the set upper-limit voltage. 
     Thus, it is preferable to include a step of performing constant-current charging within a range that does not exceed a double of a value obtained by adding a residual capacity at the time of start of charging to the charged capacity Qp 41  at the time of rise of the peak appearing on the second curved line L 3  when it is determined that the battery is in the deterioration mode brought about by enlargement of the reaction distribution. Such a method allows that, even when deterioration brought about by enlargement of the reaction distribution occurs, the constant-current charging is ended by detecting change of the active substance having the highest charging speed to the stage  2 , so that deposition of lithium metal does not occur. As a result of this, safety can be improved, and progress of deterioration can be suppressed. After the constant-current charging is ended, the charging may be ended, or alternatively the operation may be switched to the constant-voltage charging. 
     In using the battery, it is a rare case that the battery is completely discharged. Typically, charging is started in a state in which some residual capacity is still present. In this case, the charged capacity up to the charged capacity Qp 41  changes in accordance with the residual capacity at the time of start of charging. Speaking further, the capacity at which lithium metal is deposited is a charged capacity of double or more of the value obtained by adding the residual capacity at the time of start of charging (which is substantially zero when the battery is in a completely discharged state) to the charged capacity Qp 41 . Therefore, in the above step, constant-current charging is performed within a range that does not exceed a double of the value obtained by adding the residual capacity at the time of start of charging to the charged capacity Qp 41 . For this reason, there is no need to bring the battery into a completely discharged state in making deterioration diagnosis or selecting suitable charging conditions. 
     In the embodiment shown in  FIG. 2 , the polymer matrix layer  3  is attached to a wall part  28   a  of the outer casing  21  that faces the electrode group  22  in the thickness direction of the positive electrode  23  and the negative electrode  24 , that is, in the Z-direction (up-and-down direction in  FIG. 2( b ) ). The outer surface of the wall part  28   a  corresponds to the upper surface of the outer casing  21 . The polymer matrix layer  3  faces the electrode group  22  with the wall part  28   a  interposed therebetween, and is disposed parallel to the upper surface of the electrode group  22 . The electrode swelling is caused by a change in the thickness of the electrode group  22  accompanying the change in volume of the active substance, so that the action in the Z-direction is large. Therefore, in the present embodiment in which the polymer matrix layer  3  is attached to the wall part  28   a,  the electrode swelling can be detected with a high degree of sensitivity and, as a result, deterioration diagnosis can be made with a good precision. 
     In the embodiment shown in  FIG. 3 , the polymer matrix layer  3  is attached in the thickness direction of the positive electrode  23  and the negative electrode  24 , that is, in the Z-direction (up-and-down direction in  FIG. 3( b ) ), relative to the electrode group  22 . This allows that, even when the outer casino is formed of a robust material such as a metal can, the swelling of the electrode group  22  thereof, that is, the electrode swelling, can be detected with a high degree of precision and, as a result, deterioration diagnosis can be made with a good precision, 
     The detection unit  4  is disposed at a site where the change in the external field can be detected, and is preferably attached to a comparatively firm site that is hardly affected by the swelling of the secondary battery  2 . In the present embodiment, the detection unit  4  is attached to an inner surface of a case  11  of the battery module that faces the wall portion  28   a,  as shown in  FIG. 2( b ) . The case  11  of a battery module is formed, for example, of a metal or a plastic, and there may be cases in which a laminate film is used as the case  11  of the battery module. In the drawings, the detection unit  4  is disposed close to the polymer matrix layer  3 ; however, the detection unit  4  may be disposed to be distant from the polymer matrix layer  3 . 
     The present embodiment is an example in which the polymer matrix layer  3  contains a magnetic filler as the above-described filler, and the detection unit  4  detects change in a magnetic field as the above-described external field. In this case, the polymer matrix layer  3  is preferably a magnetic elastomer layer in which the magnetic filler is dispersed in a matrix that contains an elastomer component. 
     The magnetic filler may be, for example, a rare-earth-based, iron-based, cobalt-based, nickel-based, or oxide-based filler; however, a rare-earth-based filler is preferable because a higher magnetic force can be obtained. The shape of the magnetic filler is not particularly limited, so that the shape may be any one of spherical, flattened, needle-like, prismatic, and amorphous shapes. The average particle size of the magnetic filler is preferably from 0.02 to 500 μm, more preferably from 0.1 to 400 μm, and still more preferably from 0.5 to 300 μm. When the average particle size is smaller than 0.02 μm, the magnetic characteristics of the magnetic filler tend to deteriorate. On the other hand, when the average particle size exceeds 500 μm, the mechanical properties of the magnetic elastomer layer tend to deteriorate, and the magnetic elastomer layer tends to be brittle. 
     The magnetic filler may be introduced into the elastomer after magnetization; however, it is preferable to magnetize the magnetic filler after introduction into the elastomer. By magnetization after introduction into the elastomer, the polarity of the magnet can be easily controlled, and the magnetic field can be easily detected. 
     A thermoplastic elastomer, a thermosetting elastomer, or a mixture of these can be used as the elastomer component. Examples of the thermoplastic elastomer include a styrene-based thermoplastic elastomer, a polyolefin-based thermoplastic elastomer, a polyurethane-based thermoplastic elastomer, a polyester-based thermoplastic elastomer, a polyamide-based thermoplastic elastomer, a polybutadiene-based thermoplastic elastomer, a polyisoprene-based thermoplastic elastomer, and a fluororubber-based thermoplastic elastomer. Also, examples of the thermosetting elastomer include diene-based synthetic rubbers such as polyisoprene rubber, polybutadiene rubber, styrene-butadiene rubber, polychloroprene rubber, nitrile rubber, and ethylene-propylene rubber, non-diene-based synthetic rubbers such as ethylene-propylene rubber, butyl rubber, acrylic rubber, polyurethane rubber, fluororubber, silicone rubber, and epichlorohydrin rubber, and natural rubbers. Among these, a thermosetting elastomer is preferable, and this is because settling of the magnetic elastomer accompanying the heat generation or overloading of the battery can be suppressed. Further, a polyurethane rubber (which may also be referred to as a polyurethane elastomer) or a silicone rubber (which may also be referred to as a silicone elastomer) is more preferable. 
     A polyurethane elastomer can be obtained by reacting a polyol with a polyisocyanate. In the case in which the polyurethane elastomer is used as the elastomer component, a magnetic filler is mixed with a compound containing active hydrogen, and further an isocyanate component is added thereto to obtain a mixture liquid. Also, a mixture liquid can also be obtained by mixing a magnetic filler with an isocyanate component, and mixing a compound containing active hydrogen thereto. The magnetic elastomer can be produced by injecting the mixture liquid into a mold that has been subjected to a releasing treatment, and thereafter heating the mixture liquid up to a curing temperature for curing. Also, in the case in which a silicone elastomer is used as the elastomer component, the magnetic elastomer can be produced by putting a magnetic filler into a precursor of a silicone elastomer, mixing the components, putting the resulting mixture into a mold, and thereafter heating the mixture for curing. A solvent may be added as necessary. 
     A compound known in the art in the field of polyurethane can be used as the isocyanate component that can be used in the polyurethane elastomer. Examples of the isocyanate component include aromatic diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, p-xylylene diisocyanate, and m-xylylene diisocyanate, aliphatic diisocyanates such as ethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and 1,6-hexamethylene diisocyanate, and alicyclic diisocyanates such as 1,4-cyclohexane diisocyanate, 4,4′-dicyclohexylmethane diisocyanate, isophorone diisocyanate, and norbornane diisocyanate. These may be used either alone or as a mixture of two or more kinds. Also, the isocyanate component may be a modified component such as a urethane-modified, allophanate-modified, biuret-modified, or isocyanurate-modified component. Preferable isocyanate components are 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, and 4,4′-diphenylmethane diisocyanate. 2,4-toluene diisocyanate or 2,6-toluene diisocyanate is more preferable. 
     A compound typically used in the technical field of polyurethane can be used as the compound containing active hydrogen. Examples of the compound containing active hydrogen include high-molecular-weight polyols such as polyether polyols represented by polytetramethylene glycol, polypropylene glycol, polyethylene glycol, and a copolymer of propylene oxide and ethylene oxide, polyester polyols represented by polybutylene adipate, polyethylene adipate, and 3-methyl-1,5-pentane adipate, polyester polycarbonate polyols typified by reaction products of alkylene carbonate and polyester glycol such as polycaprolactone polyol or polycaprolactone, polyester polycarbonate polyols obtained by reacting ethylene carbonate with a polyhydric alcohol and subsequently reacting the obtained reaction mixture with an organic dicarboxylic acid, and polycarbonate polyols obtained by transesterification reaction of a polyhydroxyl compound and aryl carbonate. These may be used either alone or as a mixture of two or more kinds. 
     In addition to the above-described high-molecular-weight polyol components, low-molecular-weight polyol components such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, 1,4-bis(2-hydroxyethoxy)benzene, trimethylolpropane, glycerin, 1,2,6-hexanetriol, pentaerythritol, tetramethylolcyclohexane, methylglucoside, sorbitol, mannitol, dulcitol, sucrose, 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol, and triethanolamine, and low-molecular-weight polyamine components such as ethylenediamine, tolylenediamine, diphenylmethanediamine, and diethylenetriamine may be used as the compound containing active hydrogen. These may be used either alone or as a mixture of two or more kinds. Further, polyamines typified by 4,4′-methylenebis(o-chloroaniline) (MOCA), 2,6-dichloro-p-phenylenediamine, 4,4′-methylenebis(2,3-dichloroaniline), 3,5-bis(methylthio)-2,4-toluenediamine, 3,5-bis(methylthio)-2,6-toluenediamine, 3,5-diethyltoluene-2,4-diamine, 3,5-diethyltoluene-2,6-diamine, trimethyleneglycol-di-p-aminobenzoate, polytetramethyleneoxide-di-p-aminobenzoate, 1,2-bis(2-aminophenylthio)ethane, 4,4′-diamino-3,3′-diethyl-5,5′-dimethyldiphenylmethane, N,N′-di-sec-butyl-4,4′-diaminodiphenylmethane, 4,4′-diamino-3,3′-diethyldiphenylmethane, 4,4′-diamino-3,3′-diethyl-5,5′-dimethyldiphenylmethane, 4,4′-diamino-3,3′-diisopropyl-5,5′-dimethyldiphenylmethane, 4,4′-diamino-3,3′,5,5′-tetraethyldiphenylmethane, 4,4′-diamino-3,3′,5,5′-tetraisopropyldiphenylmethane, m-xylylenediamine, N,N′-di-sec-butyl-p-phenylenediamine, m-phenylenediamine, and p-xylylenediamine may also be mixed. Preferable compounds containing active hydrogen are polytetramethylene glycol, polypropylene glycol, a copolymer of propylene oxide and ethylene oxide, and 3-methyl-1,5-pentane adipate. More preferable compounds containing active hydrogen are polypropylene glycol and a copolymer of propylene oxide and ethylene oxide. 
     A preferable combination of the isocyanate component and the compound containing active hydrogen is a combination of one kind or two more kinds of 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, and 4,4′-diphenylmethane diisocyanate as the isocyanate component and one kind or two more kinds of polytetramethylene glycol, polypropylene glycol, a copolymer of propylene oxide and ethylene oxide, and 3-methyl-1,5-pentane adipate as the compound containing active hydrogen. A more preferable combination is a combination of 2,4-toluene diisocyanate and/or 2,6-toluene diisocyanate as the isocyanate component and polypropylene glycol and/or a copolymer of propylene oxide and ethylene oxide as the compound containing active hydrogen. 
     The polymer matrix layer  3  may be a foamed body containing a dispersed filler and bubbles. A general resin foam can be used as the foamed body. However, in view of the characteristics such as compression set, it is preferable to use a thermosetting resin foam. Examples of the thermosetting resin foam include polyurethane resin foam and silicone resin foam. Among these, polyurethane resin foam is preferable. The isocyanate component and the compound containing active hydrogen that have been listed above can be used for the polyurethane resin foam. 
     The amount of the magnetic filler in the magnetic elastomer is preferably 1 to 450 parts by weight, more preferably 2 to 400 parts by weight, relative to 100 parts by weight of the elastomer component. When the amount is smaller than  1  part by weight, detection of change in the magnetic field tends to be difficult. When the amount exceeds 450 parts by weight, the magnetic elastomer itself may in some cases become brittle. 
     For the purpose of preventing rusts of the magnetic filler or the like, a sealing material for sealing the polymer matrix layer  3  may be provided to such a degree that the flexibility of the polymer matrix layer  3  is not deteriorated. A thermoplastic resin, a thermosetting resin, or a mixture of these may be used as the sealing material. The thermoplastic resin may be, for example, styrene-based thermoplastic elastomer, polyolefin-based thermoplastic elastomer, polyurethane-based thermoplastic elastomer, polyester-based thermoplastic elastomer, polyamide-based thermoplastic elastomer, polybutadiene-based thermoplastic elastomer, polyisoprene-based thermoplastic elastomer, fluorine-based thermoplastic elastomer, ethylene.ethyl acrylate copolymer, ethylene.vinyl acetate copolymer, polyvinyl chloride, polyvinylidene chloride, chlorinated polyethylene, fluororesin, polyamide, polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polybutadiene, or the like. The thermosetting resin may be, for example, diene-based synthetic rubbers such as polyisoprene rubber, polybutadiene rubber, styrene.butadiene rubber, polychloroprene rubber, and acrylonitrile.butadiene rubber, non-diene-based rubbers such as ethylene.propylene rubber, ethylene.propylene.diene rubber, butyl rubber, acrylic rubber, polyurethane rubber, fluorine-containing rubber, silicone rubber, and epichlorohydrin rubber, natural rubbers, and thermosetting resins such as polyurethane resin, silicone resin, epoxy resin, or the like. These films may be stacked or may be a film including a metal foil such as aluminum foil or a metal vapor-deposition film including a metal vapor-deposited on the above-described film. 
     The polymer matrix layer  3  may be one in which the filler is unevenly distributed in the thickness direction thereof. For example, the polymer matrix layer  3  may be made of two layers, that is, a region on a one side containing a relatively larger amount of the filler and a region on the other side containing a relatively smaller amount of the filler. In the region that is located on the one side and that contains a larger amount of the filler, a large change in the external field results from small deformation of the polymer matrix layer  3 , so that the sensor sensitivity to a low internal pressure can be enhanced. Also, a region that is located on the other side and that contains a relatively smaller amount of the filler is comparatively flexible and can be easily moved. Therefore, by attaching this region located on the other side, the polymer matrix layer  3  (particularly the region located on the one side) becomes capable of being easily deformed. 
     The filler uneven distribution ratio in the region on the one side preferably exceeds 50, and is more preferably 60 or more, still more preferably 70 or more. In this case, the filler uneven distribution ratio in the region on the other side is less than 50. The filler uneven distribution ratio in the region on the one side is 100 at the maximum, and the filler uneven distribution ratio in the region on the other side is 0 at the minimum. Therefore, it is possible to adopt a stacked body structure including an elastomer layer that contains a filler and an elastomer layer that does not contain the filler. For uneven distribution of the filler, it is possible to use a method in which, after the filler is introduced into the elastomer component, the resultant is left to stand still at room temperature or at a predetermined temperature, so as to attain natural settling of the filler by the weight of the filler. By changing the temperature or time for leaving the filler to stand still, the filler uneven distribution ratio can be adjusted. The filler may be distributed unevenly by using a physical force such as a centrifugal force or a magnetic force. Alternatively, the polymer matrix layer may be composed of a stacked body made of a plurality of layers having different contents of the filler. 
     The filler uneven distribution ratio is measured by the following method. That is, the cross-section of the polymer matrix layer is observed at a magnification of 100 times by using a scanning electron microscope−energy dispersive X-ray analyzer (SEM-EDS). The existence amount of the metal element inherent in the filler (for example, Fe element in the magnetic filler of the present embodiment) is determined by element analysis for the whole region in the thickness direction of the cross-section and for each of the two regions obtained by equally dividing the cross-section into two in the thickness direction. With respect to this existence amount, the ratio of the existence amount in the region on the one side relative to the existence amount in the whole region in the thickness direction is calculated, and this is determined as the filler uneven distribution ratio in the region on the one side. The filler uneven distribution ratio in the region on the other side can be determined in the same manner. 
     The region on the other side containing a relatively smaller amount of the filler may have a structure formed of a foamed body containing bubbles. This allows that the polymer matrix layer  3  can be more easily deformed, so that the sensor sensitivity is enhanced. Also, the region on the one side as well as the region on the other side may be formed of a foamed body. In this case, the whole of the polymer matrix layer  3  is made of a foamed body. The polymer matrix layer in which at least a part thereof in the thickness direction is made of a foamed body may be composed of a stacked body including a plurality of layers (for example, a non-foamed layer that contains a filler and a foamed layer that does not contain a filler). 
     As the detection unit  4  for detecting change in the magnetic field, a magnetic resistance element, a Hall element, an inductor, an Mi element, a flux gate sensor, or the like can be used, for example. As the magnetic resistance element, a semiconductor compound magnetic resistance element, an anisotropic magnetic resistance element (AMR), a gigantic magnetic resistance element (GMR), and a tunnel magnetic resistance element (TMR) may be Mentioned as examples. Among these, a Hall element is preferable, and this is because the Hall element has high sensitivity in a wide range, and is useful as the detection unit  4 . As the Hall element, EQ-430L manufactured by Asahi Kasei Microdevices Corporation can be used, for example. 
     A secondary battery  2  in which gas swelling has proceeded may reach troubles such as ignition or rupture, so that the present embodiment is constructed in such a manner that the charging/discharging is shut off when an expansion amount at the time of deformation of the secondary battery  2  is above or equal to a predetermined level. Specifically, signals obtained by detection with the detection sensor  5  are transmitted to the controlling device  6  and, when a change in the external field above or equal to a set value is detected by the detection sensor  5 , the controlling device  6  sends signals to the switching circuit  7  and shuts off the electric current from the power generation device (or charging device)  8 , so as to bring the system into a state in which charging/discharging of the battery module  1  is shut off. This allows that the troubles caused by gas swelling can be prevented. 
     In the above-described embodiment, an example in which the secondary battery is a lithium ion secondary battery has been shown; however, the present invention is not limited to this alone. The secondary battery that is put to use is not limited to a non-aqueous electrolyte secondary battery such as a lithium ion battery, but may be an aqueous electrolyte secondary battery such as a nickel hydrogen battery. 
     In the embodiments, an example has been shown in which change in the magnetic field accompanying the deformation of the polymer matrix layer is detected with the detection unit; however, it is possible to adopt a construction in which change in another external field is detected. For example, a construction may be considered in which the polymer matrix layer contains an electroconductive filler such as metal particles, carbon black, or carbon nanotubes as the filler, and the detection unit detects change in the electric field (change in the resistance and the dielectric constant) as the external field. 
     The present invention is not limited to the embodiment mentioned above, but can be improved and modified variously within the scope of the present invention. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           1  Battery module 
           2  Sealed secondary battery 
           3  Polymer matrix layer 
           4  Detection unit 
           5  Detection sensor 
           6  Controlling device 
           7  Switching circuit 
           8  Power generation device or charging device 
           21  Outer casing 
           22  Electrode group 
           23  Positive electrode 
           24  Negative electrode 
           25  Separator 
         L 1  First curved line 
         L 2  Second curved line 
         P 1  Stage change point 
         P 2  Stage change point