Abstract:
A storage battery system includes: a secondary battery; a holding member comprising a flow channel of a cooling medium for cooling the secondary battery, which holds the secondary battery; a medium temperature measuring unit that measures temperatures T 1  and T 2  of the cooling medium at two locations in the flow channel; a battery temperature measuring unit that measures a temperature T 3  of the secondary battery; and a judging unit that repetitively calculates a coefficient α and that judges a cooling performance of the secondary battery based on a variation of the coefficient α, the coefficient α being defined as (T 3 −T 1 )/(T 2 −T 1 ), (T 3 −T 2 )/(T 2 −T 1 ), or (T 3 −T 2 )/(T 3 −T 1 ).

Description:
INCORPORATION BY REFERENCE 
     The disclosure of the following priority application is herein incorporated by reference: 
     Japanese Patent Application No. 2010-102610 filed Apr. 27, 2010. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a storage battery system using a secondary battery. 
     2. Description of Related Art 
     Performance and life time of a secondary battery strongly depend on the temperature of the secondary battery. Since the likelihood of deterioration increases at high temperatures, with a secondary battery system that requires a large current such as that of a hybrid vehicle, a cooling performance of a cooling system during charge and discharge must be accurately monitored. Cooling methods of secondary batteries can be divided into two methods, namely, an air-cooling method in which air is supplied by a fan to cool a secondary battery and a liquid-cooling method in which a liquid cooling medium is circulated to a secondary battery to transfer heat to the outside. Conceivable causes of degradation of a cooling performance in an air-cooling method include fan failure and accumulation of dust and extraneous matter on the secondary battery or in a draft air duct. Conceivable causes of degradation of a cooling performance in a liquid-cooling method include a failure of a pump that circulates a cooling medium, a change in components of the cooling medium, and deterioration due to metal corrosion in a cooling medium flow channel. As a technique for judging an abnormality in the cooling performance of such a secondary battery cooling system, Japanese Patent Laid-Open No. 2001-313092 discloses a storage battery system configured so as to estimate a temperature of a secondary battery from a charge and discharge current and a cooling state of the secondary battery, and to judge an abnormality in cooling performance by comparing the estimated temperature with a measured temperature of the secondary battery. 
     SUMMARY OF THE INVENTION 
     However, with the conventional storage battery system described above, since a battery temperature is estimated based on an amount of heat generation due to a charge and discharge current of a secondary battery, ambient temperature, an operating mode of a fan, a history of estimated temperatures, and the like, there is a problem in that an estimate contains error and whether or not the cooling performance of a battery cooling system is normal cannot be judged accurately. 
     According to the 1st aspect of the present invention, a storage battery system comprises: a secondary battery; a holding member comprising a flow channel of a cooling medium for cooling the secondary battery, which holds the secondary battery; a medium temperature measuring unit that measures temperatures T 1  and T 2  of the cooling medium at two locations in the flow channel; a battery temperature measuring unit that measures a temperature T 3  of the secondary battery; and a judging unit that repetitively calculates a coefficient α and that judges a cooling performance of the secondary battery based on a variation of the coefficient α, the coefficient α being defined as (T 3 −T 1 )/(T 2 −T 1 ), (T 3 −T 2 )/(T 2 −T 1 ), or (T 3 −T 2 )/(T 3 −T 1 ). 
     According to the 2nd aspect of the present invention, the judgment unit of a storage battery system according to the 1st aspect may judge that the cooling performance of the secondary battery is abnormal when a difference between a currently calculated value and a previously calculated value of the coefficient α exceeds a range set in advance. 
     According to the 3rd aspect of the present invention, the medium temperature measuring unit of a storage battery system according to the 1st aspect may measure temperatures T 1  and T 2  at an entry-side and an exit-side of the holding member of the flow channel passing through the holding member. 
     According to the 4th aspect of the present invention, it is preferred that the holding member of a storage battery system according to the 1st aspect holds the secondary battery via a thermal conductive sheet. 
     According to the 5th aspect of the present invention, it is preferred that in a storage battery system according to the 4th aspect, the judgment unit judges that an abnormality of the cooling performance is caused by a failure in the thermal conductive sheet when a difference between a currently calculated value and a previously calculated value of the coefficient α exceeds a range set in advance and the currently calculated value is greater than the previously calculated value. 
     According to the 6th aspect of the present invention, it is preferred that in a storage battery system according to the 1st aspect, the judgment unit judges that an abnormality of the cooling performance is caused by a failure in a cooling system of the secondary battery due to the cooling medium when a difference between a currently calculated value and a previously calculated value of the coefficient α exceeds a range set in advance and the currently calculated value is smaller than the previously calculated value. 
     According to the 7th aspect of the present invention, the judgment unit of a storage battery system according to the 6th aspect may increase the cooling performance of the cooling system when a failure of the cooling system of the secondary battery due to the cooling medium is judged. 
     According to the present invention, a cooling performance of a battery cooling system can be judged accurately. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a liquid-cooled storage battery system according to an embodiment; 
         FIG. 2  is a diagram illustrating a partial configuration of a battery module illustrated in  FIG. 1 ; 
         FIG. 3  is a side view illustrating, in enlargement, a portion of a single secondary battery of the battery module illustrated in  FIG. 2 ; 
         FIG. 4  is a top view illustrating, in enlargement, a portion of a single secondary battery of the battery module illustrated in  FIG. 2 ; 
         FIG. 5  is a diagram illustrating a thermal equivalent circuit around the single secondary battery illustrated in  FIGS. 3 and 4 ; 
         FIG. 6  is a diagram illustrating relationships of a coefficient α with respect to a cooling medium flow rate when an amount of heat generated per one secondary battery is set to 7 W and 20 W; 
         FIG. 7  is a diagram illustrating relationships of a coefficient α with respect to a cooling medium flow rate in two types of thermal conductive sheets having different thermal conductive properties; 
         FIG. 8  is a diagram illustrating a configuration of a control system of a storage battery system according to an embodiment; 
         FIG. 9  is a flow chart illustrating an example of a process for performing measurement and judgment of cooling performance; 
         FIG. 10  is a flow chart illustrating an example of a process when cooling performance is judged to be abnormal; 
         FIG. 11  is a flow chart illustrating another example of a process when cooling performance is judged to be abnormal; and 
         FIG. 12  is a perspective view illustrating a battery module of an air-cooled storage battery system according to an embodiment. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a configuration of a liquid-cooled storage battery system according to an embodiment. A storage battery system  1  according to the embodiment comprises a battery module  2 , a cooling system  3 , a cooling medium  4 , and the like. The cooling system  3  circulates the cooling medium  4  to the battery module  2 . The cooling medium  4  absorbs heat generated by the battery module  2  and transfers the heat to the outside of the battery module  2 . For example, water, an antifreeze liquid, or the like can be used as the cooling medium  4 . The liquid-cooling type cooling system  3  comprises a circulating pump  5 , a tank  6 , a heat exchanger  7 , and the like. The circulating pump  5  supplies kinetic energy to the cooling medium  4  for sending the cooling medium  4  to the battery module  2 . The tank  6  stores the cooling medium  4  to be circulated and supplies the cooling medium  4  to the circulating pump  5 . The heat exchanger  7  cools the cooling medium  4  that has been reclaimed after absorbing the heat of the battery module  2  by heat exchange with the outside, and stores the cooling medium  4  in the tank  6 . 
       FIG. 2  illustrates a partial configuration of the battery module  2 . In the battery module  2 , a plurality of secondary batteries  8  is covered by a thermal conductive sheet  9  and held by a cooling block  10 . While  FIG. 2  illustrates a part of the battery module  2  in which six secondary batteries  8  are held by the cooling block  10 , a large number of secondary batteries  8  are further built into the battery module  2  by similar structures. A cooling pipe  11  for passing the cooling medium  4  penetrates the inside of the cooling block  10 . Moreover, while  FIG. 2  illustrates an example in which the cooling pipe  11  splits into two branches at an entry-side of the inside of the cooling block  10  and converges at an exit side, a configuration and a method of piping of the cooling pipe  11  inside the cooling block  10  are not limited to those illustrated in  FIG. 2 . 
       FIGS. 3 and 4  illustrate an enlarged portion of a single secondary battery of the battery module  2 , wherein  FIG. 3  is a side view and  FIG. 4  is a top view. The cylindrical secondary battery  8  is covered by the thermal conductive sheet  9  and held by the cooling block  10 . The cooling pipe  11  for passing the cooling medium  4  is laid out inside the cooling block  10 . The shape of the secondary battery  8  is not limited to a cylindrical shape and may be another shape such as a square or laminated shape. 
     Heat generated by the secondary battery  8  during charge and discharge is primarily transferred via the thermal conductive sheet  9  surrounding a circumference of a side surface of the secondary battery  8  and via the cooling block  10 , and is absorbed by the cooling medium  4  that flows through the cooling pipe  11 . The thermal conductive sheet  9  has a thickness of around 1 mm, features superior insulating performance and thermal conductivity in a thickness direction, has high sheet surface adhesion, and facilitates heat transfer while insulating the secondary battery  8  and the cooling block  10  from each other. Moreover, with a secondary battery structured such that a surface of the secondary battery does not have a potential and the battery surface may come into direct contact with the cooling block  10  without causing any problems, the insulating performance of the thermal conductive sheet  9  is not a requisite, and a structure may be adopted in which the secondary battery  8  is brought into direct contact with and held by the cooling block  10  without using the thermal conductive sheet  9 . The cooling block  10  is formed of a metal with high thermal conductivity such as aluminum. In addition, the cooling pipe  11  is also metallic and penetrates the inside of the cooling block  10 . 
     Temperature sensors  12   a  and  12   b  are respectively installed on a cooling block entry-side pipe surface and a cooling block exit-side pipe surface of the cooling pipe  11  to measure an entry-side cooling medium temperature Tin and an exit-side cooling medium temperature Tout. It is to be noted that while the installation location of the temperature sensor  12   a  for measuring the entry-side cooling medium temperature Tin is desirably near a block inlet of the cooling pipe  11  that penetrates the inside of the cooling block  10 , a location near the block entry-side is to suffice. In a similar manner, while the installation location of the temperature sensor  12   b  for measuring the exit-side cooling medium temperature Tout is desirably near a block outlet of the cooling pipe  11  that penetrates the inside of the cooling block  10 , a location near the block exit-side is to suffice. It is to be noted that in the battery module  2  where a plurality of secondary batteries  8  is held by a single cooling block  10  as illustrated in  FIG. 2 , the temperature sensors  12   a  and  12   b  may be installed near the block entry-side and the block exit-side of the cooling pipe  11  that penetrates the cooling block  10 . 
     In addition, a temperature sensor  12   c  is installed on a surface of the secondary battery  8  to measure a surface temperature Ts of the secondary battery  8 . While the embodiment illustrates an example in which the surface temperature Ts of the secondary battery  8  is assumed to be a battery temperature, the battery temperature is not limited to the surface temperature of the secondary battery  8  and may instead be an internal temperature of the secondary battery  8 . 
       FIG. 5  is a diagram illustrating a thermal equivalent circuit around the single secondary battery illustrated in  FIGS. 3 and 4 . In  FIG. 5 , Q 0  denotes an amount of heat generated by the secondary battery  8 , Rc denotes an internal thermal resistance of the secondary battery  8 , Ra denotes an atmosphere thermal resistance, Rs denotes a thermal resistance of the thermal conductive sheet  9 , Rb denotes a thermal resistance of the cooling block  10 , and Rp denotes a thermal resistance of the cooling medium  4 . In addition, as temperatures of each node of the thermal equivalent circuit, Ta denotes an ambient temperature, Tc denotes an internal temperature of the secondary battery  8 , Ts denotes a surface temperature of the secondary battery  8 , Tin denotes a block entry-side temperature of the cooling medium  4 , and Tout denotes a block exit-side temperature of the cooling medium  4 . Furthermore, an amount of heat absorbed by cooling medium Q 1  and amount of heat discharged into atmosphere Q 2  indicating a flow of heat are defined. 
     The respective parameters of the thermal equivalent circuit illustrated in  FIG. 5  will be described in detail. The battery internal thermal resistance Rc defines a thermal resistance between the inside and the surface of the secondary battery  8 , and the atmosphere thermal resistance Ra defines a thermal resistance between the surface of the secondary battery  8  and surrounding air. The ambient temperature Ta is a temperature of air surrounding the secondary battery  8  and is treated as a voltage source in an electric circuit. In addition, the thermal conductive sheet thermal resistance Rs defines a thermal resistance from the surface of the secondary battery  8  via the thermal conductive sheet  9  to the surface of the cooling block  10 , and the cooling block thermal resistance Rb defines a thermal resistance from the surface of the cooling block  10  that is in contact with the thermal conductive sheet  9 , through the inside of the cooling block  10  and a wall of the cooling pipe  11 , and to the cooling medium  4 . The cooling medium thermal resistance Rp defines a thermal resistance of the cooling medium  4  until the cooling medium  4  releases the heat absorbed in a cooling medium circulating pathway constituted by the cooling pipe  11  and the cooling system  3 . 
     The amount of heat generated by battery Q 0  is the amount of heat generated by the secondary battery  8  during charge and discharge, and is treated as a current source in an electric current. The cooling medium exit-side temperature Tout is a temperature of the cooling medium  4  on the cooling block exit-side of the cooling pipe  11  after the cooling medium  4  has absorbed heat. In addition, the cooling medium entry-side temperature Tin is a temperature of the cooling medium  4  cooled by the cooling system  3  and flowing into a cooling block entry-side of the pipe  11 . The cooling medium entry-side temperature Tin can be arranged so as to be variable according to operation control of the heat exchanger  7  of the cooling system  3  and, in this case, is treated as a voltage source of in electric circuit. 
     The amount of heat generated by battery Q 0  first flows via the battery internal thermal resistance Rc, and subsequently splits into the amount of heat absorbed by cooling medium Q 1  that is an amount of heat absorbed by the cooling medium  4  and the amount of heat discharged into atmosphere Q 2  that is an amount of heat discharged into air. In other words, the amount of heat generated by battery Q 0  can be expressed as
 
 Q 0= Q 1+ Q 2  (1)
 
     As is obvious from the thermal equivalent circuit illustrated in  FIG. 5 , the amount of heat absorbed by cooling medium Q 1  can be expressed as
 
 Q 1=( Ts−T in)/( Rs+Rb+Rp )  (2)
 
or
 
 Q 1=( Ts−T out)/( Rs+Rb )  (3)
 
or
 
 Q 1=( T out− T in)/ Rp   (4)
 
     In addition, the amount of heat discharged into atmosphere Q 2  can be expressed as
 
 Q 2=( Ts−Ta )/ Ra   (5).
 
     Next, a method of judging deterioration of a cooling performance will be described. The superiority or inferiority of the cooling performance is judged based on whether or not the amount of heat generated by battery Q 0  is being efficiently transferred by the cooling medium  4 . In other words, it is conceivable that the higher the ratio of the amount of heat generated by battery Q 0  being transferred as the amount of heat absorbed by cooling medium Q 1 , the greater the cooling performance. As illustrated in  FIG. 5 , the battery internal thermal resistance Rc, the thermal conductive sheet thermal resistance Rs, the cooling block thermal resistance Rb, and the cooling medium thermal resistance Rp exist in a pathway through which the amount of heat absorbed by cooling medium Q 1  flows. Conceivably, an increase in these thermal resistances slows down the flow of the amount of heat absorbed by cooling medium Q 1  and causes the cooling performance to deteriorate. 
     Among the thermal resistances in the pathway through which the amount of heat absorbed by cooling medium Q 1  flows, the thermal conductive sheet thermal resistance Rs and the cooling medium thermal resistance Rp are susceptible to deterioration or variation over time. With the thermal conductive sheet thermal resistance Rs, an alteration of sheet material or a decrease in adhesion of the sheet surface makes it difficult for heat to be transferred from the surface of the secondary battery  8  to the cooling block  10 . In addition, with the cooling medium thermal resistance Rp, an increase in the thermal resistances can conceivably occur due to contamination of impurities into the cooling medium  4  or by a component change of the cooling medium  4 , an abnormal operation of the circulating pump  5 , an increase of pressure loss caused by corrosion or a form change on a surface that comes into contact with the cooling medium  4  of the pipe  11  or the like constituting the flow channel of the cooling medium  4 , and a reduction in a flow rate of the cooling medium  4  caused by these above. 
     In consideration thereof, with the storage battery system  1  according to an embodiment, in order to detect variations in the thermal conductive sheet thermal resistance Rs and the cooling medium thermal resistance Rp, temperature sensors  12   a ,  12   b , and  12   c  are installed on the battery module  2  as illustrated in  FIGS. 3 and 4  to measure the cooling medium entry-side temperature Tin, the cooling medium exit-side temperature Tout, and the battery surface temperature Ts. Furthermore, the right sides of expressions (2) and (4) that represent the amount of heat absorbed by cooling medium Q 1  are connected by an equal sign and modified to define the coefficient α as represented by expression (6) below.
 
α=( Ts−T in)/( T out− T in)=( Rs+Rb+Rp )/ Rp   (6)
 
     The coefficient α can be calculated based on measured values of the cooling medium entry-side temperature Tin, the cooling medium exit-side temperature Tout, and the battery surface temperature Ts. Since coefficient α=(Rs+Rb+Rp)/Rp, an increase in the thermal conductive sheet thermal resistance Rs due to an abnormality of the thermal conductive sheet  9  is manifested as an increase of the coefficient α. In addition, an increase in the cooling medium thermal resistance Rp due to an abnormality of the cooling medium circulating system is manifested as a decrease of the coefficient α. 
       FIGS. 6 and 7  illustrate relationships of the coefficient α with respect to cooling medium flow rate. The relationships have been created from results of a thermal analysis by simulation performed on a liquid-cooled battery module  2  mounted with 72 secondary batteries  8 . In addition, surface temperature information of a single secondary battery  8  at a representative position has been adopted as the battery surface temperature Ts. Referring to the drawings, variations in the coefficient α due to abnormalities of the thermal conductive sheet  9  and the cooling medium circulating system will be described. 
       FIG. 6  illustrates relationships of the coefficient α with respect to cooling medium flow rate when an amount of heat generation per one secondary battery  8  is set to 7 W and 20 W. These relationships correspond to cases where the amount of heat generation Q 0  of the secondary battery  8  is varied. As represented by the expression (6) above, the coefficient α does not include the amount of heat generated by battery Q 0  and therefore is not dependent on the amount of heat generated by battery Q 0 , and remains approximately constant even if the amount of heat generated by battery Q 0  varies. However, the coefficient α tends to increase as the flow rate increases. Therefore, when the cooling medium flow rate decreases due to an abnormality in the cooling medium circulating system, the coefficient α is expected to decrease. 
       FIG. 7  illustrates relationships between cooling medium flow rate and the coefficient α with respect to two types of thermal conductive sheets  9  having different thermal conductive properties. A thermal conductive sheet A represents a thermal analysis simulation performed under conditions in which a thermal conductivity of 1 W/m·K and a sheet thickness of 1.0 mm are respectively set, and a thermal conductive sheet B represents a thermal analysis simulation performed under conditions in which a thermal conductivity of 3 W/m·K and a sheet thickness of 0.1 mm are respectively set. Under these conditions, the thermal resistance Rs of the thermal conductive sheet  9  is smaller for the thermal conductive sheet B than the thermal conductive sheet A. When comparing both thermal conductive sheets at the same flow rate of the cooling medium  4 , the coefficient α is greater for the thermal conductive sheet A than the thermal conductive sheet B. In other words, in a case where the thermal resistance Rs increases due to an abnormality of the thermal conductive sheet  9 , the coefficient α is expected to increase. 
     As described above, by installing the temperature sensors  12   a ,  12   b , and  12   c  on the battery module  2  to measure the cooling medium entry-side temperature Tin, the cooling medium exit-side temperature Tout, and the battery surface temperature Ts, and calculating the coefficient α and comparing the coefficient α with a coefficient α in previous history, a state of the cooling performance of the battery module  2  can be accurately detected and the cooling performance of the battery module  2  can be accurately judged. In addition, according to an increase/decrease of the calculated coefficient α, whether a reduction in cooling performance is due to the cooling system  3  by the cooling medium  4  or to an abnormality of the thermal conductive sheet  9  can be identified. Furthermore, since the coefficient α is not dependent on the amount of heat generation Q 0  of the secondary battery  8 , the cooling performance can be detected and judged regardless of the load or operating state of the storage battery system  1 . 
     In the embodiment described above, as represented by the expression (6), an example has been illustrated where the coefficient α is defined as a ratio between (Ts−Tin) and (Tout−Tin). However, as represented by the expression (3) above, since the amount of heat absorbed by cooling medium Q 1  can also be expressed using (Ts−Tout), two coefficients α′ and α″ expressed the expressions (7) and (8) as follows can be obtained, and even when the aforementioned coefficient α is replaced with the coefficients α′ and α″, an index may be obtained for detecting an increase in the thermal conductive sheet thermal resistance Rs or the cooling medium thermal resistance Rp.
 
α′=( Ts−T out)/( Ts−T in)=( Rs+Rb )/( Rs+Rb+Rp )  (7)
 
and
 
α″=( Ts−T out)/( T out− T in)=( Rs+Rb )/ Rp   (8)
 
     In addition, in the above embodiment, an example in which the thermal conductive sheet  9  is sandwiched between the secondary battery  8  and the cooling block  10  has been described as illustrated in  FIGS. 3 to 5 . However, in a case where the secondary battery  8  is structured such that the surface thereof does not have a potential, the secondary battery  8  and the cooling block  10  may be directly bonded, metal against metal, to each other in a thermal bonding without sandwiching the thermal conductive sheet  9 . For example, conceivably, the secondary battery  8  and the cooling block  10  may be welded to each other or mechanically connected with each other by fastening using a bracket or a screw. 
     In a case where the secondary battery  8  is directly held by the cooling block  10 , the thermal conductive sheet thermal resistance Rs of the thermal equivalent circuit illustrated in  FIG. 5  can be omitted, whereby the coefficient α may be expressed as
 
α=( Ts−T in)/( T out− T in)=( Rb+Rp )/ Rp   (9)
 
     Accordingly, even when the thermal conductive sheet  9  is not present, in the same manner as the embodiment described above, an increase of the cooling medium thermal resistance Rp is manifested as a decrease in the coefficient α and an abnormality in the cooling medium circulating system can be detected. 
     Next, a configuration of a control system of the storage battery system  1  according to an embodiment will be described with reference to  FIG. 8 . Besides the components of the cooling system of the storage battery system  1  illustrated in  FIG. 1 , the control system comprises a controller  13 , a memory  14 , an indicator  15 , a pump control signal  16 , a heat exchanger control signal  17 , indication information  18 , a cooling medium entry-side temperature signal STin, a cooling medium exit-side temperature signal STout, a battery surface temperature signal STs, and the like. The controller  13  is constituted by a microcomputer including an interface such as an AD converter and controls arithmetic processing and input/output of information and signals. The memory  14  stores an initial value and historical values of the coefficient α. The indicator  15  is a device such as a display, a lamp, a speaker, or the like that visually or auditorily informs a state of the cooling performance. 
     The pump control signal  16  is a signal for controlling a motor rotation speed or the like of the circulating pump  5 , and the heat exchanger control signal  17  is a signal for controlling operations of the heat exchanger  7 . In addition, indication information  18  is information to be indicated on the indicator  15 . The cooling medium entry-side temperature signal STin, the cooling medium exit-side temperature signal STout, and the battery surface temperature signal STs are respectively outputs of the temperature sensors  12   a ,  12   b , and  12   c  in the battery module  2  illustrated in  FIGS. 3 and 4 . 
     A process performed by the controller  13  to measure and judge cooling performance will be described with reference to  FIGS. 9 to 11 .  FIG. 9  is a flow chart illustrating an example of a process for measuring and judging the cooling performance. In step  1 , the controller  13  retrieves the cooling medium entry-side temperature signal STin, the cooling medium exit-side temperature signal STout, and the battery surface temperature signal STs from the sensors  12   a ,  12   b , and  12   c , and reads a voltage value or a current value of these signals. In a next step  2 , the controller  13  obtains a cooling medium entry-side temperature Tin, a cooling medium exit-side temperature Tout, and a battery surface temperature Ts based on the signals STin, STout, and STs from the sensors  12   a ,  12   b , and  12   c . Furthermore, in step  3 , the controller  13  calculates the coefficient α=(Ts−Tin)/(Tout−Tin) by using the cooling medium entry-side temperature Tin, the cooling medium exit-side temperature Tout, and the battery surface temperature Ts. 
     In step  4 , the controller  13  reads a coefficient historical value αi of the coefficient α stored in the memory  14 , and in step  5 , the controller  13  compares the coefficient α with the coefficient historical value αi and obtains a difference Δα. In step  6 , the controller  13  sets a flag “Flag” that denotes an abnormal state of the cooling performance to 0. In step  7 , the controller  13  reads an upper limit threshold Th set in advance from the memory  14  and compares the difference Δα with the upper limit threshold Th. When the difference Δα is equal to or lower than the upper limit threshold Th, the controller  13  judges that the cooling performance is normal and proceeds to step  9 . Otherwise, the controller  13  judges that the cooling performance is abnormal and proceeds to step  8 . When the controller  13  judges that the cooling performance is abnormal, the controller  13  sets the flag “Flag” to 1 in step  8  and proceeds to step  11  in  FIG. 10  (or step  11  in  FIG. 11 ). On the other hand, when the controller  13  judges that the cooling performance is normal, in step  9 , the controller  13  reads a lower limit threshold Tl set in advance from the memory  14  and compares the difference Δα with the lower limit threshold Tl. When the difference Δα is equal to or higher than the lower limit threshold Tl, the controller  13  judges that the cooling performance is normal and ends the judgment process. Otherwise, the controller  13  judges that the cooling performance is abnormal and proceeds to step  11  in  FIG. 10  (or step  11  in  FIG. 11 ). 
       FIG. 10  is a flow chart illustrating an example of a process when the cooling performance is judged to be abnormal by the controller  13 . In step  11 , the controller  13  sets indication information  18  to be outputted to the indicator  15  according to the value of the flag “Flag” denoting an abnormal state of cooling performance. When flag “Flag”=0, the controller  13  proceeds to step  12  to indicate an abnormality “A” on the indicator  15 , and when flag “Flag”=1, the controller  13  proceeds to step  13  to indicate an abnormality “B” on the indicator  15 . An abnormality “A” indicates that the coefficient α has become abnormally smaller than the coefficient historical value αi using an error code or a message, an indication state of the lamp, or the like. On the other hand, an abnormality “B” indicates that the coefficient α has become abnormally greater than a coefficient initial value α 0  using an error code or a message, an indication state of the lamp, or the like by a method that differs from that of the abnormality “A”. An abnormality of the cooling medium circulating system is suspected in the case of the abnormality “A”, and a deterioration of the thermal conductive sheet is suspected in the case of the abnormality “B”. 
       FIG. 11  is a flow chart illustrating another example of a process when the cooling performance is judged to be abnormal by the controller  13 . In step  11 , the controller  13  sets indication information  18  to be outputted to the indicator  15  according to the value of the flag “Flag” denoting an abnormal state of cooling performance. When flag “Flag”=0, the controller  13  proceeds to step  12  to indicate the abnormality “A” on the indicator  15 , and when flag “Flag”=1, the controller  13  proceeds to step  13  to indicate the abnormality “B” on the indicator  15 . Since an abnormality of the cooling medium circulating system is suspected in the case of the abnormality “A”, depending on the degree of the abnormality, changing operations of the circulating pump  5  or the heat exchanger  7  may suffice as a countermeasure. Therefore, in step  14 , the controller  13  increases the motor rotation speed of the circulating pump  5  and controls the pump so as to increase the flow rate of the cooling medium  4 . Furthermore, in step  15 , the controller  13  controls the heat exchanger so as to lower a target temperature of the cooling medium  4  that is cooled by the heat exchanger  7 . Subsequently, the controller  13  returns to step  1  in  FIG. 9  to once again execute a calculating process of the coefficient α so that the difference Δα eventually falls between the upper limit threshold Th and the lower limit threshold Tl. 
     In the cooling performance judgment process illustrated in  FIG. 9 , an example has been presented in which the controller  13  compares a measured coefficient α with a coefficient historical value αi and judges the cooling performance based on the comparison result. The controller  13  stores the coefficient α at predetermined time intervals from an initial value as coefficient historical values αi in the memory  14 , and executes a process for detecting a tendency time-dependent variation of the coefficient historical values αi. By this detection process, the controller  13  judges whether a measured coefficient α is consistent with the variation tendency of the coefficient historical values αi or is an abrupt variation. When the coefficient α has varied abruptly, an occurrence of some kind of abnormality is presumed in cooling performance, whereby a cause of the abnormality can be identified as described above from an increase or decrease of the coefficient α. Accordingly, the accuracy of judgment of the cooling performance can be improved and abnormalities can be processed more precisely. 
     While an example of a liquid-cooled storage battery system  1  has been described as the above embodiment of the present invention, the present invention can also be applied to an air-cooled storage battery system.  FIG. 12  is a perspective view illustrating a battery module  102  of an air-cooled storage battery system  101  according to an embodiment. In the battery module  102  of the air-cooled storage battery system  101 , an approximately cuboid chassis block  110  internally holds a plurality of secondary batteries  108 . The chassis block  110  is constituted by five resin or resin-molded chassis members  110   a  to  110   e , and a plurality of cylindrical secondary batteries  108  are held within the chassis block  110 . 
     A cooling air inlet  110   f  is formed on one end in a longitudinal direction of the chassis block  110 , and a cooling air outlet  110   g  is formed on another end of the chassis block  110 . In other words, a cooling air passage is formed along the longitudinal direction in the chassis block  110 , and the plurality of secondary batteries  108  is arranged in a single row along the cooling air passage in the chassis block  110 . It is to be noted that while an example in which the secondary batteries  108  are arranged in a single row along the longitudinal direction of the chassis block  110  is shown in the air-cooled battery module  102  illustrated in  FIG. 12 , the arrangement of the secondary batteries  108  in the chassis block  110  is not limited to this example. 
     Temperature sensors  112   a ,  112   b ,  112   c , and  112   d  are installed in the chassis block  110 . The temperature sensor  112   a  is arranged near the cooling air inlet  110   f  of the chassis block  110  and outputs an entry-side cooling medium (in this case, cooling air) temperature signal STin. The temperature sensor  112   b  is arranged near the cooling air outlet  110   g  of the chassis block  110  and outputs an exit-side cooling medium (in this case, cooling air) temperature signal STout. In addition, the temperature sensors  112   c  and  112   d  are arranged on surfaces of two representative secondary batteries among the secondary batteries  108  held in the chassis block  110  and output surface temperature signals STs of the secondary batteries  108 . Output signal lines of the sensors  112   a  to  112   d  are bundled by a harness  131  and connected to the controller  13  illustrated in  FIG. 8 . 
     A cooling system (corresponding to the liquid cooling-type cooling system  3  illustrated in  FIG. 1 ) constituted by an air-conditioning duct and a cooling fan, not shown, is connected to the air-cooled battery module  102  illustrated in  FIG. 12 , and air is blown by the cooling fan to the secondary batteries  108  in the battery module  102  through the air-conditioning duct. The cooling fan is controlled by a fan control signal (not shown) from the controller  13  illustrated in  FIG. 8 . A configuration of a control system of the air-cooled battery module  102  is similar to that of the control system of the aforementioned liquid-cooled battery module  2  which is constituted by a controller, an indicator, a memory, and the like and which is illustrated in  FIG. 8 , and a depiction and a description thereof will be omitted. 
     With such an air-cooled storage battery system  101 , a thermal conductive sheet is no longer necessary. Measurement and judgment of a cooling performance of the air-cooled storage battery system  101  are similar to the measurement and judgment of the liquid-cooled storage battery system  1  with the omission of the thermal conductive sheet. 
     In the embodiment and the modification thereof described above, as illustrated in  FIG. 8 , examples have been presented where the cooling performances of the battery modules  2  and  102  are detected and judged by the controller  13  and the memory  14  provided outside of the battery modules  2  and  102 . Alternatively, a configuration may be adopted in which the controller  13  and the memory  14  are provided inside the battery modules  2  and  102  and a signal of the abnormality “A” or the abnormality “B” described above that is a judgment result is outputted to a higher-level control device located outside of the battery modules  2  and  102 . 
     It is to be noted that any combination of the embodiments and the modifications thereof described above including a combination of embodiments and a combination of an embodiment and a modification is possible. 
     According to the embodiments and modifications thereof described above, the following operational advantages can be gained. First, a secondary battery is held by a holding member (the aforementioned cooling block, chassis block, or the like) having a flow channel (the aforementioned cooling pipe, cooling air flow channel, or the like) of a cooling medium that cools the secondary battery. Next, temperatures T 1  and T 2  of the cooling medium taken at two locations in the flow channel and a temperature T 3  of the secondary battery are measured, a coefficient α defined as (T 3 −T 1 )/(T 2 −T 1 ), (T 3 −T 2 )/(T 2 −Ti), or (T 3 −T 2 )/(T 3 −T 1 ) is repetitively calculated, and the cooling performance of the secondary battery is judged based on a variation of the coefficient α. As a result, the cooling performance of the battery cooling system can be accurately judged based on measured values and without using estimated values. 
     In addition, according to an embodiment and a modification thereof, the cooling performance of the secondary battery is judged to be abnormal when a difference between a currently calculated value and a previously calculated value of the coefficient α exceeds a range set in advance. Therefore, the cooling performance of the battery cooling system can be judged accurately. 
     According to an embodiment and a modification thereof, temperatures Tin and Tout at an entry-side and an exit-side of the aforementioned holding member of a flow channel passing through the holding member are measured, whereby the coefficient α is calculated assuming that T 1 =Tin and T 2 =Tout. Therefore, the coefficient α can be calculated which enables the cooling performance of the battery cooling system to be judged accurately. 
     According to an embodiment and a modification thereof, the cooling performance of the battery cooling system can be accurately judged even with a holding member (the aforementioned cooling block, chassis block, or the like) having a structure in which the secondary battery is held via a thermal conductive sheet. 
     According to an embodiment and a modification thereof, an abnormality of the cooling performance is judged to be caused by a failure in the thermal conductive sheet when a difference between a currently calculated value and a previously calculated value of the coefficient α exceeds a range set in advance and the currently calculated value is greater than the previously calculated value. As a result, a cause of an abnormality of the cooling performance of the battery cooling system can be identified. 
     According to an embodiment and a modification thereof, an abnormality of the cooling performance is judged to be caused by a failure in a battery cooling system due to a cooling medium when a difference between a currently calculated value and a previously calculated value of the coefficient α exceeds a range set in advance and the currently calculated value is smaller than the previously calculated value. Therefore, a cause of an abnormality of the cooling performance of the battery cooling system can be identified. 
     According to an embodiment and a modification thereof, since the cooling performance of the battery cooling system is increased when a failure of the battery cooling system due to the cooling medium is judged, the failure of the battery cooling system can be solved. 
     The above described embodiments are examples, and various modifications can be made without departing from the scope of the invention.