Abstract:
The invention relates to a device for classifying a battery produced by assembling electricity-accumulating elements ( 1, 2, 3, 4, 5 ) divided into groups. In the device, a component ( 22 ) is provided for evaluating the degrees of ageing, each of which is associated with a distinct group, and for allocating, to the battery, in accordance with statistical distribution parameters of the evaluated degrees of ageing, a classification level that is representative of the potential performances of the battery during use.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is the US National Stage under 35 U.S.C. §371 of PCT/FR2010/050131 which was filed on Jan. 28, 2010 and which claims the priority of French application 0950908 filed on Feb. 13, 2009. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a method and device for classifying a battery, in particular a battery of an automotive vehicle. 
         [0003]    During use, the battery of a vehicle gradually loses its performance, with negative consequences for the operation of the vehicle, especially for a vehicle with electrical traction. 
         [0004]    In order to control this loss of performance, devices and methods have already been proposed to estimate the degree of deterioration of a battery. 
         [0005]    French Pat. No. 2841385 discloses the calculation of several degrees of deterioration, each associated with a value of the detected status. The total degree of deterioration then is calculated based on the degrees of deterioration, associated with the status values. The inconvenience of the device and method disclosed in this patent is that each degree of deterioration is related to the battery as a whole. 
         [0006]    A battery for an automotive vehicle is made in general of several modules, each consisting of several cells. 
         [0007]    U.S. Pat. No. 7,075,194 describes a method for configuring a battery in real time by connecting the cells differently with each other as a function of the use. Parallel connection of cells is advantageous for use with high current consumption. Serial connection of cells is advantageous for use requiring high voltage. 
         [0008]    Nevertheless, the cells do not age uniformly because aging depends on the specific thermal environment of each cell, the initial characteristics of the cells, the inaccuracy of the manufacturing process which is a source of early aging, and the situation in the vehicle which can cause an imbalance in the electrical circuit, for instance with respect to interconnection impedance. 
         [0009]    The dispersion of the typical characteristics of each cell is amplified over time and the battery loses part or all of its performance. The loss of battery performance has a direct effect on the performance of the vehicle, on fuel consumption and consequently on the emission of greenhouse gas. 
         [0010]    The prior state of technology involves analyzing the whole battery and evaluating its state according to overall characteristics, without taking into account the dispersion of the characteristics of its constituting cells. This approach leads to replacement of the battery when it would have been sufficient to instead replace certain cells of the battery. 
         [0011]    It is however recommended not to abandon the knowledge of the overall characteristics of the battery and to focus only on the state of the individual cells, because the overall characteristics have a direct impact outside the battery, in particular on the operation of the vehicle. 
       BRIEF SUMMARY 
       [0012]    To remedy the inconveniences of the prior state of technology, a method is disclosed for classifying a battery made by assembling electrical storage cells divided in groups. The method comprises the steps of:
       evaluating the degrees of aging associated with each distinct cell group; and   assigning to the battery, as a function of the parameters of the statistical distribution of the evaluated degrees of aging, a classification level which is representative of the potential performance of the battery when in use.       
 
         [0015]    In particular, the degree of aging of at least one cell group is evaluated by an impedance measurement method or a method for estimating the internal resistance as a function of the measured current flowing through the group of cells and the measured voltage. 
         [0016]    Advantageously, the method comprises a step of replacing at least one first group with a second group in order to maintain the classification level of the battery. 
         [0017]    In particular, the first cell group in the battery is replaced by exchanging it with the second cell group, wherein the second cell group had been exposed to utilization stresses lower than the first cell group prior to the exchange. 
         [0018]    More particularly, the utilization stresses comprise a temperature. 
         [0019]    Alternatively, the first cell group is replaced with a second cell group from another battery with lower degree of aging. 
         [0020]    Or alternatively, the first cell group is replaced with a second cell group with zero degree of aging. 
         [0021]    Among the different possible implementation methods, a cell group can comprise all of the cells of the battery, some cells of the battery or only one cell of the battery. 
         [0022]    By preference, the parameters of the statistical distribution comprise the average degree of aging and/or the highest value of the evaluated degrees of aging. 
         [0023]    A device for classifying a battery assembled from electrical storage cells divided in groups. The device comprises a component arranged for:
       evaluating the degree of aging of each distinct cell group;   assigning to the battery, as a function of the parameters of the statistical distribution of the evaluated degrees of aging, a classification level which is representative of the potential performance of the battery in use.       
 
         [0026]    In particular, the component is arranged for evaluating the degree of aging of at least one cell group by an impedance measurement method or a method for estimating the internal resistance as a function of the measured value of the current flowing through the cell group and measurement of the voltage. 
         [0027]    Advantageously, the device comprises slides which hold the electrical storage cells forming the battery and which allow replacing at least one first cell group with a second cell group in order to maintain the classification level of the battery. 
         [0028]    More particularly, the component comprises means for communicating the classification level and the evaluated degree of aging. 
         [0029]    In the device, one cell group can comprise all of the cells of the battery, some of the cells of the battery or only one cell of the battery. 
         [0030]    By preference, the statistical distribution parameters comprise the average degree of aging and/or the highest value of the evaluated degrees of aging. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0031]    The method and device will be better understood and other goals, characteristics, details and advantages will become clear upon reading the following explanatory description with reference to the attached schematic drawings, which are provided strictly as examples illustrating one implementation mode of the method and device, in which: 
           [0032]      FIG. 1  is a schematic view of the device for classifying a battery; 
           [0033]      FIG. 2  shows the steps of the method for classifying a battery; 
           [0034]      FIG. 3  is a statistical distribution curve as a function of one characteristic value; 
           [0035]      FIG. 4  shows an example of possible classification as a function of the displacement of the curve of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    With reference to  FIG. 1 , a battery is assembled from electrical storage cells  1 ,  2 ,  3 ,  4 ,  5  between two output terminals  15  and  16 . Each electrical storage cell  1 ,  2 ,  3 ,  4 ,  5  supplies electrical voltage and internal impedance between two contacts points  20  and  21 . The electrical storage cells  1 ,  2 ,  3 ,  4 ,  5  are divided into cell groups in which the cells can be connected in parallel or in series with each other and the cell groups themselves can be connected in series or in parallel with each other. 
         [0037]    In the example illustrated in  FIG. 1 , contact point  20  of one cell is connected with terminal  15  or with contact point  21  of the preceding cell. Therefore, the battery can be considered to be a group comprising five cells connected in series or five cell groups connected in series with each cell group containing only one cell. An amp meter  13  upstream of terminal  16  measures the current generated by the battery during use. Although in the illustrated example the total number of cells of the battery is five, it is clear that the total number of cells can be any number greater or smaller than five. 
         [0038]    Each electrical storage cells  1 ,  2 ,  3 ,  4 ,  5  are inserted in rails or between slides  18 , in which the cells slide easily in order to facilitate their removal, either individually or as a group. 
         [0039]    The rails or slides  18  are mounted to the vertical walls of a compartment (not shown) on the bottom of which is pressed a bus connecting the contact point of one cell with the contact point of another cell, for instance as shown in solid line in  FIG. 1 . Ideally, contact points  20 ,  21  are directed downwards to make contact with the bus when the cells are pushed in the compartment between the slides. Contact points  20 ,  21  are located under the cells and are directed downward in order to avoid short circuits, especially with the lid, which is not shown. When the lid is closed on top of the compartment, it is pressed against the electrical storage cells  1 ,  2 ,  3 ,  4 ,  5  to ensure good electrical contact between bus and contact points. After opening the lid and disconnecting the cell from the bus, the cell can be lifted out by means of a handle  19  located on top of each cell. During the lifting maneuver, there is no risk of short circuiting with the other cells because the other cells are protected by the orientation of their contact points, which are directed towards the bottom the compartment. 
         [0040]    A component  22  is arranged for implementing the process steps which will be explained now with reference to  FIG. 2 . 
         [0041]    In step  101 , the degree of aging of one distinct cell group is evaluated. In particular, the degree of aging of a cell group comprising a single cell is evaluated. 
         [0042]    The degree of aging of an individual cell can be evaluated in different ways. It can be evaluated during the use of the battery by measuring the current supplied by the battery by means of amp meter  13  and by connecting a voltmeter to contact points  20 ,  21  to measure the voltage delivered by the cell when charged, in order to deduce the degree of aging. It can be evaluated by means of a clock measuring the time separating the present moment from a prior moment in which the cell with known degree of aging was inserted in the battery basket. 
         [0043]    Advantageously, the degree of aging is evaluated by means of an impedance measuring method. The impedance measuring method consists in injecting a variable current with known amperage in the cell or group of cells to be checked and in measuring the resulting voltage variation at the terminals of the cell or the group of cells, respectively. The variable current is injected in such a manner that it is added, only in that cell or group of cells, to the zero or non-zero base current normally circulating through the battery. In this way, it is possible to measure the impedance of each cell or group of cells in isolation, without requiring knowledge of the zero load voltage or the voltage drop over the contact points of the cells caused by circulation of the base current. 
         [0044]    The component  22  is arranged for injecting the variable current by means of a current generator  23 . The current generator is constructed in a known manner, for instance by means of a bipolar electrical generator supplied with current by operational amplifiers, with voltage saturation safety. 
         [0045]    In a first implementation mode (not shown), the component  22  is a portable instrument (i.e., not on board the vehicle) comprising measuring terminals, each of which is connected to one pole of current generator  23 . The measuring terminals of the component  22  are then connected with contact points  10  and  21  of one cell or with the ends of one group of cells. Starting from the measured voltage variation between terminals, the component  22  calculates the impedance of the cell or group of cells, and uses the result to estimate the degree of aging, which is directly equal to or a function of the calculated impedance. The estimated degree of aging is saved in a memory  24  in association with a reference of the checked cell or group of cells. Communication means  17  indicate the degree of aging on a display. The measuring terminals of component  22  then are connected with contact points  20  and  21  of a next cell or the ends of a following group of cells. 
         [0046]    In a second implementation mode, illustrated by  FIG. 1 , component  22  is on board of the vehicle. One pole of generator  23  is connected to a first series of relays  6 ,  8 ,  10  and another pole of generator  23  is connected to a second series of relays  7 ,  9 ,  11 . Strictly for non-limiting illustration purposes, relay  6  in  FIG. 1  is connected to a part of the bus which connects terminal  15  with contact point  20  of cell  1 . Relay  8  is connected to a part of the bus which connects contact point  21  of cell  2  with contact point  20  of cell  3 . Relay  10  is connected to a part of the bus which connects contact point  21  of cell  4  with contact point  20  of cell  5 . Relay  7  is connected with a part of the bus which connects contact point  21  of cell  1  with contact point  20  of cell  2 . Relay  9  is connected with a part of the bus which connects contact point  21  of cell  3  with contact point  20  of cell  4 . Relay  11  is connected with a part of the bus which connects contact point  21  of cell  5  with terminal  16 . 
         [0047]    Component  22 , on board of the vehicle, is programmed to sequentially close relays  6  and  7  in order to measure the impedance of cell  1 , relays  7  and  8  in order to measure the impedance of cell  2 , relays  8  and  9  in order to measure the impedance of cell  3 , relays  9  and  10  in order to measure the impedance of cell  4 , relays  10  and  11  in order to measure the impedance of cell  5 . The impedance is measured by measuring the voltage in module and in phase by means of voltmeter  14 . The degrees of aging correlated to each impedance measurement are successively saved in a relational table indexed by cell reference. The relational table is for instance saved in memory  24  of component  22 . In the relational table, it is advantageous that each cell reference is related to a date expressed in terms of distance (e.g., kilometers) driven by the vehicle since the cell became part of the battery, a date expressed in terms of Amp hours exchanged by the cell or a date expressed in terms of elapsed time since the cell was put into service. 
         [0048]    We have developed the case in which the impedance, for instance at a given current frequency which varies in alternative form, constitutes a characteristic value of the degree of aging. Other characteristic values can be envisaged, such as for instance a time integral of the temperature experienced by the cell or the cell age. On the other hand, in the case that the voltage of each cell is checked individually, for instance in the case of a Lithium-ion battery, knowing the amperage of the current passing through the cell and by measuring its voltage, the internal resistance can be calculated and used as a characteristic for estimating the degree of aging of an cell. This operation can be carried out both while driving or in park mode at the service station. 
         [0049]    Communication means  17  is arranged for transferring information contained in memory  24 , for instance towards the diagnostic port of the vehicle (not shown). Secondarily, communication means  17  is arranged for transferring information to memory  24 , for instance from the diagnostic port of the vehicle. 
         [0050]    In step  102  a classification level is assigned to the battery. During use, the potential performance of the battery depends on the totality of cells constituting the battery but also on the individual cells. To be representative, the proposed classification level takes into account these two components which determine the performance of the battery. 
         [0051]      FIG. 3  shows a distribution curve D of the cells or groups of cells of a battery as a function of a variable characteristic Vc of the degree of aging. In general, the cells are distributed around the mean value M with a frequency of occurrence which is higher near the mean and decreases on both sides. The more homogenous the cell population, the more the values of the variable characteristic will be grouped around the mean. Inversely, the more heterogeneous the cell population, the more the characteristic variable will be dispersed with the effect of flattening the curve centered on the mean. 
         [0052]    Considering degrees of aging for which the variable characteristic is the cell impedance, the age of the cell or the number of charge and discharge cycles, the performance will be higher for the lower values of the variable characteristic on the left of the curve and lower for the higher values of the variable characteristic on the right of the curve. 
         [0053]    Component  22  contains an arithmetic and logical unit and a few lines of code for calculating the mean M of the values contained in memory  24  and measured in the preceding step. 
         [0054]    Component  22  holds in memory  24  the possible mean values M A , M B , M C . With reference to  FIG. 4 , the distribution of the cells corresponding to the mean value M A  is statistically represented by the left curve. The best battery performance can be expected from a classification level which corresponds with statistical distribution D with mean M A , because the values of the variable characteristic Vc, representing the degree of aging, are mostly lower values. The distribution of cells with mean value M c  is statistically represented by the right curve. Fair battery performance can be expected from a classification level corresponding with statistical distribution D with mean M c , because the values of the variable characteristic Vc, representing the degree of aging, are mostly higher values. The distribution of the cells with mean value M B  is statistically represented by the middle curve. The statistical distribution of D with average M B  corresponds to a classification level that may be expected in performance using the battery less than those obtained with an average M A  average but better than those obtained with the average M C . 
         [0055]    Component  22  contains a few lines of code for comparing the calculated mean value M with possible values M A , M B , and M c  within a predetermined tolerance range, in order to attribute a classification level NC to the battery, by using formulas of the following type: 
         [0000]    
       
         
           
             
               
                 M 
                 &lt; 
                 
                   
                     
                       3 
                        
                       
                         M 
                         A 
                       
                     
                     + 
                     
                       M 
                       B 
                     
                   
                   2 
                 
               
               ⇒ 
               NC 
             
             := 
             A 
           
         
       
       
         
           
             
               
                 
                   
                     
                       3 
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                         M 
                         A 
                       
                     
                     + 
                     
                       M 
                       B 
                     
                   
                   2 
                 
                 ≤ 
                 M 
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                       3 
                        
                       
                         M 
                         B 
                       
                     
                     + 
                     
                       M 
                       C 
                     
                   
                   2 
                 
               
               ⇒ 
               NC 
             
             := 
             B 
           
         
       
       
         
           
             
               
                 
                   
                     
                       3 
                        
                       
                         M 
                         B 
                       
                     
                     + 
                     
                       M 
                       C 
                     
                   
                   2 
                 
                 ≤ 
                 M 
               
               ⇒ 
               NC 
             
             := 
             C 
           
         
       
     
         [0056]    A supplementary condition can be imposed, for instance that in addition to the mean falling within one of the above defined intervals, no cell characteristic may exceed a certain threshold. 
         [0057]    If the classification level is A, B or C, respectively, and component  22  detects a cell with variable characteristic greater than a threshold S A , S B , or S c , respectively, component  22  generates an alarm and communicates the reference of the cell with the characteristic value which is too high for the class in which it is classified. 
         [0058]    The number of classification levels can be higher than three. The classification can be based on other methods than those based on the mean of the variable characteristic monitored for the different cells or groups of cells constituting the battery and where each class corresponds to a value interval of this characteristic. 
         [0059]    Another example is a method in which the classification is based on the characteristic value of the weakest cell or on the average of the x weakest cells. There are also other classification methods. 
         [0060]    If the check shows that the battery has transitioned from class A (new battery) to class B, the user has the choice of accepting the declassification of the battery or requesting an exchange of the weakest cells in order to keep the battery in class A. 
         [0061]    If the check shows that the battery is classified C, the client has the choice of accepting this classification of the battery or requesting an exchange of the weakest cells in order to keep the battery in class B or even to upgrade the battery to class A. 
         [0062]    In step  103  the faulty cells are replaced with cells which were already in service and which were recycled and classified in advance as level a, b, c and were designated as being the most suitable to be integrated in a battery with a respective classification level A, B, C. 
         [0063]    Consider the example of a cell retrieved in order to keep the battery in class A. The monitored characteristic of this cell showed an aging state greater than the criterion for class A. Let&#39;s assume that this cell is class b. This cell can be used later to upgrade a battery from class C to class B. 
         [0064]    The standard exchange of cells or groups of cells by new or reused cells is facilitated by the mechanical construction of the battery basket shown in  FIG. 1 , which is optimized for volume, weight and safety. 
         [0065]    The individual check of each cell or group of cells and the design of the battery basket facilitates the replacement of the most degraded cells or groups of cells, ensures that the battery performance can be maintained at the highest level. By retrieving the most degraded cells from the battery, during the life of the battery, all cells will age in homogeneous manner. 
         [0066]    For instance, in the case of a Lithium-ion battery, there is a requirement that during the discharge of the battery, a low voltage threshold is not exceeded for each cell from which the battery is made and that during the charge a high voltage threshold is not exceeded for each cell. Consequently, the cell with, for instance, the highest impedance or internal resistance will limit the capacity of the Lithium-ion battery. For this reason, it is important to have a chart of the characteristics linked to the performance of each cell from which the battery is made in order to be able to exchange the cells limiting the overall performance of the battery. 
         [0067]    On the other hand, the thermal environment of each cell can vary significantly from one cell to another, depending on the location of the battery and the particular cell in the vehicle, and consequently the degree of aging of each cell can be very different. One solution comprises changing the cells that are most degraded by temperature. Another solution is to periodically exchange, through preventive maintenance, the cells from the hottest locations, for instance the cells that are closest to the engine, with cells from the coldest locations, for instance the most remote from the engine, in order to achieve the most homogeneous aging of the battery. This is facilitated by checking each individual cell or each group of cells and designing a battery basket which allows the exchange of the cells.