Abstract:
A method includes providing battery cells for secondary batteries. The battery cells are charged to a fixed voltage (Vc). The battery cells are laid aside in an open circuit for a preset self-discharge time period (t). A voltage (Vt) is measured after the period t for each battery cell. A voltage difference (ΔV) is determined for each battery cell, wherein ΔV=Vc−Vt. A self-discharge rate (ρ) is measured for each ΔV. A self-discharge current (Ic) is calculated during the period t, wherein Ic is a function of ρ. The Ic is curve-fitted to the ΔV for each battery cell to obtain an Ic−ΔV equation. The Ic is divided into h number of grades. The Ic−ΔV equation is utilized to determine h number of grades of ΔV associated with each grade of Ic. Battery cells are selected having a same grade of Ic and ΔV for matching.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. application Ser. No. 14/113,287 filed Oct. 22, 2013, which is a 371 application of International Application No. PCT/CN2012/001504 filed Nov. 8, 2012, which claims priority to Chinese Application No. 201110439834.9 filed Dec. 26, 2011. The disclosures of these applications are hereby incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     Background of the Invention 
     There are a lot of patents and literatures currently available about the battery equalizing and matching, equalization of protection board, equalization of charger, and equalization of power supply system. A Chinese patent application with the Application No. 201110186716. 1, introduced a consistency screening method for the lithium secondary batteries. Charging the battery to a low state of charge via the approach of multi-stage constant current and constant voltage, and laying for a suitable period of time, selects the battery cell whose voltage is within a certain range to be used for the battery group matching. However, no matter how the battery cells are selected, two batteries always have some difference between them. So, if the requirement for consistency is set too high, the pass rate of the battery available for matching will be low; while if the requirement for consistency is set too low, the reliability of the matched battery group will be low. So, it needs to weigh the choices between the two factors. 
     Currently, the control parameters for the grade dividing and group matching of battery cell comprise capacity, voltage, internal resistance, and etc. In regard to the control for battery self discharge, it usually set the state of charge of battery cells as full charge SOC 100% or half charge SOC 50%, and then lay them at room temperature or at 45˜50° C. for a pre-determined period of time, and the battery cells with the voltage greater than or equal to the pre-determined standard value are the qualified. Alternatively, dividing the grade based on the voltage, i.e., measure the battery cell voltage and then divide the grade accordingly. The grade dividing based on the voltage has a certain degree of control to the level of battery self discharge. However, due to the fact that after charging or discharging, the voltages of various battery cells are different themselves, which is because of the difference of various battery cells and the difference of the test cabinet points. Since the voltages of the battery cells are different themselves, the grade dividing based on the voltage can not accurately divide the grades according to the battery self discharge. 
     Among the currently available battery self discharge testing and sorting system, some of them accomplish the grade dividing via the voltage difference; i.e., after measuring the battery voltage, lay it aside for a pre-set period of time, and then measure its voltage again, and based on the difference of the voltage before and after to conduct the grade dividing. Such grade dividing based on the voltage difference is a progress from the grade dividing that is based on the voltage. However, due to the facts that the battery cells from different grades of voltage difference usually have different self discharge or self discharge current; and sometimes such difference is quite large, the magnitude of the discharge current and the range of the discharge current of individual battery cell are actually unknown. So, it can not design and clearly control battery group&#39;s laying service life. The standard method for measure the self discharge is, according to the national or industry standard, measuring the charge retention capability of a battery. After the standard charging and discharging for 1 cycle with the battery or battery group, measure out the standard capacity C 5 , next perform standard charging, and then under the condition of the environmental temperature at 23° C.±2° C., lay it aside as open circuit for 28 days, next discharge it with 0.2 C 5  mA to the cut-off voltage, and measure out the capacity after the laying C 5 ′, so the charge retention capability=C 5 ′/C 5 *100%. The lost part is the self discharge rate=(C 5 −C 5 ′)/C 5 ×100%. In the industry, the processes of battery shelving, storing and assembling are usually operated at the state of full charge or half charge. Within the range of SOC 50˜100%, the voltage changes of certain types of batteries are quite insignificant, such as the fully charged 10 Ah lithium iron phosphate-graphite battery, following charging, its open circuit stable voltage is 3.338 V, then discharging for 1 h with 1 A, 1 Ah has been discharged, the SOC is 90% and the open circuit stable voltage changes to 3.334 V; then discharge it again with 1 A, and another 1 Ah has been discharged, SOC becomes 80% and its open circuit stable voltage changes to 3.332 V. Although the currently available voltage measurement technology is able to measure with the accuracy up to 0.1 mV, or even higher accuracy; as for the large scale production in the factory, the environmental temperature, air direction from the air conditioner, pressure of the measurement probe, the contacting position, and etc, may all influence the measurement accuracy for the voltage. Or the values shown in the measurement keep randomly changing, which makes it hardly can be be determined manually; even for a machine, it can only made the determination randomly. So, the actual accuracy may be just at 0.1˜1 mV. 
     In the battery group, there are always some differences of the self discharge current between individual battery cells. During the processes of storing, shelving, placing, and using, due to the difference of the self discharge current, the consistency among the individual battery cells, in particular the state of charge (SOC), keeps changing, and such difference gradually becomes bigger. Thus over a period of time, the battery group is going to fail. A battery group needs to be equipped with the equalization protection board. During the process of charging, it makes the states of charge of various individual battery cells become consistent. In the case when the equalization current of the equalization protection board does not match the range of the self discharge current of the battery cells, and the range of the self discharge current of the battery cell is too large, during charging process, it can not make the states of charge of various individual battery cells become consistent within a reasonable period of time. In addition, if the charger does not match the equalization capability of the equalization protection board, and the charging cut-off current is much larger than the equalization current of equalization protection board, then in such a case, when the equalization is just started or even has not started yet, the charging is already terminated, then during the process of charging, it can not make the states of charge of various individual battery cells become consistent, either. 
     There are mainly two equalization methods currently available. One method is the passive equalization, which only functions during the charging process. When a certain battery cell reaches the equalization initiation voltage, a part of the current will be leaked via a bypass resistor, thus to reduce the charging speed of the battery cell of a high voltage, and allow it to wait for the voltage increase of the battery cell of a low voltage. During the equalization process, it will generate heat; and its equalization efficiency is low. Yet its main advantage is the low price tag. This is the widely utilized equalization method. The other one is the active equalization method. It extracts energy from the battery cell with a high voltage via an inductance coil, and then transports it to the battery cell with a low voltage. Thus it has a very small energy loss; and generate almost no heat; as well as has a high equalization efficiency. This is a very good equality method. Nevertheless, its equalization cost is too high, which could be 3˜5 fold of that of the passive equalization method. 
     The currently available equalizing and matching methods for the lithium secondary battery have certain drawbacks. They can not ensure that the range of the self discharge current of the battery cell within a controllable range; and can not make a relational design of the range of the self discharge current of the battery cells, the equalization current of the equalization protection board and the charging cut-off current of the charger; and also can not effectively control the laying service life of a battery group. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to address the technical difficulties of the currently available technology, and provide a method for equalizing and matching lithium secondary battery with a low utilization cost. 
     The objective of the present invention is to address the technical difficulties of the currently available technology, and provide a system for equalizing and matching lithium secondary battery of a low utilization cost. 
     Studies have been conducted on the battery group with no equalization protection board for the impacts of the difference of the self discharge current, laying time, capacity difference, internal resistance difference, difference of the constant charging current ratio, to the charging and discharging performances of the battery group. It has been found that the difference of self discharge current and laying time are the major impact factors. And the impact from the difference of capacity is the next to those two factors. While the impacts from the difference of internal resistance and from the difference of constant charging current ratio, are relatively small. One of the most common reasons of battery group failure is that the voltage of a certain battery cell in the battery group becomes too low or even close to the zero voltage. When testing the discharge current of the battery cells in the same battery group, the self discharge current of that battery cell with low voltage is relatively large. During our studies about the relationship between the battery group laying service life and the range of the self discharge current of the battery cells, the obtained result has been summarized in Table 1 below. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Battery Group Laying Service Life and Range of self discharge 
               
               
                 current. 
               
             
          
           
               
                   
                   
                 Range 
                   
                 90 day 
               
               
                 Laying 
                 Range 
                 of self 
                   
                 Range of 
               
               
                 Service 
                 of self 
                 discharge 
                 30 day Range of 
                 self discharge 
               
               
                 Life 
                 discharge 
                 current 
                 self discharge 
                 Capacity 
               
               
                 Tg 
                 Rate (%) 
                 Δ Ic 
                 Capacity (mAh) 
                 (mAh) 
               
               
                 (Year) 
                 28 d 
                 (mA) 
                 with Full Charge 
                 at Full Charge 
               
               
                   
               
             
          
           
               
                 0.077 
                 20 
                 0.0002976 C 5   
                 0.214272 C 5   
                 0.642816 C 5   
               
               
                 0.102 
                 15 
                 0.0002232 C 5   
                 0.1607041 C 5   
                 0.482112 C 5   
               
               
                 0.13 
                 12 
                 0.0001785 C 5   
                 0.128520 C 5   
                 0.385560 C 5   
               
               
                 0.17 
                 9 
                 0.0001339 C 5   
                 0.096408 C 5   
                 0.289224 C 5   
               
               
                 0.25 
                 6 
                 0.0000892 C 5   
                 0.064224 C 5   
                 0.192672 C 5   
               
               
                 0.5 
                 3 
                 0.0000446 C 5   
                 0.032112 C 5   
                 0.096336 C 5   
               
               
                 1 
                 1.5 
                 0.0000223 C 5   
                 0.016056 C 5   
                 0.048168 C 5   
               
               
                 1.5 
                 1 
                 0.0000148 C 5   
                 0.010656 C 5   
                 0.031968 C 5   
               
               
                 2 
                 0.75 
                 0.0000111 C 5   
                 0.007992 C 5   
                 0.023976 C 5   
               
               
                 3 
                 0.5 
                 0.0000074 C 5   
                 0.005328 C 5   
                 0.015984 C 5   
               
               
                   
               
             
          
         
       
     
     Symbols and Terms 
     T g —laying service life: after fully charged, a battery group is laid aside, due to the range of self discharge current, differences are generated of the charge capacity of the respective battery cells, until the difference of capacity is greater than the total laying time of 0.2 C5. The method for calculation is:
 
 T   g =0.2 C   5 /(Δ I   c *24*365) unit: year (y)
 
     In turn, in order to ensure the laying Service life with the full charge state, the range of self discharge current needs to be controlled for the battery group matching is:
 
Δ I   c =0.2 C   5 /( T   g *24*365) unit: mA
 
     C 0 —the actual power of battery cell released at an actual discharge rate, unit mAh, which is the power provided for discharging from the state of charge to the termination voltage. 
     C 5 —the rated capacity of battery cell, unit mAh. It is the power should be provided of the discharge at the rate of 5 h till the termination voltage. For some specific model battery, the rated capacity is fixed. When the capacity of a particular battery cell is mentioned, C 5  stands for the standard capacity discharged by that battery cell according to the rate of 5 h (0.2 C 5 ). 
     C 5 ′—at the condition of room temperature, the retained capacity after a time period t (unit: hour) of open circuit storage of the standard charged single battery cell or battery group to make sure that there is no external Load would consume the power of the battery, unit is mAh. Wherein t could be could be any number selected from 168˜2016 hours (i.e. 7˜84 days), preferably, t could be could be any number selected from 336˜672 hours (i.e. 14˜28 days). 
     ρ—self discharge rate: at the condition of room temperature, the proportion of the lost capacity (C 5 −C 5 ′) after t ρ  (unit: hour) of open circuit storage of the standard charged single battery cell or battery group over the discharged capacity C5 from the standard discharge. It has no unit. It could be referred as self discharge.
 
ρ=( C   5   −C   5 ′)/ C   5 *100%
 
     ρ min —the minimal self discharge rate: it is the self discharge rate at the full charge state of the battery cell with the smallest self discharge rate at the full charge state in the same battery group. It has no unit. 
     ρ max —the maximal self discharge rate: it is the self discharge rate at the full charge state of the battery cell with the largest self discharge rate at the full charge state in the same battery group. It has no unit. 
     I c —self discharge current, unit mA. The discharge current calculated based on the measured self discharge rate of a battery cell with full charge being laid aside for t ρ  (unit: hour) at room temperature. Self discharge current I c  is measured at the state of full charge, thus it is sometimes called the full charge state self discharge current.
 
 I   c   =C   0   *ρ/t   ρ 
 
     t ρ —the time period selected to measure the self-discharge rate ρ. Typically t ρ  would be between approximately 14 to 84 days (336 to 2016 hours). 
     I cs —the set self discharge current standard. Its unit is mA. 
     W—coefficient of self discharge current grade dividing, no unit. 
     h—number of grade of the self discharge current grade dividing, no unit. 
     h′—any grade of the self discharge current dividing grade, unit grade. h′=(0, h] natural numbers. 
     I cmax —the self discharge current of the battery cell that has the largest self discharge current in the same battery group. Its unit is mA. It stands for the upper limit of the self discharge current grade dividing, during the process of self discharge current grade dividing and matching.
 
 I   cmax   =C   0 *ρ max   /t   ρ 
 
     I cmin —the self discharge current of the battery cell that has the smallest self discharge current in the same battery group. Its unit is mA. It stands for the lower limit of the self discharge current dividing grade, during the process of self discharge current grade dividing and matching.
 
 I   cmin   =C   0 *ρ min   /t   ρ 
 
     ΔI c —range of the self discharge current. Its unit is mA. The self discharge rate ρ measured in a time period t ρ  (unit: hour) at the state of full charge and at room temperature, of the battery cells in the same battery group. Then calculate the respective self discharge currents. The range of the self discharge current is the difference between the max value and the min value. It stands for the difference of the upper limit of the self discharge current grade dividing and the lower limit of the self discharge current grade dividing, during the self discharge current grade dividing and matching. The battery cells within the same grade of the self discharge current dividing will be selected for matching. The range of self discharge currents of a particular battery group matched according this method is less than or equal to the difference of the upper limit and the lower limit of the self discharge current of the battery cells that are within the same grade.
 
Δ I   c   =I   cmax   −I   cmin  
 
     C—battery cell capacity. It is the measured respective battery cell capacity after it has been measured separately. Its unit is mAh. 
     C s —the set capacity standard, unit mAh. 
     X—coefficient of capacity grade dividing, no unit. 
     i—grade number of the capacity grade dividing, unit grade. 
     R—internal resistance. It is the internal resistance of the battery cell. Its unit is mΩ. 
     R s —the set internal resistance standard. Its unit is mΩ. 
     Y—coefficient of internal resistance grade dividing. It has no unit. 
     j—grade number of the internal resistance grade dividing, unit grade. 
     C c —ratio of constant current: after the battery cells have been discharged according to the industry standard, it is next fully charged with constant current and constant voltage according to the industry standard. It is the ratio of the capacity being charged with the constant current over the total capacity being charged with the constant current and constant voltage. It has no unit. 
     C cs —the set constant current ratio standard. It has no unit. 
     Z—coefficient of ratio of constant current grade dividing. It has no unit. 
     k—grade number of the ratio of constant current grade dividing, unit grade. 
     V c —the voltage value of the charge at the fixed voltage value. Its unit is mV. It stands for the set charging voltage value at the voltage sensitive region of the discharge curve. 
     V 0 —the stable voltage after the fixed voltage charge. Its unit is mV. After the fixed voltage charge, it is laid aside as the open circuit to make sure that there is no external load would consume the power of the battery, the voltage will be tested and recorded for every hour. If the difference of the values from two consecutive testing is less than 1 mV, then it will be considered as the stable voltage. 
     V t —the voltage after the laying of a set laying time period post the fixed voltage charging. Its unit is mV. 
     V 7 —the voltage after the laying for 7 days post the fixed voltage charge. Its unit is mV. 
     ΔV—voltage difference; and its unit is mV. The difference of the stable voltage V 0  after the fixed voltage charge and the voltage V t  after the laying of time t post the fixed voltage charge.
 
Δ V=V   0   −V   t  
 
     V 1 —via the equation of I c −ΔV, the obtained voltage difference corresponding to I cs . Its unit is mV. 
     V 2 , V 3 —via the equation of I c −ΔV, the voltage difference grade corresponding to h′WI cs . Its unit is mV. 
     I b —the equalization current of the equalization protection board. Its unit is mA. 
     I cut —the cut-off current of the constant voltage charge by a charger. Its unit is mA. 
     T b —equalization time; its unit is h. Due to the difference of the self discharge current ΔI c  of the battery group, after laying for a time period of T 1 , the generated SOC difference between individual battery cells. It needs to make up the SOC difference via the equalization current. And T b  is the needed time for such make up. It is the total equalization time needed for making up the range of capacity after laying aside. And it is the total equalization time of the needed many times of equalizations, rather than the total time needed for one time of charge.
 
 T   b   =ΔI   c   *T   1   /I   b  
 
     n—number of the series connected individual battery cells in a battery group. 
     T 1 —laying time. Its unit is h. It comprises the time period for transportation, storage, for sale and placement. 
     SOC—state of charge. 0           SOC         1.
     t—battery cell laying time: it stands for the laying time as open circuit after the battery cell fixed voltage value charge. Its unit is h. 
     T m —maintenance period: it is also called as equalization charge period. Based on the equalization current and that usually the equalization time of a single charging is 0.5 h, in order to make up the range of capacity after the laying post full charge, it is the corresponding laying time. The equalization charge is to use the matched charger to charge the battery till the cut-off of the charger or of the protection board. It is not the cut off manually made in advance. If the laying time is longer that of the equalization charge period of time, it then needs many times of equalization charge and discharge in order to make up the range of the capacity after laying.
 
 T   m   =I   b *0.5 h /(Δ I   c *24 h ) unit: d
 
     C m —battery group dynamic capacity: after being fully charged, along with the extension of the battery laying time, the usable capacity actually keeps changing.
 
 C   m   =C   5   −I   cmax   *T   1  
 
     Voltage sensitive region (See FIGURE): based on the characteristic of battery cell discharging, usually according to the 0.2 C 5  discharging feature, and select the voltage range with the large change of discharging voltage. Generally, the voltage section is selected with the range of SOC as more than 0.005, SOC change as 0.001, and the voltage change is greater than 3 mV, voltage range is over 30 mV. Such section can be found at the end of discharging of any type of lithium secondary battery. In certain systems, such as lithium nickel manganese cobalt oxide-hard carbon, such section can be found in the entire discharging process. The battery cell is set for laying in the voltage sensitive region, via the measurement of voltage difference, it is convenient to recognize the battery cells with different self discharge current. With the selection of different voltage sensitive regions, or charging and discharging to the different voltage points in the voltage sensitive region, the fitted I c −ΔV equation will be different. 
     The discharge current of the state of full charge is generally larger than the self discharge current of the state of low charge. During the design, the self discharge current of the state of full charge has been controlled in a relatively small range. At the same level of grade, during the laying period, most of them are in the state of full charge and are prepared for use. Long time of laying with the state of full charge can ensure the battery group have an appropriate laying service life. And at the state of low charge, the battery group&#39;s laying service life can be better ensured. The range of the self discharge current of the battery cell can be selected and controlled. The equalization current of the equalization protection board can be designed and selected, and the cut-off current of the charger is also designable and selectable. 
     The present invention, a method for equalizing and matching lithium secondary battery comprising:
     charging the battery cells to a fixed voltage value V c , said V c  being a fixed voltage value selected from a voltage sensitive region;   laying the battery cells aside in an open circuit for a preset self-discharge time period t;   measuring the voltage V t , after the laying of the preset self-discharge time period t for each battery cell;   according to ΔV=V c −V t , determining a voltage difference value ΔV between the fixed voltage value V c  and V t  after the set self-discharge time period t;   measuring a self-discharge rate ρ for each ΔV of each battery cell;   calculating a self-discharge current I c  for each battery cell during the self-discharge period according to I c =C 0 *ρ/t ρ , wherein C 0  is actual power of a battery cell provided when discharging from a state of charge to a termination voltage and t ρ  is a time period selected to measure the self-discharge rate ρ;   curve fitting the values of Ic to the values of ΔV, so as to obtain an I c −ΔV equation, I c =f(ΔV), wherein I c  is a function of ΔV;   dividing the self-discharge current Ic into h number of self-discharge current grades, wherein:   a qualified self-discharge current is set as Ics, such that I c ≦I cs ,   W*I cs  defines a grade length, wherein h*W=1, and   the self-discharge current grades have self-discharge current grade levels that are arranged in a sequence as follows:
       (0 to W*I cs ] for a first grade level in the sequence,   (W*I cs  to 2*W*I cs ] for a second grade level in the sequence,   (2*W*I cs  to 3*W*I cs ] for a third grade level in the sequence, and   continuing the sequence through to ((h−1)*W*I cs  to h*W*I cs ] for the final grade level in the sequence;   
       dividing the difference value ΔV into voltage difference grade levels corresponding to those of the self-discharge current grade levels according to the Ic−ΔV equation; and
       selecting battery cells of the same grade level of the self-discharge current I c  and the same grade level of the voltage difference ΔV for matching.   
       

     A method for equalizing and matching lithium secondary battery, further including the optimization steps of: 
     measure difference value of the voltage being V t  measured and recorded after the laying of a preset laying time period t and the fixed voltage value V c , until the difference value of two consecutive measurements being less than a preset value, set the value of V t  as stable voltage V 0 , and 
     According to ΔV=V 0 −V t , determine the difference value ΔV between the stable voltage V 0  and V t  after a set laying time period t. 
     In the present invention, the grade dividing for the lithium secondary battery, according the magnitude of the self discharge current I c  dividing the battery cell into more than one grades, the number of grade being h, according the size of the voltage difference ΔV dividing the battery cell dividing the battery cell into grades that corresponding to the self discharge current, selecting the battery cells that have the same grates of self discharge current and voltage difference to conduct the matching, so the range of the self discharge current of the battery group can be controlled; 
     I c —self discharge current, unit mA, the self discharge current calculated based on the measured consumption rate of a battery cell at the state of full charge and being laid aside for a time period t ρ  (unit: hour) at room temperature; 
     ΔV—voltage difference; and its unit is mV. The difference of the stable voltage after the fixed voltage charge V 0  and the voltage after the laying of time t post the fixed voltage charge V t , ΔV=V 0 −V t ; 
     V 0 —the stable voltage after the fixed voltage charge. Its unit is mV. After the fixed voltage charge, it is laid aside as the open circuit to make sure that there is no external Load would consume the power of the battery, the voltage will be tested and recorded for every hour. If the difference of the values from two consecutive testing is less than a set value, then it will be considered as the stable voltage; 
     V t —the voltage after the laying of a set laying time period post the fixed voltage charging. Its unit is mV; 
     In the present invention, the grade dividing according to the self discharge current, setting the qualified self discharge current as I cs , and I c             I cs  being the qualified, using WIcs as the grade span length, W=0.01˜0.5, grade levels being (0, WI cs ], (WI cs , 2 WI cs ], (2 WI cs , 3 WI cs ], . . . ((h−1) WI cs , h WI cs ], h*W=1, number of grade h=1/W, number of grade h; I cs           0.0002232 C 5 , which being equal to that the battery cell with its consumption rate less than or equal to 15% as the self discharge current qualified;
     The same grade range of self discharge current ΔI c =WI cs    
     ΔI c —range of self discharge current, with the unit as mA, The self discharge currents measured in a time period t ρ  (unit: hour) at the state of full charge and at room temperature of the battery cells in the same battery group. Then calculate the respective self discharge currents. The range of the self discharge current is the difference between the max value and the min value; 
     I cs —the set self discharge current standard, unit being mA; 
     W—coefficient of self discharge current grade dividing, no unit; 
     h—grade number of self discharge current grade dividing, no unit; 
     I c —self discharge current, unit mA, the self discharge current being calculated based on the measured self discharge rate of a battery cell with full charge being laid aside for a time period t ρ  (unit: hour) at room temperature. 
     In the present invention, the grade dividing according to the voltage difference ΔV, the battery cells being charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid at room temperature for a time period t, then measuring a series of I c  values and ΔV values, determining the grades of voltage difference according to the divided grades of the range of self discharge current ΔI c , battery cells laying time at the room temperature being 1˜28 days; 
     In the present invention, the grade dividing according to the voltage difference ΔV, the battery cells being charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid with open circuit at room temperature to make sure that there is no external Load would consume the power of the battery, measuring and recording the voltage value once each hour, until the voltage difference between two consecutive measures being less than a set value, and using it as the battery cell laying initiation voltage V 0 , at room temperature laying a suitable time period t, t=1˜28 days, measuring the voltage V t  again, and the ΔV after being laid for the time period t; 
     Measuring battery cells with different ΔV, and its corresponding relationship with the self discharge current at the state of full charge I c , and then fitting the I c −ΔV equation;
 
 I   c   =f (Δ V )
 
     Fitting it until the correlation coefficient R 2             0.99, so as to fit out the I c −ΔV equation;
     Charging battery cells with constant current and constant voltage to V c , measuring and recording the stable voltage V 0 , laying time t, measuring and recording the voltage V t , calculating the ΔV; 
     ΔV=(0, V 1 ] being battery self discharge current qualified 
     ΔV=(V 2 , V 3 ] being battery self discharge current at the same grade 
     V 1  being used to control the max consumption of the battery group, which is correspondent to I cs ; V 2 , V 3  being used to control the max range of self discharge current of the battery cells in the battery group, which is correspondent to the self discharge current grade dividing h′ WI cs , h′=(0, h] natural number; in different grades, ΔV, V 2  and V 3  can be different, selecting the battery cells with the same grade of their self discharge currents to perform the matching, so as to control the range of self discharge current of the battery group; 
     In the present invention, dividing the grades of battery capacity, and setting a qualified capacity C s , C           C s  being the qualified; C s           C 5 , using XC s  as the grade span length, X=0.01˜0.1, grade levels being [C s , (1+X)C s ), [(1+X)C s , (1+2X)C s ), [(1+2X)C s , (1+3X)C s ), . . . [(1+(i−1)X)C s , (1+iX)C s ); number of grade i is related to the grade span length XC s  and the size of the capacity distributions, generally (1+iX)         1.5, iX         0.5, i         0.5/X;
     In the present invention, dividing the grades of internal resistance, and setting a qualified internal resistance R s , R           R s  being the qualified; using YR s  as the grade span length, Y=0.05˜0.5, grade levels being (0, YR s ], (YR s , 2YR s ], (2YR s , 3YR s ], . . . ((j−1) YR s , jYR s ]; jY=1, the number of grade j=1/Y;
     In the present invention, dividing the grades of constant current ratio, and setting a qualified constant current ratio C cs , C c             C cs  being the qualified; using ZC cs  as the grade span length, Z=0.01˜0.1, grade levels being [C cs , (1+Z)C cs ), [(1+Z)C cs , (1+2Z)C cs ), [(1+2Z)C cs , (1+3Z)C cs ), . . . [(1+(k−1)Z)C cs , (1+kZ)C cs ), (1+kZ)C cs           1, k         (1/C cs −1)/Z, the number of grade being k;
     Selecting the battery cells with self discharge current and the voltage difference being qualified and in the same grade, capacity being qualified and in the same grade, internal resistance being qualified and in the same grade, ratio of constant current being qualified and in the same grade to perform the matching; 
     The present invention, a system for equalizing and matching lithium secondary battery, comprises lithium secondary battery group, equalization protection board, charger, wherein: the method comprising:
         charging the battery cells to a fixed voltage value V c , said V c  being a fixed voltage value selected from a voltage sensitive region;       laying the battery cells aside in an open circuit for a preset self-discharge time period t;
       measuring the voltage V t , after the laying of the preset self-discharge time period t for each battery cell;   according to ΔV=V c −V c , determining a difference value ΔV between the fixed voltage value V c  and V t  after the set self-discharge time period t;   measuring a self-discharge rate ρ for each ΔV of each battery cell;   
       

     calculating a self-discharge current I c  of each battery cell during the self-discharge period according to I c =C 0 *ρ/t ρ , wherein C 0  is actual power of a battery cell provided when discharging from a state of charge to a termination voltage and t ρ  is a time period selected to measure the self-discharge rate ρ;
         curve fitting the values of Ic to the values of ΔV, so as to obtain an I c −ΔV equation, I c =f(ΔV), wherein I c  is a function of ΔV;       dividing the self-discharge current I c  into h number of self-discharge current grades, wherein:   a qualified self-discharge current is set as Ics, such that I c ≦I cs ,   W*I cs  defines a grade length, wherein h*W=1, and   the self-discharge current grades have self-discharge current grade levels that are arranged in a sequence as follows:
       (0 to W*I cs ] for a first grade level in the sequence,   (W*I cs  to 2*W*I cs ] for a second grade level in the sequence,   (2*W*I cs  to 3*W*I cs ] for a third grade level in the sequence, and   continuing the sequence through to ((h−1)*W*Ics to h*WIcs] for the final grade level in the sequence;   dividing the difference value ΔV into voltage difference grade levels corresponding to those of the self-discharge current grade levels according to the Ic-ΔV equation;   
       

     selecting battery cells of the same grade level of the self-discharge current I c  and the same grade level of the voltage difference ΔV for matching;
     connecting the battery cells in one of a series connection and a parallel connection; and
       assembling the lithium secondary battery with the equalization protection board and the charger.   
       

     Preferably, the system for equalizing and matching lithium secondary battery according to claim  1 , further including the optimization steps of:
         measuring difference value of the voltage being Vt measured and recorded after the laying of a preset laying time period t and the fixed voltage value Vc, until the difference value of two consecutive measurements being less than a preset value, set the value of Vt as stable voltage V0, and       According to ΔV=V0−Vt, determine the difference value ΔV between the stable voltage V0 and Vt after a set laying time period t   

     The grade dividing for the lithium secondary battery, according the magnitude of the self discharge current I c  dividing the battery cell into more than one grades, the number of grade being h, according the size of the voltage difference ΔV dividing the battery cell dividing the battery cell into grades that corresponding to the self discharge current, selecting the battery cells that have the same grates of self discharge current and voltage difference to conduct the matching, so the range of the self discharge current of the battery group can be controlled; 
     I c —self discharge current, unit mA, the self discharge current calculated based on the measured consumption rate of a battery cell at the state of full charge and being laid aside for a time period t (unit: hour) at room temperature; 
     ΔV—voltage difference; and its unit is mV. The difference of the stable voltage after the fixed voltage charge V 0  and the voltage after the laying of time t post the fixed voltage charge V t , ΔV=V 0 −V t ; 
     V 0 —the stable voltage after the fixed voltage charge. Its unit is mV. After the fixed voltage charge, it is laid aside as the open circuit to make sure that there is no external Load would consume the power of the battery, the voltage will be tested and recorded for every hour. If the difference of the values from two consecutive testing is less than 1 mV, then it will be considered as the stable voltage; 
     V t —the voltage after the laying of a set laying time period post the fixed voltage charging. Its unit is mV; 
     The grade dividing being according to the self discharge current, setting the qualified self discharge current as I cs , and I c             I cs  being the qualified, using WI cs  as the grade span length, W=0.01˜0.5, grade levels being (0, WI cs ], (WI cs , 2 WI cs ], (2 WI cs , 3 WI cs ], . . . ((h−1) WI cs , h WI cs ], h*W=1, number of grade h=1/W, number of grade h; I cs           0.0002232 C 5 , which being equal to that the battery cell with its consumption rate less than or equal to 15% as the self discharge current qualified;
     The same grade range of self discharge current ΔI c =WI cs    
     ΔI c —range of self discharge current, with the unit as mA, The self discharge currents measured in a time period t (unit: hour) at the state of full charge and at room temperature of the battery cells in the same battery group. Then calculate the respective self discharge currents. The range of the self discharge current is the difference between the max value and the min value; 
     I cs —the set self discharge current standard, unit being mA; 
     W—coefficient of self discharge current grade dividing, no unit; 
     h—grade number of self discharge current grade dividing, no unit; 
     I c —self discharge current, unit mA. The self discharge current calculated based on the measured self discharge rate of a battery cell with full charge being laid aside for a time period t at room temperature. 
     The grade dividing according to the voltage difference ΔV, the battery cells being charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid with open circuit at room temperature to make sure that there is no external Load would consume the power of the battery, measuring and recording the voltage value once each hour, until the voltage difference between two consecutive measures being less than 1 mV, and using it as the battery cell laying initiation voltage V 0 , at room temperature laying a suitable time period t, t=1˜28 days, measuring the voltage V t  again, and the ΔV after being laid for the time period t; 
     Measuring battery cells with different ΔV, and its corresponding relationship with the self discharge current at the state of full charge I c , and then fitting the I c −ΔV equation;
 
 I   c   =f (Δ V )
 
     Fitting it until the correlation coefficient R 2             0.99, so as to fit out the I c −ΔV equation;
     Charging battery cells with constant current and constant voltage to V c , measuring and recording the stable voltage V 0 , laying time t, measuring and recording the voltage V t , calculating the ΔV; 
     ΔV=(0, V 1 ] being self discharge current qualified 
     ΔV=(V 2 , V 3 ] being self discharge current at the same grade 
     V 1  being used to control the max consumption of the battery group, which is correspondent to I cs ; V 2 , V 3  being used to control the max range of self discharge current of the battery cells in the battery group, which is correspondent to the self discharge current grade dividing h′ WI cs , h′=(0, h] natural number; as for different ΔV, V 2  and V 3  can be different, selecting the battery cells within the same grade of their self discharge currents to perform the matching, so as to control the range of self discharge current of the battery group; 
     The present invention, a system for equalizing and matching lithium secondary battery, comprises equalization protection board, the equalization current of the equalization protection board being 100˜1000 fold of the range of self discharge current used for the battery cell matching design, I b =100 ΔI c ˜1000 ΔI c ; 
     The present invention, a system for equalizing and matching lithium secondary battery, comprises charger, the charge cut-off current of the charger being 1˜10 fold of the equalization current of the equalization protection board, I cut =I b ˜10 I b . 
     The more of the divided grades for the battery cell grade dividing, the more difficult of the battery management, and the more difficult to find the battery cells that are with the same grade of self discharge current, voltage difference, capacity, internal resistance, ratio of constant current, the success rate for battery matching would be low. So, it would be further optimized. 
     The grade dividing being according to the self discharge current, setting the qualified self discharge current as I cs , and I c             I cs  being the qualified, using WI cs  as the grade span length, W=0.2˜0.3, grade levels being (0, WI cs ], (WI cs , 2 WI cs ], (2 WI cs , 3 WI cs ], . . . ((h−1) WI cs , h WI cs ], h*W=1, number of grade h=1/W, number of grade h; Ics           0.0002232 C 5 , which being equal to that the battery cell with its consumption rate less than or equal to 15% as the self discharge current qualified;
     The same grade range of self discharge current ΔI c =WI cs    
     ΔI c —range of self discharge current, with the unit as mA, The self discharge currents measured in a time period t (unit: hour) at the state of full charge and at room temperature of the battery cells in the same battery group. Then calculate the respective self discharge currents. The range of the self discharge current is the difference between the max value and the min value. 
     I cs —the set self discharge current standard, unit being mA; 
     W—coefficient of self discharge current grade dividing, no unit; 
     h—grade number of self discharge current grade dividing, no unit; 
     I c —self discharge current, unit mA. The self discharge current calculated based on the measured self discharge rate of a battery cell with full charge being laid aside for a time period t (unit: hour) at room temperature; 
     In the present invention, the grade dividing according to the voltage difference ΔV, the battery cells being charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid at room temperature for a time period t, then measuring a series of I c  values and ΔV values, determining the grades of voltage difference according to the divided grades of the range of self discharge current ΔI c , battery cells laying time at the room temperature being t=7 days; 
     In the present invention, the grade dividing according to the voltage difference ΔV, the battery cells being charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid with open circuit at room temperature to make sure that there is no external Load would consume the power of the battery, measuring and recording the voltage value for each hour, until the voltage difference between two consecutive measures being less than 1 mV, and using it as the battery cell laying initiation voltage V 0 , at room temperature laying a suitable time period t, t=7 days, measuring the voltage V t  again, and the ΔV after being laid for the time period t=7 days; 
     Measuring battery cells with different ΔV, and its corresponding relationship with the self discharge current at the state of full charge I c , and then fitting the I c −ΔV equation;
 
 I   c   =f (Δ V )
 
     Fitting it until the correlation coefficient R 2             0.99, so as to fit out the I c −ΔV equation;
     Charging battery cells with constant current and constant voltage to V c , measuring and recording the stable voltage V 0 , laying time t, measuring and recording the voltage V t , calculating the ΔV; 
     ΔV=(0, V 1 ] being self discharge current qualified 
     ΔV=(V 2 , V 3 ] being self discharge current at the same grade 
     V 1  being used to control the max consumption of the battery group, which is correspondent to I cs ; V 2 , V 3  being used to control the max range of self discharge current of the battery cells in the battery group, which is correspondent to the self discharge current grade dividing h′ WI cs , h′=(0, h] natural number; as for different ΔV, V 2  and V 3  can be different, selecting the battery cells with the same grade of their self discharge currents to perform the matching, so as to control the range of self discharge current of the battery group; 
     In the present invention, dividing the grades of battery capacity, and setting a qualified capacity C s , C           C s  being the qualified; C s           C 5 , using XC s  as the grade span length, X=0.02˜0.05, grade levels being [C s , (1+X)C s ), [(1+X)C s , (1+2X)C s ), [(1+2X)C s , (1+3X)C s ), . . . [(1+(i−1)X)C s , (1+iX)C s ); number of grade I is related to the grade span length XC s  and the size of the capacity distributions, generally (1+iX)         1.5, iX         0.5, i         0.5/X;
     In the present invention, it being set that internal resistance is not divided for different grades, setting a qualified internal resistance R s , R≦R s  being the qualified; 
     In the present invention, it being set that constant current ratio is not divided for different grades, setting a qualified constant current ratio C cs , C c ≧C cs  being the qualified; 
     In the present invention, the equalization current of the equalization protection board being 200˜500 fold of the range of self discharge current used for the battery cell matching design, I b =200 ΔI c ˜500 ΔI c ; 
     In the present invention, the charge cut-off current of the charger being 2˜5 fold of the equalization current of the equalization protection board, I cut =2 I b ˜5 I b . 
     The battery system configured in the method of the present invention converts self discharge current grade dividing into voltage difference dividing after laying, so as to be convenient and swift; the range of the self discharge current of the battery cell, the laying time, the equalization charging period, the equalization current, and the charger cutoff current are all under control, and are matched with each other, the equalization cost is low, and the charging equalization time is short; the laying service life and the maintenance period of the battery group can be designed or predicted during matching, and how many times of charging/discharging is required to charge the battery group to be balanced after the battery group is laid for an extremely long time is estimated in advance; the battery group reject rate is reduced, and the battery group reliability is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
       The FIGURE is a discharge curve of phosphate lithium ion battery, the voltage sensitive region is [2.0V,3.2V], wherein one of the voltage value is 3.15V. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more specifically with reference to the following embodiments. 
     The present invention, a method and system for equalizing and matching lithium secondary battery is achieved via the steps set forth below: 
     1. Battery grade dividing and battery matching 
     1.1 grade dividing according to the self discharge current and voltage difference: setting the qualified self discharge current as I cs , and I c ≦I cs  being the qualified. I cs ≦0.0002232 C 5 . which being equal to that the battery cell with its consumption rate less than or equal to 15% as the self discharge current qualified; in order to ensure that the products comply with the industry standard, the company internal controls can be tightened. Using WI cs  as the grade span length, W=0.01˜0.5, grade levels being (0, WI cs ], (WI cs , 2 WI cs  (2 WI cs , 3 WI cs ], . . . ((h−1) WI cs , h WI cs ], h*W=1, number of grade h=1/W, the number of grade h is related to the size of the grade span length WI cs  and of the distribution of the self discharge current. Some grades set according to the theory may actually not exist in the reality. 
     The same grade range of self discharge current
 
Δ I   c   =WI   cs  
 
     can be determined based on the laying service life of the battery group. The longer the laying service life is required to be, the finer the grade dividing of self discharge current should be. The self discharge current I c  and the grade dividing of self discharge current are reflected by the voltage difference. 
     Select the voltage sensitive region with large voltage change in the discharge curve, then the battery cells are charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid at room temperature for a time period t, then measuring a series of I c  values and ΔV values, fitting the I c −ΔV equation, setting the qualified voltage difference and the grade level of voltage difference. The battery cells laying time at the room temperature is 1˜28 days. Next determine the grades of voltage difference according to the divided grades of the range of self discharge current ΔI c . 
     Example: in regard to lithium iron phosphate-graphite battery, the voltage sensitive region can be selected as 2˜3.2 V, SOC 0˜35%, and charge the battery to any voltage point in such voltage region, or discharge the battery to any voltage point in such voltage region. In regard to the lithium cobalt oxide-graphite battery, lithium manganese Oxide-graphite battery, lithium nickel manganese cobalt oxide-graphite battery, the voltage sensitive region can be selected as 2.5˜3.7 V, SOC 0˜20%, and charge the battery to any voltage point in such voltage region, or discharge the battery to any voltage point in such voltage region. In general, if the battery is charged to the section of high voltage section within the voltage sensitive region, the voltage will drop; if the battery is discharged to the section of lower voltage within the sensitive region, the voltage will then rise. 
     The specific approach is, the battery cells being charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid with open circuit at room temperature to make sure that there is no external load would consume the power of the battery, measuring and recording the voltage value once each hour, until the voltage difference between two consecutive measures being less than 1 mV, and using it as the battery cell laying initiation voltage V 0 , at room temperature laying a suitable time period t, t=1˜28 days, measuring the voltage V t  again, and the ΔV after being laid for the time period t. 
     Measuring battery cells with different ΔV, and its corresponding relationship with the self discharge current at the state of full charge I c , and then fitting the I c −ΔV equation;
 
 I   c   =f (Δ V )
 
     The trend chart in excel can be utilized to conduct the fitting. Fitting it until the correlation coefficient R 2 ≧0.99, the closer R 2  to 1, the better. When using the polynomial fitting, in general, the higher the selected order is, the closer the fitted correlation coefficient R 2  to 1; or other data processing technology can also be used for fitting the equation I c =f(ΔV). 
     Due to the differences of the selected voltage sensitive region, laying time t, laying environmental temperature, as well as the differences of the selected orders in the polynomial fitting, the equation reached from the fitting could be different. Albeit they may have a certain degree of bias, they are able to reflect the relationship between I c  and ΔV. 
     With a series of Ic and ΔV values, the equation I c =f(ΔV) can also be reached by fitting, so as to calculate the corresponding ΔV values of different I c . In regard to the voltage measurement, it is easy to achieve the accuracy to ±1 mV, yet it is hard to achieve the accuracy to ±0.1 mV. The tiny changes of the factors such as testing environmental temperature, intensity of the probe contact, position of the probe contact, impact the testing data. In general, battery&#39;s ΔV value (accurate to ±1 mV) is within a definite range, the normal ones will be all listed, and the abnormal ones will be listed as the unqualified product. 
     The battery group of series connected n piece of battery cells, the difference between the smallest self discharge current and the largest self discharge current is the range of self discharge current of battery cells
 
Δ I   c   =I   cmax   −I   cmin  
 
     In this way, the magnitudes of consumption rate and self discharge current of various battery cells which have been charged to the fixed voltage value V c  and then laid aside for a time period t and thus are with different voltage decrease ΔV, can be clearly obtained. In this way, the laying service life of the battery group can be calculated. And this can be used to configure the equalization current of the equalization protection board and the charging cut-off current of the charger. 
     The accomplishment of such basic work is helpful to understand the relationship of the values V 0 , V t , ΔV, I c  and Δ I c . And later on, the follow up work is quite simple: charge the battery cells to the V c  with constant current and constant voltage, measure and record the stable voltage V 0 , lay for time period t, measure and record voltage V t , and then calculate the value of ΔV. 
     ΔV=(0, V 1 ] being self discharge current qualified; 
     ΔV=(V 2 , V 3]  being self discharge current at the same grade; 
     V 1  has been used to control the max consumption of the battery group, which is correspondent to I cs ; V 2 , V 3  being used to control the max range of self discharge current of the battery cells in the battery group, which is correspondent to the self discharge current grade dividing h′ WI cs , h′=(0, h] natural number; as for different ΔV grade, V 2  and V 3  can be different, selecting the battery cells with the same grade of their self discharge currents to perform the matching, so as to control the range of self discharge current of the battery group. The present invention employs the self discharge current at the full charge state as the control standard, via the corresponding function relation, further uses the voltage difference as the measurement control value, so as to ensure the suitable laying service life of the matched battery group. 
     Testing and fitting the I c −ΔV equation are the fundamental work, for a particular model system, such work only needs to be done once. In the production, coverts the grade divided according to the self discharge current to the grade divided based on the voltage difference after laying, which is convenient and efficient. Utilize the bar code system of battery and the calculation function of excel, as long as the V 0  and V t  have been measured and recorded, the ΔV, I c  and the ΔI c  among individual battery cells within a matched battery group can all be calculated. 
     Select the voltage sensitive region with large voltage change in the discharge curve, then the battery cells are charged to a fixed voltage value V c  within the selected voltage sensitive region with constant current and constant voltage, the battery cells being laid at room temperature for a time period t, then measuring a series of I c  values and ΔV values, fitting the I c −ΔV equation, setting the qualified voltage difference and the grade level of voltage difference. The battery cells laying time at the room temperature is 1˜28 days. Next determine the grades of voltage difference according to the divided grades of the range of self discharge current ΔI c . Yet the voltage grades are not necessarily to be an arithmetic progression. 
     For example, concerning a 10000 mAh to lithium iron phosphate battery, as shown in Table 2, after being charged to the fixed voltage value of 3.15 V, at room temperature with open circuit laying to make sure that there is no external load would consume the power of the battery for 7 days, its ΔV are between 5˜20 mV, and the calculated corresponding self discharge currents are between 0.09˜1.371 mA. Set the battery with its self discharge current less than or equal to 1.371 mA as the qualified, I cs =1.371 mA, set W=0.333, and uses W I cs =0.457 mA as the grade span length, the divided grade levels are (0, 0.457], (0.457, 0.914], (0.914, 1.371]. By virtue of the conversion between the self discharge current Ic and the voltage difference ΔV, the actual operation would be the one with ΔV≦20 mV as the qualified. The grade levels according to the voltage difference would be (0, 12], (12, 16], (16, 20]. And the voltage grades are an arithmetic progression. 
     1.2 Dividing Grade Based on the Capacity: 
     setting a qualified capacity C s , C≧C s  being the qualified; C s ≧C 5 , using XC s  as the grade span length, X=0.01˜0.1, grade levels being [C s , (1+X)C s ), [(1+X)C s , (1+2X)C s ), [(1+2X)C s , (1+3X)C s ), . . . [(1+(i−1)X)C s , (1+iX) C s ); number of grade I is related to the grade span length XC s  and the size of the capacity distributions, generally (1+iX)≦1.5, iX≦0.5, i≦0.5/X. 
     For example, concerning a certain battery cell has a rated capacity as 10000 mAh, set the qualified capacity as ≧10000 mAh, C s =10000 mAh, set X=0.02, the grade span length X cs =200 mAh, and the qualified battery cell capacities are distributed within the range of 10000˜10980 mAh, then the grade levels would be [10000, 10200), [10200, 10400), [10400, 10600), [10600, 10800), [10800, 11000). 
     1.3 Dividing Grade Based on the Internal Resistance 
     According to the design of the respective battery cell, set a qualified internal resistance Rs, R≦R s  being the qualified; using YR s  as the grade span length, Y=0.05˜0.5, grade levels being (0, YR s ], (YR s , 2YR s ], (2YR s , 3YR s ], . . . ((j−1) YR s , jYR s ]; jY=1, the number of grade j=1/Y; which is related to the sizes of grade span length YR s  and the distribution of the internal resistance. Certain grade level set based on the theory may not exist in the reality. 
     For example, the qualified internal resistance of a battery cell R s =10 mΩ. R≦R s  as the qualified, set Y=0.2, grade span length YR s =2 mΩ, and the battery cell internal resistances are distributed in the range of 5˜10 mat. Then the divided grade levels would be (4, 6], (6, 8], (8, 10]. And the grade levels of (0, 2] and (2, 4] do not exist. 
     1.4 Dividing Grade Based on the Ratio of Constant Current 
     According to the design of the respective battery cell, set a qualified constant current ratio C cs , C c ≧C cs  being the qualified; using ZC cs  as the grade span length, Z=0.01˜0.1,grade levels being [C cs , (1+Z)C cs ), [(1+Z)C cs , (1+2Z)C cs ), [(1+2Z)C cs , (1+3Z)C cs ), . . . [(1+(k−1)Z)C cs , (1+kZ)C cs ), (1+kZ)C cs ≦1, k≦(1/C cs −1)/Z, the number of grade k is related to the sizes of the grade span length ZC cs  and the distribution of the constant current ratio. Certain grade level set based on the theory may not exist in the reality. 
     For example, a certain battery cell has been set the constant current ratio C cs =90%, C c ≧C cs  being the qualified, set Z=0.02, the grade span length ZC cs =1.8%, the distribution of the battery cell the constant current ratios are 90˜97.1%, the divided grade levels are [90%, 91.8%), [91.8%, 93.6%), [93.6%, 95.4%), [95.4%, 97.2%). And the grade level of [97.2%, 99%) does not exist. 
     Selecting the battery cells with self discharge current and the voltage difference being qualified and in the same grade, capacity being qualified and in the same grade, internal resistance being qualified and in the same grade, ratio of constant current being qualified and in the same grade to perform the matching. 
     2. The configuration of the equalization protection board: the equalization current of the equalization protection board is 100˜1000 fold of the range of self discharge current that is used for battery matching design.
 
 I   b =100 Δ I   c ˜1000 Δ I   c  
 
     3. The configuration of the charger: the charger charging cut-off current should be 1˜10 fold of the equalization current of the equalization protection board.
 
 I   cut   =I   b ˜10 I   b  
 
     Embodiment 1: 
     Model 11585135Fe lithium iron phosphate lithium ion battery produced by the company. Its nominal voltage is 3.2 V, rated capacity is 10 Ah. Based on the client&#39;s request, it has been made as battery group with 12 series connected cells, and need the matched protection board and charger. The client requires the laying service life to be more than 8 months. The end user needs to charge it from once per day to once per every 5 days; which both can charge the battery with equalization so as to function optimally. 
     Basic work: in regard to the system of Model 11585135Fe lithium iron phosphate lithium ion battery, its 0.2 C 5  discharge platform is quite flat, at the end of the discharging stage, SOC is between 0.3 and 0, the SOC variable quantity is 0.3. And its voltage drop is pretty quick, from 3.2 V drops to 2.00 V. We have selected the voltage section 2.00˜3.00 V to conduct the testing, with selecting the fixed voltage value as 3.15 V. The battery cells have been charged with constant current and constant voltage to a certain fixed voltage value V c =3.15 V of the selected voltage section, following laying with open circuit to make sure that there is no external Load would consume the power of the battery for 4 h, the voltage becomes stable, with the change rate of less than 1 mV per 1 h. Then measure and record the battery cell initial voltage as V 0 , conduct laying for a period of 7 days, measure and record the voltage V 7 , select the battery cells with ΔV=V 0 −V 7 =5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 mV: 
     1) 0.2 C 5  charge and discharge for 1 cycle, the discharge capacity is C 5 ; 
     2) 0.2 C 5  CC/CV 3.6 V, 1/20 C charge to full; 
     3) laying for 28 days, i.e. t=28*24=672 hours; 
     4) 0.2 C 5  CC 2V (discharge capacity C 5 ′) 
     5) calculating C 5 ′/C 5 , which are the charge retention capabilities of the ΔV battery cells in each grade; I c =(C 5 −C 5 ′)/(24*28), which are the full charge state self discharge current of the ΔV battery cells in each grade; 
     Using I c  to perform curve fitting to the ΔV, so as to obtain the I c −ΔV equation;
 
 I   c =8*10−6 ΔV   4 −0.0006 ΔV   3 +0.0188 ΔV   2 −0.1546 ΔV+ 0.4634
 
     Correlation coefficient R 2 =0.9992 
     In regard to a battery group of n battery cells series connected, the range of self discharge current is the difference between the max self discharge current and the min self discharge current:
 
Δ I   c   =I   cmax   −I   cmin  
 
     In this way, it can be clearly known about the size of the consumption rate and self discharge current of the battery cells with different voltage drops, after being charged to the fixed voltage value of 3.15 V and then laying for 7 days. The laying service life can be calculated. It can be configured with equalization protection board and charger. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Battery Cells ΔV and the Corresponding I c . 
               
             
          
           
               
                 ΔV (mV) 
                 I c  (mA) 
                 Ic converted to C 5  (mA) 
               
               
                   
               
             
          
           
               
                 5 
                 0.090 
                  0.000090 C 5   
               
               
                 6 
                 0.093 
                 0.0000093 C 5   
               
               
                 7 
                 0.116 
                 0.0000116 C 5   
               
               
                 8 
                 0.155 
                 0.0000155 C 5   
               
               
                 9 
                 0.210 
                 0.0000210 C 5   
               
               
                 10 
                 0.277 
                 0.0000277 C 5   
               
               
                 11 
                 0.356 
                 0.0000356 C 5   
               
               
                 12 
                 0.444 
                 0.0000444 C 5   
               
               
                 13 
                 0.541 
                 0.0000541 C 5   
               
               
                 14 
                 0.645 
                 0.0000645 C 5   
               
               
                 15 
                 0.754 
                 0.0000754 C 5   
               
               
                 16 
                 0.869 
                 0.0000869 C 5   
               
               
                 17 
                 0.989 
                 0.0000989 C 5   
               
               
                 18 
                 1.112 
                 0.0001112 C 5   
               
               
                 19 
                 1.240 
                 0.0001240 C 5   
               
               
                 20 
                 1.371 
                 0.0001371 C 5   
               
               
                   
               
             
          
         
       
     
     Based on the results from the basic work above and the actual condition of ΔV distribution, there is no battery cells in the range of ΔV≦4 mV, in the range of ΔV=[5, 6], only a few are useful, while in the range ΔV≧21 mV, only 1% battery cells have been discarded. In this project, the battery consumption rate has been controlled as less than 9.2%. Select the battery cells with ΔV≦20 mV as the qualified battery cells; and their self discharge currents Ic are distributed within the range between 0.090˜1.371 mA. The grades are divided based on the requirements set forth below: 
     1. Battery Grade Dividing and Battery Matching 
     (1) grade dividing based on self discharge current and voltage difference: the self discharge current standard is set as I cs =1.371 mA, I c ≦I cs  is the qualified one. Based on the client&#39;s requirement about the laying service life, it needs to control the range of self discharge current of the battery cells within the same grade Δ I c  less than 0.2 C 5 /(8*30*24)=0.00003478 C 5  mA=0.3478 C 5  mA; in design set W=0.25, grade span length WI cs =0.3428 mA, i.e., the designed range of self discharge current of the battery cells within the same grade ΔI c =0.3428 mA, the grades dividing based on the self discharge current is (0, 0.343], (0.343, 0.686], (0.686, 1.028], (1.028, 1.371]; 
     Converted it to the voltage difference: set ΔV≦20 mV as the qualified, the grades divided according on the ΔV are, the first grade (0, 10], the second grade (10, 14], the third grade (14, 17], the fourth grade (17, 20]; a large number of the battery cells are within the range from the first grade to the third grade, and a small number are in the fourth grade; 
     (2) Grade dividing based on the capacity: the rated capacity is 10000 mAh, set the capacity standard as C s =10000 mAh, C≧C s  as the qualified. Set X=0.05, the grade span length XC s =500 mAh; the actual capacities are distributed in the range from 10000˜10999 mAh, the grades dividing based on the capacity are the first grade [10000, 10500), the second grade [10500, 11000]; 
     (3) The internal resistance is not divided into grades: set the internal resistance standard R s =5 mΩ, R≦R s  as the qualified; 
     (4) Constant current ratio is not divided into grades: set the constant current ratio standard C cs =90%, C c ≧C cs  as the qualified; 
     Sample data are shown in Table 3; 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Parameters for Battery Cells Matching. 
               
             
          
           
               
                   
                   
                   
                 Constant 
                   
                   
                 self 
               
               
                   
                   
                 Internal 
                 Current 
                   
                 Δ V 
                 discharge 
               
               
                   
                 Capacity 
                 resistance 
                 Ratio 
                 Voltage 
                 after 7 
                 current Ic 
               
               
                 No. 
                 [mAh] 
                 [mΩ] 
                 [ % ] 
                 [V] 
                 days 
                 [mA] 
               
               
                   
               
             
          
           
               
                 A1 
                 10281 
                 4 
                 92 
                 3.132 
                 19 
                 1.240 
               
               
                 A2 
                 10253 
                 4 
                 93 
                 3.131 
                 18 
                 1.112 
               
               
                 A3 
                 10225 
                 4 
                 93 
                 3.132 
                 17 
                 0.989 
               
               
                 A4 
                 10212 
                 3 
                 92 
                 3.134 
                 19 
                 1.240 
               
               
                 A5 
                 10253 
                 4 
                 93 
                 3.126 
                 20 
                 1.371 
               
               
                 A6 
                 10233 
                 4 
                 94 
                 3.128 
                 18 
                 1.112 
               
               
                 A7 
                 10262 
                 4 
                 93 
                 3.131 
                 18 
                 1.112 
               
               
                 A8 
                 10268 
                 3 
                 93 
                 3.133 
                 17 
                 0.989 
               
               
                 A9 
                 10249 
                 4 
                 93 
                 3.125 
                 20 
                 1.371 
               
               
                 A10 
                 10218 
                 4 
                 92 
                 3.130 
                 18 
                 1.112 
               
               
                 A11 
                 10262 
                 4 
                 94 
                 3.129 
                 19 
                 1.240 
               
               
                 A12 
                 10222 
                 4 
                 93 
                 3.128 
                 18 
                 1.112 
               
               
                   
               
             
          
         
       
     
     ΔV qualified are all in the fourth grade, capacity qualified are all in the first grade, internal resistance qualified and constant current ration qualified, so as to match the battery cells; 
     2. Equalization protection board configuration: in design battery group ΔI c =0.3428 mA; 
     Select the equalization protection board with its equalization current as I b =90 mA, Ib/ΔI c =262 fold; the equalization charge period is about 5 days; 
     3. Charger configuration: select 43.8 V/2 A charger, rated charging voltage is 43.8±0.2 V, rated charging current is 2 A, red light turned to green light charging cut-off current I cut =200 mA, I cut /I b =2.2 Fold. 
     4. Verification 
     After assembled, the battery group is charge and discharge with 0.5 C for 1 cycle. Discharge capacity is 10208 mAh, charged to the full state, being laid with open circuit to make sure that there is no external Load would consume the power of the battery till the voltage becomes stable; then measure and record voltage; lay for 5 days and then measure record voltage again. Charge the battery group, measure and record the stable voltage after the equalization charging. Next discharge it, the discharge capacity being 10200 mAh. Data are shown in Table 4, the discharge capacity after equalization charging is greater than the rated capacity. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Voltages Before and After Laying and the Voltage after 
               
               
                 Equalization Charge. 
               
             
          
           
               
                   
                 Voltage before 
                   
                 Voltage after 
               
               
                   
                 laying with full 
                 Voltage after laying for 
                 equalization 
               
               
                 No. 
                 charge [V] 
                 5 days [V] 
                 charge [V] 
               
               
                   
               
             
          
           
               
                 A1 
                 3.335 
                 3.335 
                 3.336 
               
               
                 A2 
                 3.337 
                 3.336 
                 3.336 
               
               
                 A3 
                 3.334 
                 3.334 
                 3.337 
               
               
                 A4 
                 3.336 
                 3.335 
                 3.336 
               
               
                 A5 
                 2.334 
                 2.334 
                 2.335 
               
               
                 A6 
                 3.335 
                 3.334 
                 3.335 
               
               
                 A7 
                 3.333 
                 3.333 
                 3.335 
               
               
                 A8 
                 3.336 
                 3.335 
                 3.336 
               
               
                 A9 
                 3.333 
                 3.332 
                 3.333 
               
               
                 A10 
                 3.335 
                 3.334 
                 3.335 
               
               
                 A11 
                 3.337 
                 3.336 
                 3.337 
               
               
                 A12 
                 3.336 
                 3.335 
                 3.335 
               
               
                   
               
             
          
         
       
     
     The implementation methods are not limited to the foregoing examples, as long as applying the principle of the present invention to conduct lithium secondary battery equalization matching and the formed battery system, such as increase the unnecessary charge or discharge operation on purpose, or increase the unnecessary temperature treatment, all within the claimed scope of the present invention.