Abstract:
Since in existent charge-discharge control the battery direct current resistance of a lithium ion battery increases at a low temperature or in a low SOC, the battery voltage may possibly exceed greatly the maximum allowable voltage instantaneously due to over-voltage during charging or may possibly decrease greatly to less than the minimum allowable voltage during discharging. If the lithium ion battery continuously lies in such a state the battery performance is degraded abruptly. 
     The battery resistance of a lithium ion battery module is estimated in accordance with SOC and temperature of the lithium ion battery module, and current to charge is set properly based on the estimated battery resistance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a battery system using non-aqueous secondary batteries typically represented by lithium ion batteries, a battery device having the battery system mounted thereon, and a battery system-mounted vehicle. 
     2. Description of the Related Art 
     In recent years, lithium ion batteries have been utilized as a driving power supply for vehicles such as hybrid cars and electric cars and portable electronic devices such as notebook personal computers and digital cameras. 
     Degradation phenomena occur in such a battery system using lithium ion batteries. Due to repeated charge-discharge cycles, the battery capacity is lowered and the direct current resistance of the battery is increased, resulting in degradation phenomena. The degradation phenomena likely occur in charge-discharge cycles under the condition of large current and large capacity. For example, the degradation phenomena means that the capacity of insertable lithium ions to positive or negative electrode materials decreases and thereby battery performance lowers. Therefore, charging or discharging to a lithium ion battery is controlled such that the state of charge (SOC) of the battery falls within a predetermined range. Japanese Patent No. 4052080 discloses a control device that judges the state as overcharge when the voltage of a lithium ion battery exceeds a maximum allowable voltage and judges the state as over-discharge when the voltage is lower than the minimum allowable voltage, thereby stopping charging or discharging. 
     SUMMARY OF THE INVENTION 
     However, if such a charge-discharge control is employed, the battery direct current resistance increases when the lithium ion battery is at a low temperature or in a low SOC. Thus the voltage may instantaneously exceed the maximum allowable voltage greatly due to over voltage during charging, whereas the voltage may be greatly made lower than the minimum allowable voltage during discharging. If such a state continues for the lithium ion battery, then the battery performance will lower abruptly. 
     Further, as long as the upper and lower limit values of the charge and discharge current to charge and discharge and voltage are set by detecting the temperature of the lithium ion battery, the battery cannot be charged to the maximum allowable voltage higher than a restriction value. Further, if such control method is used, the voltage of the lithium ion battery may instantaneously exceed the maximum allowable voltage when the lithium ion battery is in the low SOC or at a low temperature, with the result that the battery performance may abruptly lower. 
     The present invention has been achieved based on such a finding and intends to provide a secondary battery system capable of suppressing lowering of the battery capacity and increase in the battery direct current resistance by properly controlling a value of current to charge and a charging time to a lithium ion battery. Further, the invention provides a battery device having the secondary battery system mounted thereon and a vehicle mounting the secondary battery system. 
     According to the invention, a battery resistance of a lithium ion battery module is estimated in accordance with SOC and temperature of the lithium ion battery module and current to charge is set properly based on the estimated battery resistance. 
     When the lithium ion battery module is charged, the battery resistance of the lithium ion battery module is estimated in accordance with the SOC and the temperature of the lithium ion battery module and the current to charge is properly set. Thus since the voltage does not instantaneously exceed the maximum allowable voltage due to over voltage, improvement in the life of the lithium ion battery can be expected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: 
         FIG. 1  is a partially cut-away perspective view of a cylindrical non-aqueous secondary battery; 
         FIG. 2  is a schematic configurational view of a secondary battery system; 
         FIG. 3  is a system flow chart of a secondary battery system; and 
         FIG. 4  is a correlation table of battery resistance of a non-aqueous secondary battery. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Then, an embodiment of the invention will be described with reference to the drawings. 
       FIG. 1  shows a non-aqueous secondary battery used in the invention (hereinafter also referred to simply as a battery or a lithium ion battery). A positive electrode plate  11  comprises a composite lithium oxide as an active material. A negative electrode plate  12  comprises a lithium ion retaining material as an active material. The positive electrode plate  11  and the negative electrode plate  12  are spirally wound by way of a separator  13  to provide a rolled-up electrode unit  22 . Then the rolled-up electrode unit  22  is contained inside a bottomed cylindrical battery can  26 . A negative electrode tab  24  led out from the lower part of the rolled-up electrode unit  22  is welded to the bottom of the battery can  26  and then a positive electrode tab  23  led out from the upper part of the rolled-up electrode unit  22  is welded to a battery cap  25 . A predetermined electrolyte is poured into the battery can  26 , and the battery cap  25  having an insulating gasket (not illustrated) at the periphery is attached and caulked to the opening of the battery can  26 . The side of the axis of winding  21  is defined as an inner circumferential side  31  and the outside thereof is defined as an outer peripheral part  32 . 
     The positive electrode active material coated on the positive electrode plate  11  includes, for example, lithium cobaltate and modified products thereof (products formed by solid solubilizing aluminum or magnesium into lithium cobaltate), lithium nichelate and modified products thereof (products formed by partially substituting cobalt for nickel), lithium manganate and modified products thereof, and composite oxides thereof (containing nickel, cobalt, manganese). 
     As a conductive agent, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and various kinds of graphites can be used each alone or in combination. 
     As the binder for the positive electrode, polyvinylidene fluoride (PVdF), modified product of polyvinylidene fluoride, polytetrafluoroethylene (PTFE), and particulate rubber binder comprising acrylate units can be used, in which an acrylate monomer or an acrylate oligomer introduced with a reactive functional group can also be incorporated in the binder. 
     Then, as the negative electrode active material coated on the negative electrode plate  12 , various kinds of natural graphites, artificial graphites, silicone type composite materials such as silicides, and various kinds of metal plastic materials can be used. 
     As the binder for the negative electrode, while PVdF and modified products thereof, as well as various kinds of binders can be used, it is more preferred to use styrene-butadiene copolymer rubber particles (SBR) and modified products thereof in combination with or with addition of a small amount of cellulose type resins such as carboxymethyl cellulose (CMC). 
     As the conductive agent, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and various kinds of graphites can be used, for example, each alone or in combination. 
     The separator is not restricted particularly so long as the separator has a composition endurable within the range of use of the lithium ion secondary battery. A microporous film comprising an olefinic resin, for example, polyethylene or polypropylene may be used for the separator generally and preferably each alone or in a composite manner. The thickness of the separator is not particularly restricted and is preferably from 10 to 40 μm. 
     For the electrolyte, various kinds of lithium compounds such as LiPF 6  and LiBF 4  can be used as an electrolyte salt. Further as the solvent, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC) can be used each alone or in combination. Further, vinylene carbonate (VC), cyclohexyl benzene (CHB), and modified products thereof are used preferably to form a satisfactory film on the positive electrode and the negative electrode, thereby ensuring the stability upon overcharging or over-discharging. 
     The shape of the rolled-up electrode unit in the invention is not necessarily be a normal cylindrical shape but may also be a shape such as length cylindrical shape having an elliptic rolled-up cross section or a quadrangular shape having a rectangular rolled-up cross section. A typical mode of use is as follows preferably. The rolled-up electrode unit and the electrolyte are filled in a bottomed cylindrical battery can and sealed in a state in which a tab for taking out current from the electrode plate is welded to the cap and the battery can. However, the present invention is not limited to such a mode. 
     The battery can for filling the rolled-up electrode unit is not particularly restricted and those exhibiting the good performance in strength, corrosion resistance, and machinability such as a battery can plated on iron or stainless steel battery are preferred to provide corrosion resistance. Further, for the battery can, aluminum alloys or various kinds of engineering plastics can be used for reduction in weight and various kinds of engineering plastics and metals can be used in combination. 
     Then,  FIG. 2  shows a secondary battery system. A lithium ion battery module  41  comprises a plurality of the batteries described above assembled in series, parallel, or serial-parallel connection. To detect the state of the lithium ion battery module battery  41 , the secondary battery system is provided with a control device (entire controller)  61 . The control device  61  has a battery state detection device for detecting battery voltage, charge and discharge current, and battery surface temperature of the lithium ion battery module  41  (measuring device)  42 . Further, the control device  61  calculates electric resistance, quantity of cumulative charge/discharge electricity, and cumulative operation time based on the detected values. 
     The battery resistance Z of the lithium ion battery module  41  can be represents by the formula (1) assuming a terminal voltage as V, an open voltage as V 0  and a current as I of the lithium ion battery module  41 . In the formula (1), the discharge side is defined as “+”. Further, the formula (2) is derived from the formula (1) and the battery resistance Z can be calculated.
 
 V=V   0   −I×Z   (1)
 
 Z= ( V   0   −V )/ I   (2)
 
     Further, the lithium ion battery module  41  and power switching devices  43  are combined in series, and the control device  61  transmits a control signal by way of a control signal transmission device  44  to the power switching device  43  in accordance with a detected value obtained from the battery state detection device  42  of the lithium ion battery module  41  and the amount of a demanded power from an electric load  71 . 
     The control device  61  includes a microcomputer having CPU, ROM, and RAM and operated in accordance with a predetermined program. Then, the control device  61  controls charging and discharging to the lithium ion battery module  41  based on the detected value obtained from the battery state detection device  42 . 
     A voltage detection means as the battery state detection device  42  detects the voltage on the lithium ion battery module. The battery voltage to be detected conceivably includes the voltage on one of batteries constituting the lithium ion battery module  41 , the voltage on a battery group comprising a plurality of batteries connected in series, or the voltage on assembled batteries comprising a plurality of batteries in serial and parallel connection, but it is not particularly restricted. 
     Then, a current detection device detects the value of charge and discharge current. While instruments used in the detection method conceivably include a galvanometer, a galvanometer using a shunt resistor, and a clamp meter, they are not restrictive but any of means capable of detecting current values can be used. 
     Then, a temperature detection device detects the temperature of the lithium ion battery module  41 . While devices for detecting temperature conceivably include a thermocouple, a thermister, etc., they are not restricted particularly. Examples of the place where temperature is to be detected includes the surface of the battery, the inside of the battery, the surface of a casing containing the lithium ion battery, and the environment of the lithium ion battery module  41 . 
     While the power switching device  43  may include, for example, a semiconductor switch, a mechanical switch, an inverter or a DC-DC converter as power conversion equipment, etc., the power switching device  43  are not limited thereto and any of devices capable of controlling the current value upon charging and discharging to the lithium ion battery module  41  may be used. 
     The power conversion device  51  is a device which converts a DC current obtained from the lithium ion battery into an AC current in response to the load. Usually, an inverter is preferably used as the power conversion device  51 . A capacitor  52  is connected in parallel with the power conversion device  51 . Even if power supply to the power conversion device  51  is reduced to 0 instantaneously, current is discharged from the capacitor  52 , thereby allowing stable power to be supplied to the power converter  51 . 
     The electric load  71  may be a heater, an electric brake, an electric power steering, or an electric motor, for example, in the use to an automobile. 
     Then, the charge-discharge control method by the control device  61  will be described.  FIG. 3  is a flow chart of a secondary battery system according to this embodiment. 
     First, at the starting step S 001 , an instruction for starting charging to the lithium ion battery module  41  is sent as a signal from the control device  61  to the lithium ion battery module  41  to be charged. 
     At the step S 002 , the battery resistance R of the lithium ion battery module  41  is estimated. Details as to how to estimate will be described in the step S 101 . 
     At the step S 101 , a battery resistance R just before charging is estimated. More specifically, a battery resistance R′ in a standard state is first calculated periodically, for example, upon maintenance of the lithium ion battery module  41 . The calculation method is shown by the formula (2). While the standard state is not particularly restricted, SOC of 50% and temperature of 30° C., i.e., such conditions that the degradation of the lithium ion battery does not develop as much as possible, are desirable. 
     Then, the control device  61  retrieves an initial battery resistance R 0 ′ in a standard state calculated under the same conditions of SOC and temperature as those in the standard state from a previously stored initial battery resistance correlation table ( FIG. 4 ). Then, the ratio α is calculated according to the formula (3) based on the two resistance values.
 
α=( R′/R   0 ′)  (3)
 
     Then, an inter-terminal voltage V and a battery temperature T of the lithium ion battery module  41  are detected by the battery state detection device  42  in  FIG. 2  and SOC is calculated based on the inter-terminal voltage of the lithium ion battery module  41  at the step S 011 . Then, the initial battery resistance R 0  corresponding to the current SOC and battery temperature is retrieved from the initial battery resistance correlation table in  FIG. 4 . 
     Then, the current battery resistance R of the lithium ion battery module  41  is estimated according to the formula (4).
 
 R=α×R   0   (4)
 
     At the step S 003 , charge current I to charge is represented by the formula (5) assuming the inter-terminal voltage as V, the maximum allowable voltage as V k , and the estimated battery resistance as R for the lithium ion battery module  41 . σ is an adjusting value, for which variations between batteries (standard deviation, etc.) may be used but is not particularly restricted. After setting of the charge current I, the flow proceeds to the step S 004 . 
     The charge current I is set repeatedly. The newly set value of charge current I is smaller than the previously set value.
 
 I≦ ( V   k   −V )/(σ× R )  (5)
 
     At the step S 004 , the lithium ion battery module  41  is charged. When the inter-terminal voltage of the lithium ion battery module  41  reaches a maximum allowable voltage V K , charging is stopped and a charge stop signal is sent to the control device  61 . Then, the flow proceeds to the step S 005 . When the inter-terminal voltage V of the lithium ion battery module  41  is lower than the maximum allowable voltage V k , charging is continued. 
     At the step S 005 , SOC of the lithium ion battery module  41  is calculated again. When the calculated SOC exceeds a predetermined standard SOC k , the flow proceeds to the step S 006 , at which charging is completed. When the calculated SOC is not higher than the predetermined standard SOC k , the flow proceeds to the step S 101 , at which the battery resistance R of the lithium ion battery module  41  is calculated again. 
     As described above, according to this embodiment, the lithium ion battery module  41  can be charged by the control of detecting the state of the battery of the lithium ion battery module  41  by the battery state detection device  42 , estimating the battery resistance R of the lithium battery module  41  based on the detected value and the initial battery resistance correlation table, and setting the charge current I, wherein the maximum allowable voltage is not reached due to over voltage. Accordingly, lowering of the battery capacity and increase in the battery resistance of the lithium ion battery can be suppressed, thereby providing a long life secondary battery system. 
     The present invention is not restricted to the embodiment described above and is applicable to other embodiments with appropriate modification within a range not departing from the gist of the invention. 
     For example, while the rolled-up type lithium ion battery is used as the battery in the embodiment described above, the invention is applicable also to a stacked type lithium ion battery where a plurality of positive electrode plates and a plurality of negative electrode plates are stacked alternately by way of separators respectively.