Abstract:
To provide a constant current charging device capable of shortening a total charging time of a secondary battery even if an internal impedance of the secondary battery becomes high. A constant current charging device is provided which is equipped with a charging completion voltage detector, a battery voltage drop detector, an up-down counter which adds data when the up-down counter receives a detection signal of the charging completion voltage detector, subtracts the data when the up-down counter receives a detection signal of the battery voltage drop detector, and outputs the subtracted data, and a constant current generating circuit which switches a charging current to a secondary battery according to the data outputted from the up-down counter.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2015-101397 filed on May 18, 2015, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a charging device for charging a rechargeable battery such as a lithium ion secondary battery, and more particularly to a constant current charging device suitable for quick charging. 
         [0004]    2. Background Art 
         [0005]    There are several methods to charge a secondary battery. As a typical charging method, there may be mentioned constant current charging.  FIG. 4  is a diagram illustrating a charging current I and a battery voltage VBAT in conventional constant current charging. 
         [0006]    At an initial time of charging, the secondary battery is charged with a large current. When the battery voltage VBAT rises and reaches a charging completion voltage VEND (e.g., 4.2V), the charging current is switched to a current which gradually becomes lower. At last, the secondary battery is charged with a prescribed minimum current. When the charging completion voltage VEND is reached, the charging is completed. 
         [0007]    [Patent Document 1] Japanese Patent Application Laid-Open No. H8-203563 
       SUMMARY OF THE INVENTION 
       [0008]    In a lithium ion secondary battery small in size and low in cost, the internal impedance thereof has recently been in a trend for increase as compared with other lithium ion secondary batteries. 
         [0009]    In the conventional constant current charging, when the internal impedance is high, a drop voltage of the battery voltage becomes large at timings t 1 , t 2  and so on provided for charging current switching, and the amount of the voltage charged after the switching of the charging current increases. Therefore, the time for charging at each constant current value becomes long, and the total charging time becomes long. Further, when the internal impedance becomes high, the detection of the voltage of the secondary battery becomes easy to be affected by external noise. When the voltage of the secondary battery instantaneously exceeds the charging completion voltage VEND due to the external noise, the charging current is switched irrespective of the charging completion voltage VEND being not reached, so that the time taken up to the completion of charging becomes long. 
         [0010]    The present invention has been invented to solve the above problems and provides a constant current charging device capable of shortening a charging time even though an internal impedance of a secondary battery is high. 
         [0011]    In order to solve the related art problems, the constant current charging device of the present invention which quickly charges the secondary battery is configured as follows: 
         [0012]    The constant current charging device is provided which includes a charging completion voltage detector which detects that the voltage of a secondary battery reaches a prescribed charging completion voltage, a battery voltage drop detector which detects a drop in the voltage of the secondary battery, based on a detection voltage lower than the charging completion voltage, an up-down counter which adds data when the up-down counter receives a detection signal of the charging completion voltage detector and subtracts the data when the up-down counter receives a detection signal of the battery voltage drop detector, and outputs the subtracted data therefrom, and a constant current generating circuit which switches a charging current to the secondary battery according to the data outputted from the up-down counter. 
         [0013]    Since the constant current charging device of the present invention is configured in the above-described manner, a charging time can be shortened irrespective of the internal impedance of the secondary battery. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram of a constant current charging device according to the present embodiment; 
           [0015]      FIG. 2  is a diagram illustrating a charging current and a battery voltage in constant current charging of the present embodiment; 
           [0016]      FIG. 3  is a block diagram illustrating another example of the constant current charging device according to the present embodiment; and 
           [0017]      FIG. 4  is a diagram illustrating a charging current and a battery voltage in conventional constant current charging. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The present embodiment will hereinafter be described with reference to the accompanying drawings. 
         [0019]      FIG. 1  is a block diagram of a constant current charging device according to the present embodiment. 
         [0020]    The constant current charging device  100  is equipped with a power supply circuit  103  such as a DC-DC converter, a constant current generating circuit  102 , a voltage division circuit  109 , a reference voltage generating circuit  108 , a charging completion voltage detector  106 , a battery voltage drop detector  107 , and an up-down counter  105 . 
         [0021]    The constant current generating circuit  102  is inputted with an output voltage VIN of the power supply circuit  103  and supplies current to a secondary battery  101 . The voltage division circuit  109  is connected across the secondary battery  101  and divides a battery voltage VBAT. The reference voltage generating circuit  108  generates a reference voltage VREF. The charging completion voltage detector  106  is inputted with a voltage VD 1  of a node D 1  of the voltage division circuit  109  and the reference voltage VREF and compares them with each other to detect a charging completion voltage VEND. The battery voltage drop detector  107  is inputted with a voltage VD 2  of a node D 2  of the voltage division circuit  109  and the reference voltage VREF and compares them with each other to detect a voltage VSTP lower than the charging completion voltage VEND. The up-down counter  105  performs up count on the basis of a detection signal of the charging completion voltage detector  106  and performs down count on the basis of a detection signal of the battery voltage drop detector  107 , and inputs an output signal to an input terminal of the constant current generating circuit  102 . 
         [0022]    The output signal of the up-down counter  105  is defined as a signal corresponding to the number N (where N is a natural number of 2 or more) of steps of current to be switched. As this signal, there may be mentioned N signals or a bus signal defined in binary notation, or the like. The constant current generating circuit  102  controls a current value so as to be low each time the input signal approaches N. 
         [0023]    The operation of constant current charging in the constant current charging device according to the present embodiment will next be described. 
         [0024]      FIG. 2  is a diagram illustrating a charging current and a battery voltage in the constant current charging of the present embodiment. In the constant current charging device according to the present embodiment, the number N of steps of the charging current is assumed to be 5. 
         [0025]    Since the up-down counter  105  outputs data “1” at an initial time (time up to tp 1 ) of starting charging, the constant current generating circuit  102  charges the secondary battery  101  with a charging current I 1 . The secondary battery  101  is charged with the charging current I 1  so that the battery voltage VBAT thereof gradually rises. When the battery voltage VBAT reaches the charging completion voltage VEND at the time tp 1 , the charging completion voltage detector  106  outputs a detection signal (e.g., a Hi level). In response to the detection signal of the charging completion voltage detector  106 , the up-down counter  105  adds 1 to the data “1” to output data “2”. In response to the data “2”, the constant current generating circuit  102  switches the charging current I 1  to a charging current I 2  equivalent to the data “2” and supplies the charging current I 2  to the secondary battery  101 . 
         [0026]    When the charging current is reduced, the battery voltage VBAT of the secondary battery  101  drops due to the influence of an internal impedance. When the battery voltage VBAT falls below the detection voltage VSTP, the battery voltage drop detector  107  outputs a detection signal (e.g., a Hi level). In response to the detection signal of the battery voltage drop detector  107 , the up-down counter  105  subtracts 1 from the data “2” to output data “1”. In response to the data “1”, the constant current generating circuit  102  switches to the charging current I 1  equivalent to the data “1” and supplies the charging current I 1  to the secondary battery  101  again. 
         [0027]    When the battery voltage VBAT reaches the charging completion voltage VEND again at a time tp 2 , the up-down counter  105  outputs data “2” in like manner, and the constant current generating circuit  102  reduces the charging current from the charging current I 1  to the charging current I 2 . Further, when the battery voltage VBAT drops and falls below the detection voltage VSTP, the charging current is switched to I 1 . 
         [0028]    At a time tp 3 , when the battery voltage VBAT reaches the charging completion voltage VEND, and the charging current is reduced to I 2 , the battery voltage VBAT does not fall below the detection voltage VSTP. Therefore, since the battery voltage drop detector  107  does not output the detection signal, the constant current generating circuit  102  continues charging to the secondary battery with the charging current I 2 . 
         [0029]    When the battery voltage VBAT reaches the charging completion voltage VEND again at a time tp 4 , the up-down counter  105  outputs data “3” in the same way, and the constant current generating circuit  102  reduces the charging current from the charging current I 2  to a charging current I 3 . Further, when the battery voltage VBAT drops and falls below the detection voltage VSTP, the charging current is switched to I 2 . 
         [0030]    At a time tp 5 , when the battery voltage VBAT reaches the charging completion voltage VEND, and the charging current is reduced to I 3 , the battery voltage VBAT does not fall below the detection voltage VSTP. Therefore, since the battery voltage drop detector  107  does not output the detection signal, the constant current generating circuit  102  continues charging to the secondary battery  101  with the charging current I 3 . 
         [0031]    At a time tp 6 , when the battery voltage VBAT reaches the charging completion voltage VEND again, and the charging current is reduced to I 4 , the battery voltage VBAT does not fall below the detection voltage VSTP. Therefore, since the battery voltage drop detector  107  does not output the detection signal, the constant current generating circuit  102  continues charging to the secondary battery  101  with the charging current I 4 . 
         [0032]    At a time tp 7 , when the battery voltage VBAT reaches the charging completion voltage VEND again, and the charging current is reduced to I 5 , the battery voltage VBAT does not fall below the detection voltage VSTP. Therefore, since the battery voltage drop detector  107  does not output the detection signal, the constant current generating circuit  102  continues charging to the secondary battery  101  with the charging current I 5 . 
         [0033]    The constant current charging device according to the present embodiment continues charging with the charging current I 5  with the charging current I 5  as the last charging current, and completes charging when the battery voltage VBAT reaches the charging completion voltage VEND. 
         [0034]      FIG. 3  is a block diagram illustrating another example of the constant current charging device according to the present embodiment. 
         [0035]    The constant current charging device  200  further includes a delay circuit  110  provided between an output terminal of a charging completion voltage detector  106  and an input terminal of an up-down counter  105 , and a delay circuit  111  provided between an output terminal of a battery voltage drop detector  107  and an input terminal of the up-down counter  105 . The delay circuit  110  delays a detection signal outputted from the charging completion voltage detector  106  by a preset time and inputs the same to the up-down counter  105 . The delay circuit  111  delays a detection signal outputted from the battery voltage drop detector  107  by a preset time and inputs the same to the up-down counter  105 . 
         [0036]    The delay circuits  110  and  111  have a function of stabilizing switching control of a charging current of a constant current generating circuit  102 . Setting the delay time of the delay circuit  110  longer than the delay time of the delay circuit  111  makes it possible to more stabilize the switching control of the charging current of the constant current generating circuit  102 . 
         [0037]    As described above, according to the constant current charging device of the present embodiment, when the detection voltage VSTP lower than the charging completion voltage VEND is provided, and the battery voltage VBAT falls below the detection voltage VSTP when the charging current is reduced, the procedure of returning the charging current to the current value of the previous stage is added. Consequently, it is possible to shorten the time required to charge the secondary battery irrespective of the characteristics of an internal impedance. 
         [0038]    Incidentally, the battery voltage drop detector  107  may be configured to change the detection voltage VSTP according to the charging current of the constant current generating circuit  102 . Since a drop in the battery voltage VBAT also becomes small when the charging current becomes small, it is possible to further shorten the charging time by changing the detection voltage VSTP according to the drop.