Charger for use with a nickel-cadmium battery that uses charge and discharge cycles dependent on voltage and current
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a charger used for charging a Ni-Cd (Nickel-Cadmium) battery. 
     2. Description of the Related Art 
     Heretofore, a charging device or charger used for charging a Ni-Cd battery has basically been made up of a constant-current circuit. When one attempts to charge the Ni-Cd battery using the charger, a controller of the charger detects a charging current flowing in the Ni-Cd battery and effects control for causing the charging current to reach a predetermined current value corresponding to the rated capacity of the Ni-Cd battery. At this time, an internal voltage of the Ni-Cd battery continuously supplied with the constant charging current gradually increases to reach a predetermined voltage value. Thus, the controller of the charger is activated to complete the charging of the Ni-Cd battery. 
     However, when the Ni-Cd battery is left unused for a long period of time, the electromotive force of the Ni-Cd battery is reduced. The Ni-Cd battery whose electromotive force has been reduced in this way has a high electric resistance and needs a high voltage to cause the charging current to flow in the Ni-Cd battery by the constant-current type charger. However, a problem arises in that when the high voltage is continuously supplied to a Ni-Cd battery that has been left for a long period, the Ni-Cd battery generates heat due to its high electric resistance, causing the Ni-Cd battery to deteriorate. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve at least the above problem. It is therefore an object of the present invention to provide a charger capable of recovering a Ni-Cd battery whose electromotive force is reduced because the Ni-Cd battery has been unused for a long period without deteriorating the Ni-Cd battery. 
     According to one aspect of the present invention, for achieving at least the above object, a charger is provided that is suitable for use in a Ni-Cd (Nickel-Cadmium) battery comprising charging means for causing a charging current to flow in the Ni-Cd battery and increasing an internal voltage of the Ni-Cd battery to a predetermined first voltage to thereby charge the Ni-Cd battery. Discharging means is provided for discharging an electric charge of the Ni-Cd battery charged by the charging means by a predetermined amount or quantity to thereby reduce the first voltage of the Ni-Cd battery to a predetermined second voltage. Controlling means is provided for controlling the Ni-Cd battery so that the Ni-Cd battery is initially and finally charged at least twice and for effecting control for allowing the discharging means to discharge the Ni-Cd battery during a period between the periods in which the Ni-Cd battery is being charged at least twice. 
     According to this invention, the controlling means allows the charging means to effect a charging process of increasing the internal voltage of the Ni-Cd battery to the predetermined first voltage at least twice. Further, the controlling means causes the Ni-Cd battery to be discharged once during the period between the at least two charging periods. As a result, an internal resistance of the Ni-Cd battery becomes low, and the Ni-Cd battery whose electromotive force has been reduced can be recovered without deterioration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described in detail with reference to the following figures, wherein: 
     FIG. 1 is a block diagram schematically showing the structure of a charger of the present invention; 
     FIG. 2 is a flowchart of the operation of the charger shown in FIG. 1; and 
     FIG. 3 is a graph of the waveform of a current that flows in a Ni-Cd battery by the charger shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 1, the structure of a charging device or charger in the present embodiment is schematically described. The charger comprises a switching unit or charging switch 1, a direct current (DC) supply 2, a controller 3, a current detecting resistor 5, a discharging switch 6 and a discharging resistor 7. 
     The charging switch 1 is electrically connected to the DC supply 2 and the controller 3 and is turned on and off by the controller 3. A Ni-Cd battery 4 is constructed to be connectable to the charging switch 1. When the charging switch 1 is in an &#34;on&#34; state, the DC supply 2 can apply a predetermined voltage V 0  to the Ni-Cd battery 4 so that a charging current I flows in the Ni-Cd battery 4. Thus, the Ni-Cd battery 4 is charged by the charging current I. When the charging switch 1 is in an &#34;off&#34; state, the DC supply 2 is electrically disconnected from the Ni-Cd battery 4 so that the predetermined voltage V 0  is not applied to the Ni-Cd battery 4, with the result that the Ni-Cd battery 4 is not charged. 
     The DC supply 2 is electrically connected to the charging switch 1 as described above. Further, the DC supply 2 is electrically connected to the current detecting resistor 5 and the discharging resistor 7. As mentioned above, the DC supply 2 is constructed so that the predetermined voltage V 0  can be applied to the Ni-Cd battery 4. 
     As described above, the controller 3 is electrically connected to the charging switch 1 and further connected to the discharging switch 6. The controller 3 can detect a voltage V applied to the Ni-Cd battery 4 using a voltage detection line 8 electrically connected to the controller 3 between the charging switch 1 and the Ni-Cd battery 4. Further, the controller 3 can detect the current I flowing in the Ni-Cd battery 4 using a current detection line 9 electrically connected to the controller 3 between the Ni-Cd battery 4 and the current detecting resistor 5. The controller 3 has a microcomputer incorporated therein, which comprises a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an A/D (Analog/Digital) converter, etc., all of which are not shown in the drawing but are conventional. The controller 3 effects the action of turning each of the charging switch 1 and the discharging switch 6 on and off in accordance with a control program stored in the ROM in advance to thereby enable the Ni-Cd battery 4 to be charged without being deteriorated. 
     The current detecting resistor 5 is used as a resistor for detecting the current I that flows in the Ni-Cd battery 4 when the Ni-Cd battery 4 is electrically connected to the charger and the charging of the Ni-Cd battery 4 is started. 
     As described above, the discharging switch 6 is controlled by the controller 3 to be brought into either the on or off state. When the discharging switch 6 is switched on, the electric charge stored in the Ni-Cd battery 4 is discharged through the discharging resistor 7. 
     A series of charging operations of the charger in the present embodiment is described with reference to FIGS. 1 through 3. 
     An operator first connects the Ni-Cd battery 4, whose electromotive force has been reduced, to the charger in the present embodiment. In doing so, the controller 3 starts to charge the Ni-Cd battery 4 in accordance with the control program stored in the ROM in advance. At this time, each of the charging switch 1 and the discharging switch 6 is in the off state. 
     The controller 3 first brings the discharging switch 6 to the on state and allows the discharging resistor 7 to discharge the electric charge of the Ni-Cd battery 4 (Step 1: hereinafter called &#34;S1&#34; and other Steps are abbreviated in the same manner as referred to above). At this time, the controller 3 detects the voltage V of the Ni-Cd battery 4 (S2). If the controller 3 decides that the voltage V of the Ni-Cd battery 4 has not been reduced to a predetermined voltage V L  (i.e., if the answer is negative in S2), then the controller 3 continues to detect the voltage V of the Ni-Cd battery 4. If the controller 3 determines that the voltage V has been reduced to the voltage V L  (i.e., if the answer is affirmative in S2), then the controller 3 switches the discharging switch 6 from the on state to the off state so that the Ni-Cd battery 4 stops discharging (S3). A period from S1 to S3 corresponds to a discharging period D1 shown in FIG. 3. At this tine, the current I, which flows in the Ni-Cd battery 4, is brought to a constant current I L . 
     Next, when the controller 3 switches on the charging switch 1, the DC supply 2 applies the predetermined voltage V 0  to the Ni-Cd battery 4 to thereby cause the charging current I to flow in the Ni-Cd battery 4 so that the Ni-Cd battery 4 is charged (S4). At this time, the controller 3 detects the current I flowing in the Ni-Cd battery 4 through the current detecting resistor 5 (S5). If the controller 3 decides that the current I flowing in the Ni-Cd battery 4 has not yet been raised to a predetermined rated capacity, i.e., a predetermined current value I H  (i.e., if the answer is negative in S5), then the controller 3 continues to detect the current I flowing in the Ni-Cd battery 4. If the controller 3 determines that the current I flowing in the Ni-Cd battery 4 has been raised to the predetermined current value I L  (i.e., if the answer is affirmative in S5), then the controller 3 switches off the charging switch 1 to stop the application of the voltage V 0  to the Ni-Cd battery 4 (S6). Next, the controller 3 detects the voltage V of the Ni-Cd battery 4 (S7). If the controller 3 determines that the voltage V of the Ni-Cd battery 4 has not been raised to a predetermined voltage V H  (i.e., if the answer is negative in S7), then the controller 3 switches on the charging switch 1 again when the current I flowing in the Ni-Cd battery 4 is brought to 0 and the controller 3 starts to cause the charging current I to flow in the Ni-Cd battery 4 (S4). Thus, the controller 3 repeats the above control from S4. Then, the controller 3 detects the voltage V of the Ni-Cd battery 4 while alternately switching the charging switch 1 on and off. Further, the controller 3 charges the Ni-Cd battery 4 until the voltage V of the Ni-Cd battery 4 is raised up to the predetermined voltage V H . If the controller 3 decides that the voltage V has been raised to the predetermined voltage V H  (i.e., if the answer is affirmative in S7), then the controller 3 brings the discharging switch 6 into the on state so that the electric charge of the Ni-Cd battery 4 is discharged (S8). A repetitive period from S4 to S7 corresponds to a charging period C1 shown in FIG. 3. Accordingly, the charging current I having a pulse waveform shown in FIG. 3 flows in the Ni-Cd battery 4 during the charging period C1. 
     Further, the controller 3 detects the voltage V of the Ni-Cd battery 4 (S9). If the controller 3 decides that the voltage V of the Ni-Cd battery 4 has not been reduced to the predetermined voltage V L  (i.e., if the answer is negative in S9), then the controller 3 continues to detect the voltage V of the Ni-Cd battery 4. If the controller 3 decides that the voltage V has been reduced to the predetermined voltage V L  (i.e., if the answer is affirmative in S9), then the controller 3 turns off the discharging switch 6 so that the Ni-Cd battery 4 stops discharging (S10). A period from S8 to S10 corresponds to a discharging period D2 shown in FIG. 3. At this time, the current I flowing in the Ni-Cd battery 4 is brought to the constant current I L . 
     Next, when the controller 3 switches on the charging switch 1 again, the DC supply 2 applies the predetermined voltage V 0  to the Ni-Cd battery 4 to thereby allow the charging current I to flow in the Ni-Cd battery 4 so that the Ni-Cd battery 4 is charged (S11). At this time, the controller 3 detects the current I flowing in the Ni-Cd battery 4 through the current detecting resistor 5 (S12). If the controller 3 decides that the current I flowing in the Ni-Cd battery 4 has not yet been raised to the predetermined current value I H  (i.e., if the answer is negative in S12), then the controller 3 continues to detect the current I flowing in the Ni-Cd battery 4. If the controller 3 determines that the current I flowing in the Ni-Cd battery 4 has been raised to the predetermined current value I H  (i.e., if the answer is affirmative in S12), then the controller 3 switches off the charging switch 1 to stop the application of the voltage V 0  to the Ni-Cd battery 4 (S13). Further, the controller 3 detects the voltage V of the Ni-Cd battery 4 (S14). If the controller 3 determines that the voltage V of the Ni-Cd battery 4 has not been raised to the predetermined voltage V H  (i.e., if the answer is negative in S14), then the controller 3 repeats the above control from S4. Then, the controller 3 detects the voltage V of the Ni-Cd battery 4 while repeating the action of turning the charging switch 1 on and off. Further, the controller 3 charges the Ni-Cd battery 4 until the voltage V is raised up to the predetermined voltage V H . If the controller 3 decides that the voltage V has been raised to the predetermined voltage V H  (i.e., if the answer is affirmative in S14), then the controller 3 stops the Ni-Cd battery 4 from being charged. A repetitive period from S11 to S14 corresponds to a charging period C2 shown in FIG. 3. Accordingly, the charging current I having a pulse waveform shown in FIG. 3 flows in the Ni-Cd battery 4 during the charging period C2. When the process in S14 is completed, the control program used for the charging of the Ni-Cd battery 4, which has been stored in the ROM, is finished and the charging of the Ni-Cd battery 4 is completed. 
     Thus, according to the charger of the present embodiment, the charging current I, which flows in the Ni-Cd battery 4, is not excessively increased beyond the predetermined rated capacity of the Ni-Cd battery 4, i.e., the predetermined current value I H . Therefore, the Ni-Cd battery 4 is not deteriorated due to the heat generated. 
     Since the voltage V 0  of the DC supply 2 is set to a voltage value which can provide a sufficient charging current I even if the electromotive force of the Ni-Cd battery 4 is reduced and its electric resistance is high, the capacity of the Ni-Cd battery 4 can be recovered. 
     Further, since the charging (C1) is first executed between the discharging periods D1 and D2 prior to the charging period C2 in the present embodiment, the internal electric resistance of the Ni-Cd battery 4 at the charging period C2 is lower than that of the Ni-Cd battery 4 at the charging period C1. Accordingly, the Ni-Cd battery 4 is more uniformly charged during the charging period C2. As a result, the electromotive force of the Ni-Cd battery 4 is recovered. 
     As is apparent from the above detailed description, the charger of the present invention has a controlling means for cutting off the flow of the charging current when the charging current reaches the rated capacity of the Ni-Cd battery and for subsequently effecting a repetitive control of both the charging of the Ni-Cd battery and the cutting off of the charging current. Further, since the Ni-Cd battery is charged using the current in which the rated current value of the Ni-Cd battery to be charged and the current value 0 are alternately repeated, the Ni-Cd battery whose electromotive force has been reduced and whose internal electric resistance has increased, can be charged without continuously generating heat. Besides, the capacity of the Ni-Cd battery whose electromotive force has been reduced can be recovered without any deterioration in the Ni-Cd battery. 
     Incidentally, the present invention is not necessarily limited by the embodiment described in detail above. It will be apparent to those skilled in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as set forth herein. 
     In the charger according to the present embodiment, the charging and discharging of the Ni-Cd battery 4 are respectively executed twice. However, the charger may be controlled in such a manner that the charging and discharging thereof are respectively carried out three times or more. 
     Further, the charger according to the present embodiment starts charging from or during the discharging period D1 but may start charging from or during the charging period C1 where the Ni-Cd battery 4 has been sufficiently discharged in advance.