Abstract:
There is provided a charging apparatus, which uses a charging control timer having a relatively simple constitution, capable of preventing an overcharge of a secondary battery while detecting the overcharge of the secondary battery even if a charge current value is relatively small. The charging apparatus that discontinues the charging in a preset expiration time period of a timer has a variable controller for controlling the preset expiration time period of a timer in response to an interrupted time if the charging operation that is interrupted before reaching the preset expiration time of a timer is started again.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention claims priority to its priority document No. 2003-190644 filed in the Japanese Patent Office on Jul. 2, 2003, the entire contents of which being incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a charging apparatus, and is more specifically to a charging apparatus that is capable of charging with a relatively small charging current.  
         [0004]     2. Description of Related Art  
         [0005]     In a charging apparatus for charging a secondary battery such as a nickel metal hydride battery or the like, it is typical to use, for example, a charging control timer in order to end charging operation at a predetermined constant time period after the start of charging operation (see Japanese Patent Application Publication No. S63-194533). Alternatively, charging may be terminated based on the judgment that the full charging is reached by detecting such phenomena as a minus delta V value (−ΔV detection) in voltage of the secondary battery which is being charged, a gradient of temperature rise of the battery (dT/dt detection), the upper limit of the temperature rise of the battery (TCO detection), and the like during the charging.  
         [0006]     However, in case of terminating the charging after a predetermined constant time period is passed, if the secondary battery is recharged after use or interruption of the charging operation of such battery, the charging control timer is reset regardless of the amount of the battery capacity of the secondary battery decreased by discharging, thereby carrying out the charging operation again for the same predetermined constant time period from the new start of the charging operation before termination.  
         [0007]     The constant charging time period is normally set to the time which is sufficient for charging a secondary battery, which has been discharged to the final discharge voltage, back to its rated capacity.  
         [0008]     Accordingly, if the charging operation is repeated by starting the whole charging operation before discharging to the final discharge voltage, the battery may be overcharged, thereby causing inconvenience that may lead to degradation in performance of the secondary battery.  
         [0009]     Japanese Patent Application Publication No. S63-194533 also discloses a technology which does not to reset the timer in case of short break of the AC power supply or the like during the charging operation of the secondary battery, but the timer is reset when the secondary battery is removed from the charger.  
       SUMMARY OF THE INVENTION  
       [0010]     On the other hand, in order to detect the phenomena such as the minus delta V value, the gradient of temperature rise of the battery, the upper limit of the temperature rise of the battery and the like, the charging current must be relatively large, i.e., at least 0.5C mA or larger (where, C is the rated capacity of the secondary battery) and thus not suitable for a charging apparatus with a small charging current.  
         [0011]     Moreover, since these phenomena are not necessarily accurate and reliable for detecting the fully charged state, it is common to use these phenomena together with a charging control timer, thereby encountering the above mentioned inconvenience.  
         [0012]     Accordingly, it is desirable to provide a charging apparatus using a charging control timer that has a relatively simple construction and is capable of detecting overcharging of the secondary battery even if the charging current is relatively small. The present invention is made in light of these circumstances.  
         [0013]     According to an embodiment of the present invention, there is provided a charging apparatus that discontinues a charging operation at a preset expiration time period of a timer. The charging operation is provided with variable control means for controlling the preset expiration time period of a timer in response to an interrupted time if the charging operation that is interrupted before reaching the preset expiration time period of a timer is restarted.  
         [0014]     According to the present embodiment, the charging apparatus is provided with the variable control means for controlling the preset expiration time period of a timer in response to an interrupted time if the charging operation that is interrupted before reaching the preset expiration time period of a timer is restarted. Accordingly, overcharging may be prevented even if the charging is once interrupted and restarted because the charging time is adjusted in response to an interrupted time or a discharge time of the secondary battery during the interruption. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above and other objects, features and advantages of the present invention will become more apparent from the following description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:  
         [0016]      FIG. 1  is a circuit schematic of one embodiment of the charging apparatus according to the present invention;  
         [0017]      FIG. 2  is a flowchart for describing an operation of one embodiment of the present invention;  
         [0018]      FIG. 3  is a graph for describing one embodiment of the present invention;  
         [0019]      FIG. 4  is a graph for describing one embodiment of the present invention;  
         [0020]      FIG. 5  is a graph for describing one embodiment of the present invention;  
         [0021]      FIG. 6  is a chart for describing one embodiment of the present invention; and  
         [0022]      FIG. 7  is a diagram to show another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0023]     Now, embodiments of the charging apparatus according to the present invention will be described hereunder with reference to the drawings.  
         [0024]      FIG. 1  shows an example of construction of the charging apparatus according to the present embodiment and, in  FIG. 1 , represented by numeral  1  is an electrical apparatus which is constructed to operate on electrical power from a secondary battery  2 , represented by numeral  3  is a power switch and represented by numeral  4  is an electrical circuitry of the electrical apparatus  1 .  
         [0025]     Moreover, represented by numeral  5   a  and numeral  5   b  are one and the other charging terminals which are designed to be connected to one and the other output terminals  10   a  and  10   b  of a charging apparatus  10  provided exclusively for the electrical apparatus  1 , respectively. The one charging terminal  5   a  is connected to the plus electrode of the secondary battery  2  by way of a reverse current protection diode  6  while the minus electrode of the secondary battery  2  is connected to the other charging terminal  5   b.    
         [0026]     The charging apparatus  10 , which is provided exclusively for the electrical apparatus  1 , is provided with a charging control microcomputer  11  including a counter  11   a  which constitutes a charging control timer.  
         [0027]     A secondary battery detection port  11   b  of the charging control microcomputer  11  is connected to the collector of an npn-type transistor  12  which is then connected to a power supply terminal for the microcomputer  14  for supplying positive DC voltage by way of a resistor  13 . The emitter of the transistor  12  is connected to ground and also to the other output terminal  10   b . The electrical power from the power supply terminal for microcomputer  14  is supplied to the charging control microcomputer  11 .  
         [0028]     The charging control microcomputer  11  makes a judgment that the secondary battery  2  is not connected if the secondary battery detection port  11   b  is high level “1”, while making a judgment that the secondary battery  2  is connected if the secondary battery detection port  11   b  is low level “0”.  
         [0029]     A resistor  15  is connected between the base and the emitter of the transistor  12  and the base of the transistor  12  is connected to the collector of a pnp-type transistor  17  by way of a resistor  16 .  
         [0030]     The emitter of the transistor  17  is connected to a power supply terminal  19  that supplies positive DC voltage by way of a resistor  18  and the collector of the transistor  17  is connected to ground by way of a resistor  20 .  
         [0031]     A resistor  21  is connected between the base and the emitter of the transistor  17  and also the base of the transistor  17  is connected to the one output terminal  10   a  by way of a resistor  22 .  
         [0032]     In this case, when the one and the other output terminals  10   a  and  10   b  are connected to the one and the other charging terminals  5   a  and  5   b , the transistor  17  turns on and also the transistor  12  turns on to make the secondary battery detection port  11   b  low level “0”.  
         [0033]     Also, the charging control port  11   c  of the charging control microcomputer  11  is connected to the base of an npn-type transistor  24  by way of a resistor  23  and the collector of the transistor  24  is connected to the power supply terminal  19  by way of a series circuit of a resistor  25  and a Zener diode  26 .  
         [0034]     The emitter of the transistor  24  is connected to ground and a resistor  27  is connected between the base and the emitter of the transistor  24 .  
         [0035]     The connection point of the Zener diode  26  and the resistor  25  is connected to the base of a pnp-type transistor  28 , the emitter of the transistor  28  is connected to the power supply terminal  19  by way of a resistor  29  and a resistor  30  is connected between the emitter and the collector of the transistor  28 . The collector of the transistor  28  is connected to the one output terminal  10   a.    
         [0036]     In this case, when the charging control microcomputer  11  detects that the secondary battery  2  is connected between the one and the other output terminals  10   a  and  10   b , the charging control port  11   c  becomes high level “1” and the transistor  24  turns on to supply the bias current to the transistor  28 , thereby causing the transistor  28  to supply the charging current to the secondary battery  2 .  
         [0037]     At this time, since the bias voltage of the transistor  28  is determined by the Zener diode  26  connected to the base of the transistor  28 , the bias voltage of the transistor  28  remains unchanged regardless of the voltage of the secondary battery  2 . In case when the resistance R 2  of the resistor  30  is set sufficiently larger than the resistance R 1  of the resistor  29 , i.e., R 2 &gt;R 1 , the charging current (constant current) can be determined by the Zener voltage Vz of the Zener diode  26 , the threshold voltage Vf component of the internal diode of the transistor  28  and the resistance R 1  of the resistor  29 .  
         [0038]     The charging current Ic is determined by: 
 
 Ic= ( Vz−Vf )/ R 1 
 
 If the rated capacity of the secondary battery  2  is C, the charging time required for full charging by the continuous charging is Tcf and the charging capacity is 130% of the rated capacity, the following relation is given: 
 
 C× 1.3= Ic×Tcf  
 
         [0039]     Now, the charging operation will be described by reference to the flowchart in  FIG. 2  and  FIG. 3  through  FIG. 5 .  
         [0040]     Firstly, description will be provided on a case of the continuous charging as shown in  FIG. 3 . When the power supply of the charging apparatus  10  is turned on, the full charging time Tcf required for fully charging the secondary battery  2  in the continuous charging is set to the counter  11   a  in the charging control microcomputer  11  as the initial value (step S 1 ).  
         [0041]     Then, judgment is made if the secondary battery detection port  11   b  of the charging control microcomputer  11  is low level “0”, i.e., if the secondary battery  2  is connected (step S 2 ). Subsequently, the counter  11   a  is decremented (step S 3 ) and judgment is made if the secondary battery  2  remains connected (step S 4 ). In case of the continuous charging as shown in  FIG. 3 , these steps S 3  and S 4  are repeated until the counter  11   a  reaches “0”.  
         [0042]     In this embodiment, if it is determined that the counter  11   a  becomes “0” (step  5 ), the normal charging is finished and a trickle charging is carried out thereafter (step S 6 ).  
         [0043]     In this case, judgment is made if the secondary battery  2  is connected (step S 7 ), and the trickle charging is continued as long as the secondary battery  2  remains connected.  
         [0044]     This is because, since self discharge takes place in the secondary battery  2  if it is left unused, after the full charging (fully charged condition) in this embodiment, the trickle charging is carried out in order to compensate for the self discharge.  
         [0045]     Generally, the amount of self discharge of a secondary battery per day (24 hours) is several percent of the rated capacity. In this embodiment, the trickle charging is carried out in a pulsed charging.  
         [0046]     At this time, if the amount of self discharge of the secondary battery  2  is N%, the rated capacity of the secondary battery  2  is C (mAh) and the average charging current in the trickle charging is Ict (mA), then the amount of the self discharge per day is C×0.01N and the average charging current Ict (mA) is given by: 
 
 Ict= ( C× 0.01 N )/24 
 
         [0047]     Moreover, if the normal charging current value is Ic (mA) and the optional time is T, the self discharge can be compensated by the pulsed charging (trickle charging) of the pulse of the high level time T and the low level time {(Ic−Ict)/Ict} T.  
         [0048]     The trickle charging is achieved by outputting the pulse as shown in  FIG. 6  from the charging control port  11   c  of the charging control microcomputer  11  in  FIG. 1  so that the transistor  24  is turned on and off.  
         [0049]     During the trickle charging, the count value of the counter  11   a  in the charging control microcomputer  11  is “0”.  
         [0050]     In the flowchart as shown in  FIG. 2 , when the electrical apparatus  1  is removed from the charging apparatus  10  and thus disconnecting the secondary battery  2  from the charging apparatus  10  at any time (in steps S 4 , S 7 ), it goes to step S 8 , in which the count value of the counter  11   a  is incremented (Note that the maximum value of the counter  11   a  is equal to the count value of Tcf.).  
         [0051]     In this case, judgment is made if the secondary battery  2  is connected (step S 9 ). If the secondary battery is not connected, judgment is made whether or not the count value of the counter  11   a  has reached Tcf (step S 10 ). If the count value has not reached Tcf, the count value of the counter  11   a  is incremented until the secondary battery  2  is connected again.  
         [0052]     If the secondary battery  2  is connected again, it goes to step S 3 . On the other hand, if the count value of the counter  11   a  has reached Tcf in step S 10 , it goes to step S 2 .  
         [0053]     Since the electrical apparatus  1  is assumed to be used during the time when the electrical apparatus  1  is disconnected from the charging apparatus  10 , the secondary battery  2  discharges depending on the time. Accordingly, an overcharge may be prevented by carrying out a supplemental charging to compensate for the decrease in battery capacity of the secondary battery  2  that is discharged while the electrical apparatus  1  is disconnected in addition to the charging for the calculated time required for full charging.  
         [0054]     In this case, if the charging current to the secondary battery  2  from the charging apparatus  10  is equal to the discharging current from the secondary battery  2  to the electrical apparatus  1 , a compensation for the decrease in battery capacity of the secondary battery  2  is enabled by setting the charging time equal to the time when the electrical apparatus  1  is disconnected (causing discharge) from the charging apparatus  10 .  
         [0055]     When the secondary battery is detected again (step S 9 ), since the count value of the counter  11   a  is incremented equal to the time when the electrical apparatus  1  is disconnected (step S 8 ), charging is carried out for the time equal to {(the increased count value of the counter  11   a )+(the count value of the counter  11   a  at the time when the electrical apparatus  1  is disconnected)}, i.e., {(the time during the electrical apparatus  1  is being disconnected)+(the residual charging time at the time when the electrical apparatus  1  is disconnected)}.  
         [0056]     The foregoing control is carried out repeatedly as many times as necessary in cases when the electrical apparatus  1  is disconnected from the charging apparatus  10  in any condition (in step S 4 , S 7 ) as shown in  FIG. 4  and  FIG. 5 .  
         [0057]     Continuing the description, it is assumed that the charging is interrupted by disconnecting the electrical apparatus  1  after the time Tc from the start of charging as shown in  FIG. 4 ,  FIG. 5  (Note that decrement of the count value of the counter  11   a  is carried out during the charging.). Since the count value of the counter  11   a  is decremented equal to the time Tc at the time when the electrical apparatus  1  is disconnected to interrupt the charging, the residual charging time is equal to the difference (Tcf−Tc) from the full charging time Tcf in the continuous charging.  
         [0058]     Incidentally, if the electrical apparatus  1  is used for the time Td, the battery capacity of the secondary battery  2  decreases accordingly. Therefore, at the time of restarting the charging after disconnecting the electrical apparatus  1 , the count value of the counter  11   a  is incremented by the time Td in order to compensate for the decrease in the battery capacity.  
         [0059]     At the time when recharging of the secondary battery  2  starts, the abovementioned residual charging time (Tcf−Tc) is added with the additional time Td, thereby charging for the time equal to {Td+(Tcf−Tc)} and avoiding overcharging.  
         [0060]     Such control is applied as many times as necessary if the electrical apparatus  1  is disconnected from the charging apparatus  10  in any conditions (step S 4 , step S 7 ) and overcharging can be prevented by charging for the time {(the time during the electrical apparatus  1  is being disconnected)+(the residual charging time at the time when the electrical apparatus  1  is disconnected)}.  
         [0061]     In the above description, in case that there is any difference between the charging current and the discharging current of the secondary battery  2 , a similar control to the foregoing for preventing overcharging may be realized by changing the increment or decrement per unit time of the count value of the counter  11   a.    
         [0062]     That is, if the charging current is Ic, the discharging current is Id and the time when the electrical apparatus  1  is disconnected (for use) is Td, by setting the amount of decrement of the counter  11   a  per unit time to {(Id/Ic)×(the amount of increment of the counter  11   a  per unit time)}, the charging to compensate for decrease in battery capacity of the secondary battery  2  due to disconnection and use of the electrical apparatus  1  is carried out with the charging current Ic for the time equal to (Id/Ic)×Td.  
         [0063]     Accordingly, at the time when the electrical apparatus  1  is once disconnected and the recharging is carried out, the charging time is the sum of the time {(Id/Ic)×Td} required for compensating for “the capacity decreased by discharging” and calculated time necessary for the full charging (the residual charging time at the time when the electrical apparatus  1  is disconnected).  
         [0064]     In the general full charging detection systems of the secondary battery such as the minus delta V (−ΔV detection), the gradient of temperature rise in the battery (dT/dt detection), the upper temperature limit of the battery (TCO detection) and the like, at least 0.5C mA or larger charging current is required. However, since the control is made on the charging time in the present embodiment, it is possible to obtain the charging apparatus having a function to prevent overcharging even at a relatively small charging current such as about 0.1C mA.  
         [0065]     Also, according to the present embodiment, since the increase and decrease of battery capacity of the secondary battery  2  due to charging and discharging is converted into the charging and discharging time, a reliable and stable overcharging protection may be achieved with a simple construction that requires no a construction to monitor the voltage of the secondary battery  2 .  
         [0066]      FIG. 7  illustrates another embodiment of the present invention. For describing about  FIG. 7 , portions corresponding to those in  FIG. 1  have the same reference numerals and their detailed descriptions will be omitted.  
         [0067]     In  FIG. 7 , the charging control microcomputer  11  in the embodiment in  FIG. 1  is disposed in the electrical apparatus  1  and the charging apparatus  10  is controlled by the charging control microcomputer  11 . In this embodiment, a connection point of the power switch  3  and the electrical circuitry  4  of the electrical apparatus is connected to an electrical apparatus use detection port  11   e  of the charging control microcomputer  11  so as to determine that the electrical apparatus is being used when the power switch  3  is on. Moreover, power is supplied from the secondary battery  2  to the charging control microcomputer  11  and the counter  11   a  in the charging control microcomputer  11  is not reset by turning on or off the power switch  3 .  
         [0068]     Additionally, the charging apparatus  10  is provided with a constant current generation circuit  40  for outputting the charging current to the output terminals  10   a ,  10   b . A charging control terminal  10   c  of the constant current generation circuit  40  is connected to the charging control port  11   c  of the charging control microcomputer  11 , so as to control the constant current generation circuit  40  by the charging control microcomputer  11 , while a charging detection terminal  10   d  of the constant current generation circuit  40  is connected to the charging detection port  11   d  of the charging control microcomputer  11 , thereby informing the charging control microcomputer  11  whether the secondary battery is being charged or not.  
         [0069]     In the charging control microcomputer  11 , if charging of the secondary battery  2  is detected by the charging detection port  11   d , the count value of the counter  11   a  is decremented. If no-charging is detected, the count value of the counter  11   a  is incremented.  
         [0070]     Other constructions are the same as the embodiment in  FIG. 1 . It is easily understood that the same functions and effects as the embodiment in  FIG. 1  can be achieved also by the embodiment in  FIG. 7 .  
         [0071]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.  
         [0072]     According to the embodiments of the present invention, since charging and discharging of the secondary battery are controlled on the basis of the charging and discharging time, the charging apparatus having an overcharging prevention function can be achieved even if the charging current is relatively small such as about 0.1C mA.  
         [0073]     Further, according to the embodiments of the present invention, since increase or decrease in battery capacity of the secondary battery due to charging and discharging is converted into the charging and discharging time, a reliable and stable overcharging prevention may be achieved with a simple construction that requires no construction to monitor the voltage of the secondary battery.