Abstract:
The present invention relates to an apparatus for protecting a secondary battery by preventing overcharge of the secondary battery. The apparatus for protecting a secondary battery includes: a secondary battery; a power supply, connected with both electrode terminals of the secondary battery, for providing a charging voltage and a charging current to the secondary battery to charge the secondary battery; and a secondary battery protection unit for shorting the power supply thereby to stop charging the secondary battery if a charging voltage of the secondary battery is above a predetermined voltage.

Description:
[0001]     This application claims the benefit of the filing date of Korean Patent Application Nos. 10-2005-16100 and 10-2005-65614, filed on Feb. 25, 2004 and Jul. 20, 2005, respectively in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in its entirety by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to an apparatus and a method for charging a secondary battery, and more particularly to an apparatus and a method for protecting a secondary battery by preventing an overcharge of the secondary battery.  
       BACKGROUND OF THE INVENTION  
       [0003]     Generally, a battery is widely classified into a chemical battery or a physical battery, and the chemical battery may also be classified into a primary battery, a secondary battery or a fuel cell. As the secondary battery, there are a Ni—Cd secondary battery, a Ni-Mh secondary battery, an SLA (sealed lead acid) secondary battery, a Lithium ion secondary battery, a Li-polymer secondary battery, a reusable alkaline secondary battery and the like.  
         [0004]     Particularly, when the lithium ion battery is overcharged above a given voltage, a negative reaction may occur between a cathode active material and an electrolyte. Such a negative reaction destroys the structure of the cathode active material while causing an oxidation reaction of the electrolyte. In the meantime, lithium can be deposited on an anode active material. If the voltage applied to the secondary battery continuously rises even if the secondary battery has been overcharged, accidental ignition or explosion of the secondary battery may occur.  
         [0005]     Conventionally, in order to prevent an overcharge of the Lithium ion secondary battery, a TS (thermostat) is connected between a power supply for providing a charging power and the Lithium ion secondary battery. If a temperature of the Lithium ion secondary battery rises above a predetennmied temperature due to an overcharge, the thermostat turns off thereby to stop a charging operation.  
         [0006]     In accordance with the prior art charging of the Lithium ion secondary battery is interrupted when the Lithium ion secondary battery is overheated due to the overcharge. Accordingly, although the Lithium ion secondary battery is fullly charged, charging of the Lithium ion secondary battery is not interrupted since the Lithium ion secondary battery is not overcharged yet. Therefore, a life span of the Lithium ion secondary battery may be decreased.  
         [0007]     Further, in accordance with the prior art, whether the Lithium ion secondary battery is overcharged or not is decided based on the temperature thereof Accordingly, in case of a high-temperature environment such as a tropical region or a low-temperature environment such as a polar region, it is difficult to decide whether the Lithium ion secondary battery is overcharged or not based on the temperature thereof Therefore, under such an environment, it is difficult to protect the Lithium ion secondary battery.  
         [0008]     Therefore, it is strongly required to develop a technology capable of preventing the overcharge of the Lithium ion secondary battery regardless of an ambient temperature; and stopping charging the Lithium ion secondary battery when the Lithium ion secondary battery is fullly charged or overcharged.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an apparatus and a method for protecting a Lithium ion secondary battery by automatically stopping charging the Lithium ion secondary battery when the Lithium ion secondary battery is fully charged or overcharged.  
         [0010]     Further, it is another object of the present invention to provide an apparatus and a method for protecting a Lithium ion secondary battery by preventing the overcharge of the Lithium ion secondary battery regardless of an ambient temperature.  
         [0011]     In accordance with one aspect of the present invention, there is provided an apparatus for protecting a secondary battery, including: a secondary battery; a power supply, connected with both electrode terminals of the secondary battery, for providing a charging voltage and a charging current to the secondary battery to charge the secondary battery; and a secondary battery protection unit for shorting the power supply thereby to stop charging the secondary battery if a charging voltage of the secondary battery is above a predetermined voltage.  
         [0012]     In the above-mentioned apparatus, the secondary battery protection unit includes: a switching device, coupled to the both electrode terminals of the secondary battery, for connecting the both electrode terminals of the secondary battery if a control current is applied; and a zener diode for providing the switching device with the control current if the charging voltage of the secondary battery is above the predetermined voltage.  
         [0013]     Alternatively, in the above-mentioned apparatus, the secondary battery protection unit can also includes: a switching device, coupled to the both electrode terminals of the secondary battery, for connecting the both electrode terminals of the secondary battery if a control current is applied; and a varistor for providing the switching device with the control current if the charging voltage of the secondary battery is above the predetermined voltage.  
         [0014]     In accordance with another aspect of the present invention, there is provided an apparatus for protecting a secondary battery, including: a switching device, coupled to both electrode terminals of the secondary battery, for connecting the both electrode terminals of the secondary battery if a control current is applied; and a constant voltage device for providing the switching device with the control current if the charging voltage of the secondary battery is above a predetermined voltage.  
         [0015]     In the above-mentioned apparatus, the constant voltage device is a zener diode or a varistor.  
         [0016]     In accordance with still another aspect of the present invention, there is provided a method for protecting a secondary battery, including the steps of: charging the secondary battery; if a charging voltage of the secondary battery is above a predetermined voltage, turning on a constant voltage device thereby to let a control current to flow into a switching device; if the control current is applied to the switching device, turning on the switching device thereby to stop charging the secondary battery. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a circuit diagram of an apparatus for protecting a secondary battery in accordance with a first embodiment of the present invention;  
         [0019]      FIGS. 2 and 3  are circuit diagrams showing current flows in the secondary battery protection apparatus show in  FIG. 1 ;  
         [0020]      FIG. 4  is a circuit diagram of an apparatus for protecting a secondary battery in accordance with a second embodiment of the present invention;  
         [0021]      FIGS. 5 and 6  are circuit diagrams showing current flows in the secondary battery protection apparatus show in  FIG. 4 ; and  
         [0022]      FIG. 7  is a flow chart for illustrating an operation of the apparatus for protecting a secondary battery in accordance with the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Hereinafter, an apparatus for protecting a secondary battery in accordance with a first embodiment of the present invention will be described with reference to  FIG. 1 .  
         [0024]     A power supply  100  converts a commercial electric power to a constant voltage and provides a Lithium ion secondary battery  102  with the constant voltage as a charging voltage. The power supply  100  is connected with a cathode and an anode of the Lithium ion secondary battery  102  in parallel.  
         [0025]     The Lithium ion secondary battery  102  is charged by the charging voltage provided by the power supply  100 .  
         [0026]     A secondary battery protection unit  104  is connected to the Lithium ion secondary battery  102  in parallel. The secondary battery protection unit  104  includes a zener diode  106  as a constant voltage device and a transistor  108  as a switching device. The zener diode  106  is connected between the cathode of the Lithium ion secondary battery  102  and a base terminal of the transistor  108 . An emitter terminal and a collector terminal of the transistor  108  are connected with the cathode and the anode of the Lithium ion secondary battery  102 .  
         [0027]     If a voltage not less than a breakdown voltage is applied to the zener diode  106 , the zener diode  106  let a current flow, which is referred as the zener effect. Accordingly, in order to let the current flow through the zener diode  106  when the Lithium ion secondary battery  102  is fully charged or overcharged, the zener diode  106  having a breakdown voltage corresponding to the fully-charged voltage or the overcharged voltage of the Lithium ion secondary battery  102  is selected or a voltage dividing circuit including a resistor and the like can be added to the zener diode  106 .  
         [0028]     Although the zener diode  106  is employed as a device allowing the current to flow when the Lithium ion secondary battery  102  is fullly charged or overcharged in  FIG. 1 , a varistor can also be adopted.  
         [0029]     Further, though the PNP transistor  108  is employed as a switching device in  FIG. 1 , various kinds of transistors which turn on when a current is applied to a base terminal thereof can be adopted. For example, it will be understood by those skilled in the art that an NPN transistor and an FET as well as the PNP transistor may be employed as the transistor  108  shown in  FIG. 1 .  
         [0030]     Hereinafter, an operation of the apparatus for protecting a secondary battery in accordance with the first embodiment of the present invention will be described with reference to  FIGS. 2 and 3 .  
         [0031]     The power supply  100  provides a charging voltage and then a charging current I 1  flows. The charging current I 1  is supplied to the zener diode  106  and the Lithium ion secondary battery  102 . That is, an actual charging current  13  which is equal to (I 1 - 12 ) is provided to the Lithium ion secondary battery  102 , wherein I 1  is the charging current and I 2  is a zener current provided to the zener diode  106 .  
         [0032]     If a charging voltage of the Lithium ion secondary battery  102 , which is charged by the actual charging current  13 , is equal to the fully charged voltage or the overcharged voltage thereof, the zener diode  106  allows a current to flow as a control current and then the control current flows into the base terminal of the transistor  108 . If the control current flows into the base terminal of the transistor  108 , the transistor  108  turns on and then shorts both electrode terminals of the power supply  100 . Accordingly, the charging current is not supplied to the Lithium ion secondary battery  102 .  
         [0033]     As mentioned above, by using the secondary battery protection unit  104  with a simple structure including the zener diode or the varistor and the transistor, the first embodiment of the present invention automatically stops charging the Lithium ion secondary battery when the Lithium ion secondary battery is fully charged or overcharged  
         [0034]     Further, since it is decided whether the Lithium ion secondary battery is overcharged or not based on the charging voltage thereof, the overcharge of the Lithium ion secondary battery can be prevented regardless of an ambient temperature.  
         [0035]     Hereinafter, an apparatus for protecting a secondary battery in accordance with a second embodiment of the present invention will be described with reference to  FIG. 4 .  
         [0036]     A power supply  200  converts a commercial electric power to a constant voltage and provides a Lithium ion secondary battery  202  with the constant voltage as a charging voltage. The power supply  200  is connected with a cathode and an anode of the Lithium ion secondary battery  202  in parallel.  
         [0037]     The Lithium ion secondary battery  202  is charged by the charging voltage provided by the power supply  200 .  
         [0038]     A secondary battery protection unit  204  is connected to the Lithium ion secondary battery  202  in parallel. The secondary battery protection unit  204  includes a zener diode  206  as a constant voltage device and an SCR (silicon controlled rectifier)  208  as a switching device. The zener diode  206  is connected between the cathode of the Lithium ion secondary battery  202  and a gate terminal of the SCR  208 . An emitter terminal and a collector terminal of the SCR  208  are connected with the cathode and the anode of the Lithium ion secondary battery  202 .  
         [0039]     If a voltage not less than a breakdown voltage is applied to the zener diode  206 , the zener diode  206  let a current flow, which is referred as the zener effect. Accordingly, in order to let the current flow through the zener diode  206  when the Lithium ion secondary battery  202  is fully charged or overcharged, the zener diode  206  having a breakdown voltage corresponding to the fully-charged voltage or the overcharged voltage of the Lithium ion secondary battery  202  is selected or a voltage dividing circuit including a resistor and the like can be added to the zener diode  206 .  
         [0040]     Although the zener diode  206  is employed as a constant voltage device allowing the current to flow when the Lithium ion secondary battery  202  is fullly charged or overcharged in  FIG. 4 , a varistor can also be adopted.  
         [0041]     Further, though the SCR  208  is employed as a switching device in  FIG. 4 , various kinds of switching devices which turn on when a current is applied to a gate terminal thereof can be adopted. For example, it will be understood by those skilled in the art that an N-Type SCR, a dual gate SCR, a TRIAC, a GTO (gate turn off) SCR and the like may be employed as a switching device.  
         [0042]     Hereinafter, an operation of the apparatus for protecting a secondary battery in accordance with the second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 .  
         [0043]     The power supply  200  provides a charging voltage and then a charging current  14  flows. The charging current  14  is supplied to the zener diode  206  and the Lithium ion secondary battery  202 . That is, an actual charging current I 6  which is equal to (I 4 -I 5 ) is provided to the Lithium ion secondary battery  202 , wherein I 4  is the charging current and I 5  is a zener current provided to the zener diode  206 .  
         [0044]     If a charging voltage of the Lithium ion secondary battery  202 , which is charged by the actual charging current I 6 , reaches to the fully charged voltage or the overcharged voltage thereof, the zener diode  206  allows a current to flow as a control current and the control current flows into the gate terminal of the SCR  208 . If the control current flows into the gate terminal of the SCR  208 , the SCR  208  turn on and then shorts both electrode terminals of the power supply  200 . Accordingly, the charging current is not supplied to the Lithium ion secondary battery  202 .  
         [0045]      FIG. 7  is a flow chart for illustrating an operation of the apparatus for protecting a secondary battery in accordance with the second embodiment of the present invention.  
         [0046]     The power supply  200  provides a charging voltage and a charging current to the Lithium ion secondary battery  202  (S 101 ). If the charging voltage of the Lithium ion secondary battery  202  is lower than the breakdown voltage of the zener diode  206  (S 102 ), the zener diode  206  does not allow a current to flow through the zener diode  206  and the Lithium ion secondary battery  202  is charged continuously.  
         [0047]     If the charging voltage of the Lithium ion secondary battery  202  is equal to the breakdown voltage of the zener diode  206  (S 102 ), the zener diode  206  turns on (S 103 ). Accordingly, the zener diode  206  let a control current to flow into the gate terminal of the SCR  208 .  
         [0048]     If the control current is applied into the gate terminal of the SCR  208 , the SCR  208  turns on (S 104 ). Then, both electrode terminals of the power supply  200  are shorted (S 105 ).  
         [0049]     Accordingly, charging the Lithium ion secondary battery  202  is interrupted.  
         [0050]     As mentioned above, by using the secondary battery protection unit  204  with a simple structure including the zener diode or the varistor and the SCR, the second embodiment of the present invention automatically stops charging the Lithium ion secondary battery when the Lithiun ion secondary battery is fully charged or overcharged  
         [0051]     Further, since it is decided whether the Lithium ion secondary battery is overcharged or not based on the charging voltage thereof, the overcharge of the Lithium ion secondary battery can be prevented regardless of an ambient temperature.  
         [0052]     While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and the scope of the invention as defined in the following claims.