Abstract:
A battery protection method is disclosed for protecting a battery from an over-discharge state by switching off a switching element provided between the battery and a load in accordance with the voltage of the battery. The method includes the steps of detecting application of a charging voltage and arranging a chargeable state for the battery by switching on the switching element when the application of the charging voltage is detected.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a battery protection method and a battery protection circuit, and more particularly to a battery protection method and a battery protection circuit for protecting a battery from over-discharge by switching off a switching element provided between the battery and a load in accordance with the voltage of the battery.  
         [0003]     2. Description of the Related Art  
         [0004]     Mobile phones are provided with a battery pack which serves as their electric drive source.  
         [0005]     The battery pack includes a battery and a protection IC. The battery includes a lithium ion battery having an electric chargable/dischargable configuration. The protection IC disconnects the battery from a load or a charger upon detecting overcharge, over-discharge, or overcurrent of the battery (for example, see Japanese Laid-Open Patent Application No. 11-68527).  
         [0006]      FIG. 5  is a block diagram showing an example of a conventional battery pack.  
         [0007]     In  FIG. 5 , a battery pack  1  includes a battery  11 , a discharge control transistor  12 , a charge control transistor  13 , a protection IC  14 , resistancors R 1 , R 2 , and a capacitor C 1 . The battery pack  11  is connected to a load  2  or a charger  3 .  
         [0008]     The battery  11  is, for example, a lithium ion battery having an electric chargable/dischargable configuration. In the battery  11 , the positive electrode is connected to a terminal T 1  and the negative electrode is connected to the source of the discharge control transistor  12 .  
         [0009]     The discharge control transistor  12  includes an n channel MOS field-effect transistor. In the discharge control transistor  12 , its source is connected to the negative electrode of the battery  11 , its drain is connected to the drain of charge control transistor  13 , and its gate is connected to the protection IC  14 .  
         [0010]     The charge control transistor  13  includes an n channel MOS field-effect transistor. In the charge control transistor  13 , its source is connected to a terminal T 2 , its drain is connected to the drain of the discharge control transistor  12 , and its gate is connected to the protection IC  14 .  
         [0011]     The protection IC  14  is connected to the junction point between the resistor R 1  and the capacitor C 1 , the gate of the discharge control transistor  12  and the gate of the charge control transistor  13 . Furthermore, the protection IC  14  is also connected to the terminal T 2  via the resistor R 2 . The protection IC  14  detects overcharge, over-discharge, and overcurrent, and controls the discharge control transistor  12  and the charge control transistor  13 .  
         [0012]      FIG. 6  is graph for describing operations of the conventional battery. In the graph of  FIG. 6 , the horizontal axis indicates the output voltage of the battery  11 , and the vertical axis indicates the voltage between the terminal T 1  and the terminal T 2 . Furthermore, “V 41 ” indicates over-discharge detection voltage, and “V 42 ” indicates over-discharge return voltage.  
         [0013]     An over-discharge state occurs when the voltage of the battery  11  becomes less than the over-discharge detection voltage V 41 . When the voltage of the battery  11  becomes greater than the over-discharge return voltage V 42  after the protection IC  14  reaches an over-discharge protection state, the protection IC  14  switches out of the over-discharge protection state and turns on the over-discharge control transistor  12 .  
         [0014]     The charger  3  is connected between the terminal T 1  and the terminal T 2 . The charger  3  detects over-discharge or overcharge of the battery  11  in accordance with the voltage between the terminals T 1 , T 2  and starts/stops, respectively charging the battery  11 .  
         [0015]     In the conventional battery pack  1 , even in a case where the voltage of the battery  11  is actually greater than the over-discharge return voltage V 42 , the voltage between the terminal T 1  and the terminal T 2  is decreased when the protection IC  14  reaches an over-discharge state due to forward direction voltage Vf of a parasitic diode D 12  of the over-discharge control transistor  12 . Thus, the charger  2  stops charging when the voltage between the terminal T 1  and the terminal T 2  becomes equal to or less than the over-discharge return voltage. As a result, charging cannot be sufficiently performed. That is, in the conventional battery pack  1 , the over-discharge return can only be performed in the range          V 10  shown in  FIG. 6 .  
       SUMMARY OF THE INVENTION  
       [0016]     It is a general object of the present invention to provide a battery protection method and a battery protection circuit that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.  
         [0017]     Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a battery protection method and a battery protection circuit particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
         [0018]     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of the present invention provides a battery protection method for protecting a battery from an over-discharge state by switching off a switching element provided between the battery and a load in accordance with the voltage of the battery, the method including the steps of: detecting application of a charging voltage; and arranging a chargeable state for the battery by switching on the switching element when the charging voltage is applied.  
         [0019]     In the battery protection method according to an embodiment of the present invention, the detection of the application of the charging voltage may include detection of a connection to a charger.  
         [0020]     Furthermore, an embodiment of the present invention provides a battery protection circuit for protecting a battery from an over-discharge state by switching off a switching element provided between the battery and a load in accordance with the voltage of the battery, the battery protection circuit including: a charge voltage detection circuit part for detecting application of a charging voltage; and a control circuit part for arranging a chargeable state for the battery by switching on the switching element when the charging voltage is applied.  
         [0021]     In the battery protection circuit according to an embodiment of the present invention, the detection of the application of the charging voltage may include detection of a connection to a charger.  
         [0022]     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a circuit diagram of a battery pack including a battery according to an embodiment of the present invention;  
         [0024]      FIG. 2  is a circuit diagram of a protection IC according to an embodiment of the present invention;  
         [0025]      FIG. 3  is a flowchart of an operation of a control circuit according to an embodiment of the present invention;  
         [0026]      FIG. 4  is a graph for describing operations of a battery according to an embodiment of the present invention;  
         [0027]      FIG. 5  is a circuit diagram of a conventional battery pack; and  
         [0028]      FIG. 6  is a graph for describing operations of a conventional battery. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     In the following, embodiments of the present invention are described with reference to the accompanying drawings.  
         [0030]      FIG. 1  is a block diagram showing a battery pack  100  according to an embodiment of the present invention.  
         [0031]     In this embodiment, the battery pack  100  is mounted on, for example, a mobile phone serving as its power source. The battery pack  100  includes a battery  111 , a discharge control transistor  112 , a charge control transistor  113 , a protection IC  114 , resistors R 11 , R 12 , and a capacitor C 11 .  
         [0032]     The battery  111  includes, for example, a lithium ion battery having an electric chargable/dischargable configuration. In the battery  111 , the positive electrode is connected to a terminal T 11  and the negative electrode is connected to the source of the discharge control transistor  112 .  
         [0033]     The discharge control transistor  112  includes an n channel MOS field-effect transistor. In the discharge control transistor  112 , its source is connected to the negative electrode of the battery  111 , its drain is connected to the drain of charge control transistor  113 , and its gate is connected to a terminal T 22  of the protection IC  114 .  
         [0034]     The charge control transistor  113  includes an n channel MOS field-effect transistor. In the charge control transistor  113 , its source is connected to a terminal T 12 , its drain is connected to the drain of the discharge control transistor  112 , and its gate is connected to a terminal T 23  of the protection IC  114 .  
         [0035]     In the protection IC  114 , the terminal T 21  is connected to the junction point between the resistor R 11  and the capacitor C 11 , the terminal T 22  is connected to the gate of the discharge control transistor  112 , the gate of the terminal T 23  is connected to the charge control transistor  113 , and the terminal T 24  is connected to the terminal T 12  via the resistor R 12 . Accordingly, the protection IC  114  detects overcharge and over-discharge according to the voltage of the terminal T 21  and detects overcurrent according to the current flowing at the terminal T 24 . In accordance with the detection results, the protection IC  114  controls the discharge control transistor  112  and the charge control transistor  113  by controlling the voltage of the terminals T 22 , T 23 , respectively.  
         [0036]     One end of the resistor R 11  is connected to the positive electrode of the battery  111  and the other end of the resistor R 11  is connected to the terminal T 21 . Thereby, power is supplied to the terminal T 21  whereby the voltage of the battery is lowered. One end of the capacitor C 11  is connected to the terminal T 21  and the other end of the capacitor C 11  is connected to the negative electrode of the capacitor C 11 . Thereby, the voltage of the terminal T 21  is stabilized.  
         [0037]      FIG. 2  is a circuit diagram showing an exemplary configuration of the protection IC  114  according to an embodiment of the present invention.  
         [0038]     The protection IC  114  includes a overcharge detection circuit  121 , an over-discharge detection circuit  122 , an overcurrent detection circuit  123 , a control circuit  124 , and drivers  125 ,  126 .  
         [0039]     The overcharge detection circuit  121  includes a reference voltage source  131  and a comparator  132 . The reference voltage source  131  generates a reference voltage Vref 11 . The reference voltage Vref 11  generated by the reference voltage source  131  is supplied to the inverting input terminal of the comparator  132 . The non-inverting terminal of the comparator  132  is connected to the terminal T 21 .  
         [0040]     When the voltage V 11  of the terminal T 21  is less than the reference voltage Vref 11 , the output of the comparator  132  is a low level and the battery  111  reaches an overcharge state. When the voltage V 11  of the terminal T 21  is greater than the reference voltage Vref 11 , the output of the comparator  132  is a high level. The output of the comparator  132  is supplied to the control circuit  124 .  
         [0041]     The over-discharge detection circuit  122  includes a reference voltage source  141  and a comparator  142 . The reference voltage source  141  generates a reference voltage Vref 12 . The reference voltage Vref 12  generated by the reference voltage source  141  is supplied to the non-inverting input terminal of the comparator  142 . The non-inverting input terminal of the comparator  142  is connected to the terminal T 21 .  
         [0042]     When the voltage V 12  of the terminal T 21  is greater than the reference voltage Vref 12 , the output of the comparator  142  is a low level and the battery  111  reaches an over-discharge state. When the voltage V 12  of the terminal T 21  is less than the reference voltage Vref 12 , the output of the comparator  142  is a high level. The output of the comparator  142  is supplied to the control circuit  124 .  
         [0043]     The overcurrent detection circuit  123  detects the current flowing in the terminal T 24 . When the current flowing in the terminal T 24  is less than a predetermined value, the output of the overcurrent detection circuit  123  is a low level. In the case of an overcurrent state, the current flowing in the terminal T 24  becomes greater than the predetermined value. When the current flowing in the terminal T 24  is greater than the predetermined value, the output of the overcurrent detection circuit  123  is a high level. The output of the overcurrent detection circuit  123  is supplied to the control circuit  124 .  
         [0044]     In a case where the battery  111  is in an overcharge state and the output of the overcharge detection circuit  121  is a high level, the control circuit  124  drives a driver  126  such that the voltage of the terminal T 24  becomes a low level. By setting the voltage of the terminal T 24  to a low level, the charge control transistor  113  is turned off. By turning off the charge control transistor  113 , the connection between the battery  111  and the terminal T 12  becomes disconnected and the negative electrode of the battery  111  reaches an open state. Thereby, charging the battery  111  is stopped.  
         [0045]     Furthermore, in a case where the battery  111  is in an over-discharge state and the output of the over-discharge detection circuit  122  is a high level, the control circuit  124  drives a driver  125  such that the voltage of the terminal T 23  becomes a low level. By setting the voltage of the terminal T 23  to a low level, the discharge control transistor  112  is turned off. By turning off the discharge control transistor  112 , the connection between the battery  111  and the terminal T 12  is disconnected and the negative electrode of the battery becomes an open state. Thereby, charging the battery  111  is stopped.  
         [0046]     Furthermore, in a case where an overcurrent state occurs and the output of the overcurrent detection circuit  123  is a high level, the control circuit  124  drives the driver  125  or the driver  126  such that the voltage of the terminal T 23  or the voltage of the terminal T 24  becomes a low level. By setting the voltage of the terminal T 23  or the voltage of the terminal T 24  to a low level, the discharge control transistor  112  or the charge control transistor  113  is turned off. By turning off the discharge control transistor  112  or the charge control transistor  113 , the connection between the battery  111  and the terminal T 12  is disconnected and the negative electrode of the battery becomes an open state. Thereby, the charging of the battery  111  is stopped.  
         [0047]     Next, the operation (action) of the control circuit  124  is described in a case where charging voltage is applied.  
         [0048]      FIG. 3  is a flowchart of the operation (action) of the control circuit  124  according to an embodiment of the present invention.  
         [0049]     In Step S 1 - 1  of  FIG. 3 , the control circuit  124  detects the applying of charging voltage between the terminal T 11  and the terminal T 12  (connection with the charger). When the applying of charging voltage is detected (Yes in Step S 1 - 1 ), the voltage of the terminal T 22  is compulsorily set to a high level in Step S 1 - 2 . When the voltage of the terminal T 22  is compulsorily set to a high level in Step S 1 - 2 , the discharge control transistor  112  is compulsorily turned on. If the battery  111  is not in an overcharge state, the charge control transistor  113  is also turned on. Accordingly, the battery  111  is connected to the charger via the terminal T 11  and the terminal T 12 .  
         [0050]     By connecting the battery  111  to the charger via the terminal T 11  and the terminal T 12 , an over-discharge state of the battery  111  can be detected. In this state, since the discharge control transistor  112  is switched on, the battery  111  is connected to the terminals T 11  and T 12  without passing through the parasitic diode D 112  of the discharge control transistor  112 . Accordingly, the charger can precisely detect the voltage of the battery  111  and perform charging.  
         [0051]     For example, in a case where the battery  111  is connected to the terminals T 11  and T 12  via the parasitic diode D 112 , the voltage between the terminal T 11  and the terminal T 12  is a voltage obtained by subtracting the forward direction voltage Vf of the parasitic diode D 112  from the voltage of the battery  111 . Therefore, the battery  111  is determined as having a voltage lower than the actual voltage of the battery  111 . As a result, the battery  111  is determined to be in an over-discharge state even though the voltage of the battery  111  only is not a voltage of the over-discharge state.  
         [0052]     Therefore, according to an embodiment of the present invention, in a case where the charger is connected to the battery  111 , first, the discharge control transistor  112  is compulsorily turned on, and then, the voltage of the battery  111  is applied to the terminals T 11  and T 12  without the application of the forward direction voltage Vf of the parasitic diode D 112  of the discharge control transistor  112 . Accordingly, the over-discharge state of the battery  111  can be positively detected.  
         [0053]      FIG. 4  is a graph for describing operations of the battery  111  according to an embodiment of the present invention. In the graph of  FIG. 4 , the horizontal axis indicates the output voltage of the battery  111 , and the vertical axis indicates the voltage between the terminal T 11  and the terminal T 12 . Furthermore, “V 31 ” indicates over-discharge detection voltage, and “V 32 ” indicates over-discharge return voltage.  
         [0054]     The over-discharge state occurs when the voltage of the battery  111  becomes less than the over-discharge detection voltage V 31 . When the voltage of the battery becomes greater than the over-discharge return voltage V 32  after the protection IC  114  reaches an over-discharge protection state, the protection IC  114  returns from the over-discharge protection state and it becomes possible for the charger  102  to charge the battery  111 . In this state, according to an embodiment of the present invention, when the charger  102  is connected between the terminal T 11  and the terminal T 12 , the protection IC  114  compulsorily returns from the over-discharge protection state without relying on the voltage of the battery  111 .  
         [0055]     Therefore, the protection IC  114  can return from the over-discharge state when the range of the voltage of the battery  111  is          V 0  as shown in  FIG. 4 . Since the discharge control transistor  112  is turned on when the protection IC  114  returns from the over-discharge protection state, the battery  111  is connected to the charger  102  without passing through the parasitic diode D 112  of the discharge control transistor  112 . Thereby, voltage of the battery  111  is applied to the charger  102  without attenuation of the forward direction voltage Vf of the parasitic diode D 112 . Hence, the charger  102  can precisely detect the voltage of the battery  111  and determine whether the battery  111  is in an over-discharge state, to thereby start/stop charging.  
         [0056]     Accordingly, charging can be performed on a battery  111  having a low voltage but not being in an over-discharge state.  
         [0057]     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention.  
         [0058]     The present application is based on Japanese Priority Application No. 2005-156375 filed on May 27, 2005, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.