CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is related to, claims priority from and incorporates by reference Japanese patent application number 2009-078346, filed on Mar. 27, 2009. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a battery protection circuit, a method for protecting a battery, a power supply device, and a program thereof.  
         [0004]    2. Description of Related Art 
         [0005]    A battery protection circuit that protects against degradation of a battery caused by overcharge and over discharge of a power supply device with a built-in rechargeable secondary battery, such as a battery pack described in Japanese patent number 3382002 (hereafter patent reference 1), is widely known. 
         [0006]    For example, the battery protection circuit described in patent reference 1 stops a battery from discharging immediately after (500 milliseconds) an alarm when over discharge of the battery progresses. Similarly, when overcharge of the battery progresses, charging of the battery is stopped immediately after the alarm. 
         [0007]    In the battery protection circuit described in patent reference 1, when the overcharge or over discharge progresses, a discharge path or a charge path is cut off in order to protect the battery. At this time, when it is in an over discharge condition, the battery recovers from the over discharge condition through charging of the battery by a user and becomes reusable. On the other hand, when the user leaves an over discharge condition of the battery and does not recharge, over discharge further progresses. When the over discharge progresses, eventually, the battery is degraded until it is in a nonreusable condition. 
         [0008]    When the battery is, for example, a lithium-ion battery, due to the over discharge, cobalt of a positive electrode is eluted, and copper of a collector of a negative electrode is eluted. As a result, the lithium-ion battery does not function as a secondary battery. Furthermore, it is well known that the over discharge of the lithium-ion battery could cause abnormal heating of the battery. 
         [0009]    In contrast, the condition in which a user does not recharge even though the battery is in the over discharge condition is not assumed in the battery protection circuit of patent reference 1. In other words, the battery and an over discharge detecting part (or overcharge detecting part) are constantly connected to each other in the battery protection circuit of patent reference 1. Under this structure, the battery keeps supplying power with respect to the over discharge detecting part (or overcharge detecting part) even though the over discharge progresses up to the point in which the battery is nonreusable. 
         [0010]    Accordingly, it is effective to extend a period of time to fall in a condition in which abnormal heating due to over discharge occurs by eliminating all of the elements that progress the over discharge in the over discharge detecting part (or overcharge detecting part) with respect to the battery which is in the nonreusable condition due to the over discharge. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is provided against the background discussed above. An object of the present invention is to provide a battery protection circuit, a method for protecting a battery, a power supply device, and a program that can maintain a long reusable period by recharging while avoiding abnormal heating of the battery due to the over discharge. 
         [0012]    The first aspect of the present invention is a battery protection circuit which comprises: a control part that cuts off a discharge path to a load for a battery when a voltage value of the battery is equal to or less than a first threshold value, or when the voltage value of the battery is less than the first threshold value, wherein the control part cuts off a power supply path to the control part itself when the voltage value of the battery is equal to or less than a second threshold value, or when the voltage value of the battery is less than the second threshold value which is lower than the first threshold value. 
         [0013]    In the battery protection circuit according to the present invention, it is preferable that the control part cuts off a charge path to the battery when the control part cuts off the power supply path to the control part itself. 
         [0014]    The second aspect of the invention is a method for protecting a battery which is executed by a control part that cuts off a discharge path to a load of a battery when a voltage value of the battery is equal to or less than a first threshold value, or when voltage value of the battery is the less than the first threshold value, wherein the control part executes a step to cut off a power supply path to the control part itself when the voltage value of the battery is equal to or less than a second threshold value, or when the voltage value of the battery is less than the second threshold value which is lower than the first threshold value. 
         [0015]    In the method for protecting the battery according to the present invention, it is preferable that a charge path to the battery is also cut off as a cut off processing step when the control part cuts off the power supply path to the control part itself. 
         [0016]    The third aspect of the invention is a power supply device which comprises the battery protection circuit according to the present invention and a battery which is a subject of protection by the battery protection circuit. 
         [0017]    The fourth aspect of the invention is a program that realizes a function of the battery protection circuit according to the present invention in an information processing device by being installed in the information processing device. 
         [0018]    According to the present invention, a reusable period of a battery can be maintained for a long time by recharging while avoiding abnormal heating of the battery due to over discharge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic view of an overall structure for a power supply device according to an embodiment of the present invention. 
           [0020]      FIG. 2  is a schematic view of an internal structure of a battery protection circuit of the schematic view in the overall structure for a power supply device shown in  FIG. 1 . 
           [0021]      FIG. 3  is a flow diagram showing an operation order of a control part shown in  FIG. 2 . 
           [0022]      FIG. 4  is an explanatory diagram of two threshold values of a control part shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    [Regarding Structure of Power Supply Device  1  According to Embodiment of Present Invention] 
         [0024]    As shown in  FIG. 1 , a power supply device  1  according to an embodiment of the present invention is configured with a battery protection circuit  2 , a battery  3 , a discharge switch  4 , a charge switch  5 , diodes  6  and  7 , a ground potential line  8 , terminals  9  and  10 , an alarm terminal  11 , and a load  12  or a charger  13 . The power supply device  1  is, for example, configured as a battery pack which includes the battery  3 . 
         [0025]    A structure of the battery protection circuit  2  is explained later in detail by using  FIG. 2 . The battery  3  is, for example, a lithium-ion battery. The battery  3  may be configured with a plurality of cells even through the battery  3  is shown in the figures as a single cell. The discharge switch  4  turns ON (connected)/OFF (disconnected) power supplied from the battery  3  to the load  12 . The charge switch  5  turns ON/OFF charging current supplied from the charger  13  to the battery  3 . The diode  6  prevents reverse current from flowing from the load  12  to the battery  3 . The diode  7  prevents reverse current from flowing from the battery  3  to the charger  13 . The ground potential line  8  provides “0” (V) internally to the power supply device  1 . The terminals  9  and  10  are connected to either the load  12  or the charger  13 . 
         [0026]    The alarm terminal  11  outputs an alarm signal from the battery protection circuit  2 . For example, a managing device (not shown) of an administrator is connected to the alarm terminal  11 . Or, when the load  12  has an input terminal (not shown) that receives the alarm signal of the battery protection circuit  2 , the output of the alarm terminal  11  can be connected to the load  12 . Note that both the discharge switch  4  and the charge switch  5  are normally in the turned ON condition. Even though the discharge switch  4  and the charge switch  5  are in the ON condition, the reverse current of the discharging current and charging current are blocked by diodes  6  and  7 . The discharge switch  4  and the charge switch  5  are controlled to an OFF condition by the battery protection circuit  2  at the time of over discharge or overcharge. 
         [0027]    The battery protection circuit  2  is configured with a voltage measuring part  20 , a control part  21 , a power switch  22 , and a resistor  23  as shown in  FIG. 2 . The voltage measuring part  20  measures a voltage value of the battery  3  and outputs a signal to the control part  21 . The control part  21  controls the turning ON/OFF of the charge switch  5 , the turning ON/OFF of the power switch  22 , and the turning ON/OFF of the discharge switch  4  depending on the voltage value output from the voltage measuring part  20 . The control part  21  also controls an alarm output depending on the voltage value output from the voltage measuring part  20 . The power switch  22  turns ON/OFF power supplied to the battery protection circuit  2 . The resistor  23  has an extremely large resistance value compared to a resistance value of the load  12 , and draws only a small part of current that is supplied to the load  12  by the battery  3  into the battery protection circuit  2 . 
         [0028]    [Regarding Operation of Control Part  21 ] 
         [0029]    Operation of the control part  21  is explained with reference to the flow diagram in  FIG. 3 . Note that an explanation for charge control is omitted because it is the same as conventional technology, so that discharge control is primarily explained hereafter. 
         [0030]    START: When the load  12  is connected to the power supply device  1 , power is supplied from the battery  3  to the load  12 . The control part  21  recognizes that the power supply to the load  12  from the battery  3  has started based on a change of a voltage value measured by the voltage measuring part  20 , and shifts to the processing of S 1 . 
         [0031]    S 1 : The control part  21  determines whether or not the voltage value measured by the voltage measuring part  20  is equal to or less than a threshold value Th# 1 . The control part  21  shifts to the processing of S 3  when the voltage value measured by the voltage measuring part  20  is equal to or less than the threshold value Th# 1  (YES at S 1 ). On the other hand, the control part  21  shifts to the processing of S 2  when the voltage value measured by the voltage measuring part  20  is more than the threshold value Th# 1  (NO at S 1 ). 
         [0032]    S 2 : The control part  21  maintains the discharge switch  4  ON and returns to the processing of S 1 . 
         [0033]    S 3 : The control part  21  not only turns the discharge switch  4  OFF but also outputs an alarm, and then shifts to the processing of S 4 . 
         [0034]    S 4 : The control part  21  determines whether or not the voltage value measured by the voltage measuring part  20  is equal to or less than the threshold value Th# 2 . The control part  21  shifts to the processing of S 5  when the voltage value measured by the voltage measuring part  20  is equal to or less than the threshold value Th# 2  (YES at S 4 ). On the other hand, the control part  21  returns to the processing of S 1  when the voltage value measured by the voltage measuring part  20  is more than the threshold value Th# 2  (NO at S 4 ). 
         [0035]    S 5 : The control part  21  not only turns the charge switch  5  OFF, but also turns the power switch  22  OFF, and then ends the processing (END). 
         [0036]      FIG. 4  is an explanatory diagram of two threshold values, Th# 1  and Th# 2 , of the control part  21 . As shown in  FIG. 4 , two threshold values, Th# 1  and Th# 2 , are provided in the control part  21  with respect to a discharge curve of the battery  3 . For example, when the battery  3  is a lithium-ion battery, a maximum voltage value at the time of a full charge is approximately 4.1 V; the threshold value Th# 1  is set at around 2.1 V; and the threshold value Th# 2  is set at around 1.1 V. 
         [0037]    As shown in  FIG. 4 , when the voltage value of the battery  3  is equal to or less than the threshold value Th# 1 , the discharge switch  4  is turned OFF, and a charge alarm is output to a user. At this time, because the battery protection circuit  2  is being operated, the battery  3  is chargeable by connecting the charger  13  to the terminals  9  and  10 . A period discussed above is referred to as a “charge alarm period.” In other words, just after completion of a full charge through the end point of the “charge alarm period” is referred to as a “chargeable period.” 
         [0038]    During the “charge alarm period,” when a user does not charge, the voltage value of the battery  3  will be further decreased. When the voltage value of the battery  3  is equal to or less than the threshold value Th# 2  in due time, and when there is a possibility that the voltage value becomes to the extent that it might cause degradation of the battery  3 , the control part  21  turns the charge switch  5  and the power switch  22  OFF to cut the power supply to the control part  21  itself. Accordingly, charging to the battery  3  is no longer possible. However, a period until abnormal heating occurs due to completely discharging the battery  3  can be extended compared to the conventional technology (shown in dashed line form). The period from time in which the voltage value of the battery  3  is equal to or less than the threshold value Th# 2  through the time in which the battery  3  is completely discharged is referred to as a “complete discharge extending period.” 
         [0039]    As discussed above, when the discharge condition of the battery  3  is lowered to the voltage value that could cause degradation of the battery  3 , the progress of degradation of the battery  3  can be slowed by cutting off all of the discharge paths of the battery  3 . And, when the period of time from when the voltage value of the battery  3  is equal to or less than the threshold value Th# 2  through when the battery  3  is completely discharged is short as the “conventional technology” shown with the dashed line in  FIG. 4 , a value of the threshold value TH# 2  has to be set as a higher value in view of safety. In other words, the threshold value Th# 2  is set as 1.1 V in the control part  21 ; however, this value is required to be set as a higher value in the conventional technology. 
         [0040]    In contrast, in the power supply device  1 , because the “complete discharge extending period” is longer than the conventional technology, the threshold value Th# 2  can be set as a lower value. As a result, the “charge alarm period” can be set longer so that the “chargeable period” can be set longer with respect to the battery  3 . 
         [0041]    [Other Embodiments] 
         [0042]    The embodiments of the present invention can be modified in various ways so long as such variations are not to be regarded as a departure from the sprit and scope of the invention. For example, it is explained that the alarm output is output only at the “charge alarm period” shown in  FIG. 4  and is stopped at the “complete discharge extending period.” This is because, at the “complete discharge extending period,” the power consumption is required to be none. Accordingly, when there are any alarm means in which power is not consumed, the alarm output can be continued since the voltage value of the battery  3  is equal to or less than the threshold value Th# 1 . 
         [0043]    For example, as for the alarm means that does not consume power, an alarm output device with a display content retention type is contemplated. When an input signal is received for a short period of time, magnetic energy occurs on an electromagnet. As a result, the alarm output device mechanically switches the display content “from normal to alarm” by moving a piece of iron that is drawn by the magnetic energy as a trigger. 
         [0044]    Moreover, at the processing of Si in the flow diagram in  FIG. 3 , it is possible to substitute the determination processing of “the voltage value is less than the threshold value Th# 1 ?” for “the voltage value is equal to or less than the threshold value Th# 1 ?.” Similarly, at the processing of S 4  in the flow diagram in  FIG. 3 , it is possible to substitute the determination processing of “the voltage value is less than the threshold value Th# 2 ?” for “the voltage value is equal to or less than the threshold value Th# 2 ?” 
         [0045]    The battery protection circuit, the method for protecting the battery, the power supply device, and the program being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the sprit and scope of the invention, and all such modifications as would be apparent to one of ordinary skill in the art are intended to be included within the scope of the following claims.
A reusable period of a battery can be maintained for a long time by recharging while avoiding abnormal heating of the battery due to over discharge. A battery protection circuit comprises a control part that cuts off a discharge switch which is a discharge path to a load for a battery when a voltage value of the battery is equal to or less than a first threshold value, or when the voltage value of the battery is less than the first threshold value, wherein the control part cuts off a power switch as a power supply path to the control part itself when the voltage value of the battery is equal to or less than a second threshold value, or when the voltage value of the battery is less than the second threshold value which is lower than the first threshold value.