Abstract:
A battery pack includes a plurality of lithium battery cells, an analog thermo sensing member, and an analog switch. The analog thermo sensing member detects, in an analog form, a battery temperature of at least one of the plurality of lithium battery cells, and outputs an output preventing signal if the battery temperature is over a first predetermined temperature. The analog switch element is the plurality of lithium battery cells in series, and is switched, based on the output preventing signal, to a state in which a current is prevented from flowing in the plurality of lithium battery cells.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a battery pack and an electric tool. 
         [0003]    2. Description of Related Art 
         [0004]    In a cordless electric tool, a battery as a power supply is further required to be high in capacity and reduced in weight. To cope with this, a lithium battery whose output density is high is being expected, and is gradually employed recently. 
         [0005]    On the other hand, when the overcharge, overdischarge, overcurrent occurs in a lithium battery, there is raised a fear of ignition in the worst case due to degradation of battery or temperature rise of battery. Accordingly, Japanese Patent Application Publication No. 06-141479 discloses a battery charger includes a protection circuit for observing the overcharge, overdischarge, overcurrent of respective cells in a battery pack. In this battery charger, if the battery voltage of the respective battery cells is lower or higher than a predetermined voltage value, and a current flowing through the battery cells is larger than a predetermined current, the protection circuit outputs a detection signal to shut off charge and discharge paths. In this way, a countermeasure for the safety, that is, preventing the overvoltage, overdischarge, overcurrent is performed. 
       SUMMARY OF THE INVENTION 
       [0006]    As a cordless electric tool, there are various tools such as a driver drill tool that makes a hole in a lumber or a steel plate or fastens a screw, and a circular saw tool that cuts off a lumber. In order to make a motor operate powerfully, a lithium battery pack for a cordless electric tool is required to supply a large current. In such a case, the temperature of the battery rises. The excessive heat generation causes a failure of the battery, and furthermore, may cause a fuming smoking and ignition. Therefore, it is demanded to prevent the overvoltage, overdischarge, overcurrent without fault. 
         [0007]    In view of the above-described drawbacks, it is an objective of the present invention to provide a lithium battery pack which is inexpensive and is safe and secure. 
         [0008]    In order to attain the above and other objects, the present invention provides a battery pack connectable to at least one of an electric tool having a first tool terminal and a second tool terminal and a battery charger having a first charger terminal and a second charger terminal. The battery pack includes a first battery terminal, a second battery terminal, a plurality of lithium battery cells, an analog thermo sensing member, and an analog switch element. The first battery terminal is selectively connectable to one of the first tool terminal and the first charger terminal. The second battery terminal is connectable to corresponding one of the second tool terminal and the second charger terminal. The plurality of lithium battery cells is connected between the first battery terminal and the second battery terminal. Each lithium battery cell has a positive terminal and a negative terminal. The analog thermo sensing member detects, in an analog form, a battery temperature of at least one of the plurality of lithium battery cells, and outputs an output preventing signal if the battery temperature is over a first predetermined temperature. The analog switch element is connected between the first battery terminal and the second battery terminal and in series with the plurality of lithium battery cells, and switched, based on the output preventing signal, to a state in which a current is prevented from flowing in the plurality of lithium battery cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
           [0010]      FIG. 1  is a block diagram of a lithium battery pack according to a first embodiment of the present invention; 
           [0011]      FIG. 2  is an explanatory diagram of an operation of a thermosensor; 
           [0012]      FIG. 3  is a block diagram of a lithium battery pack according to a second embodiment of the present invention; and 
           [0013]      FIG. 4  is a block diagram of a lithium battery pack according to a modification of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    A battery pack according to a first embodiment will be described while referring to  FIGS. 1 and 2 . 
         [0015]      FIG. 1  shows a block diagram of a lithium battery pack  1  (referred to as battery pack  1  briefly, hereinafter) according to a first embodiment of the present invention. The battery pack  1  can be connected to a cordless electric tool  8  (referred to as tool  8  briefly, hereinafter). The tool  8  includes a trigger switch  81 , a motor  82 , a positive terminal  83 , and a negative terminal  84 . The trigger switch  81  and the motor  82  are connected between the positive terminal  83  and the negative terminal  84  in series. 
         [0016]    The battery pack  1  includes a positive terminal  11  connectable to the positive terminal  83  of the cordless electric tool  8 , a negative terminal  12  connectable to the negative terminal  84  of the tool  8 , a battery set  2  connected between the positive terminal  11  and the negative terminal  12  and having four lithium battery cells  3  (referred to as battery cells  3  briefly, hereinafter) connected in series, a current detection resistor  4  connected to the battery cells  3  in series for detecting the current flowing in the battery cells  3  based on a voltage developed across the current detection resistor  4 , a protection circuit  5 , a charging prohibition signal terminal  13  pulled out from the protection circuit  5 . 
         [0017]    The protection circuit  5  includes a protection IC  51 , a switch element  6  connected to the battery set  2  in series, a switch element  61  connected between the protection IC  51  and the switch element  6 , resistors  62  and  63  connected to the battery set  2  in parallel, and a thermosensor  7  connected between the resistors  62  and  63  and close contact with the battery cell  3  sandwiched between another battery cells  3 . In the present embodiment, the switch elements  6  and  61  are NchFETs. Specifically, a gate of the switch element  61  is connected to the IC  51  and a source of the switch element  61  is connected to a gate of the switch element  6 . A source of the switch element  6  is connected to the battery set  2  and a drain of the switch element  6  is connected to the negative terminal  12 . 
         [0018]    The protection IC  51  includes a battery voltage detection unit  52  for detecting a battery voltage of each battery cell  3 , an overcurrent determination unit  53  for determining whether or not the current flowing in the battery cells  3  has exceeded a predetermined current, an overdischarge determination unit  54  for determining, based on the battery voltage detected by the battery voltage detection unit  52 , whether or not the battery voltage of each battery cell  3  has dropped below a first predetermined voltage, an overvoltage determination unit  55  for determining whether or not the battery voltage of each battery cell  3  has exceeded a second predetermined voltage, an overdischarge/overcurrent signal output unit  56  for outputting results determined by the overcurrent determination unit  53  and the overdischarge determination unit  54 , and an overvoltage signal output unit  57  for outputting result determined by the overvoltage determination unit  55 . 
         [0019]    Next, the operation of the protection IC  51  will be described. 
         [0020]    The battery voltage detection unit  52  constantly detects the battery voltage of each battery cell  3 , and the overdischarge determination unit  54  determines whether or not the battery voltage detected by the battery voltage detection unit  52  has dropped below the first predetermined voltage (2.0 V in the present embodiment). If the battery voltage of each battery cell  3  has dropped below the first predetermined voltage, the overdischarge determination unit  54  outputs an overdischarge signal (high signal: H) from the overdischarge/overcurrent signal output unit  56  to the switch element  61 , and the switch element  61  is turned on by the H signal. When the switch element  61  is turned on, the switch element  6  is turned off. Therefore, the output from the battery pack  1  is shut off. 
         [0021]    When the remaining capacity is reduced with discharging of the battery pack  1 , an excessive dropping of the voltage causing failure or degradation of the battery cells  3  can occur. However, the above described construction prevents the battery pack  1  from being overdischarged. 
         [0022]    Further, the overvoltage determination unit  55  determines whether or not the battery voltage detected by the battery voltage detection unit  52  has exceeded the second predetermined voltage (4.3 V in the present embodiment). If the battery voltage of each battery cell  3  has exceeded the second predetermined voltage, the overvoltage determination unit  55  outputs a charging prohibition signal (high signal: H) from the overvoltage signal output unit  57  to the charging prohibition signal terminal  13 . 
         [0023]    When the battery cells  3  are overcharged, a failure or degradation of the battery cells  3  can occur. However, if a battery charger for charging the battery pack  1  has a terminal connectable to the charging prohibition signal terminal  13  to halt the charging, the above described construction prevents the battery pack  1  from being overcharged. 
         [0024]    Furthermore, the overcurrent determination unit  53  constantly determines whether or not the current detected by the current detection resistor  4  has exceeded a predetermined current. If the current has exceeded the predetermined current, the overcurrent determination unit  53  outputs an overcurrent signal (high signal: H) from the overdischarge/overcurrent signal output unit  56  to the switch element  61 , and the switch element  61  is turned on by the H signal. When the switch element  61  is turned on, the switch element  6  is turned off. Therefore, an excessive current causing a failure or degradation of the battery cells  3  is prevented from flowing in the battery cells  3 . 
         [0025]    Next, the operation of the thermosensor  7  will be described while referring to  FIG. 2 . In  FIG. 2 , a condition in which the voltage of each battery cell  3  is in a range between the first predetermined voltage and the second predetermined voltage, and the current flowing the battery set  2  is equal to or lower than the predetermined current is assumed. 
         [0026]    The thermosensor  7  is a thermal protector of the bimetal system or a thermostat in the present embodiment. The thermosensor  7  is connected between the resistors  62  and  63  in parallel with the battery set  2  and close contact with any one of the battery cells  3 . The thermosensor  7  detects a temperature of the battery cell  3  and is opened when the temperature has exceeded a predetermined temperature. 
         [0027]    As shown in  FIG. 2 , in an ordinary temperature, the thermosensor  7  is closed (ON state). Therefore, a battery voltage of the battery cells  3  is applied to the gate of the switch element  6  to turn on the switch element  6 . When the trigger switch  81  is closed, the voltage of the battery cells  3  is applied to the tool  8 . 
         [0028]    As the tool  8  is continually used, the temperature of the battery cells  3  rises gradually. Especially, when the tool  8  is harshly used due to continuous work or heavy-loaded work, the temperature of the battery cells  3  rapidly rises. Then, when the temperature of the battery cells  3  has reached the predetermined temperature (T point in  FIG. 2 ), the thermosensor  7  is opened. When the thermosensor  7  is opened, a potential of the gate of the switch element  6  become 0 V. Thus, the switch element  6  is turned off (open state) to shut off the output from the battery pack  1 . Since the output from the battery pack  1  is halted temporarily, the temperature of the battery pack  1  is prevented from becoming high extremely. Accordingly, degrading and breaking down of the battery cells  3  are prevented. Thus, a lightweight, safe, and inexpensive lithium battery pack  1  can be provided. 
         [0029]    However, even if the switch element  6  is turned off, the switch element  6  will be turned on again if the temperature of the battery cells  3  drops below the predetermined temperature. Then, if the switch element  6  is immediately turned on again, the temperature of the battery cells  3  reaches the predetermined temperature, causing the switch element turning off. 
         [0030]    Therefore, the thermosensor  7  in the present embodiment has a hysteresis, as shown in  FIG. 2 . Due to the hysteresis, the thermosensor  7  is not turned on until the temperature of the battery cells  3  has dropped below a hysteresis temperature (H point in  FIG. 2 ) even if the temperature of the battery cells  3  has dropped below the predetermined temperature (T point). Thus, the off state of the thermosensor  7  is kept during a predetermined period that is required for dropping the temperature of the battery cells  3 . 
         [0031]    Next, a lithium battery pack  1  according to a second embodiment will be described while referring to  FIG. 3 . 
         [0032]    As shown in  FIG. 3 , a thermistor whose resistance value changes in response to changing of the temperature of the battery cells  3  is used as the thermosensor  7  in the present embodiment, while the thermal protector of the bimetal system or the thermostat is used as the thermosensor  7  in the first embodiment. 
         [0033]    The thermosensor  7  in the second embodiment is in close contact with the battery cells  3  similar to the thermosensor  7  in the first embodiment. The lithium battery pack  1  further includes a temperature output unit  9  connected to the thermosensor  7 . The temperature output unit  9  is provided with resistors  91 - 94 , a switch element  95 , a comparator  96  and a diode  97 . 
         [0034]    A voltage supplied from a power supply Vcc is divided by the resistor  91  and the thermosensor  7 , and the devided voltage is inputted into the comparator  96  as a battery temperature voltage. On the other, the voltage supplied from the power supply Vcc is also divided by the resistors  92  and  94 , and the divided voltage is also inputted into the comparator  96  as a reference voltage. The comparator  96  compares the battery temperature voltage with the reference voltage, and outputs a H signal to the gate of the switch element  61  through the diode  97  if the battery temperature voltage is larger than the reference voltage. The switch element  61  is turned on (ON state) in response to the H signal. When the switch element  61  is turned on, the switch element  6  is turned off (OFF state). Thus, the output from the battery pack  1  is shut off. 
         [0035]    Further, the comparator  96  outputs the H signal to a gate of the switch element  95  if the battery temperature voltage is larger than the reference voltage. A source of the switch element  95  is connected to the resistor  93  connected to the resistor  92  in parallel with the resistor  94 . When the switch element  95  receives the H signal, the resistors  93  and  94  are connected in parallel. When the resistors  93  and  94  are connected in parallel, the reference voltage is lowered. When the reference voltage is lowered, the comparator  96  outputs the H signal until the temperature of the battery cells  3  drops for a predetermined temperature. Thus, the temperature output unit  9  has a hysteresis characteristic. 
         [0036]    As described above, while the thermistor can be used as the thermosensor, it is preferable to use the thermal protector of the bimetal system or thermostat as described in the first embodiment due to following advantages. 
         [0037]    If the thermistor is used as the thermosensor, the temperature output unit  9  as shown in  FIG. 3  is required. However, if the thermal protector or thermostat is used as the thermosensor, the circuit configuration can be significantly simplified since the thermal protector or thermostat can be directly connected to the input unit of the switch element  6 . 
         [0038]    Further, the temperature output unit  9  as shown in  FIG. 3  requires the power supply Vcc. Thus, the current flowing the battery cells  3  is increased. However, when the thermal protector of the bimetal system or thermostat that detects the temperature by the bimetal system that is, mechanical operation is used as the thermosensor as shown in  FIG. 1 , the current flowing the battery cells is not increased since the battery pack  1  is provided with the temperature output unit  9 . Accordingly, the protection circuit  5  can saves energy. 
         [0039]    Even in the case where the switch element  6  is provided on not the battery pack  1  but the cordless electric tool  8 , it is more preferable to use the thermal protector or thermostat than to use the thermistor. 
         [0040]    While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
         [0041]    For example, as shown in  FIG. 4 , the switch element  6  may be provided on not the battery pack  1 , but the tool  8 . Further, the switch elements  6  and  61  may be another analog switch while NchFET is used the switch elements  6  and  61  in the preferred embodiment.