Abstract:
In a battery pack with battery charger, a circuit derives a relatively low constant current from the charge current of the battery charger, and this relatively low constant current is used to charge the battery pack when temperature is below a threshold. Otherwise, the charge current from the battery charger is used, at least up to a high temperature threshold.

Description:
This application claims priority from Japanese patent application no. JP2007-220904, filed Aug. 28, 2007. 
     FIELD OF THE INVENTION 
     The present invention generally relates to a charging circuit accommodated in a battery pack, and more particularly, to a charging circuit which can stably perform charging while changing the current value of a charging current based on the temperature of a battery cell. 
     BACKGROUND OF THE INVENTION 
     In a notebook personal computer (hereinafter, referred to as “note PC” for simply), which is a typical example of a portable or mobile type electronic device, a lithium ion rechargeable battery is generally used. In recent years, a number of battery pack-related fire accidents has been reported, and safety guidelines have been released for safe use of lithium ion batteries by cooperation of BAJ (The Battery Association of Japan) and JEITA (Japan Electronics and Information Technology Industries Association). Safety guidelines may be available from time to time on the associations home pages on the Internet. 
     SUMMARY OF THE INVENTION 
     It is evident that when a lithium ion battery is charged with a large charging current at low temperature, lithium ions having moved from a positive electrode to a negative electrode via organic electrolytic solution are reluctant to be absorbed in the negative electrode, resulting in occurrence of deposition of a lithium metal on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode does not return to the electrolytic solution as lithium ions. Therefore, when a flow of a large charging current is taken place at a low temperature, the deposited lithium metal will be accumulated on the negative electrode, thereby increasing a possibility that the positive electrode and the negative electrode are short-circuited. Therefore, when charging is performed at a low temperature, it is necessary to maintain the charging current at a level equal to or smaller than the maximum charging current value Imax 2 , in order to prevent lithium metal deposition on the negative electrode. 
     On the other hand, when the lithium ion battery is charged at a high temperature, the temperature of a battery cell increases with an increase in the charging current. Moreover, the ambient temperature in the battery pack is added to the battery cell temperature, and as a result, the surface temperature of the battery cell exceeds an upper limit thereof, which may put the battery pack into a critical condition or may operate a safety circuit of the battery pack so as to stop the charging. Therefore, when charging is performed at a high temperature, it is necessary to maintain the charging current at a level equal to or smaller than the maximum charging current value Imax 2 , in order to suppress a rise in the temperature of the battery cell. 
       FIGS. 6A to 6C  show typical waveforms of a DC current.  FIG. 6A  shows a waveform of a pure DC current without any AC component, and in this case, the current value can be identified as I 1 . The DC current having such a waveform may be generated by using the constant current characteristics of a transistor or may be generated as an output current of a battery.  FIG. 6B  shows a waveform of a charging current generated by a general battery charger which is operated by a switching control method. The battery charger switches or chops an input DC current at a high frequency and smooths the current in an off period by using a smoothing circuit, thereby generating the charging current. Therefore, an AC component of the charging current is small and, the difference between the peak value and the average value thereof is extremely small. The magnitude of the charging current can be identified as the average value I 2 . 
       FIG. 6C  shows a waveform of a DC current having a magnitude of I 3  when it is periodically interrupted with a duty ratio of 50 percents. The DC current has an average value of I 4  and a peak value of I 3 , and the difference between the average value I 4  and the peak value I 3  is large. In the present description of the specification, the DC current having the waveforms shown in  FIGS. 6A and 6B  will be referred to as a constant current, and the DC current having the waveform shown in  FIG. 6C  will be referred to as a switching current. Although either the constant current shown in  FIG. 6B  or the switching current shown in  FIG. 6C  contains an AC component because the currents are generated by periodically interrupting the DC current, the switching current differs from the constant current in that the switching current is generated without passing through the smoothing circuit. The switching current may flow even in an off-period due to an inductive component or a capacitive component of the circuit when the frequency increases. However, when the frequency decreases, the switching current becomes substantially zero in the off-period as shown in  FIG. 6C  and it becomes an intermittent current. In the present specification, the magnitude of the switching current will be represented by an average value I 4 . 
     A note PC having mounted thereon a battery pack of a lithium ion battery is equipped with a battery charger of the type having an operation under a constant-current/cons ant-voltage mode and thus, the battery charger is operable to output a constant current identical to a setting current value when performing a constant current control. Hitherto, no one has ever tried to limit the maximum charging current of the lithium ion battery based on the surface temperature of the battery cell and therefore, the battery charger mounted on the note PC is adapted to operate with a single setting current value. However, with the release of the safety guidelines described above, it is necessary to further strengthen the safety management of the lithium ion battery. In this regard, it is necessary to ensure the safety of a battery pack mounted on the note PC which was already shipped in accordance with the safety guidelines. Replacing the battery charger mounted on the shipped note PC with a new battery charger capable of operating with a plurality of setting current values may be a possible option to meet the safety requirement; however, this option has practical difficulties in matters including cost, replacing time, and a problem of design adaptability. 
     To solve the described problem, a method may be considered in which a new charging circuit is incorporated in the battery pack as described in Patent Documents 1 and 2, so that charging can be performed with the charging current values corresponding to the respective temperature ranges by means of the charging circuit. However, when the charging circuit, which is operated in the constant-current/constant-voltage system, similar to that mounted on the note PC, is provided in the battery pack, it may increase the size of the battery pack, and as a result, the battery pack cannot be mounted on the existing note PC. Moreover, it is necessary to switch a DC voltage at a high frequency in order to generate a constant current through switching control. However, when a switching element operating at a high frequency is provided in the battery pack, it may cause electromagnetic disturbance, leading to malfunction of processors or temperature rise due to heat generation. 
     In addition, a method can be considered in which a charging circuit capable of operating in both a low temperature range and a high temperature range is provided in a battery pack. Since lithium metal is disadvantageously deposited in the low temperature range, it is necessary for the charging circuit to control the charging current so as not to exceed the maximum charging current value Imax 2 . Therefore, in order to perform charging with the switching current in the low temperature range, the peak value of the switching current should be no more than the maximum charging current value Imax 2 . Moreover, in order to generate the switching current from the charging current supplied by the battery charger, the output current (constant current) of the battery charger should be not more than the maximum charging current value Imax 2 . In a standard temperature ranges the battery charger supplies a much greater charging current within an allowable range of the maximum charging current value Imax 1 . In such a case, however, the battery charger should be able to set therein at least two charging current values; therefore, this method cannot cope with the shipped note PC. 
     Moreover, since the difference between the average value and the peak value of the switching current is large, when the peak value is made identical to the maximum charging current value Imax 2 , the electric power used in actual charging, defined by the average value, becomes smaller, which may increase the charging time, and it is not practical. Moreover, FETs or bipolar transistors may be used as described in Patent Documents 1, and 2. However, when the FETs or the bipolar transistors are operated by a constant current control method in a continuous (constant current) manner rather than a switching manner, a large amount of heat will be generated. Therefore, it cannot be employed in a battery pack which requires strict safety management. 
     A non-limiting object of some embodiments is to provide a battery pack having accommodated therein a charging circuit capable of changing the value of a charging current based on the temperature associated with a battery cell. Another object of some embodiments is to provide a battery pack capable of increasing an internal temperature when performing charging in a low temperature range to reach a standard temperature range in a short time. A further object of some embodiments is to provide a battery pack having accommodated therein a charging circuit capable of performing charging with a plurality of charging current values without adding any modifications to an apparatus on which the battery pack is mounted. A still further object of some embodiments is to provide a method of charging a secondary battery accommodated in a battery pack based on a temperature associated with a battery cell. 
     A battery pack according to some embodiments can be charged with a charging current supplied by a DC power supply. The battery pack may include a temperature element configured to measure a temperature associated with a secondary battery, a first charging circuit, and a control unit. The first charging circuit may generate a constant current from the charging current. The control unit may control the operation of the battery pack in a manner such that the secondary battery is charged with the constant current lower than the charging current by the first charging circuit when it is determined that the temperature in connection with the secondary battery belongs to a low temperature range, while the secondary battery is charged by the DC power supply when it is determined that the temperature in connection with the secondary battery belongs to a standard temperature range higher than the low temperature range. The temperature in connection with the secondary battery may be a temperature suitable for monitoring the temperature of a battery cell and capable of being measured on the surface of a housing of the battery cell or being directly measured in the inside of the battery cell or being indirectly at a position distant from the battery cell. 
     Even when the DC power supply is a battery charger that operates with a single setting current, the first charging circuit on the battery pack side can generate a charging current as needed depending on the temperature of the battery cell and perform charging. When the first charging circuit generates the charging current by continuously controlling semiconductor elements, heat may generate from the semiconductor elements; however, in some embodiments, the heat generation is used to ensure safety. Specifically, by operating the first charging circuit only when the temperature of the battery cell belongs to the low temperature range, it is possible to increase the temperature of the battery cell in a short period of time to thusly prevent lithium metal from depositing. Moreover, after the temperature of the battery cell is increased to the standard temperature range in a short period of time, the charging can be performed with the maximum charging current allowed in the standard temperature range; therefore, it is possible to decrease the charging time. 
     The battery pack may further include a second charging circuit. The second charging circuit can generate a switching current lower than the charging current from the charging current per se. In this case, the control unit may control the operation of the battery pack in a manner such that the secondary battery is charged by the second charging circuit when it is determined that the temperature in connection with the secondary battery belongs to a high temperature range higher than the standard temperature range. The value of the switching current can be represented by the average values and the difference between the average value and the peak value may be relatively large. However, since in the high temperature range, it is only necessary to prevent any temperature rise in the battery, the charging can be performed with the switching current generated with a duty ratio that satisfies the average value determined based on the temperature rise. Moreover, since the cycle of the switching operation can be extended to a range necessary for preventing the temperature rise, there is no problem of heat generation or electromagnetic disturbance due to the switching operation. 
     The first charging circuit may be configured by using the constant current characteristics of the collector current relative to the base current in the bipolar transistor or the drain current relative to the gate current in the field-effect transistor. The second charging circuit may be provided for protection of the battery pack and can be configured as a charge protection switch which is typically installed to inhibit charging to the secondary battery. The peak value of the switching current generated by controlling turning on/off of the charge protection switch can become identical to the value of the charging current supplied by the DC power supply; however, it does not cause any problem to the charging in the high temperature range as described above. When the secondary battery is a lithium ion battery, although it is usually particularly difficult to limit the charging current relative to the temperature of the battery cell, such a difficulty can be eliminated by present principles. 
     In accordance with some embodiments, even when the DC power supply is a battery charger that is operated with a single setting current, the secondary battery can be charged with the charging current as needed depending on the temperature of the battery cell by only the components accommodated in the battery pack. Therefore, a shipped apparatus equipped with a battery charger that is operated with a single setting current can be charged with a plurality of setting current values in accordance with the temperature of the battery cell without necessity of applying any modifications to the battery charger. When the DC power supply is a battery charger which is operated by a constant-current/constant-voltage control node, the charging is performed by the first or second charging circuit during only the constant current control period, which the charging can be performed by the battery charger when the charging mode of the secondary battery enters a state wherein it switches to a constant voltage control mode. The state wherein it switches to the constant voltage control mode can be determined by the charging current or the charging voltage. Since the constant voltage control requires strict voltage management, it is desirable to perform the charging by means of the battery charger. Even when the charging mode wherein charging is performed by means of the first or second charging circuit is switched to a constant voltage control mode wherein charging is performed by means of the battery charger, the charging current is decreased to a value not more than the maximum charging current value Imax 2  that is allowed in the temperature ranges. 
     When the temperature of the battery cell belongs to the standard temperature range during charging by the first or second charging circuit, the charging can be performed by the battery charger, whereby the charging current is not limited to more than that needed, and the charging time is not increased. When the temperature of the battery cell belongs to the high temperature range while the charging is performed by means of the battery charger, the charging can be continued by the second charging circuit without necessity of stopping the charging. When the voltage of the secondary battery has reached the maximum charging voltage that is allowed in the low temperature range or the high temperature range while charging is still being performed by the first charging circuit or the second charging circuit, the charging can be stopped and therefore, the safety can be ensured. 
     In accordance with the above-mentioned various aspects of present principles, it is possible to provide a battery pack having accommodated therein a charging circuit capable of changing the current value of a charging current based on the temperature in connection with a battery cell. Further, it is possible to provide a battery pack capable of increasing an internal temperature when performing charging in a low temperature range to reach a standard temperature range in a short period of time. Furthermore, it is possible to provide a battery pack having accommodated therein a charging circuit capable of performing charging with a plurality of charging current values without adding any modifications to an apparatus on which the battery pack is mounted. Furthermore, it is possible to provide a method of charging a secondary battery accommodated in a battery pack based on the temperature associated with the secondary battery. 
     The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an outline of a charging system including a battery pack according to a non-limiting example implementation of the present invention and a note PC having mounted thereon the battery pack; 
         FIG. 2  is a block diagram illustrating an example of the battery pack according to one embodiment; 
         FIG. 3  is a flow chart illustrating non-limiting example procedures of charging the battery pack; 
         FIG. 4  is a block diagram illustrating another example of the battery pack according to an embodiment; 
         FIGS. 5A and 5B  are diagrams illustrating the relationship between the maximum values of the charging voltage and current and the surface temperature of the battery cell; and 
         FIGS. 6A to 6C  are diagrams illustrating the waveforms of a DC current. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a block diagram illustrating an outline of a charging system including an example battery pack according to present principles and a note PC having mounted thereon the battery pack. The charging system includes a note PC  10 , an AC/DC adapter  11 , and battery packs  100  and  101 . The battery pack  100  is used as a main battery pack, and the battery pack  101  is used as an auxiliary battery pack. The battery pack  100  and the battery pack  101  may have the same construction in example embodiments of the present invention; however, the charging system may be constructed by only the battery pack  100  in a state where the battery pack  101  is not mounted thereon. The note PC  10  is illustrated with only those elements that are related to present principles. The AC/DC adapter  11  is configured to be connectable to a power supply line of the note PC  10 , and the battery packs  100  and  101  are removably accommodated in a battery bay of the note PC  10 . The AC/DC adapter  11  converts an AC voltage to a DC voltage. 
     The discussion below pertains to an example non-limiting implementation. In non-limiting examples the battery charger  51  has constant-current/constant-voltage characteristics, and a charging rate thereof is fixed to about 0.7 C so that it can perform its fast charging capability. Therefore, the battery charger  51  is not operable with a plurality of charging rates. The battery charger  51  includes a switching control circuit that controls turning on/off of an FET  29  and an FET  31  in a PWM method and a smoothing circuit composed of an inductor  33  and a capacitor  34 . The battery charger  51  converts a DC voltage input from the AC/DC adapter  11  to a DC voltage suitable for charging the battery pack and outputs the converted voltage. The battery charger  51  suppresses the pulsation of the DC charging current generated through the switching control circuit by using the smoothing circuit to thereby generate a constant current. To the voltage feedback input FB-V and the current feedback input FB-I of the battery charger  51 , voltage-dividing resistors  37  and  39  and an output from the current sense resistor  35  are connected respectively, and voltages corresponding to the output voltage (charging voltage) and output current (charging current) of the battery charger  51  are input for feedback control. 
     To a current setting value input Iset and a voltage setting value input Vset of the battery charger  51 , voltages from a reference voltage source  55 , which are divided from a constant voltage generated within the note PC  10  are input. The reference voltage source  55  inputs the setting voltage Vchg to the voltage setting value input Vset and the setting current Ichg to the current setting value input Iset in accordance with instructions from an embedded controller (EC)  13 . The battery charger  51  is operated such that the output voltage or the output current is identical to either the setting voltage Vchg or the setting current Ichg. Although the battery charger  51  is operated in a constant current control mode in an initial period of charging, when the charging current decreases and becomes lower than the setting current Ichg with the progress of the charging, the battery charger  51  is automatically operated in a constant voltage control mode so that the output voltage is identical to the setting voltage Vchg. To the contrary, when due to some reasons, the charging voltage is decreased to be lower than the setting voltage Vchg during operation in the constant voltage control mode, the battery charger  51  is automatically operated in a constant current control mode so that the output current is identical to the setting current Ichg. 
     The EC  13  is an integrated circuit that controls many hardware elements of the note PC  10  as well as a power supply. The EC  13  can communicate with the battery packs  100  and  101  to thereby acquire information such as the surface temperature, battery voltage, charging current, charging power, discharging power, and remaining capacity of the battery cell generated by the battery packs  100  and  101  and the setting voltage Vchg and setting current Ichg set by the battery charger. The EC  13  delivers instructions to the reference voltage source  55  to activate or stop the battery charger  51  in accordance with the instruction from the battery packs  100  and  101 . For example, when the EC  13  is instructed by the battery packs to set the setting voltage Vchg and the setting current Ichg to zero, values of zero are programmed to the voltage setting value input Vset and the current setting value input Iset, and the operation of the battery charger  51  is stopped. When the battery charger  51  starts an operation, the EC  13  having received the instructions from the battery packs  100  and  101  programs the setting voltage Vchg and the setting current Ichg to the voltage setting value input Vset and the current setting value input Iset. 
     A DC/DC converter  53  converts the DC voltage supplied from the AC/DC adapter  11  or the battery packs  100  and  101  to a predetermined voltage and supplies the converted voltage to a device in the note PC  10 . Examples of the device include a variety of devices such as a CPU, a liquid crystal display, a wireless module, a hard disc drive, or a controller. An FET-A and an FET-B are switches for controlling charging/discharging of the main battery pack  100  and are connected to a charging/discharging circuit of the main battery pack  100 . An FET-C and an FET-D are switches for controlling charging/discharging of the auxiliary battery pack  101  and are connected to a charging/discharging circuit of the auxiliary battery pack  101 . 
     An FET-E is a switch that is connected between the battery packs  100  and  101  and the DC/DC converter  53  for forming a discharging circuit from the battery packs  100  and  101  to the DC/DC converter  53 . An FET-F is connected to a circuit for supplying electric power from the AC/DC adapter  11  to the DC/DC converter  53 . That is, the FET-F is a switch for temporarily supplying electric power from the battery packs  100  and  101  to the DC/DC converter  53  in order to perform so-called peak shifting wherein the switch suppresses the peak of an AC power source by interrupting the supply of electric power from the AC power source while electric power is being supplied from the AC/DC adapter  11  to the DC/DC converter  53 . An FET drive circuit  15  controls the FET-A to the FET-F in accordance with the instructions from the EC  13 . 
       FIG. 2  is a block diagram illustrating an internal construction of the battery pack  100  in compliance with the smart battery system (SBS) standards according to an example embodiment. The battery pack  101  has the same construction as the battery pack  100 . The battery pack  100  has a power supply line  131 , a communication line  133 , and a ground line  135 , which are respectively connected to a P terminal, a D terminal, and a G terminal of the note PC  10 . To the power supply line  131 , a charge protection switch C-FETb and a discharge protection switch D-FETb, which are configured by p-type MOS-FETs, are connected in series. To the discharge protection switch D-FETb, a battery set  106  having therein three lithium ion battery cells  103  to  105  are connected in series. The discharging current from the battery set  106  and the charging current to the battery set  106  flows between the note PC  10  and the battery set  106  via a charging/discharging circuit formed by the power supply line  131  and the ground line  135 . 
     The terminals of the battery set  106  at the voltage side of the battery cells  103  to  105  are connected to analog input terminals V 1  to V 3  of an analog interface  107 . A temperature element  110  such as one or plural thermistors is attached on the surface of the battery set  106 . The output of the temperature element  110  is connected to a T terminal of an MPU  113 . A current sense resistor  109  is connected to the ground line  135  between the negative terminal and the G terminal of the battery cell  105 . Both ends of the current sense resistor  109  are connected to the I 1  and I 2  terminals of the analog interface  107 . 
     The analog interface  107  includes analog input terminals V 1 , V 2 , and V 3  for acquiring the respective cell voltages of the battery cells  103  to  105  and analog input terminals I 1  and I 2  for acquiring potential difference across the current sense resistor  109 . The analog interface  107  also includes analog output terminals C-CTL and D-CTL for outputting signals that control turning on/off of the charge protection switch C-FETb and the discharge protection switch D-FETb. The analog interface  107  measures the cell voltages of the battery set  106 , converts the measurement values into digital values, and delivers the converted values to the MPU  113 . 
     The analog interface  107  measures the charging current and the discharging current flowing in the battery set  106  from the voltage detected by the current sense resistor  109 , converts the measurement values into digital values, and delivers the converted values to the MPU  113 . The MPU  113  is an integrated circuit in which in addition to an 8 to 16 bit CPU, a RAM, a ROM, a flash memory, and a timer are integrated into one package. The MPU  113  is configured to be able to communicate with the analog interface  107 , and calculates the amount of charged or discharged electricity based on the voltage or current measurement values delivered from the analog interface  107 . Moreover, the MPU  113  has an overcurrent protection function, an overvoltage protection function (also referred to as overcharge protection function), and an undervoltage protection (also referred to as overdischarge protection function). Upon detection of an abnormality in the battery cells  103  to  105  from the voltage or current measurement value delivered from the analog interface  107 , the MPU  113  turns off either or both of the charge protection switch C-FETb and the discharge protection switch D-FETb via the analog interface  107 . The overcurrent protection function, the overvoltage protection function, and the undervoltage protection function are implemented as a program that is executed by the MPU  113 . 
     The communication line  133  from the MPU  113  is connected to the EC  13  of the note PC  10  via the D terminal, so that the MPU  113  can communicate with the EC  13 . A clock line is included in the communication line  133 . The MPU  113  transmits the values of the setting current Ichg and the setting voltage Vchg, which are to be programmed in the battery charger  51  to the EC  13 . Then, the EC  13  programs the setting values into the battery charger  51  via the reference voltage source  55  to thereby activate or stop the operation of the battery charger  51 . 
     An npn-type bipolar transistor  115  is connected to the power supply line  131  in parallel to the series connection of the discharge protection switch D-FETb and the charge protection switch C-FETb. A resistor R 1  and an FET  121  are connected in series between the collector and the base of the transistor  115 . An FET  119  is connected between the gate of the FET  121  and the ground line  135 , and the gate of the FET  119  is connected to a CC-ON terminal of the MPU  113 . The transistor  115  has the emitter connected to one end of a resistor Re and the base connected to a series connection of diodes D 1  and D 2 . The other end of the resistor Re and the cathode of the diode D 2  are connected to the positive electrode of the battery cell  103 . The transistor  115  and the resistors, diodes, and FETs that operate the transistor  115  form a constant current circuit  111  for charging the battery set  106  in the low temperature range. The charge protection switch C-FETb is usually used for stopping the charging when the MPU  113  has detected an abnormality as to the charging voltage or charging current in the interior of the battery pack  100 . However, in the present embodiment, the charge protection switch C-FETb is also used for charging the battery set  106  in the high temperature range. It is to be noted that the transistor  115  may be a pnp-type bipolar transistor. 
     Next, a charging operation of the constant current circuit  111  to the battery set  106  will be described. The transistor  115  generates a constant current when the battery pack  100  is operated by being supplied with the charging current from the battery charger  51  of the note PC  10 . When the MPU  113  determines from the temperature detected by the temperature element  110  that the surface temperature of the battery cells  103  to  105  belongs to a low temperature range, the MPU  113  turns off the charge protection switch C-FETb while turning on the FET  119 . When the FET  119  is turned on, the FET  121  is turned on and a bias circuit of the transistor  115  is operated. The charging current supplied via the P terminal from the battery charger  51  is supplied to the battery set  106  via the collector and the emitter of the transistor  115 . 
     The diodes D 1  and D 2  cause a forward voltage drop of about 0.6 V. Since the base-emitter voltage Vbe of the transistor  115  is approximately equal to the forward voltage drop of the diode D 1 , assuming the resistance value of the resistor Rc be Re, a charging current of I=0.6/Re flows through the collector. If the charging current I increases due to some reasons, the voltage drop at the resistor Re increases and the voltage Vbe decreases. As a result, the base current decreases to suppress the increase in the charging current I. On the other hand, if the charging current I decreases due to some reasons, the voltage drop at the resistor Re decreases and the voltage Vbe increases. As a results the base current increases to suppress the decrease in the charging current I. In this manner, the transistor  115  can output a constant charging current I=0.6/Re. This charging current is the constant current described in  FIGS. 6A to 6C , and the value corresponds to the maximum charging current value Imax 2 . 
     When the constant current is generated by continuously controlling the transistor  115  rather than controlling it in a switching manner, heat corresponding to 0.6 W to 0.7 W is generated. The upper limit of heat generation in a device accommodated in the battery pack is set to 0.3 W to 0.4 W in order to prevent temperature rise in the battery cell. In this respect, the transistor  115 , which is operated in a constant current mode, is not suitable as a device accommodated in the battery pack. However, in the present embodiment, since the transistor  115  is operated only when the surface temperature of the battery cell remains in the low temperature range, the surface temperature of the battery set  103  is not increased to such a dangerous state. On the contrary, the transistor  115  increases the surface temperature of the battery cell in a short time, thereby providing an advantage that it prevents deposition of lithium metal during charging. Moreover, the surface temperature increases to the standard temperature range in a short time, and the charging can be performed with the maximum charging current value Imax 1  allowed in the standard temperature range, whereby the charging time can be reduced. 
     Subsequently, a charging operation of the charge protection switch C-FETb to the battery set  106  will be described. The charge protection switch C-FETb performs a switching operation while the battery pack  100  is being supplied with the charging current from the battery charger  51  of the note PC  10 . When the MPU  113  determines from the temperature detected by the temperature element  110  that the surface temperature of the battery cells  103  to  105  belongs to a high temperature range, the MPU  113  turns on the discharge protection switch D-FETb while controlling turning on/off of the charge protection switch C-FETb by setting the duty ratio such that the average value of the charging current supplied by the battery charger  51  becomes the maximum charging current value Imax 2  (see  FIGS. 6A to 6C ). The average value of the switching current flowing in the battery set  106  is measured by the analog interface  107  as the voltage across the current sense resistor  109  and is delivered to the MPU  113 . The MPU  113  controls the duty ratio of the charge protection switch C-FETb based on the value in a feedback manner. When the battery charger  51  is operated in a constant current control mode and is outputting a constant current of Imax 1 , the peak value of the waveform of the switching current flowing in the battery set  106  corresponds to the maximum charging current value Imax 1 . However, in the high temperature range, unlike the low temperature range, it does not cause any problem if the peak value of the current waveform exceeds the maximum charging current value Imax 2 . Therefore, when the average value is not more than the maximum charging current value Imax 2 , it is possible to suppress temperature rise, and the on/off switching cycle can be increased to about several minutes. 
     When the constant current is generated by a switching operation, it is necessary that the switching frequency is increased to about 100 KHz or more and that a smoothing circuit is provided. In such a case, in addition to a space problem in the battery pack, there is a fear of electromagnetic disturbance in the operation of the MPU  113 ; for this reason, up to this far, it was difficult to generate the charging current by the switching operation of the charge protection switch C-FETb. However, in the present embodiment, the charging by means of the charging protection switch C-FETb is performed only when the surface temperature of the battery cells  103  to  105  remains in the high temperature range. Therefore, as long as the switching current is generated such that the surface temperature does not exceed the upper limit temperature, it is possible to maintain the switching frequency at a sufficiently low level. Thus, there is no problem of electromagnetic disturbance or heat generation. 
       FIG. 3  is a flow chart illustrating the procedures of charging the battery set  106  by means of the charging system shown in  FIG. 1  having the battery pack  100  mounted thereon. In block  201 , the battery pack  100  is attached to a battery bay of the note PC  10 . The MPU  113  measures the voltages of the battery cells  103  to  105  to thereby determine whether or not charging is required. When it is determined that the charging is required, in block  203 , the MPU  113  determines from the temperature detected by the temperature element  110  whether the surface temperature of the battery cells belongs to either of the three temperature ranges shown in  FIGS. 5A and 5B . In one example embodiment, the maximum value of the charging current is set to 0.3 C in the low temperature range and the high temperature range, while the maximum value of the charging current is set to 0.7 C in the standard temperature range. It is to be noted that the present invention is not limited to the three example temperature ranges illustrated and that the setting value of the charging current in each temperature range is not limited to these values. 
     In block  203 , when the MPU  113  determines that the surface temperature of the battery cells  103  to  105  belongs to the low temperature range, in block  205 , the MPU  113  turns off the charge protection switch C-FETb while turning on the FET  119  to thereby activate the bias circuit of the transistor  115 . Subsequently, in block  207 , the MPU  113  issues a charge request by instructing the EC  13  to set the setting current Ichg and the setting voltage Vchg in the battery charger  51 . The setting current Ichg is set to a fixed rate of 0.7 C, and therefore, the battery charger  51  outputs a charging current of 0.7 C when it is operated in the constant current control mode. When the setting current Ichg and the setting voltage Vchg are programmed in the current setting value input Iset and the voltage setting value input Vset, the battery charger  51  starts its operation. 
     In block  209 , the transistor  115  is operated in a constant current control mode to thereby generate a constant current of 0.3 C from the constant voltage supplied from the battery charger  51 , and therefore, the battery cell,  103  to  105  are charged with the constant current of 0.3 C. The MPU  113  is periodically monitoring the surface temperature of the battery cells  103  to  105  during charging. When it is determined in block  209  that the surface temperature has reached the standard temperature range, the flow proceeds to block  237 , where the MPU  113  stops the operation of the transistor  115  and turns on the charge protection switch C-FETb, thereby switching a charging mode to a mode wherein charging is performed by means of the battery charger  51 . The charging by means of the battery charger  51  is carried out via a path formed by the P terminal, the charge protection switch C-FETb, the discharge protection switch D-FETb, the battery set  106 , the current sense resistor  109 , and the G terminal. Since the transistor  115  also functions as a heating element, the battery pack  100  can shorten the time until the surface temperature reaches the standard temperature range from the low temperature range to thereby suppress the deposition of lithium metal. Moreover, since the surface temperature can be shifted from the low temperature range to the standard temperature range in a short time, the charging can be performed with a charging current of 0.7 C by means of the battery charger  51 , thereby shortening the charging time. 
     When it is determined in block  209  that the surface temperature remains in the low temperature range, the flow proceeds to block  211 , where the MPU  113  determines based on the values detected by the current sense resistor  109  whether the charging current is decreased to a level at which the battery charger  51  switches to a constant voltage control mode. When it is determined in block  211  that the charging current is decreased up to such a level, the flow proceeds to block  217 , where the MPU  113  turns off the FET  119  to stop the operation of the transistor  115 , while turning on the charge protection switch C-FETb to thereby switch the charging mode to a mode wherein charging is performed by means of the battery charger  51 . When it is determined in block  211  that the charging current is not decreased up to such a level, the flow proceeds to block  213 , where the MPU  113  determines whether the charging voltage has reached the maximum charging voltage value Vmax 2  in the low temperature range shown in  FIG. 5B . 
     When it is determined in block  213  that the charging voltage has reached the maximum charging voltage value Vmax 2 , the operation of the transistor  115  is stopped in block  215 , and the operation of the battery charger  51  is stopped in block  245 , thereby stopping the charging to thereby prevent deposition of the lithium metal. When it is determined in block  213  that the charging voltage has not reached the maximum charging voltage value Vmax 2 , the flow returns to block  209  to continue the charging by means of the transistor  115 . 
     When the MPU  113  determines in block  203  that the surface temperature of the battery cells  103  to  105  belongs to the standard temperature range, the flow proceeds to block  235 , where the MPU  113  issues a charge request to the EC  13  to activate the battery charger  51 , and at the same time, in block  237 , the MPU  113  stops the operation of the transistor  115  and turns on the charge protection switch C-FETb. In block  239 , the battery charger  51  charges the battery set  106  in a constant current control mode with a charging rate of 0.7 C. Since the MPU  113  is monitoring the surface temperature during charging, when it is determined in block  241  that the surface temperature belongs to the low temperature range, the flow proceeds to block  205 , while when it is determined that the surface temperature remains in the standard temperature range, the flow proceeds to block  243 , and when it is determined that the surface temperature belongs to the high temperature range, the flow proceeds to block  265 . When as a result of the progress of charging, the charging current is decreased to a level at which the battery charger  51  cannot be operated in a constant current control mode, in block  243 , the battery charger  51  is automatically operated in a constant voltage control mode so that the output voltage is identical to the setting voltage Vchg. When the charging current is decreased to a predetermined value, the operation of the battery charger  51  is stopped in block  245  and the charging is completed. 
     When the MPU  113  determines in block  203  that the surface temperature belongs to the high temperature range, the flow proceeds to block  265 , where the MPU  113  controls turning on/off of the charge protection switch C-FETb with a cycle of several seconds to several minutes by setting the duty ratio such that the average value of the charging current becomes 0.3 C. Subsequently, in block  267 , the MPU  113  issues a charge request by instructing the EC  13  to set the setting current Ichg and the setting voltage Vchg in the battery charger  51 . The setting current Ichg is set to a fixed rate of 0.7 C, and therefore, the battery charger  51  outputs a charging current of 0.7 C when it is operated in the constant current control mode. When the setting current Ichg and the setting voltage Vchg are programmed in the current setting value input Iset and the voltage setting value input Vset, the battery charger  51  starts its operation. 
     In block  269 , the battery set  106  is charged with a charging current (switching current) having an average value of 0.3 C and a peak value of 0.7 C by the switching operation of the charge protection switch C-FETb. Since there is no problem of deposition of lithium metal in the high temperature range, it does not cause any problem if the peak value exceeds the maximum charging current value Imax 2 . When it is determined in block  209  that the surface temperature has decreased to the standard temperature range, the flow proceeds to block  237 , where the charge protection switch C-FETb stops the switching operation and maintains an on state, whereby the charging mode is switched to a mode wherein charging is performed by means of the battery charger  51 . Since the on/off switching cycle of the charge protection switch C-FETb can be increased, the amount of heat generation is small and the temperature rise in the battery pack can be suppressed. 
     When it is determined in block  269  that the surface temperature remains in the high temperature range, the MPU  113  determines in block  271  whether the charging current is decreased to a level at the battery charger  51  switches to a charging voltage control mode. When it is determined in block  271  that the charging current is decreased tip to such a level, the flow proceeds to block  277 , where the charge protection switch C-FETb stops the switching operation and maintains an on state, whereby the charging mode is switched to a mode wherein charging is performed by means of the battery charger  51 . When it is determined in block  271  that the charging current is not decreased up to such a level, the flow proceeds to block  273 , where the MPU  113  determines whether the charging voltage has reached the maximum charging voltage value Vmax 3  in the high temperature range shown in  FIG. 5B . 
     When it is determined in block  273  that the charging voltage has reached the maximum charging voltage value Vmax 3 , the charge protection switch C-FETb stops the switching operation and maintains an off state in block  275 , and then the flow proceeds to block  245 , where the operation of the battery charger  51  is stopped to thereby ensure the safety against the temperature rise. When the MPU  113  determines in block  273  that the charging voltage has not reached the maximum charging voltage value Vmax 3 , the flow returns to block  269 , where the charging is continued by means of the switching operation of the charge protection switch C-FETb. 
     The program for executing the procedures described above is stored in a ROM of the MPU  113 . The charging current in the low temperature range or the high temperature range is generated by the constant current circuit or the switching circuit accommodated in the battery pack  100 . The information or instructions delivered from the battery pack  100  to the EC  13  are the same as those of the conventional note PC  10  equipped with the battery charger which is operated with a single setting current Ichg. Therefore, the above procedures can be executed by only mounting the battery pack  100  on the note PC  10 , which was already shipped, without needing to apply any modifications thereto. 
       FIG. 4  is a block diagram of a battery pack, showing another example of the constant current circuit that is operated in a low temperature range. The constant current circuit  152  shown in  FIG. 4  is different from the constant current circuit  111  shown in  FIG. 2 , in that the constant current circuit  111  is mainly configured by the transistor  115  while the constant current circuit  152  uses a p-channel MOS-FET  155 , which is an enhancement-type field-effect transistor. An FET  153  has one end thereof connected to the power supply line  131  and the other end connected to the drain of the MOS-FET  155 . A current sense resistor Rs has one end thereof connected to the source of the MOS-FET  155  and the other end connected to the positive terminal of the battery cell  103 . An FET  151  is connected between the gate of the FET  153  and the ground line  135 , and the gate of the FET  151  is connected to the CC-ON terminal of the MPU  113 . 
     The cathode of a zener diode  159  is connected to a plus (+) terminal of an operational amplifier  157 , and the anode of the zener diode  159  is connected to a positive terminal of the battery cell  103 . The zener diode  159  supplies a reference voltage Vz to the operational amplifier  157 . The minus (−) terminal of the operational amplifier  157  is connected to the drain of the MOS-FET  155 . The output terminal of the operational amplifier  157  is connected to one end of a resistor  161 , and the other end of the resistor  161  is connected to the gate of the MOS-FET  155 . 
     Similar to the constant current circuit shown in  FIG. 2 , configured by the transistor  115 , the constant current circuit  152  shown in  FIG. 4 , configured by the MOS-FET  155 , is operated only when the surface temperature of the battery cells  103  to  105  is in the low temperature range. When the MPU  113  turns on the FET  151 , the FET  153  is turned on, whereby the operation of the constant current circuit  152  is started. Assuming the resistance value of the current sense resistor Rs be Rs and the charging current flowing through the MOS-FET  155  be I, by selecting the resistance value of the current sense resistor Rs and the breakdown voltage of the zener diode  159  so as to satisfy the relationship of IRs=Vz, the gate voltage of the MOS-FET  155  is controlled such that the charging current has a constant value. It is to be noted that a junction-type FET may be used instead of the MOS-type FET. Although the MOS-FET  155  generates heat when it is operated in the continuous constant current mode rather than a switching manner, the heat is advantageously used in a manner similar to the constant current circuit  111  shown in  FIG. 2 . 
     The present invention can be applied to a battery pack in which a charging Current is required to be changed in accordance with the temperature of the battery cell. The present invention can be applied to a battery pack particularly useful in a shipped apparatus equipped with a battery charger which is operated with a single setting current. 
     While the particular BATTERY PACK AND CHARGING METHOD is herein shown and described in detail, it is to be understood that the subject matter which is encompassed by the present invention is limited only by the claims.