Abstract:
In a charging device, a terminal is configured to connect a rechargeable battery. A first power feeding unit is configured to charge the rechargeable battery connected to the terminal. A controller is configured to control the first power feeding unit. A second power feeding unit is configured to feed electrical power to the controller and a monitoring unit. The monitoring unit includes a monitoring portion and a switching element. The monitoring portion is configured to monitor at least one of the rechargeable battery, the first power feeding unit, and the controller. The switching element is configured to interrupt the second power feeding unit to feed the electrical power to the monitoring portion.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a charging device that charges a rechargeable battery used as a power source of a cordless electric power tool. Further, the present invention relates to a charging device capable of selectively charging battery packs of different rated voltages. 
       BACKGROUND ART 
       [0002]    Various types of cordless electric power tools are used depending on the intended use. For examples, the power tools varies in sizes, shapes, and power levels. Further, rechargeable batteries of various rated output voltages and capacities are used in the power tools. For example, a lithium-ion battery pack whose rated voltage is 3.6V is used for light-work, and a lithium-ion battery pack whose rated voltage is 36V is used for hard-work. 
         [0003]    A recharging device that is dedicated to a single type of the various battery pack is provided. Further, a multi-type charging device capable of charging a plurality of battery packs is provided for improving convenience of users. For example, Japanese Patent Application Publication No. 2009 178012 discloses a multi-type charging device that can charge the battery packs of different rated voltages. This multi-type charging device identifies the type and voltage of the battery in order to charge the battery pack appropriately. More specifically, the battery pack has an identification element such as a resistor, which varies according to the type of the rechargeable battery or a charge-discharge voltage. At the time of charging, the charging device identifies the identification element. 
         [0004]    When a high-voltage battery pack or large-capacity battery back is charged, or when quick charging is carried out, output power of the charging device tends to become larger, thereby requiring not only monitoring temperatures of the battery pack but also monitoring temperatures of components inside the charging device. Therefore, the charging device determines the temperature of the battery pack, and the temperature inside the charging device. If the temperatures are greater than or equal to predetermined temperatures, the charging device reduces an output charging current, thereby reducing generation of heat. 
       SUMMARY OF INVENTION 
     Solution to Problem 
       [0005]    Recently, the charging device is increasingly required to reduce power consumption for reasons such as environmental consciousness. In particular, power that is consumed by the charging device in a standby mode after the charging is completed does not contribute to supply of energy to the battery pack. Therefore, it is desirable that the value be as low as possible. However, the above conventional battery type determination method and temperature monitoring method do not pay little attention to a reduction in power consumption. 
         [0006]    In view of the foregoing, the object of the present invention is to provide a charging device that can reduce power consumption while reliably carrying out identification of a plurality of battery-voltage types and monitoring of temperatures. 
         [0007]    Further, a growing number of battery packs have adopted high-performance rechargeable batteries such as lithium-ion batteries in order to meet demand for higher output and larger capacity (longer work time). The high-performance rechargeable batteries require strict conditions for charging and discharging in order to sufficiently ensure life and performance thereof. Attention needs to be paid not only to a voltage during charging that is set based on the rated voltage and a voltage at which the charging is completed, but also to the state and voltage of the battery at the start of the charging. Unlike a charging device dedicated to a specific battery pack with preset charging conditions, the multi-type charging device needs to determine the state of a plurality of battery packs and carry out charging in accordance with the type of the battery packs. 
         [0008]    In view of the foregoing, another object of the present invention is to provide a charging device that can selectively charge a plurality of battery packs of different rated voltages, appropriately determine the state of a battery at a time when charging of a battery pack is started and a voltage thereof, and carry out charging under suitable charging conditions. 
         [0009]    The present invention features a charging device. The charging device includes a terminal, a first power feeding unit, a controller, a monitoring unit, and a second power feeding unit. The terminal is configured to connect a rechargeable battery. The first power feeding unit is configured to charge the rechargeable battery connected to the terminal. The controller is configured to control the first power feeding unit. The second power feeding unit is configured to feed electrical power to the controller and the monitoring unit. The monitoring unit includes a monitoring portion and a switching element. The monitoring portion is configured to monitor at least one of the rechargeable battery, the first power feeding unit, and the controller. The switching element is configured to interrupt the second power feeding unit to feed the electrical power to the monitoring portion. 
         [0010]    The present invention further features a charging device. The charging device includes a terminal, an identifying unit, a charging unit, a threshold selecting unit, and a determining unit. The terminal is configured to connect a batter pack. The identifying unit is configured to identify a rated voltage of the battery pack connected to the terminal from among a plurality of rated voltages. The charging unit is configured to charge the battery pack connected to the terminal. The threshold selecting unit is configured to select a threshold value of the battery pack connected to the terminal from among a plurality of threshold values based on the identified rated voltage. The determining unit is configured to determine whether a battery voltage of the battery pack is less than the selected threshold value. 
       Advantageous Effects of Invention 
       [0011]    According to the present invention, the electrical power to the monitoring portion can be shutoff at an appropriate timing, thereby reducing power consumption. 
         [0012]    Further, according to the present invention, the threshold value of the battery pack can be selected depending on the rated voltage. Accordingly, the over-discharged state of the battery pack can be properly determined. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a circuit diagram according to a first embodiment according of the present invention. 
           [0014]      FIG. 2  is a flowchart illustrating a charging operation according to the first embodiment. 
           [0015]      FIG. 3  is a circuit diagram according to a second embodiment of the present invention. 
           [0016]      FIG. 4  is a flowchart illustrating a charging operation according to the second embodiment. 
           [0017]      FIG. 5  is a circuit diagram according to a third embodiment of the present invention. 
           [0018]      FIG. 6  is a flowchart illustrating a charging operation according to the third embodiment. 
           [0019]      FIG. 7  is a flowchart illustrating a charging operation according to a modification of the third embodiment. 
           [0020]      FIG. 8  is a circuit diagram according to a fourth embodiment of the present invention. 
           [0021]      FIG. 9  is a flowchart illustrating a charging operation according to the fourth embodiment. 
           [0022]      FIG. 10  is a circuit diagram according to a fifth embodiment of the present invention. 
           [0023]      FIG. 11  is a flowchart illustrating a part of charging operation according to the fifth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0024]    Hereinafter, a charging device  1  of a first embodiment of the present invention, and a battery pack  2  that is mounted on the charging device  1  will be described with reference to the accompanying drawings.  FIG. 1  is a circuit diagram showing a situation where the battery pack  2  is mounted on the charging device  1 . The battery pack  2  is used as a power source of a cordless tool, which is not shown in the diagram. 
         [0025]    First, the battery pack  2  that is to be charged will be described. As shown in  FIG. 1 , the battery pack  2  includes a battery set in which a plurality of battery cells  2   a  are connected in series; a battery type identification resistor  7 ; a thermistor  8  that is a temperature sensing element; and a protection IC  2   b . According to the present embodiment, an example of the battery pack  2  including lithium-ion battery cells  2   a  will be described. However, the type of battery cells to be charged is not specifically limited and any type of secondary battery may be used. The battery type identification resistor  7  has a unique resistance value that varies according to the type of the battery pack  2  (such as rated voltage or the number of battery cells that are connected in series). Based on the resistance value, the type of the battery pack, such as rated voltage and the number of battery cells  2   a  that are connected in series, can be determined. The thermistor  8  is so placed as to be in contact with the battery set, or near the battery set, to detect a temperature of the battery set. The protection IC  2   b  monitors voltage of each of the battery cells  2   a , and prevents any one of the battery cells  2   a  from becoming unusual state (or error state) due to overcharge or over-discharge. As the battery cells  2   b  is charged, the voltage of the battery cells  2   b  increases. When the voltage has reached a threshold voltage indicative of full charge as a result of charging, the protection IC  2   b  outputs a signal corresponding to the full charge. Further, the protection IC  2   b  outputs a signal even when at least one of the battery cells  2   a  goes down to a threshold voltage (discharge limit voltage), because there is a risk that the battery cell  2   a  is over-discharged. In a state where the voltage has reached the threshold voltage indicative of full charge, a state where the voltage is less than or equal to the discharge limit voltage of the battery cells  2   b , or a normal state, the protection IC  2   b  outputs signals corresponding to the states. 
         [0026]    The battery pack  2  includes terminals that correspond to a charge plus terminal and charge minus terminal provided in the charging device  1 , a temperature detection terminal, and a battery type information input terminal. As the battery pack  2  is mounted on the charging device  1 , the terminals of the battery pack  2  are connected to the corresponding terminals of the charging device  1 . 
         [0027]    Then, the charging device  1  will be described. The charging device  1  includes a power source section, a microcomputer  50 , various detection sections connected to input ports of the microcomputer  50 , and controlled sections connected to output ports of the microcomputer  50 . 
         [0028]    The power source section includes a main power source that supplies charging power, and an auxiliary power source that applies drive voltage to the microcomputer  50 . The main power source is a power source that charges the battery pack  2 , and includes a first rectifying and smoothing circuit  10 , a switching circuit  20 , and a second rectifying and smoothing circuit  30 . 
         [0029]    The first rectifying and smoothing circuit  10  includes a full-wave rectifier circuit  11  and a smoothing capacitor  12 . The full-wave rectifier circuit  11  full-wave rectifies an AC voltage supplied from an AC power source  500 . The smoothing capacitor  12  smooths the voltage, and outputs a DC voltage. The AC power source  500  is an external power source such as a commercial power source. 
         [0030]    The switching circuit  20  is connected to an output side of the first rectifying and smoothing circuit  10 , and includes a high-frequency transformer  21 , a MOSFET  22 , and a PWM control IC  23 . The PWM control IC  23  changes a drive pulse width inputted to the MOSFET  22 . In accordance with the drive pulse width, the MOSFET  22  carries out switching, thereby converting a DC output from the first rectifying and smoothing circuit  10  into a voltage of pulse-train waveform. The voltage of pulse-train waveform is applied to a primary winding of the high-frequency transformer  21 , and the voltage is stepped up (or down) by the high-frequency transformer  21  and then is outputted to the second rectifying and smoothing circuit  30 . 
         [0031]    The second rectifying and smoothing circuit  30  includes a diode  31 , a smoothing capacitor  32 , and a discharge resistor  33 . The second rectifying and smoothing circuit  30  is configured to rectify and smooth an output voltage obtained from the secondary side of the high-frequency transformer  21  and generate a DC voltage, and output the DC voltage through the plus and minus terminals of the charging device  1 . 
         [0032]    The charging device  1  further includes an auxiliary power source  40  and a rectifying and smoothing circuit  6 . The auxiliary power source  40  is a constant-voltage power supply circuit connected to the first rectifying and smoothing circuit  10  and the switching circuit  20  and receives power, and applies a stabilized reference voltage Vcc to various circuits such as the microcomputer  50  or operational amplifiers  61  and  65 , which will be described later. The auxiliary power source  40  includes transformers  41   a ,  41   b , and  41   c , a switching element  42 , a control element  43 , a rectifying diode  44 , a three-terminal regulator  46 , oscillation prevention capacitors  45  and  47 , and a reset IC  48 . The reset IC  48  is an IC that outputs a reset signal to the microcomputer  50  at a time when the charging device  1  is connected to an AC power source. 
         [0033]    The rectifying and smoothing circuit  6  is connected to the auxiliary power source  40  and the switching circuit  20 , and serves as a power source of the PWM control IC  23 . The rectifying and smoothing circuit  6  includes a secondary coil  6   a  of the transformer  41   a , a rectifying diode  6   b , and a smoothing capacitor  6   c.    
         [0034]    The microcomputer  50  includes a first output port  51   a , a second output port  51   b , A/D input ports  52  ( 52   a  and  52   b ), and a reset port  53 . The microcomputer  50  processes various signals inputted to the A/D input ports  52 , and outputs various resulting signals to each of the various controlled sections through the first output port  51   a  and the second output port  51   b . In this manner, the microcomputer  50  controls the operation of the charging device  1 . 
         [0035]    The charging device  1  further includes a charging current setting circuit  70 , a current detection resistor  3 , a battery type determination circuit  9 , a battery temperature detection circuit  80 , a battery voltage detection circuit  90 , a component temperature detection section  700 , a charging current signal transmission section  5 , a charging voltage control circuit  100 , a charging current control circuit  60 , a charging control signal transmission section  4  and a display section  120 . 
         [0036]    The second output port  51   b  includes a plurality of ports, one of which is connected to a charging current setting circuit  70 . 
         [0037]    The charging current setting circuit  70  includes resistors  71  and  72  that are connected in series between the reference voltage Vcc and the ground, and a resistor  73 . The charging current setting circuit  70  sets a prescribed current value of the charging current. A connection point of the resistors  71  and  72  is connected to the resistor  73  and a non-inverting input terminal of the operational amplifier  65  in a charging control circuit  60 . The resistor  73  is connected one port of the output port  51   b.    
         [0038]    According to the present embodiment, the charging current setting circuit  70  selectively sets one of two types of current values J 1  and J 2  as a set current for charging. More specifically, when a high signal is output from one of ports of the output port  51   b  connected to the resistor  73 , a value obtained by dividing the reference voltage Vcc with the resistors  71  and  72  is used as a reference value for setting the set current as the current value J 1 . According to the present embodiment, the charging current value J 1  is set to 3 A as one example. 
         [0039]    As a low signal is output from one of ports of the second output port  51   b  connected to the resistor  73 , a value obtained by dividing the reference voltage Vcc with the resistor  71  and parallel resistance of resistors  72  and  73  is used as a reference value for setting the set current as the current value J 2 . The charging current J 2  is smaller than the charging current J 1 . According to the present embodiment, the charging current J 2  is set to 1 A as one example. 
         [0040]    As described above, the charging control circuit  60  is connected to the charging current setting circuit  70 , and controls the charging current based on settings by the charging current setting circuit  70 . The charging control circuit  60  includes the operational amplifiers  61  and  65 , resistors  62 ,  63 ,  64 ,  66 , and  67 , and a diode  68 . Incidentally, the A/D input port  52   a  includes a plurality of ports, one of which is connected to an output side of the operational amplifier  61 . 
         [0041]    The current detection resistor  3  is connected between the second rectifying and smoothing circuit  30  and a charging voltage control circuit  100 , and detects the charging current flowing through the battery pack  2 . 
         [0042]    The A/D input port  52   a  of the microcomputer  50  includes a plurality of ports, which are respectively connected to the battery type determination circuit  9 , a battery temperature detection circuit  80 , and the battery voltage detection circuit  90 . 
         [0043]    The battery temperature detection circuit  80  includes resistors  81  and  82  connected in series between the reference voltage Vcc and the ground. A connection point of the resistors  81  and  82  is connected to the thermistor  8  of the battery pack  2 , and to one of ports of the A/D input port  52   a  of the microcomputer  50 . As the temperature of the battery set  2   a  of the battery pack  2  changes, a voltage value of the thermistor  8  corresponding to the temperature change is applied to a corresponding one of the ports of the A/D input port  52   a  of the microcomputer  50 . In this manner, the charging device  1  can detect the temperature of the thermistor, that is, the temperature of the battery set  2   a.    
         [0044]    The battery voltage detection circuit  90  is connected to a plus terminal of the battery set when the battery pack  2  is mounted on the charging device  1 . The battery voltage detection circuit  90  includes resistors  91  and  92 . The voltage applied to the battery pack  2 , or, the voltage of the battery pack  2 , is divided by the resistors  91  and  92 , and a value thereof is input as battery voltage information to one of the ports of the A/D input port  52   a  of the microcomputer  50 . When no power is supplied to the battery pack  2 , information indicative of the voltage of the battery pack  2  is input as battery voltage information to one of the ports of the A/D input port  52   a  via the battery voltage detection circuit  90 . In the present invention, the battery voltage indicates a value one-to-one corresponding to a battery voltage that is actually detected from the battery pack  2 , or a voltage of the actual battery. 
         [0045]    The battery type determination circuit  9  includes resistors  9   a ,  9   b , and  9   c , and a FET  9   d . A source of the FET  9   d  is connected to a terminal that is connected to the battery type identification resistor  7 . A gate of the FET  9   d  is connected to one of the ports of the second output port  51   b . As a low signal is output from one of the ports of the second output port  51   b  that is connected to the FET  9   d , the FET  9   d  is turned ON. As a high signal is output from one of the ports of the second output port  51   b  that is connected to the FET  9   d , the FET  9   d  is turned OFF. When the battery pack  2  is connected and when the FET  9   d  is ON, the microcomputer  50  identifies the type of the connected battery pack  2  (such as rated voltage or the number of battery cells that are connected in series) on the basis of a value obtained by dividing the reference voltage Vcc with the resistor  9   a  and the battery type identification resistor  7 . 
         [0046]    The first output port  51   a  of the microcomputer  50  includes a plurality of ports, which are respectively connected to the charging control signal transmission section  4  and the display section  120 . The charging voltage control circuit  100  and the charging current setting circuit  70 , and the battery type determination circuit  9  are connected to corresponding one of the ports of the second output port  51   b  of the microcomputer  50 . The constant-voltage power supply circuit  40  is connected to the reset port  53 . 
         [0047]    The charging control signal transmission section  4  is connected to the switching circuit  20  and the microcomputer  50 . The charging control signal transmission section  4  includes a photo coupler that transmits a signal for controlling a process of turning the PWM control circuit  23  ON/OFF, and a FET  4   a  that is connected to a light-emitting element in the photo coupler  4  and controls a process of turning the light-emitting element ON/OFF. The first output port  51   a  includes a plurality of ports, one of which is connected to a gate of the FET  4   a . When a high signal is output from one of the ports of the output port  51   a  that is connected to the FET  4   a , the FET  4   a  is turned ON, and the photo coupler  4  is turned ON. As a result, the PWM control circuit  23  is activated, and the charging starts. When a low signal is output from one of the ports of the output port  51   a  that is connected to the FET  4   a , the FET  4   a  is turned OFF, and the photo coupler  4  is turned OFF. As a result, the PWM control circuit  23  is stopped, and the charging is stopped (or ended). 
         [0048]    The component temperature detection section  700  includes a resistor  703 , a thermistor  701 , and a FET  702 . The resistor  703 , the thermistor  701 , and the FET  702  are connected between the reference voltage Vcc and the ground. A connection point of the resistor  703  and thermistor  701  is connected to the A/D input port  52   b . A gate of the FET  702  is connected to one of the ports of the first output port  51   a  that is also connected to the FET  4   a . That is, one of the ports of the first output port  51   a  is shared by the FET  4   a  and the FET  702 . Therefore, the process of turning the FET  702  ON/OFF is in synchronization with the process of turning the FET  4   a  ON/OFF. Only during the charging, the thermistor  701  is driven, that is, the electrical power is supplied to the thermistor, and an internal temperature of the charging device  1  is detected. That is, as a high signal is output from one of the ports of the first output port  51   a  that is connected to the gates of the FET  702  and FET  4   a , the FET  702  is turned ON, and current flows from the reference voltage Vcc to the resistor  703  and the thermistor  701 . At this time, the microcomputer  50  determines the temperature of the thermistor  701  based on a value obtained by dividing the reference voltage Vcc with the resistor  703  and the thermistor  701 . As a low signal is output from a port a of the first output port  51   a  connected to the gates of the FET  702  and FET  4   a , the FET  702  is turned OFF, and the current from the reference voltage Vcc to the resistor  703  and the thermistor  701  is blocked. The thermistor  701  is so placed as to be in contact with a component that generates heat in the charging device  1  and is likely to rise in temperature, or near the component. In one example, according to the present embodiment, the thermistor  701  is placed near the PWM control circuit  23 . 
         [0049]    The charging current signal transmission section  5  is connected to the switching circuit  20 , the charging voltage control circuit  100 , and the charging current control circuit  60 . The charging current signal transmission section  5  includes a photo coupler that feeds a signal of the charging current back to the PWM control IC  23 . 
         [0050]    The display section  120  is a circuit for displaying a charging state, and includes a LED  121  and resistors  122  and  123 . When a high signal is output from one of the ports of the first output port  51   a  connected to the resistors  122 , the LED  121  emits red light. When a high signal is output from one of the ports of the first output port  51   a  connected to the resistors  123 , the LED  121  emits green light. When a high signal is output from both the ports, the LED  121  emits orange light. According to the present embodiment, the microcomputer  50  controls the LED  121  to emit red light before the charging starts, such as when the battery pack  2  is not connected or when the device is in a charging standby mode. The microcomputer  50  controls the LED  121  to emit orange light during the charging by simultaneously turning on two light-emitting elements of the LED  121 . After the charging is completed, the microcomputer  50  controls the LED  121  to emit green light. 
         [0051]    The charging voltage control circuit  100  is connected to the second rectifying and smoothing circuit  30 , and controls the charging voltage. The charging voltage control circuit  100  includes resistors  101 ,  103 ,  105 ,  106 ,  107 ,  108 ,  110 ,  111 ,  113 ,  114 ,  115 ,  118 ,  119 , and  130 , a potentiometer  102 , FETs  109 ,  116 ,  117 , a capacitor  104 , a shunt regulator  112 , and a rectifier diode  111 . The resistors  108 ,  115 , and  119  are respectively connected to a plurality of ports that the second output port  51   b  has. Based on a signal from the second output port  51   b  of the microcomputer  50 , the charging voltage is set by setting a reference value of the shunt regulator  112  to a voltage value divided by the series resistance of the resistor  101  and potentiometer  102  and the parallel resistance of the resistor  105  and any one of resistors  106 ,  113 , and  130 . For example, a value determined by the series resistance of the resistor  101  and potentiometer  102  and only the resistor  105  (when the FETs  109 ,  116 ,  117  all are OFF) is used to charge a two-cell lithium-ion battery. A value determined by the series resistance ( 101 ,  102 ) and the parallel resistance of the resistors  105  and  106  (or the parallel resistance at a time when the FET  109  is turned ON) is used to charge a three-cell lithium-ion battery. Similarly, a four-cell lithium-ion battery is supported when the FET  116  is ON; a five-cell lithium-ion battery is supported when the FET  117  is ON. 
         [0052]    With reference to a flowchart of  FIG. 2 , a charging process by the charging device  1  will be described. In Step S 201 , the microcomputer  50  outputs a high signal from one of the ports of the first output port  51   a  connected to the resistors  122 , thereby controlling the LED  121  to emit the red light and notifying a user of the fact that the charging is not yet started. In Step  202 , the microcomputer  50  outputs a low signal from one of the ports of the second output port  51   b  connected to the resistor  9   c , thereby turning the FET  9   d  ON and supplying the current from the reference voltage Vcc to the battery type determination circuit  9 . In Step  203 , the microcomputer  50  determines whether the battery pack  2  is mounted on the charging device  1 . The determination is made by determining whether a signal is input from the battery temperature detection circuit  80 , the battery type determination circuit  9 , and the battery voltage detection circuit  90  to corresponding ports of the A/D input port  52   a . When the inputting is detected in the circuits, the microcomputer  50  determines that the battery pack  2  has been mounted. When a negative determination is made in Step S 203  (S 203 : NO), the process goes back to step S 201 , and the device then is in a standby mode. When an affirmative determination is made in Step S 203  (S 203 : YES), the microcomputer  50  in Step  204  outputs a high signal to both the ports of the output port  51   a  connected to the resistors  123  and  122 , thereby controlling the LED  121  to emit the orange light and notifying a user of the fact that the battery pack  2  is in the process of being charged. 
         [0053]    In Step  205 , based on a value obtained by dividing the reference voltage Vcc with the resistor  9   a  of the battery type determination circuit  9  and the battery type identification resistor  7 , the microcomputer  50  identifies the type of the connected battery pack  2  (such as rated voltage or the number of battery cells that are connected in series). In Step  206 , based on the number of cells of the battery pack  2  that are identified, the microcomputer  50  sets the charging voltage of the charging voltage control circuit  100 . More specifically, the microcomputer  50  identifies that the battery cells  2   a  that are connected in series in the charging pack are lithium-ion batteries, and identifies the number of battery cells connected in series. The microcomputer  50  sets the charging voltage based on those identified information, and controls the driving of the FETs  109 ,  116 , and  117 , as described above, in accordance with the number of battery cells connected in series in order to set the prescribed charging voltage. 
         [0054]    In Step S 207 , the microcomputer  50  outputs a high signal from one of the ports of the first output port  51   a  connected to the gate of the FET  4   a , thereby activating the PWM control circuit  23 . As a result, the process of charging the battery pack  2  is started. In response to a high signal from one of the ports of the first output port  51   a  connected to the gate of the FET  4   a , the FET  702  is simultaneously turned ON. As a result, the thermistor  701  is activated, and a voltage corresponding to the temperature of the thermistor  701  is input to the A/D input port  52   b . The microcomputer  50  starts monitoring the internal temperature of the charging device  1  through the thermistor  701 . 
         [0055]    In Step  208 , the microcomputer  50  determines whether the battery pack  2  is fully charged. For example, one way to make the determination is to invert and amplify the potential detected by the current detection resistor  3  by using the operational amplifier  61 , and input the potential to a corresponding port of the A/D port  52   a , thereby monitoring the charging current. According to the present embodiment, as one example of charging control, a constant current &amp;#8211; constant voltage control method is performed. That is, the charging is started in a constant current mode. As the battery is charged, the voltage of the battery rises. When the voltage has reached a predetermined voltage, a constant-voltage charging mode is started. During a constant-voltage charging period, as the charging is carried out, the current decreases. Therefore, when the current value is less than or equal to a predetermined value, it is determined that the battery is fully charged. According to the present embodiment, while the details will be given later, depending on the internal temperature of the charging device  1 , two types of current values J 1  (e.g. 6 A) and J 2  (e.g. 3 A) are set as the charging current. Therefore, the predetermined value varies according to the type of the current value that is set as the charging current. For example, in the case of the current value J 1 , a terminal current value is 3 A in one example whereas in the case of the current value J 2 , the terminal current value is 1 A. The terminal current values may be equal for two types of current values J 1  and J 2  (e.g. 1 A). The above constant current—constant voltage control method is one example of the charging control method. Any other charging methods that are used for charging secondary batteries may be employed, such as those featuring only constant-voltage control or constant-current control. 
         [0056]    In Step  208 , the microcomputer  50  determines whether or not the battery is fully charged. When a negative determination is made in Step  208  (S 208 : NO), the microcomputer  50  in Step  209  determines whether the current value J 2  is set by the charging current setting circuit  70  as the set current. When an affirmative determination is made in Step  209  (S 209 : YES), the charging continues with the current value J 2 , and the process returns to step  208 . When a negative determination is made in Step  209  (S 209 : NO), the microcomputer  50  determines, by using the thermistor  701 , whether the temperature of the charging device  1  is greater than or equal to a predetermined value. When a negative determination is made in Step  210  (S 210 : NO), the charging continues with the current value J 1 , and in Step  208  the microcomputer  50  determines again whether the battery is fully charged. When an affirmative determination is made in Step  210  (S 210 : YES), in Step  211  the microcomputer  50  changes the set current of the charging current setting circuit  70  to the current value J 2 , which is smaller than the current value J 1 , in order to reduce a rise in the internal temperature of the charging device  1 . 
         [0057]    When an affirmative determination is made in Step  208  (S 208 : YES), or when it is determined that the battery is fully charged, the microcomputer  50  in Step  212  controls the LED  121  to emit the green light, notifying a user of the fact that the charging is completed. 
         [0058]    In Step  213 , in response to the full-charge detection in Step  208 , the microcomputer  50  outputs a low signal from the first output port  51   a  and turns the FETs  4   a  and  202  OFF. As a result, the charging is ended. The current from the reference voltage Vcc to the resistor  703  and the thermistor  701  is blocked, thereby reducing power consumption by the thermistor  701 . 
         [0059]    In Step  214 , a low signal is output from one of the ports of the second output port  51   b  connected to the FET  9   d , and turns the FET  9   d  OFF. As a result, the current from the reference voltage Vcc to the resistor  9   a  and the battery type identification resistor  7  is blocked, thereby reducing power consumption. The FET  9   d  may be turned OFF not only in Step  214  of the present embodiment, but at any time after the type of the battery is identified in Step S 205 . 
         [0060]    In Step  215 , the microcomputer  50  determines whether the battery pack  2  is removed. The microcomputer  50  waits until the battery pack  2  is removed. After the battery pack  2  is removed (S 215 : YES), the charging conditions are reset, and the process returns to step  201 . Incidentally, even if the battery pack  2  is removed from the charging device  1  at a timing not shown in the flowchart, as in the case of the above step  215 , the charging device resets a series of charging conditions, goes back to step S 201 , and waits. 
         [0061]    The above charging device  1  can turn the FET  9   d  OFF at any given time after the process  205  of determining the type of the battery, thereby blocking the current flowing through the battery type determination circuit  9 . In this manner, after the type of the battery pack  2  is determined, the route from the reference voltage Vcc to the ground via the battery type determination circuit  9  is cut off to further reduce power consumption by the charging device  1 . 
         [0062]    Moreover, the FET  702  that controls the thermistor  701  is turned ON and OFF in synchronization with the FET  4   a  that controls the charging. Therefore, when the charging is not carried out, the current flowing through the resistor  703  and the thermistor  701  can be blocked. Therefore, power consumption when the device is in a standby mode can be reduced in a more effective manner. 
         [0063]    To reduce power consumption when the device is in a standby mode, in addition to the battery type determination circuit  9  and the component temperature detection section  700 , which are illustrated in the above embodiment, a FET for cutting off a circuit may be provided for other circuits included in the charging device  1 . For example, a FET may be provided for the battery temperature detection circuit  80  or the battery voltage detection circuit  90 . When the charging is not carried out, the battery temperature detection circuit  80  connected to the thermistor  8  of the battery pack  2 , or the battery voltage detection circuit  90  connected to a charging terminal of the battery pack  2  may be cut off. More specifically, as in the case of the battery type determination circuit  9  or the component temperature detection section  700 , a FET is provided at a position where a current path can be cut off (for example, a position between the reference voltage Vcc and the resistor  81 , the position between the resistors  81  and  82 , or one of end positions of the resistor  91 ). In response to a control signal from the output port  51   a  or  51   b  of the microcomputer  50 , the FET is preferably so controlled as to be turned ON only when necessary. This configuration can reduce power consumption in a more effective manner. 
         [0064]    When a plurality of positions where temperatures are detected are required in the charging device  1 , a plurality of component temperature detection sections  700  may be provided. In this case, between the gate of the FET  4   a  and a corresponding port of the first output port  51   a , a plurality of component temperature detection sections  700  may be connected in parallel. Alternatively, the plurality of FETs may be provided and connected to the plurality of component temperature detection sections  700 . The microcomputer  50  may output different control signals to drive the component temperature detection sections  700  separately by controlling each of the plurality of FETs. 
       Second Embodiment 
       [0065]    Next, a second embodiment of the charging device  1  will be described. The following description of the second embodiment will focus on points of difference from the first embodiment, wherein like parts and components are designated with the same reference numerals to avoid duplicating description. 
         [0066]    In the second embodiment, the protection IC  2   b  outputs a high signal for a normal working voltage when the battery pack  2  is neither over-discharged nor fully charged. In an unusual or error state such as when the over-discharge or full-charge is informed, the protection IC 2   b  outputs a low signal such as ground voltage. 
         [0067]    As shown in  FIG. 3 , in the second embodiment, the charging device  1  does not includes the component temperature detection section  700 . The microcomputer  50  does not includes the A/D input port  52   b . Because the A/D input port  52   b  is not included, the A/D input port  52   a  is denoted simply the A/D input port  52  in the following description. 
         [0068]    In the second embodiment, the battery type determination circuit  9  includes the reference resistor  9   a  positioned between the power supply voltage Vcc and the A/D input port  52 . In the second embodiment, the battery type determination circuit  9  does not include resistors  9   b , and  9   c , and a FET  9   d . When the battery pack  2  is mounted, the battery type identification resistor  7  and the reference resistor  9   a  of the battery type determination circuit  9  are connected in series. A divided voltage obtained by dividing the power supply voltage Vcc with the resistor  9   a  and the battery type identification resistor  7  is input to the microcomputer  50  (the A/D input port  52   a ). Based on the divided voltage value, the microcomputer  50  determines the type of the connected battery pack  2  (such as rated voltage or the number of battery cells that are connected in series). 
         [0069]    The charging control signal transmission section  4  is connected to the switching circuit  20  and the microcomputer  50 . The charging control signal transmission section  4  includes a photo coupler  4  that transmits a signal for controlling a process of turning the PWM control circuit  23  ON/OFF, and a FET  4   a  that is connected to a light-emitting element making up the photo coupler  4  and controls a process of turning the light-emitting element ON/OFF. The gate of the FET  4   a  is connected to the first output port  51   a  via a diode  4   b . When a high signal is output from the output port  51   a , the FET  4   a  is turned ON, and the photo coupler  4  is turned ON. As a result, the PWM control circuit  23  is activated, and the charging starts. When a low signal is output from the output port  51   a , the FET  4   a  is turned OFF, and the photo coupler  4  OFF. As a result, the PWM control circuit  23  is stopped, and the battery charge is stopped. Furthermore, when a FET  210  of a threshold voltage setting circuit  25 , which will be detailed later, is turned ON, a high signal that is output from the output port  51   a  is not input to the FET  4   a , but is supplied to the ground via the diode  4   c  and the FET  210 . As a result, the FET  4   a  is not driven, and the photo coupler  4  is turned OFF. Accordingly, the PWM control circuit  23  is stopped, and the battery charge is stopped. 
         [0070]    The charging voltage control circuit  100  is connected to the second rectifying and smoothing circuit  30 , and controls the charging voltage. The charging voltage control circuit  100  includes resistors  101 ,  103 ,  105 ,  106 ,  107 ,  108 , and  110 , a potentiometer  102 , a FET  109 , a capacitor  104 , a shunt regulator  112 , and a rectifier diode  111 . In the second embodiment, the charging voltage control circuit  100  does not include resistors  113 ,  114 ,  115 ,  118 ,  119 , and  130 , FETs  109 ,  116 ,  117 . Based on a signal from the second output port  51   b  of the microcomputer  50 , the charging voltage is set by setting a reference value of the shunt regulator  112  to a voltage value divided by the series resistance of the resistor  101  and the potentiometer  102  and the parallel resistance of the resistors  105  and  106 . 
         [0071]    In the second embodiment, the charging device  1  further includes a threshold voltage setting circuit  25 . 
         [0072]    The threshold voltage setting circuit  25  includes an operational amplifier  220 , resistors  200 ,  203 ,  206 ,  207 ,  209 ,  211 , and  212 , FETs  208 ,  210 , and  213 , zener diodes  201  and  204 , and diodes  202  and  205 . The threshold voltage setting circuit  25  determines whether or not the battery pack  2  is being over-discharged. The two zener diodes  201  and  204  has breakdown voltages (zener voltages) different from each other. According to this configuration, two discharge limit voltages are set in the threshold voltage setting circuit  25 . Here, the discharge limit voltages are used for determining an over-discharge state that depends on the rated voltage of the battery pack  2  mounted on the charging device  1 . 
         [0073]    The threshold voltage setting circuit  25  is provided between the reference potential (hereinafter, the reference potential indicates ground potential) and a node A. The battery voltage of the battery pack  2  is an input voltage of the threshold voltage setting circuit  25 . That is, the battery voltage of the battery pack  2  applies between the node A and the reference potential. 
         [0074]    As a route for a first threshold voltage, the following components are sequentially connected in series in the following order from a high potential side (the node A) to the reference potential: the resistor  200 , the zener diode  201 , the diode  202 , and the resistor  207 . A cathode of the zener diode  201  is connected to the resistor  200 , and an anode of the zener diode  201  is connected to an anode of the diode  202  at a node B. A zener voltage V 1  of the zener diode  201  corresponds to a discharge limit threshold voltage of the battery pack  2  when five battery cells of the battery pack  2  are connected in series, for example. According to the present embodiment, the zener voltage V 1  is 9V for example. 
         [0075]    In the threshold voltage setting circuit  25 , as a route for a second threshold voltage, the following components are sequentially connected in series in the following order from a high potential side (node A) to the resistor  207 : the resistor  203 , the zener diode  204 , and the diode  205 . The route for the second threshold voltage is parallel to the rout for the first threshold voltage. A cathode of the zener diode  204  is connected to the resistor  203 , and an anode of the zener diode  204  is connected to an anode of the diode  205 . A zener voltage V 4  of the zener diode  204  has a value that is higher than the zener voltage of the zener diode  201 . The zener voltage V 4  corresponds to a discharge limit threshold voltage of the battery pack  2  when ten battery cells of the battery pack  2  are connected in series, or 18V, for example. 
         [0076]    Furthermore, the resistor  206  and the FET  208  are sequentially connected in series and in this order from the power supply voltage Vcc to the reference potential. A drain of the FET  208  is connected to the resistor  206 , and a source of the FET  208  to the reference potential, and a gate of the FET  208  to a connection point of the diode  205  and resistor  207 . 
         [0077]    A drain of the FET  210  is connected to the first output port  51   a  of the microcomputer  50  via the diodes  4   b  and  4   c  as an output of the threshold voltage setting circuit  25 . Here, the diode  4   c  prevents a backward current flowing from the threshold voltage setting circuit  25  to the microcomputer  50 . A source of the FET  210  is connected to the reference potential, the gate of the FET  210  to a connection point of the drain of the FET  208  and the resistor  206 . 
         [0078]    In the threshold voltage setting circuit  25 , the operational amplifier  220  is a logical operation circuit. A divided voltage obtained by dividing the power supply voltage Vcc with the battery type identification resistor  7  of the battery pack  2  and the reference resistor  9   a  is input to a non-inverting terminal of the operational amplifier  220 . A divided voltage obtained by dividing the power supply voltage Vcc with the resistors  211  and  212  is input to the inverting terminal of the operational amplifier  220 , as a reference. Therefore, the operational amplifier  220  determines whether the electric potential between the register  9   a  and the battery type identification resistor  7  of the mounted battery pack  2  (the potential of the non-inverting terminal) is larger or smaller than the reference of the operational amplifier  220 . If a high-rated-voltage battery pack  2  in which ten cells are connected in series is mounted, the battery type identification resistor  7  has a relatively large resistance value of 1,000 (kilo ohm) as one example. Therefore, a voltage higher than the reference of the operational amplifier  220  is input to the non-inverting terminal, and the operational amplifier  220  outputs a high signal. If a low-rated-voltage battery pack  2 ′ in which five cells are connected in series is mounted, the battery type identification resistor  7  has a smaller resistance value than that of the above battery pack  2 , e.g.  500  (kilo ohm). Therefore, a voltage lower than the reference voltage is input to the non-inverting terminal, and the operational amplifier  220  outputs a low signal. 
         [0079]    An output terminal of the operational amplifier  220  is connected to the gate of the FET  213 . A drain of the FET  213  is connected to the connection point of the anode of the zener diode  201  and the anode of the diode  202 , and a source of the FET  213  is connected to the reference potential. Accordingly, when a signal output from the operational amplifier  220  is a high signal, the FET  213  is turned ON. When the signal is a low signal, the FET  213  is turned OFF. That is, according to the present embodiment, when the battery pack  2  is a low-rated-voltage battery pack  2 ′ in which five battery cells  2   a  are connected in series, the FET  213  is turned OFF. When the battery pack  2  is a high-rated-voltage battery pack  2  in which ten battery cells  2   a  are connected in series, the FET  213  is turned ON. As a result, the route that defines the second threshold value of a low rated voltage (the route including the zener diode  201 ) does not contribute to control of the FET  208  because the route is connected to the ground via the node B and the FET  213 . 
         [0080]    The operation of the charging device  1  will be described with reference to  FIGS. 3 and 4 . 
         [0081]    First, the case where a low-rated-voltage battery pack is connected to the charging device  1  will be described. After the battery pack  2  is mounted on the charging device  1  (Step S 1 : YES), the battery type identification resistor  7  of the battery pack  2  is connected in series to the reference resistor  9   a  of the charging device  1 . In the threshold voltage setting circuit  25 , a divided voltage obtained by dividing the power supply voltage Vcc with the battery type identification resistor  7  and the reference resistor  9   a  is input to the non-inverting input terminal of the operational amplifier  220 . At this time, if the number “a” of battery cells  2   a  connected in series is five, the value of the divided voltage is smaller than the reference, and the operational amplifier  220  therefore outputs a low signal (Step S 2 : Low). In response to the low signal output from the operational amplifier  220 , in Step S 3  the FET  213  is turned OFF. At this time, a voltage corresponding to the battery voltage is applied to the zener diodes  201  and  204 . Therefore, the FET  208  is turned ON/OFF depending on the magnitude relation between the zener voltage V 1  of the zener diode  201  and the battery voltage. The reason is that the zener voltage V 1  of the zener diode  201  is smaller than the zener voltage V 4  of the zener diode  204 . That is, in Step S 4  the threshold voltage setting circuit  25  sets the threshold voltage to the zener voltage of the zener diode  201 . Therefore, the battery voltage corresponding to the zener diode  201  with a low breakdown voltage (zener voltage) is used as a threshold value in controlling the FET  208 . 
         [0082]    According to the above configuration, the breakdown voltage V 1  of the zener diode is used as a threshold voltage to determine a discharge limit of the battery pack  2 , and the threshold voltage setting circuit  25  determines whether or not an over-discharge state exists. When the battery voltage is less than the breakdown voltage V 1  of the zener diode  201 , i.e. when the battery pack  2  is less than or equal to the discharge limit voltage (Step S 5 : YES), in Step S 6  the FET  208  is turned OFF, and in Step S 7  the FET  210  is turned ON. As a result, in the charging control signal transmission section  4 , a high signal output from the output port  51   a  is supplied to the ground via the diode  4   c  and the FET  210 , thereby blocking an input to the FET  4   a . Therefore, even if the charging of the battery pack  2  already has started, in Step S 8  the battery charge is immediately stopped. Here, the breakdown voltage of the zener diode  204  is higher than the breakdown voltage of the zener diode  201 . Therefore, at voltage V 1 , the route going through the zener diode  204  does not become conductive, making no contribution to the control of the FET  208 . 
         [0083]    When the battery voltage is greater than or equal to the zener voltage V 1  (Step S 5 : NO), the battery pack  2  is not in an over-discharge state, and thus in Step S 9  the FET  208  is turned ON, and in Step S 10  the FET  210  is turned OFF. As a result, the threshold voltage setting circuit  25  is disconnected from the charging control signal transmission section  4 , and the output port  51   a  outputs a high signal. If the charging process of the battery pack  2  is started, in Step S 11  the charging continues. If the battery voltage is higher than the breakdown voltage V 4  of the zener diode  204 , the route of the zener diode  201 , as well as the route of the zener diode  204 , becomes conductive. Through any of the routes, the FET  208  should be driven. 
         [0084]    On the other hand, if a high-rated-voltage battery pack is connected, i.e. if the number “a” of battery cells  2   a  connected in series is ten in Step S 2 , a value of the divided voltage obtained by dividing the power supply voltage Vcc with the battery type identification resistor  7  and the reference resistor  9   a  is larger than the reference, and the operational amplifier  220  therefore outputs a high signal (Step S 2 : High). In response to the high signal from the operational amplifier  220 , the FET  213  is turned ON (step S 12 ). As the FET  213  is turned ON, as described above, the route of the zener diode  201  is connected to the ground via the FET  213 . As a result, the zener diode  204  becomes dominant for the process of turning the FET  208  ON/OFF. Therefore, in Step S 13  the threshold value of the battery voltage is dependent on the zener voltage V 4  of the zener diode  204 . After Step S 13 , as in the case of the above low-rated-voltage battery pack, a comparison is made between the battery voltage and the zener voltage of the zener diode  204  to determine whether the charging should be stopped or continue (Steps S 5  to  11 ). 
         [0085]    As described above, the threshold voltage setting circuit  25  sets the threshold value of the battery voltage based on the number of battery cells  2   a . The number of battery cells  2   a  indicates the rated voltage of the battery pack  2 . Thus, the threshold voltage setting circuit  25  sets the threshold value of the battery voltage based on the rated voltage of the battery pack  2 . 
         [0086]    Accordingly, in the case where the low-rated-voltage battery pack  2  is mounted to the charging device  1 , the zener voltage of the zener diode  201  can be used as a threshold voltage. In the case where the high-rated-voltage battery pack  2  is mounted to the charging device  1 , the zener voltage of the zener diode  204 , which is higher than that of the zener diode  201 , can be used as a threshold voltage. That is, in accordance with the rated voltage of the battery pack  2  (or the number of cells connected in series), a discharge-limit threshold voltage for determining whether or not an over-discharge state exists can be selectively set. 
       Third Embodiment 
       [0087]    A third embodiment of the present invention will be described with reference to  FIGS. 5 and 6 . The following description of the third embodiment will focus on points of difference from the second embodiment. In the second embodiment, the threshold voltage setting circuit  25  has a plurality of zener diodes of different breakdown voltages which is used to set a threshold voltage. However, the present invention is not limited thereto. According to the third embodiment, instead of the threshold voltage setting circuit  25 , the microcomputer  50  of a charging device  1  determines whether or not an over-discharge state of a battery pack  2  exists. Therefore, in the charging device  1  shown in  FIG. 5 , the function of the threshold voltage setting circuit  25  is incorporated into the microcomputer  50 , and the charging device  1  does not includes the threshold voltage setting circuit  25 . The configuration of the other portions is the same as that of the charging device  1  shown in  FIG. 3 . 
         [0088]    The operation of the charging device  1  shown in  FIG. 5  will be described with reference to  FIG. 6 . In the operation of the charging device  1  shown in  FIG. 6 , when the microcomputer  50  determines that the battery voltage of the battery pack  2  is less than or equal to a discharge limit voltage, the charging of the battery pack  2  is not carried out. 
         [0089]    First, when the battery pack  2  is mounted on the charging device  1  (Step S 21 : YES), the microcomputer  50  reads, from a battery type identification resistor  7  of the battery pack  2 , the number of lithium-ion batteries connected in series in the battery pack  2  and a rated voltage. Based on the rated voltage of the battery pack  2  that is read, in Step S 22  the microcomputer  50  sets a threshold voltage of discharge limit used to determine whether the battery pack  2  is in an over-discharge state. For example, in the case where a battery pack with a rated voltage of 14V in which five lithium-ion batteries are connected in series is mounted to the charging device  1 , the threshold voltage is set to 9V. In the case where a battery pack with a rated voltage of 36V in which ten lithium-ion batteries are connected in series is mounted to the charging device  1 , the threshold voltage is set to 18V. 
         [0090]    In Step S 23  the microcomputer  50  compares the battery voltage of the battery pack  2  detected by the battery voltage detection circuit  90  to the threshold voltage, and determines whether or not the battery pack  2  is in an over-discharge state. When the battery pack with a rated voltage of 14V is mounted, the microcomputer  50  compares the battery voltage with the threshold voltage of 9V. When the battery pack with a rated voltage of 36V is mounted, the microcomputer  50  compares the battery voltage is compared with threshold voltage of 18V. That is, the microcomputer  50  compares the threshold voltage that is set in accordance with the rated voltage of the battery pack with the actual battery voltage. If the battery voltage is less than or equal to the threshold voltage (Step S 23 : YES), the microcomputer  50  determines that the battery pack  2  is in an over-discharge state, and in Step S 26  the battery charge is not carried out (ended). If the battery voltage is greater than the threshold voltage (Step S 23 : NO), in Step S 24  the microcomputer  50  starts charging the battery pack  2 . 
         [0091]    When the charging of the battery pack  2  continues, and the microcomputer  50  determines, based on the battery voltage detected by the battery voltage detection circuit  90 , that the battery pack  2  is fully charged (Step S 25 : YES), then in Step S 26  the charging of the battery pack  2  is ended. If the battery pack  2  is not yet fully charged (Step S 25 : NO), the microcomputer  50  continues the charging until the battery pack  2  is fully charged. After the battery pack  2  is removed from the charging device  1  (Step S 27 : YES), the microcomputer  50  waits for the next battery pack  2  to be mounted. Although not shown in the flowchart, when the battery pack  2  is removed from the charging device  1  prior to step S 27 , the charging device  1  resets the conditions, and enters a standby mode to wait for the next battery pack  2  to be mounted. 
         [0092]    In that manner, the threshold voltage which is a discharge limit voltage for determining whether the battery pack  2  is in an over-discharge state can be changed according to the rated voltage of the battery pack  2 . Therefore, one charging device  1  can properly set the threshold value of the discharge limit voltage corresponding to the rated voltage of the battery pack  2  mounted to the charging device  1 , thereby increasing the life of the battery pack  2 . 
         [0093]    Next, a modified example of the charging operation of the charging device  1  shown in  FIG. 5  will be explained with reference to  FIG. 7 . In the modified example, the charging device  1  pre-charges a battery pack  2  if the charging device  1  estimates that the battery pack  2  is in the over-discharge state, that is, the battery voltage of the battery pack  2  is less than or equal to the discharge limit voltage. Subsequently, the charging device  1  determines whether or not the charging should continue based on a progression or result of the pre-charging, that is, based on how the pre-charging is performed. 
         [0094]    First, when the battery pack  2  is mounted on the charging device  1  (Step S 31 : YES), the microcomputer  50  reads, from a battery type identification resistor  7  of the battery pack  2 , the number of lithium-ion batteries connected in series in the battery pack  2  and a rated voltage. Based on the rated voltage of the battery pack  2  that is read, in Step S 32  the microcomputer  50  sets a threshold voltage of discharge limit used to determine whether the battery pack  2  is in an over-discharge state. For example, in the case where a battery pack with a rated voltage of 14V in which five lithium-ion batteries are connected in series is mounted to the charging device  1 , the threshold voltage is set to 9V. In the case where a battery pack with a rated voltage of 36V in which ten lithium-ion batteries are connected in series is mounted to the charging device  1 , the threshold voltage is set to 18V. 
         [0095]    Then, in Step S 33  the microcomputer  50  compares the battery voltage of the battery pack  2  detected by the battery voltage detection circuit  90  with the threshold voltage corresponding to the rated voltage. That is, the microcomputer  50  compares the threshold voltage with the actual battery voltage, and determines whether or not the battery pack  2  is less than or equal to the discharge limit voltage. When the battery voltage is less than or equal to the threshold voltage (Step S 33 : YES), the battery pack  2  is probably in an over-discharge state. Thus, instead of normal charging conditions, in Step S 34  the microcomputer  50  starts pre-charging of the battery pack  2 . Here, the pre-charging is a charging method performed when degradation of battery performance is anticipated. The degradation of battery performance is occurred when the battery voltage of the battery pack  2  is less than or equal to the discharge limit voltage, for example. Compared with the normal battery charge performed when the battery pack  2  is not in an over-discharge state, the pre-charging is performed under “mild” charging conditions that low current flows to the battery pack  2  or low voltage is applied to the battery pack  2 , for example. In the present embodiment, the microcomputer  50  sets the charging current to J 1  when performing normal charging and sets the charging current to J 2  that is lower than J 1  when performing pre-charging. 
         [0096]    After the pre-charging of the battery pack  2  is started, in Step S 35  the microcomputer  50  continuously or intermittently detects the battery voltage of the battery pack  2  while performing pre-charging. If the detected battery voltage is greater than the threshold voltage (Step S 36 : YES), in Step S 37  the microcomputer  50  judges that the battery pack  2  is normal, and continues the battery charge after switching to the charging current J 1  that is larger than the charging current J 2 , and proceeds to Step S 42 . 
         [0097]    If the detected battery voltage is not greater than the threshold voltage (Step S 36 : NO), the microcomputer  50  proceeds to Step S 38 , and in Step S 38  determines whether or not a predetermined time has elapsed since the pre-charging is started. If the predetermined time already has elapsed (Step S 38 : YES), it is suspected that the battery cells have run into some trouble, and in Step S 43  the microcomputer  50  stops the battery charge. If the predetermined time has not yet elapsed since the pre-charging is started (Step S 38 : NO), the process returns to Step S 36 . Thus, Step S 36  is repeated, and the microcomputer continues monitoring of the battery voltage of the battery pack  2 . 
         [0098]    On the other hand, if the battery voltage detected is greater than the threshold voltage (Step S 33 : NO), it is determined that the battery pack  2  is not in an over-discharge state, and then in Step S 40  the microcomputer  50  determines whether or not a signal is supplied from the protection IC  2   b  of the battery pack  2 . If no signal is supplied from the protection IC  2   b  (Step S 40 : NO), in Step S 41  the microcomputer  50  starts the battery charge with the normal charging current J 1 . In S 42  the microcomputer  50  continues charging the battery pack  2 , and determines whether the battery pack  2  is fully charged. When the battery pack  2  is fully charged (Step S 42 : YES), then in Step S 43  the microcomputer  50  stops the battery charge. After that, when the battery pack  2  is removed from the charging device  1  (Step S 44 : YES), the microcomputer  50  waits for the next battery pack  2  to be mounted. Here, as in the case of the third embodiment, when the battery pack  2  is removed before the charging is ended, the charging device  1  resets the conditions, and enters a standby mode to wait for the next battery pack  2  to be mounted. 
         [0099]    If the signal supplied from the protection IC  2   b  (Step S 40 : YES), the battery already has been fully charged, or the protection IC  2   b  stops the battery charge for some reason. Accordingly, the microcomputer  50  does not perform the charging of the battery pack  2 , and in Step S 43  ends the battery charge. 
         [0100]    The microcomputer  50  appropriately changes the threshold voltage for determining whether the battery pack  2  is in an over-discharge state depending on the rated voltage of the battery pack  2 , and therefore can properly determine the over-discharge state of the battery pack  2 . When it is determined that the battery pack  2  is in an over-discharge state, the pre-charging is performed over a predetermined period of time. Based on how the voltage of the battery pack  2  has risen, the microcomputer  50  determines whether or not the charging should continue by checking whether or not the battery pack  2  is normal. 
       Fourth Embodiment 
       [0101]    A fourth embodiment of the present invention will be described with reference to  FIGS. 8 and 9 . According to the fourth embodiment, a threshold voltage is set in the threshold voltage setting circuit  25  for determining an over-discharge state such that the threshold voltage can be changed between a battery pack with a large number of cells connected in series and a battery pack with a small number of cells connected in series. Moreover, the charging device  1  tries to pre-charge and charge the battery pack  2  in which the battery voltage of one of the battery cell is less than or equal to a discharge limit voltage and in which the protection IC  2   b  outputs a low signal indicating some alerts. The charging device  1  of the fourth embodiment is basically the same with the charging device  1  of the second embodiment shown in  FIG. 3 , however the charting device of the fourth embodiment further includes an error signal processing circuit  250 . The following only describes portions that are different from those of the charging device  1  shown in  FIG. 3 . 
         [0102]    In the fourth embodiment, the microcomputer  50  outputs a high signal to the charging control signal transmission section  4  when the microcomputer  50  receives the low signal from the node C via the A/D input port  52 . On the other hand, the microcomputer  50  stops to output a high signal to the charging control signal transmission section  4  via the output port  51   a  when the microcomputer  50  receives the high signal from the node C via the A/D input port  52 . 
         [0103]    As shown in  FIG. 8 , the alert (or “some error”) signal processing circuit  250  includes resistors  214 ,  215 , and  217 , and FETs  216  and  218 . The error signal processing circuit  250  is inserted between the threshold voltage setting circuit  25  and the first output port  51   a  of a microcomputer  50 . Based on a signal from a protection IC  2   b  and the threshold voltage setting circuit  25 , the error signal processing circuit  250  inputs a signal for stopping the battery charge into an A/D input port  52  of the microcomputer  50 , and blocks a signal output from the microcomputer  50  to a charging signal transmission section  4 . 
         [0104]    In the error signal processing circuit  250 , from a power supply voltage Vcc to a reference potential, the resistor  214  and the FET  216  are sequentially connected in series and in this order. A drain of the FET  216  is connected to the resistor  214 , and to the A/D input port  52  of the microcomputer  50 . A source of the FET  216  is connected to the reference potential, and a gate of the FET  216  is connected to the protection IC  2   b  of the battery pack  2 . The resistor  215  is connected between the gate and source of the FET  216 . A drain of the FET  218  is connected to an output line of the first output port  51   a  of the microcomputer  50  via a diode  4   c , a source of the FET  218  is connected to the reference potential, and a gate of the FET  218  is connected to a node C, which is a connection point of the resistor  214  and the drain of the FET  216 . The resistor  217  is connected between the source and gate of the FET  218 . 
         [0105]    The operation of the charging device  1  shown in  FIG. 8  will be described with reference to  FIG. 9 . 
         [0106]    First, the case where a low-rated-voltage battery pack in which the small number of battery cells  2   a  is connected in series, is connected to the charging device  1  will be described. After the battery pack  2  is mounted on the charging device  1  (Step S 51 : YES), the battery type identification resistor  7  of the battery pack  2  is connected in series to the reference resistor  9   a  of the charging device  1 . In the threshold voltage setting circuit  25 , a divided voltage obtained by dividing the power supply voltage Vcc with the battery type identification resistor  7  and the reference resistor  9   a  is input to the non-inverting input terminal of the operational amplifier  220 . At this time, if the number “a” of battery cells  2   a  connected in series is five, the value of the divided voltage is smaller than the reference, and the operational amplifier  220  therefore outputs a low signal (Step S 52 : Low). In response to the low signal output from the operational amplifier  220 , in Step S 53  the FET  213  is turned OFF. At this time, a voltage corresponding to the battery voltage is applied to the zener diodes  201  and  204 . Therefore, the FET  208  is turned ON/OFF depending on the magnitude relation between the zener voltage V 1  of the zener diode  201  and the battery voltage. That is, in Step S 54  the threshold voltage setting circuit  25  sets the threshold voltage to the zener voltage of the zener diode  201 . In other words, the zener voltage V 1  is set as the threshold voltage that is used to determine whether the battery pack  2  is in the over-discharge state. 
         [0107]    When the battery voltage is less than the breakdown voltage V 1  (Step S 55 : YES), in Step S 56  the FET  208  is turned OFF, and in Step S 57  the FET  210  is turned ON. At this time, a signal warning of over-discharge (low signal) is also output from the protection IC  2   b  (Step S 58 : Low), and in Step S 59  the FET  216  is turned OFF. Because the FET  216  is turned OFF, the node C is not connected to the reference potential via the FET  216 . However, as described above, because the FET  210  of the threshold voltage setting circuit  25  is turned ON, that is, the node C is connected to the reference potential via FET  210 , no signal is applied to the gate of the FET  218 . Thus, the FET  218  remains the OFF state. Accordingly, in Step S 60  a low signal is inputted to the A/D input port  52  of the microcomputer  50 . Though the low signal is inputted to the A/D input port  52  from the node C, in Step S 61  the microcomputer  50  outputs the high signal toward the charging control signal transmission section  4  via the output port  51   a  based on the battery voltage of the battery pack  2  detected by the battery voltage detection circuit  90  in order to pre-charges the battery pack  2 . The precharging of the battery pack  2  is performed similarly to the third embodiment. The high signal outputted from the output port  51   a  is not lowered to the reference potential by the FET  218 , and is transmitted to the charging control signal transmission section  4 . Accordingly, the microcomputer  50  can pre-charge the battery pack  2  based on the battery voltage of the battery pack  2  detected by the battery voltage detection circuit  90 . 
         [0108]    When the low signal is not output from the protection IC  2   b  of the battery pack  2 , that is, the high signal is output form the protection IC  2   b  (Step S 58 : High), in Step S 62  the FET  216  is turned ON. Because the FET  216  is turned ON, the node C is connected to the reference potential through the FET  216 , and in Step S 63  the FET  218  is turned OFF. The low signal is inputted to the A/D input port  52  from the node C. In Step S 61  the microcomputer  50  outputs the high signal toward the charging control signal transmission section  4  via the output port  51   a  based on the battery voltage of the battery pack  2  detected by the battery voltage detection circuit  90  in order to precharges the battery pack  2 . The high signal output from the output port  51   a  is not lowered to the reference potential by the FET  218 , and transmitted to the charging control signal transmission section  4 . Thus, the microcomputer  50  can pre-charge the battery pack  2  based on the battery voltage of the battery pack  2  detected by the battery voltage detection circuit  90 . 
         [0109]    When the battery voltage is greater than or equal to the zener voltage V 1  (Step S 55 : NO), the battery pack  2  is not in an over-discharge state, and thus in Step S 64  the FET  208  is turned ON, and in Step S 65  the FET  210  is turned OFF. That is, the threshold voltage setting circuit  25  is electrically disconnected from other components of the charging device  1 . At this time, when a signal (low signal) is output from the protection IC  2   b  (Step S 66 : Low), in Step S 67  the FET  216  is turned OFF. As the FET  216  is turned OFF, in Step S 68  a high signal is input to the A/D port  52  of the microcomputer  50  to stop the charging, and the FET  218  is turned ON at the same time. Accordingly, based on the signal for stopping the charging (high signal inputted from the A/D port  52 ), in S 69  the microcomputer  50  stops an output from the output port  51   a . Even if the output port  51   a  of the microcomputer  50  keeps outputting the high signal, the high signal is lowered to the reference potential by the FET  218 . Accordingly, the charging of the battery pack  2  is forcibly stopped. 
         [0110]    When the low signal is not output from the protection IC  2   b  (Step S 66 : high), that is, the high signal is output from the protection IC  2   b  (Step S 66 : High), the normal charging is available. Then, in Step S 70  the FET  216  is turned ON, and in Step S 71  a low signal is inputted to the A/D port  52  of the microcomputer  50 , and the FET  218  is turned OFF at the same time. Accordingly, the charging device  1  continues the charging of the battery pack  2 . 
         [0111]    In Step  52 , if the number a of battery cells  2   a  connected in series that constitute the battery pack  2  is 10, that is, the battery pack  2  is the high-rated-voltage, the operational amplifier  220  outputs a high signal because the value of the divided voltage is greater than the reference voltage (Step S 52 : High). In response to the high signal output from the operational amplifier  220 , in Step S 73  the FET  213  is turned ON. As the FET  213  is turned ON, the zener diode  204  becomes dominant for the process of turning the FET  208  ON/OFF. Therefore, in Step S 72  the process is dependent on the zener voltage V 4  of the zener diode  204 . That is, the zener voltage V 4  of the zener diode  204  is used as a threshold voltage for determining whether or not the battery pack  2  is in an over-discharge state. In the subsequent processes following Step S 74 , determinations with respect to pre-charging, stop of charging, or continuation of charging are made similarly to the above low-rated-voltage battery pack  2  (Steps S 55  to S 72 ). 
         [0112]    Accordingly, when the number of battery cells  2   a  of the battery pack  2  that are connected in series is small, the zener voltage of the zener diode  201  can be used as a threshold voltage for determining whether or not the battery pack  2  is in an over-discharge state. When the number of battery cells  2   a  of the battery pack  2  that are connected in series is large, the zener voltage of the zener diode  204 , which is higher than that of the zener diode  201 , can be used as a threshold voltage for determining whether or not the battery pack  2  is in an over-discharge state. That is, depending on the number of cells of the battery pack  2  that are connected in series, a threshold voltage for determining whether or not the battery pack  2  is in an over-discharge state can be selectively set. 
         [0113]    In a normal charging device, if the battery pack  2  is less than or equal to the discharge limit voltage, and a signal warning of over-discharge is generated from the protection IC  2   b  of the battery cells  2   a , the charging is stopped. However, according to the present embodiment, even in such cases, the microcomputer  50  pre-charges the battery pack  2 , and can continue the charging of the battery pack  2 . 
         [0114]    Without using the microcomputer  50 , the threshold voltage setting circuit  25  and the error signal processing circuit  250  determines whether the battery pack  2  is in an over-discharge state. Therefore, even if a failure occurs in the microcomputer  50 , the threshold voltage for determining whether the battery pack  2  is in the over-discharge state is set based on the number of battery cells of the battery pack  2  that are connected in series. 
       Fifth Embodiment 
       [0115]    A charging device  1  of a fifth embodiment of the present invention will be described with reference to  FIGS. 10 and 11 . The configuration of the charging device  1  shown in  FIG. 10  is basically the same as that of the charging device  1  shown in  FIG. 8 . In the threshold voltage setting circuit  25  shown in  FIG. 8 , one operational amplifier  220  is used, and resistance values of the battery type identification resistor  7  of the battery pack  2  are classified into two, large and small. That is, in the fourth embodiment, two threshold voltages for determining whether the battery pack  2  is in an over-discharge state can be selected depending on the number of zener diodes. However, in the present embodiment, the charging device  1  includes a threshold voltage setting circuit  25 A shown in  FIG. 10  instead of the threshold voltage setting circuit  25 . The threshold voltage setting circuit  25 A includes two operational amplifiers  220  and  224 , and classifies resistance values of a battery type identification resistor  7  of a battery pack  2  into three types. In order to set three threshold voltages for determining an over-discharge state, the threshold voltage setting circuit  25 A further includes three zener diodes  201 ,  204 , and  225 . A threshold voltage can be selected from the three threshold voltages. 
         [0116]    The threshold voltage setting circuit  25 A further includes resistors  200 ,  203 ,  206 ,  207 ,  209 ,  211 ,  212 ,  221 , and  222 , FETs  208 ,  210 ,  213 , and  223 , and diodes  202 ,  205 , and  226 . The zener voltages of the zener diodes  204  is largest among the zener diodes  201 ,  204 , and  225 . The zener voltage of the zener diode  225  is the smallest. A reference voltage inputted to an inverting input terminal of the operational amplifier  220  is larger than a reference voltage inputted to an inverting input terminal of the operational amplifier  224 . 
         [0117]    The operation of the charging device  1  shown in  FIG. 10  will be described with reference to  FIG. 11 . 
         [0118]    First, the case where a low-rated-voltage battery pack in which the number of battery cells  2   a  connected in series is five for example, is connected to the charging device  1  will be described. After the battery pack  2  is mounted on the charging device  1  (Step S 81 : YES), the battery type identification resistor  7  of the battery pack  2  is connected in series to the reference resistor  9   a  of the charging device  1 . In the threshold voltage setting circuit  25 A, a divided voltage obtained by dividing the power supply voltage Vcc with the battery type identification resistor  7  and the reference resistor  9   a  is input to the non-inverting input terminal of the operational amplifiers  220  and  224 . At this time, if the number “a” of battery cells  2   a  connected in series is five, the value of the divided voltage is smaller than references of the operational amplifiers  220  and  224 , and the operational amplifiers  220  and  224  therefore output low signals (Step S 82 : Low). In response to the low signals output from the operational amplifier  220  and  224 , in Step S 83  the FET  213  and  223  are turned OFF. At this time, a voltage corresponding to the battery voltage is applied to the zener diodes  201 ,  204 , and  225 . Therefore, the FET  208  is turned ON/OFF depending on the magnitude relation between the battery voltage and the zener voltage of the zener diode  225 , which has the smallest zener-diode breakdown voltage. That is, in Step S 84  the threshold voltage setting circuit  25 A sets the threshold voltage by the zener voltage of the zener diode  225 . In other words, the zener voltage of the zener diode  225  is used as the threshold voltage for determining whether or not the battery pack  2  is in an over-discharge state. 
         [0119]    In a case where a medium-degree rated-voltage battery pack  2  in which the number a of battery cells  2   a  are connected in series is seven is connected to the charging device  1 , a high signal is output from the operational amplifier  224  due to the divided voltage based on the identification resistor  7  (Step S 82 : High), while a low signal is output from the other operational amplifier  220  (Step S 85 : Low). In this case, in Step  86  the FET  223  is turned ON, but the FET  213  remains OFF. At this time, the FET  208  is turned ON/OFF depending on the magnitude relation of the battery voltage and the zener voltage of the zener diode  201 , which has a medium-level breakdown voltage. That is, in Step S 87  the threshold voltage setting circuit  25 A sets the threshold voltage by the zener voltage of the zener diode  201 . The zener voltage of the zener diode  201  is used as the threshold voltage for determining whether or not the mounted battery pack  2  is in an over-discharge state. 
         [0120]    In a case where the high-rated-voltage battery pack  2  in which the number a of battery cells  2   a  are connected in series is 10 is connected to the charging device  1 , the operational amplifier  224  outputs a high signal based on the divided voltage based on the identification resistor  7  (Step S 82 : High), and the operational amplifier  220  also outputs a high signal (Step S 85 : High). Therefore, in Step S 88  both the FETs  223  and  213  are turned ON. At this time, the FET  208  is turned ON/OFF depending on the magnitude relation between the battery voltage and the zener voltage of the zener diode  204 , which has the highest breakdown voltage. That is, in Step S 89  the threshold voltage setting circuit  25 A sets the threshold voltage by the zener voltage of the zener diode  204 . In the words, the zener voltage of the zener diode  204  is used as a threshold voltage for determining whether or not the mounted battery pack  2  is in an over-discharge state. 
         [0121]    Accordingly, in accordance with the number of battery cells  2   a  of the battery pack  2  that are connected in series, and using the three zener diodes  201 ,  204 , and  225 , an appropriate threshold voltage is selected from the three threshold voltages. Then, the process proceeds to step S 55  shown in  FIG. 9 , and, in accordance with an over-discharge state of the battery pack  2  and a signal from the protection IC  2   b , pre-charging, stop of charging, or normal charging is carried out. 
         [0122]    Therefore, in accordance with the number of battery cells of the battery pack that are connected in series, a threshold voltage of discharge limit voltage can be selected. 
         [0123]    The above described embodiments only illustrate one form of the present invention. The battery pack  2  may include any number of battery cells  2   a  connected in series. 
         [0124]    In the above embodiments, the number of zener diodes in the threshold voltage setting circuit is two or three. However, the present invention is not limited thereto. A plurality of zener diodes may be provided. The threshold voltage setting circuit sets the threshold voltage for determining the over-discharge from the plurality of threshold voltages that depends on the plurality of zener diodes. Further, in the above described embodiments in which the pre-charge is performed, the charging device  1  always performs the pre-charge irrespective of the value of the set (selected) threshold voltage. However, the charging device  1  may performs the pre-charge only when the set (selected) threshold voltage satisfies a prescribed condition. For example, the charging device  1  performs the pre-charge only when the set (selected) threshold voltage is a specific value, or one of specific values. Or, the charging device  1  performs the precharge only when the set (selected) threshold voltage is not a specific value. 
       REFERENCE SIGN LIST 
       [0000]    
       
         
           
               1  charging device 
               2  battery pack 
               7  identification resistor 
               50  microcomputer 
               90  battery voltage detection circuit 
               9  battery type determination circuit 
               700  component temperature detection section