Patent Publication Number: US-2012037385-A1

Title: Electric power tool powered by a plurality of single-cell battery packs

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese patent Application No. 2010-180527 filed on Aug. 11, 2010, the contents of which are hereby incorporated by reference into the present application. 
     1. Technical Field 
     The present invention relates to an electric power tool powered by a plurality of rechargeable cells. 
     2. Description of the Related Art 
     U.S. Pat. No. 7,414,337 discloses an electric power tool. This electric power tool is provided with a tool main body and a battery pack that can be detachably attached to the tool main body. The battery pack has a housing that can be detachably attached to the tool main body and a plurality of rechargeable cells housed inside the housing and supplies electric power as a power source of the power tool to the tool main body. 
     SUMMARY OF THE INVENTION 
     In an electric power tool powered by a battery pack, a battery pack having a nominal voltage corresponding to a rated voltage of the electric power tool is used. For example, in the electric power tool having the rated voltage of 14.4 V, a battery pack having the nominal voltage of 14.4 V is used; and in the electric power tool having the rated voltage of 18 V, a battery pack having the nominal voltage of 18 V is used. In other words, in the electric power tool having the rated voltage of 14.4 V, the battery pack having the nominal voltage of 18 V cannot be used, and in the electric power tool having the rated voltage of 18 V, the battery pack having the nominal voltage of 14.4 V cannot be used. Therefore, when the user of the electric power tool having the rated voltage of 14.4 V newly purchases an electric power tool having the rated voltage of 18 V, the user should purchase also the battery pack having the nominal voltage of 18 V. In addition to this, the battery pack having the nominal voltage of 14.4 V cannot be used for the electric power tool having the rated voltage of 18 V, even if there is no problem with the internal rechargeable cells. 
     In order to resolve the abovementioned problem, with the present technique, when an electric power tool is powered by a plurality of rechargeable cells, the plurality of rechargeable cells is not housed in a single battery pack, as in the conventional configuration. Instead, each rechargeable cell is configured to be individually housed in a battery pack that can be detachably attached to the tool main body. With such a configuration, each battery pack housing a single rechargeable cell can be commonly used in electric power tools having different rated voltages. For example, let us assume that a user of an electric power tool having the rated voltage of 14.4 V replaces this tool with a new electric power tool having the rated voltage of 18 V. In this case, the user can simply purchase a limited number of battery packs for the lacking 3.6 V, combine the purchased battery packs with the battery packs for 14.4 V that have been used heretofore, and use the combination of battery packs in the electric power tool having the rated voltage of 18 V. 
     The abovementioned single rechargeable cell may be a nickel hydride cell, a lithium ion cell, or a rechargeable cell of another type. When the battery pack houses a single lithium ion cell, the nominal voltage of the battery pack is 3.6 V. Therefore, an electric power tool having the rated voltage of 14.4 V can be driven by four battery packs, and an electric power tool having the rated voltage of 18 V can be driven by five battery packs. Thus, when the user using the electric power tool having the rated voltage of 14.4 V replaces it with the electric power tool having the rated voltage of 18 V, the user may simply purchase just one battery pack and continue using the already available four battery packs. Further, instead of the battery pack incorporating a single lithium ion cell, it is possible to buy three battery packs each incorporating a single nickel hydride cell (nominal voltage 1.2 V). 
     The below-described electric power tool can be realized on the basis of the above-described technique. This electric power tool includes a tool main body; a plurality of battery packs that is detachably attached to the tool main body; and a controller that controls discharges of the battery packs attached to the tool main body. Bach battery pack has a housing and a single rechargeable cell housed within the housing. 
     With the electric power tools of the above-described configuration becoming widespread, the users will be able to use a plurality of battery packs (rechargeable cells) for a plurality of electric power tools with different rated voltages. As a result, it will be possible to use the rechargeable cells completely to the limit of the service life thereof and the number of wastefully discarded rechargeable cells can be expected to decrease. 
     In order to use the battery packs more effectively, it is preferred that the above-described electric power tool is capable of using various battery packs having different characteristics in the rechargeable cells. In this case, it is preferred that the discharges of the battery packs be controlled according to characteristics of the rechargeable cells incorporated in the battery packs. For this reason, in one embodiment of the present technique, each battery pack can have a memory device that stores characteristic data indicative of the characteristics of the rechargeable cell. In this case, the controller can access the memory device of each of the battery packs attached to the tool main body and can be configured to control the discharges of the battery packs on the basis of the characteristic data stored in the memory devices. 
     The abovementioned memory device may store at least one characteristic value from among, for example, an upper limit in a voltage of the rechargeable cell, a lower limit in the voltage of the rechargeable cell, a maximum limit in a discharge current of the rechargeable cell, a maximum limit in the charge current of the rechargeable cell, an upper limit in a temperature of the rechargeable cell, a lower limit in the temperature of the rechargeable cell, and a capacity of the rechargeable cell as the characteristic of the rechargeable cell. 
     In one embodiment of the present technique, the memory device of each battery pack preferably stores at least the lower limit in the voltage of the rechargeable cell. In this case, it is preferred that the controller is capable of measuring the output voltage of each battery pack attached to the tool main body and inhibiting or restricting the discharges of the plurality of battery packs when the measured output voltage of at least one battery pack becomes lower than the lower limit in the voltage stored in the memory device of the battery pack. With such a configuration, overdischarge of the battery pack (rechargeable cell) can be prevented, and deterioration or damage of the battery pack (rechargeable cell) can be suppressed. 
     In one embodiment of the present technique, the controller preferably stores a maximum limit in an input voltage of the tool main body and inhibits or restricts the discharges of the plurality of battery packs when a total value of the measured output voltages of the battery packs becomes higher than the maximum limit in the input voltage of the tool main body. With such a configuration, excess voltage is prevented from being applied to the tool main body, and the motor and other electric components of the tool main body can be prevented from being damaged. 
     In another embodiment of the present technique, the memory device of each battery pack preferably stores at least an upper limit in a discharge voltage of the rechargeable cells. In this case, it is preferred that the controller is capable of measuring the discharge current produced by a plurality of battery packs attached to the tool main body and inhibiting or restricting the discharges of the plurality of battery packs when the measured discharge current becomes higher than the upper limit in discharge current stored in a memory device of at least one battery pack. With such a configuration, the overcurrent of the battery pack (rechargeable cell) can be prevented, and the deterioration or damage of the battery pack (rechargeable cell) can be suppressed. 
     In the above-described embodiment, it is preferred that the controller stores a maximum limit in input current of the tool main body and inhibit or restrict the discharges of the plurality of battery packs when the measured discharge current becomes higher than the maximum limit in input current of the tool main body. With such a configuration, the excess current is prevented from being supplied to the tool main body, and the motor and other electric components of the tool main body can be prevented from being damaged. 
     In another embodiment of the present technique, it is preferred that each battery pack further have a temperature measuring element that measures the temperature of rechargeable cells and the memory device of each battery pack store at least the upper limit in the temperature of the rechargeable cell. In this case, it is preferred that the controller is capable of being connected to the temperature measuring element of each battery pack attached to the tool main body and inhibiting or restricting the discharges of the plurality of battery packs when the temperature measured by the temperature measuring element of at least one battery pack becomes higher than the upper limit in the temperature stored in the memory device of the battery pack. With such a configuration, the overheating of the battery pack (rechargeable cell) can be prevented and the deterioration or damage of the battery pack (rechargeable cell) can be suppressed. 
     In the above-described embodiments, it is preferred that after the discharges of the plurality of batteries packs have been inhibited or restricted, the inhibition or restriction be continued till the main switch of the tool main body is turned off. With such a configuration, the inhibition or restriction of the discharges of the battery packs is prevented from being canceled by the user at an unintended timing, and the tool main body is prevented from being unexpectedly actuated. 
     In the electric power tool according to the present technique, a plurality of battery packs may be configured to be detachably attachable to the electric power tool by a pack holder. In this case, the pack holder may be detachably attachable to the tool main body and may be detachably attachable to the plurality of battery packs. Where the pack holder is used, it is possible to use the tool main body of the conventional electric power tool, that is, the tool main body having a single battery pack having a plurality of rechargeable cells, with the plurality of battery packs each having a single rechargeable cell. 
     In the electric power tool according to the present technique, at least a part of the controller can be incorporated in the tool main body. Alternatively, at least another part of the controller can be incorporated in the above-described pack holder. In one embodiment according to the present technique, one part of the controller is incorporated in the tool main body and the other part of the controller is incorporated in the pack holder. The configuration is such that the one part of the controller incorporated in the tool main body is communicatively connected to the other part of the controller incorporated in the pack holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an electric power tool according to Embodiment 1 in which three battery packs are attached to a tool main body. 
         FIG. 2  shows the electric power tool according to Embodiment 1 in which the three battery packs are attached to the tool main body. 
         FIG. 3  is a view of the tool main body taken from the III-III direction in  FIG. 2  and showing an internal structure of a battery attachment portion. 
         FIG. 4  is a cross-sectional view taken along the IV-IV line in  FIG. 2  and showing the internal structure of the battery attachment portion. 
         FIG. 5  is a perspective view illustrating an external appearance of the battery pack. 
         FIG. 6  is a cross-sectional view taken along the VI-VI plane in  FIG. 5  and showing an internal structure of the battery pack. 
         FIG. 7  is a circuit diagram illustrating a circuit configuration of the electric power tool according to Embodiment 1. 
         FIG. 8  shows an electric power tool powered by five battery packs which is a variation example of Embodiment 1. 
         FIG. 9  shows an electric power tool according to Embodiment 2. In this configuration, a battery holder is attached to a tool main body, and three battery packs are attached to a pack holder. 
         FIG. 10  shows the electric power tool according to Embodiment 2. In this configuration, the battery holder is attached to the tool main body, and the three battery packs are detached from the pack holder. 
         FIG. 11  is a view of the pack holder taken from the XI-XI direction in  FIG. 10  and showing an internal structure of a battery attachment portion. 
         FIG. 12  is a cross-sectional view taken along the XII-XII line in  FIG. 10  and showing the internal structure of the battery attachment portion. 
         FIG. 13  is a circuit diagram illustrating a circuit configuration of the electric power tool according to Embodiment 2. 
         FIG. 14  is a circuit diagram illustrating a variation example of the circuit configuration of the electric power tool according to Embodiment 2. 
         FIG. 15  is a circuit diagram illustrating another variation example of the circuit configuration of the electric power tool according to Embodiment 2. 
         FIG. 16  shows an electric power tool powered by five battery packs which is a variation example of Embodiment 2. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Representative, non-limiting examples of the present invention will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved electric power tools, as well as methods for using and manufacturing the same. 
     Moreover, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 
     EMBODIMENT 1 
     An electric power tool  10  according to Embodiment 1 will be described below with reference to the drawings.  FIGS. 1 and 2  show an external appearance of the electric power tool  10 . As shown in  FIGS. 1 and 2 , the electric power tool  10  has a tool main body  12  and a plurality of battery packs  100 . The tool main body  12  is provided with a tool holder  14  for detachable attachment of the tool, a main switch  16  operated by the user, and a grip  18  held by the user. A motor  50  (see  FIG. 7 ), which drives the tool holder  14 , and a circuit board  22  are housed in the tool main body  12 . 
     Three battery attachment sections  30  are provided in the tool main body. The three battery attachment sections  30  are positioned in the end portion of the grip  18 . Each battery attachment section  30  is configured such that one battery pack  100  can be detachably attached thereto. A release member  24  for releasing the battery pack  100  from the battery attachment section  30  is provided at the tool main body  12  for each battery pack  100 . As shown in  FIGS. 3 and 4 , each battery attachment section  30  is provided with a positive electrode input terminal  32 , a negative electrode input terminal  34 , a first communication terminal  36 , a second communication terminal  38 , and a third communication terminal  40 . These terminals extend from the circuit board  22  to the battery attachment section  30 . 
     As shown in  FIGS. 5 and 6 , the battery pack  100  is provided with a housing  102  and a single rechargeable cell  110  housed within the housing  102 . The housing  102  has a columnar shape and can be detachably attached to the battery attachment section  30  of the tool main body  12 . The rechargeable cell  110  is a lithium ion cell and a nominal voltage thereof is 3.6 V. Therefore, a nominal voltage of the battery pack  100  is also 3.6 V. A rated voltage of the tool main body  12  is 10.8 V. 
     The battery pack  100  has a positive electrode output terminal  122  and a negative electrode output terminal  124 . The positive electrode output terminal  122  is disposed at one end of the housing  102  and electrically connected to a positive terminal  110   a  of the rechargeable cell  110 . The negative electrode output terminal  124  is disposed at the other end of the housing  102  and electrically connected to a negative electrode  110   b  of the rechargeable cell  110 . The positive electrode output terminal  122  and the negative electrode output terminal  124  are housed within the housing  102  and exposed to the outside through an opening formed within the housing  102 . 
     The battery pack  100  has a circuit board  112 . The circuit board  112  is housed within the housing  102 . A memory device (EEPROM)  114 , a thermistor  116 , a first communication terminal  126 , a second communication terminal  128 , and a third communication terminal  130  are provided at the circuit board  112 . The thermistor  116  is an element for measuring a temperature of the rechargeable cell  110  and is disposed close to the rechargeable cell  110 . A resistance value of the thermistor  116  changes according to the temperature of the rechargeable cell  110 . The first communication terminal  126 , second communication terminal  128 , and third communication terminal  130  are exposed to the outside through an opening provided within the housing  102 . 
     The memory device  114  stores characteristic data indicative of characteristics of the rechargeable cell  110 . The characteristic data include characteristic values such as an upper limit in a voltage of the rechargeable cell  110 , a lower limit in the voltage of the rechargeable cell  110 , a maximum limit in a discharge current of the rechargeable cell  110 , a minimum limit in a charge current of the rechargeable cell  110 , an upper limit in the temperature of the rechargeable cell  110 , a lower limit in the temperature of the rechargeable cell  110 , and a capacity of the rechargeable cell  110 . 
       FIG. 7  is a circuit diagram showing an electric configuration of the electric power tool  10 . As shown in  FIG. 7 , in the battery pack  100 , the first communication terminal  126  is connected to the memory device  114  of the circuit board  112 , the second communication terminal  128  is connected to a ground terminal (not shown in the figures) of the circuit board  112 , and the third communication terminal  130  is connected to the thermistor  116 . 
     The tool main body  12  is provided with the motor  50 , a power supply circuit  52 , a main switch detection circuit  54 , a controller  60 , a first multiplexer  62 , a second multiplexer  64 , a buffer circuit  66 , and an amplification circuit  68 . The power supply circuit  52  electrically connects the positive electrode input terminal  32 , negative electrode input terminal  34 , and motor  50 . 
     Where each of the three battery packs  100  are attached to the tool main body  12 , the positive electrode output terminal  122  of the battery pack  100  is electrically connected to the positive electrode input terminal  32  of the tool main body  12 , and the negative electrode output terminal  124  of the battery pack  100  is electrically connected to the negative electrode input terminal  34  of the tool main body  12 . Inside the tool main body  12 , the power supply circuit  52  connects the three battery packs  100  in series to the motor  50 . Thus, the three rechargeable cells  110  are connected in series to the motor  50 . 
     The main switch  16  is provided at the power supply circuit  52 . As a result, where the user turns on the main switch  16 , the power supply circuit  52  is electrically closed (connected), and where the user turns off the main switch  16 , the power supply circuit  52  is electrically open (disconnected). Further, the main switch  16  is a variable-speed switch and outputs a speed command signal correspondingly to the operation amount of the turn-on operation performed by the user. The speed command signal of the main switch  16  is inputted to the controller  60 . 
     The power supply circuit  52  is provided with a FET (field-effect transistor)  58 . A gate of the FET  58  is connected to the controller  60 . The controller  60  electrically opens and closes the power supply circuit  52  by turning on/off the FET  58 . By controlling the FET  58 , the controller  60  can inhibit and restrict the discharges of the three battery packs  100  (such control will be described below in greater detail). 
     The power supply circuit  52  is provided with a shunt resistor  70 . The shunt resistor  70  is a resistor element for measuring the current flowing in the power supply circuit  52 . The current flowing in the power supply circuit  52  is a discharge current produced by the rechargeable cell  110  and supplied to the motor  50 . The voltage appearing on the shunt resistor  70  is inputted to the controller  60  via the amplification circuit  68 . The controller  60  can measure the current flowing in the power supply circuit  52  on the basis of the voltage appearing on the shunt resistor  70 . 
     The first communication terminals  126  of the three battery packs  100  are connected to respective three first communication terminals  36  of the tool main body  12 . The three first communication terminals  36  of the tool main body  12  are connected to the controller  60  via the first multiplexer  62 . With such a configuration, the controller  60  can access the memory devices  114  of the battery packs  100  and can acquire the characteristic data stored in the memory devices  114 . The controller  60  can also write data into the memory devices  114  of the battery packs  100 . 
     The second communication terminals  128  of the three battery packs  100  are connected to respective three second communication terminals  38  of the tool main body  12 . The three second communication terminals  38  of the tool main body  12  are grounded inside the tool main body  12 . With such a configuration, the circuit boards  112  of all of the battery packs  100  are grounded to the same potential as that of the controller  60  of the tool main body  12 . 
     The third communication terminals  130  of the three battery packs  100  are connected to respective three third communication terminals  40  of the tool main body  12 . The three third communication terminals  40  of the tool main body  12  are connected to the controller  60  via the first multiplexer  62 . With such a configuration, the controller  60  can be connected to each of the thermistors  116  of the three battery packs  100  and can measure the temperature of the rechargeable cell  110  of each battery pack  100 . 
     The buffer circuit  66  is connected to the positive electrode input terminal  32  and negative electrode input terminal  34  of each battery attachment portion  30 . The buffer circuit  66  is selectively connected to the electrode input terminal  32  and negative electrode input terminal  34  of one battery attachment portion  30  and outputs a signal corresponding to a voltage between the electrode input terminal  32  and the negative electrode input terminal  34 . The output signal of the buffer circuit  66  is inputted to the controller  60 . The controller  60  can measure the output voltage of each battery pack  100  (that is, rechargeable cell  110 ) by controlling the second multiplexer  64  and receiving the output signal of the buffer circuit  66 . 
     The main switch detection circuit  54  detects the ON/OFF state of the main switch  16 . With the circuit configuration shown in  FIG. 7 , the main switch detection circuit  54  outputs a high-level voltage signal (Vcc) to the controller  60  as long as the main switch  16  is turned off, and outputs a low-level voltage signal (GND) to the controller  60  as long as the main switch  16  is turned on. The controller  60  can detect the ON/OFF state of the main switch  16  on the basis of the output signal of the main switch detection circuit  54 . 
     In the electric power tool  10  according to the present embodiment, the controller  60  controls the discharges of three battery packs  100  attached to the tool main body  12 . The controller  60  can access the memory device  114  of each battery pack  100  attached to the tool main body  12  and can control the discharges of the three battery packs  100  on the basis of the characteristic data stored in the memory device  114 . A prime example of discharge control executed by the controller  60  will be explained below. 
     The controller  60  can measure the output voltage of each battery pack  100  by using the buffer circuit  66  and inhibits the discharges of the battery packs  100  by turning off the FET  58  when the measured value of the output voltage of at least one battery pack  100  becomes lower than the lower limit in the voltage stored in the memory device  114  of this battery pack  100 . As a result, the overdischarge of the battery pack  100  (rechargeable cell  110 ) is prevented, and the deterioration or damage of the battery pack  100  (rechargeable cell  110 ) is suppressed. The controller  60  may partially restrict the discharges of the battery packs  100  by intermittently turning off the FET  58 , without completely inhibiting the discharges of the battery packs  100 . 
     In addition, the controller  60  stores the maximum limit in input voltage of the tool main body  12  and can partially restrict the discharges of the battery packs  100  by intermittently turning off the FET  58  when a total value of the measured output voltages of the battery packs  100  becomes higher than the maximum limit in the input voltage of the tool main body  12 . Thus, by performing PWM control of the FET  58 , the controller  60  can reduce the voltages supplied from the three battery packs  100  to the tool main body  12  to a value equal to or lower than the maximum limit input voltage of the tool main body  12 . As a result, the excessive voltage can be prevented from being supplied to the tool main body  12 , and the motor  50 , main switch  16 , and the like can be prevented from being damaged. The controller  60  may also completely turn off the FET  58  and inhibit the discharges of the battery packs  100 . 
     The controller  60  measures the discharge current created by the three battery packs  100  by using the shunt resistor  70  and can inhibit (interrupt) the discharges of the battery packs  100  by turning off the FET  58  when the measured discharge current becomes larger than the upper limit in the discharge current stored in the memory device  114  of at least one battery pack  100 . With such a configuration, the overcurrent of the battery packs  100  (rechargeable cells  110 ) is prevented, and the deterioration or damage of the battery packs  100  (rechargeable cells  110 ) is suppressed. The controller  60  may partially restrict the discharges of the battery packs  100  by intermittently turning off the FET  58 , without completely inhibiting the discharges of the battery packs  100 . 
     In addition, the controller  60  stores data indicative of a maximum limit in input current of the tool main body  12  and can inhibit (interrupt) the discharges of the battery packs  100  by turning off the FET  58  when the measured discharge current becomes larger than the maximum limit in input current of the tool main body  12 . As a result, an excessively high current can be prevented from being supplied to the power supply circuit  52  of the tool main body  12 , and the motor  50 , main switch  16 , and the like can be prevented from damage. Further, the controller  60  may partially restrict the discharges of the battery packs  100  by intermittently turning off the FET  58 , without completely inhibiting the discharges of the battery packs  100 . 
     The controller  60  is connected to the thermistor  116  of each of the battery packs  100  and can inhibit the discharges of the battery packs by turning off the FET  58  when the value measured by the thermistor  116  of at least one battery pack  100  becomes higher than the upper limit in temperature stored in the memory device  114  of the battery pack. As a result, overheating of the battery packs  100  (rechargeable cells  110 ) can be prevented and deterioration or damage of the battery packs  100  (rechargeable cells  110 ) is suppressed. The controller  60  may partially restrict the discharges of the battery packs  100  by intermittently turning off the FET  58 , without completely inhibiting the discharges of the battery packs  100 . 
     As described hereinabove, the controller  60  can inhibit or restrict the discharges of the battery packs  100  according to the voltage, current, and temperature of the battery packs  100  (rechargeable cells  110 ). Here, the controller  60  is configured such that once the discharges of the battery packs  100  have been inhibited or restricted, the inhibition or restriction of the discharges is continued till the main switch  16  is detected by the main switch detection circuit  54  to be turned off. As a result, the inhibition or restriction of the discharges of the battery packs  100  is prevented from being canceled by the user at an unintended timing, and the tool main body  12  is prevented from being unexpectedly actuated. 
     In the above-described electric power tool  10 , the rated voltage of the tool main body  12  is 10.8 V and the nominal voltage of the battery pack  100  is 3.6 V. Therefore, three battery packs  100  are used. It goes without saying that the rated voltage of the tool main body  12  is not limited to 10.8 V and the number of battery packs  100  used is also not limited to three. For example, the rated voltage of the tool main body  12  may be 18 V and the number of battery packs  100  used may be five or more, as in an electric power tool  11  shown in  FIG. 8 . 
     The battery packs  100  can be commonly used by the user in the electric power tool  10  having the rated voltage of 10.8 V and the electric power tool  11  having the rated voltage of 18 V. Therefore, the user can effectively use the available battery packs  100 . For example, the user using the electric power tool  10  having the rated voltage of 10.8 V replaces it with the electric power tool  11  having the rated voltage of 18 V. In this case, the user can simply purchase just two more battery packs  100  that are lacking, combine them with the three battery packs  100  that have been heretofore used, and use the battery packs altogether in the electric power tool  11  having the rated voltage of 18 V. Further, the battery packs  100  can be replaced in the order from that in which the internal rechargeable cells  110  have completely deteriorated. 
     EMBODIMENT 2 
     An electric power tool  200  according to Embodiment 2 will be explained below with reference to the drawings.  FIGS. 9 and 10  illustrate the external appearance of the electric power tool  200  according to Embodiment 2. As shown in  FIGS. 9 and 10 , the electric power tool  10  has a tool main body  212 , a plurality of battery packs  100 , and a pack holder  214 . By contrast with the electric power tool  10  according to Embodiment 1, the electric power tool  200  according to Embodiment 2 is configured such that the three battery packs  100  are detachably attached to the tool main body  212  by the pack holder  214 . The electric power tool  200  according to Embodiment 2 is described below in greater detail, but components common with the electric power tool  10  according to Embodiment 1 are assigned with same reference numerals and the explanation thereof is omitted. 
     One battery attachment portion  216  is provided in the tool main body  212 . A tool connector  218  is provided on the upper surface of the pack holder  214 . The tool connector  218  of the pack holder  214  can be detachably attached to the battery attachment portion  216  of the tool main body  212 . Three battery attachment portions  30  are provided on the lower surface of the pack holder  214 . As shown in  FIGS. 11 and 12 , the battery attachment portion  30  of the pack holder  214  has a configuration identical to that of the battery attachment portion  30  of the electric power tool  12  explained in Embodiment 1. One battery pack  100  can be detachably attached to each battery attachment portion  30 . The conventional battery pack in which a plurality of rechargeable cells is housed in a single housing can be attached, instead of the pack holder  214 , to the battery attachment portion  216  of the tool main body  212 . 
       FIG. 13  is a circuit diagram showing the electric configuration of the electric power tool  200 . The tool main body  212  is provided with a motor  50 , part of a power supply circuit  52 , a tool controller  220 , and a memory device  222 . The tool controller  220  is connected to a main switch  16 , and a speed command signal outputted by the main switch  16  is inputted to the tool controller  220 . Characteristic data of the tool main body  212  is stored in a memory device  222 . The characteristic data includes a maximum limit in input voltage of the tool main body  212  and a maximum limit in input current of the tool main body  212 . 
     In addition, the tool main body  212  is provided with a positive electrode input terminal  232 , a negative electrode input terminal  234 , a first communication terminal  236 , a second communication terminal  238 , a third communication terminal  240 , and a fourth communication terminal  242 . These terminals are disposed at the battery attachment portion  216  of the tool main body  212 . The positive electrode input terminal  232  and the negative electrode input terminal  234  are electrically connected to the motor  50 . The first communication terminal  236  is connected to the motor  50  side of the main switch  16 . The second communication terminal  238  is electrically connected to the tool controller  220 . The third and fourth communication terminals  240 ,  2421  are electrically connected to the memory device  222 . 
     The pack holder  24  is provided with part of the power supply circuit  52 , a main switch detection circuit, a controller  60 , a first multiplexer  62 , a second multiplexer  64 , a buffer circuit  66 , and an amplification circuit  68 . The power supply circuit  52  is provided with a FET  58  and a shunt resistor  70 . 
     In addition, the pack holder is provided with a positive electrode output terminal  252 , a negative electrode output terminal  254 , a first communication terminal  256 , a second communication terminal  258 , a third communication terminal  260 , and a fourth communication terminal  262 . These terminals are disposed at the tool connector  218  of the pack holder  214 . The positive electrode output terminal  252  is electrically connected to the positive electrode input terminal  32  via the power supply circuit  52 . The negative electrode output terminal  254  is electrically connected to the negative electrode input terminal  34  via the power supply circuit  52 . The first communication terminal  256  is electrically connected to the main switch detection circuit  54 . The second communication terminal  258  and the third communication terminal  260  are electrically connected to the controller  60 . The fourth communication terminal  262  is electrically connected to a ground potential of the circuit of the back holder  214 . 
     As shown in  FIG. 13 , where the pack holder  214  is attached to the tool main body  212 , the terminals  252 ,  254 ,  256 ,  258 ,  260 ,  262  of the pack holder  214  are electrically connected to the corresponding terminals  232 ,  234 ,  236 ,  238 ,  240 ,  242  of the tool main body  212 . As a result, the controller  60  of the back holder  214  is communicatively connected to the tool controller  220  and memory device  222  of the tool main body  212 . The controller  60  of the pack holder  214 , and the tool controller  220  and the memory device  222  of the tool main body  212  cooperatively function similarly to the controller  60  explained in Embodiment 1. 
     It should be noted that, in the electric power tool  200  of the present embodiment, the controller  60  of the pack holder  214  accesses the memory device  222  of the tool main body  212  and acquires characteristic data (maximum limit in input voltage and maximum limit in input current) of the tool main body  212 . The FET  58  is then turned on/off and the discharges of the battery packs  100  are controlled on the basis of the acquired characteristic data of the tool main body  212 . Therefore, the pack holder  214  and the plurality of battery packs  100  are not limited to a specific tool main body  212  and can be commonly used in a plurality of tool main bodies  212 . 
     In the electric power tool  200  according to Embodiment 2, the circuit configuration shown in  FIG. 13  can be changed as appropriate. For example, as shown in  FIG. 14 , the FET  58  may be disposed on the tool main body  212  and the control of the FET  58  may be performed by the tool controller  220 . Alternatively, the circuit configuration may be changed to that shown in  FIG. 15 . In such circuit configuration, the controller  60  of the pack holder  214  measures indexes such as the output voltage and discharge current of the battery pack  100  and outputs a command signal for inhibiting or restricting the discharges of the battery packs  100 . This signal is transmitted to the tool controller  220  of the tool main body  212  via the communication terminals  258 ,  238 . The tool controller  220  receives the signal from the controller  60  of the pack holder  214  and performs the processing of inhibiting or restricting the discharges of the battery packs  100 , that is, turns off the FET  58 . With such a configuration, the number of communication terminals between the tool main body  212  and the pack holder  214  can be reduced. 
     In the above-described electric power tool  10 , the rated voltage of the tool main body  212  is 10.8 V and the nominal voltage of the battery pack  100  is 3.6 V. Therefore, three battery packs  100  are used. It goes without saying that the rated voltage of the tool main body  212  is not limited to 10.8 V and the number of battery packs  100  used is also not limited to three. For example, the rated voltage of the tool main body  212  can be 18 V and the number of battery packs  100  used can be five or more, as in an electric power tool  201  shown in  FIG. 16 .