Patent Publication Number: US-2007122692-A1

Title: Battery venting system

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates generally to a battery venting system that is useful for cordless power tools. The system includes one or more of battery cells, sleeves that may surround the battery cell, a carrier that can position and hold a number of battery cells to form a cell pack, and a charger useful with the battery cells and/or carrier.  
      Rechargeable nickel-cadmium cells and nickel-metal hydride cells are widely used as a battery power source for portable apparatus, such as power tools. Typically, nickel-cadmium cells or nickel-metal hydride cells are used in the form of a cylindrical cell that has a cylindrical shape. In the portable apparatus, the cylindrical cells are used alone or more typically in the form of a cell pack in which a plurality of cells are connected and are contained within a housing that can be removably attached to an apparatus.  
       FIG. 1  shows a typical cylindrical cell, which has a cylindrical case  1  that contains the cell materials in a closed fashion by a cover  11 . A conductive projection  11   a  is formed in the cover  11 , and a safety valve for releasing gasses is disposed in the projection  11   a.  Generally, the cover  11  having the projection  11   a  the positive electrode and the case  12  is the negative electrode. A sleeve  13  usually surrounds the peripheral side of the cell and is formed from an electrically insulating material so that when the battery cells are intentionally or unintentionally touching each other at the peripheral side face, the battery cells will not short out or discharge.  
      When the cylindrical cell  1  is to be used as a power source for an apparatus, a plurality of cells are connected to each other to form a cell pack, as shown in  FIG. 2 . Adjacent cells  1  are bridged by an electrically conductive plate  9 , such as a nickel plate. The plate may be attached by spot-welding or other methods of attaching to the projection  11   a  of the cover  11  of one of the cells and the bottom face of the case  12  of the other cell, respectively. To further secure and position the plurality of cells, it is known to wrap the periphery of the outermost cells with a shrinkable plastic or tape.  
      A disadvantage to providing an electrically insulating case around each cell is that when a plurality of cells are provided to form a cell pack, heat generated upon discharge and upon recharging operations is not easily dissipated. Likewise, when a cell pack is wrapped with shrinkable plastic or tape, the generated heat is further hindered from dissipation. When the temperature of the cell is raised as a result of the heat generation, self-discharge may reduce the cell capacity or shorten the service life of the cell. This problem is also applicable to sealed-type cells or other types such as rectangular cells.  
      Accordingly, there have been attempts to address this issue by providing cell holders such as that shown in U.S. Pat. No. 5,578,392, particularly  FIG. 13 . There, a cell holder is provided to hold and position individual battery cells. In this cell holder, an upper and a lower plate is provided to respectively receive the upper and lower portion of the individual cells. Each plate has a plurality of spaced apart cell holes into which a portion of the upper or lower portion of a battery cell can be received. Accordingly, a gap is formed between adjacent cells to facilitate heat dissipation during discharge and charging operations. Although this solution may be useful, the plates take up space within the battery pack housing and by virtue of their size still hinder heat dissipation.  
     BRIEF SUMMARY OF THE INVENTION  
      With the above in mind, in one aspect of the present invention, a battery cell having a first terminal and a second terminal is provided without an outer case or covering. In one embodiment, the battery cell may be cylindrically shaped with a first end having a first terminal and a second end spaced from the first end and having a second terminal. A longitudinal axis extends from the first end to the second end. A peripheral side face is disposed between the first end and the second end.  
      In another aspect of the present invention, a battery cell is provided with a sleeve to surround the peripheral side face of the cell. The sleeve is formed from an electrically insulating material and has a plurality of apertures. The sleeve may be formed form paper, plastic, or any other suitable electrical insulating material. The sleeve may also be formed of a plastic mesh such as a molded plastic mesh.  
      The apertures may have a variety of suitable shapes such that the peripheral side face of the cell is sufficiently exposed to allow heat to dissipate while still allowing the cell to be insulated when contacted by a similar cell having a sleeve. Suitable aperture shapes include those selected from a circle, ellipse, parabola, crescent, obround, disc, triangle, rectangle, polygon, and mixtures thereof. Where the aperture has a shape that provides a longer side (or a pair of longer sides) and a short side (or a pair of shorter sides, like an obround, the longer side(s) may be arranged to be parallel to the longitudinal axis. Alternatively, the longer side(s) may be arranged to be normal to the longitudinal axis.  
      In yet another aspect of the present invention, a carrier of a plurality of battery cells is provided to arrange and hold the plurality of battery cells to define a cell pack. The carrier may be used with battery cells without an outer case, with battery cells having an outer case, or with battery cells having electrically non-conductive sleeves according to the present invention. In one embodiment, the carrier causes the terminals on each end of the cells to be positioned in a substantially same plane.  
      In this cell pack, a plurality of cells are arranged side-by-side and are generally parallel to their longitudinal axis. An electrically conductive connecting member connects the cells with each other. The electrically conductive material may be configured of a flexible material. Even when vibration or shock is applied to the cell pack, therefore, the force acting between the cylindrical cells is absorbed by the flexible material of the connecting member.  
      It is a still further object of the invention to provide a cell carrier in which upper and lower end portions of sealed-type cells are held by a respective upper carrier and lower carrier to define a vent space between adjacent battery cells, thereby allowing heat generated from the sealed-type cells to be dissipated to the exterior via the vent hole. The upper carrier and lower cell carrier are provided with apertures that are aligned with the vent space to define an elongated fluid flow path to allow heat to be dissipated. The upper carrier and lower carrier are also provided with a connecting member receiving area.  
      According to the present invention, a plurality of battery cells are held between the pair of upper and lower carriers and heat generated from the cells can be dissipated via the elongated fluid flow path. Since the outward-directed peripheral side faces of the battery cells may be free of electrically insulating material and are exposed to the exterior between the pair of upper and lower carriers, heat can also be dissipated from the peripheral side faces of each battery cell. Alternatively, if a sleeve according to the present invention is provided, i.e., with a plurality of apertures, a substantial portion the peripheral side faces of the battery cells will be exposed to the exterior between the pair of upper and lower carriers so that heat can be dissipated from the peripheral side faces of each battery cell.  
      The venting effect can be further enhanced by forcing a fluid such as air past and through the cell pack. For example, in another aspect of the present invention, a charger is provided with a fan to force a fluid into the cell pack where it can pass through adjacent cells. Advantageously, the charger may be configured to provide charging of more than a single cell pack, either sequentially or simultaneously. The charger is provided with a mechanism to simultaneously force fluid into two separate cell, while charging.  
      The cell pack may be provided with contacts that operatively associated with the cell pack to act as a conduit to transfer electricity to the motor of a power tool or to receive a charge from a charger. In addition, the cell pack may be provided with an outer casing or housing, typically formed of plastic to define a battery pack.  
      Additional aspects, objects, and advantages of the invention will become apparent from the detailed description, the appended claims, and accompanying drawings, as well as by practice of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings illustrate an embodiment of the invention and together with the description serve to explain the principles of the invention. In the drawings, the same reference numerals indicate the same parts.  
       FIG. 1  is a perspective view of a prior art cylindrical battery cell.  
       FIG. 2  is a perspective view of a plurality of cylindrical battery cells electrically connected to define a cell pack.  
       FIG. 3  is a perspective view of a plurality of cylindrical battery cells having sleeves according to the present invention and being electrically connected to define a cell pack.  
       FIG. 4  is a perspective exploded view of a carrier for a plurality of cylindrical battery cells electrically connected to define a cell pack.  
       FIG. 5  is a perspective view of a carrier for a plurality of cylindrical battery cells electrically connected to define a cell pack.  
       FIG. 6  is a perspective view of a battery charger useful for simultaneously charging two batteries and containing a venting system according to one aspect of the present invention.  
       FIG. 7  is a rear cut-away view of the charger with elements removed to better illustrate the venting system of the present invention. One embodiment of the duct that forms a portion of the venting system is shown with a portion removed to better illustrate the flow of fluid through the duct and to a battery pack that houses the carrier of the present invention. A portion of the battery pack housings are cut-away to show the carrier. In addition, the battery pack housings are is shown just before engagement with the charger.  
       FIG. 8  is a bottom view of a portion of a venting system for a battery charger according to one aspect of the present invention.  
       FIG. 9  is a bottom view of one embodiment of a duct that forms a portion of the venting system useful in a battery charger that is capable of receiving two battery packs.  
       FIG. 10  is a bottom view of a battery charger that is capable of receiving two battery packs and the duct of  FIG. 9  that forms a portion of the venting system. Certain parts of the battery charger and venting system are not shown to better illustrate the duct. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring now to the drawings and initially to  FIG. 1 , a prior art battery cell is shown. The prior art battery cell  1  has a cylindrical case that contains the cell materials in a closed fashion by a cover  11 . A conductive projection  11   a  is formed in the cover  11 , and a safety valve for releasing gasses is disposed in the projection  11   a.  Generally, the cover  11  having the projection  11   a  is the positive electrode and the case  12  is the negative electrode. Generally, an outer sleeve  13  is formed from an electrically insulating material so that when the battery cells are intentionally or unintentionally touching each other at the peripheral side face, the battery cells will not discharge.  
      When the prior art cylindrical cell  1  is to be used as a power source for an apparatus, generally, a plurality of cells are connected to each other to form a cell pack, as shown in  FIG. 2 . Two adjacent cells  1  are bridged by an electrically conductive plate  9 , such as a nickel plate. The plate  9  may be attached by spot-welding or other method of attaching the plate  9  to the projection  11   a  of the cover  11  of one of the cells and the bottom face of the case  12  of the other cell, respectively. The sleeve  13  acts to prevent adjacent cells from shunting. In the description of the embodiments of the present invention, components having the same function as those of the prior art described above with respect to  FIG. 1  and  2  are designated by the same reference numerals.  
      Turning now to  FIG. 3 , one embodiment of the present invention is shown. Although the cell  1  is depicted with a cylindrical shape, it is to be understood that the cell  1  can have any suitable shape presently known or that can be contemplated in the future. In general, the cell  1  has a first end  2  end and a second end  3  spaced from the first end  2  to define a peripheral side face  4  that extends from the first end  2  to the second end  3 . A longitudinal axis  5  extends from the first end  2  to the second end  3  of the cell  1 . The first end  2  has a cover  11  from which a projection  11   a  extends. The projection  11   a  defines a positive electrode or terminal. The second end  3  defines a negative electrode or terminal. While the cell  1  may be a nickel-cadmium cell or a nickel-metal hydride cell, the present invention is not restricted those types of cells.  
      According to another embodiment of the present invention, the cell  1  is provided with a sleeve  30  that surrounds the peripheral side face  4  of the cell  1 . The sleeve  30  made be made of any suitable electrically insulating material. For example, the sleeve  30  may be made from paper, plastic, or any other suitable insulating materials. The sleeve  30  may be a molded plastic mesh or similar construction.  
      Desirably, the sleeve  30  is provided with a plurality of apertures  32  to allow heat to dissipate from the cell  1 . The apertures  32  may have any suitable shapes such that the cell  1  is sufficiently exposed to allow heat to dissipate while still allowing the cell  1  to be insulated when contacted by a similar cell  1  having a sleeve  30 . Suitable shapes include those selected from a circle, ellipse, parabolic, crescent, obround, disc, triangle, rectangular, polygonal, and mixtures thereof. Where the aperture  32  has a shape that provides a longer side (or a pair of longer sides) and a short side (or a pair of shorter sides, like an obround, the longer side(s) may be arranged to be parallel to the longitudinal axis  5 . Alternatively, the longer side(s) may be arranged to be normal to the longitudinal axis  5 .  
      To form a cell pack  20 , a plurality of individual cells  1  are electrically connected to each other.  FIG. 3  shows the case where a cell pack is configured by six cylindrical cells  1 . One skilled in the art will understand that this embodiment of the present invention is applicable to any number of battery cells and is applicable to any shape of battery cell. Of course, in the case where two or more cells  1  are used, a cell pack  20  is configured by connecting the cells  1  to each other by an electrically conductive connecting member  9 . Instead of connecting the cells  1  in series, the cells  1  may be connected in parallel or in a combination of series and parallel connections. Accordingly, the connecting member  9  may connect adjacent positive terminals, adjacent negative terminals, both, or adjacent positive and negative terminals.  
      The electrically conductive connecting member  9  may be formed from any suitable electrically conductive material. Non-limiting examples include nickel, nickel and steel composites, steel, or nonferrous conductive material. The electrically conductive connecting member  9  may be attached by spot welding or by any other suitable attachment method such as laser welding.  
      Next, with reference to  FIGS. 4 and 5 , an embodiment of the carrier of the present invention in which fifteen cylindrical cells are held will be described. In the present invention, however, the kind and shape of the cell  1  are not restricted to nickel-cadmium or nickel-metal hydride and conventional manganese cells or sealed-type cells of other types such as those of rectangular cells may be used. In addition, while the present invention will be illustrated and described with fifteen cells, one skilled in the art will appreciate that the carrier will be suitable for use with any number of cells, for example, twelve, ten, seven, four, or even two depending on the desired voltage output.  
      As shown in  FIG. 4 , the fifteen cells  1  are arranged in three rows with each row consisting of five cells  1 . The fifteen cells  1  are sandwiched between an upper carrier  40  and a lower carrier  50  to define a carrier  60 . Desirably, the upper carrier  40  and lower carrier  50  are made of an electrically insulating material such as a synthetic resin. The upper carrier  40  and the lower carrier  50  have a significant portion that is substantially flat. The thickness of the upper carrier  40  and the lower carrier  50  ranges from about 0.25 mm to about 25 mm, desirably from about 0.5 mm to about 10 mm, more desirably from about 0.75 mm to about 1.25 mm. The upper carrier  40  and lower carrier  50  are used while being disposed opposing each other.  
      The upper carrier  40  has an upper surface  41  and a lower surface  42 , which faces the battery cells  1 . The upper carrier  40  has a plurality of ribs  43  extending outward from the lower surface  42 . The ribs  43  are located on the upper carrier  40  in certain areas to cooperatively interact with and engage the battery cells  1  to correctly position them with respect to each other. In this regard, the ribs  43  are spaced from each other and therefore they do not completely surround the cell  1 .  
      The number of ribs  43  provided will be that necessary to selectively position the number of cells  1  forming the desired cell pack  20 . in addition, when the upper carrier  40  is placed on the plurality of battery cells  1  the ribs  43  extend from upper carrier  40  a distance substantially less than the distance from the top  2  to the bottom  3  of the cell  1 . Because the cell  1  may have any of a variety of different shapes and heights, the length of the ribs  43  will vary depending on the shape of the cell  1 . Desirably, the ribs  43  extend a distance less than one-half the distance from the top  2  to the bottom  3  of the cell  1 . For example, the ribs  43  extend from the lower surface  42  a distance ranging from about 0.5 mm to about 10 mm, desirably from about 3 mm to about 6 mm.  
      The upper carrier  40  is provided with shaped cut-outs  44  that substantially conform to the shape of the electrically conductive connecting member  9 . Thus, when the connecting member  9  is rectangular, the cut-out  44  will likewise have a rectangular shape with dimensions substantially the same (slightly larger) as the dimensions of the connecting member  9 . The thickness of the upper carrier  40  may be substantially the same as the thickness of the connecting member  9  so that when the upper carrier  40  is placed on top of the plurality of cells  1  forming the cell pack  20 , the upper surface of the connecting member  9  and the top surface  41  of the upper carrier  40  will lie in substantially the same plane. Alternatively, the connecting member  9  can have a thickness slightly less than the thickness of the upper carrier  40  so that the connecting member  9  does not extend beyond the top surface  41  of the upper carrier  40 .  
      The upper carrier  40  is also provided with a plurality of vent openings  46 . In general, the vent openings  46  are provided on the upper carrier  40  at a location that is aligned with a gap  6  formed by adjacent cells  1 . For example, referring to  FIG. 4 , a gap  6  is formed by three adjacent cells  1 . By providing vent openings  46  on the upper carrier  40 , fluid flowing through the gap  6  can also flow through the vent openings  46  to more effectively dissipate heat generated by the cells  1 .  
      The lower carrier  50  has a lower surface  51  and an upper surface  52 , which faces the battery cells  1 . The lower carrier  50  has a plurality of ribs  53  extending outward from the upper surface  52 . The ribs  53  are located on the upper carrier  50  in certain areas to cooperatively interact with and engage the battery cells  1  to correctly position them with respect to each other. In this regard, the ribs  43  are spaced from each other and therefore they do not completely surround the cell  1 . In addition, the ribs  53  are also located in areas on the lower carrier  50  so that when the upper carrier  40  and the lower carrier  50  are placed on the cells  1  to effectively sandwich the cells  1 , the cells  1  will be aligned substantially vertically and parallel to each other, i.e., parallel to the longitudinal axis  5 .  
      The number of ribs  53  provided will be that necessary to selectively position the number of cells  1  forming the desired cell pack  20 . When the lower carrier  50  is placed on the plurality of battery cells  1  the ribs  53  extend from lower carrier  40  a distance substantially less than the distance from the bottom  3  to the top  2  of the cell  1 . Because the cell  1  may have any of a variety of different shapes and heights, the length of the ribs  53  will vary depending on the shape of the cell  1 . Desirably, the ribs  53  extend a distance less than one-half the distance from the bottom  3  to the top  2  of the cell  1 . For example, the ribs  53  extend from the upper surface  52  a distance ranging from about 0.5 mm to about 10 mm, desirably from about 3 mm to about 6 mm.  
      The lower carrier  50  is provided with shaped cut-outs  54  that substantially conform to the shape of the electrically conductive connecting member  9 . Thus, when the connecting member  9  is rectangular, the cut-out  54  will likewise have a rectangular shape with dimensions substantially the same (slightly larger) as the dimensions of the connecting member  9 . The thickness of the lower carrier  50  may be substantially the same as the thickness of the connecting member  9  so that when the lower carrier  50  is placed on the plurality of cells  1  forming the cell pack  20 , the exposed surface of the connecting member  9  and the lower surface  51  of the lower carrier  50  will lie in substantially the same plane. Alternatively, the connecting member  9  can have a thickness slightly less than the thickness of the lower carrier  50  so that the connecting member  9  does not extend beyond the lower surface  51  of the lower carrier  50 .  
      The lower carrier  50  is also provided with a plurality of vent openings  56 . In general, the vent openings  56  are provided on the lower carrier  50  at a location that is aligned with a gap  6  formed by adjacent cells  1 . For example, referring to  FIG. 4 , a gap  6  is formed by three adjacent cells  1 . In addition, the vent openings  56  provided on the lower carrier  50  are aligned with the vent openings  46  provided on the upper carrier  40 . By aligning the vent openings  46 ,  56  and the gap  6 , fluid flowing through the gap  6  can also flow through the vent openings  46 ,  56  to more effectively dissipate heat generated by the cells  1 .  
      As shown in  FIG. 5 , the carrier  60  causes the cells  1  to be positioned in such a manner that the end portions  2  and  3  are in the substantially same plane. In this case, the electrically conductive connecting member  9  is not required to be bent and can be made flat so that the terminals are easily connected to each other.  
      In addition, In the cell pack shown in  FIG. 5 , in which the cylindrical cells  1  are held by the carrier  60 , as described above, ventilation is enabled between the exterior and the vicinity of the inward-directed peripheral side faces of the cylindrical cells  1 , via the vent openings  46 ,  56  provided in the upper carrier  40  and lower carrier  50 , respectively, which as noted above are longitudinally aligned. Consequently, heat generated from the cylindrical cells  1  can be rapidly dissipated to the exterior and the temperature rise of the cylindrical cells  1  is reduced so that it does not exceed the specified range.  
      It is believed that the cells  1  closest to the center of the cell pack  20  tend to cool slower than those cells farthest from the center of the pack. Therefore, the size of the vent openings  46 ,  56  can be varied so that the vent openings  46 ,  56  closest to the center of the cell pack  20  are larger than the vent openings farthest from the center of the cell pack  20 .  
      The ventilation due to the vent openings  46 ,  56  and the like can occur as a result of natural convection. When the amount of generated heat is expected to be large or is large, the natural convection can be enhanced by providing a fluid moving device to force a fluid around the outward directed peripheral side face  4  of the cells  1  and through the vent openings  46 ,  56  provided in the upper carrier  40  and lower carrier  50 .  
      The carrier  60  of the present invention may be used with cells  1  that are bare or do not have any insulating cover. In this instance, the ribs  43 ,  53  are located on the upper carrier  40  and lower carrier  50 , respectively so that adjacent cells  1  do not touch. As a result, effective heat dissipation can occur. Alternatively, the carrier  60  of the present invention may be used with cells  1  that are provided with sleeves  30  according to the present invention or with standard known cells.  
      A filter (not shown) may be placed adjacent each of the upper carrier  40  and lower carrier  50  to reduce or prevent dust entering the cell pack  20 . The filter may be any suitable material such as a synthetic fiber mesh.  
      The cell pack may also have a temperature indicating device to indicate the temperature of one or more cells  1  in the cell pack  20 . The temperature indicating device may be thermistor, a capacitor, a theromostat, or other temperature indicating device.  
      One skilled in the art will understand that the cell pack of the present invention can be provided with one or more electrical contacts that operatively associated with the cell pack  20 . The electrical contacts act as a conduit of electricity from the cell pack  20  to the motor of a power tool with which the cell pack  20  is associated. In addition, the electrical contacts act to transfer the charge from a charger so that the cells in the cell pack  20  can be recharged.  
      In addition, it is customary to provide a housing  100  that surrounds the cells  1  and cell pack  20  to define a battery pack  102 . In this regard,  FIG. 6  shows a battery charger  200  incorporating a venting system  220  and having two battery packs  102  operatively associated with the charger  200 . The housing  100  may be formed from any suitable material but is typically formed of a hard plastic to aid in protecting the cells. The upper  110  and lower  120  faces of the battery pack  102  may contain vents  112 ,  122  to allow fluid to pass through the housing  100  and the vents  46 ,  56  provided on the upper carrier  40  and the lower carrier  50 . Although it is understood that the upper  110  and lower  120  faces of the battery pack  102  are relative, for ease of description, the upper face  110  of the battery pack  102  will be considered to be that face adjacent the upper carrier  40  and the lower face  120  of the battery pack  102  will be considered to be that face adjacent the lower carrier  50 . The battery pack  102  may be slideably engaged with a power tool such that the battery pack  102  can be removed from the power tool and placed in a charger  200  for recharging the cells  1 .  
      The charger  200  includes a housing having one surface that receives a battery pack. For convenience the charger  200  will be described as having a top  202 , a bottom  204 , and at least one side  208 . It will be understood, however, that the configuration of the charger  200  is not particularly important so long as the features described below are incorporated. In addition, for convenience, the one surface will be referred to as the top surface  202 . The top surface  202  is provided with at least one slot  210  to removably receive a battery pack. In a desired embodiment, the charger  200  is provided with two slots  210   a ,  210   b  to simultaneously receive two separate battery packs  102 . Each slot  210  has respective contacts to couple with the electrical contacts  70  associated with the cell pack  20  to charge the cells  1 . The charging of the cells  1  may be controlled by a switch, a microcontroller, a circuit or the like. Methods of designing and operating the charging mechanism are known and any of several different operating schemes may be used.  
      The charger  200  further includes a venting system  220  that includes an inlet  222 , an outlet  224 , a fan  226 , and a duct  230  to direct fluid from the fan  226  to the outlet  224 . The outlet  224  is associated with vents  122  provided on the lower face  120  of the battery housing  100  so that fluid flows from the fan  226 , through the duct  230 , into the battery housing  100  and through the vents  56  in the lower carrier  50 , across the cells  1  and out the vents  46  in the upper carrier  40  and the vents  112  in the upper face  110  of the battery housing  100 . Alternatively, the fan  226  may be arranged to suck fluid from the outlet  224  to the inlet  222  so that the fluid is moved from the vents  112  on the battery housing  100  through the vents  46  in the upper carrier  40 , across the cells  1  through the vents  56  in the lower carrier  50  and into the duct  230 . It will be understood by one skilled in the art the terms inlet and outlet are relative to the direction of the fan flow. For ease description, the fan flow will be described as moving fluid from the charger housing  200  into the battery housing  100 . Accordingly, the outlet  224  of the charger housing  200  will be disposed on the top  202  of the charger housing  200  and will be associated with the vents  122  provided on the lower face  120  of the battery housing  100 .  
      As noted above, the venting system  220  includes an inlet  222  that may be provided on the top  202  of the charger housing  200 , the bottom  206  of the charger housing  200 , the side(s)  208  of the charger housing  200  or each of them. The inlet  222  may be provided by a grill, slots, or other types of openings such that fluid, typically air, can pass into the charger housing  200  to the fan  226 . A filter may be provided adjacent the inlet to reduce or prevent particles such as dust from entering the charger housing  200 .  
      The outlet  224  is provided on the top  202  of the charger housing  200  such that when a battery pack  102  is positioned on the charger  200 , the outlet  224  is aligned with the vents  122  provided on lower face  120  of the battery pack  102  so that fluid may enter the battery pack  102 . For example, the top  202  of the charger  200  has a slot  210  to receive the battery pack  102 . The outlet  224  may be provided within the confines of the slot  210  or adjacent to the slot  210  so long as the outlet  224  is adjacent the vents  122  provided on the lower face  120  of the battery pack  102 .  
      In the desired embodiment, the charger  200  is provided with two slots  210   a  and  210   b  so that two battery packs  102  may be simultaneously located on the charger  200 . In this instance, at least one outlet  224   a  and  224   b  will be respectively associated with each slot  210   a  and  210   b.  A screen or grill may be located adjacent the outlet  224  to reduce or prevent dust, chips, or other things from entering the battery pack  102 .  
      A duct  230  fluidically connects the fan  226  to the outlet  224 . The duct  230  has a bottom wall  232  with a side wall  234  extending from the periphery of the bottom wall  232 . The bottom wall  232  is provided with a central portion  236  having a first arm  240  and a second arm  244  connected with and extending from the central portion  236 . The duct  230  is desirably formed with smooth contours to minimize flow and pressure gradients. The bottom  232  may be T-shaped, Y-shaped, or shaped in a similar fashion such that fluid can flow from a central portion  236  to the first  240  and second  244  arms. The bottom wall  232  has an aperture  238  to receive the fan  226 . Desirably, the aperture  238  is provided in the central portion  236  so that fluid is distributed about equally to each of the first arm  240  and second arm  244  of the duct  230 .  
      The first arm  240  terminates at a terminal end  242  and the second arm  244  terminates at a terminal end  246  with the terminal ends  242 ,  246  located adjacent respective outlets  224   a,    224   b.  A seal may be provided between the terminal end of the arms  242 ,  246  and the outlet  224  to reduce or prevent fluid from by passing the outlet  224 .  
      The duct  230  is desirably located within the charger housing  200  such that the underside  204  of the top  202  of the charger housing  200  forms a top surface of the duct  230 . In this way, the duct  230  will be closed except for the portion that is adjacent to the outlet  224 . Alternatively, the duct  230  can be provided with a top housing (not shown) to enclose the side wall  234  except at the terminal end of the arms  242 ,  246 . It will be appreciated that when the fan  226  is activated, fluid is directed from the fan inlet, into the duct  236  and through the terminal ends of the arms  242 ,  246  to the respective outlet  224   a,    224   b  and through the vents  122  on the lower face  120  of the battery housing  100 .  
      The fan  226  can be activated as desired. For example, the charger electronics could be coupled with a sensor inside the battery pack  102  that would be activated through the electrical contacts. The sensor could sense a property of the cell pack  20  such as voltage of one or more cells  1 , temperature within the cell pack  20 , or other property. Based on the output of the sensor, the fan  226  could be activated to run. Alternatively, the fan  226  could be activated to continuously be activated while the charger is operating. The fan  226  could also be activated through the use of a manual switch provided on the charger housing  200 .  
      In yet another alternative, a mechanical switch may be provided such that when a battery pack  102  is positioned within a slot  210  of the charger housing  200 , the fan  226  can be activated. In this embodiment, when the battery pack  102  is removed from the slot  210 , the fan  226  will be de-energized.  
      Other fan modulation process may also be used. For example, the fan  226  may be turned on for a predetermined period and turned off before a battery pack  102  is disposed on the charger  200 . This period could occur when the charger  300  is either turned on, connected to an electrical outlet or when a button on the charger  200  is pushed. This would blow foreign particles, such as dust, that has settled on the charger  200  and or the slot  210 . Accordingly, the particles or dust would not be blown into the battery pack  102  during charging. This result can also be achieved if the fan  226  is always on, on after the battery pack  102  has been removed, or if the fan  226  is periodically turned on and off when the battery pack  226  is not disposed in the charger  200 .  
      In addition, rather than fan  226  being completely turned off, it may be expedient to just regulate the power sent to the fan  226  so that the fan  226  rotates at a lower speed. Accordingly, the fan  226  can rotate at a first speed before the battery pack  102  is disposed on the charger  200 . When the battery pack  102  is disposed on the charger  200 , the fan  226  can rotate at a second speed, which is higher than the first speed. When the battery pack  102  is removed, the fan  226  can be turned off completely or brought back to a lower speed. This would also help in maintaining the duct  230  free of dust.  
      It may be desirable to turn the fan  226  on at a high first speed for a predetermined period and then lower the speed before a battery pack  102  is disposed on the charger  200 . This period could occur when the charger  200  is either turned on, connected to an electrical outlet or when a button on the charger  200  is pushed.  
      Furthermore, the charger  200  can control the speed of the fan  226  by using information from the temperature indicating device. For example, the charger  200  would receive information from the temperature indicating device. If the battery pack  102  is below a first desired temperature, the charger  200  would lower the speed or stop the fan  226 . Similarly, the charger  200  can control the fan  226  to maintain the temperature of battery pack  102  near a predetermined temperature.  
      In addition, the charger  200  can control the fan  226  to obtain accurate information from the temperature indicating device. For example, if the temperature indicating device was exposed to the air or fluid flow, the temperature indicating device might indicate a cell temperature lower than the actual cell temperature. Such result can be minimized or avoided if the charger  200  periodically lowers the speed of or stops the fan  226  for a predetermined period of time. This would allow the temperature indicating device to indicate or show a more accurate cell temperature, which can then be read by the charger  200  and used in its temperature analysis. The charger  200  can then increase the speed of or start the fan  226  until the next time the charger  200  needs temperature information.  
      While the invention has been described in conjunction with specific embodiments it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing detailed description. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.