CROSS-REFERENCE 
     The invention described and claimed hereinbelow is also described in German Patent Application DE 10 2004 043821.8, filed Sep. 10, 2004. This German Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119 (a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for locking a power tool to a battery pack, and also relates to a battery pack. 
     Known locking devices for locking cordless power tools, such as handheld power drills, drill hammers, and the like, to a battery pack serving to supply current to them, have one or more locking bars in most cases, which are each supported movably, counter to the force of a spring, in a housing of the power tool and are brought by the spring force into engagement with a detent recess in the battery pack when the battery pack is thrust into a guide of the power tool, in order to connect the battery pack to the power tool. For undoing the connection, the housing of the power tool is equipped with an unlocking device, which is typically located, in the form of a push button, in the vicinity of the guide and is actuated by hand in order to disengage the locking bar from the detent recess. 
     In another known locking device, which makes so-called one-handed operation of the battery pack possible, the locking bars and the springs that act on the locking bars are integrated, together with two unlocking buttons serving to release the battery pack, with the battery pack, so that when the battery pack is released, the power tool can be firmly held with one hand while the battery pack is grasped with the other hand, the two locking bars are disengaged by finger pressure or thumb pressure on the unlocking buttons from the detent recesses that are recessed out of the housing of the power tool, and the battery pack can then be pulled out of the guide of the power tool. This kind of one-handed operation is indeed sought, but it has the disadvantage that the production costs for the battery pack are higher because of the integration of the locking bars, springs and unlocking devices, and because of the requirement for higher-precision tolerances. Nevertheless, the battery pack should be as inexpensive as possible, since most customers consider the battery pack, unlike the power tool, to be a disposable item that has to be replaced after a certain time in use. The readiness to buy a new battery pack is all the greater, the more inexpensively the battery pack can be procured. Moreover, because of the integration of the locking bars, springs and unlocking devices with the battery pack, the requisite installation space and the weight of the battery pack are increased, which is also seen as a disadvantage. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a locking device for a power tool to a battery pack, and a battery pack, which avoids the disadvantages of the prior art. 
     When the device and the battery pack are designed in accordance with the present invention, they provide the advantage above all that, despite the possibility of convenient one-handed operation, the production costs of the battery pack can be reduced not inconsiderably in comparison to those for battery packs having the last-mentioned locking device above, since now only the unlocking device or unlocking devices themselves have to be mounted on the battery pack, while the spring-loaded locking bar or locking bars are located in the housing of the power tool, and accordingly the tolerances that must be adhered to for the battery pack can be greater. 
     Shifting the spring-loaded locking bar or locking bars into the housing of the power tool can further reduce both the installation space required and the weight of the battery pack, and the unlocking device or devices can be integrated with the plastic housing of the battery pack in a simple, inexpensive embodiment. Moreover, by the combination of characteristics according to the invention, it can be reliably prevented that the battery pack will be released unintentionally from the power tool if an unlocking device is damaged, worn or missing. Compared with the restoring action of an unlocking device integrated along with the locking bar in the battery pack, it is possible, last but not least, for the restoring action of an unlocking device integrated with the battery pack without a locking bar to be considerably less, since the unlocking device needs to assure only its own restoration. 
     This restoring action of the unlocking device is preferably obtained by means of an elastically deformable element, which upon a manual actuation of the unlocking device is deformed for disengaging the locking bar, and whose elastic restoring force moves the unlocking device back into its unactuated outset position. The elastically deformable element may expediently comprise an elastomer material, which is embodied for example in the form of a soft film of thermoplastic elastomer (TPE) and connects an unlocking button of the unlocking device, which button is accessible from the outside of the battery pack, to a housing of the battery pack in a dustproof and movable manner. 
     Alternatively, however, the elastically deformable element may also comprise a tongue of elastically deformable plastic material, which is selectively integrated with a housing of the battery pack and acts on the unlocking button, or is integrated with the unlocking button and acts on a housing of the battery pack, and which upon actuation of the unlocking button is elastically deformed and when the unlocking button is released effects the restoration of the unlocking button to the outset position. In principle, it is even conceivable to use the spring, which acts on the locking bar and presses the locking bar into the detent recess, as the elastically deformable element; in that case, however, the restoration of the unlocking button by the locking bar engaging the detent recess does not occur until upon locking of the battery pack to the power tool. 
     In a further preferred feature of the invention, the unlocking device has a contact-pressure face, which can be pressed against the locking bar, and preferably against its free face end engaging the detent recess, in order to move the locking bar out of the detent recess so far that the contact-pressure face is located between two diametrically opposed, preferably flat guide faces of the battery pack and of the power tool, or is aligned with one of these guide faces, so that the battery pack together with the unlocking device can be displaced along these guide faces for the sake of being released from the power tool. 
     To prevent the contact-pressure face of the unlocking device, upon the actuation of the latter, for moving past the plane of the guide faces, a stop for the locking bar or for the unlocking device is preferably provided. 
     A further preferred feature of the invention provides that the unlocking device is accessible from the outside of the battery pack and can be moved preferably transversely to the insertion direction and in the direction of motion of the locking bar into the detent recess, in order to put the contact-pressure face into contact with the free face end of the locking bar that engages the detent recess and then to press the locking bar out of the detent recess, until the contact-pressure face and the tip, contacting it, of the locking bar are located between the diametrically opposed guide faces or are aligned with one of these faces. 
     The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary cutaway perspective view of a connection between a power tool and a battery pack, having a locking device according to the invention; 
         FIG. 2  is a sectional view of the locking device along the plane II-II of  FIG. 1 , after the power tool and the battery pack have been locked; 
         FIG. 3  is a view corresponding to  FIG. 2 , but upon actuation of two unlocking buttons of the locking device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The locking device shown in the drawing serves the purpose of mutually locking a cordless power tool  2 , such as a handheld drill hammer, and a battery pack  4  required for supplying current to the power tool  2 . 
     As best shown in  FIG. 1 , the power tool  2 , in a known manner, has a protruding guide rail  8  on the free lower end of its handle  6 , which guide rail can be inserted in the direction of the arrow A into a guide groove  10 , recessed out of the upper end of the battery pack  4 , that has a cross section complementary to the cross section of the guide rail  8 . The insertion motion of the guide rail  8  into the guide groove  10  is limited by cooperating stops (not shown), which contact one another when the battery pack  4  is properly locked to the power tool  2  and an electrical connection is made between connection contacts of a current-storing means of the battery pack  4  and a current circuit of a consumer of the power tool  2 . 
     The locking device  12  serving to lock the battery pack  4  to the power tool  2  here includes a total of four locking bars  14 , which are located in pairs, one behind the other, in the insertion direction of the battery pack  4  in associated locking bar guides  16  in the interior of the guide rail  8 . Two each of the locking bars  14  have aligned longitudinal axes and are forced apart to opposite sides transversely to the insertion direction (arrow A), each by a respective helical compression spring  18 , so that their outward-pointing free face ends  20 ,  22  protrude past adjacent lateral guide faces  24 ,  26  of the guide rail  8 . Once the battery pack  4  is locked to the power tool  2 , the face ends  20 ,  22  of one pair of locking bars engages diametrically opposed detent recesses  28 ,  30  ( FIG. 2 ) located in the interior of the guide groove  10  of the battery pack  4 . 
     The locking device  12  is designed for so-called two-stage locking, in which in a first stage assures the snapping of the locking bars  14 , of the front pair of locking bars in the insertion direction, into the detent recesses  28 ,  30  such that the battery pack  4  is secured against unintentional release relative to the handle  6  of the power tool  2 , for instance for shipping, but no electrical connection is yet made between the connection contacts of the current-storing means of the battery pack  4  and the current circuit of the consumer of the power tool  2 . This connection is not made until the battery pack  4 , in the second stage, is inserted farther until it reaches the terminal position shown in  FIG. 2 , in which the locking bars  14  of the rear pair of locking bars, in the insertion direction, snap into the detent recesses  28 ,  30  and assure a final locking of the battery pack  4  relative to the power tool  2  while simultaneously making an electrical connection (not shown) between them. 
     The free face ends  20 ,  22  of the locking bars  14  have run-up chamfers  34 , which point in the insertion direction and assure that all the locking bars  14 , when the battery pack  4  is slipped onto the guide rail  8  of the power tool  2  and the front locking bars  14 , on passing the detent recesses  28 ,  30 , are automatically displaced inward in their guides  16 , counter to the force of the spring  18 , until their tips  36  are aligned with the guide faces  24 ,  26 . These guide faces extend parallel to diametrically opposed side faces  38 ,  40  of the guide groove  10  and have a slight spacing from them. 
     The locking bars  14  are provided in the interior with a cylindrical hollow chamber  42  for the associated helical compression spring  18 , which is braced by its outward-pointing face end against the bottom of the hollow chamber  42  and by its inward-pointing face end against a stationary abutment  44  in the interior of the guide rail  8 . The spacings between the inward-pointing face ends of the locking bars  14  and the adjacent, outward-pointing face ends of the abutments  44  are dimensioned such that the locking bars  14  strike the abutments  44  when their tips  36  are aligned with the guide faces  24 ,  26  (see  FIG. 3 ). On their rear face ends, the locking bars  14  have laterally protruding lugs  46 , which cooperate with the locking bar guide  16  and limit the outward path of motion of the locking bars  14 , to prevent the locking bars  14  from falling out when the battery pack  4  is removed. 
     The two detent recesses  28 ,  30 , located diametrically opposite one another in the interior of the guide groove  10 , extend as far as the adjacent outside of a housing  56  of the battery pack  4  and open in the interior into the two diametrically opposed side faces  38 ,  40  of the guide groove  10 . Each of the detent recesses  28 ,  30  accommodates an unlocking button  50 ,  52 , which is accessible or actuatable from the outside of the housing  56  and is displaceable inside the detent recess  28 ,  30  transversely to the insertion direction, so that the locking bars  14 , of each pair of locking bars, engaging the detent recesses  28 ,  30  can be manually disengaged from the two recesses  28 ,  30  by means of a finger or thumb pressure exerted simultaneously on both buttons  50 ,  52 , in order first to unlock the battery pack  4  and then release it from the power tool  2 . 
     Each of the two unlocking buttons  50 ,  52  is joined in dustproof fashion to the surrounding edge of the detent recess  28 ,  30 , on the outside of the housing  56 , by a soft film  54  that is square in outline and is made of an elastomer material, such as a thermoplastic elastomer (TPE). The soft film  54  is glued or welded in its middle to the outside of the button  50 ,  52 , while its peripheral edge is glued or welded to the edge of the housing  56  surrounding the detent recess  28 ,  30 . In the undeformed state ( FIG. 2 ), the film  54  has an inward-protruding bead  58 , which protrudes into an indentation that extends around the buttons  50 ,  52  and is defined by diametrically opposed shoulders of the button  50 ,  52  and of the housing  56 . In the region of the bead  58 , the film  54  is reversibly elastically deformable, so that the unlocking buttons  50 ,  52 , with deformation of the bead  56 , can be pressed inward into the detent recesses  28 ,  30  and, because of the elastic restoring forces of the deformed film  54 , can automatically return to their outset position shown in  FIG. 2 , even if no locking bar  14  is pressing from the inside against the respective button  50 ,  52 . 
     On their inside, the unlocking buttons  50 ,  52  have a flat contact-pressure face  60 , which is diametrically opposite the free face end  20 ,  22  of the locking bar  14 . Upon an actuation of the unlocking buttons  50 ,  52 , the contact-pressure face  60  is pressed against the tip  36  of the adjacent locking bar  14  and, for disengaging its face end  20 ,  22  from the detent recess  28 ,  30 , displaces the locking bar inward in the locking bar guide  16 , counter to the force of the spring  18 , until the locking bar  14  strikes the abutment  44 , as shown in  FIG. 3 . In this position, the contact-pressure faces  60  are aligned with the adjacent side faces  38 ,  40  of the guide groove  10 , and the tip  36  of the locking bar  14  is aligned with the guide faces  24 ,  26  of the guide rail  8 , so that the battery pack  4  can be pulled off the power tool  2  in the direction of the arrow B. 
     Besides the detent recesses  28 ,  30 , the side faces  38 ,  40  each have one further recess  62 , which is located behind the detent recesses  28 ,  30  in terms of the insertion direction and which is engaged by the locking bars  14  of the front pair of locking bars, for relieving their compression springs  18  without locking, once the battery pack  4 , in the second stage, is locked in final fashion to the power tool  2 . 
     The term “battery pack”  4  as used within the scope of this application is meant to refer primarily to a pack of rechargeable current-storing means (accumulators), but also to a pack of disposable current-storing means (batteries). Moreover, instead of the geometry shown in the drawing for the connection between the battery pack  4  and the power tool  2 , some other geometry may be employed, such as a shaft in the power tool  2 , into which the battery pack  4  is thrust partway. Moreover, the described locking device  12  is suitable not only for locking battery packs  4  to power tools  2  but also for locking them to arbitrary other cordless electrical devices. 
     It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
     While the invention has been illustrated and described as embodied in a device for locking a power tool to a battery pack, and battery pack, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of reveal present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of the invention.
A device for locking a power tool to a battery pack has at least one movable locking bar mountable on the power tool and upon locking engaging a detent recess of the battery pack, and at least one unlocking device for manually disengaging the locking bar from the detent recess, wherein the unlocking device is mountable on the battery pack.