Abstract:
A battery charger for simultaneously charging a plurality of rechargeable batteries for example of AA or AAA size. The invention allows such batteries to be fully charged in up to about half the time previously considered to be possible without damaging or destroying the batteries. It has been found that by mounting the batteries on the base of a charger such that they are substantially exposed and spaced apart, an average charging current of up to about twice that previously considered to be possible can be applied without overheating the batteries.

Description:
BACKGROUND 
     The present invention relates to a charger for simultaneously charging a plurality of rechargeable batteries. 
     Many portable electronic products use rechargeable batteries as their energy source. Examples of such batteries are of the AA or AAA size, for example of the Nickel Metal Hydride (NiMH) type. In charging a rechargeable battery, electrical energy, that is an electrical current is passed through the battery in a reverse direction to the discharge current direction and is converted into chemical energy internally of the battery, which process is accompanied by generation of heat energy causing a rise in the temperature of the battery being charged, both internally and on its surface. As the battery becomes more fully charged, the internal chemical reaction slows and more of the electrical energy input is converted into heat energy. This continues until, at full charge, the chemical reaction effectively ceases and virtually all of the electrical energy input is converted into heat energy, which stage is manifested by a significant increase in the surface temperature of the battery. This heat generation during charging limits the rate at which a battery can be charged and thus the time taken to fully charge a battery. An International Electrotechnical Commission (IEC) “standard charging current” for rechargeable batteries specifies a charging current of 0.1 I t  (where I t  is the reference test current specified in IEC International Standard 61436) that will take a long time, for example 16 hours, to fully charge a rechargeable battery and thus avoids problems associated with heat generation during the charging process. Consumers in today&#39;s world, however, are seeking to have their electronic products and thus the batteries that energise them available for the maximum amount of time possible and are thus requiring shorter and shorter charging times for their rechargeable batteries. Shorter charging times are achievable by increasing the charging current. However the greater the increase in the charging current, the greater is the heat generation problem, which can be so great as to destroy a battery. 
     In a current charger for AA and AAA sized batteries, a minimum charging time of approximately one hour is possible. Generally, charging times that are shorter than this carry a high risk of causing damage to or destroying the battery concerned because of the heat generated by the increased charging current necessitated by the shorter charging time. This problem is worsened in a charger for simultaneously charging a plurality of batteries, for example four batteries, because there is greater heat generation from the plurality of batteries than from one battery due to a localisation of the batteries to meet size and cost limitations for the charger for consumer acceptance. 
     An object of the present invention is to provide a charger of consumer acceptable size and cost for simultaneously charging a plurality, for example four, rechargeable batteries that allows a significantly increased charging rate and thereby much shorter charging time than is generally currently achievable for chargers of comparable size and cost. 
     SUMMARY OF THE INVENTION 
     The present invention provides a charger for simultaneously charging a plurality of rechargeable batteries, each battery having a nominal charging current as determined by its capacity rating in milliamp hours at one hour, the charger comprising 
     a base, 
     a charging circuit contained within the base and connectable to a power supply, 
     the base including means for individually mounting thereon a plurality of re-chargeable batteries, the mounting means for each battery including contacts connected to the charging circuit for contacting the terminals of the battery for passage of a charging current through the battery, 
     wherein the mounting means for each battery are located relative to the base and to each other such that a plurality of batteries mounted on the base are substantially exposed and spaced apart whereby the plurality of batteries are chargeable without over-heating with an average charging current of up to about twice their nominal charging current. 
     It has been discovered that within consumer acceptance constraints of size and cost for a charger, a plurality of rechargeable batteries can simultaneously tolerate a charging current of about twice that which is commonly considered to be maximally appropriate for a fast charge via an equivalently sized charger, provided the plurality of batteries can be adequately cooled. It has furthermore been discovered that such adequate cooling can be achieved without significantly changing the physical size of a charger compared to a said equivalently sized charger or unacceptably increasing the cost as by the addition of a cooling fan accessory. This cooling is achievable when the batteries are individually substantially exposed to ambient cooling air and are substantially spaced apart, that is, when they are maximally spaced apart according to the dictates of the shape of the base. For example, for a base that is of rectangular parallelepiped shape of substantially 120×80×34 mm, four batteries for simultaneously charging can each be aligned along a top edge of the base instead of lying parallel to each other across a top surface as in prior art chargers. Recesses, each opening to a top and adjacent side surface of the base, can be provided at each top edge for accommodating the batteries and ensuring exposure of each battery at the top and adjacent side surfaces for cooling via ambient air flow. Additionally each recess may be provided with a slot extending from a bottom surface of the slot and opening to a bottom surface of the base, thereby further exposing a battery in the recess to cooling ambient air flow. For a base of generally cylindrical or frustoconical shape for example, four batteries for simultaneous charging can be held in an upright orientation and spaced equally around the outer curved surface of the base. This arrangement also facilitates the use of mounting means for the batteries, for example brackets or clips, for holding the batteries off the curved outer surface, that is for spacing the batteries from the curved outer surface by a small amount for further exposure of the batteries for cooling. 
     Preferably the batteries are chargeable with an average charging current greater than 1.0 I t  and up to about 2.0 I t , where It is the reference test current defined in paragraph 2 of IEC Standard 61434:1996, that is:            I   t        A     =         C   n        A                 h       1                 h                              
     where 
     I t  is the reference test current, in amperes (A); 
     C n  is the rated capacity of the cell or battery as declared by the manufacturer, in ampere-hours (Ah); 
     n is the time base (hours) for which the rated capacity is declared. 
     According to the invention, the charging current may vary in a predetermined manner during the charging process. For example, the charger may charge at 4.0 I t  for 10 minutes and then at 1.0 I t  for 20 minutes. 
     For a better understanding of the invention and to show how it may be carried into effect, preferred embodiments thereof will now be described by way of non-limiting example only with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a battery charger according to a first embodiment of the invention. 
     FIG. 2 is a view similar to that of FIG. 1 showing batteries accommodated within the charger for charging. 
     FIG. 3 is an underneath view of the charger of FIG.  1 . 
     FIG. 4 schematically illustrates another embodiment, and 
     FIG. 5 schematically illustrates a third embodiment. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A battery charger  10  according to a first embodiment of the invention (see FIGS. 1 to  3 ) comprises a base  11  which is of rectangular parallelepiped shape. An ideal size for the base is 120×80×34 mm. The base  11  is a two-part casing formed by a top part  12  and a bottom plate  13  held together by appropriate screws, as is known (holes  15  for such screws can be seen in FIG.  3 ). The top part  12  and bottom plate  13  may be formed by moulding a suitable plastics material such as ABS (Acrylonitrile Butadiene Styrene). The top part  12  of base  11  has four rounded upper edges  14 ,  16 ,  18 ,  20 . The upper edges  14 - 20  are defined by the rounded transitions between a top surface  22  of the base  11  and, respectively, four side surfaces  24 ,  26 ,  28 ,  30  of the base  11 . The base  11  also has a bottom surface  32 , that is, the lower facing surface of bottom plate  13 —see FIG. 3. A charging circuit, schematically indicated by (but not necessarily bound by) dotted outline  34  in FIG. 1, is included within the base  11  and is connectable to a suitable power supply either built-in or via a socket  35 . As is known, a suitable power supply may be provided from a mains power supply via a power cord having a combined plug/transformer for connection to the mains at one end and a low voltage plug at the other end for connection to socket  35 . 
     The base.  11  includes means for individually mounting on the base  11  four rechargeable batteries. Such means are contained within recesses  36 ,  38 ,  40 ,  42  at, respectively, each upper edge  14 ,  16 ,  18 ,  20 . Each of the recesses  36  to  42  is open to the top surface  22  and to a respective side surface  24 ,  26 ,  28  or  30  of the base  11 . The means for mounting include a rest or seat  44  (best seen in recess  36 ), raised above a bottom surface  45  or floor of each recess  36  to  42  for supporting an AA sized battery at its side adjacent an end face of the battery that includes the positive terminal. The means for mounting furthermore includes another generally semi-circular seat  46  (best seen in recess  38 ) for locating and supporting an AA size (or MA size) battery at its side adjacent an end face of the battery that includes the negative terminal. A contact  50  is located adjacent rest  44  by a wall  52  of each recess and an opposite contact  47  is located adjacent the seat  46  of each recess for an AA sized battery to be located therebetween. The contact  47  is shaped to have a convexly curved portion  48  directed towards the contact  50  to provide a spring bias on the battery facing end (that is, on the negative terminal end of the battery). This curved portion  48  of contact  47  urges the battery towards the other contact  50 . Thus each recess  36  to  42  contains oppositely located contacts  47  and  50  between which a rechargeable AA sized battery is locatable, resting upon rest  44  and seat  46 . The rest  44  in each recess  36  to  42  extends towards the contact  47 - 48  a short distance to a substantially vertical wall  54  (best seen in recess  36 ) which extends to the bottom surface  45  of each recess  36  to  42 . Wall  54  locates another contact  56  for the positive terminal of a AAA sized battery, wherein the negative end of the battery also contacts the contact  47 - 48 , which biases the AAA sized battery towards the contact  56 , the AAA sized battery for charging lying at an angle within a recess resting on the seat  46  and the bottom surface  45  of each recess  36  to  42 . FIG. 2 illustrates an AA sized battery  57  located for charging in recesses  38  and  42  and a AM sized battery  59  located for charging in recesses  36  and  40 . The batteries  59  are located at an angle due to the different heights of seat  46  near contact  47 - 48  and floor surface  45  near contact  56 . Thus each recess  36  to  42  of the battery charger  10  is formed to accommodate either an AA or an AAA sized battery for charging. 
     The base  11  of charger  10  furthermore includes a number of slots  60 , each of which extends from the bottom surface  45  of a respective recess  36  to  42  through to the bottom surface  32  of the base  11  and preferably also opens to the adjacent side surface  24 ,  26 ,  28  or  30  of the base  11  as indicated by references  62  (best seen in FIG.  3 ). The base  11  also includes four pads  64  on its bottom surface  32  generally near the corners of such surface such that when placed on a bench top for example, the bottom surface  32  of base  11  will be spaced away a small distance from such bench top. This spacing allows for improved ambient air circulation through the slots  60  thus further exposing the AA or AAA sized batteries contained in the recesses  36  to  42  for cooling. The top part  12  of base  11  is formed with vertically extending channels  66  along a rear wall of each of the recesses  36 ,  38 ,  40  and  42  which further assist convective air flow and thus cooling of the batteries  57  and  59 . The bottom plate  13  of base  11  also includes slots  68  passing therethrough for circulation of ambient air through the interior of base  11  for cooling the charging circuit. 
     As is known, the charger  10  also includes a small lamp  70 , which is for example green, to indicate that power to the charger  10  is on, and a small lamp  72  for each charging position, which are for example red, to indicate that a battery is being charged, which lamps  72  may be arranged to blink or be off when the associated battery has reached full charge. Also, given charging circuits for such chargers are well known by persons skilled in this field, further detailed description of the charging circuit  34  is unnecessary apart from noting that it may be a constant current charging circuit providing four series connected charging stations for simultaneously charging four batteries. It may also include monitoring and control circuitry for isolating one or more of the batteries when they become fully charged whilst the others continue to be charged. Clearly the lamps  72  would be operated by such monitoring and control circuitry. 
     FIG. 4 schematically illustrates a charger  80  having a base  82  of frustoconical shape. Base  82  may be of two-part construction (not shown) similar to base  11  of charger  10  of FIGS. 1 to  3 , thereby having a hollow interior for containing the charging circuitry (not shown). The curved outer surface  84  of base  82  includes means thereon in the form of brackets  86  and  88  arranged in pairs and between each pair of which a battery  90  for charging is removably locatable. The pairs of contacts  86  and  88  of which there may be, for example four pairs, are evenly spaced around the curved surface  84  and they are shaped so as to hold a battery  90  in position a small distance away from surface  84 . Thus the batteries  90  are substantially exposed and spaced apart for cooling whilst being charged. 
     Each of the brackets  86  provides a rigid mount for an associated electrical contact  92 . An opposite electrical contact for each battery is provided by the opposite bracket  88  as such of each pair. Each bracket/contact  88  may be a metal strip extending as a cantilever from the curved surface  84 , thereby providing a spring structure, and having a curved portion  94  for contacting the negative terminal end of a battery  90 . Thus each bracket/contact  88  biases an associated battery  90  towards the opposite contact  92 / 86  structure. As is known, the contacts  88  and  92  are associated with the charging circuit within base  82  for a charging current to be passed through a battery  90  located therebetween. 
     The charger  80  will include means (not shown) as known for supplying power to its charging circuit from a suitable power supply and may include other features, such as slots (not shown) for passage of air through the base  82  for cooling the charging circuit, or indicator lamps as for the charger  10  of FIGS. 1 to  3 , as are also known. Furthermore the means for individually mounting the batteries  90  may take various forms. For example, annular ledges around curved surface  84  may be provided which locate and support appropriately positioned contacts. Although charger  80  as illustrated is for accommodating batteries of one size, for example AA or AAA, it may be structured to accommodate both sizes of battery. This may be provided for example by each contact  88  being carried by a slide having two indent positions, whereby the contacts  92  and  88  may be spaced to accommodate an AA sized battery or may be more closely spaced (by sliding contact  88  on its slide towards contact  92 ) to accommodate an AAA sized battery. 
     FIG. 5 schematically illustrates a further embodiment of a charger  100  according to the invention having a base  102  of cylindrical shape. Otherwise the charger  100  is similar to the charger  80  of FIG.  4  and thus corresponding componentry has been correspondingly referenced. 
     For a battery having a rated capacity of 1800 mAh (milliamp hours) which means it takes one hour to fully charge or discharge the battery at a current of 1800 milliamps, 30 minute charging requires a charging current of 3.6 amps. For a battery of rated capacity 2100 mAH (currently the maximum rating for AA type NiMH batteries) 30 minute charging requires a charging current of 4.2 amps. Experiments have shown that it is possible with an embodiment of the present invention as in FIGS. 1-3 to charge such batteries to rated capacity within 30 minutes without damaging the batteries. For example, for 1800 mAh NiMH rechargeable batteries, according to these experiments, the average temperature rise under a charging current of 3.6 amps is reduced by about 5 to 10° C. when compared with a prior art charger. The design of a charger as in FIGS. 1 to  3  also provides the advantage of easy installation and removal of the batteries from the charger. 
     The experimental results were as follows: 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Conditions - Batteries AA size 1800 mAh (NiMH type) 
               
               
                 Charging current: 3.6 A 
               
               
                 Charging time: 30 mins 
               
               
                 Charger size: 120 × 80 × 34 mm 
               
               
                 Ambient temp: 25° C. 
               
               
                   
               
             
             
               
                 Prior Art Layout of batteries (i.e. side by side) 
               
             
          
           
               
                   
                 Battery No. 
                 A 
                 B 
                 C 
                 D 
               
               
                   
                   
               
               
                   
                 Battery Temp ° C. 
                 68 
                 72 
                 72 
                 68 
               
               
                   
                   
               
             
          
           
               
                 Layout of batteries as in FIG. 1 
               
             
          
           
               
                   
                 Battery No. 
                 A 
                 B 
                 C 
                 D 
               
               
                   
                   
               
               
                   
                 Battery Temp ° C. 
                 62 
                 62 
                 62 
                 62 
               
               
                   
                   
               
             
          
         
       
     
     Each of the battery temperatures shown in the above tables is the average of the temperatures taken on a number of test batteries after each has been charged for 30 minutes. 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.