Patent Publication Number: US-2007099073-A1

Title: Cell connection straps for battery cells of a battery pack

Description:
PRIORITY STATEMENT  
      This application claims the benefit under 35 U.S.C. §119(e) to the following U.S. Provisional Patent Applications: Ser. No. 60/731,501, filed Oct. 31, 2005 to Daniel J. White et al. and entitled “FUSE PROTECTION FOR BATTERY PACKS HAVING LI-ION CELLS IN PARALLEL”; and Ser. No. 60/836,396, filed Aug. 9, 2006 to Steven J. Phillips et al. and entitled “WELD STRAP IMPROVEMENTS FOR BATTERY CELLS”. The entire contents of each of these provisional applications are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Example embodiments in general relate to battery cell connection straps for electrically connecting battery cells in a battery pack.  
      2. Description of Related Art  
      As cordless power tools begin to push new, higher power to density (mass and volume) limits, new battery chemistries are being investigated as a viable power sources for high-power (such as above 18V) cordless power tool applications. Such battery packs should provide much higher power but not add significantly to total tool system weight. Conventional battery packs for power tools include NiCd, NiMH and lead acid battery packs.  
      Another cell chemistry that has been conventionally used for powering lower voltage devices (such as cell phones and laptops) and lower voltage power tools, such as screwdrivers) are cells having a lithium-ion (Li-ion) chemistry. A challenge of using Li-ion cells in a much higher-power environment, such as in a battery pack for supplying power to cordless power tools having current draws at multiple orders or magnitudes in excess of that of a typical cell phone or laptop (0.1-1A)), is that Li-ion cells may pose a safety issue for the end user at these higher currents. In the Li-ion industry, the issue of concern is referred to as “thermal runaway”. A thermal runaway condition is caused by overheating of a Li-ion battery cell (either due to a malfunction or discrepancy/damage in the cell itself or due to some external source causing the damage or malfunction in the cell) until a reaction occurs inside the cell. The reaction causes the release of not only the stored energy in the cell, but also the chemical energy in the lithium metal and the fuel energy of the flammable electrolyte. These reactions happen quickly (on the order of seconds) and have caused several recalls of products and/or injuries to end users.  
      A strategy used by Li-ion cell manufacturers in order to release safer Li-ion battery packs into the market is to utilize smaller cells (lower capacity) in the design of a battery pack used in higher power electrical devices such as power tools. These smaller cells each contain less energy than corresponding larger cells. Thus, in a condition where a given cell would generate heat due to some external damage, for example, a smaller cell would produce less heat and be less likely to heat to a point where the cell goes into thermal runaway. Also, using smaller cells in parallel would cut the current going to each cell in half relative to a single serial string battery pack, thus further reducing heat generation.  
      Using smaller cells (less capacity) has proven safer in the industry and has been used in several power tool applications. There is a drawback to lower capacity cells, however. Capacity is a major request from a power tool customer and directly correlates to the amount of run time a cordless power tool will provide to the end user.  
      In an effort to address this concern, some manufacturers are using serial strings of smaller cells in parallel or parallel strings of series cells. This does not solve the safety problem; however, because when one cell is in parallel with another, it is possible to have a condition where one cell becomes damaged and the energy of the other “good” cell is dumped into the damaged cell. With the exception of minor losses in cell connection straps which connect the battery cells in parallel together, this condition would be the same as having one large cell, hence an equal likelihood of reaching a thermal runaway condition.  
      In addition to thermal runaway concerns, another concern is that the cell straps in a battery pack designed for higher-power applications such as those performed by a cordless power tool can become disconnected from the cells due to pack vibrations during tool use and/or a drop or impact of the battery pack, which is foreseeable in a power tool environment. In a battery pack for a cordless power tool having multiple cells, these cell connection straps connect the battery cells together in series and/or parallel. The cell straps are typically attached to the tops of cell housings which enclose the battery cells. This housing that contains the cell is referred to as a “can”.  
      These welds can be particularly weak when the cell can is constructed with a highly electrically conductive material such as aluminum. During a drop of the battery pack, as would be seen in a power tool environment, these welds tend to fail. A common failure mechanism is that the weld joint breaks when it is stressed as the cells move relative to one another within the power tool battery pack. The rigid cell straps that connect adjacent cells translate all of the relative motion (and therefore stress) to the relatively weak weld joint, causing the cell strap to become disconnected or detached from the cell cans within the pack.  
     SUMMARY OF THE INVENTION  
      An example embodiment of the present invention is directed to a cell connection strap for electrically connecting a pair of adjacent battery cells within a battery pack. The strap includes a body having a length extending between a first and second end and a width between sides thereof. A fuse link is disposed in the body between the first and second ends.  
      Another example embodiment is directed to a battery pack. The pack includes a housing and a plurality of cells disposed in the housing and configured in a serial-parallel arrangement within the housing. The pack includes a plurality of cell connection straps for electrically connecting pairs of adjacent battery cells within a battery pack. A cell strap connecting a pair of cells in parallel in the pack includes a fuse link therein.  
      Another example embodiment is directed to a cell connection strap for electrically connecting a pair of adjacent battery cells within a battery pack. The strap includes a body having a length extending between a first and second end and a width between sides thereof. The body includes a plurality of raised features formed thereon between the first and second ends.  
      Another example embodiment is directed to a battery pack which includes a housing and a plurality of cells disposed in the housing and configured in a serial-parallel arrangement within the housing. The pack includes a plurality of cell connection straps for electrically connecting pairs of adjacent battery cells within a battery pack. Each cell strap has a body terminating between first and second ends. A cell strap connecting a pair of cells in parallel in the pack includes a fuse link and a plurality of raised features formed in its body between the first and second ends.  
      Another example embodiment is directed to a cell connection strap for electrically connecting a pair of adjacent battery cells within a battery pack. The strap includes a body having a length extending between a first and second end and a width between sides thereof. The strap includes a first pair of slits, with each slit of the first pair formed in a lengthwise direction of the body at a corresponding end. The strap includes a second pair of slits formed in a widthwise direction of the body between the first and second ends, with one slit formed in each side of the body.  
      Another example embodiment is directed to a battery pack having a housing and a plurality of cells disposed in the housing and configured in a serial-parallel arrangement within the housing. The pack includes a plurality of cell connection straps for electrically connecting pairs of adjacent battery cells within a battery pack. Each cell strap comprises a body having a length extending between a first and second end and a width between sides thereof, a first pair of slits and a second pair of slits. Each slit of the first pair is formed in a lengthwise direction of the body at a corresponding end of the body. The second pair of slits are formed in a widthwise direction of the body between the first and second ends, with one slit of the pair formed in each side of the body.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The example embodiments of the present invention will become more fully understood from the detailed description given herein below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limitative of the example embodiments of the present invention.  
       FIG. 1  is a top view showing cell connection straps within a battery pack in accordance with an example embodiment.  
       FIG. 2A  is a top view of a cell connection strap for cells of a battery pack in accordance with another example embodiment.  
       FIG. 2B  is a side view of the cell strap in  FIG. 2A .  
       FIG. 3  is a top view of a cell connection strap for cells of a battery pack in accordance with another example embodiment.  
       FIG. 4  is a top view of a cell connection strap for battery cells of a battery pack in accordance with another example embodiment.  
       FIG. 5  is a top view of a cell connection strap having a fuse link in accordance with another example embodiment.  
       FIG. 6  is a top view of a cell connection strap having a fuse link in accordance with another example embodiment.  
       FIGS. 7A-7H  are top views of cell strap variations in accordance with the example embodiments. 
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS  
       FIG. 1  is a top view showing cell connection straps within a battery pack that has a pair of serial strings of cells  120  connected in parallel. Power terminal leads are not shown for purposes of clarity, it being understood that a pair of power leads may be connected to cells  120  on an opposite side of the pack  100 , in place of a given cell strap  110  between two cells, as is known. In  FIG. 1 , there is shown only a bottom housing of a battery pack  100  (top housing removed for clarity) containing a plurality of cells  120  in two serial strings that are connected in parallel. This is just an example connection scheme, as battery pack  100  may be configured to have additional cells  120  in series or parallel. The cells  120  may be oriented sideways within the housing of pack  100  instead of upright as shown in  FIG. 1 .  
      In one example, the battery pack  100  may include high power density (mass), Li-ion cells  120 , which have a much higher power density as compared to NiMH or NiCd cells. The example embodiments are not so limited, however, as the cells  120  may have a NiMH, NiCd, lead acid or another lithium or lithium Ion chemistry such as Lithium sulfur dioxide in alternative embodiments. A battery cell is typically housed in what is referred to as a can. The can represents the outer shell of the cell  120  and is typically steel or aluminum, for example but cold be made of a variety of other materials, alloys, foils or polymers.  FIG. 1  shows top ends of the cans housing cells  120 . To increase the power density (mass) even further, aluminum cans may be used for cells  120  to reduce the weight of the cell, instead of steel cans, which conventionally have been used for enclosing such cells  120 . For ease of description, it is understood each cell  120  is contained within a metal can.  
      A typical battery cell  120  in pack  100  may be configured as having a separator provided between a positive electrode (cathode) and negative electrode (anode) in a spiral round configuration, which is an electrode structure of high surface area created by winding the electrodes and separator into a spiral-wound, jelly-roll configuration. In an example battery pack for supplying power to cordless power tools, these cells can be configured as cylindrical cells having a jelly roll configuration.  
      In  FIG. 1 , there is shown a plurality of cell straps  110  electrically connecting adjacent cells  120 . Each cell strap  110  may be embodied as a metallic strip having a body  111  terminating at ends  113  and  114 . End  113  makes contact with a terminal on one cell  120  and end  114  makes contact with a terminal on a second adjacent or adjoining battery cell  120  for making an electric connection between the two adjacent cells  120 , as is known. Thus, each cell strap  110 / 110 A has a length that is sufficient to make connection, both physically and electrically, with two adjacent cells  120 .  
      In an example, the cell straps  110 ,  110 A may be formed of a material having desirable electrical conducting characteristics. One example material is a nickel 200 or 201 alloy, although other materials such as copper, aluminum, steel, etc.. Clad and plated materials such as nickel clad copper can also be used, which combine the desirable qualities of one material with the desirable qualities of the other. In this example, the nickel allows for good weldability, whereas the copper give superior current carrying capability. could be used as the material for cell straps  110 ,  110 A.  
      Connection may be a series electrical connection or a parallel electrical connection, connecting terminals of adjacently positioned cells  120  within pack  100 . In the example of  FIG. 1 , pairs of serially-arranged cells  120  are connected by cell straps  110  and parallel-arranged cells  120 A and  120 B are connected by cell strap  110 A. The arrangement of serial strings of cells  120  in parallel as shown in the pack  100  of  FIG. 1  provides a total voltage that is increased over the voltage of the individual cells.  
      Physical connection between cell straps  110  or  110 A and individual cells  120  may be accomplished by resistive, or ultrasonic welding to connect a terminal on the can surface of a cell  120  to a strap  110 / 110 A at an interface referred to as weld joint  118 , as is known in the art. In  FIG. 1 , the weld joint  118  is shown generally by the dotted line circles  118  beneath straps  110 / 110 A. In an alternative, laser welding may be desirable to weld cell straps  110 ,  110 A at the weld joint  118  to aluminum cans enclosing cells  120 . While electrically connecting the cells  120  in the example serial-parallel arrangement as shown in  FIG. 1 , cell straps  110  and  110 A also physically connect adjoining cells  120 .  
      Each cell strap  110 / 110 A can have a centrally located slit  112  at each end  113  and  114  thereof dividing a positive electrode leg  117  from a negative electrode leg  119  of the strap  110 . Alternatively, a slit can be provided across a width of the strap  110  between the points on the strap  110  where the welds are to be made. For example, slits  112  can be formed in the middle of the strap  110 / 110 A.  
      As shown in  FIG. 1 , the cell strap  110 A between two cells  120 A and  120 B in parallel includes a fuse link  115 . The fuse link  115  is characterized as being a substantially narrowed or thinning portion at the center of body  111  of the cell strap  110 A between the two parallel-connected cells  120 A and  120 B. If desired, strap  110 A could additionally replace standard cell connection straps  110  between serially-connected cells.  
       FIGS. 5 and 6  illustrate cell straps with alternative fuse link designs. In  FIGS. 5 and 6 , only the cell strap is shown for purposes of clarity, it being understood that the cell strap configurations shown in  FIGS. 5 and 6  are applicable to pack  100  in  FIG. 1 .  
      As shown in  FIG. 5 , the fuse link  115  alternatively can be embodied as a conventional fuse  115 ′ attached between metal end portions  113  and  114  of an alternative cell strap  110 A′ construction. The metal end portions are composed of an electrically conducting material such as nickel or an alloy thereof, for example. As shown in  FIG. 6 , fuse link  115  further can be embodied as a positive temperature coefficient (PTC) device  115 ″ attached between end portions  113  and  114  of another alternative cell strap  110 A″ construction, for example. Fuse  115 ′ and PTC device  115 ″ can be attached to end portions  113 ,  114  via resistive, ultrasonic or laser welding, for example.  
      By configuring cell strap  110 A to include a fuse link  115 , if one cell (in this example cell  120 B (or vice versa) is damaged, the other, undamaged cell  120 A that is connected thereto will begin to quickly dump its energy into the damaged cell  120 B. This, the current will climb in the fuse link  115  to blow open fuse link  115 , thus preventing the energy from being transferred from the “good” cell, in this example cell  120 A, to the damaged cell  120 B. Accordingly, a thermal runaway condition in a damaged cell  120  of pack  100  can be avoided.  
      Similarly, if a fuse  115 ′ is provided between end portions  113  and  114  of alternative strap  110 A′, as cell  120 A in the example pack  100  of  FIG. 1  goes to quickly dump its energy into the damaged cell  120 B, the current will climb in the fuse  115 ′ to blow the fuse  115 ′, thus preventing the energy from being transferred from the cell  120 A to the damaged cell  120 B. A thermal runaway condition is thus avoided.  
      If instead of using a narrowed width cell strap  110 A between two cells in parallel as shown in  FIG. 1  or a fuse  115 ′ between strap end portions  113  and  114  of a cell strap  110 A′ that connects cells  120 A and  120 B, as shown in  FIG. 5 , a PTC device  115 ″ can be attached between end portions  113  and  114  at the center of the alternative cell strap  110 A″ configuration shown in  FIG. 6 . PTC devices are devices whose resistance increases as the temperature of the device increases. For example, if cell  120 B is damaged and adjacent, undamaged cell  120 A begins to dump its energy into its parallel-connected adjoining cell  120 B, the PTC device  115 ″ formed centrally in the alternative cell strap  110 A″ configuration between the cells  120 A,  120 B increases in resistance to limit the current to cell  120 B and avoid and/or possibly prevent a thermal overload condition.  
      Although strap  110 A and its fuse link variants are described as between parallel-connected cells  120 A and  120 B in  FIG. 1 , strap  110 A can replace conventional straps  110  between serially-connected cells  120 . Thus, all cell straps connecting cell in series or parallel within pack  100  can be embodied as cell strap  110 A and its variants in  FIGS. 5 and 6 .  
      In one particular example, cell straps  110 A″ with PTC device  115 ″ therein can be used as a fuse link between cells  120  in series, in addition to between parallel-connected cells  120  within pack  100 . Power PTC devices are being developed that can placed in series with cells  120  within battery pack  100   
       FIGS. 7A-7H  illustrate alternative cell strap configurations. The cells straps shown in  FIGS. 1, 5  and  6  may alternatively have no slits therein, or slits which are formed in the sides or at a middle of a strap. Further, an example fuse link (narrowed strap portion, fuse or PTC device) may be formed on a side of the strap, or off-center closer to one side of the cell strap and hence closer to one of the cells  120 . For example, the fuse link  715  in  FIG. 7A  is formed in a cell strap  710 A which has no slits. In  FIG. 7B , slits  712  are shown formed in the sides of strap  710 A rather than the ends.  FIGS. 7C and 7D  show a fuse link  715  that is formed at an end of a cell strap  710 A, with centrally located slits  712  and without slits  712  therein.  
       FIGS. 7E and 7F  show example cell strap variants of  FIG. 5 . In  FIG. 7E , fuse  715 ′ is attached closer one cell (not shown) between a shorter portion  713  and a longer end portion  714  of cell strap  710 A′. Fuse  715 ′ is shown on a side of cell strap  710 A′ in  FIG. 7F , with slits  712  formed within a middle portion of cell strap  710 A′.  
       FIGS. 7E and 7F  show example cell strap variants of  FIG. 5 . In  FIG. 7E , fuse  715 ′ is attached closer one cell (not shown) between a shorter portion  713  and a longer end portion  714  of cell strap  710 A′. There are no slits  712  in the cell strap  710 A′ of  FIG. 7E . Fuse  715 ′ is shown on a side of cell strap  710 A′ in  FIG. 7F , with slits  712  formed within a middle portion of cell strap  710 A′.  
       FIGS. 7G and 7H  show additional example cell strap variants of  FIG. 6 . In  FIG. 7G , PTC device  715 ″ is attached closer one cell (not shown) between a shorter end portion  714  and a longer end portion  713  of cell strap  710 A″. There are slits  712  formed in the middle of cell strap  710 A″, instead of at the ends.  FIG. 7H  illustrates a similar cell strap  710 A″ but with no slits formed therein, and with PTC device  715 ″ attached off-center between end portions  713  and  714 . These additional cell strap variants are merely exemplary, other configurations would be evident to one of ordinary skill in the art.  
      In addition to being configured as described above in order to avoid or prevent a thermal overload condition in one of the cells, it is also desirable to reduce the rigidity of the cell straps  110  and/or  110 A so as to avoid a failure at a weld joint between a given strap  110  and the can surface of its connected cell  120 , should the weld joint become stressed as the cells move relative to one another within the power tool battery pack due to pack vibration and or a drop or impact of the pack.  
       FIG. 2A  is a top view of a cell connection strap for cells of a battery pack in accordance with another example embodiment, and  FIG. 2B  is a side view of the cell strap in  FIG. 2A . The strap configuration shown in  FIGS. 2A and 2B  is applicable as a substitute configuration for one or more of the straps  110 / 110 A shown in the pack  100  of  FIG. 1 . In  FIG. 2A , the weld joint is generally indicated by element  218 , and the cell  120  is not shown for purposes of clarity. In  FIG. 2A , cell strap  210  has a body  211  terminating at ends  213 ,  214 . Each end  213 ,  214  has a centrally located slit  212  dividing a positive electrode leg  217  from a negative electrode leg  219  of the strap  210 . Cell strap  210  includes a plurality of crumple zones (indicated generally at  216 ) formed between the slits  212  in the ends of the strap  210  in a third dimension.  
      As best shown in  FIG. 2B , the crumple zones are characterized as one or more raised features  216  which can be stamped into the cell strap  210  during manufacture. The raised features  216  extend across the entire width of the strap  210  so as to be perpendicular to the slits  212  and extend vertically upward from a plane of the strap  210  (shown by “X” in  FIG. 2B ). The inclusion of these raised features  216  permit for movement of the strap  210  along the strap plane X. Introducing these features  216  in strap  210  helps the strap  210  to divert stresses away from the more fragile welds at weld joint  218  at the interface between strap  210  and a can surface of cell  120 . These stresses can be induced by cell movement relative to one another within the battery pack  100  due to pack vibration and or a drop or impact of the pack.  
      Although strap  210  is described as attached between parallel-connected cells  120 A and  120 B in  FIG. 1 , strap  210  can replace conventional straps  110  between serially-connected cells  120 . Thus, all cell straps connecting cell in series or parallel within pack  100  can be embodied as cell strap  210 .  
      Of course, the location of slits  212  and raised features  216  in  FIGS. 2A and 2B  is merely exemplary. These locations can be varied in a somewhat similar fashion as shown in  FIGS. 7A  to  7 H. Slits  212  can be formed in the middle or sides of cell strap  210 , or no slits  212  may be formed therein. In another example, the raised features  216  may be formed at ends of strap  210 , and multiple raised features  216  (&gt;2) can be formed across the width of cell strap  210 , with or without slits  212  formed therein.  
       FIG. 3  is a top view of a cell connection strap for cells of a battery pack in accordance with another example embodiment.  FIG. 3  combines the crumple zones shown in  FIGS. 2A and 2B  with the fuse link shown in  FIG. 1  (and alternative fuse link embodiments  115 ′ and  115 ″ in  FIGS. 5 and 6 ) to provide a cell strap that has a reduced rigidity and which also is configured to prevent the occurrence of a thermal runaway condition in a connected cell  120 . In  FIG. 3A , the weld joint is generally indicated by dotted line circle  318 , and the adjoining cells  120  are not shown for purposes of clarity. The strap configuration shown in  FIG. 3  is thus applicable as a substitute configuration for one or more of the straps  110 / 110 A shown in the pack  100  of  FIG. 1 .  
      The cell strap  310  has a body  311  terminating at ends  313  and  314 . In this particular example, each end  313 ,  314  has a slit  312  formed in a lengthwise direction of the body  411  for dividing a positive electrode leg  317  from a negative electrode leg  319  of the strap  310 . Of course, the location of slits  312  can be in the sides or middle of cell strap  310 , as shown in the cell strap variants of  FIGS. 7A  to  7 H.  
      As shown in  FIG. 3 , raised features  316 , which provide crumple zones, are formed across the width of the cell strap  310 . In this particular example, the raised features  316  are shown formed between the slits  312  of the strap  310  and the central narrowed portion of body  311  which forms the fuse link  315  in the body  311  of strap  310 . The raised features  316  may be formed as shown and described in  FIGS. 2A and 2B .  
      In one example, parallel connected cells  120 A/ 120 B in  FIG. 1  may be connected by strap  310  in  FIG. 3 , and serially connected cells  120  in  FIG. 1  may be connected by straps  210  as shown in  FIGS. 2A and 2B . In another example, each of the cells  120 ,  120 A and  120 B may be connected by straps  310  shown in  FIG. 3 . Thus, all cell straps connecting cell in series or parallel within pack  100  can be embodied as cell strap  310 .  
      In another example, cell strap configurations  110 A′ and  110 A′ in  FIGS. 5 and 6  (alternative fuse link designs) may include the raised features  216 / 316  formed across the width of straps  110 A′,  110 A″ on each end portion  113 ,  114  between the slits  112  and corresponding centrally located fuse link (which could also be fuse  115 ′ of  FIG. 5  or PTC device  115 ″ of  FIG. 6 ).  
      Of course, the location of slits  312 , fuse link  315  (i.e., narrowed portion of body  311 ) and raised features  316  in  FIG. 3  is merely exemplary. These locations can be varied in a somewhat similar fashion as shown in  FIGS. 7A  to  7 H. The narrowed portion of body  31  forming the fuse link could be formed off-center or at one of the ends of strap  310 . Further, as noted above a fuse  115 ′ or PTC device  115 ″ in  FIGS. 5 and 6  may be substituted for the fuse link  315  shown in  FIG. 3 . Slits  312  can be formed in the middle or sides of cell strap  310 , or no slits  312  may be formed therein. In another example, the raised features  316  may be formed at ends  313 ,  314  of strap  310 , and/or multiple raised features  316  (&gt;2) can be formed across the width of cell strap  310 , with or without slits  312  formed therein. Such variations would be evident to one of ordinary skill in the art.  
      Accordingly, the cell strap  310  of  FIG. 3  can divert stresses away from more fragile welds at the weld joints  318  that are induced due to pack vibration, drop or impact, while also reducing the likelihood of a thermal runaway condition occurring in a damaged cell connected thereto.  
       FIG. 4  is a top view of a cell connection strap for battery cells of a battery pack in accordance with another example embodiment. Instead of employing a fuse link as shown in  FIGS. 1 and 3 , singly or in combination with the reduced rigidity provided by the raised features  216 / 316  in  FIG. 3 , the cell straps  110  and/or  110 A of battery pack  100  in  FIG. 1  can be configured in another construction which also enhances deformation of the strap under stress to protect the welds at weld joints  418 , while acting as a fusible link between cells  120  to avoid a thermal runaway condition in a damaged cell  120  that is connected thereto.  
      In  FIG. 4 , the cell strap  410  includes a body  411  terminating at ends  413 ,  414  with a slit  412  formed lengthwise in the body  411  in each end  413 ,  414 , dividing a positive electrode leg  417  from a negative electrode leg  419  of the strap  410 . However, the body  411  of strap  410  includes a second pair of slits  416 , one slit formed on each opposite side along the length of strap  410 . Slits  416  are formed between the slits  412  that are formed in the ends  413 ,  414  of body  411 . The second pair a slits  416  are formed in a widthwise direction of the body  411 , generally in a central area or portion of the body  411  of strap  410  so as to be perpendicular to slits  412 . The slits  416  of the second pair are offset from one another, with a portion of the body  411  between the offset slits  416  forming a fuse link in the strap  410 .  
      The inclusion of slits  416  decreases the overall rigidity of the strap  410 . The slits  416  provide crumple zones where the strap  410  can more easily deform to allow for relative movement between the cells  120  without translating the stress to the welds at weld joints  418 .  
      Additionally the inclusion of slits  416  enables the cell strap  410  to be used as a fusible link, as the area of the body  411  between the offset slits  416  is reduced. Thus, in the scenario described in the example of  FIG. 1 , as an undamaged cell  120 A that is connected to strap  410  begins to quickly dump its energy into the damaged cell  120 B, the current will climb in the portion of strap body  411  between the slits  416  to blow open that portion of the strap  410 , thus preventing the energy from being transferred from the undamaged cell  120 A to damaged cell  120 B. This may prevent a thermal runaway condition in a damaged cell  120  or  120 A/B of pack  100  from occurring. Accordingly, so long as a desired fuse-blow current is significantly higher than the operating current, this type of fusible link in strap  410  of  FIG. 4  may provide desired reliability.  
      The example embodiments of the present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the example embodiments of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims herein.