Abstract:
A battery pack includes at least one battery module comprising a plurality of unit cells stacked together; and at least one thermoelectric module on the at least one battery module, wherein the thermoelectric module may include a Peltier device having an input terminal configured to receive a polarity-convertible current.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0048185, filed on May 24, 2010, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to battery packs including secondary batteries used in electric cars or electric bicycles. 
     2. Description of the Related Art 
     Secondary batteries refer to batteries that may be rechargeable. The secondary batteries are the primary components for providing power to portable electronic appliances such as cellular phones, laptop computers, or camcorders. Also, secondary batteries are used as primary components for providing power to environment-friendly transportation devices such as electric cars or electric bicycles. 
     Unit cells of the secondary battery include an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator therebetween. The electrolyte is filled between the positive electrode plate and the negative electrode plate of the electrode assembly. Lithium ions contained in the electrolyte pass through the separator and move between the positive electrode plate and the negative electrode plate to thereby cause charging or discharging. During charging, electric energy is converted to chemical energy, and during discharging, chemical energy is converted to electric energy. During the energy conversion, heat is generated in the unit cells due to, for example, collision of lithium ions. 
     For high power output and large capacity of an electric bicycle or an electric car, a plurality of unit cells of secondary batteries may be stacked horizontally and/or vertically. 
     In a battery module, heat may be generated in the unit cells during charging and discharging and also due to changes in the external environment or due to external impact. 
     If the heat is not efficiently discharged from each of the unit cells, the unit cells may be over-heated and thus the charging or discharging performance may be degraded, and at worst, the battery module may explode. Meanwhile, if the ambient temperature of the battery module is low, the output power may be low. 
     SUMMARY 
     One or more embodiments of the present invention provide battery packs with increased stability and power output characteristics by maintaining an appropriate temperature around unit cells of the battery packs. 
     According to an aspect of the present invention, a battery pack includes at least one battery module comprising a plurality of unit cells stacked together; and at least one thermoelectric module on the at least one battery module. In one embodiment, the at least one thermoelectric module includes a Peltier device comprising an input terminal configured to receive a polarity-convertible current. 
     The battery pack may also include a battery management system having a switching device configured to convert a polarity of a current applied to an electrode of the Peltier device and a temperature sensor, wherein the at least one thermoelectric module is configured to convert a polarity of a current applied to an electrode of the Peltier device based on a temperature measured by the temperature sensor. 
     In one embodiment, the battery pack includes a plurality of thermoelectric modules, wherein each of the thermoelectric modules is configured to operate independently. Further, each of the thermoelectric modules may include at least one thermoelectric device including a positive input terminal and a negative input terminal, a first heat transfer member contacting a first surface of the at least one thermoelectric device, and a second heat transfer member contacting a second surface of the at least one thermoelectric device. In one embodiment, a first thermoelectric module is located on a first end of the unit cells and a second thermoelectric module is located on a second end of the unit cells to fix the unit cells together. 
     Further, in one embodiment, the battery pack includes a plurality of heat pipes on the battery module and contacting the first heat transfer member and the battery module, wherein the heat pipes are accommodated in grooves on the first heat transfer member. The heat pipes may extend along the battery module in a first direction and in a second direction substantially perpendicular to the first direction. Additionally, the battery pack may be configured to power an electric bicycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a perspective view illustrating a battery pack according to an embodiment of the present invention; 
         FIG. 2  is a perspective view of an interior of the battery pack of  FIG. 1 ; 
         FIG. 3  is a partially exploded perspective view illustrating the battery pack of  FIG. 1 ; 
         FIG. 4  is a partially exploded perspective view illustrating a battery pack according to another embodiment of the present invention; 
         FIG. 5  is a cut-away perspective view illustrating a battery pack according to still another embodiment of the present invention; 
         FIG. 6  is a partially exploded perspective view illustrating a battery pack according to another embodiment of the present invention; 
         FIG. 7  is a schematic view illustrating an operational principle of a thermoelectric device according to an embodiment of the present invention; 
         FIG. 8  is a perspective view illustrating a thermoelectric device according to an embodiment of the present invention; 
         FIG. 9  is a circuit diagram illustrating a switching circuit of a battery management system that converts a polarity of a current applied to a thermoelectric module according to an embodiment of the present invention; and 
         FIG. 10  illustrates a direction in which heat is discharged in a unit cell including an electrode assembly according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
       FIG. 1  is a perspective view illustrating a battery pack  1  according to an embodiment of the present invention. The battery pack is surrounded by or substantially encompassed by a housing  5 . For example, at least two battery modules  11  may be mounted in the battery pack  1  and then substantially covered by the housing  5 . Descriptions hereon will describe one battery module  11  but it will be understood that all of the battery modules  11  can be substantially similar. A plurality of unit cells of the battery module  11  are stacked in a first direction. In addition, a heat sink  14  may protrude from each of two lateral sides of the battery module  11  in the first direction. When the battery pack  1  is installed in an electric bicycle, the heat sink  14  may be directly exposed to outside air and thus if cold air passes around the heat sink  14 , the battery pack  1  is cooled efficiently. 
     In one embodiment, an extension direction of heat dissipation pins  14   a  of the heat sink  14  may vary according to a position and/or an orientation of the battery pack  1  of an electric product, for example, an electric bicycle. For example, the extension direction of the heat dissipation pins  14   a  may be set as close as possible to the direction in which outside air may pass through the battery pack  1 . Thus, when the electric bicycle is moving forward, air may actively dissipate heat and thus may effectively cool the battery pack  1 . 
     Further, if the battery pack  1  is attached in such a way that it is not directly exposed to outside air, the heat sink  14  may be oriented in such a way that it does not protrude outside of the housing  5 . In this case, a heat dissipation fan can be additionally attached to the heat sink  14 , and outer air holes may be formed in a portion of the housing  5  to discharge warm air transferred by the heat dissipation fan. 
       FIG. 2  is a perspective view illustrating the battery pack  1  of  FIG. 1 . 
     The battery module  11  includes a plurality of unit cells  11  a stacked in the first direction, that is, a length direction. The unit cells  11   a  are electrically connected in series and/or parallel according to the purpose of the battery pack  1 . 
     Thermoelectric modules  12 ,  13 , and  14  ( FIGS. 2 and 3 ) are located on each of the two lateral sides of the battery module  11  in the longitudinal direction. Hereinafter, the sides of the battery module  11  in the longitudinal direction (i.e., at each end of the battery module) will be referred as first side portions. The thermoelectric modules  12 ,  13 , and  14  may be connected to a side plate  15  and an upper plate  16 . 
     Accordingly, by compressing the stacked unit cells  11   a  between the thermoelectric modules  12 ,  13 , and  14  on opposing first side portions, the stacked unit cells  11  a may be fixed to each other. The thermoelectric modules  12 ,  13 , and  14  on the opposing first side portions may be connected to each other by using the side plate  15  and/or the upper plate  16 . That is, the thermoelectric modules  12 ,  13 , and  14  may function as end plates. 
     In one embodiment, the thermoelectric modules  12 ,  13 , and  14  may be located on only one of the sides of the battery module  11 . Further, the thermoelectric modules  12 ,  13 , and  14  may be connected only to the side plate  15 , only to the upper plate  16  and/or also connected to a lower plate. 
     The thermoelectric modules  12 ,  13 , and  14  according to an embodiment of the present invention will be described in detail with reference to  FIG. 3 . 
     In the current embodiment, the thermoelectric modules  12 ,  13 , and  14  include a Peltier device  13 , a first heat transfer member  12 , and a second heat transfer member  14 . 
     The first heat transfer member  12  is formed of a material having good heat conductivity. The first heat transfer member  12  has a flat surface in order for a first surface of the Peltier device  13  to be located thereon. The first heat transfer member  12  also includes a curved side portion  12   a  and an curved upper portion  12   b  that are respectively formed on a boundary of a side surface and a boundary of an upper surface of the first transfer member  12  and are each bent at substantially a right angle. A screw hole  12   aa  corresponding to a screw hole  15   a  formed in the side plate  15  may be formed in the curved side portion  12   a . A screw hole  12   ba  corresponding to a screw hole  16   a  formed in the upper plate  16  may be formed in the curved upper portion  12   b . Also, a curved portion  12   c  that supports another battery module  11  may be formed in an upper portion of the first heat transfer member  12  so that another battery module  11  may be mounted on the first heat transfer member  12 . 
     The second heat transfer member  14  may be the heat sink  14  described above. An inner surface of the heat sink  14  is flat and contacts a second surface of the Peltier device  13 , and the heat dissipation pins  14   a  may be thin and may be formed on an outer surface of the heat sink  14  to increase its surface area. 
     During assembly, the Peltier device  13  is inserted between the first heat transfer member  12  and the second heat transfer  14  to contact them both. Accordingly, the Peltier device  13  is in surface contact with the first heat transfer member  12  and the second heat transfer member  14 , thereby forming a heat transfer path. 
     The thermoelectric modules  12 ,  13 , and  14  are maintained in contact with the first side portions of the battery module  11  by being screw-coupled to the side plate  15  and the upper plate  16  via screws  17  and  18 , respectively. 
     The Peltier device  13  is operated according to the principle as shown in  FIG. 7 . The Peltier effect refers to a phenomenon where end portions of two different metals are bonded and a current is applied thereto, causing heat to be absorbed in one bonding portion, and heat to be generated by the other bonding portion. Metals  13   d  and  13   c  are respectively bonded to ends of an n-type semiconductor  13   b  and a p-type semiconductor  13   a  while having a metal  13   e  located between the n-type semiconductor  13   b  and the p-type semiconductor  13   a , with a direct current circuit being connected to the metals  13   d  and  13   c . Bonding portions serially formed between the metal  13   e , the n-type semiconductor  13   b , and the metal  13   d  may each be a Peltier device, and bonding portions serially formed between the metal  13   e , the p-type semiconductor  13   a , and the metal  13   c  may each be a Peltier device. All the elements together may form the Peltier device  13 . A current may be applied to each bonding portion. When free electrons flow from the metal  13   e  to the n-type semiconductor  13   b , electrons on the Fermi level need to transfer to a conduction band of the n-type semiconductor  13   b  and thus the average movement energy of the electrons in the bonding portion between the n-type semiconductor  13   b  and the metal  13   e  is increased to absorb heat from the surroundings. When free electrons flow from the semiconductor  13   b  to the metal  13   d , the movement energy of the electrons is reduced and thus heat is dissipated to the surroundings, thereby generating heat. If a current is applied to the two metals  13   d  and  13   c  in opposite directions, heat generation and heat absorption occurs in the same direction. 
     That is, when a positive voltage and a negative voltage are applied to the metal  13   d , that is, a left input terminal  13   d , and the metal  13   c , that is, a right input terminal  13   c , heat is absorbed from an upper surface  13   g  and then the absorbed heat is dissipated onto a lower surface  13   f  as illustrated by solid arrows in  FIG. 8 . On the other hand, when a negative voltage and a positive voltage are respectively applied to the left input terminal  13   d  and the right input terminal  13   c , heat is absorbed from the lower surface  13   f  and heat is dissipated onto the upper surface  13   g  as illustrated by dotted arrows in  FIG. 8 . 
     As described above, by converting a polarity of a current applied to the left and right input terminals  13   d  and  13   c  of the Peltier device  13 , heat absorption or heat generation may be controlled to selectively occur using the same current. For example, referring to  FIG. 3 , when the battery module  11  is to be cooled, a polarity of current to be applied is adjusted to be such that heat absorption occurs on the first surface of the Peltier device  13  contacting the first heat transfer member  12 , and heat is generated at the surface of the Peltier device  13  contacting the second heat transfer member  14 . Accordingly, the heat generated in the battery module  11  is quickly and efficiently transferred to the heat sink  14  and discharged outside therethrough. On the other hand, if the battery module  11  is to be heated, a polarity of current to be applied is adjusted to be such that heat is generated at the surface of the Peltier device  13  contacting the first heat transfer member  12 , and heat absorption occurs on the surface of the Peltier device  13  contacting the second heat transfer member  14 . Accordingly, heat around the battery module  11  is taken and transferred to the battery module  11  to heat the battery module  11 . 
       FIG. 9  is a circuit diagram illustrating a switching circuit of a battery management system that may convert a polarity of a current applied to a thermoelectric module. The battery management system may be installed inside the housing  5 . Meanwhile, the thermoelectric module may be formed of one Peltier device  13 , but may also be formed of a plurality of Peltier devices  13  as illustrated in  FIG. 9 . 
     Positive input terminals of the Peltier devices  13  may be connected to each other, and negative input terminals of the Peltier devices  13  may be connected to each other. When a third switch SW 3  and a fourth switch SW 4  are opened and a first switch SW 1  and a second switch SW 2  are shut, a positive voltage is applied to the positive (+) input terminal of the Peltier device  13 , and a negative voltage is applied to the negative (−) input terminal of the Peltier device  13 . On the other hand, when the first switch SW 1  and the second switch SW 2  are opened and the third switch SW 3  and the fourth switch SW 4  are shut, a negative voltage is applied to the positive (+) input terminal of the Peltier device  13 , and a positive voltage is applied to the negative (−) input terminal of the Peltier device  13 . Accordingly, in the above-described manner, the thermoelectric module may be controlled in such a way that heat generation and heat absorption selectively occur. 
     The switching circuit illustrated in  FIG. 9  is exemplary and other equivalent electric circuits may also be formed, as is well known to one of ordinary skill in the art. 
       FIG. 4  is a cut-away perspective view illustrating a battery pack  110  according to another embodiment of the present invention. 
     In the battery pack  110 , heat pipes  115   a ,  115   b , and  115   c  are further included as closed pipes. An operating fluid in the heat pipes  115   a ,  115   b , and  115   c  is transferred from a high temperature portion to a low temperature portion through phase change cycles, and then to the high temperature portion again. The operational fluid in the heat pipes  115   a ,  115   b , and  115   c  dissipates heat absorbed in the high temperature portion to the low temperature portion through the phase change cycles. The heat pipes  115   a ,  115   b , and  115   c  have about 40 times higher efficiency than copper in terms of heat transfer. 
     The heat pipes  115   a ,  115   b , and  115   c  may be, for example, vacuum-sealed containers having a porous wick installed on an inner wall thereof and have a small amount of liquid operating fluid saturated in the porous wick. A container evaporation unit in the heat pipes  115   a ,  115   b , and  115   c  is a vapor path of the operating fluid. The heat pipes  115   a ,  115   b , and  115   c  may be formed of three elements, for example, an evaporation unit, a condensation unit, and a heat insulation unit. When heat is applied to the evaporation unit, the heat is absorbed by the liquid and the heat is used as evaporation heat to evaporate the liquid into a vapor, and the vapor is transported to the condensation unit. In the condensation unit, heat is dissipated, and vapor is condensed into a condensate and then absorbed into the wick. The condensate may return to the evaporation unit due to a difference in capillary pressures formed at a gas-liquid interface of the wick between the condensation unit and evaporation unit. As the operating fluid goes through phase change cycles, heat may be transported from the evaporation unit to the condensation unit without requiring external power. 
     The heat pipes  115   a ,  115   b , and  115   c  are located along a first side portion and a second side portion of a battery module  111 . For example, the heat pipe  115  may be bent at substantially a right angle at a corner of the battery module  111  and thus a first end portion of the heat pipe  115   a  may be located on the first side portion of the battery module  111 , and a second end portion of the heat pipe  115   a  may be located on the second side portion of the battery module  111 . A plurality of the heat pipes  115   a  may be separately located along a height direction of the battery module  111 . The heat pipes  115   a ,  115   b , and  115   c  may be bent at each corner of the battery module  111 . 
     In one embodiment, the heat pipes  115   a  and  115   b  located along the second side portion may be connected to each other via a connection member  118 . Also, a coupling plate  119  may protrude from a curved portion of the heat pipes  115   a  toward thermoelectric modules  112 ,  113 , and  114 . A screw hole  119   a  formed in the coupling plate  119  and a screw hole  112   aa  that is formed in a curved side portion of the first heat transfer member  12  and corresponds to the screw hole  119   a  are aligned, and a screw  117  may be inserted into the screw holes  119  and  112   aa  and coupled thereto. 
     Accordingly, the thermoelectric modules  112 ,  113 , and  114  may function as end plates, and the heat pipes  115   a  and  115   b  may function as side plates. Thus, the battery module  111  may be firmly fixed and closely adhered to a surface of the battery module  111 , thereby facilitating heat transfer in the battery module  111 . 
     The heat pipes  115   a ,  115   b , and  115   c  may have a substantially circular inner cross-section. Alternatively, the heat pipes  115   a ,  115   b , and  115   c  may instead have an ovular inner cross-section. The heat pipes  115   a ,  115   b , and  115   c  having an ovular inner cross-section have a greater contact surface area for contacting a surface of the battery module  111  than the heat pipes  115   a ,  115   b , and  115   c  having a circular inner cross-section, and thus may further improve heat transfer performance. 
     Also, grooves  112   g  having a shape corresponding to the cross-sections of the heat pipes  115   a ,  115   b , and  115   c  may be formed on the surface of the first heat transfer member  12  contacting the battery module  111 . Accordingly, the contact surface between the heat pipes  115   a ,  115   b , and  115   c  and the first heat transfer member  12  is maximized, thereby further promoting heat transfer. 
     While varying according to the type of the battery module  111 , usually more heat is generated in the second side portion than the first side portion of the battery module  111  because, as illustrated in  FIG. 10 , opened upper and lower portions of an electrode assembly  111   b  mounted in each unit cell  111   a  are directed toward the second portion of the unit cells  111   a . That is, heat generated in the electrode assembly  111   a  is easily transferred to the opened upper and lower portions of the electrode assembly  111   b , and thus an amount of heat Q 1  that comes out to the second side portion of the unit cell  111   b  may be the greatest. 
     Consequently, the second side portion of the battery module  111  illustrated in  FIG. 4  is warmer than the first side portion thereof. Furthermore, heat absorption by the thermoelectric modules  112 ,  113 , and  114  occurs on the first side portion of the battery module  111 , and thus a difference in temperatures of the first side portion and the second side portion of the battery module  111  increases. 
     Thus, the heat pipes  115   a  may quickly transfer the heat generated in the second side portion, which is a high temperature area of the battery module  111 , to the first side portion, which is a low temperature area of the battery module  111 , and thus the heat may be efficiently discharged out of the first side portion through the thermoelectric modules  112 ,  113 , and  114 . Thus, the battery module  111  may be efficiently and easily cooled. 
       FIG. 5  is a cut-away perspective view illustrating a battery pack  210  according to another embodiment of the present invention. 
     The battery pack  210  is different from the battery pack  10  of  FIG. 3  in that thermoelectric modules  222  and  224  are further disposed on a second side portion of a battery module  211 . A first heat transfer member  212  located on the second side portion may be coupled to a side plate  215 , for example, by screw-coupling. 
     As described above, the second side portion of the battery module  211  usually generates more heat than a first side portion thereof. Thus, by locating the thermoelectric modules  222  and  224  on the second side portion, heat dissipation may quickly occur in the entire battery module  211 . 
     Likewise, if the ambient temperature of the battery module  211  is low and is to be increased, and if heat that is generated in the thermoelectric modules  212 ,  214 ,  222 , and  224  and transferred to an electrode assembly in the battery module  211  is provided to the second side portion, the heat is more effectively transferred to the electrode assembly. Accordingly, the temperature of the battery module  211  may also be quickly increased. 
     In one embodiment, when the thermoelectric modules  212 ,  214 ,  222 , and  224  are located not only along the first side portion but also the second side portion of the battery module  211 , the thermoelectric modules  212 ,  214 ,  222 , and  224  may be independently cooled or heated. An optimum temperature is thereby distributed over the entire battery module  211 , thus improving the performance of the battery pack  210 . To this end, the thermoelectric modules  212 ,  214 ,  222 , and  224  each include a temperature sensor and operate based on a measured temperature value. 
       FIG. 6  is a cut-away perspective view illustrating a battery pack  310  according to another embodiment of the present invention. 
     The battery pack  310  is different from the battery pack  110  of  FIG. 4  in that thermoelectric modules  322  and  324  are also located on a second side portion of a battery module  311 . 
     The thermoelectric modules  322  and  324  installed on the second side portion of the battery module  311  may operate when it is difficult to reduce a temperature of the second side portion of the battery module  311  using heat pipes  315   a ,  315   b , and  315   c  only. Also, to increase a temperature of the second side portion of the battery module  311 , the thermoelectric modules  322  and  324  may operate to transfer heat absorbed from outside. Methods of operating the thermoelectric modules  322  and  324  installed on the second side portion of the battery module  311  described above are exemplary and may be modified according to necessity, as is well known in the art. 
     In one embodiment, thermoelectric modules as described above may also be located only on the second side portion of a battery module  311 . In this embodiment, a portion of a heat sink of the thermoelectric modules installed on the second side portion protrude from the housing and thus be naturally air-cooled. 
     The above-described battery packs  10 ,  110 ,  210 , and  310  may be used in electric bicycles, but are not limited thereto. For example, battery packs may be used in electric cars. In this case, the battery packs may be modified as described above to be used in the electric cars. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.