Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. As a result, kinds of applications using the secondary battery are being increased owing to advantages of the secondary battery, and hereafter the secondary battery is expected to be applied to more applications and products than now.
As kinds of applications and products, to which the secondary battery is applicable, are increased, kinds of batteries are also increased such that the batteries can provide outputs and capacities corresponding to the various applications and products. Furthermore, there is a strong need to reduce the sizes and weights of the batteries applied to the corresponding applications and products.
For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, and laptop computers, use one or several small-sized, light cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, medium- or large-sized devices, such as electric bicycles, electric vehicles, and hybrid electric vehicles, use a battery module (or battery pack) having a plurality of cells electrically connected with each other because high output and large capacity is necessary for the medium- or large-sized devices. The size and weight of the battery module is directly related to the receiving space and output of the corresponding medium- or large-sized device. For this reason, battery manufacturers are trying to manufacture small-sized, light battery modules. Furthermore, devices, which are subject to a large number of external impacts and vibrations, such as electric bicycles and electric vehicles, require stable electrical connection and physical connection between components constituting the battery module. In addition, a plurality of cells are used to accomplish high output and large capacity, and therefore, the safety of the battery module is regarded as important.
Generally, a medium- or large-sized battery module is prepared by mounting a plurality of unit cells in a case (housing) having a predetermined size and electrically connecting the unit cells. Rectangular secondary cells or pouch-shaped secondary cells, which can be stacked with high integration, are used as the unit cells. Preferably, the pouch-shaped cells are used as the unit cells, since the pouch-shaped cells are light and inexpensive.
FIG. 1 is a typical plan view illustrating a conventional pouch-shaped cell 100. Referring to FIG. 1, the pouch-shaped cell 100 includes an electrode assembly (not shown), which comprises a cathode, a separation film, and an anode, mounted together with an electrolyte in a sealed pouch-shaped case 110, which is made of an aluminum laminate sheet. From the electrode assembly extends electrode taps, which protrude from the upper end of the cell to constitute plate-shaped electrode terminals 120 and 130. Alternatively, the plate-shaped electrode terminals 120 and 130 may be constituted by electrode leads attached to the electrode taps, which protrude from the upper end of the cell.
However, several problems are raised when preparing a battery module using the above-described cell as the unit cell.
Specifically, the mechanical strength of the cell sheath (case) is lower, and therefore, it is difficult to prepare a structurally strong battery module using a plurality of such cells. Also, the cell itself does not include a structure for coupling the cells to each other, and therefore, an additional coupling member is necessary to prepare the battery module.
In the conventional art, cells are mounted in a cartridge, which is capable of receiving one to three cells, a plurality of cartridges are stacked (piled) one on another, and the stacked cartridges are mounted in a module case to prepare a battery module. In other words, the plurality of cartridges and the module case are used to prepare the battery module including the plurality of cells. Consequently, the size of the battery module is increased, and a process for preparation of the battery module is complicated.
Also, it is difficult to electrically connect the plate-shaped electrode terminals in series or in parallel with each other. Consequently, a process for electrical connection between the electrode terminals is also complicated. Generally, the electrode terminals are connected with each other using wires, plates, or bus bars by welding. For this reason, the plate-shaped electrode terminals are partially bent, and the plates or the bus bars are connected to the bent parts of the plate-shaped electrode terminals by welding, which requires skilled techniques. Also, this connecting process is very complicated. In addition, the connected parts may separate from each other due to external impacts, which results in increase in the number of defective products.