Rechargeable, secondary batteries such as lithium-polymer batteries, show great promise in providing power sources for a variety of applications such as portable electronic devices. Such batteries may be formed as a thin, somewhat rectangular form factor with large available power density. Once the battery is formed, it is then subjected to a formation or charging process.
Battery formation is the process in which secondary cells are energized to effectuate their capability to deliver electrical energy. Secondary cells and batteries are assembled with electronically inert chemical compounds. In order to energize the devices by converting the inert substances into electroactive species and to prepare them for service, one must provide an initial, slow charge called formation or forming. This process is normally executed in a two-phase process. First, a constant current is applied to the cell or battery up to a predetermined voltage limit. At that point, the voltage applied to the battery terminals is maintained at a constant value and the current trails off to a low value. Formation can then be terminated based upon a total number of ampere-hours input into the cell or group of cells or a current limit as the charging current decreases with the internal resistance of the cell.
While this procedure is implemented routinely for a wide range of battery chemistries, there are a number of associated shortcomings, particularly with cells based upon lithium chemistry. The first shortcoming relates to the overall system for connecting the cells to the charge apparatus for the formation process. Commonly, the cells being formed or charged are connected in series to one another as part of a single regulated circuit. In this configuration there can be no provision for treating a potentially weak cell without affecting, perhaps adversely, other cells in the circuit. Moreover, for large batches of cells, such as more than 100 cells or batteries, the voltage requirements may become extreme, and the power supply specifications prohibitively expensive and dangerous.
The second shortcoming of the standard formation process and instrumentation is the means of electrically and mechanically coupling a battery or cell to the formation circuit. Most lithium cells, especially lithium-polymer cells, are manufactured in custom form factors without standardized, rigidly defined dimensions, particularly in length and width. Also, the electrical contacts or terminals of the cells are generally flexible metallic foil tabs which are difficult to localize. That means that interfacing these cells with the formation circuit poses a particular challenge. Clamps can be used, but labor becomes intensive as the number of cells to be formed increases for a particular batch of cells. Connection through probes oriented normal to the plane of the terminal tabs is less laborious, but stabilizing the flexible strips becomes critical.
Thus, there remains a need for a lithium battery formation system in which large batches of cells or batteries may be formed and charged safely and effectively and in which the cells in the batch are more uniformly charged in the process. There is a further need for a system for safely and effectively connecting lithium batteries into the formation and charging system. The system should make the coupling of the cells into the system easy and thus eliminate the difficulties in the art of interfacing the cells, should makes registration of the cells in the system easy to accomplish, and should eliminate the need to stabilize the flexible strips of the battery cells.