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Using cell balancing to maximize the capacity of multi-cell Li-ion battery packs

Posted: 12 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:cell balancing? Li-ion battery packs? multi-cell battery?

Common multiple cell configurations for LiIon cells in battery packs consist of three or four cells in series, with one or more cells in parallel. This combination gives both the voltage and power necessary for portable computer, medical, test and industrial applications. While these configurations are common today, they may not be as efficient as they could be. The reason for this is that any capacity mismatch between cells in a series connection of cells results in a reduction of overall pack capacity.

There are two kinds of mismatch in the pack, state-of-charge (SOC) and capacity/energy (C/E) mismatch. Each problem limits the pack capacity (mAh) to the capacity of the weakest cell. It is important to recognize that the cell mismatch results more from limitations in process control and inspection than from variations inherent in the Li-ion chemistry. As such, these types of cell-to-cell variation more likely occur in Li-ion prismatic cells, due to more extreme mechanical stresses, and in Li-ion polymer, due to the newer processes involved.

The use of cell balancing can improve the performance of series connected Li-ion cells by addressing both SOC and C/E issues. SOC mismatch can be remedied by balancing the cell during an initial conditioning period and subsequently only during the charge phase. C/E mismatch can be remedied by balancing during both charge and discharge periods. Even though the defect level for a given cell manufacturer may be very low, the extra confidence that a pack will not be returned, because of an early end of life, may be well worth the extra effort. It provides another level of quality assurance.

Please view the PDF document for more information.

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