Abstract: The cell inconsistency of series battery pack is one of the key issues in the group application of Li-ion batteries, which is currently often regulated by intervention of battery equalization techniques. However, existing equalization strategies mostly focus on the maximum utilization of single cycle capacity and energy, ignoring the consistent evolution of the battery pack over a long-time scale. In an attempt to achieve better battery pack cycle life, the influence of cycling state of charge ranges on battery aging is studied experimentally, the consistency evolution of battery pack under traditional top and bottom equalization is also discussed, and the life equalization strategy aiming to optimize the worst cell working range is proposed. It is discovered that the used Li-ion batteries have faster capacity fade when cycling under higher state of charge range and higher depth of discharge. Although the top equalization and bottom equalization can maximize the single cycle performance of the battery pack, the consistency continues to deteriorate, indicating the need for the life equalization. The proposed life equalization strategy allows the worst cell to cycle at the lowest working range, reducing its capacity decay rate, thus effectively increasing the accumulated discharge of the battery pack. Finally, experiments have confirmed that the proposed strategy can significantly improve the capacity consistency of battery pack, and a corresponding system implementation plan is designed. The life equalization strategy proposed also provides a new direction for future battery pack equalization research.

Key words: series battery pack, state of charge range, capacity fade, capacity consistency, life equalization