Among series-connected cells in large battery packs, such as those found in electric vehicles, a charge imbalance develops over time due to manufacturing and temperature variations. Therefore, active balancing strategies can be employed in Battery Management Systems (BMSs) to attain a charge balance among cells by transferring charge between them, maximizing the usable capacity of the battery pack. Recently, decentralized BMS architectures with smart battery cells have been developed, in which balancing strategies can operate by local cooperation between the cells without requiring global coordination. In this paper, we propose a decentralized active balancing strategy for smart cells where we identify boundary cells having special properties. These boundary cells enable to divide the global balancing problem into independent subproblems, where local decisions on charge transfers eventually converge to a globally balanced battery pack. The proposed strategy is implemented in a simulator framework and compared with two decentralized state-of-the-art strategies. Our results show significantly improved performance and scalability of the proposed strategy in terms of charge transfer losses and communication overhead between cells, while maintaining a comparable time to balance.