A Hierarchical Active Balancing Architecture for Lithium-Ion Batteries

This paper proposes a hierarchical active balancing architecture for the series-connected lithium-ion batteries. The key point of the architecture is that by grouping the battery string into different packs and introducing the top layer, the coupled influence among the cells in different packs is eliminated, which reduces the required balancing time and the energy loss. The repeated charging and discharging problem is avoided, which is beneficial for increasing the state-of-health. Moreover, the proposed architecture can lower the current rating of the balancing circuits, which helps decrease the required cost and improve the system efficiency. On the basis of the architecture, a balancing control using the state-of-charge of each cell is proposed to achieve the balance for all the cells. Furthermore, to deliver the energy from one pack to any other pack bidirectionally, a multidirectional multiport converter along with the current control method is proposed to serve as the top layer. This converter is different from the conventional three-port converter as it can achieve the arbitrary current flow direction control for any number of ports. The experimental results based on 24 series-connected 200-Ah lithium-ion batteries verified the benefits of the proposed architecture and the balancing circuits. After balancing, 4.1% of the total energy of the battery string is saved. Compared to the conventional adjacent cell-to-cell architecture, the proposed architecture can decrease the balancing time and the energy loss during the balancing process by 27.6% and 44.0%, respectively. In addition, the current rating and the cost of the balancing circuits in the proposed architecture is only 37.5% and 11.5% of that in other references.