Active Balancing Management And Efficiency Optimization For Lithium-ion Batteries

The battery enerey storage system plays significant roles in renewable erengy generation units. This paper mainly focuses on the load distribution control strategy to improve the efficiency of paralleled micro-converters in the distributed lithium-ion battery energy storage system and the active balancing architecture for series connected lithium-ion batteries.In order to solve the problems in the existing active balancing system, a hierarchical battery balancing architecture for the series connected batteries is proposed. The battery cells are grouped into different packs and the bottom layer is the Adjacent Cell-to-Cell structure consisting of the packs. The top layer is connected to different packs and can deliver the energy from one pack to any other pack bi-directionally, leading to high flexibility. A multi-directional multi-port converter is proposed to serve as the top layer. With the hierarchical architecture, the balanced energy transfer of the cells in different packs can be decoupled, which avoid the repeated charging and discharging during the balancing process. This is beneficial for lengthening the battery lifetime and increasing the SOH. Moreover, the proposed architecture can lower the current rating of the balancing circuits, which helps decrease the required cost and improve the system efficiency. The experimental results verified the effectiveness of the proposed architecture. Compared to A-C2 C architecture, the proposed architecture can decrease 27.6% of the balancing time and 44.0% of the energy loss during the balancing process. Compared to the balancing methods in other references, the proposed architecture can reduce the current density and the cost of the balancing circuits to 37.5% and 11.5% respectively.This paper analyzes the efficiency problem in the load distribution of the paralleled micro-converters system. A universal loss model is proposed to establish the analytical function between the loss and the input/ output condition directly under the system level. Based on the model, a self-adaptive load distribution control strategy is proposed and it can distribute the load of different converters dynamically to follow the minimum total loss point in wide range, so that the system-level efficiency can be optimized. An experimental testing platform consisting of three 200 W high voltage gain LLC micro-converters in parallel was built to verify the benefits of the proposed control strategy. The experimental results showed that the system efficiency was improved in wide load range and the peak efficiency improvement of the system is 10.1%.