This paper presents a hierarchical balancing algorithm for use in large scale reconfigurable AC battery packs. Using a decentralised Battery Management System (BMS) the pack is organised in control layers with each layer controller responsible for a small number of slave controllers, thereby significantly reducing communication and centralised processing requirements. The hierarchical balancing algorithm uses all available voltage steps in the pack ensuring a high quality sinusoidal output voltage while balancing the objects of different layers. Balancing priority can be swapped dynamically between layers according to the maximum SoC variation between the objects of each layer while also ensuring that the output voltage reference is always met even when a cell failure occurs, as long as there are an adequate number of cells available in the pack. An experimental demonstration of the proposed balancing algorithm is presented using an AC battery pack comprising 144 20Ah lithium titanate cells. Cell balancing to within 2 mV of the cell open circuit voltage is achieved between cells with 20% initial State of Charge variation and 5% capacity difference.
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