Series/Parallel Li-ion Battery Modules Active Equalisation Considering Load and No-Load Operation

Series and series/parallel connection of Li-ion modules is required to build Li-ion energy storage systems (ESS) up to the desired voltage level and energy capacity. Embedding several modules and the required electrical connections into a complete ESS is an endeavour engineering problem that raises the need for equalisation techniques. This paper proposes an equalisation method for Li-ion energy modules using a voltage-sharing technique under load and no-load conditions. The proposed system uses module-level dc/dc power converters for power exchange and equalisation. The proposed method is validated by simulations and experimentally, including the integration with the hierarchical control system typically used in ESS. The experimental validation is implemented using 2-connected series modules and 2-parallel strings, even though it can be easily extended to an arbitrary number of modules. The proposed no-load equalisation method allows a limited balancing capability even with the energy modules and the dc/dc converter disconnected from the subsequent power conversion stages, typically an inverter.