Efficiency Trade-off-Oriented Analysis for the integration of DC-DC Converter and Battery Pack in V2G Applications

Growing concerns about environmental aspects and clean energy generation call for effective solutions to reduce the demand for fossil fuels and greenhouse gas emissions. One solution of particular interest is the use of electric vehicles (EVs), which are considered one of the most favourable strategies for reducing pollution in the transport sector. Besides battery charging process, the EV battery pack can serve as an energy storage system (ESS) to support the grid, thanks to vehicle-to-grid (V2G) ancillary services. During the EV power exchange with the main grid, the overall efficiency depends not only on the converter but also on the battery. Typically, power converters, in particular dual-active-bridge (DAB) converters, present a low efficiency at light loads (i.e. low C-rates) and higher values at high power levels. The battery efficiency, on the other hand, decreases almost linearly as the power increases. Therefore, there is an optimum C-rate that could be selected to operate the converter and the battery when they are connected to the grid in order to minimize the losses of the overall system. In this context, the paper aims to analyze the trade-off between several designed lithium-ion battery packs and DAB efficiencies to find the best compromise. Both simulation and experimental results are presented to validate the correctness of the theoretical analyses, which also lead to an efficiency-focused design method for V2G applications.