$\eta _{max}$-Charging Strategy for Lithium-Ion Batteries: Theory, Design, and Validation

This paper introduces a charging strategy for maximizing the instantaneous efficiency ( $\eta _{max}$ ) of the lithium-ion (Li-ion) battery and the interfacing power converter. A theory based on the trade-off between several designed Li-ion battery packs and dual-active-bridge (DAB) converter efficiencies is established to find the best compromise. The proposed framework enables Vehicle-to-Everything (V2X) functionality for an electric vehicle providing energy services. Typically, power converters, in particular DAB converters, present low efficiency at light loads and higher values at high power levels. On the other hand, the battery efficiency decreases linearly as the current increases. Therefore, an optimum C-rate could be selected to operate the converter and the battery to minimize the overall losses. Both simulations and experimental results are presented to validate the correctness of the theoretical analyses. The implementation and the experimental results of the $\eta _{max}$ -charging strategy are explained, by showing superior performance compared to conventional CC and CP charging strategies while preserving the material lifetime compatibility.