This paper presents a novel adaptive charging strategy for a three-phase bidirectional on-board electric vehicle (EV) charger. The charger system enables seamless grid-to-vehicle (G2V) and vehicle-to-grid (V2G) operations while enhancing battery longevity and grid compatibility. It integrates real-time battery state of charge (SOC) and temperature feedback, with a predictive first-order thermal model to dynamically adjust charging parameters, mitigating degradation. Featuring a three-phase LCL filter, an AC–DC converter and a buck-boost DC–DC converter, the charger employs proportional-integral (PI) control for the AC–DC converter and an adaptive controller for the DC–DC converter. It achieves low total harmonic distortion (THD) of 1.33% (G2V) and 1.7% (V2G), 98.3% efficiency and a 30% reduction in charging time (5 h for 20%–80% SOC) compared with conventional methods. MATLAB/Simulink simulations confirm robust performance under sensitivity analysis, demonstrating stability. The system’s unified control framework, combining SOC-based mode switching, thermal derating and harmonic suppression, outperforms existing methods, offering a scalable solution for smart grid integration and sustainable EV charging infrastructure.