In this study, we report a three-dimensional thermal-fluid-coupled finite element model (FEM) for a lithium-ion-battery liquid-cooling system, explicitly incorporating solar irradiance as a boundary condition. The model is validated computationally and used to investigate the pack’s thermal behavior under solar exposure. Two designs of the cooling architecture, focusing on flow channel geometry and manifold configuration, are proposed to enhance thermal management. Numerical analysis shows that the designs significantly improve performance. At an ambient temperature of 35 ℃, the optimized design reduces the maximum and average volumetric temperatures by 40.91 and 48.64%, respectively, compared with a reference configuration. It also achieves a peak temperature reduction of 25.42 ℃ under 2C discharge. We conclude that solar heating can raise the casing temperature of a pack to 79 ℃, driving inward heat conduction, and that optimizing inlet geometry to shorten flow paths is effective in mitigating axial temperature gradients. Furthermore, we propose the present simulation methodology as a practical, time-efficient alternative to year-long outdoor aging tests for battery thermal and degradation evaluation, enabling accelerated testing with rationally selected environmental profiles.