A combined preheating approach for fast charging of lithium-ion batteries at sub-zero temperatures

Fast charging of lithium-ion batteries at subzero temperatures is limited by increased polarization, reduced charge acceptance, and accelerated degradation. This study develops and validates a combined preheating framework for commercial 21700 NMC811/graphite cells by integrating adaptive internal pulse-current heating with external phase-change composite (PCC)-based heating. Experiments were conducted at initial SOC levels of 30%, 50%, and 80% under − 20 ° C ambient conditions. A control-oriented equivalent-circuit model, parameterized using hybrid pulse power characterization and electrochemical impedance spectroscopy data, was coupled with a three-dimensional thermal model to predict temperature rise, thermal non-uniformity, and energy demand. A voltage-constrained adaptive pulse-current strategy maximized internal heat generation while limiting the absolute SOC drift to within 0.43%. Combined preheating outperformed internal-only and external-only approaches across all SOC levels. At 30% SOC, the strategy raised the cell above 35 ° C in about 15 min, achieved a heating rate of 3.61 ° C / min , reduced preheating time by about 30%, and decreased the temperature difference from 2.5 ° C to 0.9 ° C . After 200 equivalent full cycles (EFC) at − 20 ° C , the combined-preheated cell retained 86% state of health, compared with 50% for the unheated cell.