The automotive lithium-ion batteries suffer severe capacity and power degradation at subzero temperatures, leading to serious "range anxiety" of the electric vehicles (EVs). Therefore, the onboard battery preheating equipment is essential for EVs at cold climates. In this study, an interleaved resonant onboard battery self-heater is developed for internally preheating the automotive batteries without external power supplies, thereby providing great flexibility for EVs at different parking areas. By properly adjusting the switching frequency, the self-heater can achieve the zero-current-switching (ZCS) to improve the energy consumption and eliminate the voltage spikes during the turn-off. Meanwhile, a detailed guideline for optimizing the resonant tank parameters is presented so that the efficiency of the self-heater can be further developed by reducing the circulating current. The experimental validation on 18650 cells demonstrates the proposed heater can preheat the battery from -20°C to 0°C within only 3.5 minutes, and only consumes 5% of the cell energy.