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Preheating batteries is crucial to improve the performance and lifetime when using lithium-ion batteries in cold weather conditions. Even though the immersing preheating system (IPS) has demonstrated attracting advantages, there is still lack of systematical evaluation about its performance and factors affecting the performance. To bridge the knowledge gap, this work considered the following key performance indicators: the rate of temperature rise, the temperature uniformity of the cell and the pack and the energy storage density; and the influences of the inlet flow rate and inlet temperature of heat transfer fluid (HTF), the gap between the batteries, the number of the batteries and the location of the HTF inlet and outlet on the preheating performance were investigated. A 3D CFD model was developed, which has been validated against experiments. Based on simulations, it was found that the IPS can achieve a high rate of temperature rise, which is up to 4.18 °C/min, and a small temperature difference in the battery pack, which is less than 4 °C. The number of batteries has been identified to have the biggest impact on the rate of temperature rise and the uniformity of the battery pack. Allocating the inlet on the left/right faces of IPS can effectively reduce both maximum temperature difference of the cell and the pack.