Experimental Study on the Mechanism of Extra-Heat in High-Frequency Preheating of Lithium-Ion Battery at Low Temperature

Lithium-ion batteries suffer severe energy loss, significant pulse power decline, and reduced life cycle under cold weather. It is imperative to preheat the lithium-Ion batteries for electric vehicles (EVs) at low temperatures. At present, various internal alternative current (AC) heating methods have been proposed to achieve fast and high-efficiency battery preheating. However, there is still confusion in the selection of the AC frequency. Researchers have found that the simplified Bernard heat generation model is not always suitable for AC preheating, especially under high AC frequencies. Under these scenarios, a frequency-dependent heat (nominate “extra-heat”) is generated, making the traditional thermal model inapplicable. In this paper, the source of “extra-heat” generated under high frequency is analyzed experimentally. Based on the electrochemical impedance spectrum (EIS) data and entropy thermal coefficient of the cell in the experiment, controlled experiments in different frequencies are developed to compare the cell heat generation rate. The experiments showed that the heat generation rate is highly correlated to its frequency, and the “extra-heat” generation is not due to the Joule effect and electrochemical reaction, which are the two primary sources of heat generation in the Bernard's heat generation model