•Controlled experiments to explore the source of frequency-dependent heat have developed.•The heat generation mechanism of lithium-ion batteries under high-frequency AC excitation is different from those under low-frequency.•Frequency-dependent heat partly due to current harmonics, and partly from an unknown mechanism. 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 Bernardi heat generation model is not always suitable for AC preheating, especially at high AC frequencies. Under these scenarios, a frequency-dependent heat is generated, making the traditional thermal model inapplicable. In this paper, the source of frequency-dependent heat 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. Further analysis show that the frequency-dependent heat is not due to electrochemical reaction, partly due to Joule effect by current harmonics, and partly from unknown mechanism (nominate “extra-heat”).