<p>Accurate measurement of battery heat generation is essential for optimizing battery thermal management systems. While small-capacity lithium-ion batteries are widely used in drones and portable electronic devices, limited research exists on their thermal characteristics, particularly under low-temperature conditions. In this study, the heat flux of a 5 Ah pouch-type lithium-ion battery was measured under different temperatures (−20 °C, 0 °C, 20 °C) and discharge rates (0.5C, 1C, 2C). The experiment was conducted using an isothermal immersion calorimeter, where the battery was submerged in a non-conductive liquid to maintain a stable thermal environment. The results indicate that the highest heat flux (657 W/m²) occurred at −20 °C and a 2C discharge rate, while the lowest (58 W/m²) was recorded at 20 °C and 0.5C discharge. These findings provide crucial insights for designing efficient thermal management systems for small lithium-ion batteries.</p>