In this study, we demonstrate first time the application of confinement tests under excessive heating to track the thermal responses during the thermal runaway in hard prismatic lithium-ion batteries used in smart phones. Seven hard prismatic lithium-ion batteries used in smart phones of iPhone 5, iPhone 6, Redmi 2, SAMSUNG Note 3, SAMSUNG S5, SONY C3 and SONY Z3 at full-charged state have been studied. Characteristics in relation to thermodynamics such as onset temperature, crucial temperature, maximum self-heat rate and maximum temperature are measured and assessed. SAMSUNG S5 shall carry the most unstable feature with an exothermic onset temperature as low as 117 °C. SAMSUNG Note 3 displays the worst-case scenario by possessing the maximum temperature and maximum self-heat rate reaching the extremity of 675.6 °C and 11,860.0 °C min−1.