Isothermal Calorimetry can aid in the design and performance of 18650 cells. Using the VariPhi 3D sensor in the ARC® 254, true isothermal tests were conducted on a Li-ion 18650 cell. Heat is measured directly using this technique, there is no need to assume or calculate a heat capacity. The blue “baseline” shown in the diagram represent the amount of heat to hold the cell isothermally at 50C. Endothermic behavior, highest during the start of the charge cycle can be see as heater power above the baseline, exothermic behavior, as seen in the discharge cycles is below the baseline. Isothermal testing gives you the heat-output as a function of time, temperature, and cycling conditions. This data can be used to calculate thermal loads and battery efficiency.