The state of health (SOH) is a crucial indicator of lithium-ion batteries. A battery cycle and calendar life are critical for electric vehicle batteries. Complex interactions occur between the SOH and internal resistance of a battery. In this study, several ternary lithium-ion battery charge discharge experiments were performed to investigate the effects of the ambient temperature, discharge rate, and depth of discharge on a battery's internal resistance. An SOH prediction model was then constructed and used to evaluate the remaining capacity of the electric vehicle battery. The model was verified through various experiments, and a comparison of experimental and model-derived data revealed a favorable agreement. Thus, the model accurately predicted the SOH of a ternary lithium-ion battery.