Abstract: Electric vehicles (EVs) are developed rapidly due to the energy problem and the environment problem, and lithium-ion batteries play an important role in energy storage system of EVs. The characteristics of power lithium-ion batteries are closely connected to ambient temperature. The discharge capacity characteristic and the state of charge-open circuit voltage (SOC-OCV) curve are important parameters to represent the performance of power batteries and to design battery management systems (BMS). The experiments of 18650 cells and packs are carried out for the laws between SOC and OCV, also for the discharge capacity with different rates and ambient temperatures.