Energy Storage System is increasing to grid system and ESS accidents are also increased. So, it need to analyze safety standard in ESS system. To find safety element of ESS system, this paper analyze lithium-ion battery internal resistance. Experiments are studied from 18650 battery and pouch type battery internal resistance with correlation of temperature. The experiment result show that temperature is increased and internal resistance of battery is decreased both 18650 battery and pouch type battery. This means that temperature is great effect of internal resistance. This paper installed temperature sensor in operating ESS site and monitoring data for 20 days. Experiment result is showed that variation of temperature is large between top position and bottom position. So, It needs to control temperature in energy storage system and maintain stable environment. This paper analyze correlation between internal resistance and temperature and it is important to control stable temperature. Future studies will analyze various environment element which effect to internal resistance.