In hydropower plants it is necessary to always have local power supply. Therefore, the plants are equipped with batteries as stationary back up power. Vattenfall Vattenkraft is using lead acid batteries but has been investigating alternatives to replace them. The aim of this study is to investigate the potential of using Li-ion technology as back up power. The study showed that Li-ion batteries have many qualities. The Li-ion technology will decrease the space and maintenance demand. The technology also eliminates many of the personal safety risks that lead acid batteries may cause. Unfortunately overcharging or overheating the Li-ion battery may result in fire or explosion. Those risks are reduced by an electronic battery management system and mechanical protection. The result from the cost comparison between the two battery technologies showed that the maintenance costs decreases with 70 % when using Li-ion battery. The purchase cost of Li-ion batteries is almost three times higher compared to lead acid batteries. But prices will probably drop drastically in the coming years. According to this study Li-ion has great potential to replace the lead acid battery.  Vattenfall Vattenkraft should wait until the technology develops further and prices decrease before any larger installations are made. But small installations should be considered in order to get more experience of the technology.