Since the Fukushima nuclear accident, the interest in securing a reliable emergency power has increased, and as follow-up measures, each country is conducting various studies for enhancement of safety such as re-examination of EDG (emergency diesel generator) capacity, securement of additional emergency power, etc. This study was performed to find the way to implement a reliable DC system in coping with 72 hours extended station blackout for APR1400 within the battery room size. By using the largest capacity battery in the market which satisfies the requirements for Class 1E to perform a duty cycle for 24 hours, after the decay of the qualified class-1E lead acid battery a Non-Class 1E Lithium ion based battery system can be used as a backup system for 48 hours.