Series is calculated using data from the H.3 release as Total reserve balances maintained (http://research.stlouisfed.org/fred2/series/RESBALNSW) less Reserve balance requirements (http://research.stlouisfed.org/fred2/series/RESBALREQW).
Effective February 2, 1984, reserve computation and maintenance periods have been changed from weekly to bi-weekly. Series with data prior to February 2, 1984 have different values reported from one week to the next. After February 2, 1984, the value repeats for 2 consecutive weeks.
Please note though that this historical concept of "excess reserves" no longer has the same meaning following phase two of the simplification of reserves administration. https://federalregister.gov/a/2012-8562
Federal Reserve Bank of St. Louis, Excess Reserves of Depository Institutions [EXCSRESNW], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/EXCSRESNW/, August 27, 2015.