This paper proposes a sub-synchronous resonance (SSR) mitigation controller for battery energy storage system (BESS). It is a fact that series compensated long transmission lines may interact with the generator shaft and cause SSR oscillations. With the high penetration of BESS in smart grids, auxiliary damping controllers can be designed in order to contribute to the SSR mitigation in wind farms or synchronous generators. This paper designs a damping controller for a grid connected BESS using the wide area measurement system (WAMS) data. The designed damping controller will be supplemented to the active power control of the BESS. Fast Fourier transform (FFT) analysis is conducted to analyze the performance of the designed damping controller. Time domain simulations with MATLAB/Simpscape toolbox on the modified IEEE second benchmark model (SBM) are performed to support the claims.