First, this paper focuses on how to use a battery energy storage system (BESS) to enhance performance in secondary regulation of a coal-fired generator. Second, the probability characteristics of automatic generation control (AGC) instruction duration period, interval period, regulation rate and regulation direction are modeled based on real AGC instructions. AGC instructions with random regulation direction will lead to frequent switching between charging and discharging states of the BESS, thus quickly depleting the battery. To address this problem, the BESS is divided into two parts when connecting to the frequency regulating generator. The two parts of the BESS are respectively in charging and discharging states for supporting up and down AGC instructions. Once any part of the BESS arrives at full charge or discharge, two parts of the BESS switch their charging/discharging states immediately. The performance on secondary frequency regulation of the coal-fired-generator with the BESS and tearing losses of the BESS are estimated based on simulating the operation of a hybrid coal-fired-generator and BESS power station during a typical day by the sequential Monte Carlo simulation (SMCS) method. Finally, the simulation experiments based on a real frequency regulating generator show that BESS can enhance performance on secondary regulation of the frequency regulating generator and the strategy developed in this paper can effectively reduce cycle life loss of the BESS.