A useful and systematic dynamic model of a battery energy storage system (BES) is developed for a large-scale power system stability study. The model takes into account converter equivalent circuits, battery characteristics and internal losses. Both charging mode and discharging mode are presented. The model is expressed in equivalent transfer function blocks, and it can be easily used in dynamic stability analysis of a power system. To examine the dynamic behaviour of the model, applications to the damping of turbogenerator torsional oscillations are performed. Active and reactive power modulation by the BES can be controlled according to system requirements. Eigenvalue analysis and dynamic simulations are performed to demonstrate the damping effect of the BES.