This paper investigates modeling and control of a battery management system used in a microgrid for both grid-connected and autonomous modes. The paper has three salient contributions: 1) An aggregated battery circuit model with the open circuit voltage as a nonlinear function of the state of the charge (SOC) is derived and modeled in PSCAD. 2) Closed-loop feedback control strategies of the battery system are developed for the microgrid under both operation modes. At the grid-connected mode, power control is employed while at the autonomous mode, voltage and frequency control is employed for the battery to act as a synchronous generator by providing voltage and frequency support. 3) An upper level SOC based management system is also developed. Since SOC cannot be directly measured, an estimation scheme is derived based on power and voltage measurements from the battery. The overall management system is demonstrated to be effective by six case studies at different microgrid operation modes.