This paper presents the modeling and control of a battery energy storage system (BESS) connected to the grid by a single-phase current source inverter. A bidirectional DC/DC converter connects the battery bank to the dc bus of the inverter allowing power flow in both directions. An integrated non-linear control strategy is proposed to control both converters and to manage the power flow direction between the BESS and a stiff grid. Simulations show the BESS achieves a smooth power transition from 100[%] of power injection to 100[%] of power absorption without adversely affecting the voltage at the PCC or the current at the DC side. These results suggests that the proposed control strategy allows a seamless connection to the grid of the BESS, maintaining a high power quality during its entire operation.