A battery assembly including: at least one rechargeable lithium battery including a negative electrode including a silicon-containing negative active material selected from silicon, a silicon-carbon composite, and a combination thereof, and a positive electrode including a positive active material; a circuit board electrically connected to the battery assembly; and an outer terminal electrically connecting the battery assembly to an outer power or an outer load, wherein the circuit board includes a charge/discharge element for charging and discharging the battery assembly and a charge/discharge controller electrically connected to the battery assembly and the charge/discharge element, wherein the charge/discharge controller controls the charge and discharge of the battery assembly, and wherein a discharge cut-off voltage of the charge/discharge controller is predetermined as a voltage when LixSi present in the negative electrode during the discharge has an x value of less than or equal to about 1.25.