A battery module system includes a battery module and a control member. The battery module comprises battery units are connected in series. Each of the battery units comprises a unit cell having a voltage variation rate A (mV/% SOC) at a full charge voltage V.sub.H1 (V) is larger than 20 (mV/% SOC), which is a value obtained when the unit cell is charged at a current of 1 C at 25.degree. C. The control member controls current to a current I.sub.1 until a maximum value V.sub.max (V) among the voltage of each unit cell reaches the full charge voltage V.sub.H1 (V) and then controls a voltage of the battery module to a voltage V2 (V) given by the following equation (1): V2=V.sub.H2.times.n (1).