Operating batteries in parallel improves the battery power system management and resolves the problems of conventional battery banks that arrange batteries in series. The discharging currents of the batteries are independently controlled, but coordinated to provide a full amount of the load current. Batteries connected in parallel do not suffer from charge imbalance, and thus, can avoid being overcharged or overdischarged. Sophisticated discharging profiles can be realized to efficiently utilize the available stored energy in batteries. Some of the batteries may take rest or be isolated from the system for the open-circuit measurement during the operation period, thus, facilitating the estimation of the state-of-charge and the evaluation of state-of-health.