An automatic equivalent-circuit parameter-estimation system for a Lithium-ion battery including battery temperature controller is developed in this paper. The system consists of a thermostatic chamber and estimating programs for parameters of an internal voltage and of an internal impedance, which is expressed by two resistors and a capacitor. These resistances increases by more than 60% and above, while the temperature decreases from 20 to 0degC, and the capacitance becomes about 80% of that at 20degC. The proposed equivalent circuit reproduces the transient characteristics including the temperature dependent effect. An estimating method of SOC (State of Charge) using instantaneous terminal voltage and current of the battery is proposed in this paper. The internal voltage is a function of the SOC and it can be evaluated by the equivalent circuit. The system is applicable to the other types of secondary batteries. The proposed numerical model and its parameters including the temperature dependent effect is useful for simulations of the apparatuses, and for an estimation of SOC without any integration of a battery current.