A plurality of resistance value tables of a battery group with respect to temperature and SOC of the assembled battery, which are measured in advance when currents of different change patterns flow in the assembled battery, are stored. A resistance value for the assembled battery is calculated, based upon its voltage and current. From among the plurality of resistance value tables, a resistance value table is selected so that it corresponds to the change pattern of the current flowing in the assembled battery when the voltage and the current of the assembled battery were measured, and a resistance value is searched for from that resistance value table corresponding to the temperature and the SOC of the assembled battery. Then the calculated resistance value and the one from the table are compared to determine the life of the assembled battery.