1. A secondary cell system for estimating a secondary cell status comprising: a secondary rechargeable cell comprising a plurality of battery blocks connected in series; voltage detection means connected to the secondary cell for detecting a voltage of the secondary cell; current detection means connected to the secondary cell for detecting a current of the secondary cell; a vehicle engine control unit (ECU) and a battery control unit; and the battery electronic control unit is connected to the voltage detection means and the current detection means and that estimates the secondary cell status by use of a plurality of models, the battery electronic control unit performing steps including: optimizing parameters used respectively in the plurality of models by use of the voltage value detected by the voltage detection means, after optimizing the parameters, selecting, as an optimum model, a model involving least change in an amount of parameter correction from among the plurality of models having the optimized parameters, and then estimating the secondary cell status by use of the selected optimum model, wherein the plurality of models is selected from at least two of the following: a first model in which an estimated voltage is obtained by adding an estimated voltage drop due to an internal resistance (DCIR) of the secondary cell, an estimated linear polarization value and an estimated hysteresis value estimated by the use of a predetermined polarization voltage-model, and an electromotive voltage based on an SOC obtained by integrating a current of the secondary cell; a second model in which the estimated voltage is obtained by adding the estimated DCIR voltage drop, an estimated short-term linear polarization value and an estimated long-term linear polarization value estimated by the use of the polarization voltage model, and the electromotive voltage based on the SOC; and a third model in which the estimated voltage is obtained by adding the estimated DCIR voltage drop, the estimated short-term linear polarization value and the estimated long-term polarization value estimated by the use of the polarization voltage model, and an electromotive voltage obtained by the use of a predetermined dynamic battery model; and transmitting, at least, the cell status to a vehicle ECU, wherein the vehicle ECU controls the charging and discharging of the secondary cell; based at least on the cell status, the ECU outputs a control command discharging the secondary cell to an DC to AC inverter to power an electric motor; based at least on the cell status, the ECU outputs a control command charging the secondary cell by actuating an engine and generator.