In an apparatus, a circuit model includes a DC resistance model, and a reaction impedance model having a nonlinear relationship between a first potential difference across a reaction resistance and a current flowing through the secondary battery. The nonlinearly relationship depends on a temperature of the secondary battery. The circuit model includes a diffusion impedance model. The apparatus predicts a future remaining voltage across the secondary battery at a future timing when a predetermined duration will have elapsed since a prediction time. The apparatus calculates a future polarization voltage across the secondary battery at the future timing. The apparatus predicts a target power parameter of the secondary battery at the future timing according to the future remaining voltage, the future polarization voltage, a first potential difference across the reaction resistance, and a second potential difference across the DC resistance model.