1. A battery internal state estimating apparatus that estimates an internal state of a battery based on a simulation model of the battery, comprising: a storing section that stores a plurality of parameters of the simulation model; a detecting section that detects a discharge current flowing from the battery to a load; a selecting section that selects a parameter to be subjected to adaptive learning based on a value of the discharge current detected by the detecting section; and an adaptive learning section that performs adapting learning on a parameter selected by the selecting section.