State of charge estimating apparatus and method for a secondary cell

In charge rate estimating apparatus and method for a secondary cell, a current flowing through the secondary cell is measured, a voltage across terminals of the secondary cell is measured, an adaptive digital filtering is carried out using a cell model in a continuous time series shown in an equation (1), all of parameters at one time are estimated, the parameters corresponding to an open-circuit voltage which is an offset term of the equation (1) and coefficients of A(s), B(s), and C(s which are transient terms, and, the charge rate is estimated from a relationship between a previously derived open-circuit voltage V0 and the charge rate SOC using the open-circuit voltage V0, V = B(s) DIVIDED A(s)•I+1 DIVIDED C(s)•V0 wherein s denotes a Laplace transform operator, A(s), B(s), and C(s) denote poly-nominal functions of s.