Correlations between a residual capacity of a secondary battery and
4 parameters of an internal resistance, temperature, a discharge current and one
of an open circuit voltage and a discharge voltage of the secondary battery are
previously determined and a contour map of the residual capacity and 2 of the parameters
is made, for example. Then, at least 2 varying parameters selected from the 4 parameters
are detected while the secondary battery is in use and collated with the correlations
to estimate the residual capacity of the secondary battery.
