An apparatus estimates charge/discharge electricity amount without being
affected by current measurement error. Under specific selection
conditions, a no-load voltage calculation part uses pairs of data
consisting of current data I(n) and voltage data V(n) corresponding to
the current data to calculate a no-load voltage V.sub.sep as the voltage
intercept at a current of zero in a straight-line approximation obtained
by statistical processing such as regression analysis using a least
squares method. If specific current conditions exist for a certain amount
of time, an open circuit voltage calculation part calculates the terminal
voltage of the secondary battery as the open circuit voltage V.sub.oc. An
estimated charge/discharge electricity amount calculation part uses a
preset change-in-voltage adjustment constant .DELTA.V.sub.bc/adjustment
coefficient K.sub.b, change-in-electromotive-force constant K.sub.eq and
polarization voltage generation constant K.sub.pol to calculate the
estimated charge/discharge electricity amount .DELTA.Q.sub.e as a
function of the change .DELTA.V.sub.b in the no-load or open circuit
voltage over a predetermined period of time.