Abstract: A method for determining the state-of-charge for zinc mercuric oxide type primary cells. I-V curves of the cells are plotted and the nearly constant slopes of these I-V curves above 75mA are measured graphically. These slopes are used to define effective differential DC resistance values (.DELTA.V/.DELTA.I).sub.75mA which have a linear relationship to charge expended, Q(A-h), which is graphically produced by plotting (.DELTA.V/.DELTA.I).sub.75mA versus Q(A-h). This relationship is used as a basis for a state-of-charge indicator to determine the remaining charge to about 12%. One can also subtract the internal resistance of the cell (measured with an a-c bridge technique) from (.DELTA.V/.DELTA.I).sub.75mA and obtain a new parameter, R.sub.p, which is related to the polarization resistance of the cell. The maximum uncertainty can be reduced to less than 10% if the linear curve is produced by plotting polarization resistance R.sub.p of the cell versus the charge expended Q(A-h).