A readily instrumented and conducted nondestructive technique to determine the state of charge of zinc‐mercuric oxide primary cells has been developed and preliminary results obtained. Measurements of the current vs.voltage (I‐V) characteristics, the a‐c impedance, the electronically sampled derivative at different sampling currents and frequencies, and the discharge‐time characteristics of the cells as a function of charge expended have been made and reported. The nearly constant slopes of the I‐V curves above 75 mA were measured graphically. It was found that these slopes have a linear relation to charge expended and can be used as a basis for a simple state‐of‐charge indicator which determines remaining charge to about 12%. By combining this criterion with a‐c resistance measurements the maximum experimental deviation was found to be less than 10% with 90% of the data points deviating less than 6% from the mean linear curve. These studies indicate that this technique constitutes a useful, easily applied state‐of‐charge test.