A technique which combines nonlinear potential-step excitation and impedance response measurements is presented. The basic difference to similar techniques is that it is applied on time scales which are of the same order of magnitude as the typical time constants of the system under investigation. The technique is applied to Li batteries to monitor the time development of the impedance spectra during the so-called potential-delay effect typically observed on a time scale of 1 to 100 s after a potential step excitation. Experimental conditions that minimize the error of impedance response measurements are discussed theoretically and checked experimentally using an equivalent circuit consisting of a diode, a resistor, and capacitors. With the suggested technique it is shown that the non-LTI (non-Linear and time invariant") characteristic of the potential-delay effect is due to time variability rather than nonlinearity of Lithium batteries. A possible physical reason for such behavior is indicated. For comparison, an equivalent circuit including a field-effect transistor is constructed.