DIFFERENTIAL MEASUREMENT OF NONLINEAR ELECTRICAL RELAXATION OF LITHIUM BATTERIES

A technique which combines nonlinear potential-step excitation and imp edance response measurements is presented. The basic difference to sim ilar 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 un der investigation. The technique is applied to Li batteries to monitor the time development of the impedance spectra during the so-called po tential-delay effect typically observed on a time scale of 1 to 100 s after a potential step excitation. Experimental conditions that minimi ze the error of impedance response measurements are discussed theoreti cally and checked experimentally using an equivalent circuit consistin g of a diode, a resistor, and capacitors. With the suggested technique it is shown that the non-LTI (non-Linear and time invariant'') charac teristic of the potential-delay effect is due to time variability rath er 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.