Monitoring State-of-charge and State-of-health of VRLA Batteries Using Impedance Spectroscopy: Experimental Analysis

Abstract

In this paper the impedance measurements of 24Ah valve regulated lead acid batteries are reported and analyzed. The monitoring process is performed using a designed and tested laboratory spectroscope during discharge process. The tests are done with constant rates within the range C/200 to C/5 and under ambient temperature. A simplified impedance model is used according to the relatively high excitation frequency range exploited (from few Hz to several kHz). Using the obtained results we would discuss the eventual correlation between battery impedance and its state of charge (SOC) and its state of health (SOH). The investigation clearly reveals the mainly resistive behaviour of the battery impedance. However even if the internal resistance is found to be function of the battery state of charge, a large part of this profile is found to be quasi-flat. Moreover the high influence of plate’s pore blocking process strongly related to the cycle rate is confirmed. A last set of extensive tests performed during a long period time have clearly shown that high frequency impedance could be an efficient battery state of health indicator since all battery history is stored in its variation.