Journal of Power Sources 127 (2004) 105–111 Monitoring state-of-charge of Ni–MH and Ni–Cd batteries using impedance spectroscopy Abderrezak Hammouche ∗ , Eckhard Karden, Rik W. De Doncker Institute for Power Electronics and Electrical Drives, Aachen University of Technology, Jägerstrasse 17–19, D-52066 Aachen, Germany Abstract This paper reports on laboratory studies into the ac impedance spectra of nickel–metal hydride and nickel–cadmium batteries, aiming at finding out possible correlation between electrical parameters, extracted directly from the high frequency region, and the battery state-of-charge (SoC). Impedance diagrams were recorded immediately after interrupting the dc charge, or discharge, current. The study revealed that the series resonance frequency, at which the dynamic cell behavior switches from an inductive character (Z ′′ > 0) to a capacitive one (Z ′′ < 0), varied monotonously as a function of state-of-charge. This behavior was reproducible after intermittent charge and discharge. Half-cell measurements were also conducted to associate the cell impedance with either processes occurring at the positive or negative plates. © 2003 Elsevier B.V. All rights reserved. Keywords: Ni–MH; Ni–Cd; Battery state-of-charge; Impedance spectroscopy; Series resonance frequency 1. Introduction Sealed nickel–metal hydride and nickel–cadmium sec- ondary batteries, have demonstrated a rapid expansion in many areas of battery usage. For practical purposes, a tech- nique capable of predicting the residual capacity with suffi- cient accuracy is urgently required. This applies particularly to the new 42 V-system where the battery is strained to a state-of-charge (SoC) as low as 40%. Traditional solutions used for vented lead–acid cells, such as specific gravity or electrolyte resistance measurements, cannot be applied to these sealed systems because the cell electrolyte composition remains virtually constant during charge–discharge cycles. The only reliable procedure to determine battery SoC and aging effects is to perform a complete discharge–charge cy- cle. This operation is obviously expensive, time-consuming and leaves the batteries to be tested out of service during the testing time. Impedance spectroscopy is used to make a fast, non-destructive and reliable method for such a characteriza- tion. The objective consists of inferring from the impedance spectra, plotted in different conditions, one or many electri- cal parameters, which increase or decrease monotonically as a function of the battery SoC. Such parameters should ∗ Corresponding author. Tel.: +49-241-80-96973; fax: +49-241-80-92203. E-mail address: hm@isea.rwth-aachen.de (A. Hammouche). be: (i) simple to measure; (ii) reproducible; and (iii) quickly accessible. Research in laboratories on impedance parameters related to state-of-charge or state-of-health of battery cells has been carried out over the last two decades [1–4]. Hampson et al. [5], Huet [6] and Rodrigues et al. [7] reviewed the pub- lished data on impedance studies of batteries and battery electrodes, undertaken for this purpose. It has been shown that various parameters may be useful to estimate the status of battery systems under different experimental conditions (under load or at open circuit after a period of rest), particu- larly of lead–acid and nickel–cadmium secondary batteries, but little thorough work on impedance parameters related to Ni–MH batteries has been done so far [8]. Earlier measurements conducted on alkaline batteries [2,3,9,10], investigating cell response in the high frequency domain, were not conclusive because the electrolyte does not participate to the overall electrode processes, whereas a pronounced effect of SoC was observed on those electrical quantities extracted from the low frequency region of the impedance diagrams, such as the impedance modulus, the phase angle, the equivalent series and parallel capacitance of the cell. However, it should be noted that measurements at low frequencies are time-consuming and, hence, less suitable for routine monitoring of cells in service. Besides, most of these studies were concerned with small capacity batteries, typically less than 10 Ah. Such cells gen- erally present impedance magnitudes ranging from some 0378-7753/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2003.09.012