Investigation of the high-frequency resistance of a lead-acid battery
Introduction
The need of a reliable real-time method for estimating the state of health (SOH) or state-of-charge (SOC) of lead-acid batteries (LAB) has accompanied their development in the last decades [1], [2]. Particularly, the possibility of using an electrochemical parameter as a reliable gauge for monitoring both the SOC and SOH has been the matter of several studies [3], [4], [5], [6], [7]. Nevertheless, no definitive method has been undoubtedly validated and the quest of an accurate and easily implemented SOH and/or SOC indicator remains a subject of great interest [8]. Many studies were based upon the electrochemical impedance technique that provides a complete description of the dynamic behaviour of a metal–electrolyte interface from the low-frequency limit (steady state) to the purely resistive high-frequency limit. The high-frequency resistance, RHF, of a LAB cell (also called internal resistance and related to the conductance 1/RHF) has been found to give distinct values for fully charged and fully discharged LAB cells and was then proposed as a potentially interesting parameter for SOC- and SOH-monitoring [9], [10], [11].
In the present work, the evolution of the high-frequency resistance of a LAB cell of 45 Ah capacity has been investigated during the charge and discharge of the cell at various rates in order to determine the origin of the RHF variations and verify the feasibility of SOC or SOH diagnosis based on the measurement of the mean value of RHF and of a parameter characterizing its time fluctuations. The influence of the temperature and conductivity of the bulk electrolyte on the RHF evolution is also discussed.
Section snippets
Experimental
Tubular 2 V LAB cells of 45 Ah capacity from CEAC-Exide were studied in the present work. The electrochemical impedance measurements were performed using a white noise as excitation signal. The overall LAB impedance was measured from the voltage, V, at the terminals of the cell and the impedances of the positive and negative plates were obtained from their potential, Vpos and Vneg, respectively, measured against a saturated mercurous sulphate reference electrode (ESS) immersed above the plates.
Results and discussion
In a first set of experiments, the overall electrochemical impedance of the cell and the impedances of the negative and positive plates were measured in order to determine the characteristic frequency, FHF, at which the RHF value should be measured. FHF corresponds to the frequency at which the impedance diagram in the Nyquist plot intercepts the real axis at high frequency, as explained in Fig. 1 that shows that the impedance diagram of the cell strongly depends on the charging (Fig. 1A) or
Conclusions
The high-frequency resistance of 45 Ah flooded tubular LAB cells has been investigated with the aim of determining the origin of its variations with the cell SOC and evaluating the possibility of devising a reliable SOC indicator based on RHF measurements.
While RHF is independent of the charging or discharging current at the time it is measured, RHF clearly varies with the SOC of the cell in a way that strongly depends on the charge or discharge regime, indicating that RHF is a function of the
Acknowledgements
The authors would like to acknowledge the financial support of CEAC and ADEME (Convention No. 99-05-081).
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