An electrochemical technique for state of charge (SOC) probing of positive lead–acid battery plates Paul J. Blood a , Sotiris Sotiropoulos b,* a School of Chemical, Environmental and Mining Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, UK b Physical Chemistry Laboratory, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece Received 12 December 2001; received in revised form 26 March 2002; accepted 3 April 2002 Abstract Electrochemical experiments that intend to characterise the state of charge (SOC) of lead–acid battery positive plates are presented. These experiments are designed for potential battery plate production quality control and in situ monitoring of battery condition. A small size probe, consisting of a counter and a reference electrode encased in a glass body ending to a fine aperture tip and pressed onto the specimen, was used to apply cathodic galvanostatic pulses on positive plate battery samples. Although this probe arrangement is similar to that of a coulometric thickness gauge, the porous nature of the battery plate results eventually in full discharge of the entire specimen. However, during the initial stages of specimen discharge using the contact probe, a potential arrest was observed for fully charged and partially discharged samples and it was attributed to the time needed for the thickness of a PbSO 4 film formed during discharge and the corresponding resistance under the probe’s tip to reach a critical value for the discharge to spread to the rest of the sample. The duration of this potential arrest was found to be related to the positive plate’s SOC indicating the possibility of using the technique in positive plate quality control or in situ monitoring. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Lead–acid batteries; State of charge sensors; Coulometric gauge 1. Introduction Rechargeable batteries are receiving increasing attention as the demand for environmentally clean energy sources expands. Traditional applications include starting, lighting and ignition of automobiles, electric traction and uninter- ruptible power supplies used in telecommunications, sub- stations, chemical plants and nuclear installations [1–5]. The emerging electric vehicle technology aiming to the development of fuel-cell/battery hybrid electric cars [6] has further stimulated research into rechargeable batteries. Despite the emergence of new battery types and fuel-cells, the lead–acid battery is still the most attractive option from an economic point of view and is expected to dominate the above mentioned applications for the foreseeable future. The electrochemistry of both positive and negative plate electrodes and lead–acid battery grids has been thoroughly studied in the past and manufacturing of the main compo- nents of the various types that are commercially available is a mature technology [1–3]. However, there is ongoing research into new charging and monitoring circuits and strategies to improve battery lifetime and performance. To that direction, a useful parameter in evaluating the battery’s state of health and optimising its utilisation is the state of charge (SOC), defined as the ratio of remaining available capacity (at a certain point of its lifetime) to the maximum attainable capacity (under certain discharge conditions). Simple monitoring of the voltage difference across the battery terminals, although in principle the most direct method for SOC estimation, is not very useful in early diagnosis of battery failure since significant voltage changes during discharge only occur abruptly and just before the battery’s life end. Therefore, other techniques which follow phenomena directly or indirectly linked to the SOC have been developed as diagnostic tests: specific gravity tests and acid stratification monitoring using simple or optical hydro- meters [7,8] or laser interferometry [9,10], pressure trans- ducers to detect gassing during charging [11], humidity sensors [12], electrochemical noise monitoring for early detection of imminent failure [5], ac impedance techniques [13–15] or internal resistance measurements by dc current- interrupt techniques [16]. The first four of these techniques require the incorporation of additional devices in the cell body and provide indirect information about the SOC. From the electrochemical techniques, ac impedance Journal of Power Sources 110 (2002) 96–106 * Corresponding author. Tel.: þ30-310-997742; fax: þ30-310-443922. E-mail addresses: eczss@chem.auth.gr, eczss@otenet.gr (S. Sotiropoulos). 0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-7753(02)00231-8