Multi-scale model of a valve-regulated lead-acid battery with electromotive force characterization to investigate irreversible sulphation

Abstract

Valve-regulated lead-acid (VRLA) batteries are commonly used for energy storage because they are inexpensive and easy to use. Combined with an immobile electrolyte, a VRLA battery has almost no risk of an acid spill. The use of VRLA batteries is expected to grow even though, in renewable energy systems, more than a third fail prematurely due to incorrect or abusive operation. A major cause of premature capacity loss in lead-acid batteries is a damage mechanism called irreversible sulphation (IS). This damage mechanism occurs on a microscopic scale in the electrical double-layer (EDL) during unobservable processes. On the observable macroscopic scale, measurable quantities during operation are used to calculate the battery's state-of-charge (SOC). Charge controllers use the SOC in an attempt to avoid the well-known operating modes resulting in IS, yet many batteries still fail. An improved understanding between microscopic processes in the EDL and observed macroscopic phenomena is necessary. The primary research contribution of this study is a multi-scale electrochemical model of a VRLA battery with an immobile electrolyte and its analysis. The model's input parameters are subjected to elementary effects analysis and a reduced set of the most influential parameters are used in variance-based model sensitivity analysis. The time and complexity associated with parameter estimation are reduced by electromotive force (EMF) characterization. The EMF of the battery is characterized using an accurate concentration-based method presented in this thesis as a secondary contribution. The validated multi-scale model is then used to simulate an operating mode that leads to IS while changes in the active surface area of the electrodes are observed. It was found that the available active surface area suffers irreversible decreases due to minor errors in SOC indication. Additionally, the internal resistance during the initial voltage drop increases from one discharge to the next. It was concluded that IS cannot be prevented satisfactorily using SOC information because SOC is not indicative of a specific damage mechanism. The curve of EMF versus electrolyte concentration resulting from EMF characterization is more descriptive of the battery's internal state than the SOC. Future work should include the development and application of a health-conscious charge control algorithm using the EMF curve because it requires very basic measurement data. With a thorough understanding of premature failure due to irreversible sulphation in VRLA batteries, charge controllers can be improved. This will ensure that the end-user has no opportunity for incorrect or abusive operation of the battery. Contributions to the mature field of lead-acid batteries are, in essence, advances in energy storage technology. As such, this study is part of a global effort towards a sustainable energy future. Keywords: valve-regulated lead-acid battery, irreversible sulphation, state-of-charge, multi-scale model, parametric analysis, electromotive force characterization