Premature failure of industrial lead–acid batteries can arise not only from conventional aging but also from atypical environmental exposure that changes the chemistry and morphology of corrosion products. This study presents a focused post mortem investigation of three industrial flooded lead–acid batteries that underwent premature failure shortly after building renovation, despite differing in manufacturer, nominal capacity, operational target, and service time. Corrosion products collected from electrolytes, bridges, and terminals were examined by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and complementary electrochemical testing of lead-based alloys in sulfuric acid. Raman spectra indicated mixtures of α- and β-PbO₂, PbSO₄, and trace PbS, whereas XRD identified the dominant crystalline phases as PbO₂ polymorphs, PbO, PbSO₄, and Pb₃O₄. The absence of a strong PbS signal in XRD is consistent with its low amount, poor crystallinity, and surface-localized occurrence. SEM images showed loose corrosion products rather than a dense protective layer. The combined evidence indicates an atypical, sulfide-associated corrosion pattern. External H₂S exposure is considered as one possible source of sulfur-containing species; however, no atmospheric H₂S measurements, renovation-material analyses, or controlled exposure tests were performed.

Lead–acid batteries remain widely used as backup power sources in distributed industrial infrastructure, where reliable State-of-Health (SoH) assessment is sometimes complicated by poorly documented service histories and uncontrolled environmental exposure. This study examines 12 field-deployed 12 V valve-regulated lead–acid (VRLA) batteries repres1enting six commercial models, withdrawn from medium-voltage grid-management devices after five years of service. Their condition was evaluated using temperature-dependent capacity testing, open-circuit voltage (OCV) measurements, and electrochemical impedance spectroscopy (EIS). The measurements revealed pronounced and highly non-uniform degradation. At −25°C, none of the tested battery types delivered more than 30% of their nominal capacity. More importantly, several batteries exhibiting apparently normal OCV values of 13.2–13.3 V retained only 10–30% of their nominal capacity during discharge, demonstrating that voltage measurements alone may substantially overestimate the availability of backup power. Advanced degradation was also manifested by non-linear discharge profiles and anomalous, physically non-interpretable impedance spectra, providing diagnostic information not evident from OCV measurements. These findings show that OCV-based screening alone is insufficient for reliable SoH assessment of aged VRLA batteries used in emergency power applications. For distributed industrial installations, practical monitoring protocols should therefore combine temperature-aware rapid electrical diagnostics, particularly impedance- or internal resistance-based screening, with periodic reference capacity tests and replacement criteria based on application-specific minimum-capacity requirements.