Industrial Ageing of Field-Deployed Lead–Acid Batteries I—A Case Study of Post-Industrial VRLA Batteries

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study examines 12 field-deployed 12 V valve-regulated lead–acid (VRLA) batteries repres1enting six commercial models, withdrawn from medium-voltage grid-management devices after five years of service.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.