Capacity retention of absorbent glass mat lead acid batteries against electrolyte freezing

Lead acid batteries operating in cryospheric environments are susceptible to electrolyte freezing, particularly the Absorbent Glass Mat (AGM) Valve Regulated Lead Acid (VRLA) type. A myriad of failure mechanisms may be postulated within the multivariate state space, porous electrode structure, and complex chemistry of the AGM VRLA cell. The lack of published data has resulted in suboptimal usage patterns in Antarctica. This study therefore performed an iterative experiment ranging from 0 to −40 ∘ C, alternating 24 vs 72 h storage periods, and a sample size of 14 batteries stored at varying State of Charge (SoC). Capacity trends are compared against the mass fraction of ice formed, which depends on SoC and temperature, but is measurable by electromotive force under open circuit conditions. These exploits in dimensional reduction and measurement allow for an accurate, finite range, and feasible experimental methodology. The results found no correlation between ice crystallisation and capacity loss. Theoretical analysis additionally refute several postulations of failure mechanisms on macrophysical and thermodynamic grounds. The evidence and theory therefore supports that AGM VRLA electrolytes can be repeatedly frozen without permanent capacity loss. Intelligent cryospheric battery management strategies may therefore allow for such frozen states with confidence of long-term performance in light of these findings.