Aging mechanisms and service life of lead-acid batteries

In lead-acid batteries, major aging processes, leading to gradual loss of performance, and eventually to the end of service life, are: Anodic corrosion (of grids, plate-lugs, straps or posts). Positive active mass degradation and loss of adherence to the grid (shedding, sludging). Irreversible formation of lead sulfate in the active mass (crystallization, sulfation). Short-circuits. Loss of water. Aging mechanisms are often inter-dependent. For example, corrosion of the grids will lead to increased resistance to current flow, which will in turn impede proper charge of certain parts of the active mass, resulting in sulfation. Active mass degradation may lead to short-circuits. Sulfation may be the result of a loss of water, and so forth. The rates of the different aging processes strongly depend on the type of use (or misuse) of the battery. Over-charge will lead to accelerated corrosion and also to accelerated loss of water. With increasing depth-of-discharge during cycling, positive active mass degradation is accelerated. Some aging mechanisms are occurring only upon misuse. Short-circuits across the separators, due to the formation of metallic lead dendrites, for example, are usually formed only after (excessively) deep discharge. Stationary batteries, operated under float-charge conditions, will age typically by corrosion of the positive grids. On the other hand, service life of batteries subject to cycling regimes, will typically age by degradation of the structure of the positive active mass. Starter batteries are usually aging by grid corrosion, for instance in normal passenger car use. However, starter batteries of city buses, making frequent stops, may age (prematurely) by positive active mass degradation, because the batteries are subject to numerous shallow discharge cycles. Valve-regulated batteries often fail as a result of negative active mass sulfation, or water loss. For each battery design, and type of use, there is usually a characteristic, dominant aging mechanism. determining the achievable service life. Temperature has a strong influence on aging. Grid corrosion rates, and rates of water loss due to evaporation or hydrogen evolution at the negative plates (self-discharge), increase with increasing temperature. On the other hand, a (moderate) temperature increase may improve service life in applications involving severe cycling. Aging also depends on acid concentration (and concentration variations, for instance due to acid stratification). In general, very low acid concentrations, as prevailing in the discharged state, are harmful to the grids. On the other hand, at very high acid concentrations, service fife also decreases, in particular due to higher rates of self-discharge, due to gas evolution, and increased danger of sulfation of the active material. (C) 2003 Elsevier B.V. All rights reserved.