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Hydrogen evolution at the negative electrode and corrosion of the positive grid are unavoidable secondary reactions in lead-acid batteries. Both cause water loss, that gradually changes the cell balance in valve-regulated types, because it cannot be compensated for by refilling. Thus, charge and discharge behaviour of valve-regulated Pb-acid batteries gradually changes with service life. The impact of water loss on discharge behaviour is different between batteries based on absorbent-glass-mat separators and those with gelled electrolyte. The authors describe how this has been elucidated with batteries that had lost a considerable amount of water by special pretreatment. Both effects can, at least partly, be explained by the pore structure of the medium between the electrodes. The pore sizes in absorbent-glass-mat separators are coarse compared to those of the active material or both electrodes. So, capillary forces of the active material are higher, and water loss occurs mainly at the expense of the water content in the separator. In gelled electrolyte, a real pore structure cannot be ascertained by direct measurement but the electrolyte behaves in the same way as in a felt with a pore system one order of magnitude finer than that of the absorbent-glass-mat separator. Thus, 'capillary competition' between active material and immobilized electrolyte are less significant.>
Berndt et al. (Wed,) studied this question.