Capacity effects on the determination of the state-of-charge in lead-acid cells

Abstract

The state-of-charge in lead-acid cells was the aim of a previous paper [C. Armenta, J. of Power Sources 27, 297 (1989)] in which the use of a reference coltage improved the calculation of the state-of-charge related to the use of density measurements [C. Armenta et al., J. of Power Sources 27, 189 (1989)]. The method predicted good results within a low error, maximum 7.5%, for depth of discharge between 15% and 100%. The method, developed for conventional and uncommon applications, is of good merit if applied to non-aged batteries. Recent experiments carried out in our laboratory have shown increasing deviation from experimental results, with increasing depth of discharge of aged lead-acid batteries because of the influence of discharge rate and capacity on the empirical coefficients used to determine the state-of-charge in lead-acid cells. This paper presents a correction factor, based on the ratio of capacities for standard and non-standard discharge rates, which should be used in combination with the expression proposed in previous paper [C. Armenta, J. of Power Sources 27, 297 (1989)], for the correct estimation of the real state-of-charge of lead-acid cells. The results obtained with the correction factor are encouraging, reducing the estimated error to less than 5% on average.