The variable float voltage characteristics of VRLA cells

A review of past INTELEC papers indicates that many references have been made to the wide variation in the operating float voltages observed with VRLA cells. This paper addresses the wider excesses of the initial variation range by reference to the degree of electrolyte saturation in AGM separator types, and to the state of charge of the product on entry into service. The effect of electrolyte saturation is explained by reference to Tafel plots from which it is shown that a mixture of “wet” and “starved” VRLA cells can produce a wide range of float voltage values (±100 mV about the mean) for any given current value. It has shown that as the mean float voltage increases so the range gets wider in accordance with the divergence of the Tafel plot slopes. The same Tafel plots also show that there is a float voltage level at which the range of float voltage values is a minimum irrespective of whether the VRLA cells are “wet” or “starved”. Unfortunately, this value is too low to effect a recharge of the cells in an acceptable time. The wide range of float voltage values generated by the degree of electrolyte saturation is further compounded by the effect of the state of charge of cells should they be put into service without any pre-commissioning charge, (which is usually the case), and this totally excludes the use of float voltage measurements as a reasonable means of battery acceptance procedure