VRLA batteries design

The number of VRLA batteries used in stationary applications is increasing rapidly. They account for more than 5.2% of the total US standby power production and more than 60% of Japanese and European production. Fig. 5.15 shows a VRLA battery design for telecommunications standby power. Over the past decade, VRLA batteries have been scaled to sizes up to 3000 Ah for industrial applications. Although the original... 

The cells are vacuum treated. 35% of the electrolyte volume is filled and the cells are vacuum treated again. Another 35% of the electrolyte is added. The cells are vacuum treated and the remaining 30% of the electrolyte volume is filled. Most often, however, this procedure is reduced to two vacuum treatments only. The method is applied mainly to gel designs of VRLA battery. There are two types of vacuum treatment (i) soft vacuum , in which a pressure slightly lower than atmospheric pressure is maintained in the cells (ii) hard vacuum , in which the cell pressure is < 15 mm Hg column. The hard vacuum treatment ensures faster filling of the plate group with electrolyte. 

The majority of VRLA batteries produced today are used in standby applications to provide a reliable source of power in the event of failure of the mains supply. Discharges are infrequent and the batteries are maintained by float charging at a preset voltage. For example, European practice is to use parallel strings (usually 48 V) across a 54.5 V supply (2.27 V per cell). Adequately designed new cells, after conditioning and free of impurities, have a float current of < 1 mA per Ah at 20°C. 

R.F. Nelson, J.B. Olson, E.D. Sexton, A.A. Pesaran, M.A. Keyset, ALABC Project No. B-007.2, Development of improved cycle life by design of charge algorithms specifically aimed at VRLA batteries. Progress Report 21 July to 20 September 1999, Advanced Lead-Acid Battery Consortium, Research Triangle Park, NC, USA, 1999. 

Two VRLA battery technologies are currently predominant, i.e., absorptive glass mat (AGM) and gel designs. In the former, the AGM immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica, and an additional separator is required to fix the plate distance and to prevent electronic shorts. 

Clearly, Cl may not be a universal algorithm, but it would be interesting to see what cycle-lives could be achieved with this procedure on thicker-plate VRLA batteries, where the oxygen cycle is not as active as in the Optima and Genesis products. It should be noted that each product may require a unique approach, as design dictates maximum current levels and recharge times in VRLA products. This has been demonstrated in the Cominco ALABC work on fast charging of Optima (thin-plate) and Delphi (thick-plate) batteries [61]. 

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