IEEE 1635-2012 pdf download IEEE/ASHRAE Guide for the Ventilation and Thermal Management of Batteries for Stationary Applications
5.1.1.1 Vented lead-acid (VLA) batteries
The VLA battery is simply a lead-acid battery with a free flowing liquid electrolyte that allows any gasesgenerated during charging to be vented directly to the atmosphere. That volume below the cover and abovethe electrolyte level can be up to 67% hydrogen and 33% oxygen by volume, which is an explosivemixture. Typically these gases are vented via a flame arresting (safety) vent to prevent ignition from aspark or flame outside the cell from entering the cell container and igniting the contained gases.
For VLA batterics being overcharged the principal overcharge reaction involves gas production as inEquation (6). Recombination as in Equation (5) docs occur, but at such a low rate that it is can bedisregarded. The reason is that oxygen gas diffuses at a very slow rate through liquid electrolyte. During arecharge cycle the rate of gas production is dependent on a number of factors involving characteristics ofboth the batteries and the charger. In general, peak gas production occurs at the end of the cycle as thebatteries near their full state of charge. At this point, most of the charge energy results in electrolysis ofwater
VLA batteries use a varicty of lead alloys, which impactandproducedduring an overcharge condition. The alloys discussed in the tables in Clauseare lcad-calcium.leadantimony，and lead-selenium. Lead-calcium batteries havelow float currents, Lead-antimony and leadselenium have higher float currents, which increase over the life of the battery. Pure lead cells have thelowest float current All lead-acid batteries are capable of producing hydrogen sulfide gas (toxic andcorrosive) and acid mist under extreme overcharge conditions, but such failure modes are beyond the scopeof this document. See Annex D for additional information.
5.1.1.2 Valve-regulated lead-acid (VRLA) batteries
VRLA batteries are designed to take advantage of the naturally occuring recombination cycle whereinternally generated oxygen gas molecules are recombined with hydrogen ions to form water moleculesKey construction elements of these batteries are pressure relief valves and special internal construction topermit oxygen gas migration to the negative plate.
To minimize electrolysis of the electrolyte and the resulting water loss, VRLA batteries are designed tohave low float current Current VRLA battery technologies are based on lead-calcium, lead-tin, or purelead grid alloys. The electrolyte is immobilized in either a glass mat (absorbed glass mat (AGM)] or a silicagel (gelled cell). These construction methods achieve rates of oxygen gas migration sufficient to achieverecombination at a level to substantially reduce or eliminate the need for watering for the life of the battery.
The recombination cycle comes into play during charging, especially at the end of a recharge cycle andduring periods of elevated charging for equalization or freshening. In the recombination cycle oxygen gasgenerated at the positive plates migrates to the negative plates for recombination with hydrogen ions toform water molecules. The principal limiting factor to recombination is the sustainable rate of migration ofoxygen gas molecules to the negative plate.
The pressure relicf valve allows excess internal pressure from generated gascs to be relieved whilpreventing atmospheric air from entering the battery. When the valves do function to relieve internalpressure, the gases that escape directly equate to a loss of water from the battery that generally cannot bereplaced
The recombination reaction is cxothermic releasing heat at the negative plate. The greater the overchargecurrent, the greater the oxygen gencrated at the positive plate. This will result in greater heating at thenegativc plate and that hcat must be dissipated. If thermal cquilibrium cannot be reached, thermal runawaymay occur (SCe Annex E). Regardless of whether or not thermal runaway occurs with incrcasing ovcrchargc