Flooded batteries sulfuric acid

Electrolyte stratification has been responsible for the failure of many flooded battery banks. In simple terms, this phenomenon can be defined as a build-up of higher strength acid at the bottom of the battery. Stratification occurs because sulfuric acid has a higher density than water and, when formed during the charging process, will sink to the bottom of the battery container. Such behaviour results in a decrease in battery capacity due to uneven utilization of the active material [12]. Moreover, if the resulting concentration gradient is allowed to remain for extended periods, premature failure of the battery can occur. 

The sulfuric acid used in lead-acid batteries is a combination of sulfuric acid (or dihydrogen sulfate, (H2SO4) and water (H2O)). Acid concentrations in automotive batteries are about 35% H2SO4. The cells are flooded with excess electrolyte to prevent the battery from drying out during use. 

Figure 9.6 Comparison of a flooded system and a valve regulated lead acid system in a lead/sulfuric acid battery with fixed electrolyte.

Gel silicon electrolyte was introduced in battery design to eliminate spilling and the need for constant maintenance. A gel battery (also known as a gel cell) is a VRLA battery with a gelled electrolyte the sulfuric acid is mixed with silica fume or silica additives, which makes the resulting mass gel-like, stiff, and immobile. Unlike a flooded wet-cell lead-acid battery, these batteries do not need to be kept upright. Gel batteries reduce the electrolyte evaporation and the spillage (and subsequent corrosion issues) common to the wet-cell battery, and they boast greater resistance to extreme temperatures, shock, and vibration. Chemically they are the same as wet (nonsealed) batteries except that the antimony in the lead plates is replaced by calcium. 

In a flooded lead-acid battery, the sulfuric acid serves as both the electrolyte for conductance of the ions and it also promotes the electrochemical reaction. To achieve optimum performance from a battery, the amount of sulfuric acid should be stoichio-metrically balanced around the other reactants, namely, the positive and negative active material. The amount of electrolyte between electrodes is fixed by the 3D structure of the flooded lead-acid separator. Normally facing the positive electrode, there are ribs protruding off the planar surface of the substrate that serve to fix the distance between the electrodes and thus the volume available for the electrolyte [27]. Figures 4.9 and 4.10 illustrate some typical profiles. 

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