Flooded electrolyte lead acid

Lead-acid batteries can be classified into three major types or categories, namely, automotive (SLI), stationary, and motive power (industrial). In addition, there are many special batteries that cannot be easily categorized as either of the above types. As these types of batteries are constructed with different materials and design to meet the requirements of their intended end uses, each requires a particular separator with specific material composition, mechanical design, and physical, chemical, and electrochemical properties that are tailored for the battery and its relevant specific uses. These batteries are generally available in flooded electrolyte or valve regulated (sealed) versions. In this section the types... 

Flooded battery — A battery (or a cell) containing an excess of electrolytic solution (in contrast to starved electrolyte batteries). Usually relevant to rechargeable -> lead-acid and -> nickel-cadmium batteries. Flooded battery design is typically applicable for heavy duty batteries, equipped with a vent valve that releases pressure buildup due to gas evolution. The excess electrolyte affects more sturdy batteries to become less susceptible to damage due to overcharge. In addition, the thermal conductivity of the electrolyte affords more efficient heat dissipation and thus higher -> power densities. 

Starved electrolyte battery — A -> battery with minimum amount of -> electrolyte. The electrolyte in starved electrolyte cells or batteries exists in the porous structure of the - electrodes and absorbed in the separator, so it contains little or no free fluid electrolytic solution. This type of batteries is used in certain constructions of sealed - lead-acid and -> nickel-cadmium batteries that rely on gas diffusion and recombination on the electrodes during charging or overcharging in order to maintain maintenance-free conditions, and to suppress pressure buildup. Starved electrolyte batteries benefit from larger - energy density due to the reduced amount of electrolyte. This design may suffer from poor heat dissipation compared with -> flooded batteries, thus for high power applications this point has to be taken into account. 

Different types of lead-acid batteries have been developed as energy sources for many power applications, like traction and backup or standby power systems. The flooded lead-acid batteries have an excess or flooded electrolyte and they were the largest used at the beginning of the last century for many applications. Valve-regulated lead-acid (VRLA) batteries were developed as an alternative to the flooded lead-acid batteries, in order to maintain levels of distilled water and prevent drying of cells, which means safe operation for battery packs in electric... 

The classical scheme for the manufacture of flat pasted plates for automotive, traction and stationary lead-acid batteries is shown in Fig. 3.1. There is no difference between the technology of plate manufacture for conventional (flooded) and valve-regulated (VRLA) lead acid batteries. The two versions do differ, however, in the method of separation of the plates, the quantity and type (hquid or gel) of electrolyte, and in the design of the battery itself. 

In a flooded lead-acid system, the two plates charge independently of one another due to the saturated electrolyte condition and to the low diffusion rates of oxygen and hydrogen gases in sulfuric acid. Thus, these gases escape from the cell before they can interact with the opposite electrode. Both are still affected by impurity effects on overpotentials and thus perfectly efficient recharge of both plates is not guaranteed. 

Acid drainage is an extreme case of gravity effects in VRLA cells. In very tall cells or in cells built with low surface-area or glass/organic hybrid separators combined with low compression levels electrolyte can actually pool in the bottom of the cell, even at moderate or high saturation levels. This creates a condition where the bottom of the cell charges as if it were a flooded lead-acid cell (which it is) and the upper portions of the cell try to function as a normal. 

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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