The recombinant cell and the flooded cell are two different types of commercially available lead-acid battery designs that are used in many automotive and industrial (e.g., forklift) applications. Both types include adjacent positive and negative electrodes that are separated from each other by a porous battery separator. The porous separator prevents the adjacent electrodes from coming into physical contact and provides space for an electrolyte to reside. Such separators are formed of materials that are sufficiently porous to permit the electrolyte to reside in the pores of the separator material, thereby permitting ionic current flow between adjacent positive and negative plates.
The first type of lead-acid battery, the recombinant battery, or valve-regulated lead-acid battery, typically has an absorptive glass mat (AGM) separator composed of microglass fibers. While AGM separators provide high porosity (>90%), low ionic resistance, and uniform electrolyte distribution, they are relatively expensive and still do not offer precise control over oxygen transport rate or the recombination process. Furthermore, AGM separators exhibit low puncture resistance that is problematic for two reasons: (1) the incidence of short circuits increases, and (2) manufacturing costs are increased because of the fragility of the AGM sheets. In some cases, battery manufacturers select thicker, more expensive separators to improve the puncture resistance, while recognizing that the ionic resistance increases with thickness.
In the case of a recombinant battery using an AGM, the sulfuric acid is essentially “immobilized” within the three-dimensional glass mat structure, enabling the battery to be positioned without concern of acid spillage. In an alternate version of a recombinant battery, the sulfuric acid is mixed with fumed silica under high shear and temperature to form a mixture that “gels” after injection into a battery containing microporous polyethylene separators between the plates. In this case, a thixotropic gel without chemical cross-links is formed because the silica provides a significant increase in the viscosity of the acid, making it less susceptible to spillage. This latter description is often referred to as a gel battery.
The second type of lead-acid battery, the flooded battery, has only a small portion of the electrolyte absorbed into the separator. The remaining portion of the acid between the electrodes is in a continuous liquid state. Flooded battery separators typically include porous derivatives of cellulose, polyvinyl chloride, rubber, and polyolefins. More specifically, microporous polyethylene separators are commonly used because of their ultrafine pore size, which inhibits dendritic growth while providing low ionic resistance, high puncture strength, good oxidation resistance, and excellent flexibility. These properties facilitate sealing of the battery separator into a pocket or envelope configuration into which a positive or negative electrode can be inserted.
Most flooded lead-acid batteries include polyethylene separators. The term “polyethylene separator” is something of a misnomer because these microporous separators require large amounts of precipitated silica to be sufficiently acid wettable. The volume fraction of precipitated silica and its distribution in the separator generally control its ionic permeability, while the volume fraction and orientation of polyethylene in the separator generally control its mechanical properties. The porosity range for commercial polyethylene separators is generally 50%-60%.
A sub-category of the flooded lead-acid battery is the dry-charged battery. This battery is built, charged, washed and dried, sealed, and shipped without electrolyte. It can be stored for up to 18 months. Before use, liquid electrolyte (acid) is added and the battery is given a conditioning charge. Batteries of this type have a long shelf life. Motorcycle batteries are typically dry charged batteries. The acid between the electrodes and the separator is in a continuous liquid state.
In response to the increased price of lead and new start-stop applications in which the lead-acid battery operates in a partial stage of charge, battery manufacturers are seeking new ways to separate electrodes while achieving low ionic resistance and minimizing acid stratification.