A gel battery is classified as a VRLA, valve regulated lead-acid battery, which is a category of low maintenance lead-acid battery. These types of batteries are pre-sealed; the owner does not need to maintain them by checking their water levels. A gel battery contains far less acid than a traditional battery. In fact, they are filled with a gel substance, referred to as a gel electrolyte, that replaces what some of us know as battery acid.
The gel electrolyte is sulfuric acid combined with silica. The gel is immobile, like gelatin. Traditional batteries, however, are liquid or wet-cell batteries. A gel battery is referred to as a sealed battery, but this isn't completely true. A truly sealed gel battery would create gas build up, and dangerous levels of pressure. Gel batteries regulate their gas build up through openings in the seal.
Battery electrolyte is generally a solution of 35 percent sulfuric acid and 65 percent water. The solution produces electrons through a chemical reaction, which creates the "juice" that makes a battery work. Adding fumed silica to the sulfuric acid adds a thickening effect to the electrolyte, and suspends the acid. Fumed silica is quartz sand that has been cooked in a 3000 degree electric arc. By thickening the electrolyte, the battery does not require the monitoring that traditional batteries require. The electrolyte gel does not need to be replenished, because it does not evaporate.
Gel batteries contain a valve regulating system. This system allows gas to be released or expelled when needed. Along with the gas being expelled, the gel substance contains calcium, instead of the antimony that is found in traditional batteries. The calcium keeps the gel from gassing in the same way a traditional battery might.
Overcharging a gel battery will dry out the gel electrolyte, causing holes in the gel that don't fill back up (heal). Undercharging can be just as harmful. If undercharging becomes a habit, the positive plate of the battery will form a layer of sulfate. Over time, the sulfate build-up prevents the ability to recharge the battery. Over- or undercharging can be prevented by using a temperature-compensated and voltage-regulated charger. Constant current chargers are not to be used on a gel battery.
Gel batteries have a few strong advantages that keep them on the market. They don't typically leak, or produce spills or corrosion. Even if they are cut open, the gel does not flow out of the battery. The batteries are not known to evaporate, which is a constant issue with traditional wet batteries. They have a better resistance to extreme temperature changes, and are more vibration-resistant and shock-resistant than other types of batteries.
