Knowledge What Is a Gel Battery


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A gel battery is a sealed lead-acid (SLA) battery with a gelled electrolyte. In lead acid batteries, the electrolysis of the electrolyte during charging causes decomposition of water in the electrolyte. The gas products generated during this process are either recombined internal to the battery cell as in sealed batteries, or released through the cell vents in flooded batteries. Sealed gel technology (commonly referred to as “gel cell” technology) was developed to facilitate oxygen recombination reactions between the positive and negative plates. The recombination reaction is used to prevent the escape of oxygen and hydrogen gases normally lost in a flooded lead-acid battery. Gas recombination technology removes the need for regular water addition and allows lead acid batteries to be operated in any orientation without fear of acid spill.

The sealed lead-acid battery is unique in that its electrolyte is immobilized and each cell contains a one-way, pressure-relief valve system. The combination of these two features drives oxygen recombination cycles which, under normal circumstances, will prevent the escape of gases and thus eliminate the need to replenish the electrolyte water supply. SLA batteries are also called valve regulated lead acid (VRLA) batteries because of the characteristic valve built into the cell. VRLA batteries fall into two categories: gel batteries and absorbed glass mat (AGM) batteries. Both types are regulated by a safety valve. The valve regulating mechanism is a critical feature of both gel and AGM batteries. The valve keeps the cell sealed and pressurized, otherwise the recombination of oxygen and hydrogen gases will not take place. The valve must safely release excess pressure from the chemical reactions in cases of extended overcharge, incompatible battery/charger combinations, or a charger failure that causes an over current condition. Under these abnormal conditions, it is possible that oxygen, and hydrogen, will be generated at a rate faster than what it can diffuse through the separator system. When there's excessive gas pressure buildup inside the battery, the one-way valve will open to release excessive gas. The vent reseals once the internal pressure falls within the limit.

The gel cell is similar to the AGM in construction, but the electrolyte is immobilized in a different way. An AGM battery has its electrolyte soaked in in a porous absorptive glass mat separator. All of the liquid sulfuric acid electrolyte is trapped in the sponge-like matted boron-silicate glass fibers. Technically the AGM battery is still considered to be a wet cell although they don't leak or spill acid like a flooded battery. The electrolyte in a gel cell is a solid mass which contains sulfuric acid mixed with a silica gel. The oxygen transfer in gel batteries takes place through the cracks and voids in the gelled electrolyte. The effectiveness of oxygen recombination of VRLA-batteries using a gelled electrolyte is typically 98%.

Gel batteries have nearly the same volume of electrolyte as flooded batteries. The high electrolyte volume makes the gel battery well suited to deep discharge applications. When compared to an AGM battery, the gel battery is less prone to thermal runaways as the significantly less volume of electrolyte in the AGM battery reduces the efficiency of heat transfer. Gel batteries can have over twice the cycle life of a conventional wet battery and usually outlast AGM batteries in deep-cycle applications. However, gel batteries are more expensive than all other types of lead acid batteries. These batteries have poor specific power and power density. Gel batteries do not perform as good as AGM batteries in high current, high power applications and in extremely cold environments. Another disadvantage of gel lead-acid batteries is that they must be charged at a slower rate. The charging voltage must be carefully regulated and temperature compensated.