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Author Topic:   Gelled Electrolyte Lead-Acid Battery
jimh posted 06-04-2013 09:11 AM ET (US)   Profile for jimh   Send Email to jimh  
A gelled electrolyte battery is typically a lead-acid battery in which the electrolyte has been formulated to be in a gelled state, and the battery is not vented. The attraction of having the gelled electrolyte is a reduction in the risk of spill. In conventional vented flooded-cell lead-acid batteries, the electrolyte can spill if the battery is turned sideways or upside down.

The principal problem with a gelled electrolyte lead-acid battery is its sensitivity to charging voltage. The chemistry of all lead-acid batteries causes the electrolyte to out-gas hydrogen (and oxygen) if the charging current pushes the battery too far in the chemical reaction of storing energy in the electrolyte. The electrolyte boils out hydrogen gas. In a vented flooded-cell lead-acid battery, the gas escapes to the atmosphere, and the electrolyte level is reduced. The remedy is to add more electrolyte.

In a gelled-electrolyte lead-acid battery, the gas creates bubbles in the gelled electrolyte, but because of the gelled nature, these bubble cannot migrate. They remain near the lead plates of the battery, and their effect is to reduce the surface area of the lead plates. As more trapped gas bubbles occur, the surface area of the lead plates in the battery is reduced, and the capacity of the battery to store electrical energy is reduced. If the over-charging is chronic, eventually too much electrolyte is lost, there are too many bubbles, and the battery capacity is reduced too far, making the battery useless.

In an absorbed glass mat (AGM) battery, the electrolyte is a non-gelled liquid, and, like the gelled electrolyge battery, the AGM uses a sealed non-vented case. If an AGM battery is overcharged, the same hydrogen and oxygen gases will be produced. However, the gases are free to move away from the plates. AGM batteries, like all sealed or maintenance-free batteries, employ a recombinant mechanism that works to capture the hydrogen and oxygen gase, combine them back into water, and return the water to the electrolyte. The effect is to permit AGM batteries to survive some over-charging without damage. This same design is used in flooded-cell batteries with a sealed case. A recombinant mechanism tries to capture gases and return them to the electrolyte. In all these designs, if too much gas is produced, eventually a pressure-controlled release valve on the case permits the gases to escape to the atmosphere. This occurs when the recombinant mechanism has been overwhelmed by too much gas.

The gelled electrolyte batteries are typically noted as requiring a lower charging voltage than other lead-acid batteries. This provision is required to avoid reaching the step in the chemical reaction of re-charging when hydrogen and oxygen gas will be formed. By holding the maximum charging voltage below this threshold level, the gelled-electrolyte battery can avoid creating gas bubbles.

In applications with outboard engines, the regulation of the charging current from the outboard engine is typically pre-set and cannot be adjusted by the user The outboard engine charging current may cause a lead-acid battery connected to the outboard to produce gas during the charging cycle. If charging a flooded-cell vented lead-acid battery, the electrolyte lost to a gas can be replaced by adding electrolyte to the battery. If charging an AGM battery, the lost electrolyte can be captured by the battery's recombinant mechanism and returned to the cells. If charging a gelled electrolyte battery, the electrolyte lost tends to remain a bubbles in the gel, causing the battery to lose capacity.

Many older outboard engines have very poor voltage regulation of their charging current, and they will chronically over-charge their batteries. Newer outboard engines have better regulated charging outputs, but they are not designed with the strict regulation needed for use with gelled electrolytes. For this reason, using a gelled electrolyte battery with an outboard engine may result in chronic over-charging of the battery, with the corresponding loss of storage capacity and eventual ruin of the battery. AGM batteries are at risk for this, too, but their incorporation of a recombinant mechanism to return gas to the electrolyte tends to protect them against this problem. However, if chronically overcharged, an AGM battery can be ruined from creation of too much gas, too. For this reason, AGM batteries should not be used with older outboard engines whose charging current is not well regulated.

Because there is not a particular advantage of using a gelled electrolyte battery compared to an AGM battery in the electrical performance as an energy storage device, and because the gelled electrolyte battery is at more risk to being damaged by over-charging, I don't see any particular incentive to using gelled electrolyte batteries. Typically an AGM battery will provide equivalent electrical performance and avoid the risk of damage inherent in the gelled electrolyte battery.

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