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Sealed Lead Acid Batteries


Sealed Lead Acid batteries come in a variety of technologies. Each technology has its attributes, advantages and disadvantages in any given application - however, they all remain 'Lead Acid' batteries.  


They are known as;


SLA (Sealed Lead Acid)


VRLA (Valve Regulated Lead Acid)


AGM (Absorbed Glass Mat)


Gel (Gelled electrolyte)


Hybrid (Gel/AGM Hybrid)



Where a regular flooded battery has the electrolyte in a 'wet' fluid state, also referred to as 'free electrolyte',  the essence of a sealed battery is to immobilise the electrolyte. However, it is necessary that the gasses generated during charging are recombined in a so called 'oxygen cycle' to the fluid and negative plate. Should the gasses escape, a gradual drying out would occur. To assist in the recombination of the water's constituents, oxygen and hydrogen, the sealed case is held in a positive pressure of 2 or 3lbs above atmospheric pressure.

In the case of rapid generation of oxygen gas exceeding the absorbing capacity, the pressure relief valve will open to release the excessive gasses.




SLA (Sealed Lead Acid) batteries otherwise known as VRLA (Valve Regulated Lead Acid) were originally known as "Dry Batteries". They were introduced in the 1950's and at that time untilised a Gel electrolyte. The otherwise free acid was immobilised with a fine silica powder and formed a gel substance. In the 1970's technology moved to AGM (Absorbed Glass Mat) where the separators between the plates were made of a felt of micro-fine glass fibers absorbing and immobilising the acid.



AGM has since become the preferred VRLA technology for use in stand-by or float applications.  They are used in multi unit power packs for UPS support in the telecommunications, power, and indeed any mission critical industry where the power supply must not be interrupted.

Gel batteries are also used in UPS stand-by duty but it is argued that neither are the complete answer.

Hence the development of the Gel/AGM Hybrid


Gel technology has experienced good results in cyclic applications and used extensively in boats, motor homes, golf trundlers and carts - the list is long.


Nothing is perfect

The various shortfalls of AGM and Gel batteries are well documented.

Research has shown that considerable compressive force is needed for the AGM mat to make good electrolyte/plate contact. It looses compression as the cells age affecting performance.


On the other hand, the immobilised gel electrolyte suffers shrinkage over time due to loss of water and dry out or sulphating of the negative electrode. The gelled electrolyte inherently has a lower recombination efficiency compared with AGM.

And further, the gel will not tolerate gassing bought on by excessive voltage while charging. However,  this issue is manageable with appropriate regulation of alternators and mains-powered chargers.


The Hybrid break-through

In spite of the problems discussed above,  the AGM is still the preferred material for immobilising the  electrolyte in VRLA batteries. To further improve gel technology, a colloidal poly-silica gel has been developed  and this in combination with AGM technology has given rise to Hybrid Gel technology.


Excess gel surrounding the cell element provides additional reserve and superior thermal properties. And while the AGM-based technology has the preferred approach to immobilising the electrolyte, the combining of the two technologies is suggested as being the method to improve the sealed lead acid battery. Further development has been done to find a glass matting with better memory under compressive loads ensuring retention of plate contact for efficient ion transfer.


Thus, the Hybrid has arrived offering:


Superior cyclic service even in adverse environmental conditions.


Improved thermal properties extending long-term reliability and performance.


Hybrid Application

Hybrid suggests duality and this battery does have dual applications. It enjoys superior cyclic life but also excellent in standby applications.


In the case of the marine house bank, one generally talks 'deep cycle'. But is it really cyclic use of a battery when it is subjected to stand-by for weeks at a time and then used over several days? One could rightly say the battery bank requires stand-by and cyclic attributes. The solution?  The hybrid battery.


Even when the vessel has full time residents, it will be powered by shore-power at the dock and cycled -batteries during oceanic cruising. The solution?  The hybrid battery.


There are obviously many cyclic and stand-by applications one could conjure up and perhaps the hybrid, being generally superior in both roles is the better solution to power requirements. 



To choose a hybrid - click 


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