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.
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
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.
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
The Hybrid break-through
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
Thus, the Hybrid has arrived offering:
Hybrid suggests duality and this battery does have dual applications. It
enjoys superior cyclic life but also excellent in standby applications.
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
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.
choose a hybrid - click