Valve Regulated lead acid battery:
It is more commonly known as a sealed battery.Because of its construction,VRLA battery do not require regular addition of the water into the cells & venting gases ,compare to flooded lead acid battery.
The reduced venting is an advantage since they can be used in confined or poorly ventilated space.
But sealing cells & preventing access to the electrolyte also has several disadvantage.
VRLA Batteries are commonly classified as:
1. Absorbed glass Mat(AGM) Battery
2. Gel Battery (gel cells)
An absorbed glass mat battery has the electrolyte absorbed in fiber glass mat separator.
A gel cell has the electrolyte mixed with silica dust to form an immobilized gel.
While these batteries are often colloquially called sealed lead acid battery,they always include safety pressure relief valve. As opposed to vented(also called flooded) batteries, a VRLA cannot spill its electrolyte if it is inverted.
Because AGM VRLA Batteries use much less electrolyte than traditional lead acid batteries ,they are sometimes called an Acid starved design.
They are actually recombinant batteries ,which means that the oxygen evolved at the positive plates will largely recombine with the hydrogen ready to evolve on the negative plates creating water & preventing water loss.
The valve is a safety feature in case the rate of hydrogen evolution becomes dangerously high.
In flooded cell the gases escape before they can recombine,so water must be added periodically.
Gel Battery:
It is a VRLA Batetry with a gelified electrolyte,the sulphuric acid is mixed with silica fume,which makes the resulting mass gel like immobile. Unlike a flooded lead acid battery ,these batteries do not need to keep upright.Gel batteries reduce the electrolyte evaporation,spillage (& subsequent corrosion)common to the wet cell battery & boast greater resistance to extreme temp, shock & vibration.
Chemically they are almost the same as wet batteries except that the antimony in the lead plates is replaced by calcium & gas recombination can take place.
Comparison with Flooded lead acid Battery:
Advantage:
1.Can be mounted in any position
2.Since the battery system is designed to be recombinant & eliminate the emission of gasses on overcharge,room ventilation requirements are reduced & no acid fume is emitted during normal operation.
3.The volume of free electrolyte that could be released on damage to the case or venting is very small.
4.There is no need to check the level of electrolyte or to top up water lost due to electrolysis,reducing inspection & maintenance.
5.Have shorter recharge time.
Disadvantage:
1. Because of calcium added to its plates to reduce water loss,a sealed battery recharges much more slowly than a flooded lead acid battery.
2.Compare to flooded batteries VRLA Batteries are more vulnerable to thermal run away during abusive charging.
3.Less reliable than flooded lead acid cell
4.Can not tolerate overcharging ;its lead to premature failure.
5. Have shorter useful life ,compared to properly maintained wet cell battery.
6. Discharge significantly less hydrogen gas.
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