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VRLA Battery or Valve Regulated Lead Acid Battery

The VRLA battery means the Valve Regulated Lead Acid Battery. This type of battery is sealed from outside. So there is no provision of topping up water or acid during its lifespan. It is a maintenance-free battery.

Types of VRLA Battery

Mainly two varieties of VRLA battery are available in the market. These are Gel Cell and Absorbent Glass Mat Cell.

Gel Cell Type VRLA Battery

Here we mix silica powder to the electrolyte. So it forms a putty-like gel. Due to the presence of silica powder in the electrolyte, sometimes we also call it a silicone battery.

Absorbent Glass Mat Cell Type VRLA Battery

Here we use fine fiberglass mesh to hold the liquid electrolyte. We place the fiberglass mesh between each pair of electrode plates.

Advantages of VRLA Batteries

The main benefits of VRLA Batteries are

  1. It is maintenance free. Although, regular cleaning of the cells is also essential. In addition to that, continuous monitoring of voltage per cell is necessary. Because, if we see any abnormality in any cell of the battery assembly, we must immediately replace the cell. Otherwise, it may affect the lifespan of the other cells in the row.
  2. Another advantage of VRLA batteries is that we can orient a VRLA cell in any orientation. Because of the chance of spilling of electrolyte from these cells is nil.
  3. These batteries are free from spilling of electrolyte. Hence, we can safely use them in any vibratory movable system.

Disadvantages of VRLA Batteries

The electrolyte inside the VRLA battery cells is immobile. Because of immobility, the sudden increase in the rate of electrolysis does not happen. It is the main disadvantage of the VRLA design. Because of that, we can not use VRLA batteries in high peak power applications. For example, we usually do not use Valve Regulated Lead Acid (VRLA) Battery for the starting purpose of vehicle engines. Because starting of an engine requires high current pulses. We only use VRLA where charging and discharging is slower and consistent.

Working Principle of VRLA Battery

The charging and discharging phenomena of a VRLA battery are identical to that of a conventional lead-acid battery.  Indeed the basic working principle of a lead acid battery is the same as that of a VRLA battery.  Like a conventional lead-acid cell, in VRLA Cell produces, oxygen, and hydrogen gas during overcharging.  But in VRLA cell the gases cannot escape from the electrolyte in bubble forms. In other words, the gas can not come upwards directly in bubble forms. Instead, the porous medium of electrolyte traps the gases.

The trapped oxygen gas from positive plates comes in contact with the negative plate through the porous medium of electrolyte.  Then, the oxygen reacts with the spongy lead and forms lead oxide. Then sulfuric acid reacts with this lead oxide and produces water. Therefore a portion of the negative plate becomes lead sulfate. That is the partially discharged condition of the negative plate. In this way, both hydrogen and oxygen recombine in water. So a VRLA battery cell does not lose its water (moisture) even during prolonged overcharging. Therefore, we do not need to feed water to the battery cells. To conclude we can say it is a maintenance-free battery.

Valve Regulated Means

This type of battery cell consists of a valve on the top cover. The valve provides a way of releasing excess gas which may be formed in an abnormal overcharging condition. Although in VRLA cell normally over gassing does not happen even in overcharging condition. But still, this valve is provided for better safety.

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