Water loss : Water in a vented lead acid cell is lost during float or over charge by a process known as electrolysis . In this process water is converted in to oxygen at the +ve plate and to Hydrogen at the ve plates . These gasses are allowed to vent out of the cell into atmosphere, resulting in loss of water in a lead acid cell . The oxygen recombination cycle limits the water loss.

Oxygen recombination : Perfect water to oxygen - to water cycle , all the oxygen produced at the + ve plate on float or overcharge would be transported to the ve plate and converted back to water, with no water being lost in the cell. This is the basis for VRLA cell technology. ( IEEE Std 1189-1996)

Gelled electrolyte technology : Gelled electrolyte cells are designed such that voids ( gas passages ) develop in the gel. These voids serve as passages through which oxygen transport to the negative plates is enhanced.Absorbed electrolyte technology : Absorbed electrolyte cells are designed with an absorbent separator that is approximately 95% filled with liquid electrolyte. The remaining voids provide for optimized oxygen transport from positive to negative plates (AGM technology : Absorbent Glass Mass technology )

Pressure regulation valve: The internal cell pressure caused by the evolved gases is regulated by a valve that allows them to escape periodically. This is the origin of the term valve regulated. VRLA valves operate over a relatively narrow range, typically within the limits of 0.5 50 psig, depending upon design, allowing the escape of unrecognized gases and preventing the backflow of air into the cell

Thermal runawayWhen a VRLA cell is operating on float or overcharge in a fully recombinant mode, almost all of the overcharge energy results in heart generation . If the design of the system and environment are such that the heat produced can be dissipated and thermal equilibrium can be reached , then there is no thermal runaway problem. However if the recombination reaction gives rise to a rate of heat evolution that exceeds the rate of heat dissipation, the battery temperature will rise and more current will be required to maintain the float voltage. The additional current results in still more recombination and heat generation further raises battery temperature , and so on. The net effect can be accelerated dry out/or melting of the battery. The problem of thermal runaway can be minimized by use of appropriate ventilation between and around the cells and limiting charging current .

Effect of temperature on capacity of cell:

The temperature operation will shorten VRLA battery life. As a general rule , prolonged use at elevated temperature will reduce the battery life by approximately 50% for every 8 degree centigrade raise in temperature .

StorageIf the batteries are not to be used immediately , they should be stored in a cool, dry location , protected from direct sunlight and rain.Do not stack the packing boxes one on the top of the other since this may cause damage to the batteries.Storage period :Batteries upon storage lose capacity due to self discharge. The rate of self discharge increase with temperature.Batteries need to be charged periodically , in case of prolonged storage, failing which the battery capacity may become irrecoverable due to irreversible salvation.The are supplied from the factory in charged condition and can be stored up to a maximum of 6 months before installation at an ambient temperature of 27 deg. C In general , a 7 t0 10 deg.C rise in ambient temperature will have storage time reduced from 6 months to 3 months.

Initial charging ( freshening )The battery system may require an initial charge if the voltage per cell drops to 2.1 Volts while the battery is in storage prior to installation.Set the Voltage to 2.3 volts x No. of cells with a current limit of .2 C10 Amps max. After attaining the required voltage for three consecutive hours, the system has to be switched to float mode ( 2.25 V per cell) and the load may be connected to battery.However , initial commissioning of batteries may be done in the presence of supplier service engineer.

Recharging details for the discharged batteriesCharging must be carried out at constant potential with the current limit to .2 C10 Amps. Charge in boost mode at a charge voltage of 2.3 volts per cell for a period of 16 hours followed by charge in float mode at a float voltage of 2.25 volts per cell for a period of 72 hours before connecting to load . The duration of charge varies with the depth of discharge.

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