In a VRLA battery
system is designed in such a way that negative plates are never fully charged
even when the cell is fully charged and hence almost no hydrogen Gas generates
from the negative plates although oxygen is generated from positive plate. This
oxygen gas generated at the positive plate migrates towards the negative plate
and reacts with the freshly fongy lead and turns into lead monoxide. The lead
monoxide is turn reacts with the sulphuric acid to turn into lead sulphate
resulting in the negative plate to be partially discharged.
To summarize the oxygen evolved at the positive plate is
absorbed by the negative plate without being released to the outside. The
negative being always in states of partial discharge never generate hydrogen.
This completely prevent loss of water.
The recombination principle may be expressed by:-
In addition to no
loss of water a VRLA battery also have other improved feature in order to achieve
reliable service and loner life; some of them are described here.
a) Safety
valve: When
the internal pressure increases abnormally, the safety valve opens to release
gas from the cell to restore the normal pressure.
b) Flame
arresting vent plug:
provides with the explosion proof filter constructed of aluminium oxide.
a)
Container:
Made of polyproplyne co-polymer.
b) Positive
plate: with lead calcium tin alloy grid
providing lower corrosion and less self-discharge rates.
c) Separator: Made of high absorbent glass mat
(nonwoven) with excellent porosity(AGM type).
d) Negative
plate: With
lead calcium tin alloy grid providing lower corrosion and less self discharge
rates.
e) Electrolyte: Dilute sulphuric acid without
any impurity keeps regenerating by recombination principle.
Freshening
charge / commissioning charge:
Batteries should not be stored for more than 6 months
without a freshening charge. For ambient temperature above 35C, freshening
charge should be given every 3 months.
Freshening charge should be given at 2.3 volt per all (1.28/8
volt for 56cell battery set for 12 hrs.) and the charging current should be
limited to a maximum of 20% or C10 in amps. When the charging current has
tapered and stabilized, charge for 12hrs or until the lowest cell voltage
ceases to rise. To determine lowest cell the monitoring should be performed
during the final 10% of charge time.
Normal charge:- Batteries should be recharged
within 24hrs don’t keep the batteries in discharged condition. Battery may be
given a terminal/boost charge at 2.30 volt per cell based on the battery
condition .Charge should be given using constant voltage , current limited
charger.
Terminal/boost
charge:-
Under normal condition terminal/boost charge is not required.
Terminal/boost charge is a special charge given to a battery when non-uniformity
in voltage has developed between cells. It is given to restore all cells to a
fully charged condition. Non-uniformity of cells may result from low float
voltage due to improper adjustment of the alternator or panel voltmeter which
reads an incorrect (higher) voltage. Variation in cell temperature greater than
3C in battery bank due to environment or module can also cause low cells.
Pilot cells:-
A pilot cell is selected in the battery bank to reflect the
general condition of all cells in battery. The cell selected should be the
lowest cell voltage in series string following the initial charge.
Charging
method:-
A voltage of 2.30 volt per cell (128.8 volt for 56 cell battery
bank) for a period of 12hrs with charging current limited to a maximum of 20%
of rated capacity at 10hrs rate of C10 in amps. When charging current has
tapered and stabilized (no further reduction for 3hrs) charge for 12hrs or
until the lowest cell voltage cease to rise. To determine the lowest cell.
Monitoring should be performed during the final 10% of the charge time.
