Valve-regulated Lead-acid Battery are maintenance free batteries These batteries are generally known as VRLA batteries. Through these batteries are similar to conventional lead-acid batteries in working principal but their construction and way of monitoring is quite different. This write-up deals with these details and procedure to be followed for monitoring of the state of these VRLA batteries.
Battery/Cell
A battery or cell may be defined as a device capable of supplying positive charges in a given time to a connected load. Flow of positive charges is known as current or flow of electricity . Cell is the smallest unit of the device. A group of cells is collectively known as battery of cells or a battery.
Cells have positive charges accumulated in it and a process takes place inside it producing +ve charges for the required time. This process is a chemical process which takes place in the form of chemical reaction among chemicals present in the cells. The process is known as discharging.
If chemical reaction inside the cell is reversible than the cell is known as rechargeable cell and the chemicals inside the cells can be regenerated making them ready for production of charges again. The process is known as charging.
Conventional Lead-Acid Battery
Lead-acid cell or battery is rechargeable cell also known as accumulator.
It consists of electrodes made of PbO2 and Pb immersed in dilute sulphuric acid (H2SO4and water) known as electrolyte. The specific gravity of fully charged cell is 1.20 to 1.28. PbO2 acts as positive electrode and Pb as negative electrode.
While discharging (supplying current to load), the SO4-2 ions, produces by disintegration of electrolyte, move toward the negative (Pb) electrode, give up the negative charge and produces PbSO4 there.
2H2SO4 4H+ + SO4-2
Pb + SO4-2 PbSO4 + 2 e-
The H+ ions, produces by disintegration of electrolyte, move to the PbO2 electrode, give up the positive charge and reduces PbO2 to PbSO2 as follow:-
PbO2 +4H+ + SO4-2 + 2 e- PbSO4 + 2H2O
As the sulphuric acid is used up in discharging, the specific gravity of the acid decreases. When the specific gravity falls to 1.15, the cell is considered to be fully discharged and any further current drawn from it may permanently damage the electrodes.
The charging process is reverse of discharging process. During charging process current is forced from the positive electrodes to the negative electrode inside the cell.
H+ ions move towards the negative electrodes and react with PbSO4 present there generates Pb (thus restoring negative electrode) there :-
PbSO4 + 2 H+ Pb + H2SO4
At the positive electrode, SO4-2 ions reacts with PbSO4 present there and produces PbO2 (thus restoring positive electrode) there:-
PbSO4 + SO4-2 + 2H2O PbO2 + 2H2SO4
Thus, the PbSO4 deposited during discharging process at the two electrodes is dissolve during charging process.
When the battery charging approaches its final stage the charging current is consumed solely for electrolytic decomposition of water. The generated gases due will escape outside the cell through vents causing decrease of electrolyte quantity in the cell. Hence the cell needs to be topped up with water.
In this way the capacity of the cell is restored for providing the current again.
The emf of a charged cell is about 2.05 V and that of discharged cell is 1.8 V.
The other type chargeable cells are Ni-Cd, Ni-MH, Li-Ion etc. These cells are generally of small capacity and small in size for same capacity and charging time. But these are costly compare to the lead- acid.
As lead-acid cells contains liquid H2SO4 and requires frequent topping up with water and cleaning of sulphate formation on the outer side terminals. Furthermore, charging and discharging processes produce acid fumes (due to heating) and hence have to be installed in separate well ventilated rooms.
Due to above disadvantages of conventional lead-acid cells, innovations have been done and the sealed and maintenance free lead-acid cells have been developed. Since these cells have sealed container, they did not generate any fumes and hence can be installed anywhere.
VRLA Type Lead-Acid Battery
The cells used for the backup battery of UPS and 48V DC supply under ULDC scheme are modified lead-acid type. These cells are VRLA type i.e. Valve Regulated Lead Acid type.
These cells contain liquid acid with highly absorbent glass mat type separator with very high porosity designed to have pore volume in excess of the liquid acid volume making it nearly dry type.
In conventional lead-acid cells oxygen gas evolves at the positive plate and bubbles upwards through the electrolyte and releases through vents. In typical charging conditions, oxygen at the positive plate occurs before hydrogen evolution at the negative plate. This feature is utilized in VRLA cells.
In VRLA cells the oxygen gas transported through the separator medium to the negative plate. There the oxygen gets reduce by reactionwith lead of negative plate turning a part of it into partially discharged condition, there by effectively suppressing the hydrogen gas evolution at the negative plate. This is known as oxygen recombination principal.
Reaction at positive plate:
H2 O2 + 2 H+ + 2e-
Reaction at negative plate :
Pb O2 + Pb
Pb + H2SO4 PbSO4 + H2O
PbSO4 + 2 H+ + 2e- Pb + H2SO4
The part of negative plate which was partially discharged is then reverted to original lead by subsequent charging.
Reaction at negative plate :
PbSO4 + 2 H+ + 2e- Pb + H2SO4
Hence no gas leaves the cells. Due to this these cells do not need topping up with water.
Monitoring of VRLA Lead-Acid Battery
The battery should be monitored regularly for obtaining satisfactory performance through out its operating life. These batteries do not require watering or specific gravity measurement.
10. Once in a month terminal voltage of pilot cell along with atmospheric temperature and battery terminal voltage should be taken to monitor the condition of the battery.
11. A pilot cell is selected in a series string of the cells to reflect the general condition of all the cells in the battery. The cell selected should be the lowest cell voltage in the series string following freshening charge. Freshening charge is the charging of the battery at the time of installation.
12. Once every three months a complete set of readings should be taken of individual cell along with the atmospheric temperature and battery terminal voltage. This three month is the absolute minimum to protect the warranty.
13. Periodical cleaning of the cell covers with a dry 50mm paintbrush should be done to remove accumulated dust. If any cell parts appear to be damp with electrolyte or show sign of corrosion, the same should be reported to the manufacturer.
14. Periodically battery terminals and intercell connections should be inspected. These should be corrosion free and tight fro trouble free operation. If corrosion is present, disconnect the connector from terminal. Gently clean the affected area using a brush or scouring pad. Apply a thin coating of petroleum jelly to the cleaned contact surfaces, after making the connection.
Determination of State of Charge of VRLA Batteries
The maintenance free feature of the VRLA batteries often raises the practical problem in the field. How can the battery bank be monitored?. In conventional batteries, the specific gravity of the electrolyte gives fairly good indication of the state of charge of the battery. However, in VRLA batteries, it is not possible to measure the specific gravity of the electrolyte since it is completely absorbed in the spun glass micro porous separator.
16. However the fact that terminal voltage is directly related to the concentration of the electrolyte may be helpful for state monitoring. Terminal voltage should be Open circuit Voltage and readings should be taken 24 hrs. after charging is discontinued as below:-
% state Open Circuit
of Charge voltage + 0.05 -0.02
100 2.15
90 2.13
80 2.11
70 2.09
60 2.07
50 2.05
40 2.03
30 2.01
20 1.99
10 1.97
0 1.95
However, generally it is not possible to disconnect the batteries. Then the pattern of charging current delivered by a temperature compensated voltage regulated charger after a discharge provides the alternate method for determining the full state of charge. The temperature compensation factor is -3mv per cell 0C rise from the ambient temperature of 27 0C.
Under normal conditions the batteries are floated at around 2.25 volts per cell i.e. in a 48V DC system cells are floated at 53.5 volts. During charging as the cells approach full charge, the battery voltage rise to approach the charger output voltage i.e. 53.5V and charging current decreases to the float current value around 50mA/100 AH for VRLA batteries. So, when the charging current has stabilized at the float current for three consecutive hours or voltage across the battery bank terminals is constant for six consecutive hours then the battery bank can be considered as having reached full state of charge.
