What is the working Principle of valve-regulated lead-acid (VRLA) Battery?
Working Principle of valve-regulated lead-acid VRLA Battery:
MAINTENANCE – FREE SECONDARY CELLS (VRLA Battery)
Maintenance free, valve-regulated lead-acid – VRLA batteries ensure a reliable, effective and user friendly source of power. It is spill proof, leak proof and explosion resistant and there is no need to add water or to clean terminals. It has low self-discharge rate which eliminates the need for equalizing charges. The container is made of polypropylene. Each plate is individually wrapped by a highly absorbent, micro porous glass separate developed specially for VRLA batteries. The chemically inert glass ensures life long service. The absorbed electrolyte ensures that there is no spillage even in the unlikely event of puncture of the cell. Gas evolution under float conditions is negligible. The water loss throughout life due to gassing is roughly 0.1% of the total electrolyte present in the cell. This will in no way affect performance and also eliminate the need for specially ventilated battery room and acid resisting flooring. As the batteries can be installed in stacks, there will be considerable space saving also.
Various capacities of Batteries are 120 AH, 200 AH, 400 AH, 600 AH, 1000 AH, 1500 AH, 2000 AH, 2500 AH, 3000 AH, 4000 AH and 5000 AH.
VRLA Technology – A brief review of Chemical Reaction
The electrode reactions in all lead acid batteries including VRLA battery is basically identical. As the battery is discharged, the lead dioxide positive active material and the spongy lead negative active material react with the sulphuric acid electrolyte to form lead sulphate and water. During charge, this process is reversed. The Columbic efficiency of the charging process is less than 100% on reaching final stage of charging or under over charge conditions, the charging energy is consumed for electrolytic decomposition of water and the positive plates generate oxygen gas and the negative plates generate hydrogen gas.
Under typical charging conditions, oxygen at the positive plate occurs before hydrogen evolution at the negative. This feature is utilized in the design of VRLA batteries. In flooded cells, the oxygen gas evolved at the positive plate bubbles upwards through the electrolyte and is released through the vents. In MF-VRLA batteries the oxygen gas evolved, at the positive plate, instead of bubbling upwards is transported in the gas phase through the separator medium to the negative plate. The separator is a highly absorbent glass matrix type with very high porosity, designed to have pore volume in excess of the electrolyte volume (starved electrolyte design), due to which the oxygen gas finds an unimpeded path to the negative plate. The oxygen gas gets reduced by reaction with the spongy lead at the negative plate, turning a part of it into a partially discharged condition, there by effectively suppressing the hydrogen gas evolution at the negative plate. This is what is known as the oxygen recombination principle. The part of negative plate which was partially discharged is then reverted to the original spongy lead by subsequent charging. Thus, a negative plate keeps equilibrium between the amount which turns into spongy lead by charging and the amount of spongy lead which turns into lead sulphate by absorbing the oxygen gas generated at the positive plate. The oxygen recombination principle can be shown by the following reaction :
Thus, the oxygen recombination technology makes the battery virtually maintenance Free.
Section 3 :
Technical Specification of 1000 AH Battery
1. Capacity of the Battery @ 10 Hr. rate discharge to 1.75 ECV : 1000 AH
2. Nominal Voltage per cell of fully charged battery at 27oC : 2.0 V
3. Open Circuit Voltage (OCV) of fully charged battery at 27oC : 2.15 V
4. Recommended Float Voltage Condition
(i) Terminal Voltage of Charger 2.25 V/Cell
(ii) Float charging current at 2.25 V/cell :Maximum current to be limited to 20% of the rated AH
- Recommended Boost charging condition for quick charging at 27oC : 2.30 V/Cell
- Internal resistance of the cell : 0.257 milli ohms
- Life Expectancy of the Battery : 4000 Cycles at 20% Depth of Discharge or 20 years under Float condition
- Containers :
(i) Material: Polypropylene (Spl. Grade)
(ii) Thickness of wall:2.3 mm (Approx.)
Container and cover – Polypropylene Co-polymer housed in a steel tray
Separator – Spun glass micro porous matrix
Safety valve – Explosion proof, pressure-regulating and self-resealing type
Positive plate – Patented MFX alloy
Negative plate – Lead Calcium alloy
Terminal – Integral lead terminal with solid copper core
Self discharge – Less than 0.5% per week
Charging – Current limited, constant potential
Float charge – 2.25 VPC at 27oC with a max. current limit of 20% of rated capacity in amperes
Boost charge – 2.30 VPC at 27oC with a max. current limit of 20% of rated capacity in amperes
Connectors – Heavy-duty, lead plated copper connectors
Life expectancy – Float service at 27oC – upto 20 years
– Cycle duty at 27oC – 80% DOD – 1200 Cycles
– Cycle duty at 27 oC – 20% DOD – 4000 Cycles
Section 4 – Freshening Charge
Batteries lose some charge during transportation as well as during the period prior to installation. A battery should be installed and given a freshening charge after receipt as soon as possible. Battery positive (+) terminal should be connected to charge positive (+) terminal and battery negative (-) terminal to charger negative (-) terminal.
The charge intervals for storage is given in the table below.
Temp in 0 c Charging interval in months
Storage beyond this period without freshening charge can result in excessive sulphation of the plates.
It is preferable to have the following characteristics in the battery charger being used with these batteries
- High voltage cut-off at 2.37 VPC
- Low voltage trip at 60 VPC
- Voltage ripple to be limited to 2 % of rms.
Voltage regulation to be limited to 1 %
Constant Voltage Method
Constant voltage is the only charging method recommended. Most modern chargers are of the constant voltage type.
Determine the maximum voltage that may be applied to the system equipment. This voltage, divided by the number of cells connected in series, will establish the maximum volts per cell (VPC) that may be used.
Table B lists recommended voltages and charge times for the freshening charge. Select the highest voltage the system allows but not exceeding 2.37 volts per cell to perform the freshening charge in the shortest time period. The charging current should be limited to a maximum of 20% of the rated capacity in Amps.
Table – B
Note : Time periods listed in Table B are for temperatures from 15oC to 40oC. For temperatures below 15oC double the number of hours.
The charging current should be limited to a maximum of 0.2 times of AH Capacity. Widely accepted charging methods use a current of 0.1xC10 (C10 = AH Capacity when discharged at 10 hr rate) Example:
A 2000 AH VRLA battery is to be charged as under
0.2 x 2000
0.2 x 2000 = 400 Amps or
0.1 x 2000 = 200 Amps ( as per TEC recommendation)
Raise the voltage to the maximum value not exceeding 2.37 volts per cell permitted by the system equipment. When charging current has tapered and stabilized (no further reduction for three hours), charge for the hours shown in the above table or until the lowest cell voltage ceases to rise. Correct charge time for the temperature at the time of stabilization. To determine lowest cell, monitoring should be performed during the final 10% of the charge time.