In this lecture, we are going to learn about the VRLA better, the working principle of the VRLA battery, technical specifications, charging methods, and many more details about VRLA better. So let’s start to form a brief introduction to VRLA battery.
Brief Introduction of VRLA Battery
- The Full form of VRLA battery is Valve Regulated Lead Acid Battery.
- Maintenance-free, valve-regulated lead-acid (VRLA) batteries ensure a reliable, effective, and user-friendly source of power.
- It is spill-proof, leakproof, and explosion resistant and there is no need to add water or clean terminals. It has a 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, microporous glass separately developed especially for VRLA batteries.
- The chemically inert glass ensures lifelong service. The absorbed electrolyte ensures that there is no spillage even in the unlikely event of a 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 rooms 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 the VRLA battery are 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 sulfate and water. During charge, this process is reversed.
- The efficiency of the charging process is less than 100% on reaching the final stage of charging or under overcharge conditions, the charging energy is consumed for the 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, thereby effectively suppressing the hydrogen gas evolution at the negative plate. This is what is known as the oxygen recombination principle.
- The part of the 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 the spongy lead by charging and the amount of spongy lead which turns into lead sulfate by absorbing the oxygen gas generated at the positive plate. The oxygen recombination principle can be shown by the following reaction :
1. Reaction at the positive plate: H2O = ½ O2 + 2e–
2. Reaction at the negative plate :
Pb + 1/2O2 = PbO … (2)
PbO+H2SO4 = PbSO4 + H2O … (3)
PbSO4 + 2H+ + 2e– = Pb + H2SO4 … (4)
3. The total reaction at the negative plate: ½O2 +2H+ = H2O
- Thus, the oxygen recombination technology makes the battery virtually maintenance Free.
Technical Specification of 1000 AH Battery
|Sr. No||Specifications of Battery||Values of Battery|
|1.||The 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
(ii) Float charging current at 2.25 V/cell
Maximum current to be limited to 20% of the rated AH
|5.||Recommended Boost charging condition for quick charging at 27oC||2.30 V/Cell|
|6.||The internal resistance of the cell||0.257 milli ohms|
|7.||Life Expectancy of the Battery||4000 Cycles at 20% Depth of Discharge or 20 years under Float condition|
(ii) Thickness of wall
Polypropylene (Spl. Grade)
2.3 mm (Approx.)
|9.||Container and cover||Polypropylene Co-polymer housed in a steel tray|
|10.||Separator||Spun glass microporous matrix|
|11.||Safety valve||Explosion-proof, pressure-regulating, and self-resealing type|
|12.||Positive plate||Patented MFX alloy|
|13.||Negative plate||Lead Calcium alloy|
|14.||Terminal||Integral lead terminal with solid copper core|
|15.||Self-discharge||Less than 0.5% per week|
|16.||Charging||Current limited, constant potential|
|17.||Float charge||2.25 VPC at 27oC with a max. the current limit of 20% of rated capacity in amperes|
|18.||Boost charge||2.30 VPC at 27oC with a max. the current limit of 20% of rated capacity in amperes|
|19.||Connectors||Heavy-duty, lead-plated copper connectors|
|20.||Life expectancy||Float service at 27oC – up to 20 years|
Cycle duty at 27oC – 80% DOD – 1200 Cycles
Cycle duty at 27oC – 20% DOD – 4000 Cycles
Freshening Charge of VRLA Battery
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. The battery positive (+) terminal should be connected to the charge positive (+) terminal and the battery negative (-) terminal to the charger negative (-) terminal.
The charge intervals for storage are given in the table below.
|Temp in oC||Charging interval in months|
- Storage beyond this period without freshening charge can result in excessive sulphation of the plates
Requirement of Charger
- 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 1.60 VPC
- Voltage ripple is to be limited to 2 % of rms.
- Voltage regulation is 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 the 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 the lowest cell, monitoring should be performed during the final 10% of the charge time.
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