What is VRLA Battery?

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.

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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 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 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.

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Technical Specification of 1000 AH Battery

Sr. NoSpecifications of BatteryValues of Battery
1. The capacity of the Battery @ 10 Hr. rate discharge to 1.75 ECV1000 AH
2.Nominal Voltage per cell of fully charged battery at 27oC2.0 V
3.Open Circuit Voltage (OCV) of fully charged battery at 27oC2.15 V
4.Recommended Float Voltage Condition
(i)  Terminal Voltage of Charger
(ii)  Float charging current at 2.25 V/cell
2.25 V/Cell
Maximum current to be limited to 20% of the rated AH
5.Recommended Boost charging condition for quick charging at 27oC2.30 V/Cell
6.The internal resistance of the cell0.257 milli ohms
7. Life Expectancy of the Battery4000 Cycles at 20% Depth of Discharge or 20 years under Float condition
8.Containers :
(i)  Material
(ii)  Thickness of wall
Polypropylene (Spl. Grade)
2.3 mm (Approx.)
9.Container and coverPolypropylene Co-polymer housed in a steel tray
10.SeparatorSpun glass microporous matrix
11.Safety valveExplosion-proof, pressure-regulating, and self-resealing type
12.Positive platePatented MFX alloy
13.Negative plateLead Calcium alloy
14.TerminalIntegral lead terminal with solid copper core
15.Self-dischargeLess than 0.5% per week
16.ChargingCurrent limited, constant potential
17.Float charge2.25 VPC at 27oC with a max. the current limit of 20% of rated capacity in amperes
18.Boost charge2.30 VPC at 27oC with a max. the current limit of 20% of rated capacity in amperes
19.ConnectorsHeavy-duty, lead-plated copper connectors
20.Life expectancyFloat 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 oCCharging 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
  1. High voltage cut-off at 2.37 VPC
  2. Low voltage trip at  1.60 VPC
  3. Voltage ripple is to be limited to 2 % of RMS.
  • Voltage regulation is to be limited to 1 %

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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

Cell VoltsTime
2.2530 hrs
2.3012 hrs
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.


The service life of a VRLA battery depends as much on usage as on design and manufacturing quality. Unmanned sites and no redundancy have become commonplace in telecom installations. Knowledge of certain simple points will help to extend the battery service life and improve reliability.

Charging voltage

  • Float charge voltage shall be set at 2.250 ± 0.005 V/cell at 27° C
  • Boost charge voltage shall be set at 2.300 ± 0.005 V/cell at 27° C
  • Equalizing charge voltage shall be set at 2.350 ± 0.005 Vicella 27° C Charge voltage (Float / Boost / Equalizing charge) temperatures other than 27° C shall be corrected using the empirical formula.

Battery path current

  • For float applications – Shall be a minimum of 10% of battery capacity
  • For semi-cyclic / cyclic applications – Shall be a minimum of 15 to 20% of battery capacity

Recharge Time

  • Necessary arrangements shall be made so that the battery receives full charge after every discharge. Discharging a battery before receiving a full charge leads to gradually reduced available capacity.

Charge settings for batteries under Partial State of Charge (PSOC) operation

  • In cyclic applications, the battery may suffer from undercharging. So appropriate charging voltage and current shall be selected (specific to the application) for optimum performance & life.
  • Please consult HBL for guidance on the charge settings so that the impact of PSOC operation on battery life can be minimized. Also please provide the details on operating conditions (Load current, backup time/Discharge duration, Cut off voltage, Power plant features – Max, voltage, Max. current, Boost to float change over criteria, etc. & Operating temperature) while seeking recommendations.

Low Voltage Disconnect (LVD) Settings

  • LVD value corresponding to the selected DOD shall be set on the power plant. For optimum performance and life, the battery shall be cycled at 50 to 60% DOD.

Equalizing Charge

  • Equalizing charge shall be given as per Section 8.3 at regular intervals.

Enhance the battery capacity

  • Battery capacity shall be enhanced when the operating conditions (Load current, Back up time, Recharge time, Battery path current limit, etc.) change from the ones considered during sizing without which battery life is shortened.

Frequently Asked Questions on VRLA Battery

What is meant by VRLA battery?

A valve-regulated lead–acid (VRLA) battery, is commonly known as a sealed lead–acid (SLA) battery.

How long does VRLA battery last?

The typical design life of VRLA batteries to back up UPS systems is 10 years. However, a battery’s actual service life is usually only 3-5 years. The end of life is commonly defined as the point when a battery can only be charged to 80% of its rated capacity.

Can the VRLA battery be charged?

Constant current charging usually is used for charging these batteries. Constant voltage with a current limiting process is recommended for charging these batteries.

Is VRLA battery maintenance free?

The VRLA battery is the most popular reserve power design because the electrolyte is captive, preventing it from spilling even when the case is punctured. VRLA batteries are considered “maintenance free” and require no addition of electrolytes or water.

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Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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