White Paper on Battery Technologies

8/10/2019 White Paper on Battery Technologies

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POWER PLANTS ______________________________________________________________________________

Evaluation of

Different Battery Technologies

Used in Indian Telecom Network

_____________________________________________________________________________

Telecommunication Engineering CentreKhurshid Lal Bhavan, Janpath

New Delhi-110001, India


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CONTENTS

S. No Topic Page No.

1. Introduction 1

2. Chronicle 1

3. Comparative analysis of VRLA (AGM and GEL) technologywith respect to Flooded type batteries (Tubular/Flat pasted)

2

4. Tabular comparison of batteries based on differenttechnologies (VRLA AGM, Tubular VRLA GEL and Flooded)

4

5. Present Field Scenario 5

6. Conclusions 5

7. Suggestions of TEC 6


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Introduction

This paper details the chronicle of developments in technology of batteries used in IndianTelecom Network. It deliberates on the comparative study of various technologies used. It givesthe present state of batteries under use in the telecom network. It also analyses the scope ofswitchover from one technology to another.

Chronicle

The battery has been the integral part of the telecom system since the beginning. It was used asthe main power source in the initial stages, and as standby power source, later on.

In the initial stages of standby power source, flooded flat-pasted type batteries were used in theIndian telecom network. Later on flooded tubular positive plates batteries were used. All thesebatteries were being procured and tested as per respective IS standard IS-1650. Thecertification of these batteries was being done by QA wing of DoT (presently BSNL).

Both these type of batteries require a separate battery room, because they release a significantamount of sulphuric acid fumes.

These batteries also require periodical special charging process at comparatively higher voltageof 2.7V/cell (a total voltage of 65 V), also known as boost-charging, to agitate the electrolytethoroughly to prevent stratification of electrolyte, as well as to reduce sulpahtion of plates. Asthe telecom equipment can not withstand such a high voltage, the battery under boost-chargingand the charger have to be isolated from the exchange equipment.

Maintenance of these batteries needs more efforts and is more labour oriented.

With the expansion of telecom network, large number of small exchanges came into existence.These exchanges were mostly installed in the rented buildings of one or two rooms. It becamenecessary to explore the battery technology which can be accommodated with the power plant,if possible with the telecom equipment also.

In 1991 TEC was entrusted the job of studying the various technologies and also prepare theGR of the battery technology after approval.

In 1992 first edition of the GR (No. GR/BAT-01/01 AUG 1992) for VRLA batteries based on AGM (Absorbent Glass Mat) technology was issued. The department started procuring thebatteries instantly on experimental basis and later on in bulk.

It is understood that the department for the last so many years has not procured any PowerPlants compatible with conventional flooded type batteries, it appears that department, long

back, has decided to go for Power Plants compatible with VRLA Batteries only. Moreover,presently except for a few conventional (SCR/Thyristor Controlled) Power Plants, all the PowerPlants in the field are SMPS Power Plants compatible with VRLA Batteries only.

TEC studied the VRLA battery technology in detail and issued a Monitoring and planningguidelines for VRLA batteries in 2003. The same was circulated to all the telecom circles.

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Malfunctioning of some of the VRLA batteries were reported from the field units. On analysis byTEC it was found that in most of the cases, the failures were due to shortcomings in themanufacturing designs of a manufacturer.

At the time of revision of the GR issued in 2004, manufacturing design parameters were alsoincluded in the GR. Since then, procurements of batteries have been as per the new GR and nofield complaints has been received by TEC.

TEC has also issued a GR for Tubular VRLA batteries based on GEL technology (No. GR/BAT-03/01 MAR 2006) in March 2006, which has been introduced, in BSNL /MTNL on experimentalbasis. This technology has been chosen because it can work with the existing SMPS PowerPlant in the field, irrespective of the version of the existing Power Plant. It is also claimed that ithas all the advantages of AGM VRLA technology, along with the following additionaladvantages :

a. Better thermal management, because of more electrolyte.b. Better performance at high temperature as the life loss for every 10 C is 30% against

50% for AGM batteries.c. Cyclic life is 20% to 30% higher than AGM batteries (1400 cycles in AGM and 2100

cycles in Tubular GEL).d. Self discharge is lower than AGM batteries and can be stored up to one year against six

months as in AGM.e. Same charging techniques as for AGM but slightly low float and charge voltage.f. Slow rate of discharge performance is excellent, suits rural application.

Decision to make bulk procurement shall be taken after successful completion of field trials andsubsequent evaluations.

Comparative analysis of VRLA (AGM and GEL) technology with respect to flooded typebatteries (Tubular/Flat-pasted) :

a. VRLA batteries have a pressure regulated valve which does not allow the gasses toescape unless the set pressure is exceeded. As the charging voltage is kept at lowerlevel, there is negligible gassing. Moreover, recombination of hydrogen and oxygen isachieved by, which further results in reduction in loss of water. As the amount ofreleased gasses is negligible, release of acid fumes is also reduced. Hence thesebatteries can also be installed in the equipment room. Whereas, in flooded batteries(tubular/flat-pasted batteries) a large amount of sulphuric acid fumes are released, dueto which a separate battery room with exhaust fans is essential.

b. Use of VRLA batteries results in :

Saving in space as the battery, power plant and equipment can be installed in thesame room.

Saving in bus-bar/cable material. Low voltage drop resulting in better utilisation of battery capacity and increase in

efficiency of equipment.

c. Charge efficiency of VRLA batteries is excellent, i.e., 6 to 8 hours for 90% recovery, asagainst 12 to 14 hours for flooded batteries. Recouping of capacity is quicker as shorter

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time is sufficient as compared to flooded batteries, suiting the rural power supplyconditions particular.

d. VRLA batteries have high charge density, resulting in smaller battery size. Moreover, thebattery can be discharged to lower end-cell voltage, viz., 1.75V/cell against 1.85V/cell forflooded batteries.

e. VRLA batteries can be stacked horizontally and the cell can be stacked one above theother, which reduces the battery space requirement by 30-60%.

f. VRLA battery performance is better under partial state of charge condition as thesebatteries are less prone to sulphation. They suite the telecom requirements, wherepartial state of charge of batteries is quite common.

g. VRLA batteries have high cyclic life - 1400 cycles for AGM and 2100 cycles for GEL- atan average temperature of 35C, as compared to 2000 cycles at 27C for floodedbatteries.

h. Longer storage in charged condition for VRLA batteries, up to six months for AGM andone year for Tubular GEL at 35C, is possible without permanent loss of capacity,against 28 days at 27C for flooded batteries.

i. VRLA batteries can be transported in the charged condition as there is no danger of thespillage of the electrolyte, and hence, initial charging at site is not required. Floodedbatteries can not be transported in charged condition, hence assembly and charging atsite is essential. Moreover, it will be more difficult for the flooded batteries to meet thepollution norms, issued by Ministry of Environment and Forest.

j. Capacity of VRLA batteries can be recovered even after a storage period of sixmonths/one year within three to four charge discharge cycles, while in flooded batteries

it very difficult even after one month, due to sulphation of the plates while in idlecondition.

k. Stratification is unavoidable in flooded batteries because of the settling down of the acidin the lower portion of the cell which causes sulphation in both lower and upper parts ofthe plates. To avoid stratification, periodic boost-charging of these batteries at 2.7V/cellis absolutely essential, for agitating the electrolyte. In VRLA batteries stratification isruled out as electrolyte is captive either in AGM or in Gel.

l. In VRLA batteries, internal short-circuiting does not occur, while it is quite common inflooded batteries due to sedimentation of shed active material of the positive plate.

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Tabular comparison of Batteries based on different technologies (VRLA AGM, TubularVRLA GEL and Flooded)

S.No. Feature VRLA (AGM) Tubular GEL VRLA Tubular Flooded1. Gassing/

fumingNo gassing/fuming,can be installed

anywhere

No gassing/fuming,can be installed

anywhere.

High gassing/fuming,separate battery room

with exhaust system isessential.

2. Topping up ofelectrolyte

No topping-up requirednormally

No topping-uprequired normally

Topping up requiredfrequently

3. Chargingcurrent level

High Lower Lowest

4. Spacerequirement

Small cell size, Lowspace requirement.

Small cell size, Lowspace requirement.

Large cell size, Largespace required.

5. Stacking Horizontal or vertical Up to 1500 AH :Horizontal or vertical>1500 AH: Vertically,in tiers

Vertical stacking only.Tier stacking notpractical for large size.

6. Transportation

in chargedcondition

Easy Easy Not possible.

Transportation inuncharged (unfilled)condition recommended.

7. Self-dischargeduring storage,at an averagetemperature of35C.

50% self-discharge in6 months. Recoveryeasy.

50% self-discharge inone year. Recoveryeasy.

Self-discharge is veryhigh. Long durationstorage notrecommended.Recovery difficult.

8. Cyclic Life(to 80% DoD).

1400 cycles at anaverage temperatureof 35C in normalenvironmentalcondition

Better than 2100cycles at an averagetemperature of 35C innormal environmentalcondition

Theoretically maximum2000 cycles at 27C.

9. Float life at 35C Good Good Not known10. Hightemperatureperformance

Average, buttemperaturecompensationprovision made in thePower Plants

Good Good

11. Lowtemperatureperformance

Good Good Poor

12. Stratification Negligible, no boostcharging required.

Negligible, no boostcharging required.

Prominent, requiresfrequent boost chargingfor prevention.

13. End cell

voltage

1.75V/cell 1.75V/cell 1.85V/cell

14. Capacity atvery low rate ofdischarge

Good Good Average

15. Deep dischargerecovery

Average, after 4 to 5charge/dischargecycles

Average, after 4 to 5charge/dischargecycles

Poor, hard sulphationprevents recovery.

16. Chargeefficiency

Excellent, 6 to 8 hoursfor 90% recovery.

Slightly poor, 8 to 10hours for 90% recovery

Poor, 12 to 14 hours for90% recovery.

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S. No. Feature VRLA (AGM) Tubular GEL VRLA Tubular Flooded

17. Under-chargedperformance

Average Good Poor

18. Overcharging Poor, damages thebattery.

Over charge protectionprovision made inSMPS Power Plants.

Good Good

19. Performanceunder partialstate of charge

Good Good Poor

20. ChargingRequirement

Constant voltagecharging by SMPSPower plants

Constant voltagecharging by SMPSPower plants

Periodical boostcharging at 2.7V/cellessential

21. Thermalrunaway

Probable, yet rare Not possible Not found

22. Risk of internalshort-circuiting

Remote Remote High, due to activematerial shedding.

Present field scenario:

Presently in both BSNL and MTNL, most of the batteries are based on AGM VRLAtechnology.

Almost all the Power Plants in the field are SMPS Power Plants, compatible with VRLAbatteries.

The SMPS Power Plants in the field are as per five GRs issued from 1994 to 2005. Up-gradation or modification of the earlier version SMPS Power Plants, to make a provision

for boost charging may not be feasible.

Replacement of these power plants may require a huge investment.

Conclusions

Change-over to flooded batteries (low maintenance or otherwise) may not be feasible, becauseof the following reasons:

Flooded batteries cannot work with the existing SMPS Power Plants because the lattercannot provide output of 65 V required for boost-charging.

Replacement of existing SMPS Power Plants may not be feasible as it shall involve ahuge investment. It may also not be possible and practical to modify the existing powerplants.

Sufficient man-power may not be available to maintain flooded batteries as it requiresmore stringent maintenance schedule of periodic monitoring of SG, electrolyte level, andperiodic boost-charging.

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There may be problems in compliance of pollution norms because of fuming in floodedbatteries.

There may be problems in providing a separate battery room in rented buildings.

Suggestions of TEC

Power plant must be fully compatible with the battery, and all the parameters must beset at desired levels.

It must be ensured that both float and charge voltages of the Power Plants are set atdesired levels.

Charge current to each battery must be set as per the battery capacity.

It must be ensured that temperature compensation mechanism is installed and is

functional. Voltages must be set at desired levels commensurate with the temperatures,as mentioned in monitoring guidelines.

Maintenance and planning guidelines issued by TEC may be followed to ensure properfunctioning of the batteries and power plants.

Places where electric supply condition is erratic, the charge condition of the battery mustbe monitored. If required, the batteries should be refurbished by charging it fullyperiodically with the help of mobile DG set, for a group of exchanges.

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White Paper on Battery Technologies