Batteries for UPS.pdf

7/27/2019 Batteries for UPS.pdf

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The UPS People

Microaids 2007 1 of 4

Selecting Batteries for Your UPS

Performance of any UPS systemdepends largely on performance ofbatteries connected to it. As suchselection of right type and rating ofbattery for your UPS is very critical.Regular maintenance of installedbatteries also should be given due

importance.

Battery Specifications andCharging

Batteries are accumulators ofelectrical energy. Battery stores

electrical energy in chemical form.Battery specification consists of theirvoltage and ampere-hour (AH)

capacity. Thus 12v 100AH batterymeans one can draw 10 Amperes for

about 10 hours. This relationship isactually non-liner. In other wordsyou cannot draw 100 amperes from100 AH battery for one hour.Typically battery manufacturers givedischarge curves as per their designspecification for each rating. A 12V

battery typically consists of 6 cells of2 volts each connected in series andmounted in hard rubber or PPcontainer. Charging methods forthese batteries are also specified bymanufacturers and one has to followthem strictly to get maximum life outof these batteries. UPS systemsprovide sophisticated electronic

battery charger to ensure propercharging of batteries. Chargingcurrent restrictions decide recharging

time for batteries. (This specificationbecomes critical when you haveregular long load shading periodsduring the day)

One has to understand that batteryis a consumable item and hasdefinite life, which is much less thanlife of the UPS. A normal life spanfor well-designed lead acid battery isabout 2-3 years and depends uponselection of proper ampere-hour(AH) capacity for given dischargecurrent. Long life (5 to 6 years)

batteries are available but at extra

cost. In developing countries, longerbackup period is required for UPSsystems. As such, the investment in

batteries is at times almost equal to(or even more than) the UPS.

Types of BatteriesThree basic types of batteries are

available in the market:

1. Automotive Lead acidbatteries (Wet)

A typical wet battery is made of

positive and negative plates, made oflead and lead oxide. These plates arearranged in hard rubber or PVC

container. These plates areimmersed in dilute sulphuric acidserving as liquid electrolyte. Theperformance of these batteriesgreatly depends on purity of leadused in battery plates and quality ofsulphuric acid used. Hence, we findthat branded batteries in thiscategory perform much better than

local makes.

Automotive batteries are mainlyproduced by manufacturers for use

in automobiles, where the mainfunction of the battery is to start theengine. This requires battery todeliver very high current to startermotor for short duration, whilestarting the automobile. In runningcondition these batteries are floated

on alternator rectifier connected to

engine, which charges the battery.As such it does not undergo highcurrent discharge for longer time.Manufacturers use battery plateswith less cross sectional area toserve this purpose and reduce thecost.

The electrolyte evaporates duringcharging and discharging changingits specific gravity. We have to adddistilled water at regular intervals toensure proper performance of thesebatteries.


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Generally these batteries are notrecommended for serious UPSinstallations. However if the cost

constraints are stringent one can usethese batteries with UPS of lower

ratings. The capacity should be soselected that that the dischargecurrent is not more than 1/100 ofbattery rating.

2. Stationary Tubular Leadacid batteries (Wet)

Tubular batteries are similar toautomotive batteries but with somesignificant differences in their

construction. These batteries have

larger cross sectional area for platesand larger volume to accommodatemore electrolyte. In case of UPSlarge current discharge is requiredfor longer durations to support theloads. This naturally demands more

cross sectional area for batteryplates to give longer battery life.

Tubular batteries provide this type ofconstruction and thus are bettersuited for UPS applications. Provisionof more electrolyte avoids frequent

topping up.

These batteries are available in 2V or12V models with ampere-hourcapacities ranging from 40AH -200AH.

As the volumes getting sold are

increasing, local reliable sourcesfrom unorganized sector are

developing in many areas for thesetype of batteries.

A variant called Thick plate orPasted Plate battery is alsoavailable in the market. Quality andcost wise it falls between automotiveand tubular batteries.

User can choose the battery afterensuring the reputation and track

record of the manufacturer.

3. Sealed Maintenance free

(SMF) batteries (Dry - ValveRegulated Lead Acid Batteries)

These batteries use plates similar towet batteries, but the electrolyteused is generally in gel form. Thebattery is sealed and does notrequire any electrolyte fillingthroughout its lifespan.

These batteries have predicted life offixed number of discharge cycles,typically 300 discharge cycles at 250cambient. One discharge cycle istypically discharging battery up to60% and charging it again at

recommended charging current. Intropical countries like India, thistemperature restriction cannot be

achieved unless batteries are used inair-conditioned atmosphere. As suchwe get average life of about 200discharge cycles.

Any repairs during the life span arenot possible in SMF batteries. Thuswithin the life span but after theguarantee period, if any battery is

found faulty then it needs totalreplacement.

These batteries are available in the

capacities ranging from 7 AH to 100AH. Few manufacturers provide

factory assembled, 2 Volt stacks ofthese batteries (at almost double thecost) with special constructionaldifference to give 5 7 years life.

With the constraints as explained

above these batteries arerecommended for backuprequirements of about 20-30

minutes.

Sophisticated electronic chargers aremust for these batteries to avoidbursting or bulging due toovercharging. These batteries shouldnot be kept in discharged conditionfor longer duration.

Computation of battery requirementDC voltage of any UPS is design parameter decided by UPS manufacturer.Voltages ranging from 12V to 768V (64 Batteries of 12V in series) are used by


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various manufacturers for different ratings of UPS. Keeping this trend in mind,user should always specify UPS rating and backup time requirement and leavethe selection of DC voltage to the UPS manufacturer. One can further compare

costs and ratings of batteries quoted by various vendors for same backup

Computation of Battery Requirement:Tubular Batteries:

Following parameters are design specifications for UPS and are available incatalog.UPS DC to AC efficiency =%

UPS DC voltage = VDC volts.Max Output Load = W watts. (One should take actual load in case it is different than Max)

Following formula is used to calculate DC input currant of UPS.UPS DC input current = IDC Amps.

Dc

DC

UPSoutputWatts WI = =INputDCvolts V

Amps. .(I)

Following Empirical formula is used to calculate required battery rating in Ah.

1DC

Ah

UPSDCcurrentI = Backup time in hours. ..(II)

Example:Calculate battery rating for following situation:

UPS rating = 3 KVA supporting 2100 Watts.

UPS DC voltage = 96 volts. (12V, 8 Batteries.)

Maximum battery voltage = 12 8 96 .volts =

Min battery voltage = 10.5 . (Battery manufacturer will not allow discharge8 84volts =beyond this voltage.)

So average DC voltage = (96+84)/2=90 volts.UPS efficiency = 92%Back up required = 2 hours.

Using above formula (I)

2100

25.360.92 90DCI Amps= =

Using above formula (II)

1 225.36

3 25.36 76.08

Ah

Ah

=

= =

Nearest available battery rating is 80 Ah. So above UPS will require 12V / 80Ah 8batteries to give 2 hours back up.

This Empirical formula takes care of nonlinear discharge characteristics of batteries and

also ensures that maximum discharge current is limited to maximum permissible value asspecified by battery manufacturer.


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SMF VRLA Batteries: (Sealed Maintenance free or Valve regulated Lead Acid Batteries.)

For calculating required rating of these batteries, one has to first calculate IDC using sameformula ( I ) as above. After knowing DC current one has to refer to dischargecharacteristics given by manufacturer. (Graph of back up time Vs discharge current drawnfor different Ah capacities).

Few Dos and Donts for UPS

batteries

Batteries should be kept asnear to UPS as possible.

It is always recommendedthat required number of batteriesshould be connected in series. Asfar as possible series parallelcombination should be avoided(Series parallel combination may

not ensure proper charging andequal discharging, thus could

reduce life of the batteries.)

As far as possible do not mixold batteries with new batteries or

batteries of unequal rating, whileconnecting in series to avoidunequal charging.

For easy maintenance andfrom the point of view of safety

batteries should be kept in steel orwooden racks duly designed tosuite the site. Such racks shouldbe kept away from heat and anyother chemical fumes or chemicalstorage.

DC voltages higher than 24Vmay prove to be fatal for

unauthorized handling. As such thebattery racks should have safety

covers with lock.

Battery rooms should be

accessible to authorized personsonly. These rooms should be wellventilated with provision for dry typefire extinguisher. Periodic visualchecking for bulging, fuming, looseconnections is recommended.

Batteries should be connected

to UPS through a manual isolatingswitch so that in case of emergencybattery bank can be quicklydisconnected from UPS.

In case of wet batteries theracks should be designed to provideenough height above batteries tofacilitate regular gravity checkingand filling of distilled water.

A periodic record in thefollowing form if maintained helps inassessing battery performance.

Date: Checked By:

BatteryNo.

No Load voltage( To be noted with

mains and UPSoutput off)

On loadvoltage

Specific Gravity(all cells)

Terminal & linkcondition

Documents

Batteries for UPS.pdf