Sealed Lead Acid Batteries (SLAB) Explained
Sealed Lead Acid Batteries (SLAB)
Explained
DDB Unlimited
8445 Highway 77 North
Wynnewood, OK 73098
800-753-8459
405-665-2876
www.ddbunlimited.org
[email protected]
SEALEDLEADACIDBATTERIES(SLAB)EXPLAINED
ThisdocumentisintendedtoprovidetheuserwithanoverviewoftheoperationofSealedLeadAcid
Batteries(SLAB)anddoesnotgetintothechemicalconsiderationsofthedesignandmanufacturethat
arequitecomplex.ThecurrentdaySLABbatteryhasevolvedoverthepast30yearsintoaproductthat
isreliableandsafetouseinstationaryplatformsthatrequirelongtermbatterybackupapplications.
TherearemanyotherchemistriesofbatteriessuchasLithiumIonthatcanprovidesuperior
performancebutatthesametimearemoreexpensiveandhavesomesafetyconcerns.Therecent
LithiumIonapplicationat30,000sitesofAT&Tisagoodexamplewheretheyhavehadfiresand
explosionsatanumberofsitesisthattheproductmaynotbeassafeaspreviouslythought.
Themajorchangeoccurredintheautomotiveindustrywhenthechargingmethodwaschangedfroma
DCgeneratortoanACalternatorwhichwasdrivenbytheincreasedrequirementsofstablevoltage,
muchhighercurrentdemandsandpreventionofbatteryovercharging.Diodeisolationofthealternator
simplifiedthebatterydischargeproblemofthealternatorandtheelectronicregulatorprovidedthe
stableandaccuratevoltagerequiredfortheelectronicsinthevehicleespeciallythecomputerwhich
controlstheengineandothervehiclefunctions.TheintroductionoftheSLABormaintenancefree
batteryoccurredduringthegeneratorphaseandsincetherewasnomethodofinsuringoverchargingof
thebatteryitwasnotwellreceived.Themajorproblemwithoverchargingisboilingandevaporationof
electrolyte.
TheSealedLeadAcidBattery(SLAB)usedintelecomandindustrialapplicationsreallybecamepopularin
themid90’sduetothefactthatitwassafertohandleandsignificantlyreducedthepresenceof
hydrogenoutgassingwhichcanleadtoexplosionsandpotentialpersonnelhazards.TheSLABbattery
beingsealedalsopreventedthepotentialforelectrolyteleakswhichalsorequiresspecialOSHA
requirementforspillprevention.TheSLABhasalsoincludedthevalveregulated(VRLA)releaseof
batterypressurewhenthebatteryinternalpressureexceeded5PSI.
Themajormaintenanceissuewithanybatteryarrayistheconnectionstothebattery.Corrosionis
typicallythecriticalissuethatoccursbecauseofelectrolysisandexposuretooxygenandcontainments.
TheuseofNO-OXorsimilarantioxidantmaterialwilltypicallypreventthisfrombecominganissue.
Insuringthattheconnectionsaresecureandtightalsoreducestheresistancebuildupthatistypically
dissipatedinheatandlostcurrentcapacity.
AsocalledmaintenancefreebatterySLABdependsonanintelligentchargingmethodthatnearly
eliminatesoverchargingwhichistheprimaryreasonfortheelectrolytetoboilandevaporate.Most
currentdayrectifiersuseathreestepchargingprocesswhichistypicallyaclosedloopfeedbackmethod
(1)initialhighcurrentcharge,(2)topoffchargeatreducedchargecurrent,(3)floatchargewhichis
typicallyintherangeof1to3ampsdependingontheamphourratingofthebattery.Initiallythereis
thedeepchargingcyclewherethecurrentprovidedtothebatteryunderchargemaybeashighas10to
20ampswhichistypicallycurrentlimitedbytherectifier.Twofactorsdeterminethisperiodwhichare
theinitialvoltageofthebatteryandtheamountofchargingcurrent.Theindustrystandardforcharging
is0.1timestheamphourratingofthebattery.MostSLABbatteriescanacceptupto0.2oftheamp
hourratingforashortperiodoftime(lessthan2hours)withoutdamagingthebattery.Extended
periodsofhighcurrentchargingwilltypicallyshortenthelifecycleofthebattery.Asthebatterybegins
toacceptthechargingcurrenttheamountofcurrentisreducedovertime.Thetopoffusuallyoccursin
thechargingcurrentrangeofabout3to5ampsandistypicallythelongestperiodofthecharging
process.Thefloatchargealsoreferredtoasthestandbychargingisthefinalphaseofthecharging
processandistypicallyintherangeof1to3amps.
ThetypicalchargingprocessofaLeadAcidbatterydependsontheinitialfloatvoltageofthebattery
afterdischarge.ThosesystemsthathaveLowvoltageDropOut(LDO)insurethatthebatteryarray
voltagewillneverbebelowtheLDOvoltagevalue.ThosesystemswithoutLDOareindeterminateasto
whatvoltagemayexistinthebatteryarrayandassuchmayrequireamuchhigherchargecurrentto
chargethearraythanwithLDO.Thetypicalchargingperiodisbetween12to14hoursforLDOsystems
andmaybemuchlongerwithoutLDO.
ThelifecycleofatypicalSLABbatteryisabout5yearsduetothefactthatasthebatteryagesthe
efficiencyofthebatterytoprovidecurrentisreducedbyabout5%peryearofoperationassuminga
nominalamountofbatterybackupusage.Agreaterfrequencyofbatterybackupusagewilldepletethis
lifecyclefurther.Atypicalbatterybackupwouldbeabout90%ofthetimethechargeisinthefloat
modewhichislikeshelflifeforstandardalkalinebatteries.Thenormalbrownoutandtransmissionline
switchingwhichprovidemomentarylossofpowerisaccountedforintheestimate.Theextended
batterybackupofstormsthatmightextendthebatterybackupisconsideredtobenomorethan10
periodsperlifecycleyear.Thoseareasthathavegreaterthannormalstormsactivitymayrequirea
shorterlifecycleconsideration.
Selectingtherectifiercurrentrequirementsshouldincludeasafetymarginofabout20%toallowfor
variationsofequipmentcurrentduetovariationintrafficflowandoperationoverthetemperature
extremesoftheequipment.Inadditiontotheequipmentcurrentrequirementthereisthechargingof
thebatterybankthatneedstobeconsidered.Keepinmindthatthecurrentwillalwaysflowfromthe
highestvoltagesourceandinthecaseoftherectifierbydesignwillalwaysbethehighestvoltagesource
aslongastherectifierisbeingpowered.Mostrectifiershaveacurrentlimitforchargingthebatteries
whichistypicallyintherangeof10to20amps.IfthesystemisequippedwithLDOthenthelowest
voltageofthebatterybankwouldbe42volts(maybeadjustable)ina48voltsystem.WithouttheLDO
thebatterybankmayrequirehigherinitialcurrentduetolowervoltage.Thisisbasedontheindustry
standardthattheindividualbatterycellisatorlessthan1.75voltspercellisthedefinitionofafully
dischargedbattery.Thischargingcurrentmustbeaddedtotheequipmentcurrentinordertoproperly
sizetherectifier.Thecurrentlimitofchargingthebatteriesmayrequirelongerthanthetypical12to14
hourchargetimetofullyrecoverthebatteryarray.
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