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StateofChargeandLithiumionBatteries
Definition,Measurement,andDangerousGoodsRegulations
INTRODUCTION
TherecentlyreleasedaddendumtotheICAOTechnicalInstructionfortheSafe
TransportofDangerousGoodsbyAir(2015‐2016Edition)willlimitthestateofcharge
(“SOC”)oflithiumion(Li‐ion)batteries(UN3480)toamaximumof30%whenoffered
forairtransport.ImportantquestionshavebeenraisedastomeasuringSOCandhow
compliancewiththisnewregulationwillbedeterminedbyenforcementagencies.This
paperexploreswhatSOCmeans,howSOCisutilizedinequipment,andaddressessome
ofthechallengesassociatedwithmeasuringandenforcingthisnewrequirement.Itis
importanttonotethattheSOClimitdoesnotapplytoLi‐ionbatteriesthatare
packedwithorinstalledinproducts(e.g.,equipment(UN3481),vehicles(UN3171)).
DEFININGSTATEOFCHARGE
Simplydefined,stateofcharge(“SOC”)ofaLi‐ionbattery(orcell)isthepercentageofits
totalenergycapacitythatisstillavailabletodischarge.Thisislooselycomparabletoan
automobile’sfueltankinthesensethat0%indicatesanemptytankand100%indicates
afulltank.Infact,theindicationoftheSOCremaininginaproductisoftenreferredto
as“fuelgauging.”However,unlikemeasuringthefuelremaininginanautomobile’s
tank,whichhasafixedvolume,thereisnodirectwaytomeasureSOCofaLi‐ionbattery.
Factorssuchasageofthebattery,chargevoltage,temperature,dischargerate,and
productusepatternsaffectthemeasurement.Theknowledgeandtrainingoftheperson
makingthemeasurementandtheequipmentavailablearealsosignificantfactors.This
makesdevelopinganindicationofSOCaverycomplexprocess,theaccuracyofwhichis
heavilydependentonthefuelgaugingmethodchosen.Infact,theindicationofSOCis
oftenreferredtoasanestimateratherthanameasurementordeterminationasthereis
nosinglecorrectprocesstobeemployed.
Theaddendumtothe2015‐2016EditionoftheICAOTechnicalInstructionsrefersto
“guidanceandmethodology”thatcanbefoundintheUNManualofTestsandCriteria
fordeterminingratedcapacityofLi‐ionbatteries,whichisusefulforestimatingSOC.
TheUNManualliststhefollowingIECbatterystandardsforthisguidanceand
methodology:IEC61960,IEC62133,andIEC62660.
STATEOFCHARGEINMANUFACTURING
ReputableLi‐ionbatterymanufacturers,eachwithadeepunderstandingoftheir
product’scharacteristics,developfactorycontrolsandmeasurementmethodsto
demonstratecompliancewithapplicableSOCspecifications.Forexample,cell
manufacturersoftendevelopcomplexcorrelationsbetweencellvoltageandestimated
SOCtocharacterizetheircells.Thesecorrelationstypicallyinvolvesubjectinganumber
ofcellstoentirecharge‐dischargecyclestonotonlydeterminetherelationship,butalso
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gaininsightintotheassociateduncertainty.Themanufacturingprocessisthen
designedtoensurecellsleavingtheproductionfloorareatacarefullyselectedvoltage,
whichtakesintoaccountthedetermineduncertaintytoensuremeetingtheSOCtarget.
BatterymanufacturersdevelopsimilarprocessestodeterminetheSOCofthebattery,
whichcanbeevenmorecomplicatedastheyalsoneedtoensureproductintegrityand
thatprotectivedevicesandelectroniccontrolsareoperatingproperly.
ESTIMATINGSTATEOFCHARGEINAPRODUCT
Aspreviouslynoted,theSOClimitadoptedbyICAOdoesnotapplytoLi‐ionbatteries
thatarepackedwithorcontainedinproducts.However,variousmethodsfor
reasonablyestimatingtheSOCofabatteryinaproducthavebeendeveloped.Products
suchaselectricvehicles(“EVs”),notebookPCs,tablets,andsomehigh‐end,handheld
devicesutilizecomplexalgorithmsthatrelyonacombinationofbatterycurrent,cell
voltage,batteryimpedance,productusepatterns,andtemperaturetoestimateSOC.
Thesemethodsmaydeliverapproximately1‐5%accuracyandcandisplaydetailed
estimatesofSOC.Ontheotherendofthespectrumareproductssuchaslow‐endpower
banks,RCtoys,andinexpensivecellularphonesthatusesimplevoltagemeasurements
toilluminate3or4LEDstoindicateanestimateofSOChaving25‐33%accuracy.Inthe
middleareanumberofproductsthatuseavariationonthesemethodstoestimateSOC
toaccuracylevelsbetweentheseextremes.
REGULATINGSTATEOFCHARGEINTRANSPORT
Thechallengefacingenforcementagencieswhenattemptingtoensurecompliancewith
anySOCrequirementsmaybeobvious.HowcancompliancewithanSOClimitbe
determinedwhentheshippedbattery’sSOCcannotbedirectlymeasuredinthefield?
Thereisaninherentandacceptedleveloftrustfactoredintoallofthedangerousgoods
regulations.Forexample,electricdoublelayercapacitors(UN3499)havebeen
regulatedforseveralyearsontheassumptiontheyareshippedatan“unchargedstate.”
Shippersoffully‐regulateddangerousgoods(e.g.,Liionbatteriesshippedinaccordance
withSectionsIAorIBofPackingInstruction965oftheICAOTechnicalInstructions)
mustsignandself‐certifyonaShipper’sDeclarationforDangerousGoodsthata
shipmentbeingofferedfortransportisinallrespectscompliantwiththeapplicable
regulations.Li‐ionbatteryshipmentsofferedinaccordancewithSectionIIofPacking
Instruction965oftheICAOTechnicalInstructionsmustincludeareferenceto
“compliancewithSectionIIofPI965”onanairwaybill,whenanairwaybillisused.
Thisself‐certificationofcompliancewiththeregulationswillnowapplytothenewSOC
requirementforLi‐ionbatteries.Allofthisobviouslyrequiresaleveloftrustbetween
thebatterymanufacturers,shippers,airlines,andenforcementagencies.
Relianceonthefuelgaugingmethodprovidedbythemanufacturermaybeanadditional
componentofacomplianceprogram.Sincethecorrelationsdevelopedduringthe
productdevelopmentprocessareoftenleveragedtodesignthefuelgauge,itsindication
coupledwithinformationfromthemanufacturerinproduct’sliteratureorcompany’s
website,couldprovideareasonablemethodfordeterminingSOCofthebatterypriorto
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beingofferedfortransport.Thiswillbeabenefittoregulatorsaswellasshippers
downstreaminthelogisticschain(e.g.,batteryandproductdistributors).
Anothercomponentofacomplianceprogrammaybeinspectionsbydangerousgoods
enforcementagenciesatcellandbatterymanufacturingfacilities.Theinspectorcould
workwiththemanufacturertoverifytheyhavereasonableprocessesinplacetoensure
cellsandbatteriesarelimitedto30%SOCbeforebeingofferedfortransport.For
reputableLi‐ioncellandbatterymanufacturers,theprocessofestablishinganSOClimit
willbepartoftheirwrittenstandardoperatingprocedureandeasilyverifiable.
SUMMARY
ThereisnosinglemethodforestimatingtheSOCofaLi‐ionbattery(orcell).Inaddition,
thereisnouniversalinstrumentformeasuringSOC.Inordertocomplywiththe30%
SOClimitthattakeseffectonApril1,2016,shippersofLi‐ionbatteriesshouldconsider
thefollowingbestpractices:
1. Checkwiththeoriginalbatterymanufacturertodetermineiftheyhave
implementedapolicyoflimitingtheirLi‐ionbatteriesto30%SOCwhenshipped
fromthefactory.
2. IfyourcompanyiscapableofmanagingoraccuratelyestimatingtheSOCofaLi‐
ionbattery,considerpreparingastandardoperatingprocedureformanagingor
estimatingtheSOCthatcanbeprovidedtocustomers,enforcementagencies,
freightforwarders,airlines,andotherentitiesuponrequest.
3. IfaLi‐ionbatteryutilizesLEDstoindicateanestimateofSOC,adoptthe
necessaryprocedurestoensurethemaximumnumberofLEDsisnotexceeded
beforeofferingthebatteryfortransport.
4. Ifanaccuratefuelgaugeisincorporatedintoaproductthatispoweredbythe
Li‐ionbatterybeingshipped,considerusingthisfuelgaugetoestimatetheSOCof
thebatterybeforeofferingthebatteryfortransport.
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