Series Testing of aircraft Batteries V 3.1 SERIES TESTING OF AIRCRAFT
NICKEL-CADMIUM BATTERIES
Note:
ThisdocumentisaguidelinefortheuseofCharger‐AnalyzersmanufacturedbyJFM
EngineeringforthetestingandcertificationofaircraftLead‐Acidbatteries.
CMMsandBatteryManualsfrommanufacturersmustalwaysbeconsultedtodeterminethe
exactconditionsrequiredtoservicetheirparticularbatteries.
Warning:
Batteriesarecapableofveryhighdischargecurrents.Protectfromshorts!
Batteriescontainelectrolytethatcanproducechemicalburns,Exercisecaution!
Alwayswearprotectiveequipmentanduseappropriatetools.
Insurethatbatterytestingisperformedinwellventilatedareas.
OnlySealedLead‐AcidbatteriescanbeinthesameenvironmentalongwithNickel‐Cadmium
Batteries.
VentedLead‐Acidtypesneedtheirown,wellventilatedarea
JFM Engineering, Inc.
8030 NW 67th Street
Miami, Florida 33166
305-592-2272
www.jfmeng.com
Page 1 of 4 Series Testing of aircraft Batteries V 3.1 1. Introduction:
CanNickel‐CadmiumBatteriesbechargedinseries?
The answer is both yes and no as it depends entirely on the charging method and equipment.
2. OperationDescription:
Nickel‐Cadmiumbatteriescanbechargedinseriesif,andonlyifitisdonewithconstantcurrent.
Amorecompleteanswer,however,dependsonthepurposeofthecharging.
Ifthepurposeistosimplychargeadischargedbattery,anymethodavailablewilldo(willhave
todo).
But,ifthepurposeistochargeabatteryfortestpurposes(todetermineifthebatteryis
airworthy)thenconstantcurrentistheonlyway,asitisestablishedbythemanufacturersofthe
batteries.
Whenabatteryischargedinconstantvoltage(orothervoltagedependentmethods)thenthe
individualcellsthatmakeupthebatterycaneasilybecomeimbalanced1.Thisisoneofthe
reasonswhyaperiodicdeepcycleisneeded.
WhenaNickel‐Cadmiumbatteryischargedatconstantcurrent,allcellsarefreetodevelopa
voltagethatbecomesindicativeoftheirchargeacceptance.ItisforthisreasonthatNickel‐
Cadmiumbatteriescanbechargedperfectlywellinseries2.
Whychargebatteriesinseries?Strictlyforefficiencyreasons.ThetypicalchargetimeforaSaft
batteryis6hours3,therefore,iftwobatteriescanbechargedsimultaneously,thereisa
significantgainintime.
ChargingtwobatteriesinseriesrequiresahighercapabilityintheCharger‐Analyzeranda
correspondinghighersafetymargin.TheSuperseder/MasterChargerwasdesignedforboth.It
canprovideconstantcurrentchargingfromonecellupto50cellsanditcanonlyoutputcurrent
whileconnectedtoabattery(nodangerous“hot”terminals).
Whataboutthetaskofmeasuring40cells,notjust20?
Yes,measuringthevoltageoneachcellislaboriousanderrorpronebutitcanbedone(notjust
on20cellsbutonall40–thereisplentyoftime).Toalleviatethisburden,theBTAS16Battery
ManagementSystemwasdesignedtoautomaticallytakeallnecessarymeasurements,accurately
andefficiently(seehttp://jfmeng.com/btas16.htm).
1
Note that Lead-Acid batteries can also suffer from the same imbalance problem, although to a lesser extent.
Note that a typical aircraft battery can be looked at as a series string of 20 individual batteries.
3
Two hours of Main Charge (C/2) and four hours of Topping Charge (C/10).
2
Page 2 of 4 Series Testing of aircraft Batteries V 3.1 3. Whataboutdischargingtwobatteries?
Dischargingoftwobatteries,forthepurposeofCapacityTesting,presentsadifferentsituation.
Atconstantcurrent,itbasicallydoesnotmatterhowmanybatteriesareconnectedbutthereis
apowerdissipationlimitation.
ThecurrentmodelofSuperseder/MasterChargerislimitedtoamaximumof60A
forone24Vbatteryor30Afortwo24Vbatteries.Atahighervoltageand/orcurrent,theheat
dissipationwillexceedthecoolingcapabilityintheloadbanks.
Yes,itispossibletolowerthecurrentandextendthetimetoaccommodatebatteriesthatwould
otherwiseexceedthepowerdissipationlimit,butitmaybenecessarytoaccountforpossible
performancedifferencesatthelowercurrent.
But,thereisamorefundamentalissuehere.Capacitytestingisnotsimplybasedonabattery
terminalmeasurement;itrequiresthatallcellsbemeasured,becausethebatterywillfail
capacitythemomentthatanyoneofthecellsfailscapacity,thusrequiringrapidmeasurement
ofallcellvoltages.
Itisalreadyquitedemandingtoquicklymonitor20cells,letalone40ofthem.
TheBTAS16systemagaincomesintoplaybyprovidinganeasymethodtomeasureallcells,not
just20but40!
4. Summary:
4.1. SerieschargingofNickel‐Cadmiumbatteriesisperfectlynormalprovidedthatitisdone
withconstantcurrent.
4.2. Seriesdischarging,however,isnotthatsimple,asitisaffectedbyotherfactorsthatlimit
thegainintestingefficiency.
4.3. Thefollowingarticles,availablefromtheJFMEngineeringWebsite,willprovide
additionalinformationonaircraftbatterytesting:
4.3.1.
4.3.2.
4.3.3.
4.3.4.
AircraftBatteryTestingDefinitionsandNotes
BatteryTestingGuidelines
TestingIntervalsforAircraftNickel‐CadmiumBatteries
TheImportanceofWaterLevelling
Page 3 of 4 Series Testing of aircraft Batteries V 3.1 5.
REVISIONINDEX
Table 1 - Index of Revisions
REVISION
V3.0
V3.1
DATE
30 September 2014
11 October 2014
NOTES
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Minor text formatting corrections
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