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8/13/2019 PluginHwy PHEV2007 PaperReviewed Valoen

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THE EFFECT OF PHEV AND HEV DUTY CYCLES ON BATTERY ANDBATTERY PACK PERFORMANCE

Lars Ole Valenaand Mark I. ShoesmithbaMilj Innovasjon AS, Nystrandvegen 59, N-3944 Porsgrunn, Norway

b-!ne Moli nergy "#anada$ %td&, '(,((( Stewart #res)ent, Ma*le +idge, #, '. 9/

ABSTRACT

Comparing different batteries or different battery chemistries for real life duty cycles isan onerous process and requires each battery to undergo cycling for each specific dutycycle. he present paper is a first step to!ards simplifying this process by de"eloping ametric for describing ho! a duty cycle current profile de"iates from the constant currentstate. his quantity# the duty cycle eccentricity# is then utili$ed to compare the efficienciesof commercial %iCd# %iM& and Li'ion po!er tool batteries. Li'ion battery efficiency!as found to be higher than that of %iM& and %iCd batteries.

KEYWORDSLi'ion# batteries# duty cycles# efficiency# &(V# )&(V

INTRODUCTION

Coulombic efficiency can be seen as a measure of ho! much of the electric charge that isused for the intended purpose of an electrical de"ice. %on'aqueous battery chemistriestend to run "ery small side reaction currents# hence achie"ing near *++, coulombicefficiency# e"en for real life duty cycles as seen in po!er tools or electric "ehicles. heenergy efficiency is the fraction of the total stored energy in a cell that is measurable aselectrical energy. %o battery chemistry can obtain *++, energy efficiency# for the simple

reason that there !ill al!ays be energy dissipated as heat through internal impedances.-enerally# the efficiency drops as the current is increased because the rate dependentcomponent of electrode o"erpotentials and the formation of concentration gradients !illconsume energy. In order to effecti"ely design and utili$e batteries for real life# po!er'demanding applications# some understanding of ho! the heat to electrical energy ratio"aries !ith current and duty cycle is required.

he aim of the present paper is to demonstrate a methodology for adequately comparingdifferent batteries or different battery chemistries for real life type of duty cycles !ithoutha"ing to deal !ith the eact details of such duty cycles

/uty cycles are often defined in terms of their a"erage current I 0 ho!e"er# this isinsufficient to characteri$e pulsed duty cycles that may contain numerous steps. hestandard de"iation gi"es a representation of the departure from the constant currentstate. o make it more uni"ersally applicable and to enable scaling !e define the dutycycle eccentricity 1/C(2 as a unitless quantity gi"en by

0#=1

I=

I3I3

I

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%iM& batteries. 6or the %iCd batteries# the charge efficiency !as some!hat lo!er usingCA*+ as a charge current and >+, SOC !as chosen as the fully charged# reference state.

6or all cells# a charge approimately equal to *+, of the battery capacity !as remo"edduring the pulsed discharge step. he cells !ere then charged up replacing an equalamount of charge compared to !hat !as remo"ed during the discharge step. 6or the%iM& and the %iCd batteries# a charge current of CA*+ !as selected !hereas the Li'ionbatteries could sustain *C charge current !ithout the coulombic efficiency dropping. ocompensate for limitations in the testing equipment# some of the tests !ere conductedusing %iM& and %iCd 7 cell series packs.

6or characteri$ing pulsed charge# a lo!er cutoff "oltage !as selected and used to definethe discharged state. 6or %iCd and %iM discharged state !as defined as a CA*+discharge to +.9V. 6or Li'ion# the cells !ere discharged to a lo!er cutoff "oltage of 3.BVusing a C'rate current. his !as defined as the fully discharged reference state. 8ftere"ery pulsed charge an equal amount of charge !as remo"ed before the net charge !asinitiated.

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RESULTS AND DISCUSSION

6igures *# 3# and 7 sho! the coulombic efficiencies for the three battery types using aCA*+ constant charge current.

2igure NiM +ound tri* )oulo6bi) e77i)ien)y and dis)8arge )a*a)ity as a 7un)tion o7)8arged )a*a)ity 7or #( )urrents&

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2igure ' Ni#d +ound tri* )oulo6bi) e77i)ien)y and dis)8arge )a*a)ity as a 7un)tion o7)8arged )a*a)ity 7or #( )urrents&

It can be obser"ed that both full efficiency and full discharge capacity are unobtainable

e"en for this relati"ely lo! current. In particular for %iM& 16igure *2 and %iCd 16igure32 cells the discharge efficiency drops rapidly as the cell approaches its fully chargedstate. he onset of this efficiency loss !as chosen as the fully charged reference state forenergy efficiency tests. his corresponds to approimately 9+, of the maimumdischarge capacity for the %iM& cell and >+, for the %iCd cells.

2igure 3 %i-ion +ound tri* )oulo6bi) e77i)ien)y and dis)8arge )a*a)ity as a 7un)tion o7 )8arge voltage 7or#( )urrents& :nli;e NiM and Ni#d %i-ion batteries are )8arged using a ##-# )8arging routine& 2.

B


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In 6igure ;# the measured round'trip efficiency as a function of the duty cycle eccentricityis sho!n. In all tests the a"erage current !as kept to 7.7C 1*+82. 6or the tests !ith"ariable current on charge# the discharge current !as kept constant at *C and for the tests!ith "ariable current on discharge0 the charge current !as kept constant at *C.

2igure 4 Measured round-tri* )8arge and dis)8arge e77i)ien)ies as a 7un)tion o7 duty)y)le e))entri)ity 7or 3&3# "(A$ dis)8arges&

6or constant current# Li'ion sho!s a higher efficiency than %iM& and %iCd. 8s the dutycycle eccentricity increases the energy efficiency drops for all chemistries tested. he%iM& packs drops off faster than %iCd and Li'ion. 8s pre"iously described# !e canpredict an ideal cur"e for efficiency "s. duty cycle eccentricity# denoted DohmicE in theplot. It can also be obser"ed that the discharge efficiency de"iates from predicted ohmicbeha"ior once the eccentricity is greater than one. his is an indication that non'ohmic

processes# such as diffusion or reaction kinetics# start to significantly influence the cellperformance abo"e that point.

6rom 6igure ;# it can be obser"ed that as the duty cycle eccentricity of the current profileincreases# the efficiency decreases for all the battery systems tested. he efficiency forthe Li'ion batteries is higher than for the %iM& and the %iCd batteries for the constantcurrent discharge. he %iM& batteries sho!ed a higher decline in efficiency compared tothe Li'ion and the %iCd batteries !hereas the latter !ere comparable. It is also

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!orth!hile to obser"e that the measurements performed using 7 cell series packs for%iCd and %iM& cells yielded the same efficiencies as the measurements performed forsingle cell tests.

Since the coulombic efficiency is "ery close to * for Li'ion batteries# it is also possible toanaly$e the efficiency as a function of duty cycle eccentricity for the charge current in a"ery straightfor!ard# simple !ay. his is sho!n in 6igure B.

2igure 5 Measured )8arge e77i)ien)ies as a 7un)tion o7 ratio o7 duty )y)le e))entri)ity7or Ni#d, NiM and %i-ion batteries

6rom 6igure Bit can be obser"ed that the beha"ior is fundamentally different for thedifferent battery chemistries. Li'ion has a "ery high charge efficiency# decreasing slightlyas the duty cycle eccentricity increases. 6or the %iCd batteries# the charge efficiency issubstantially lo!er. he %iCd efficiency sho!s a peak for an intermediate duty cycleeccentricity. 6or constant current# the coulombic efficiency is quite lo!# increasing !ith

increasing duty cycle eccentricity of the current profile. &o!e"er# as the duty cycleeccentricity increases# the heat production increases and offsets the gain in coulombicefficiency.

he importance of the findings presented in 6igure Bare of less importance for a batterycharged using abundant po!er from the grid than for a battery charged from a limitedenergy source such as recaptured energy from regenerati"e braking. 6or a plug'in hybridor electric "ehicle# this !ill impact the all'electric'range 18(


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8(< etension pro"ided by the regenerati"e braking significantly contribute to !hichbattery chemistry !ould constitute the ideal selection for such "ehicles.

In 6igure =# the efficiency as a function of the depth of discharge 1/O/2 is gi"en. heheat production is measured using the accelerated rate calorimetry 18


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CONCLUSION

In the present paper# a methodology for comparing different batteries# including differentbattery chemistries for real life parameters is presented. 8"erage current !as found to beinsufficient for characteri$ing real life duty cycles. he performance of any battery ishighly dependent on the duty cycle and kno!ledge of the po!er profile is crucial forbattery and battery pack design. /e"elopment of standard tools for characteri$ing andgrouping together duty cycles is important to be able to design optimal batteries andbattery packs.

It is also sho!n that the charge and discharge efficiencies decrease as the duty cycleeccentricity of the current profile is increasing. his holds for all three batterychemistries tested. he energy efficiency of Moli IM2. &andbook of battery materials# 5iley'VC&

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