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International Journal of Scientific and Research Publications, Volume 4, Issue 12, December 2014 1

ISSN 2250!15!

Neural Network Based State of Charge (SOC) Estimation

of Electric Vehicle Batteries

J.A..S. Ja!asinghe" ...#. Nadishan

De"artment of #lectronic and $elecommunication #n%ineerin%, &ni'ersit( of )oratu*a, Sri +ana

Abstract$ -ccurate estimation of state of the char%e .S/ is

'ital for electric 'ehicle batteries $his "a"er "resents a no'elmethod to estimate the S/ based on a neural net*or *hich

can be "ro%ramed into a lo* cost microcontroller $he

microcontroller monitors the batter( 'olta%e and taes four

sam"les immediatel( after the batter( is disconnected from theload and monitors the stead( state terminal 'olta%e *hen the

'ehicle is "ared for more than !0 minutes to train a neural

net*or #ach time the batter( is disconnected from the load

*hile dri'in%, the microcontroller taes four 'olta%e sam"les toestimate the S/ usin% the "re'iousl( trained neural net*or In

order to increase the accurac(, the batter( tem"erature is also

taen as an in"ut

Index Terms 3atter( State of har%e, #lectric Vehicle,

)icrocontroller, Neural Net*or

I IN$R/D&$I/N

e"letin% fossil fuel sources made researchers to loo for

alternati'e ener%( sources to meet the e'er increasin%

trans"ortation needs (dro%en fuel "o*ered 'ehicles, batter(

"o*ered electric 'ehicles are t*o maor ne* technolo%ies Plu%in #lectric Vehicles, (brid #lectric Vehicles are alread( in the

maret $o(ota Prius and onda Insi%ht are "o"ular h(brid

electric 'ehicles $esla Roadster and Nissan +eaf are "lu%in

electric 'ehicles but their "o"ularit( is not as %ood as $o(otaPruis or onda Insi%ht )aor reasons for this are hi%h "rice,

limited ran%e and "rolon% char%in% time In h(brid electric

'ehicles, ran%e is enhanced b( a %asoline en%ine and hence

char%in% time is not a crucial issue -s h(brid electric 'ehiclesuse %asoline en%ines *hich runs at much better fuel econom(,

researchers ha'e found a solution onl( to reduce the demand on

fossil fuel &sin% rene*able ener%( sources such as solar, *ind

or bio mass for char%in% "lu%in electric 'ehicles is a lon% lastin%"ermanent solution for de"letin% fossil fuel sources $here are

man( issues (et to be sol'ed to mae such a solution as a 'iable

re"lacement for fossil fuel 3atteries ha'in% 'er( hi%h ener%(densities, 6uic char%in% time and reliable estimate of remainin%

batter( ca"acit( are fe* such issues related the batteries used in

"lu%in electric 'ehicles In a %asoline "o*ered 'ehicle, a

'ariable resistor actuated b( a floater is sufficient to dri'e the

fuel %au%e fi7ed on the dashboard Such an indicator %i'es a'er( reliable estimate of the remainin% amount of fuel and hence

dri'er can refuel his 'ehicle to reach his destination #stimatin%

the remainin% ca"acit( of a batter( is not a sim"le as abo'e8ettin% a 'er( reliable estimate of the remainin% batter( ca"acit(

D

is 'er( im"ortant for electric 'ehicles as the batter( rechar%in%

time is hi%h com"ared to the %asoline refuelin% time Pan% et al 9!: "ro"osed an al%orithm for estimatin% the

S/ throu%h an e7tended ;alman filter b( measurin% the

terminal 'olta%e and dischar%e current $he e7tended ;alman

filterin% is com"utational incenti'e al%orithm and is not suitablefor 'ehicles Sato and ;a*amura 9h?% =urther lithium ion batteries create leas

amount of ha@ardous *aste durin% dis"osal $herefore, man(manufacturers of chea" electric 'ehicles such as electric scooters

are usin% lead acid batteries *hile lithium ion *ill be most the

"rominent batter( in future electric 'ehicles =or lead acid and lithium ion batteries, o"encircuit 'olta%eis considered as an accurate indicator of the batter( S/

follo*in% char%e or dischar%e "rocess "ro'ided sufficient time is

allo*ed to stabili@e the chemicals in the batter( =or a lead acid

batter(, the S/ is linearl( de"endent on o"encircuit 'olta%eand s"ecific %ra'it( of the batter( electrol(te $he stead(state

o"encircuit 'olta%e .E of a lead acid batter( and s"ecific

%ra'it( .SG of the electrol(te is %i'en b( 95:A

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International Journal of Scientific and Research Publications, Volume 4, Issue 12, December 2014 2

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$he S/ and stead(state o"encircuit 'olta%e is %i'en b(

91:A

*here aand bare t*o constants determined b( 'olta%es of

full and @ero ca"acities =or lead acid batter( the stead(state

o"encircuit 'olta%e 'aries bet*een 1B! to 21C 'olts "er cella""ro7imatel( form 10100 S/ and it taes more than !0

minutes to reach the stead(state o"encircuit 'olta%e $he

"arameters aand b'ar( *ith the batter( a%e =or a lithium ion batter(, the stead(state o"encircuit'olta%e 'aries bet*een !4 41 'olts "er cell a""ro7imatel( form

10100 S/ 94:, and the batter( sho*s a dual %radient linear

relationshi" from 10E0 and E0 100 *ith t*o different

%radients $he batter( taes more than 20 minutes to reach itsstead( state o"en circuit 'olta%e after char%in% or dischar%in% In

+ithiated Nicel /7ide .N- batteries, a sin%le %radient linear

relationshi" bet*een the stead(state o"en circuited 'olta%e and

S/ e7tendin% from 10100 92: If the 'ehicle is sto""ed for more than !0 minutes the

batter( *ill reach the stead(state o"encircuit 'olta%e and a 'er(

%ood estimate for the S/ can be obtained o*e'er our attem"t

is to obtain an accurate S/ estimate *hile the 'ehicle is in useDue to obstacles such as hea'( traffic, traffic si%nals, road bends

etc it is usual that dri'er tem"orar( taes the foot off the

accelerator "addle In case of an electric 'ehicle, this *illdisconnect the batter( from the motor load and hence batter(

o"encircuit 'olta%e can be measured In our "ro"osed method,

o"encircuit 'olta%e is measured at four re%ular sam"lin% instants

and a trained neural net*or is used to obtain an accurateestimate of the S/ $he neural net*or is trained e'er( time

the 'ehicle is sto""ed for more than !0 minutes

=i%ure 1 de"icts the "ro"osed S/ estimation al%orithm -

sam"lin% cloc acti'ates the al%orithm to measure the batter('olta%e and current at re%ular inter'als and com"utations are

acti'ated *hen the current dra*n from the batter( reaches @ero

$he al%orithm stores the o"encircuit batter( 'olta%e V(nT) forinitial four sam"les and a trained neural net*or is used tocom"ute the S/ and u"date the dis"la( $he sam"lin% inter'al

is in the order of 1 second and hence e'er( time the load current

is disconnected for more than 4 seconds, the S/ dis"la( is

u"dated $he al%orithm continues to measure the o"encircuit'olta%e to find the steadstate o"encircuit 'olta%e >hen the

measured o"encircuit 'olta%e remains constant for fe*

measurements under noload condition, al%orithm decides that

sufficient time has ela"sed and batter( has reached itFs stead(state o"encircuit 'olta%e $his measured o"encircuit 'olta%e

*ith the "re'iousl( measured initial four sam"les is a trainin%

data set &sin% this trainin% data set, neural net*or is retrained

$his retrainin% "rocess occurs e'er( time the dri'er sto"s the'ehicle for more than !0 minutes and hence the reduction of the

batter( ca"acit( due to a%in% is taen into account -s the batter(

S/ is also de"ends on the batter( tem"erature, batter(

tem"erature is also measured and used as an in"ut for the neural

net*or

%igure &' Algorithm %low Chart

=i%ure 2 de"icts the bloc dia%ram of the neural net*or

*ith fi'e in"ut nodes, one out"ut node and hidden la(er *ith si7

nodes /ur studies ha'e re'eal that onl( fi'e in"ut nodes and

hidden la(er *ith si7 nodes are sufficient to %et a S/ estimate*ith 1 accurac(, *hereb( entire al%orithm can be coded into a

lo* cost microcontroller

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International Journal of Scientific and Research Publications, Volume 4, Issue 12, December 2014 !

ISSN 2250!15!

%igure ' Neural Network

III I)P+#)#N$-$I/N

$he bloc dia%ram the S/ estimator based on

-$)e%aCCP- microcontroller is de"icted in =i%ure ! It has three

analo% in"ut "orts to measure the batter( 'olta%e, current andtem"erature It has a B

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International Journal of Scientific and Research Publications, Volume 4, Issue 12, December 2014 4

ISSN 2250!15!

%igure ' S!stem Block #iagram

%igure *' +ain Controller Block #iagram

$he "hoto%ra"h in =i%ure 5 sho*s the "rotot("eim"lementation Due to the sim"licit( of the al%orithm the code

is stored in a lo* cost microcontroller containin% C b(tes

"ro%ram memor(, 512 b(tes ##PR/) for storin% data, ! timer

counters for "ro%ram flo* control $he cost of the "rotot("e is

Rs 2000?G and hence can be used e'en *ith lo*end electric'ehicles such as electric scooters

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International Journal of Scientific and Research Publications, Volume 4, Issue 12, December 2014 5

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%igure ,' -hotograh of the rotot!e

IV /N+&SI/N

- neural net*or based al%orithm and its im"lementation on

a lo*cost microcontroller for %ettin% 'er( reliable estimate of

the S/ of lead acid and lithium ion batteries used in electric'ehicles is "resented $he sim"licit( of the al%orithm maes it

"ossible to im"lement the s(stem usin% a lo* cost

microcontroller *hile the robustness of the al%orithm mae it

"ossible to %et a 'er( reliable estimate for the S/ e'en tain%the batter( a%in% and tem"erature effects

R#=#R#N#S

91: D +inden and $3 Redd(, andboo of 3atteries, )c8ra*ill 3ooom"an(, 2002

92: J )cDo*all, &nderstandin% +iIon $echnolo%(, Proceedin% of 3attcon200C, "" B1 B10

9!: S Pan%, J =arrell, J Du and ) 3arth, H3atter( State of har%e

#stimation, Proceedin%s of the -merican ontrol onference 2001, ""1