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8/13/2019 00022379 http://slidepdf.com/reader/full/00022379 1/3 1 a 2 SERVICE LIFE LIMITATIONS OF FLOODED LEAD ACID BATTERIES IN UPS SERVICE W T. Rutledge PACIFIC BELL BBSTR CT This paper describes difficulties that have evolved in the application of flooded lead acid batteries to Uninter- ru pt ib le P ow er Systems (UPS) service. Industry pressures fo r greater power densities appear to have resulted in developments that may have been coun- terproductive to the service life of the UPS cells. The high current, short term discharge characteristics that are desirable for UPS applications result in battery designs tuned to less than optimum life. Uninterruptible Power Systems are generally applied to large computer data centers. The data center managers, who administer these applica- tions, tend to be less informed on the life/ construction characteristics of batteries that can result in unforeseen and severe economic losses for their mana ge men t . This paper describes efforts by the IEEE to provide appli- cations documentation designed to re- duce this information gap, bring the matter out into the open, and reduce these competitive pressures on the bat- te ry and UPS industries. HISTORY Historically, the major commercial battery applications h av e been in the railroad, telecommunications and elec- tric utility markets. Each of these market segments is populated by enough relatively large companies who can af- ford the technical expertise to effect control of product quality through their purchasing policies. UPS The emerging UPS battery market, unlike the previous markets, is consi- derably less sophisticated in its basic knowledge of battery technology. New or replacement batteries are often pro- cured by Data Center managers or build- ing maintenance persons. The demands of th is market have driven the battery manufacturing industry into a competi- tive race to provide the highest pos- sible p ow er density. T he UPS users have not recognized the battery design trade-offs that can result in reduced battery life. UPS user grou ps are now forming to di sc us s and compare common problems. Unfulfilled battery life has surfaced as their foremost difficulty. Initial in- vestigations by West coast UPS users and several UPS maintenance contractors have defined the problem as follows: HIGH POWER DENSITY BATTERIES After initial UPS i n s t a l l a t i o n s were equipped with general purpose telecom- munications type cells, the battery man- ufacturers, in an effort to meet cus- tomer d em an d for more Watts per unit volume responded with several design changes. First, electrolyte specific gravities were increased to provide bet- ter short term discharge rate perfor- mance. Second, plate grid designs were improved to better collect energy from areas more distant from the battery posts. A heavier overall current col- lection system was built into the cells including copper inserts in the posts to a cc om mo da te th e very high short term discharge rates. Third, plates were thinned down to allow the greater sur- face discharge action that accompanies high discharge rates. Also, i n some cases the density of the active material pasted into the plates was decreased to allow better conductivity to the inner surface areas of the active material. Finally, the heavier, fiber glass, p os - itive plate retaining mats that hold the delicate lead dioxide charge crystals against the surfaces of the positive plates were either thinned to the thick- ness of tissue paper or completely removed. Each of the design changes that so enhanced th e high rate UPS performance also tended to decrease the cell life and limit its ability to h an dl e the deeper discharges that also tend to accompany high rate usage. The users fo r the most part, however, were no t made aware of the consequences of their high power density selections on the expected battery service life. Often, in new UPS installations, the battery is purchased through the UPS supplier who may be unaware of the high power density risks or he is under pres- sure of price competition and may tend to cut corners with the battery. 379 CH2653 4/88/0000 0379 01 00 C 1988 IEEE
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Transcript of 00022379
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1a 2
SERVICE LIFE LIMITATIONS OF
FLOODED L EAD ACID BATTERIESI N UPS SERVICE
W T . Rut ledge
PACIFIC BELL
BBSTR CT
Thi s paper d e sc r ib e s d i f f i c u l t i e st h a t h av e e v ol ve d i n t h e a p p l i c a t i o n off l oo d ed l e a d a c i d b a t t e r i e s t o Un in te r -ru pt ib le Power Systems (UPS) se rv ic e .I n d u s t r y p r e s s u r e s f o r g r e a t e r p owerd e n s i t i e s appear t o ha ve r e s u l t e d i ndevelopments t h a t may have been coun-t e rp r od u ct iv e t o t h e s e r v i ce l i f e oft h e UPS c e l l s . The h i g h c u r r e n t , s h o r tterm d i sc h a rg e c h a r a c t e r i s t i c s t h a t a red e s i r a b l e f o r UPS a p p l i c a t i o n s r e s u l ti n b a t t e r y d e s i g ns t un e d t o l e ss t h a noptimum l i f e . Uni n t e r rup t i b l e P owerSystems a r e g e n e r a l l y a pp l i e d t o l a r g ecompu te r da t a cen t e r s . The da t a cen t e rmanagers, who adm inis ter th es e a p p l i c a -t i o n s , t e nd t o b e l e ss in formed on thel i f e / c o n s tr u c t io n c h a r a c t e r i s t i c s o f
b a t t e r i e s t h a t ca n r e s u l t i n u nf or es ee na nd s e v e r e e co no mi c l o s s e s f o r t h e i rmanage ment . T h i s p a p e r d e s c r i b e se f f o r t s by t h e I EEE t o p r ov i de a p p l i -ca t i ons docum en t a t ion des i gned t o re-d u ce t h i s i n f o rm a t io n g a p , b r i n g t h ematter ou t i n t o t h e open , and r educet h e s e c o m p e t i t i v e p r e s s u r e s on t h e b a t -te ry and UPS in du s t r i es .
HISTORY
Hi s t o r i ca l l y , t h e m a jo r comm erc i alb a t t e r y a p p l i c a t i o n s h av e b ee n i n t h era i l road , t e l ecom m un i ca t i ons and elec-t r i c u t i l i t y m a r ke t s. E ac h o f t h e s emarket segments is populated by enoughre l a t i ve l y l a r ge compan ies who can a f -f o r d t h e t e c h n i c a l e x p e r t i se t o e f f e c tc o n t r o l of p r o d uc t q u a l i t y t h r o u ght h e i r p u r c ha s in g p o l i c i e s .
UPS
The emerging UPS ba t t er y marke t ,u n l i k e t h e p r e v io u s m a r k et s , is cons i -d e r a b l y less s o p h i s ti c a t e d i n i t s b a s i cknowledge of b at te ry t echnology. N e w
o r r e pl ac e me n t b a t t e r i e s are of t en p ro -cured by Data C en t e r m anagers o r bu i l d -in g maintenance pe rson s. The demandso f t h i s m ar ke t h av e d r i v e n t h e b a t t e r ym a n u f a c t u r i n g i n d u s t r y i n t o a competi-t i v e race t o p r o v id e t h e h i g h e s t p os -
s i b l e power den s i t y . TheUPS
u s e r sh av e n o t r e c o g ni z e d t h e b a t t e r y d e s i g nt r a d e- o f f s t h a t c an r e s u l t i n r ed uc edb a t t e r y l i f e .
UPS u s e r grou ps a re now forming t odi sc us s and compare common problems .U n f u l f i l l e d b a t t e r y l i f e h as s u r f a c e d ast h e i r f o re mo st d i f f i c u l t y . I n i t i a l i n -v e s t i g a t i o n s b y West co as t UPS use rs andse ve ra l UPS main tenance co nt r ac tor s havedef i ned t he p rob l em as fo l l ows :
H I G H POWER DENSITYBATTERIES
A f t e r i n i t i a l UPS i n s t a l l a t i o n s wereequipped wi t h g ene ra l purpose te lecom-m u n ic a t io n s t y p e c e l l s , t h e b a t t e r y man-u f a c tu r e rs , i n an e f f o r t t o meet cus-tomer demand f o r more Watts per u n i tvolume responded wi t h se ve ra l des ignc ha ng es . F i r s t , e l e c t r o l y t e s p e c i f i c
g r a v i t i e s were i n c r e a s e d t o p r o v i d e b e t -t e r s h o r t term d i s c h a r g e r a t e p e r f o r -mance. Second, p l a t e g r i d d e s i g ns wereimproved to b e t t e r c o l l e c t e n e r g y f r o ma r e a s more d i s t a n t f ro m t h e b a t t e r yp o s t s . A h e a v i er o v e r a l l c u r r e n t c o l -l e c t i o n s ys te m was b u i l t i n t o t h e c e l l s
i n c l u di n g c o pp er i n s e r t s i n t h e p o s t s t oaccommodate th e very h ig h sh o r t termd i s c h a r g e r a t e s . T h i r d , p l a t e s weret h i n ne d down t o a l l o w t h e g r e a t e r s u r -f a c e d i s c h a r g e a c t i o n t h a t a cc om pa ni esh i g h d i s c h a r g e r a t e s . Also , i n somec a s e s t h e d e n s i t y o f t h e a c t i v e m a t e r i a lp a s te d i n t o t h e pla tes was d e c r e a s e d t oa l lo w b e t t e r c o n d uc t i vi t y t o t h e i n ne rs u r f a c e a r e a s of t h e a c t i v e m a t e ri a l .F i n a l l y , t h e h e a v i e r , f i b e r g l a s s , p os -i t i v e p l a t e r e t a i n i n g mats t h a t h ol d t h ed e l i c a t e l e a d d i o xi d e c h ar g e c r y s t a l sa g a i n s t t h e s u r f a c e s o f t h e p o s i t i v ep la te s were e i t h e r t hi n ne d t o t h e t h i c k -n e s s o f t i s s u e paper or com pl e t e l yremoved.
Each of t h e d e s i g n c ha ng es t h a t s oenhanced th e h ig h r a t e UPS performancea l s o t en de d t o d e c r e a se t h e c e l l l i f eand l i m i t i t s a b i l i t y t o h an dl e t h ed e ep e r d i s c h ar g e s t h a t a l s o t e n d t oaccompany high r a t e usage.
The u s e r s f o r t h e most par t , however ,were no t made aware of the consequenceso f t h e i r h i g h p ower d e n s i t y s e l e c t i o n so n t h e e xp e c t e d b a t t e r y s e r v i c e l i f e .
Oft en , i n new UPS i n s t a l l a t i o ns , t he
b a t t e r yis
purch ased thro ugh t h e UPSsu pp l i er who may be unaware of t he h ig hpower d e n s i t y r i s k s o r he is under pres -s u r e o f price com pet i t io n and may ten dt o c u t c o r n er s w i t h t h e b a t t e r y .
379 CH2653 4/88/0000 0379 01 00 C 1988 IEEE
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WARRANTIES
A l l o f t h e s e f a c t o r s h av e f o r c e dt h e b a t t e r y m a n u fa c tu r er s i n t o a h i g h l yc o m p et i t iv e e f f o r t t o o f f e r h i gh erp ow er d e n s i t i e s a t l o w er c o s t s . Onem ig ht e x p e c t t h a t a d r o p i n w a r r a n t y
i n t e r v a l s w ould f o l l o w a l o n g w i t h t h i se f f o r t . Tha t , howe ve r, d id no t oc c u r .The use r saw t h e same twe n ty year war-r a n t y . Some manufacturers addedc l au s e s t o l i m i t l i a b i l i t y . Fo r i n s-t a n c e , w i t h h i g h er s p e c i f i c g r a v i t y ,t h e u s e r m ig ht b e c o n s t r a i n e d , u n de rwa r r a n ty , t o ke ep th e t e m pe r a tu r e downt o 5 80F t o e x p e c t f u l l t we nt y yearb a t t er y l i f e .
EAR LY POSITIVEFAILURES
PLATE
A s t h e UPS use r s ba nde d toge t he r t ocompare maintenance no te s , t soon be-
came e v i de n t t h a t t h e in d u s t r y was ex-p e r i e n c i n g a v e r y h i g h i n c i d e n c e o fe a r l y b a tt e r y f a i l u r e s . A t t h i s p o i ntthe t e l e phone c om pa ny UPS u s e r s c a l l e dupon t h e i r more e xpe r i e nc e d ba t t e r yp eo pl e t o f u r t h e r d e f i n e t h e r o o tproblem.
I n i t i a l i n s p e c ti o n o f s e v e r a ls t r i n g s of t h e f a i l i n g c e l l s showed ex-c e s s i v e a mo un ts o f p o s i t i v e p la t e ma-t e r i a l s l o u gh e d o f f i n t o t h e s e d i me n tba s i ns . The m a nuf a c tu r er s f e l t t h a tt h e p l a t e f a i l u r e s were t h e r e s u l t o fe x c e s s i v e d e e p d i s c h a r g e c y c l i n g . C al -cium c e l l s t e n d t o b e h e a v i l y d e p e nd e n to n f i b e r g l a s s p o s i t i v e p l a t e w r ap t os u p p o r t d e e p d i s c h a r g e c y c l i n g c h a r a c-t e r i s t i c s . C e r t a i n l y , w i t h t h e h i g h
r a t e UPS d i s c h a r g e c h a r a c t e r i s t i c s , t
is easy t o l og many he avy d i sc ha r g e sd u r i n g t h e UPS i n s t a l l a t i o n / t e s t i n t e r -v a l b e f o r e t h e u s e r e v en p l a c e s t h esys tem i n s e r v i c e . A ls o, a wellmeaning us e r who wishes t o be ce r t a i nt h a t h i s c o n n e ct i o ns a re p r o p e r l yt o r q u e d a nd c a p a b l e o f c a r r y i n g t h eh i g h r a t e l o a d s , may u s e t h e l oa d t omake voltage drop measurements of t h ei n t e r c e l l c o n n e c ti o n s . Th e p u z z l i n gq u e s t i o n w i t h t h i s s c e na r i o is t h a t t h eus ers group had members wi t h mu l t ip lea ged id e n t i c a l UPS m odu les , a l l i de n -t i c a l l y t r e a t e d , whose b a t t e r y f a i l u r emodes d i f f e r e d g r e a t l y . W r e c o r de do ne l o c a t i o n w i t h 4 year old te lecom-m unic a t ions c e l l s t h a t were s t i l l per -f o r m ing ve r y well. A few f e e t away, i nt h e same ba t t e r y r oom , we obse r ve d anewer des ign UPS b a t t e r y t h a t h adTOT LLY f a i l e d a f t e r o n l y s e ve n y e a rso f a p p a r e n t l y i d e n t i c a l t r e a t m e n t ?
Tests of t h e f a i l i n g c e l l s r e v e a l e dAC r i p p l e c u r r e n t s o f as much a s 400
times t h e e x pe ct ed f l o a t c u r r e n t s f o rt h e f l o a t v o l t ag e s a t w hich t h e c e l l swere b e i n g o p e r a t e d . The o l d e r , t e l e -communications c e l l s a l s o r e c o r de d w ha twe c o n s i d e r e d v e ry h i g h r i p p l e c u r -
re nt s . They were , however , magni tudeslower than the newer UPS d e s i g n s .Measurements a t s e v e r a l o t h e r u s e r l o -c a t i o n s r e v e a l e d s i m i l a r h i g h r i p p l ecu r r en ts i n th e new UPS c e l l d e s i g n s .
We, a t f i r s t , s u s p e c t e d p o or m ain -t e n an c e o f t h e UPS r e c t i f i e r s t a g e f i l -t e r c a p a c i t o r s . F u r t h e r t e s t s showed,h ow ev er , t h a t t h e f i l t e r b an ks werep u l l i n g t h e i r p ro p er s h a r e o f t h e r i p p l ec u r r e n t s . F u r t h e r c o n s i d e r a t i o n o f t h eb a t t e r y i n d u s t r i e s a t t e m p t s t o p r o vi d et h e h i g h e s t p o s s i b l e s h o r t term c u r r e n tr a t e s , l e d us t o b e li e v e t h a t t h ene c e ssa r y lower b a t t e r y impedance ha dpr ov ide d a b e t t e r s h o r t c i r c u i t p a t h f o rt h e r i p p l e c u r r e n t s t h a n t h e i n t en d e dr e c t i f i e r f i l t e r c a p a ci t o r s. A ls o, t
would seem l o g i c a l t h a t t h e s e r i p p l ec u r r en t s c ou ld s t r i p t h e d e l i c a t e l ea dd i o x i d e ( c h a r g e ) c r y s t a l s f ro m t h e nowu n p ro t ec t ed s u r f a c e s o f t h e p o s i t i v ep l a t e s . T h i s p o s s i b i l i t y g e n e r at e d , asyou might expec t , g rea t i n t e r e s t f ro mt h e b a t t e r y a nd UPS s u p p l i e r s .
UPS, A DIFFERENT LOAD
The UPS i n v e r t e r p r e s e n t s a f a rd i f f e r e n t l o a d t o a b a t t e r y t h a n a ny o fo u r p r e vi o u s i n d u s t r y a p p l i c a t i o n s . Re
c o g ni z e t h a t p r e v i o u s b a t t e r y l o a d s wereg e n e r a l l y ran do m i n n a t u r e ( t e n s o fthousa nds o f t e l e pho ne r e l a y s ) and
t en d ed t o p r e s e n t a r e l a t i v e l y s m o o t hl o ad t o t h e b at t e r y . A ls o, t h er e c t i f i e r s were well a b l e t o k e ep u pw i t h t h e s l o w e r r a t e o f c u r r e n tchanges . The In ve r t e r s tage of t h e UPS,however, draws massive amounts of energya t t h e 60/360 Hz r a t e . I n t h e case of a
s o f t AC power l i ne , t h i s e ne r gy m ustcome f ro m e i t h e r t h e f i l t e r c a p a c i t o r si n t h e r e c t i f i e r s t a g e o r f ro m t h ebatteru The new lowe r impedance ba t-t e r i e s who can s o r e a d i l y r es po nd t o t h en ee d o f t h e i n v e r t e r a p p ea r t o p r o vi d et h e s e s h o r t term e ne r gy r e qu i r e m e n t s .
One UPS m a n u fa c t ur e r f e l t t h a t r i p p l ec u r r e n t s were of to t a l l y no conseque nc e
s t a t i n g t h a t b a t t e r i e s were e ve nknown t o be f l o a t ed on ha l f wave rec-t i f i e d power w i t h o u t a ny a d v e r s e a f f e c t .
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UPS, A DIFFERENT LOAD
(Cont.
T h i s s of c our se ve r y t r ue . Wi th nol o ad o n t h e b a t t e r y , l i k e a c a p a c i t o r ,t c h ar ge s t o a p e ak v a l u e w i t h l i t t l e
d i sc ha r ge f r om i t s i n t e r n a l l o s s e s.Eve n though the r i p p l e VOLTAGE s ex-
cee din gl y HIGH, t h e r i p p l e CURRENTS areve r y LOW. The case f o r i n v e rt e r s is
j u s t t h e o p p o s i t e . The r e c t i f i e r s pre-s e n t a n e x t r e m e l y l ow r i p p l e v o l t a g ef ro m t h e i r s i x p ha se b r i d g e well f i l -t e r e d systems. However, even th e small
va l l e y s i n t h e wavef or m c a n be dep -l e t e d by t h e h ea v y, c y c l i c a l e n e r g y re-qu i r e m e n t s of t h e i n v e r t e r g e n e r a ti n ge v e n h i g h e r r i p p l e CURRENTS t h a n mi gh tbe measured i n an unloaded , ha l f wavec h a r g e d b a t t e r y s y s t e m .
DO RIPPLE CURRENTS DAMAGE?
One m ig ht c e r t a i n l y e x p e c t t h i s t o
b e t h e case s i n c e t h e b a t t e r y manu-f a c t u r e r s h av e f o r so l ong p l a c e ds t r i n g e n t r i p p l e v o l t a g e c o n s t r a i n t s o nt h e a p p l i c a t i o n o f t h e i r p r o d uc t s . I tis p o s s i b l e , t h ou g h, t h a t t h e s e c on -s t r a i n t s were d i c t a t e d by o ne of t h e i rm a jo r m a r ke t s , t h e t e l e c om m unica t ionsin du st ry , who s i m p l y w is he d t o l i m i t
t h e hum i n t h e s u b s c r i b e r s e ar s. We,t h e u s e r s , a re a w a i t i n g w i t h grea ti n t e r e s t , t h e p u b l i c a ti o n o f d e t a i l e d ,d e f i n i t i v e , t e s t r e s u l t s on t h e ef fec t sof r i p p l e a l o n e a nd r i p p l e / c y c l e com-b ine d e xpe r im e n t s on t he UPS l i n e ofce l l s w i t h o u t f i b e r g l a s s r e t a i n e r s a ndw i t h a l l o f t h e o t h e r c h an g es e mb od ie di n t h e c u r r e n t UPS l i n e o f c e l l s .P r e v ious r i p p l e / l i f e papers ha ve no twell d e f in e d t h e e f f e c t s o f r e t a i n i n g
m a t s , p o s i t i v e p l a t e e n c a p s u l a t i o n , o rg r i d a l l o y s t h a t makes l i f t t r u c k c e l l ss o r e s i s t a n t t o c yc l in g .
UPS us er s cont inu ed t o be confused byf a i l u r e s of b a t t e r y s t r i n g s ( Some i no n l y 3 years) wit h no known his to ry of
c y c l i n g . User g r ou p i n q u i r i e s d i s c l o s e ds i m i l a r e a r l y f a i l u r e s i n s e v e r a l manu-f a c t u r e r s UPS p r oduc t l i n e s a nd wereve r i f i e d by se ve r a l UPS ba t t e r y m a inte -n a n c e c o n t r a c t o r s .
I EEE INVOLVEMENT
A t t h i s p o i n t , UPS d e s i g n c o ns u l -t a n t s , u nd er f i r e from t h e i r c l i e n t s ,a s k e d t h e I EEE t o c on v en e a g r ou p t os tudy the p r ob le m . A m aj or d i f f i c u l t y ,t was d i s c e r n e d , stems from th e new and
smaller b a t t e r y u s e r s who t e n d t o b eu n f am i l i ar w i t h t h e e f f e c t s of t h e UPSb a t t e r y d e s i g n c h a n ge s t h a t t h e y h av ep r e s s e d i n t o e x i s t e n c e . An I EEE t a s kf o r c e h as b ee n f or me d u n de r t h e S t a t i o nB a t t e r y Working Subgroup. Th is grou p is
i n t h e p r o c e s s o f p r e p a r in g a u s e r l e v e li n f o r m a t i v e d oc um en t t h a t w i l l preparet h e new UPS market t o se l e c t t h e b e s te n e r g y s t o r a g e system t o meet t h e i r
ne e ds .
W T . R u t l e d g e , P a c i f i c B e l l
R e f e r e nc e : I n f lue nc e O f SuperimposedAl t e r n a t i ng C u r r e n t On Capaci ty andC yc le L i f e For Lead Acid Ba t te r ies ,S. Oka z a k i , S Higuc h i , 0. Nakamura,S Ta ka ha sh i ,J o u r n a l of A p p l i e d E l e c t r o c h e m i s t r y-16 1986) 894-898)
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