Feldebene
Frank Blettenberger
Achilles Heel
Battery
Uninterruptible Power Supplies
(UPS) protect machines and
devices from unexpect ed power
outages and dist urbances in power
grids. The purpose of UPS
syst ems is self defeating if they
also unex pectedly fail when they
are most needed. This makes the
perpetual monitoring and
management of UPS systems a
giv en obligation.
tatic
Uninterr uptible
Power
Supplies ( UPS), which include
everything from Mini-UPS for the offic e
to UPS w ith more than 100 KVA
capacity, use batteries f or storing the ir
electrica l char ge . These batter ies ar e to
guarantee the continual supply of
alternating c urr ent upon which the
power consuming device s that are
attached
to
them depend
for
unobstructe d operation. This make s
batter ies a cr uc ial point in the
conceptual design of a UPS system, a
point that is hardly considered in places
where stable power supply systems ar e
present like Germa ny. But a UPS
without working batteries is not only
useless, but wor se, it also represents a
risk.
www. elekt roni knet .de . C omput er&AUTOM ATI ON . 2/05
This is where battery monitoring
systems come in. These devices
continually ga uge and record the status
values of the batteries. Depending on the
scope of the battery monitoring systems
functionality,
the se
devices
can
prediagnose which batteries will cause
eventual problems and in the best case
even prohibit or eliminate arising problems
altogether.
Decisive for the effectivene ss of suc h
ba ttery monitoring and manageme nt
systems is the systems capability to
monitor each individual battery and not
just the va lue of their voltage. Mor e so,
such a system must c onduc t a long- term
monitoring of both each individual
ba tteries te mperature and volta ge for the
sa ke of delive ring reliable values.
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In optimal cases the monitor ing system
also measures and records each
individual ba tteries inter nal re sistance. In
most UPS, seale d lea d gell cell batteries
with battery lives between five and ten
years are used. The spec ified ba ttery life
is howe ver no indication of how long the
ba tteries will last. This specifie d term
merely defines the duration under which
the device qualifie s as battery per
definition as to have the capacity to store
and pr ovide a n electrica l charge in
general. The term „battery life“ actually
pertains to the physica l phenomena
limiting the ba tteries qualification as a
ba ttery per se over which no battery
management system can have any
influence. Thus, the term battery life as
specifie d serves of no statistical value in
regards to practical purposes.
Of much greater importanc e is the
battery’s „dur ability": this sets down the
per iod in which the battery is actually
usable , as to fulf il the charging and
discharging capacitie s set out in its
specifications. The ba ttery durability
depends on the type of char ging units in
use, the ba ttery quality, the se verity and
frequency of disc harging, the temperature
and the early de tection of other defect
batter ies in its battery bank.
Goal: Homogenit y
The foremost bla tant problem
affecting batter y dura bility in UP S
ba ttery banks is the charging unit.
Proper battery care in the proce sse s of
charging a nd disc harging is a cr ucial
fac tor that a ma na geme nt system must
attain for. Especially by lead ge ll cells,
inc orrec t char ging can cause over and
under charging. Over char ging ca n
de stroy batteries after only a short while.
Theref ore, a battery manager must
charge each individual ba ttery until a
homogenous bond ha s been ac hieved in
which each ba ttery is on the same level.
Such homoginisa tion c an for example be
performed by the „BA CS BatteryMana ger" from Generex. This pa tent
pe nding func tion ter med „Equalizing"
ba lances out the differences in the
individual batteries and transf orms
inhomogenous battery
banks into
homogenous battery groups.
Unhomogenise d charging can cause
the most da mage: charging currents that
are too high f or the batte rie s can lea d to
a c ondition in whic h the batteries start
emitting gases. When the batte ry is then
una ble to reach the target capacity set by
the c ha rging unit, this gas emission
cause s the ba ttery to c orrode and
eventually short c irc uit. On the other
hand, if the batteries reach capac ity too
quickly, this is an indication that the
batter ies are e ither already corroded or
tha t the lead plate s are defect or that
the re is a short circuit. I n this case the
battery will indica te that it is fully
charged when in fact it is simply defect.
Also prolonged state s of dorma ncy, as
is the case with storage in which the
batteries remain unchar ged, can cause
battery damage. A battery manage r must
be capable of dete rmining such
scenarios a nd be able to go
automatica lly into a self hibernation
mode as to preserve ba ttery ca pacity.
The system must be capable of recor ding
the le ngth of such states in order to
resume monitoring after the system has
been reac tivate d f or rec harging.
Furthe r, BACS will de termine the
degree to which corrosive deposits have
acc umulated and will ena ct a ba ttery
discharge followed by a rec harge that is
gauged towards e liminating the corrosion.
If the amount of corrosive build-up
inhibits discharging, BA CS will at lea st
send out a warning to maintenance.
BACS-C2-Module ins tallation on a
Problem Area: Discharge
Battery discharge is not necessarily
negative for a ba ttery, but on the c ontrary
it is good for a battery. When the
discharging a nd charging are conducted
correc tly the battery will operate well in
accordance with the numbe r the number
of charging cyc les it ha s bee n through.
Howe ver, damage does occ ur whenever
more than 60% of the batte ry’s capacity
is extracte d. Anytime this limit is
exceeded, an estimated battery dama ge of
up to 30% could be inflic ted. After three
or four time s of this oc currence the
battery will be in danger of fa ilure.
Since the batter ies are inside of a UPS
battery bank, the elec tronics of the UPS
are not ca pa ble of determining which
batteries fall into this ca tegory since the
stronger batterie s will compensate f or
this malfunctioning. This mea ns that an
uninte ntional and uncontrolled dischar ge
will continue to progress until the
complete battery bank as a whole has
reached the targete d c harging level. It is
not evident for the UPS that several
batter ies are being damaged in the process
of doing so.
On the other hand, a battery manager
will control and stop discharging when it
become s necessary in order to prevent
ba ttery
dama ge.
This
ha s the
disadvantage of slightly lower ing the
autonomy time , but it in trade it
lengthens the overall durability of the
ba tteries and ups system as a whole.
www. elekt roni knet .de . C omput er&AUTOM ATI ON . 2/05
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batte ry. The large cooling plate insure s
an adequa te thermal distributi on for the
dissi pa tion of heat energy. Under neath
the C2- Module lies a „Carrier " plate
that separ ates the module from the
batte ry’ s regulator va lv e.
Battery Qualit y
Ba ttery quality is a diffic ult subject
beca use
not even
the
battery
manufacturers can make any va lid
conclusions about the quality of their own
batteries. Even when new, the quality of
each individua l battery tends to be
different.
The ma in problems ste ms from the
production process used for sea led
batterie s: first of all, there are various
grades of lead ava ilable on the market.
This lead is formed into lead rods with a
tolerance of ±5 mm and in prac tice; the
ac tua l var iance lies much higher than
what can be technically achieved. In
addition the r ods are ground into a paste
using sulfuric acid. This proce ss often
produces an irregular inhomoge nous
suspe nsion. This lea d gell is then cast
into casings by machines. De pending on
several factors like the conveyor belt
speed, c onsistency of the lead ge ll batch
and
the
spreading
mecha nism,
Feldebene
Feldebene
different application thicknesses result
each time. Thus, tolerances of ±20 mm
can only be avoided with great diffic ulty.
These forms are then presse d into shape
and c overed with gla ss matting. Additional
variances in forming pressure, temperature
and time cause further irregularities. The
curing process tha t follows also affects
whether the lead will shrink or expand
while it dries out. Last but not lea st,
during the a ssembly in which the cell unit
is place d into its casing, connec ting the
ce ll with the battery posts prese nts a final
diffic ulty in which faulty placement ca n
lead to c orrosion or larger transition
resistance.
This imponderability of the batteries is
where the „evil“ begins. If there is just one
battery in a battery bank tha t does not
maintain the same charging performance
as the re st of the batter ies, this battery will
negative ly affect the re st of the batter ies in
the long run. A norma l char ging unit can
not detect the wea kest member of the
UPS, and this weak battery will continue
to deteriorate until it affects the complete
battery bank.
UP S devices are then not primarily
de pendent on the quality of the
individual ba ttery, but ra ther on the
homogenised qua lity of all batteries
bonded together. An a ppr opriate
indicator of battery ca pacity a nd as such
an indicator of battery c ondition is the
internal resista nce . If the internal
resistance rises during operation, the
capacity
sinks
pr oportionate ly
diminishing
the
batte ries
active
substanc e. This makes an increase in
internal battery resistance valid
A BACS-C1-Modul e installed in a 2 000 -VA-UPS: Battery type module adj ustment
can be perfor me d usi ng a notebook. Later, the note book will be repl aced by the
BACS-Web-Manager.
evide nce of an approaching failur e
although the operating volta ge is
correc t. Still, the inter nal ba ttery
resista nce is r are ly measure d due to the
eff orts involved in deter mining this
value . A modern ba ttery mana geme nt
system de tects increases in internal
resista nce, determines whic h battery ce ll
is defect and warns ma inte na nce to
replace the battery cell. This way it is
possible to avoid having to replace a ll
batter ies and simply pinpoint the defect
batter ies that nee d replac ing before they
adver sely affec t the other batter ies.
A Matter of Temperature
for change s in temperatur e by monitoring
the te mperature on each individual batte ry.
Communicating with the charging unit via
ne twork or se ria l c ontact for example, each
system batte ry unit can signal for a
reduc tion in charging current as a means of
lowe ring the temperature . After receiving a
warning, ma intenance can ta ke active
measure s to c ool down the system. BACS is
also ca pable of te mporarily re duc ing the
charging current independently of the
charging unit.
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More informa tion:
www.gen ere x.de
Te mperature has a considerable
influe nce on the battery dura bility. In
UPS, ba tteries are pac ked close together
in small spaces needing to withstand
tempera ture s of 30 °C and above .
Howeve r, any degree ove r 25 °C
shorte ns the batteries durability and
temperature over 40 °C will cut the
durability in ha lf. This mea ns that a
batte ry ma na ge ment system must also
account
Frank
Ble ttenberger
Genera l manager
Gen ere x, Ha mburg.
The „Batt ery Analyzing and Care Syst em" BACS
The princ iple item in the battery m anagement
s ystem from Generex is an intelligent c harging
dis tribution
mechanis m
that
works
independently of the attac hed c harging unit
us ing an adaptive regulator that adjusts the
c harging rate of each individual battery until a
uniform charging sequenc e is attained. This
process termed „Equalizing" prohibits the total
dis charging as well as over c harging of the
individual batte ries capping the c harging level
at its maxim um tolerable value. The B ACS
c harging regulato r at tempts a t firs t to b ring
the weakes t batte ries up to level with the
others. During a c omplete charging phas e,
through a process of s ucc ess ive
app roximation, the batteries a re b rought into a
uniform s tate and are at the s ame time
elevated s tep by step to the targeted level of
c apac ity. If uniform batte ry c onditions are
obtained afte r s eve ral c harging cyc les, the
c harging c apacity will be rais ed until the
maximum values are achieved. B y optimising
c harging c urrent and charging voltage to
matc h the battery c onditions , the bat teries are
„trained " and c on ditioned to reac h their
opti mal c apac ity .
BA CS is c om posed of battery modules , a
c entral proc ess ing unit, the B ACS -Controller
and a uniform bus c able that c onnec ts the
modules with one another.
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Each bat tery in the sys tem is equipped with
its own battery module. The c ent ral unit is
us ed to c onfigure the parameters of the
individual batte ries : limit values for c harging
voltage, inte rnal res is tanc e and tem pe rature
as well as fo r s etting the paramete r profiles
of each diffe rent t ype o f battery . The
controller manages the battery modules and
reads out the m eas urements taken fro m the
batteries and c onduc ts a c om paris on with
the r eferenc e values for the batteries
corres ponding type. Values exceeding
tolerance le vels will invo ke ala rms that will
be read out on the LCD-Display , and an
additional alarm will be triggered.
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