BATTERY (AND DC POWER) SOLUTIONS
This catalog features a comprehensive look at the importance of batteries and routine battery testing and
maintenance. In addition to this reference guide, you can go to our website anytime . . . www.avointl.com . . .
for the latest in new battery testing products, applications, and other helpful ideas.
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WHY TEST BATTERY SYSTEMS
Batteries are complex chemical mechanisms. No matter how well batteries are manufactured, there is still some amount of “black art” to them
(and all chemical processes). A battery is basically two dissimilar metals in an electrolyte. In fact, you can put a penny and a nickel in half of a
grapefruit and you now have a battery. Obviously, an industrial battery
is more sophisticated than a grapefruit battery and is not without its
maintenance needs. A good battery maintenance program may prevent,
or at least, reduce the costs and damage to critical equipment due to an
ac mains outage.
Why Test Systems:
• To insure the supported equipment is adequately backed-up.
• To prevent unexpected failures.
• To forewarn death.
Battery systems are installed
for only two reasons:
• To protect and support critical
equipment during an ac outage.
Volta invented the battery in 1800. Planté perfected the lead-acid battery in 1859. It was a good design then, but with refinements over the
decades, it has become a critically important back-up power source.
The refinements include improved alloys, grid designs, jar and cover
materials and improved jar-to-cover and post seals. The most revolutionary development was the valve-regulated battery. Nickel-cadmium
has seen many of the same types of changes over the years.
• To protect revenue streams due
to the loss of service.
Lead-acid Overview
• When will it need to be replaced?
The basic chemical reaction is:
• What is its condition now?
PbO2 + Pb + 2H2SO4 <—> 2PbSO4 + 2H2 + 2O2
in a sulfuric acid electrolyte, where the sulfate of the acid is part of the
reaction. The acid is depleted upon discharge and regenerated upon
recharge. Hydrogen and oxygen form during recharge and float charging. In flooded batteries, they escape and water must be periodically
added. In sealed batteries, they recombine to form water. In valve-regulated lead-acid batteries, the acid is immobilized by an absorbed glass
matte (AGM). The matte is much like the fiberglass insulation in your
house. It traps the hydrogen and oxygen formed during discharge and
allows them to migrate so that they react back to form water. This is
why VRLA never need water added compared to flooded (vented) leadacid batteries. A battery is alternating positive and negative plates separated by microporous rubber in flooded lead-acid or absorbed glass
matte in VRLA or plastic sheeting in NiCd. All of the like-polarity plates
are welded together and to the post. In the case of VRLA cells, some
compression of the plate-matte-plate sandwich is exerted to maintain
good contact between them. There is also a self-resealing pressure relief valve to vent gases when over-pressurization occurs.
Lead-acid (flooded) Failure Modes
• Positive grid corrosion
There are only four questions
a battery user will ask. The
first two are very important:
• What is the capacity of the battery now?
• What can be done to improve,
not reduce its life?
There are several main types
of batteries with subtypes:
• Lead-acid
• Flooded
• Lead-calcium
• Lead-antimony
• VRLA (sealed)
• AGM
• Gel
• Flat plate
• Tubular
• Nickel-cadmium
• Flooded
• Sealed
• Sediment (shedding) buildup
• Pocket plate
• Top lead corrosion
• Flat plate
• Plate sulphation
• Hard shorts (paste lumps)
1
IMPEDANCE:
WHAT IS IT AND
WHY IT WORKS SO WELL
Impedance is resistance in ac
terms. With regard to dc battery
systems, it indicates the condition
of batteries without harming or
stressing them in any way. Since
it tests the condition of the entire
electrical path of a battery from
terminal plate to terminal plate,
impedance can find weaknesses in
the battery easily and reliably.
Applying an ac test signal to the
terminal plates performs the test.
Then measure both the total ac
current in the string and the voltage drop of each unit in the string
by measuring each cell and intercell connector consecutively until the entire string is measured.
Impedance is calculated, displayed and stored. As cells age,
the internal impedance increases
(see the graph to the right). By
measuring impedance, the condition of each cell in the string can
be measured and trended to determine when to replace a cell or
the string aiding in planning budgetary needs.
The impedance test is a true fourwire, Kelvin-type measurement to
provide excellent reliability and
highly reproducible data on which
to base sound decisions with regard to battery maintenance and
replacement.
Each battery type has many failure modes, some of which are more prevalent than others. In the case of flooded lead-acid, the predominant failure mode is plate growth. The growth rate has been well characterized
and is taken into account when designing batteries. In many battery data
sheets, there is a specification for clearance at the bottom of the jar to
allow for plate growth in accordance with its rated lifetime, for example
20 years.
The normal failure mode is positive grid corrosion. The grids are lead
alloys (lead-calcium, lead-antimony, lead-antimony-selenium) that convert to lead oxide over time. Since the lead oxide is a bigger crystal than
lead metal alloy, the plate grows. Battery manufacturers design for extra
space in the jars to account for this growth. At the “designed end-of-life”
the plates will have grown sufficiently to pop the tops off of the batteries. But excessive cycling, temperature and overcharging can also increase the speed of positive grid corrosion.
% Battery Life
Figure 1: Changes in impedance as a result of battery capacity
Sediment buildup (shedding) is a function of the amount of cycling a
battery endures. This is more often seen in UPS batteries but can be
seen elsewhere. Shedding is the sloughing off of active material from the
plates, converting to white lead sulfate. Sediment buildup is the second
reason battery manufacturers have space at the bottom of the jars to
allow for a certain amount of sediment before it builds-up to the point of
shorting across the bottom of the plates rendering the battery useless.
The float voltage will drop and the amount of the voltage drop depends
upon how “hard” the short is. Shedding, in reasonable amounts, is normal.
Some battery designs have “wrapped” plates such that the sediment is
held against the plate and is not allowed to drop to the bottom. So sediment does not buildup in wrapped plate designs. The most common
application of wrapped plates is UPS batteries.
2
Top lead corrosion, like slivering, is more of a manufacturing defect. This
is hard to detect even with a visual inspection since it occurs near the
top of the battery and is hidden by the cover. Nearing a full failure, impedance may find this defect. But it will surely fail due to the high current draw when the ac mains drop off. The heat buildup when discharging will most likely melt the crack open and then the entire string drops
off-line — a catastrophic failure.
Plate sulphation is one of the easiest failure modes to find with impedance. A thorough visual inspection can sometimes find traces of plate
sulphation. Sulphation is the process of converting active plate material
to inactive lead sulfate. Since impedance finds electrical path failures
very well (rather than mechanical failures, unless they are manifested
as electrical path failures), sulphation is one of the electrical path problems which is easily found. Sulphation is due to low charger voltage settings or incomplete recharge after an outage. Sulfates form when the
battery is not being kept fully charged.
Lead-acid (VRLA) Failure Modes
• Dry-out (a.k.a. Loss-of-Compression)
• Plate Sulphation (see above)
• Soft and Hard Shorts
• Post leakage
• Thermal runaway
• Positive grid corrosion (see above)
Dry-out is a phenomenon that occurs due to excessive heat (lack of
proper ventilation), over charging, which can cause elevated internal
temperatures, high ambient (room) temperatures, etc. At elevated internal temperatures, the sealed cells will vent through the PRV. When sufficient electrolyte is vented, the glass matte no longer is in contact with
the plates, thus increasing the internal impedance and reducing battery
capacity. In some cases, the PRV can be removed and distilled water
added (but only in worst case scenarios and by an authorized service
company since removing the PRV may void the warranty). This failure
mode is easily detected by impedance and is one of the more common
failure modes of VRLA batteries.
Soft (a.k.a. dendritic shorts) and hard shorts occur for a number of reasons. Hard sorts are typically caused by paste lumps pushing through
the matte and shorting out to the adjacent (opposite polarity) plate.
Soft shorts, on the other hand, are caused by deep discharges. When the
specific gravity of the acid gets too low, the lead will dissolve into it.
Since the liquid, and dissolved lead, is immobilized by the glass matte,
when the battery is recharged, the lead comes out of solution forming
dendrites inside the matte. In some cases, the lead dendrites short
through the matte to the adjacent plate. The float voltage may drop
slightly but impedance can find this failure mode easily but is a decrease
in impedance, not the typical increase as in dry-out. See Figure 1, “Abnormal Cell”.
IEEE RECOMMENDED
PRACTICES
IEEE 450-1995
“IEEE Recommended Practice for
Maintenance, Testing and Replacement of Vented Lead-acid
Batteries for Stationary Applications” describes the frequency
and type of measurements that
need to be taken to validate the
condition of the battery. The frequency of tests ranges from
monthly to annually. Some of the
monthly tests include string voltage, appearance, ambient temperature, float current, etc. Quarterly tests include specific gravity,
cell voltage and temperature
(=10% of cells). Annual tests are
performed on the entire string.
Additionally, the resistance to
ground of the battery rack and intercell connection resistance need
to be measured. Other tests made
need to be performed based on
the values measured during periodic tests and battery usage (cycling history).
IEEE 1106-1995
“IEEE Recommended Practice for
Installation, Maintenance, Testing
and Replacement of Vented NickelCadmium Batteries for Stationary
Applications” has similar recommended practices as IEEE 450.
IEEE 1188-1996
“IEEE Recommended Practice for
Maintenance, Testing and Replacement of Valve-Regulated
Lead-Acid Batteries for Stationary
Applications” defines the recommended tests and frequency.
VRLA cells have been classified
into tiers of criticality of the installation. The frequency and type of
tests vary based on the battery’s
tier.
3
INSULATION RESISTANCE
Leaking cells can lead to reduced
battery capacity, system inefficiencies, conditions hazardous to
personnel and, in rare cases, lead
to fires. A cell can leak for a variety of reasons including mishandling during shipping or installation, poor post-seals, overcharging, excessive plate growth, etc.
This off-line test applies a dc voltage, say 500 V dc, between the
buss and the rack, then measures
the dc leakage current to calculate
resistance in MΩ or GΩ. The
higher the resistance is the better.
This test is recommended at installation and whenever a leak
may be suspected (from telltale
signs such as salt buildup). This
test must be performed while the
battery or cell is off-line to prevent
shorting the battery since a dc
voltage is applied to the battery.
GROUND FAULTS
ON DC SYSTEMS
When ground faults occur on floating battery systems and, in some
cases, grounded systems, critical
components in the system may
not operate properly. A reduction
in system readiness may then occur. Ground faults in excess of 100
kΩ are rarely worrisome. Sometimes, in less critical systems, a
single fault is not worrisome, either. But when the total ground
faults are less than 30-50 kΩ, system reliability is suspect.
A fault is traced by applying an ac
(25 Hz) current signal into the dc
buss. Simply trace the circuits
with the highest current values
until the faults are located. Faults
can be traced easily regardless of
the number of distribution panels
or circuits because the “tracer” is
merely following the strength of
the ac signal. System integrity is
maintained because it is an on-line
ac test and is designed to prevent
system trips.
4
Thermal runaway is when a battery’s internal components melt down in
a self-sustaining reaction. Normally, this phenomenon can be predicted
by as much as four months which is one of the reasons why AVO International recommends quarterly VRLA impedance testing. The impedance
will increase in advance of thermal runaway as does float current. Thermal runaway is relatively easy to avoid, simply by using temperaturecompensated chargers and properly ventilating the battery room/cabinet. Temperature-compensated chargers reduce the charge current as
the temperature increases. Remember that heating is a function of the
square of the current.
Nickel-Cadmium Overview
NiCd chemistry is similar in some respects to lead-acid in that there are
two dissimilar metals in an electrolyte. The basic reaction is:
2 NiOOH + Cd +2 H2O <—> Ni(OH)2 + Cd(OH)2
in a potassium hydroxide (alkaline) electrolyte. In NiCd batteries, the
KOH does not enter the reaction like sulfuric acid does in lead-acid batteries. The construction is similar to lead-acid in that there are alternating positive and negative plates submerged in an electrolyte. Rarely seen,
but available, are sealed NiCd batteries.
Nickel-Cadmium Failure Modes
NiCd batteries seem to be more robust than lead-acid. They are more
expensive to purchase but the cost of ownership is similar to lead-acid,
especially if maintenance costs are used in the cost equation. Also, the
risks of catastrophic failure are considerably lower than VRLA.
The failure modes of NiCd are much more limited than lead-acid. Some
of the more important modes are:
• Gradual loss of capacity
• Carbonation
• Floating effects
• Cycling
• Iron poisoning of positive plates
• Service at elevated temperatures
• Memory effects
Gradual loss of capacity occurs from the normal aging process. It is irreversible but is not catastrophic.
Carbonation is reversible and is gradual. But without proper maintenance,
this can cause the load to not be supported, which can be catastrophic.
This can be reversed by exchanging the electrolyte.
Floating effects are the gradual loss of capacity due to long periods on
float without being cycled. Through routine maintenance, this can be
avoided and is easily found by impedance testing. Floating effects are
reversible by deep-cycling the battery once or twice.
Iron poisoning is caused by corroding plates and is irreversible. This is
typically a “pocket-plate” design failure mode.
CONVERTING DATA INTO
INFORMATION
When taking measurements, there
are two main things to know about
accuracy and repeatability:
1. They are not the same thing
2. One is more important than the
other.
Poor Accuracy, Poor Repeatability (Worst Case)
Excellent Accuracy, Poor Repeatability (Bad Case)
To the left are four graphs depicting the difference between accuracy and repeatability.
Naturally, everyone wants the
“best case” but it is not always
practical to achieve. So let’s make
it easy to understand. Accuracy is
the closeness of a measurement to
the true value, in this example,
zero. Repeatability is the closeness of readings to each other. The
difference between repeatability
and accuracy is called “bias”
which can be easily calculated but
only if there is excellent precision.
In the case of trending data, precision is far more important since
you want to make sure that the
result you get is close to the result you got last time unless real
changes have truly occurred. Only
through solid repeatability can
one make sure that the trend is
due to real changes, not due to
poor equipment.
Poor Accuracy, Excellent Repeatability (Better Case)
Excellent Accuracy, Excellent Repeatability (Best Case)
5
FAQS ABOUT BATTERIES
What is the value of taking specific gravity readings?
Traditionally, specific gravity has not provided much value in determining impending battery failure. In fact, it changes very little after the initial three to six months of the battery’s life. This initial change is due to
the completion of the formation process which converts inactive paste
material into active material by reacting with the sulfuric acid.
What does float voltage of a cell tell me?
?
Float voltage indicates that the charger is working, that is, state-of-charge.
It does not indicate the state-of-health (condition) of the cell. It indicates that the cell is fully charged but don’t confuse “fully charged” with
“full capacity.” There have been many times that the float voltage is within
acceptable limits and the battery fails. A low float voltage may indicate
that there is a short in the cell. This is evidenced by a float voltage at
about 2.06 or below for lead-acid (if the charger is set for 2.17 V per cell).
In some cases a cell floats considerably higher than the average. This
may be caused by the high float blotage cell compensating for another
cell that is weak and is floating low. It is possible that one cell floats
much higher to compensate for several cells floating a little low. The
total of all cells’ voltages must equal the charger setting.
What are the recommended maintenance practices for the
different types of batteries?
IEEE recommended (Maintenance) practices cover the three main types
of batteries: Flooded Lead-acid (IEEE 450), Valve-regulated Lead-acid (IEEE
1188), and Nickel-cadmium (IEEE 1106). Generally speaking, maintenance
is essential to ensure adequate backup time. There are differing levels of
maintenance and varying maintenance intervals depending upon the battery type, site criticality, and site conditions. For example, if a site has
an elevated ambient temperature, then the batteries will age more quickly
implying more frequent maintenance visits and battery replacements.
How important is intercell connection resistance?
Our experience has found that many battery failures are due to loose
intercell connections that heat up and melt open rather than cell failure.
Whether a cell is weak or an intercell connector is loose, one “bad apple”
does spoil the whole bushel.
When lead acid batteries are frequently cycled, the negative terminal
may “cold flow,” thus loosening the connection.
What are the most common failure modes and how can
impedance find them?
There are numerous failure modes, again depending upon battery type.
Briefly, some failure modes for flooded are: plate sulphation, sediment
buildup and positive grid corrosion. For VRLA, some failure modes include: dry-out (loss-of-compression), positive grid corrosion and cell
leakage. Please refer to the “Battery Failure Modes” Application Note
which can be found on the AVO website (www.avointl.com) for more
detailed information.
6
FAQS ABOUT BATTERIES
How often should impedance readings be taken?
The frequency of impedance readings varies with battery type, site conditions, and previous maintenance practices. AVO International recommends that VRLA batteries are measured quarterly due to their unpredictable nature while semiannually for NiCd and flooded lead-acid types.
How do I interpret data?
There are three general modes of data interpretation: instantaneous,
short term, and long term. For more details, please refer to the Data
Interpretation Application Note which can be found on the AVO website
(www.avointl.com).
?
What does ripple current tell me?
Ripple current is a manifestation of the rectifier/charger which converts
ac into dc. No charger is 100 percent efficient. Therefore, some ac “carryover” occurs. The level of this carry-over depends upon the quality and
features of the charger itself. The most basic chargers do not have filters
to remove ripple components and, therefore, apply higher levels of ripple
to the battery. This is typical in UPS systems. Because ripple creates
hum on telephone lines, telco rectifiers are very well filtered. As the
charger ages though, the ripple current increases.
Battery manufacturers have defined a loose guideline of 6 A rms ripple
for every 100 Ah of battery capacity. Above this level, heating of the
battery may occur, thus shortening the life of the battery. The 10 A Biddle®
BITEs measure 60 Hz component of ripple. The 60 Hz component is well
correlated to total rms ripple and gives reliable data.
How can I predict when I need to change a cell or the entire
battery?
Even though there is not a perfect mathematical correlation between
battery capacity and impedance, the amount of increase in impedance
is a strong indicator of battery health. AVO has found that a 20 percent
increase in impedance for flooded lead-acid generally correlates to 80
percent battery capacity. In VRLA, that increase is closer to 50 percent
from the battery’s initial impedance or from the manufacturer’s baseline
values.
At what point should I stop changing cells and replace the entire
battery?
In shorter strings (less than 40 cells/jars), the entire battery should be
replaced when three to five units have been replaced. In longer strings, a
similar percentage that is replaced is the criterion.
For additional details on battery solutions and applications, contact the
factory directly via email:
battery@avointl.com
7
BATTERY TEST EQUIPMENT
Regardless of whether you are
testing flooded lead-acid, VRLA or
Ni-Cd cells, AVO International has
the right equipment for your battery maintenance requirements.
The products and associated accessories provides meaningful
data on battery health without significant expense or any reduction
in remaining battery capacity.
Interruption in service can cause
disaster to supported equipment
and facilities. Consequently, a dependable backup power system is
critical so that when AC mains fail,
costly service interruptions can
be avoided. The battery impedance test helps to identify weak
cells before they cause problems.
Taking the battery off-line for testing is time consuming and adds
risk to the process. This process
is unnecessary with the on-line
testing capabilities of the AVO
family of battery test products.
The highly repeatable instruments
help reduce downtime and avoid
personal injury and equipment.
MBITE Miniature Battery Impedance Test Equipment
The Biddle® MBITE is a lightweight, easy-to-use battery tester ideal for
determining the health of flooded and sealed lead-acid and Ni-Cd batteries up to 2500 Ah in many telco, switchgear, motive power, airline, etc.
applications.
In addition to measuring battery impedance and interconnection resistance, the MBITE measures individual dc terminal voltages. Furthermore,
since impedance does not stress the battery, terminal voltages do not
increase during the test and may be used for documentation purposes.
All three parameters can be stored (up to 1000 lines of readings) onboard for immediate review at the test site on the 3-1/2 in. LCD or from
the built-in printer. The stored data can be downloaded via the RS-232
connector to a PC using “AVOLink” download software or other communications packages to spreadsheet applications for further analysis or
databasing.
BITE2 Battery Impedance Test Equipment
The Biddle® BITE2 is designed primarily for testing larger cells found in
telco COs and MTSOs, UPS and substation. However, it also can be used
in the smallest of battery systems. Because the BITE2 has a larger
memory, more data can be stored from more sites before downloading
to a PC.
A unique feature of the BITE2 is that if while in the middle of a test, you
receive an emergency call to go elsewhere, simply turn off the Receiver,
pack up and go. When you return to finish the test, simply reconnect the
source current leads and turn on the Receiver. The Receiver will remember where you left off. Just continue measuring from that point forward.
C-BITE Compact Battery Condition Tester
The Biddle® C-BITE is ideally suited to help determine the condition of
batteries in distributed power (outside plant), wireless, customer
premise, smaller telecommunications installations, emergency lighting
systems, small UPS systems, and many other smaller installation types.
The C-BITE measures the internal resistance, interconnection resistance,
float voltage and temperature (with an optional probe) of secondary
batteries. An ac four-wire measurement (Kelvin-type) is utilized to eliminate any errors due to lead wire resistance. Another advantage of ac
resistance is that the dc voltage of the battery system does not affect it.
MBITE
BITE 2P Battery Impedance Test Equipment
BITE2
The Biddle® BITE 2P determines the condition of lead-acid and nickelcadmium cells up to 7000 Ah. An advanced feature set has been developed that includes Pass/Warning/Fail calculations based on a user-entered baseline value, advanced printing functions and more. The case of
the BITE 2P consists of both the transmitter and a carrying case for all of
the standard accessories and some optional accessories, in an all-in-one
unit.
The BITE 2P receiver stores the readings in its internal memory. These
measurements, along with other maintenance data such as ambient and
pilot cell temperatures and ac ripple current, assist in determining the
overall condition of battery systems.
C-BITE
8
Battery Ground-Fault Locator
The Multi-Amp® Battery Ground-fault Locator (BGL) is useful for locating grounds on any type of battery system, including those in refineries,
mines, utilities, UPS, and continuous process systems. The BGL operates on battery systems up to 260 V dc that are either floating or grounded
through a resistor.
An additional feature of the BGL is its ability to measure battery system
total capacitance to ground or the capacitors of any branch of the system. This allows the operator to determine the maximum practical faultresistance range and provides the user with information on the battery
system.
Battery Ground Fault Tracer
The Biddle® Battery Ground Fault Tracer (BGFT) identifies, tracks, and
locates ground faults in ungrounded dc battery systems without the need
to go off-line. The BGFT speeds up fault location by eliminating trial-anderror procedures and is particularly useful in any industry where
uninterruptable power is critical. Those industries include:
• Power generation
• UPS systems
• Mines
• Continuous process systems
• Nuclear power plants
• Utility substations
• Refineries
• Naval and shipping operations
• Telecom backup systems
Digital Low Resistance Ohmmeters (DLROs)
There are two DLROs that are very appropriate for intercell connection
resistance — the Megger® Ducter® DLRO®10 and DLRO10X. The portability of the instruments allows effortless mobility around battery strings.
An automatic test mode reduces the amount of control manipulations.
DH4 handspikes available with the DLRO10 and DLRO10X include indicator lights in one of the probes that duplicate warning and signal lights
on the instrument. These lights indicate that P and C probe contact is
adequate, that there is voltage across the sample under test, discharge
current is still flowing, and when the test is complete.
INTERCELL CONNECTION
RESISTANCE
Many times batteries fail not because of weak cells but due to
weak intercell connections. Torquing is a mechanical method to ensure that the electrical path resistance is very low. But it does not
truly indicate the quality of the
electrical path resistance. The
only true method is to measure
each intercell connection resistance with a Digital Low Resistance Ohmmeter. Additionally, it
is recommended that this be done
before the battery is commissioned. This method will find if a
washer is stuck on the No-Ox® between the post and the intercell
connector whereas torquing will
not. IEEE Recommended Practices
specify that ten percent of the intercell connectors be measured
quarterly and all intercell connectors annually. They further specify
that the variation of intercell connection resistance be less than 10
percent. This translates into 7 µΩ
on a 70-µΩ intercell connection
resistance.
DLRO10
In addition to its features similar to the DLRO10, the DLRO10X has the
ability to download test results either in real time, or after storage in on
board memory. In real time, data is output to the RS-232 port as ASCII
text, suitable for printing on a serial printer or for capture by a suitable
PC program.
Battery Ground Fault Tracer
BITE 2P
Battery Ground-fault Locator
9