Network
Connectivity
Care and Feeding of
DC Power Plant and
UPS Batteries
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MasterAgent
Site
Controller
BatteryAgent
Sensor
Units
Up To 6
Battery
Strings
RJ-11
“Daisy Chain”
Battery String #1
Battery String #2
Overview
• Present and future of battery
backup
• Battery theory
• Battery failure causes
• Load vs AC characteristics
testing
• Current preventative
maintenance practices
• Remote monitoring
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The Need for Batteries…
Ancient technology, bright future
• The market for batteries is
growing, not shrinking:
– Stationary batteries are >$3
billion/yr business; expected to be
>$7 billion by 2010 (BCC Research Group)
•
•
•
•
•
Telecom, cell sites, cable headends
Industrial, IT infrastructure
Automotive & Hybrid vehicles,
Alternative energy systems:
Wind, solar, fuel-cell, etc, all need
batteries for storage
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Batteries in Broadband
•
Legacy headend UPS power
plants:
– Develops backup source of
single-phase or 3-phase AC
– Typically up to 40 12V batteries
in series (~500VDC)
•
Modern 48VDC headend power
plants:
– Newer equipment designed to
operate from 48VDC
– Typically 24 2-volt batteries in
series
•
Outside plant standby power
– Another story for another day
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Anatomy of a UPS
Standby
Mode
Normal Mode
AC IN
Rectifier
Rectifier
Rectified AC input
powers inverter and
charges batteries
DC
Inverter
Inverter
AC OUT
AC OUT
Charge
Current
Batteries power inverter
Batteries
Batteries
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Some Battery Terminology
•
•
•
“Cell”
– Simple form of energy storage device typically
comprised of positive and negative plates,
separators, electrolyte and a container.
– This device can be placed in series with other
cells to form a monobloc or battery
– Lead-acid cells are typically about 2.1 vdc
“Monobloc” (sometimes called a module)
– A number of cells connected (typically in series)
and packaged together a single container
– What is commonly thought of as a 12vdc battery
can also be thought of as a 6-cell monobloc
“Battery”
– Combination of “monobloc” modules placed in
series or parallel, the total of which forms a
battery
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The Lead-Acid Battery
What’s in the box?
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Lead-Acid Battery Types
Many sizes & shapes…
•
“Flooded” or “Wet” Cells
– The cell plates (commonly a lead alloy)are
suspended in a bath of liquid electrolyte
(typically sulphuric acid)
•
“Gel” Cells
– The liquid electrolyte is replaced with a
thick gel electrolyte
•
“AGM” (Absorbed Glass Mat) Cells
– The space between plates is filled with a
mat-like material that holds liquid electrolyte
•
Gel and AGM are sealed-cell
technologies
– Maintenance free
– Sometimes called VRLA (Valve Regulated
Lead-Acid)
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Capacity Metrics
• “Amp-Hours” (AH)
– A constant that describes how long a cell can supply a
specified amount of current before reaching its “end
voltage”. This is the most common capacity metric.
• “Cold Cranking Amps” (CCA)
– The number of amps a new, fully charged battery can
deliver at 0°F for 30 seconds, while maintaining a
voltage of at least 7.2 volts, for a 12 volt battery. Used
by the automotive industry,
• “MCA & “CA” (Marine Cranking
Amps/Cranking Amps)
– The load in amperes which a battery at 32°F , can
continuously deliver for 30 seconds and maintain
a terminal voltage equal or greater than 1.2 volts
per cell. Used by the marine industry
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Capacity Limitations
Why it’s not a perfect voltage source
• An ideal cell would have unlimited
capacity.
• Capacity is limited by non-ideal
internal elements
– Rmetal is a very low resistance
comprised of strap, post, plate &
electrolyte resistances
– Relectrolyte is known as charge transfer
resistance or contact resistance
between plate and electrolyte
– Rleakage is a very high resistance that
causes self-discharge
– C is the battery’s inherent capacitance
which is about 1.5 farads per 100 AH
capacity
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• As batteries age they lose
some ability to deliver power
• According to IEEE 450
“2002” when a battery has
lost 20% of its capacity it is
no longer viable
Discharge Behavior
• Initial “Coup de Fouet”
– Sudden deep drop, then some
recovery over several seconds
• Linear voltage decay until “cutoff
voltage” is reached.
• Fast voltage drop after cutoff time
• Deep discharge is bad
• Excessive discharge rate is bad
• The discharge rate must be kept
within manufacturer’s ratings
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Charging Considerations
• Ideal charger has 3 states:
– Bulk: Constant current quick charge
‘till voltage rises
– Absorption: Constant voltage ‘till
current drops
– Float: Low-current maintenance
charge
• Excessive charge current
causes heat and “gassing”
• Overcharging causes dry-out
• Undercharging leads to
sulphation
• Charge rate and voltage are
temperature dependent
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Why Batteries Fail
“Treat them kindly”
•
Heat:
– For every additional 15 degrees of heat
over 77 deg F, lead acid battery life
(regardless of type) is cut in half.
•
Overcharging:
– Overcharging causes heat and ‘gassing’ –
not good.
•
Undercharging:
– Leads to sulphation of plates
•
Deep-discharging:
– The first time a lead-acid cell is
discharged by 80%, its life expectancy is
halved
•
Mechanical Deterioration
– Corrosion of straps & posts, sulphation of
grids
Field studies have shown VRLA
batteries last approximately 3-8 years if
treated properly
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The Battery Management Conundrum
• Stationary batteries are expensive
• Batteries need regular checking and
maintenance to achieve their rated
life
• Operators are being driven to
increase system availability while
reducing maintenance costs
• When budgets are cut, maintenance
is the first to go
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Costs
Availability
Preventative Maintenance Practices
“No Maintenance”
• The most common practice
• Batteries are replaced when they fail
• The most costly practice
• Power failures result in downtime & loss of
revenue
• The least cost-effective practice
• Lack of vigilance can result in undetected
deterioration
• Lack of maintenance can result in catastrophic
failure
• A genuine job-security threat
• A headend outage can be a career-ender
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Preventative Maintenance Practices
“Rip n’ Tear”
• Time based replacement
– Based on projected 3-8 year life
expectancy
– Commonly called the “rip n’ tear” approach
• The problem with rip n’ tear:
– Replacement too early is costly and
inefficient
– Waiting too long to replace will
cause loss of services & revenue
• TB replacement is gambling!!
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Preventative Maintenance Practices
“Periodic Maintenance”
• Requires regular site visits
– Quarterly
•
•
•
•
•
Visual inspection
AC Characteristic measurement
Voltages/Current
Corrective action
Manual data logging
– Annual/ Semi/Tri annual (2-3 yrs)
• Capacity testing (load)
• Other similar to quarterly
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Battery Test Methods
DC Load Testing
• Designed to test battery capacity
in amp-hours
– A heavy load is placed on the
battery and the time to discharge
to the end-voltage is measured
• Manual and intrusive testing
– Any discharge event is a potential
outage-causing event.
• Expensive and time consuming
– Requires special equipment and
personnel
• Should not be performed within
72 hours of a discharge event
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More on DC Load Testing …
• IEEE recommends Time adjusted
or Rate adjusted testing for
capacity testing
– Accomplished by taking batteries
off line and testing with a constant
load to specified terminal voltage
– Time adjusted: Greater than one
hour; no correction, except
temperature
– Rate adjusted: Less than one hour
and requires the battery’s
published specs and corrections
for time and temperature
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Manual Test Methods
”AC Characteristics” Testing
C1
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C1
R1
R2
Remote vs Manual
3200
3000
Conductance
• Commonly known as “impedance” or
“conductance” testing
• Characterizes the cell’s internal
resistances
• Non intrusive measurement (manual or
remote)
• Designed to provide battery “State of
Health” (SOH) information
• Can be performed with handheld
instruments or via a remote monitoring
system
• Generally accepted as the best SOH test
method
2800
Remote
Manual
2600
2400
2200
2000
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65
two day interval
AC Characteristics Testing
How it works…
• Forces a known AC current
through the battery terminals
AC Current
Source
(Iac)
AC Voltage
Amplifier
Gain = A
– Causing a small AC voltage to be
developed
• AC signal can easily be separated
from large DC component
• AC voltage is amplified and
measured
Vac
AC Voltage
Meter
(Vac)
1.0 amp
– Rb = Vac/Iac
– Example: If Iac = 1amp and Vac =
0.001volt, then Rb = 0.001 ohm
Vcell
Rb = R1 + R2
C1
C1
R1
R2
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Current Preventative Maintenance Practices
Manual Methods Summary
• Manual testing is expensive & some tests are intrusive
• Data logged manually and transferred to software
program (MS excel) manually.
• Quarterly tests do not provide enough data for
meaningful trending analysis (and so will miss
impending failures)
• Provides a good opportunity for visual inspection
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The Need for Monitoring…
• Mission-critical
infrastructure elements
must be monitored,
maintained, and
managed.
• Major outages can be
apocalyptic.
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Remote Battery Monitoring
• Much more comprehensive means of
looking at battery state of health
• Provides operators with instant status
update on entire enterprise
• Reduces/eliminates unnecessary PM
site visits
• Provides asset management &
inventory control
• A more intelligent and cost effective
means of determining battery
replacement
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Remote vs Manual
“The devil is in the details”
Remote vs Manual
3200
Conductance
3000
2800
Remote
Manual
2600
2400
2200
2000
1 5
9 13 17 21 25 29 33 37 41 45 49 53 57 61 65
two day interval
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Remote Monitoring Legacy
“Too much data – not enough information”
• Cumbersome
– Slow serial based communications
– Alarm storms
– Poor correlation and analysis capability
• Expensive
• Proprietary in nature
• Complex installation and maintenance
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Remote Monitoring Today
“User Friendly – Standards-based”
• New technology has dramatically
reduced cost & complexity
• Standards based systems (HTML,
TCP/IP, SNMP)
• Intelligent reporting
• Provides real time status of hundreds or
thousands of battery plants
instantaneously
• Trend analysis and correlation
• Information available to many vs few
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Web based Clients
Battery Monitoring System
Architecture
Network
Connectivity
•
•
•
•
Monitoring systems are
typically comprised of
sensors, site controllers,
and software
Sensors make
impedance
measurements
Communications
between controller and
NOC software uses
SNMP
Clients can use a
browser to access the
NOC software, or
directly access the site
controller.
MasterAgent
Site
Controller
BatteryAgent
Sensor
Units
Up To 6
Battery
Strings
RJ-11
“Daisy Chain”
Battery String #1
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Battery String #2
The Battery Sensor
• Connects to battery post
• Measures battery temperature,
voltage, & impedance
• Low current: <10ma idling; typ 1
amp during test
• Each sensor is addressed by the
site controller
• Site controller determines when
tests are made and collects data
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The Site Controller
• Manages multiple strings of
sensors
• Can be powered from battery
string or from wall-transformer
• Communicates with NOC via
Ethernet
• Sends alarm traps if any
measurement is abnormal
• Built-in web page
• Built-in email client
• Fully SNMP compliant
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Built-in Web Server
• Site summary page with
alarm color coding
• String summary page with
alarm color coding
• Battery details page with
individual battery real-time
measurements
• Complete provisioning of
text labels and alarm
thresholds via web –
password protected
• Provisioning can also be
done via SNMP from NOC
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Benefits of remote monitoring
“Continuous testing; It just makes sense”
• Remote monitoring is the best way to determine
comprehensive state of battery health
• Real time visibility of enterprise DC power plants
• Reduced maintenance costs (fewer site visits)
• Fewer outages
• More efficient use of resources during crises
• Proactive vs reactive maintenance
• Asset management/inventory control
• Enterprise wide accessibility
Costs
Availability
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Benefits of remote monitoring
“A more rational approach”
• Eliminates the need for manual data
logging and analysis
• Eliminates data overload – provides
useful information
• Provides historical data for warranty
claims
• Consistent measurements (eliminates
human errors)
• Alarm notification and routing
• Eliminates site access problems
(manpower/security)
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Summary
•
•
•
•
•
Batteries are a growth industry, not a dying
technology
As batteries age they will fail to deliver
expected run time
Manual testing has proven to be at best
only partially effective
Remote monitoring combined with yearly
inspection offers the most comprehensive
and effective method for assessing battery
replacement
Remote monitoring will allow operators to
be proactive thereby reducing the number
of system outages and realizing significant
savings in battery replacement
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