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The Lead-Acid Battery - Chemical Reaction
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Lead Acid Battery Basics
Lead Acid Batteries are Electro-chemical devices
As such, they are designed to fail over time Life expectancy is primarily a function of the
thickness of the positive plate and electrolyteconfiguration
Traditional flooded lead acid batteries are designedto last 20 years in normal float conditions at 77degrees F.
Valve Regulated Lead Acid batteries (VRLA) typically
experience 5-12 years of life, with 6&12 voltmonoblocs failing at the lower end of this lifeexpectancy range and 2 volt cells being a bit morerobust
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Lead Acid Battery Basics
Stationary Lead Acid Batteries come in a variety of designs& Chemistries: Flooded
Plate Design: Flat plate, tubular plate and Plante
Positive Plate Thickness: Long Duration, General Purpose, High Rate
Positive Plate Alloy: Lead Calcium, High or Low percentage LeadAntimony, Lead Selenium, Pure lead (Plante)
Electrolyte: Aqueous H2SO4Specific Gravity varies
VRLA Plate Design: Flat plate, tubular plate
Positive Plate Thickness: Long Duration, High rate
Positive plate alloy: Lead/tin, Lead/Calcium, pure lead
Electrolyte: Immobilized H2SO4 Gelled or Absorbed Glass Mat(starved electrolyte) AGM, Specific Gravity varies
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Lead Acid battery Basics
Battery design parameters will dictate the
charge or float voltage of the battery e.g., LeadCalcium vs. Lead Antimony
Temperature of the installation will dictate thecharge or float voltage of the battery
Specific Gravity of the electrolyte will dictate thecharge or float voltage of the battery
Installation configuration e.g., distance frombattery to charge source will dictate voltagesetting
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System Analysis
Measure actual plant load ( in DC amps)
Document installed batterys rated capacity
Identify required battery run time (in hours)
Multiply the DC load by run time to determine site amp
hours required, to proper end voltage Example:
28 amps x 8 hrs. = 224 site amp hours
Then add 25 % for end of life consideration!!
Over sizing by 25% will insure 100% coverage of the load at
the IEEE specified 80% end of life condition
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Condition of Power Plant
Things to look for:
Any Power Plant warning lights/alarms Proper charging voltage for batteries
Normal DC charge current
Physical damage
Battery physical condition, leaks or bulges
Loose or broken hardware
Review of site records, are they easy to access?
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Relevant IEEE Maintenance
Standards
IEEE450 Recommended Practice for Maintenance, Testing,
and Replacement of Vented Lead-Acid Batteries for StationaryApplications
The purpose of this recommended practice is to provide the user with
information and recommendations concerning the maintenance, testing, and
replacement of vented lead-acid batteries used in stationary applications.
IEEE-1188 Recommended Practice for Maintenance, Testing,
and Replacement of Vented Lead-Acid Batteries for Stationary
Applications
This recommended practice is limited to maintenance, test schedules, andtesting procedures that can be used to optimize the life and performance of
valve-regulated lead-acid (VRLA) batteries for stationary applications. It also
provides guidance to determine when batteries should be replaced.
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Reference IEEE Standards
IEEE Std 485 IEEE Recommended Practice for Sizing Lead-AcidBatteries for Stationary Applications.1, 2
IEEE Std 484-1996, IEEE Recommended Practice for InstallationDesign and Installation of Vented Lead- Acid Batteries for StationaryApplications (ANSI/BCI). 1,2
IEEE Std. 1187 IEEE Recommended Practice for Installation Designand Installation of Valve-Regulated Lead-Acid Storage Batteries forStationary Applications.
IEEE Std 1189 IEEE Guide for Selection of Valve-Regulated Lead-Acid
(VRLA) Batteries for Stationary Applications
IEEE publications are available from the Institute of Electrical andElectronics Engineers, Inc., 445 Hoes Lane,Piscataway, NJ 08854, USA(http://standards.ieee.org/)
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Maintenance of VRLA Batteries
Monthly- Overall float voltage measured at battery
terminals, Charger output and voltage, ambienttemperature, visual inspection, DC float current
Quarterlyohmic value, temperature of batteries
at negative terminal, voltage of individual batteries YearlyIn addition to above items, intercell
resistance values, AC ripple/current on batteries,
typically around 50 mA/ 100Ah of capacity is
normal, values 3X this range would be a concern,
check manufacturers guideline for this
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Maintenance of Flooded Lead Acid batteries
MonthlyString float voltage measured at the battery terminals, general
appearance: cleanliness, water levels, appearance of battery plates, signs of
post corrosion or leaking. Charger output, ambient temperature, voltage and
temperature of Pilot cell if used, battery float charging current or pilot cell
specific gravity (temperature corrected), for antimony cells SG preferred.
Grounding, any monitoring system if installed operational
Quarterly - Individual battery voltages (cell), lead antimony check specific
gravity of 10% of the cells and float charge current, other floodedtechnologies check 10% of SG if float current is not used for state of charge
indication, check temperature on 10% of string. Refer to manufacturers
literature and/or IEEE 450 for temperature correction factors for voltage and
specific gravity.
YearlyAdd to quarterly routine: SG on all antimony cells, if not using floatcurrent for other types of cells check all SG, detailed visual inspection of all
cells, Cell to cell and terminal connection resistance values, structural integrity
of racks.
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Ohmic Testing Methods
Conductance:
A low AC voltage signal is impressed across the batteryterminals and the AC current response is measured. Theconductance is the ratio of the AC test current response tothe impressed AC voltage
DC Resistance:
Short duration DC load on the cell/unit to measure stepchange in current and voltage. By dividing the change involtage by the change in current, a DC resistance iscalculated using Ohms Law
Impedance:
Performed by sending an AC current of a known frequencyand amplitude, into the cell/unit and measuring the ACvoltage drop. Compute the resulting impedance usingOhms Law
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Capacity correlation performed by HBL Battery, India
480 VRLA batteries in 200 to 300 Ah range. Correlation approximately 90%
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%R
atedCapacity
%O
riginalValue
% Rated Capacity
100
95
90
85
80
120
110
100
90
80
% Life
10 20 30 40 50 60 70 80 90 100
Conductance
Source: Johnson Controls Form 41-7271 Rev.8/94
CONDUCTANCE CORRELATION
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Ohmic Testing
IEEE 1188 addresses ohmic testing of VRLA batteries
No one method is specifically endorsed Goal is to provide a consistent method of quantifying these ohmic
values
When taken, the values obtained, equipment used and location test
points should be recorded for consistent procedures
Trending of data is key, establish a baseline value & trend against this
value going forward
Substantial changes (typically 30% or more +/-) generally indicate it is
time to change the batteries
Installation variations will effect ohmic valuesparallel strings canproduce an ohmic signature substantially different from series
connected cells
Understand that all Ohmic testers may cause some Voltage Creep
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Probe Placement
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Consistent Probe Placement
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Ohmic Testing & Reference
Values
Baseline or benchmark value. Measurements of
known good batteries are taken to create this value. They come from battery manufacturers, Midtronicslab, customer testing, discharge results.
Important to note that a reference value is an
estimate where the batteries should be, not anexact value.
Trending new batteries is the best method.
To trend or establish a reference value, you can take
measurements within the first 1 year, preferablywithin the first 90 days for VRLA batteries. For wetcell or lead acid batteries you can establishreadings within 3 years.
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