Maintenance & Safety of Stationary Lead Acid Batteries UTC October 2012

The Lead-Acid Battery - Chemical Reaction

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 electrolyte configuration Traditional flooded lead acid batteries are designed to last 20 years in normal “float” conditions at 77 degrees F. Valve Regulated Lead Acid batteries (VRLA) typically experience 5-12 years of life, with 6&12 volt monoblocs failing at the lower end of this life expectancy range and 2 volt cells being a bit more robust

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 Lead Antimony, Lead Selenium, Pure lead (Plante’) Electrolyte: Aqueous H2SO4 – Specific 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

Lead Acid battery Basics Battery design parameters will dictate the charge or float voltage of the battery e.g., Lead Calcium vs. Lead Antimony Temperature of the installation will dictate the charge or float voltage of the battery Specific Gravity of the electrolyte will dictate the charge or float voltage of the battery Installation configuration e.g., distance from battery to charge source will dictate voltage setting

Measure actual plant load ( in DC amps) System Analysis Measure actual plant load ( in DC amps) Document installed battery’s 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

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?

Relevant IEEE Maintenance Standards IEEE – 450™ Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications “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, and testing 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.”

Reference IEEE Standards IEEE Std 485™ IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications.1, 2 IEEE Std 484™-1996, IEEE Recommended Practice for Installation Design and Installation of Vented Lead- Acid Batteries for Stationary Applications (ANSI/BCI). 1,2 IEEE Std. 1187™ IEEE Recommended Practice for Installation Design and Installation of Valve-Regulated Lead-Acid Storage Batteries for Stationary 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 and Electronics Engineers, Inc., 445 Hoes Lane,Piscataway, NJ 08854, USA (http://standards.ieee.org/)

Maintenance of VRLA Batteries Monthly- Overall float voltage measured at battery terminals, Charger output and voltage, ambient temperature, visual inspection, DC float current Quarterly – ohmic value, temperature of batteries at negative terminal, voltage of individual batteries Yearly – In 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 manufacturer’s guideline for this

Maintenance of Flooded Lead Acid batteries Monthly – String 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 flooded technologies check 10% of SG if float current is not used for state of charge indication, check temperature on 10% of string. Refer to manufacturer’s literature and/or IEEE 450™ for temperature correction factors for voltage and specific gravity. Yearly – Add to quarterly routine: SG on all antimony cells, if not using float current 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.

Ohmic Testing Methods Conductance: A low AC voltage signal is impressed across the battery terminals and the AC current response is measured. The conductance is the ratio of the AC test current response to the impressed AC voltage “DC” Resistance: Short duration DC load on the cell/unit to measure step change in current and voltage. By dividing the change in voltage by the change in current, a DC resistance is calculated using Ohm’s Law Impedance: Performed by sending an AC current of a known frequency and amplitude, into the cell/unit and measuring the AC voltage drop. Compute the resulting impedance using Ohm’s Law

Capacity correlation performed by HBL Battery, India 480 VRLA batteries in 200 to 300 Ah range. Correlation approximately 90%

CONDUCTANCE CORRELATION 100 95 90 85 80 % Rated Capacity % Rated Capacity 120 110 100 90 80 % Original Value Conductance 10 20 30 40 50 60 70 80 90 100 % Life Source: Johnson Controls Form 41-7271 Rev.8/94

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 values – parallel strings can produce an ohmic signature substantially different from series connected cells Understand that all Ohmic testers may cause some “Voltage Creep”

Probe Placement

Consistent Probe Placement

Ohmic Testing & Reference Values Baseline or benchmark value. Measurements of known good batteries are taken to create this value. They come from battery manufacturers, Midtronics lab, customer testing, discharge results. Important to note that a reference value is an estimate where the batteries should be, not an exact value. Trending new batteries is the best method. To trend or establish a reference value, you can take measurements within the first 1 year, preferably within the first 90 days for VRLA batteries. For “wet cell” or lead acid batteries you can establish readings within 3 years.

Ohmic Testing Alternatives Traditional Ohmic testing uses hand held devices to capture conductance, resistance or impedance information. Variations in testing procedures and failure to properly record and trend data can introduce error. Batteries can fail in between testing routines Installed 24/7 monitoring reduces human error and gaps in routines Monitoring systems should allow for verification of cell health, intercell connection integrity, temperature on each cell, ambient temperature, cell and bus voltage, charge current, and capture discharge event data. Monitoring systems should allow for alarming to all personnel remotely, and allow for ability to “drill down” to cell level Options for communication protocol, ease of installation, and data capture for regular reporting and filing need to be considered