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Philadelphia Scientific. Advances in the Design and Application of Catalysts for VRLA Batteries. Harold A. Vanasse – Philadelphia Scientific Robert Anderson – Anderson’s Electronics. Presentation Outline. A Review of Catalyst Basics Advances in the Catalyst Design
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Philadelphia Scientific Advances in the Design and Application of Catalysts for VRLA Batteries Harold A. Vanasse – Philadelphia Scientific Robert Anderson – Anderson’s Electronics
Presentation Outline • A Review of Catalyst Basics • Advances in the Catalyst Design • Hydrogen Sulfide in VRLA Cells • Catalyst Poisoning • A Design to Survive Poisons • Advances in the Field Application • Catalysts in Canada – Lessons Learned • Review of 3 Year Old Canadian TestSite
Catalyst Basics • By placing a catalyst into a VRLA cell: • A small amount of O2 is prevented from reaching the negative plate. • The negative stays polarized. • The positive polarization is reduced. • The float current of the cell is lowered.
Catalysts in the Field • 5 years of commercial VRLA Catalyst success. • A large number of cells returned to good health. • After 2-3 years, we found a small number of dead catalysts. • Original unprotected design. • Indicated by a rise in float current to pre-catalyst level.
Dead Catalysts • No physical signs of damage to explain death. • Unprotected catalysts have been killed in most manufacturers’ cells in our lab. • Catalyst deaths are not certain. • Length of life can be as short as 12 months. • Theoretically catalysts never stop working …. unless poisoned. • Investigation revealed hydrogen sulfide (H2S) poisoning.
H2S Produced on Negative Plate • Test rig collects gas produced over negative plate. • Very pure lead and 1.300 specific gravity acid used. • Test run at a variety of voltages. • Gas analyzed with GC.
Test Results • High concentration of H2S produced. • H2S concentration independent of voltage. • H2S produced at normal cell voltage!
Lead oxides make up positive plate active material. Lead oxides absorb H2S. Test Results
Test Results • H2S clearly being removed in the cell. • 10 ppm of H2S detected when gassing rate was 1,000 times normal rate of cell on float!
GC Analysis of VRLA Cells • Cells from multiple manufacturers sampled weekly for H2S since November 2000. • All cells on float service at 2.27 VPC at either 25°C or 32° C. • Results: • H2S routinely found in all cells. • H2S levels were inconsistent and varied from 0 ppm to 1 ppm, but were always much less than 1 ppm.
H2S in VRLA Cells • H2S can be produced on the negative plate in a reaction between the plate and the acid. • H2S is absorbed by the PbO2 of the positive plate in large quantities. • An equilibrium condition exists where H2S concentration does not exceed 1 ppm.
How do we protect the Catalyst? • Two possible methods: • Add a filter to remove poisons before they reach the catalyst material. • Slow down the gas flow reaching the catalyst to slow down the poisoning.
Basic Filter Science • Precious metal catalysts can be poisoned by two categories of poison: • Electron Donors: Hydrogen Sulfide (H2S) • Electron Receivers: Arsine & Stibine • A different filter is needed for each category.
Our Filter Selection • We chose a dual-acting filter to address both types of poison. • Proprietary material filters electron donor poisons such as H2S. • Activated Carbon filters electron receiver poisons.
Slowing Down the Reaction • There is a fixed amount of material inside the catalyst unit. • Catalyst and filter materials both absorb poisons until “used up”. • Limiting the gas access to the catalyst slows down the rate of poisoning and the rate of catalyst reaction.
Microcat® Catalyst Design • Chamber created by non-porous walls. • Gas enters through one opening. • Microporous disk further restricts flow. • Gas passes through filter before reaching catalyst. Gas / Vapor Path Porous Disk Filter Material Catalyst Material Housing
How long will it last? • Theoretical Life Estimate • Empirical Life Estimate
Theoretical Life Estimate • Microcat® catalyst theoretical life is 45 times longer than original design. • Filter improves life by factor of 9. • Rate reduction improves life by factor of 5.
Empirical Life Estimate: • Stubby Microcat® catalysts developed for accelerated testing. • 1/100th the H2S absorption capacity of normal. • All other materials the same. • Placed in VRLA cells on float at 2.25 VPC & 90ºF (32ºC). • Two tests running. • Float current and gas emitted are monitored for signs of death.
Stubby Microcat®Catalyst Test Results • Stubby Microcats lasted for: • Unit 1: 407 days. • Unit 2: 273 days. • Translation: • Unit 1: 407 x 100 = 40,700 days = 111 yrs • Unit 2: 273 x 100 = 27,300 days = 75 yrs.
Catalyst Life Estimate • Life estimates range from 75 years to 111 years. • We only need 20 years to match design life of VRLA battery. • A Catalyst is only one component in battery system and VRLA cells must be designed to minimize H2S production. • Fortunately this is part of good battery design.
Catalyst Design Summary • Catalysts reduce float current and maintain cell capacity. • VRLA Cells can produce small amounts of H2S, which poisons catalysts. • H2S can be successfully filtered. • A catalyst design has been developed to survive in batteries.
Catalysts in Canada – Lessons Learned • Anderson’s Electronics has been adding water and catalysts to VRLA cells in Canada for over 3 years. • Main focus with catalysts has been the recovery of lost capacity of installed VRLA cells. • Their technique has been refined and improved over time. • The following data was collected by Anderson’s from sites in Canada.
Steps to Reverse Capacity Loss • Assess the state of health of the cells. • Trended Ohmic Measurements & Capacity Testing • If necessary, rehydrate the affected cells to gain immediate improvement. • Install a Catalyst Vent Cap into each cell to address root cause of problem. • Inspect cells over time.
Factors to Consider when Qualifying a VRLA Cell • Age of cell: Cells from 1994 to 1998 were successfully rehydrated this year. • Cell Leaks: The cell must pass an inspection including a pressure test in order to qualify for rehydration. • Physical damage: Positive Plate growth should not be in an advanced stage – no severely bulging jars or covers.
Do Ohmic Readings Change After Catalyst Addition & Rehydration? • “Ohmic” refers to Conductance, Impedance or Internal Resistance. • Data must be collected over time and trended to get best results. • Rehydration significantly improves ohmic readings for cells that are experiencing the “dry-out” side effect of negative plate self discharge.
Ohmic Change after Catalyst/Rehydration Process(1995) 530 Ah Cells
A More Exact Way to Rehydrate VRLA Cells? • Anderson’s Electronics believes that VRLA cells dry out at different rates and should not be rehydrated using the same amount of water in each cell. • The rehydration tuning procedure has been further refined since last year to produce even more uniform readings.
Observations after Rehydrating 3,500 Canadian VRLA cells. • Age of cells worked on: 1994 to 1998. • All cells showed signs of improvement. • Newer cells (1997–1998) did not exhibit the same amount of ohmic improvement. • We believe that these cells were not as dried out as older cells. • Older cells (1994-1996) recovered with enough capacity to remain in service and provide adequate run times for the site loads.
Update on 3 Year Old Test Site • 2 year old data from this Canadian site presented at last year’s conference. • All cells are VRLA from 1993 and same manufacturer. • Cells were scheduled to be replaced but catalysts and water were added to each cell as a test.
W Site Load Test Run Time Change(Minutes before 1.90 VPC at 3 Hour Rate)
W Test Site Summary • The improvements are still being maintained after 3 years. • This string was about to be recycled, however 3 years later it remains in service. • Site load being protected for the required amount of time (8 hours). • During the recent blackout this site was without power for 5 hours and the load was successfully carried by this string.
Conclusions • The new generation of Microcat®catalyst product is engineered to survive real world conditions for the life of the cell. • Retrofitting your cells and rehydrating can: • Restore significant capacity for 3 years or more. • Save money on replacement batteries. • Help you get the capacity you need. • How did your non-Catalyst “protected” VRLA cells perform in the blackout?