Philadelphia Scientific Advances in the Design and Application of Catalysts for VRLA Batteries

Harold A. Vanasse – Philadelphia Scientific Robert Anderson – Anderson’s Electronics Philadelphia Scientific © Philadelphia Scientific 2003

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 Test Site Philadelphia Scientific © Philadelphia Scientific 2003

Catalyst Basics

• By placing a catalyst into a VRLA cell: – A small amount of O 2 is prevented from reaching the negative plate. – The negative stays polarized.

– The positive polarization is reduced. – The float current of the cell is lowered. Philadelphia Scientific © Philadelphia Scientific 2003

Catalyst Basics

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Advances in the Catalyst Design

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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.

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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 (H 2 S) poisoning. Philadelphia Scientific © Philadelphia Scientific 2003

H

2

S 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.

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Test Results

600 500 400 300 200 100 0 2.25

2.35

2.45

2.55

Cell voltage (V) 2.65

2.75

• High concentration of H 2 S produced.

• H 2 S concentration independent of voltage.

• H 2 S produced at normal cell voltage!

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H

2

S Absorbed by Positive Plate

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Test Results

• Lead oxides make up positive plate active material. • Lead oxides absorb H 2 S.

Test Material

Empty PbO PbO 2

Amount (grams)

0.0

2.2

2.0

Breakthrough Time (minutes)

0.01

120 360 Philadelphia Scientific © Philadelphia Scientific 2003

H

2

S

Absorbed in a VRLA Cell Philadelphia Scientific © Philadelphia Scientific 2003

Test Results

100 90 80 70 60 50 40 30 20 10 0 0 5 H2S Concentration (ppm) Gas Flowrate (ml/min) 10 15 Time (hours) 20 25 160 140 40 20 30 0 120 100 80 60

• H 2 S clearly being removed in the cell.

• 10 ppm of H 2 S detected when gassing rate was 1,000 times normal rate of cell on float! Philadelphia Scientific © Philadelphia Scientific 2003

GC Analysis of VRLA Cells

• Cells from multiple manufacturers sampled weekly for H 2 S since November 2000. • All cells on float service at 2.27 VPC at either 25 °C or 32° C.

• Results: – H 2 S routinely found in all cells.

– H 2 S levels were inconsistent and varied from 0 ppm to 1 ppm, but were always much less than 1 ppm.

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H

2

S in VRLA Cells

• H 2 S can be produced on the negative plate in a reaction between the plate and the acid. • H 2 S is absorbed by the PbO 2 of the positive plate in large quantities.

• An equilibrium condition exists where H 2 S concentration does not exceed 1 ppm. Philadelphia Scientific © Philadelphia Scientific 2003

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.

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Basic Filter Science

• Precious metal catalysts can be poisoned by two categories of poison: – Electron Donors: Hydrogen Sulfide (H 2 S) – Electron Receivers: Arsine & Stibine • A different filter is needed for each category.

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Our Filter Selection

• We chose a dual-acting filter to address both types of poison.

– Proprietary material filters electron donor poisons such as H 2 S.

– Activated Carbon filters electron receiver poisons. Philadelphia Scientific © Philadelphia Scientific 2003

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.

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Microcat

®

Catalyst Design

Gas / Vapor Path

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Porous Disk Filter Material Catalyst Material

• Chamber created by non-porous walls.

• Gas enters through one opening.

• Microporous disk further restricts flow.

• Gas passes through filter before reaching catalyst.

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How long will it last?

• Theoretical Life Estimate • Empirical Life Estimate

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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.

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Empirical Life Estimate:

• Stubby Microcat ® catalysts developed for accelerated testing. – 1/100 th normal.

the H 2 S absorption capacity of – All other materials the same. – Placed in VRLA cells on float at 2.25 VPC & 90 ºF (32ºC).

– Two tests running.

• Float current and gas emitted are monitored for signs of death. Philadelphia Scientific © Philadelphia Scientific 2003

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.

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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 H 2 S production. – Fortunately this is part of good battery design. Philadelphia Scientific © Philadelphia Scientific 2003

Catalyst Design Summary

• Catalysts reduce float current and maintain cell capacity.

• VRLA Cells can produce small amounts of H 2 S, which poisons catalysts. • H 2 S can be successfully filtered.

• A catalyst design has been developed to survive in batteries. Philadelphia Scientific © Philadelphia Scientific 2003

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Advances in the Field Application of Catalysts

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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.

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Steps to Reverse Capacity Loss

1. Assess the state of health of the cells.

• Trended Ohmic Measurements & Capacity Testing 2. If necessary, rehydrate the affected cells to gain immediate improvement. 3. Install a Catalyst Vent Cap into each cell to address root cause of problem. 4. Inspect cells over time. Philadelphia Scientific © Philadelphia Scientific 2003

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. Philadelphia Scientific © Philadelphia Scientific 2003

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. Philadelphia Scientific © Philadelphia Scientific 2003

Ohmic Change after Catalyst/Rehydration Process (1995) 530 Ah Cells 80% 70% 60% 50% 40% 30% 20% 10% 0%

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Cell #

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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.

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Example of Uniform Rehydration (1994) 615 Ah Cells

300 200 100 0 900 800 700 600 500 400

Before After

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Cell #

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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. Philadelphia Scientific © Philadelphia Scientific 2003

Average Ohmic Improvement after Catalyst/Water Addition

30.0% 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% 1994 1995 1996 Year of Manufacture 1997 1998

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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.

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W Site Conductance Change

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W Site Load Test Run Time Change (Minutes before 1.90 VPC at 3 Hour Rate) © Philadelphia Scientific 2003 Philadelphia Scientific

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. Philadelphia Scientific © Philadelphia Scientific 2003

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?

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