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Transcript of © Philadelphia Scientific 2003 Philadelphia Scientific Advances in the Design and Application of...
© Philadelphia Scientific 2003 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 Philadelphia Scientific
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 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
Catalyst Basics
© Philadelphia Scientific 2003 Philadelphia Scientific
Advances in the Catalyst Design
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
Test Results
• High concentration of H2S produced.
• H2S concentration independent of voltage.
• H2S produced at normal cell voltage!
0
100
200
300
400
500
600
2.25 2.35 2.45 2.55 2.65 2.75
Cell voltage (V)
H2S
con
cent
ratio
n (p
pm)
© Philadelphia Scientific 2003 Philadelphia Scientific
H2S Absorbed by Positive Plate
Material to be tested
Reactor
H2 +
H2S
10
0 p
pm
GC
H2 Gas with 100 ppm of
H2S
© Philadelphia Scientific 2003 Philadelphia Scientific
Test Results
• Lead oxides make up positive plate active material.
• Lead oxides absorb H2S.
Test Material
Amount (grams)
Breakthrough Time
(minutes)
Empty 0.0 0.01
PbO 2.2 120
PbO2 2.0 360
© Philadelphia Scientific 2003 Philadelphia Scientific
H2S Absorbed in a VRLA Cell
H
2 +
H2S
10
0 p
pm
H2 Gas with 100 ppm of
H2S
VRLA cell
GC
2.27V
© Philadelphia Scientific 2003 Philadelphia Scientific
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!
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30
Time (hours)
H2S
co
nce
ntr
ati
on
in
th
e o
utl
et
ga
s (p
pm
)
0
20
40
60
80
100
120
140
160
Inle
t ga
s flow
rate
(ml/m
in)
H2S Concentration (ppm)
Gas Flowrate (ml/min)
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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
FilterMaterial
Catalyst Material
Housing
© Philadelphia Scientific 2003 Philadelphia Scientific
How long will it last?
• Theoretical Life Estimate
• Empirical Life Estimate
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
Advances in the Field Application of Catalysts
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
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 2003 Philadelphia Scientific
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 2003 Philadelphia Scientific
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 2003 Philadelphia Scientific
Ohmic Change after Catalyst/Rehydration Process
(1995) 530 Ah Cells
0%
10%
20%
30%
40%
50%
60%
70%
80%
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 #
% O
hm
ic C
ha
ng
e
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
Example of Uniform Rehydration
(1994) 615 Ah Cells
0
100
200
300
400
500
600
700
800
900
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 #
Inte
rnal
Res
ista
nce
Before After
© Philadelphia Scientific 2003 Philadelphia Scientific
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 2003 Philadelphia Scientific
Average Ohmic Improvement after Catalyst/Water Addition
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
1994 1995 1996 1997 1998
Year of Manufacture
% O
hm
ic I
mp
rove
men
t
© Philadelphia Scientific 2003 Philadelphia Scientific
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.
© Philadelphia Scientific 2003 Philadelphia Scientific
W Site Conductance Change
© Philadelphia Scientific 2003 Philadelphia Scientific
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 2003 Philadelphia Scientific
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?