© Philadelphia Scientific 2002 Philadelphia Scientific
Catalysts in Canada: The Science Behind 2 Years of
Canadian VRLA Cell Rehydration and Catalyst Addition
Harold A. Vanasse – Philadelphia Scientific
Robert Anderson – Anderson’s Electronics
Philadelphia Scientific
© Philadelphia Scientific 2002 Philadelphia Scientific
Presentation Outline
• An example of real batteries
• Polarization made simple
• How a cell works with a catalyst
• Two years of Canadian field data
• Fine tuning rehydration
© Philadelphia Scientific 2002 Philadelphia Scientific
Overview
© Philadelphia Scientific 2002 Philadelphia Scientific
A Tale of Two Batteries
• Test has been running for 6 years.
• All cells at 90ºF and 2.27 VPC.
• 2 Groups of 6 cells each.
• High Quality 2 Volt VRLA AGM Cells.
© Philadelphia Scientific 2002 Philadelphia Scientific
A Tale of Two Batteries
• Parameters Frequently Measured:• Gas evolved.• Conductance• Capacity• Polarization • Float Current
• Cells never boost charged --- just float charged!
© Philadelphia Scientific 2002 Philadelphia Scientific
What did we find? Cell Tear Down
• Tear down analysis of all but two catalyst cells conducted in years 4 & 5.
• Catalyst cells looked great.
• Non-Catalyst cells had:– Massive corrosion of negative grid. – Dry out.
© Philadelphia Scientific 2002 Philadelphia Scientific
What did we find? Capacity
Capacity Test Results
100% 101%
65%
12%
0%
20%
40%
60%
80%
100%
120%
Year 4 Year 5
Ca
pa
cit
y
Catalyst Cells Non-Catalyst Cells
© Philadelphia Scientific 2002 Philadelphia Scientific
What did we find? Gas Evolution
Gas Evolution vs. TimeVRLA Cells at 2.27 VPC and 90°F
0
20
40
60
80
100
120
0 52 104 156 208 260 312Elapsed Time (weeks)
Ga
ssin
g R
ate
(m
l/10
0A
h/d
ay) Non-Catalyst Cells
Catalyst Cells
© Philadelphia Scientific 2002 Philadelphia Scientific
What did we find? Conductance
Conductance vs. TimeVRLA Cells at 2.27 VPC and 90 °F
0.0
0.5
1.0
1.5
2.0
2.5
0 52 104 156 208 260 312
Elapsed Time (Weeks)
Co
nd
ucta
nce (
Km
ho
s)
Old Conductance MeterNew Conductance Meter
Non-Catalyst Cells
Catalyst Cells
© Philadelphia Scientific 2002 Philadelphia Scientific
What did we find? Float Current
Float Current vs. TimeVRLA Cells at 2.27 VPC and 90 °F
0
100
200
300
400
500
600
700
800
900
0 52 104 156 208 260 312
Elapsed Time (Weeks)
Flo
at
Cu
rren
t (m
A/1
00 A
h)
Non-Catalyst Cells
Catalyst Cells
© Philadelphia Scientific 2002 Philadelphia Scientific
Why is Float Current Important?
• A lowered float current is beneficial to the cell … but not just by adjusting the charger.
• Lowered float current reduces:– Positive plate corrosion.– Amount of gas generated by the cell
(Water loss).– Cell dryout (better conductance).– Risk of thermal runaway.
© Philadelphia Scientific 2002 Philadelphia Scientific
What did we find? Polarization of Plates
• Non-catalyst cell: All the overvoltage is on the positive.
• Catalyst cell has a better distribution.
Non-Catalyst Cell
Catalyst Cell
Neg. 0 mV -20 mV
Pos. 120 mV 100 mV
© Philadelphia Scientific 2002 Philadelphia Scientific
Polarization Made Simple
• A reference electrode measures the voltage on the positive plate and the voltage on the negative plate.
• Combining these two voltages gives you the total cell voltage.
• A depolarized plate is not good for the cell.
© Philadelphia Scientific 2002 Philadelphia Scientific
And Now For Some Math …
• An example:Float Voltage 2.27 Volts
Open Circuit Voltage 2.15 Volts
Overvoltage 0.12 V or 120 mV
• The overvoltage is what overcomes the cell’s self-discharge.
© Philadelphia Scientific 2002 Philadelphia Scientific
Polarization Fundamentals
• Using a reference electrode we can determine how the overvoltage is split among the plates.
• Optimally, the positive plate receives the majority.– Too little and it won’t charge properly– Too much and corrosion will be
accelerated.
© Philadelphia Scientific 2002 Philadelphia Scientific
Lander Curve
Optimum Positive Plate Polarization
© Philadelphia Scientific 2002 Philadelphia Scientific
Tafel Curve
• A diagram that relates polarization and current … among other things.
• The next slide shows how an increase in positive plate polarization leads to higher cell current.
Warning: Electrochemical diagram ahead!
© Philadelphia Scientific 2002 Philadelphia Scientific
Tafel Curve Series
© Philadelphia Scientific 2002 Philadelphia Scientific
Lessons from the Tafel
• Positive plate polarization and float current are directly related.
• Lower positive plate polarization leads to lower float current.
• If negative plate is polarized it will automatically reduce the polarization on the positive plate. This leads to lower float current.
© Philadelphia Scientific 2002 Philadelphia Scientific
Remember what we found?
• Non catalyst cells had higher float current because all of the overvoltage was on the positive.
• Catalyst cells had lower float current because overvoltage was distributed.
Non-Catalyst Cell
Catalyst Cell
Neg. 0 mV -20 mV
Pos. 120 mV 100 mV
© Philadelphia Scientific 2002 Philadelphia Scientific
Take Home Point
• Keeping the negative plate polarized is the key to lowering the float current.
© Philadelphia Scientific 2002 Philadelphia Scientific
How does a catalyst help do this?
• Must start with some VRLA basics:– H2 produced on the negative plate.
– O2 produced on the positive plate.
– O2, hydrogen ions and electrons recombine on the negative plate to form water.
© Philadelphia Scientific 2002 Philadelphia Scientific
How does a catalyst help do this?
– When O2 reaches negative plate it causes a reduction in the polarization.
– While the cell is on float, a portion of the charge current will try to polarize the negative.
• There is a balancing act between the last two points
© Philadelphia Scientific 2002 Philadelphia Scientific
Putting it all together
• 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 2002 Philadelphia Scientific
Science Summary
• The catalyst prevents some oxygen from reaching the negative plate.
• Too much oxygen reaching the negative plate causes depolarization.
• A depolarized negative plate causes a higher cell current due to excessive polarization of the positive plate.
• Test shows better performance in cells with catalysts.
© Philadelphia Scientific 2002 Philadelphia Scientific
Catalysts in Canada
• Anderson’s Electronics has been adding water and catalysts to VRLA cells in Canada for over two years.
• The following data was collected by Anderson’s from sites in Canada.
© Philadelphia Scientific 2002 Philadelphia Scientific
What is Rehydration?
• Rehydration is adding water to VRLA Cells that have dried out.
• Contact between AGM and plates is restored as AGM swells with water.
• Improved ohmic measurements and capacity are generally observed.
• Improvements are only temporary until cell dries out again.
© Philadelphia Scientific 2002 Philadelphia Scientific
Fine Tuning Rehydration
• A consistent amount of water is usually added to each cell.
• Two years of field experience has resulted in a more precise method.
• All cells don’t dry out at the same rate. • A unique amount of water is added to
each cell to optimize rehydration.
© Philadelphia Scientific 2002 Philadelphia Scientific
Conductance withoutTuning
0
500
1000
1500
2000
2500
3000
3500
4000
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 Number
Co
nd
uct
ance
© Philadelphia Scientific 2002 Philadelphia Scientific
Conductance with Tuning
0
500
1000
1500
2000
2500
3000
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 Number
Co
nd
uct
ance
© Philadelphia Scientific 2002 Philadelphia Scientific
Why Tune Rehydration?
• Cells in a string do not dryout at the same rate.
• A more consistent end result for all cells in the string.
• Too much or too little water is never added.
© Philadelphia Scientific 2002 Philadelphia Scientific
The Real World
• Two test sites in Canada each two years old. • All cells are VRLA from 1993 and same
manufacturer. • Data collected from these sites:
– Conductance– Capacity– Run Time
• Catalysts and water added to each cell.
© Philadelphia Scientific 2002 Philadelphia Scientific
P SiteConductance Change
3100
3200
3300
3400
3500
3600
3700
3800
3900
4000
4100
Before After 1 Year After 2 Years
Co
nd
uct
an
ce
© Philadelphia Scientific 2002 Philadelphia Scientific
P Site Capacity Change
0
10
20
30
40
50
60
70
80
90
100
Before After 1 Year After 2 Years
Ca
pa
city
(%
)
© Philadelphia Scientific 2002 Philadelphia Scientific
P Site Load Test Run Time Change(Minutes before 1.90 VPC at 3 Hour Rate)
0
20
40
60
80
100
120
140
Before After 1 Year After 2 Years
Ru
n T
ime
(M
inu
tes)
© Philadelphia Scientific 2002 Philadelphia Scientific
W Site Conductance Change
0
500
1000
1500
2000
2500
3000
3500
Before After 1 Year After 2 Years
Co
nd
uct
an
ce
© Philadelphia Scientific 2002 Philadelphia Scientific
W Site Capacity Change
0
10
20
30
40
50
60
70
80
90
100
Before After 1 Year After 2 Years
Ca
pa
city
(%
)
© Philadelphia Scientific 2002 Philadelphia Scientific
W Site Load Test Run Time Change(Minutes before 1.90 VPC at 3 Hour Rate)
0
20
40
60
80
100
120
Before After 1 Year After 2 Years
Ru
n T
ime
(M
inu
tes)
© Philadelphia Scientific 2002 Philadelphia Scientific
Technical Summary
• Conductance is improved and maintained for two years thus far.
• Capacity is greatly improved and maintained for two years thus far.
• Run time is restored and now meets customer requirements. – 8 hours of real run time.
© Philadelphia Scientific 2002 Philadelphia Scientific
Common Sense Summary
• This customer did not need to replace the batteries.
• They have lasted two years longer than expected and will most likely continue.
• Deferred capital expenditure.• The procedure was successful.
• The Customer Was Happy!
© Philadelphia Scientific 2002 Philadelphia Scientific
Conclusions
• Water added to cells gives immediate improvements.
• Catalysts maintain the improvement because the root cause is addressed.
• Catalysts prevent negative plate from depolarizing, which lowers float current.
• Batteries remain in service longer. • Water and Catalysts must be added together.
© Philadelphia Scientific 2002 Philadelphia Scientific
Thank You
Philadelphia Scientific
Top Related