© Philadelphia Scientific 2002 Philadelphia Scientific Catalysts in Canada: The Science Behind 2 Years of Canadian VRLA Cell Rehydration and Catalyst Addition

© 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


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


Overview


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.


A Tale of Two Batteries

• Parameters Frequently Measured:• Gas evolved.• Conductance• Capacity• Polarization • Float Current

• Cells never boost charged --- just float charged!


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.


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


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


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


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


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.


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


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.


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.


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.


Lander Curve

Optimum Positive Plate Polarization


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!


Tafel Curve Series


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.


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


Take Home Point

• Keeping the negative plate polarized is the key to lowering the float current.


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.


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


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.


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.


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.


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.


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.


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


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


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.


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.


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


P Site Capacity Change

0

10

20

30

40

50

60

70

80

90

100


Ca

pa

city

(%

)


P Site Load Test Run Time Change(Minutes before 1.90 VPC at 3 Hour Rate)

0

20

40

60

80

100

120

140


Ru

n T

ime

(M

inu

tes)


W Site Conductance Change

0

500

1000

1500

2000

2500

3000

3500


Co

nd

uct

an

ce


W Site Capacity Change

0

10

20

30

40

50

60

70

80

90

100


Ca

pa

city

(%

)


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

0

20

40

60

80

100

120


Ru

n T

ime

(M

inu

tes)


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.


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!


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.


Thank You

Philadelphia Scientific

Documents

© Philadelphia Scientific 2002 Philadelphia Scientific Catalysts in Canada: The Science Behind 2 Years of Canadian VRLA Cell Rehydration and Catalyst Addition