© philadelphia scientific 2004 a case study: four years of performance data at a canadian...

© Philadelphia Scientific 2004

A Case Study: Four Years of Performance Data at a Canadian

Rehydration and Catalyst Addition Site

Harold A. Vanasse – Philadelphia Scientific

Robert Anderson – Anderson’s Electronics

Philadelphia Scientific


Presentation Outline

• Site Description.

• Process Description.

• The Science Behind the Process.

• Site Results.

• Financial Impact.

• Trends from 10,000 cells.


Site Description

• Central Office site owned by a major telecom.

• One string of 24 cells. – “20-Year Design” VRLA Product.– 900 Ah cells.– Major US Manufacturer.

• Cells were installed in 1993.• Temperature controlled at 21 – 27 ºC.


Site Description

• Good maintenance practices followed.– Dedicated power technicians.– Annual re-torqing of intercell connectors.– Semi-annual conductance testing, voltage

measurement and visual inspection.

• About as good as it gets for batteries!


Test Site as found in 2000

• Slight bulging of jar cover indicating positive plate growth – not severe at all.

• Capacities near 5%!

• Customer not happy.


Decision Process

Scrap cells and replace with new cells.

OR

Try adding water & catalysts and save the cells.


Decision Process

• Decision: Water and catalysts were added to each cell as a test of the process.

• Telecom felt they had nothing to lose.


Process Steps

Philadelphia Scientific

Step 1: Cell Inspection


Cell Inspection

• Cell Leaks: The cell must pass a pressure test in order to qualify.

• Physical damage: Positive Plate growth should not be in an advanced stage – no severely bulging jars or covers.

• Cell voltage measurement.

• Cell temperature measurement.


Process Steps Continued

2. State of health determined. – Ohmic measurements.– Capacity Test.

3. Cells Rehydrated -- Water added to each cell.

4. Catalyst Vent Cap installed into each cell.

5. Annual follow-up inspections.


What Happens Inside the Cell at Each Step?

(This is the technical section!)


What Water Addition Does – Part 1

• Dry out occurs because oxygen and hydrogen gas vent from the cell over time.

• As float current rises dry out process accelerates.

• The water that was added replaced the water that was lost.– Maintains proper electrolyte.


What Water Addition Does – Part 2

• When glass mat separator dries out it shrinks.– Electrical contact between plates is

disrupted (conductance lower).

• Water added is absorbed by glass mat.• Glass mat swells like a sponge and

restores electrical contact between plates.– Conductance higher/better.


What Catalyst Addition Does

• Negative plate self discharge is a fundamental problem with VRLA cells.

• Too much oxygen reaches negative plate and causes it to discharge.

• This occurs while battery is on float charge!

• Polarization of individual plates tells the story.


What is Polarization?

• A measure of the voltage on the positive plate and the voltage on the negative plate.

• Cell over-voltage is divided between positive and negative plates.

• We want to know how the voltage is distributed among the plates.


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 of Plates

• Results of a long term lab test serve as example.

• 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


The Positive Plate and the Lander Curve

Optimum Positive Plate Polarization


Tafel Curve

• A diagram that relates polarization and current … among other things.

• The next slide shows:– The difference between a healthy and non-

healthy distribution of voltage.– How a decrease in positive plate

polarization leads to lower cell current.


Tafel Curve Series


Catalyst Addition

• 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.


Putting It All Together


Ongoing Inspections

• Site inspected each September from 2001 to 2004.

• Parameters:– Visual inspection.– Conductance. – Capacity Test. – Temperature.– Float Voltage.


September 2004: Positive Plate Growth Not Progressing


Site Conductance Change

2306

31923257

3153 31913070

0

500

1000

1500

2000

2500

3000

3500

Before 4 Days 1 Year 2 Years 3 Years 4 Years

Co

nd

uct

ance


Site Load-Test Run Time Change

(Minutes before 1.90 VPC at 3 Hour Rate)

9

67

108 108

120116

0

20

40

60

80

100

120

140


Min

ute

s


Site Run Time Change(Minutes at Actual 62 Amp Load -- Calculated)

0.8

6.0

9.6 9.6

10.710.3

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0


Ho

urs


Anecdotal Evidence

• During the August 2003 blackout the battery string at this site did not drop the load.

• Site was powered by the battery for 5+ hours until generator arrived and was on-line.


Test Site Data Interpretation

• Immediate improvements (within 6 months) result of water addition.

• Long term improvements (6 months to 4 years) result of catalyst addition.

• The improvements are still being maintained after 4 years.

• Site load being protected for the required amount of time (8 hours).


Financial Impact

• This string was about to be recycled, however 4 years later it remains in service.

• The end user did not need to buy new cells for this site – this purchase has now been deferred for 4 years.


Financial Impact

• Based on a financial analysis of actual work at over 375 sites across multiple customers in North America:– For every $1,000 spent on this process

$13,000 has been deferred in battery replacement costs.

– Typical payback in 4 to 8 months.


Another Way to Look at the Financials

• Assume:– Site equipment has 20 year life. – Batteries have 7 year life.

• 3 strings of batteries will be purchased throughout the life of the site.

• If batteries can last 10 years only 2 strings would be purchased.

• Test site is now at 11 years of life.• The requirement to purchase 1 string of

batteries has been eliminated.


Trends from 10,000 Cells

• Rehydration and catalyst addition process completed on 10,000 cells so far.

• Ages range from 1993 to 2001.

• Four trends identified:1. Cell dry out (or loss of compression) starts

earlier then most people believe.


Trends from 10,000 Cells

• Trends continued:2. New cells are not immune to the problems

presented. Negative Plate Self Discharge begins within the first few years.

3. Ohmic measurements (conductance, resistance, impedance) are good tools to identify problems if data is trended.

4. By customizing the amount of water added to each cell uniform recovery can be obtained across an entire string.


One Last Bit of Data

• 180 cell UPS Site

• Internal Resistance data trended over the last 7 years.

• Water and Catalysts added in 2002.

• Improvements seen in resistance measurements.


180 Cell UPS SiteAverage Internal Resistance

(mOhms)

653 681 708744

856

578 602

0100200300400500600700800900

1000

1997 1998 1999 2000 2001 2002 2003


Conclusions

• VRLA Cells can be recovered from Negative Plate Self Discharge.

• Water and Catalyst Addition process can defer replacement of cells that are “failing”.

• Our test site is still looking good after 4 years.

• Ohmic measurements can provide early warning if data is trended.

© philadelphia scientific 2004 a case study: four years of performance data at a canadian...

Documents