Keeping up the PressureKeeping up the Pressure

Strategies to Maintain Plate Group Pressure and Extend the Cycle Life

of VRLA Batteries

M.J.Weighall

MJW Associates

ALABC ResearchALABC Research

• Importance of high plate group pressure– eliminate premature capacity loss (PCL)– Maintain +ve plate integrity– oppose expansion of +ve active material

• Must set plate group pressure early in life and maintain throughout life.

Plate Group PressurePlate Group Pressure• Influenced by many factors:

– Control of plate and separator thickness– Assembly process– Container design and materials– Separator properties

• All glass

• “Hybrid” - glass + other material

• Alternative material

– Compression devices• Internal compression plate

• external clamping

The ideal AGM separatorThe ideal AGM separator

• Compressive Properties reproducible when dry, and the separator should recover to its original thickness when the compressive force is released.

• Separator should not contract when wetted with acid and should show the same compressive properties as the dry separator.

• The slope of the compression curve should accommodate small changes in plate thickness without an appreciable change in the compressive force

Typical Compression Curve for Typical Compression Curve for AGM SeparatorAGM Separator

Compression Curve - 200g/m² - 1.8m²/g

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 10 20 30 40 50 60 70 80 90 100

Compression - kPa

Thic

kn

ess

- m

m

Dry Down Dry Up Wet Down Wet Up

Acknowledgement: Hollingsworth & Vose Company

AGM Compression CurvesAGM Compression Curves• Separator thickness reduces with increasing applied

force– up to and possibly beyond 50 kPa

• Separator recovery is incomplete when applied force is released– does not return to original thickness

• Wet separator tends to contract.– Lower thickness than dry separator for given applied force

• Applied force in excess of 50 kPa may be needed to achieve a compression of 30%

Alternative SeparatorsAlternative Separators

• Less compressible separators may maintain higher plate group pressure

• Less tolerant of plate thickness variations

• May need to be used in conjunction with compression plate.

• May require other battery design or assembly changes.

• Volume porosity may be lower than AGM

Amer-Sil Proposal AAmer-Sil Proposal A

Acknowledgement: BE97-4085 Task 1(a)

Compression curves for some Compression curves for some alternative separatorsalternative separators

Acknowledgement: CSIRO Project B-001.2

–

Alternative SeparatorsAlternative Separators

• Ceramic separator– Totally incompressible– Currently available material unsuitable for

use as a battery separator

• Daramic AJS– Good results in EALABC programme

• Microporous separator sandwiched between AGM separator (Amer-Sil)– Encouraging results in EALABC programme

Alternative SeparatorsAlternative Separators• SLGM separator (H & V)

– Encouraging results in CSIRO ALABC Project - merits further study

• Staflex (Entek)– Interesting thickness/ pressure behaviour, let

down by poor oxygen recombination.

Other Separator Options?Other Separator Options?

• Owens Corning have developed a bi-component glass fibre, trade marked “Miraflex™”– two different formulations of glass fibre have been

engineered into a single filament

– the resultant fibre is extremely compactable and able to recover its original form after being compressed.

• Could the coarse and fine glass fibres used in the AGM separator similarly be engineered into a single filament?

• Other innovative fibre manufacturing/ processing techniques?

Cell VariabilityCell Variability

“For battery packs with 156 cells in series, a cell reliability of 99.5% means that 50% of the battery packs will fail”

J.Wallace, Ford Motor Company, ALABC Members & Contractors Conference, March 2000

Plate TolerancesPlate Tolerances

• Plate thickness tolerances are quite wide compared with tolerances on engineered components in many other industries

• This will have a significant influence on the actual plate group pressure within the cell.

• Variable plate group pressure may have an adverse impact on the cycle life of the battery.

Impact of Plate Thickness Tolerance on Impact of Plate Thickness Tolerance on plate group pressureplate group pressure

• Assume ± 0.05mm tolerance on plate thickness.• Assume 13 plate battery with 2.1mm +ve plates,

1.4mm -ve plates, 1.1mm interplate spacing.• H & V separator 200g/m2, 1.8m2/g.• Plate group pressure may vary from 85 kPa to 60

kPa (dry separator)• Plate group pressure may vary from 65 kPa to 40

kPa (wet separator)

Battery ReliabilityBattery Reliability

• Tighter tolerances on plate weight and thickness, and on separator thickness, are needed.

• This will help the battery manufacturer to control separator compression and plate group pressure.

• This may require changes to plate making techniques

Container designContainer design

• Container “draft” will result in variable plate group pressure within the cell.– It may be possible to compensate for this by

introducing a “wedge” into the battery container.

• May be possible to overcome this problem by assembling single cells rather than monoblocs– e.g. Accuma have a Motive Power battery cell

with 3mm thick walls which is ejected from the mould utilising a 0°draft angle inside the cell

Container Design (2)Container Design (2)• A container design is available from Accuma in

which there are internal vertical, flexible ribs.• These ribs engage the element stack at an oblique

angle.• The ribs can fold out of the way as they are not

connected to the case at the bottom of the cell.• The ribs provide a constant force to the element

group to hold it in place.

Plastic moulding materialsPlastic moulding materials

• Thin walled polypropylene containers are unsuitable for advanced VRLA batteries

• Container materials for VRLA batteries require a high flexural modulus to achieve stiff end walls and withstand high internal battery pressures.

• A typical FR-ABS material has double the flexural modulus of a typical polypropylene material.

WP Compression Plate

Compression Devices (CSIRO)

–

LS Compression Plate

Compression Devices (CSIRO)–

Other compression DevicesOther compression Devices

• Flexible hydrostatic pressure-bag– the pressure inside the bag can be adjusted

by introducing or removing compressed air

• Elastic compression device– this envelopes the whole cell group and is

like a thick, porous stocking.

–

(Ref: CSIRO ALABC Project AMC-007)

Cell AssemblyCell Assembly

• Need to set, control and monitor plate group pressure prior to insertion in cell container. This may be higher than in the actual container

• Implications for damage or irreversible crushing of the separator material.

Cell Assembly OptionsCell Assembly Options• Slide the compressed cell into the container while

maintaining it under pressure• Force the uncompressed cell through a tapered

“compression shoe” into the container• Force an uncompressed cell into a tapered container

(which would yield uneven pressure vertically in the cell).

• "Overcompress" the cell in a clamping device to make it sufficiently smaller than the container opening in order to be able to extract the clamping means from the cell space after insertion.

Automated Automated Battery AssemblyBattery Assembly

• Example:– Sealed Energy Systems (SES) assembly

method and apparatus.– US Patent 5,344,466 & 5,407,450– Enables application of controlled

compression to the cell group and automatic insertion of the compressed cell into the case

– Enables automated assembly, high production efficiency and product consistency

Cell/ Battery FormationCell/ Battery Formation• Advanced VRLA batteries require great care

if “jar formation is used:– thin plate designs with close plate spacing– high plate group pressures

• high compression reduces porosity./ pore size

• wicking properties of the electrolyte adversely affected

– May be residual dry areas after filling• risk of massive grid corrosion

– Changes in plate group pressure caused by changes in plate volume during formation

Formation IssuesFormation Issues

• Consider dry charge instead of “jar” formation.

• Look at possibility of pre-compressing the separator

• Look at possibility of pre-wetting the separator– may create additional assembly problems

Cylindrical Cell DesignCylindrical Cell Design

• Favoured for high rate or automotive applications

• Assists in maintaining high, uniform plate group pressure

• Internal spring at centre of cell may assist in maintaining controlled plate group pressure– this concept is used in some lithium cell designs

ConclusionsConclusions

• Advanced VRLA batteries need changes to battery design and assembly techniques, to enable high plate group pressures to be set and maintained.

• Advanced separator designs may enable higher plate group pressures to be achieved. This may also require an internal compression device.

Conclusions (2)Conclusions (2)

• The battery design and assembly processes may need to be tailored to the separator being used, based on the characteristics of the separator.

• The plate group pressure needs to be set at the optimum value to match the properties of the separator being used

Conclusions (3)Conclusions (3)

• Consider novel assembly and formation techniques– pre-compressed and pre-wetted separators– dry charged plates vs. jar formation– new battery case designs

• Modify negative grid design and negative expander formulation to avoid adverse impact on negative plates of high plate group pressures

Conclusions (4)Conclusions (4)

• Raw materials suppliers, component suppliers and equipment suppliers need to work closely with battery manufacturers to develop advanced VRLA batteries in which high plate group pressures are set and maintained.

Keeping up the PressureKeeping up the Pressure

Strategies to Maintain Plate Group Pressure and Extend the Cycle Life

of VRLA Batteries

M.J.Weighall

MJW Associates