Techniques for the Formation Techniques for the Formation of VRLA Batteriesof VRLA Batteries

M.J.Weighall

MJW Associates

Why is it more difficult to form Why is it more difficult to form VRLA Batteries?VRLA Batteries?

VRLA Battery FormationVRLA Battery Formation• Filling is more difficult because:

– The separator completely fills the space between the platesThe separator completely fills the space between the plates

– The separator controls acid flowThe separator controls acid flow

– The separator controls distribution of acid between the The separator controls distribution of acid between the positive plate, negative plate and separatorpositive plate, negative plate and separator

• There is a lower limit on the maximum formation temperature

• There is a greater risk of localised low acid density and hydration shorts/ dendrite formation

• Accurate control of the final acid content is required (~ 95% saturation)

Battery Design ParametersBattery Design Parameters

• Cylindrical or prismatic

• Plate thickness and interplate spacing

• Plate height/ plate spacing ratio

• Battery case draft

• Filling port position

• Active material additives

Separator Design ParametersSeparator Design Parameters• Volume porosity and pore structure

• Caliper

• Grammage

• Surface area/ fibre diameter

• Saturation

• Compression

• Fibre structure– ratio of coarse/ fine fibresratio of coarse/ fine fibres– synthetic fibressynthetic fibres

Gravity Top FillGravity Top Fill

• Simple

• Filling is slow (10 - 40 minutes)

• Slow heat generation– may need to chill electrolyte for larger may need to chill electrolyte for larger

batteriesbatteries

• Trapped gas pockets may result in incomplete wetting

Soft-vacuum fill (>~20mm Hg)Soft-vacuum fill (>~20mm Hg)

• Moderate filling rate (30-60 seconds)

• Moderate vacuum level– Element “sucks up” electrolyte at its own rateElement “sucks up” electrolyte at its own rate

• Non-uniform electrolyte distribution– push-pull (pressure-vacuum) finishing step to push-pull (pressure-vacuum) finishing step to

help diffusionhelp diffusion

• Thermal management needed– chilled electrolytechilled electrolyte– chilled water bathchilled water bath

Hard-vacuum fill (<~10mm Hg)Hard-vacuum fill (<~10mm Hg)

• Very fast e.g. 1-10 seconds for 1.2-25Ah

• Uniform electrolyte distribution

• Rapid heat generation– Use only on small batteries (<50Ah)Use only on small batteries (<50Ah)– Careful thermal management neededCareful thermal management needed– Risk of hydration shortsRisk of hydration shorts

– COCO22 may be liberated from plates may be liberated from plates

Vacuum vs. non-Vacuum fillVacuum vs. non-Vacuum fill

The Filling ProcessThe Filling Process

Vacuum Filling EquipmentVacuum Filling Equipment

Back View

• Kallstrom SF4-8D

• Vacuum filling equipment.

• Volume measured by mass flow density transmitter, enables pre-selected volume of acid to be metered into each cell.

• Pulse filling: alternating between vacuum and atmospheric pressure

Vacuum Filling EquipmentVacuum Filling Equipment

Front View

• Kallstrom SF4-8D

• Vacuum filling equipment.

Initiation of Formation ChargeInitiation of Formation Charge• A. Low current

– Minimises temperature rise at Minimises temperature rise at start of formation.start of formation.

– Compensates for high battery Compensates for high battery resistanceresistance

• B. Ramp-current– Ramp up over an hour or soRamp up over an hour or so

• C. High Current– Reduces total formation timeReduces total formation time

– High initial voltageHigh initial voltage

– Initial temperature rise may be Initial temperature rise may be excessiveexcessive

Formation Profiles: CVFormation Profiles: CV

• A. Single Step CV– Initial constant current until voltage Initial constant current until voltage

limit is reached, then taperslimit is reached, then tapers

– Need electronic integration of Ah Need electronic integration of Ah inputinput

– Long charge “tail”Long charge “tail”

• B. Stepped CV/CC– Current stepped down in stages as Current stepped down in stages as

voltage limits are reached, then voltage limits are reached, then tapers at final CV limittapers at final CV limit

– More control over total formation More control over total formation timetime

– Still need electronic integration of Ah Still need electronic integration of Ah inputinput

CC Algorithms and Ideal Formation CurveCC Algorithms and Ideal Formation Curve

• Multi-step constant current algorithm is much closer to the ideal formation curve than conventional CC formation

• Multi-step algorithm is very practical with modern computer controlled formation equipment

Rests and DischargesRests and Discharges• Allows time for water and acid to diffuse into the

plate interior– acid can react with any PbO left in the platesacid can react with any PbO left in the plates– use at fixed point in formation or initiated by “trigger” voltageuse at fixed point in formation or initiated by “trigger” voltage

• Use of significant “off” time can actually result in faster, more complete formation process.

• Rest period simpler than discharge– discharge more complex in capital equipment requirements discharge more complex in capital equipment requirements

and will lengthen formation timeand will lengthen formation time

Constant Current AlgorithmConstant Current Algorithm• Algorithm A:

– High temperature towards High temperature towards end of formationend of formation

– high overcharge and gassing high overcharge and gassing levelslevels

• Algorithm B:– Higher initial current, Higher initial current,

slightly lower current for slightly lower current for bulk chargebulk charge

– May improve pore structureMay improve pore structure

CV/ Taper Charge AlgorithmCV/ Taper Charge Algorithm

• A. One-step CV– Requires more time or a higher Requires more time or a higher

inrush current than CC or inrush current than CC or stepped CC formationstepped CC formation

• B. One-step taper current– High inrush current but only High inrush current but only

tapers to about 30% of initial tapers to about 30% of initial valuevalue

– Results in higher Ah input and Results in higher Ah input and shorter formation timeshorter formation time

– at expense of higher at expense of higher temperature and more gassingtemperature and more gassing

Algorithm with Rests or DischargeAlgorithm with Rests or Discharge

• A. CC/rest – rest period provides time rest period provides time

for electrolyte penetrationfor electrolyte penetration

– also keeps temperature also keeps temperature downdown

• B. CC/ discharge– Will require higher charge Will require higher charge

current or longer current or longer formation timeformation time

– discharge data can be used discharge data can be used to match battery modulesto match battery modules

Programmed FormationProgrammed Formation• Up to 50 steps per formation schedule• Precise control of:

– currentcurrent– voltagevoltage– temperaturetemperature

• Display:– step timestep time currentcurrent voltagevoltage– ampere-hoursampere-hours watt-hourswatt-hours cyclecycle– step no.step no. scheduleschedule temperaturetemperature

• Temperature probe– allows charge current adjustment up or down depending on allows charge current adjustment up or down depending on

battery temperaturebattery temperature

Programmed FormationProgrammed Formation

Temperature limits for VRLA Jar Temperature limits for VRLA Jar FormationFormation

• Conventional flooded batteries can tolerate maximum formation temperatures up to 65°C

• For VRLA batteries high formation temp:– may result in formation of lead dendrites/ hydration may result in formation of lead dendrites/ hydration

shortsshorts

– may have adverse effect on negative plates (decrease in may have adverse effect on negative plates (decrease in surface area)surface area)

• Keep maximum temperature below 40°C if possible– will require external cooling e.g water or forced air.will require external cooling e.g water or forced air.

Electrolyte AdditivesElectrolyte Additives• 1% sodium sulphate is

normally added to the electrolyte– ““common ion” effect common ion” effect

prevents the harmful prevents the harmful depletion of sulphate depletion of sulphate ionsions

– the graph shows that the graph shows that PbSOPbSO44 solubility solubility

increases significantly as increases significantly as HH22SOSO44 density decreases density decreases

Separator Surface AreaSeparator Surface Area

• There is a relationship between mean pore size and surface area– related to ratio of related to ratio of

coarse/fine fibrescoarse/fine fibres

• Smaller pore structure results in a lower wicking rate but a higher ultimate wicking height

Separator Wicking HeightSeparator Wicking Height

• A higher surface area correlates to a smaller pore structure and results in a lower wicking rate, but a greater ultimate wicking height

• Taller batteries may require higher surface area separator, but filling time will be longer

Separator with 2.2m2/g SA wicks to greatest height

Vertical Wicking SpeedVertical Wicking Speed

• The influence of fibre mix and segregation on the vertical wicking speed is shown– slowest wicking is slowest wicking is

with 100% fine fibreswith 100% fine fibres

Oriented vs. Non-Oriented FibresOriented vs. Non-Oriented Fibres

• Multi-layer AGM with oriented fibres wicks to a greater height in a given time.

• AGM with oriented fibres also has advantages in “fill and spill” formationThe “oriented” separator has

separate layers of coarse andfine fibres

Separator CompressionSeparator Compression

• High compression designs are more difficult to fill– reduction in pore size and electrolyte reduction in pore size and electrolyte

availability results in slower wicking and availability results in slower wicking and lower fill rateslower fill rates

• Plate group pressure may change during formation– reduction in plate group pressure may reduction in plate group pressure may

adversely affect battery lifeadversely affect battery life

Plate Group PressurePlate Group Pressure

• To minimise the risk of loss of plate group pressure during jar formation:– Assemble cells with the maximum practicable Assemble cells with the maximum practicable

plate group pressure (> 40 kPa)plate group pressure (> 40 kPa)– maximise available acid volume and increase maximise available acid volume and increase

separator grammage to >= 2g/Ahseparator grammage to >= 2g/Ah– Increase the fine fibre content of the separatorIncrease the fine fibre content of the separator– Use a formation algorithm that minimises Use a formation algorithm that minimises

gassing at the end of chargegassing at the end of charge

CommentsComments• The VRLA battery design needs to take into

account the requirements of VRLA jar formation

• The separator properties are critical

• This presentation has given suggestions for filling techniques and formation algorithms

• The battery manufacturer can use these suggestions as a basis but needs to experiment to find the optimum formation algorithm for his specific battery design and application

AcknowledgementsAcknowledgements

• Bob Nelson, Recombination Technologies, provided most of the figures and a lot of the detailed information.

AcknowledgementsAcknowledgements

• This paper is based on a project initiated by Firing Circuits Inc.