techniques for the formation of vrla batteries l.
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Techniques for the Formation of VRLA Batteries - PowerPoint PPT Presentation

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Techniques for the Formation of VRLA Batteries. M.J.Weighall MJW Associates. Why is it more difficult to form VRLA Batteries?. VRLA Battery Formation. Filling is more difficult because: The separator completely fills the space between the plates The separator controls acid flow

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vrla battery formation
VRLA Battery Formation
  • Filling is more difficult because:
    • The separator completely fills the space between the plates
    • The separator controls acid flow
    • The separator controls distribution of acid between the positive 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 parameters
Battery 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 parameters
Separator Design Parameters
  • Volume porosity and pore structure
  • Caliper
  • Grammage
  • Surface area/ fibre diameter
  • Saturation
  • Compression
  • Fibre structure
    • ratio of coarse/ fine fibres
    • synthetic fibres
gravity top fill
Gravity Top Fill
  • Simple
  • Filling is slow (10 - 40 minutes)
  • Slow heat generation
    • may need to chill electrolyte for larger batteries
  • 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 rate
  • Non-uniform electrolyte distribution
    • push-pull (pressure-vacuum) finishing step to help diffusion
  • Thermal management needed
    • chilled electrolyte
    • chilled 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)
    • Careful thermal management needed
    • Risk of hydration shorts
    • CO2 may be liberated from plates
vacuum filling equipment
Vacuum Filling Equipment
  • 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

Back View

vacuum filling equipment12
Vacuum Filling Equipment
  • Kallstrom SF4-8D
  • Vacuum filling equipment.

Front View

initiation of formation charge
Initiation of Formation Charge
  • A. Low current
    • Minimises temperature rise at start of formation.
    • Compensates for high battery resistance
  • B. Ramp-current
    • Ramp up over an hour or so
  • C. High Current
    • Reduces total formation time
    • High initial voltage
    • Initial temperature rise may be excessive
formation profiles cv
Formation Profiles: CV
  • A. Single Step CV
    • Initial constant current until voltage limit is reached, then tapers
    • Need electronic integration of Ah input
    • Long charge “tail”
  • B. Stepped CV/CC
    • Current stepped down in stages as voltage limits are reached, then tapers at final CV limit
    • More control over total formation time
    • Still need electronic integration of Ah input
cc algorithms and ideal formation curve
CC 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 discharges
Rests and Discharges
  • Allows time for water and acid to diffuse into the plate interior
    • acid can react with any PbO left in the plates
    • use 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 and will lengthen formation time
constant current algorithm
Constant Current Algorithm
  • Algorithm A:
    • High temperature towards end of formation
    • high overcharge and gassing levels
  • Algorithm B:
    • Higher initial current, slightly lower current for bulk charge
    • May improve pore structure
cv taper charge algorithm
CV/ Taper Charge Algorithm
  • A. One-step CV
    • Requires more time or a higher inrush current than CC or stepped CC formation
  • B. One-step taper current
    • High inrush current but only tapers to about 30% of initial value
    • Results in higher Ah input and shorter formation time
    • at expense of higher temperature and more gassing
algorithm with rests or discharge
Algorithm with Rests or Discharge
  • A. CC/rest
    • rest period provides time for electrolyte penetration
    • also keeps temperature down
  • B. CC/ discharge
    • Will require higher charge current or longer formation time
    • discharge data can be used to match battery modules
programmed formation
Programmed Formation
  • Up to 50 steps per formation schedule
  • Precise control of:
    • current
    • voltage
    • temperature
  • Display:
    • step time current voltage
    • ampere-hours watt-hours cycle
    • step no. schedule temperature
  • Temperature probe
    • allows charge current adjustment up or down depending on battery temperature
temperature limits for vrla jar formation
Temperature limits for VRLA Jar Formation
  • 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 shorts
    • may have adverse effect on negative plates (decrease in surface area)
  • Keep maximum temperature below 40°C if possible
    • will require external cooling e.g water or forced air.
electrolyte additives
Electrolyte Additives
  • 1% sodium sulphate is normally added to the electrolyte
    • “common ion” effect prevents the harmful depletion of sulphate ions
    • the graph shows that PbSO4 solubility increases significantly as H2SO4 density decreases
separator surface area
Separator Surface Area
  • There is a relationship between mean pore size and surface area
    • related to ratio of coarse/fine fibres
  • Smaller pore structure results in a lower wicking rate but a higher ultimate wicking height
separator wicking height
Separator 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 speed
Vertical Wicking Speed
  • The influence of fibre mix and segregation on the vertical wicking speed is shown
    • slowest wicking is with 100% fine fibres
oriented vs non oriented fibres
Oriented 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” formation

The “oriented” separator has

separate layers of coarse and

fine fibres

separator compression
Separator Compression
  • High compression designs are more difficult to fill
    • reduction in pore size and electrolyte availability results in slower wicking and lower fill rates
  • Plate group pressure may change during formation
    • reduction in plate group pressure may adversely affect battery life
plate group pressure
Plate Group Pressure
  • To minimise the risk of loss of plate group pressure during jar formation:
    • Assemble cells with the maximum practicable plate group pressure (> 40 kPa)
    • maximise available acid volume and increase separator grammage to >= 2g/Ah
    • Increase the fine fibre content of the separator
    • Use a formation algorithm that minimises gassing at the end of charge
  • 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
  • Bob Nelson, Recombination Technologies, provided most of the figures and a lot of the detailed information.
  • This paper is based on a project initiated by Firing Circuits Inc.