Transcript Techniques for the Formation of VRLA Batteries

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
– 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
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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
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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
• 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)
• 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)
• Very fast e.g. 1-10 seconds for 1.2-25Ah
• Uniform electrolyte distribution
• Rapid heat generation
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Use only on small batteries (<50Ah)
Careful thermal management needed
Risk of hydration shorts
CO2 may be liberated from plates
Vacuum vs. non-Vacuum fill
The Filling Process
Vacuum Filling Equipment
• Kallstrom SF4-8D
• Vacuum filling
equipment.
• Volume measured by
mass flow density
transmitter, enables preselected volume of acid
to be metered into each
cell.
• Pulse filling: alternating
between vacuum and
atmospheric pressure
Back View
Vacuum Filling Equipment
• Kallstrom SF4-8D
• Vacuum filling
equipment.
Front View
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
• 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
• 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
• 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
• 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
• 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
• 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
• Up to 50 steps per formation schedule
• Precise control of:
– current
– voltage
– temperature
• Display:
– step time
– ampere-hours
– step no.
current
watt-hours
schedule
voltage
cycle
temperature
• Temperature probe
– allows charge current adjustment up or down depending
on battery temperature
Programmed 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
• 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
• 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
• 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
• 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
The “oriented” separator has
separate layers of coarse and
fine 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
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
• 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
Comments
• 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
Acknowledgements
• Bob Nelson,
Recombination
Technologies,
provided most of the
figures and a lot of
the detailed
information.
Acknowledgements
• This paper is based
on a project initiated
by Firing Circuits
Inc.