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 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 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 ( 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 seconds for Ah Uniform electrolyte distribution Rapid heat generation –Use only on small batteries (<50Ah) –Careful thermal management needed –Risk of hydration shorts –CO 2 may be liberated from plates

Vacuum vs. non-Vacuum fill

The Filling Process

Vacuum 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 Equipment Front View Kallstrom SF4-8D Vacuum filling equipment.

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 timecurrentvoltage –ampere-hourswatt-hourscycle –step no.scheduletemperature 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 PbSO 4 solubility increases significantly as H 2 SO 4 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 2.2m 2 /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 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 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.