Download Charging VRLA Batteries

Charging VRLA Batteries
Jan 10, 2016
ensitepower.com
CONFIDENTIAL
©2015 EnSite Power, Inc.
What’s So Special About VRLA?
• It’s not just “A battery is a battery is a
battery!”
• VRLA design is “different” than VLA design.
• VRLA is considered as “Starved Electrolyte”
design.
• VRLA, by design, is a recombinant system.
• VRLA is more sensitive to temperature.
• In some cases, very minor design changes can
affect life.
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CONFIDENTIAL
Difference between Flooded and AGM’s
• Liquid electrolyte
• Evolved gases escape to
atmosphere
• Can be topped up with water
• Cannot be inverted
• Electrolyte absorbed in glass
mat
• Evolved gases recombine
• Cannot be topped up with
water
• Can be inverted
Definitions
• Nominal Voltage – 2V for a lead-acid cell
• Open Circuit Voltage OCV – Voltage of the
battery while at rest, i.e. no current flow
• Float Voltage – Voltage applied to the
battery by the rectifier or charger
• Polarization (η) – Difference between the
applied voltage and the OCV, this causes
the current to flow – Relationship known as
the Tafel Plot
η = a ± b log(I)
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Polarization vs. Current
Charge
Polarization
OCV


Battery on-float
Low-rate discharge

Higher-rate discharge
Discharge
Current
Difference between Flooded and AGM’s
Flooded
• Liquid electrolyte
• Evolved gases escape to
atmosphere
• Separate battery room required with
ventillation system for dispersal of
hydrogen
• Can be topped up with water
• Can only be operated upright
• Spill containment
system is required
Difference between Flooded and AGM’s
AGM’s
• Electrolyte absorbed in
glass mat
• No spill containment
required
• Evolved gases recombine
and stay within the battery
• No need to top up with
water
• Can be operated in any
orientation
Difference between Flooded and AGM’s
Mass Balance - Flooded
PbO2 + Pb + 2H2SO4  2 PbSO4 + 2H2O
Excess
• Flooded LA’s have an excess of electrolyte
• Easier to over discharge the positive electrode
• Higher heat capacity, 30-40% since there is relatively more
liquid…
Difference between Flooded and AGM’s
Mass Balance - AGM
PbO2 + Pb + 2H2SO4  2 PbSO4 + 2H2O
Excess
Limited
• VRLA’s are acid starved
• Positive is protected by lack of acid
• Can see alkaline conditions following a deep discharge
which can result in dendritic growth upon recharge
• Can see higher temperatures due to lower heat capacity
Difference between Flooded and AGM’s
Overcharge - Flooded
Positive
Negative
Total
2H2O  O2 + 4H+ + 4e4H+ + 4e-  2H2
2H2O  O2+ 2H2
Vented batteries lose water through
electrolysis when overcharged
Difference between Flooded and AGM’s
Overcharge - AGM
Recombination
2H2O  O2 + 4H+ + 4ePositive
Negative 2Pb + O2 + 2H2SO4  2PbSO4 + 2H2O + heat
Pb + H2SO4  PbSO4 + 2H+ + 2e- + heat
Total
Thanks to the recombination process AGM batteries do not
electrolyze water, however recombination;
• Discharges the negative electrode
• Generates heat
Difference between Flooded and AGM’s
What does all this mean to a standby application?
• The negative electrode is depolarized due to the recombination
process – it is in danger of being discharged, hence purity is far
more critical for AGM’s
• The positive electrode is more anodically polarized in an AGM,
hence more prone to grid corrosion
• Overcharge is converted to heat in an AGM due to the
recombination process
• AGM’s have less electrolyte therefore a lower heat capacity, thus
they require less heat to raise their temperature
• Thermal management is far more critical for AGM’s…
Why Are VRLAs So Finicky?
• VRLAs are actually several distinct designs.
• Gel Cells
• Absorbed Glass Mat (AGM)
• Lead-Calcium
• Pure Lead
• “Fire Fly” (A separate Subject!)
• Each design has it’s own specific charging
regime.
• Not only Float voltage, but recharge voltage, as
well.
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CONFIDENTIAL
Why Are VRLAs So Finicky? (cont’d)
• Accuracy of the charge voltage is more critical
in VRLA designs.
• Recombination efficiency is directly affected by
charge accuracy.
• Balancing positive and negative plate
polarization, while achieving high
recombination efficiency is walking a razor’s
edge!
• Typical VRLA Float voltage has an acceptable
voltage tolerance of ~ +.01/-.005 V.
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CONFIDENTIAL
VRLAs Are Several Distinct Designs
• Gel Cells
•
•
•
•
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Typically require a lower charge voltage.
If charged at too high a rate, gel cells tend to
form bubbles within the gel electrolyte which
can cause permanent damage.
Typical recommendation is maximum of 2.3 to
2.35 VPC @ 10% of rated ampacity.
If charged at these voltage levels, charge voltage
must be reduced to recommended float voltage
as full charge level is reached.
CONFIDENTIAL
VRLAs Are Several Distinct Designs
• Absorbed Glass Mat (AGM)
•
•
•
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Typical maximum recommended charge voltage is
2.35 to 2.45 VPC @ 10% of rated ampacity.
If charged at these voltage levels, charge voltage
must be reduced to recommended float voltage
as full charge level is reached.
Temperature compensated charging is highly
recommended, based on battery temperature
instead of ambient.
CONFIDENTIAL
VRLAs Are Several Distinct Designs
• Pure Lead
•
•
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Typical maximum recommended charge
voltage is 2.45 to 2.65 VPC, with some
manufacturers specifying C/5 and higher CC
charge rates.
Temperature compensated charging is highly
recommended.
CONFIDENTIAL
VRLA Caveats
• Manufacturers virtually always recommend
Constant Voltage Charging, as opposed to
Constant Current charging.
• Manufacturers recommend float charging, as
opposed to intermittent charging regimes.
• Temperature and float voltage control are
critical in order to maximize VRLA life.
• Microprocessor controlled chargers can
provide optimum control of charging cycle.
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CONFIDENTIAL
Additional VRLA Topics
• Charging Parallel Strings
• Battery Monitoring
• Equalize Charging
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