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Charging VRLA Batteries

Charging VRLA Batteries

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Charging VRLA Batteries

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  1. Charging VRLA Batteries Jan 10, 2016 CONFIDENTIAL

  2. 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. CONFIDENTIAL

  3. 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

  4. 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) CONFIDENTIAL

  5. Polarization vs. Current Charge OCV   Battery on-float Low-rate discharge  Higher-rate discharge Discharge

  6. 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 containmentsystem is required

  7. 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

  8. PbO2 + Pb + 2H2SO4 2 PbSO4 + 2H2O Excess Difference between Flooded and AGM’sMass Balance - Flooded • 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…

  9. PbO2 + Pb + 2H2SO4 2 PbSO4 + 2H2O Excess Limited Difference between Flooded and AGM’sMassBalance - AGM • 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

  10. Positive 2H2O  O2 + 4H+ + 4e- 4H+ + 4e- 2H2 2H2O  O2+2H2 Negative Total Difference between Flooded and AGM’sOvercharge - Flooded Vented batteries lose water through electrolysis when overcharged

  11. Recombination Positive Negative Total Difference between Flooded and AGM’sOvercharge - AGM 2H2O  O2 + 4H+ + 4e- 2Pb + O2 + 2H2SO4 2PbSO4 + 2H2O + heat Pb + H2SO4 PbSO4 + 2H+ + 2e- + heat • Thanks to the recombination process AGM batteries do not electrolyze water, however recombination; • Discharges the negative electrode • Generates heat

  12. Difference between Flooded and AGM’sWhat 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…

  13. 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. CONFIDENTIAL

  14. 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. CONFIDENTIAL

  15. VRLAs Are Several Distinct Designs • Gel Cells • 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

  16. VRLAs Are Several Distinct Designs • Absorbed Glass Mat (AGM) • 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

  17. VRLAs Are Several Distinct Designs • Pure Lead • 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

  18. 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. CONFIDENTIAL

  19. Additional VRLA Topics • Charging Parallel Strings • Battery Monitoring • Equalize Charging CONFIDENTIAL