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. η = a ± b log(I) 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
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

7. Difference between Flooded and 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. 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…

9. 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

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

11. Difference between Flooded and AGM’s Overcharge - AGM
Positive
Negative
Total
Recombination
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’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…

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 % 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 % 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