1. AGM Separator Properties Influence on Formation
Presenter: Jack Reesman, Undergraduate Student, Mechanical Engineering, University of Wisconsin – Milwaukee
Faculty Advisor: Deyang Qu, Mechanical Engineering, University of Wisconsin - Milwaukee
Conclusions
Introduction
Experimental
Lead acid battery technology has been around for over 100 years. In the last 10 years advanced lead acid batteries have been developed with a fiberglass mat separator in-between the lead plates. The separator helps fight acid stratification and plate exfoliation and increases cycle life by allowing the oxygen gas generated on the cathode to migrate through the separator and be reduced to water on the negative plate. This keeps the battery from drying out due to
gassing.
Before the batteries are shipped to the consumer they must be filled with acid and formed. This formation process is a prolonged charging step, which generates a significant amount of gas as a side reaction (H2O decomposition) which is vented from the box bringing the separator to a final saturation percentage of 95%. The final weight of the battery is a critical metric of the manufacturing process.
The objective of this project is to understand the basic causes of weight loss during formation and its relation to AGM separator properties. With the goal of identifying separator properties that affect formation gas generation. With the findings from this project we will make a recommendation to modify the existing separator qualification specification to to make it a more accurate separator qualification tool.
The following separator characteristics were measured with following techniques. All of the measurements were completed at UWM (.
Specification
Feature
Units
Thickness (20 kPa) mm
Grammage
g/m^2
Density (20 kPa)
g/m^2/mm
Wicking Height (2 min)
mm/2 min
Wicking Height (24 hr)
mm/24 hr
Porosity (Volume) %
Air Permeability
cm^3/s/cm^2
Pore Size (minimum) μm
Pore Size (maximum)
Pore Size (mean)
Characteristic
Measruement
Micro and Meso Porosity
Nitrogen adsorption
Macro Porosity
Mercury inrutsion / porosimery
Surface chemical composition
FTIR and XPS
Wetting rate
Dynamic balance
Surface morphology
SEM imaging
Surface profile
Optical digital microscopy
Permeability
Capillary Flow
Filling process
3-plate stack cell
Gas evolution and plate potential
Formation cycle
Results
Using the knowledge gained in this study we made a recommendation to JCI for techniques to use to qualify separators for production. The thickness should be measured at higher levels of compression (i.e. 120 kPa) which more closely represents the thickness the separator is used at in the battery. A dynamic mechanical analyzer should be used to measure the force needed to compress the separator to desired thickness.
The wetting rates should be measured with a mass balance and under compression as opposed to visually measuring the height in an uncompressed state. The Z plane permeability should be measured under compression and the XY plane should also be measured under compression (new technique developed during this project).
Porosity/Pore size distribution should be measured using mercury porometry and the target pore volume and diameter should more closely resemble their current supplier.
At the completion of this project we identified separator features critical to AGM battery scale 3 weights. We assisted in qualifying 20 separators from 4 different suppliers and tested them in lab built single cell testing apparatus, measuring there affect on formation gas generation.
We found The battery weight is dependent upon the synergetic effects of multiple separator properties. The following three properties had the largest impact on formation weight loss.
Pore Volume
Mercury porosimetry of maco pores
Target pore volume: 4.85 ± 0.15 ml/g
Wetting Rate
Dynamic wetting balance
Wetting is measured under compression
High wetting rates are desirable; target values ≥ 4.2mm/s0.5
Compression
Dynamic mechanical analyzer
Force vs thickness curves produced. Higher forces at a fixed distance are preferred.
Experimental
A resilient single cell AGM testing apparatus was designed and fabricated to allow the duplication of the formation process. This apparatus also allowed the measurement of the temperature, half cell potentials and full cell potential simultaneously. The vented gas is collected and measured to assist in weight loss calculation.
Acknowledgements
I would like to thank my parents, Dr. Deyang Qu and Joshua Harris for their support. I would also thank Johnson Controls for supplying the funding for this project. A big thank you to the UWM machine shop for their hard work.