Penn State researchers developing methods to prolong operating life of valve regulated lead-acid batteries; electric locomotive as example
A team of Penn State researchers is developing more cost-effective ways to prolong the life of valve regulated lead-acid (VRLA) batteries, such as those applied in Norfolk Southern Railway No. 999—the first all-electric, battery-powered locomotive in the United States. (Earlier post.)
A leading cause of damage and death in lead-acid batteries is sulfation, a degradation of the battery caused by frequent charging and discharging that creates an accumulation of lead sulfate. In a recent study, the researchers looked for ways to improve regular battery management practices. The methods had to be nondestructive, simple and cheap—using as few sensors, electronics and supporting hardware as possible while still remaining effective at identifying and decreasing sulfation.
Christopher Rahn, professor of mechanical engineering, along with mechanical engineering research assistants Ying Shi and Christopher Ferone, cycled a lead-acid battery for three months in the same way it would be used in a locomotive. They used electroimpedance spectroscopy and full charge/discharge to identify the main aging mechanisms. Through this, the researchers identified sulfation in one of the six battery cells.
They then designed a charging algorithm that could charge the battery and reduce sulfation, but was also able to stop charging before other forms of degradation occurred. The algorithm successfully revived the dead cell and increased the overall capacity. The researchers, who report their results in the current issue of the Journal of Power Sources, then compared the battery to a new battery. They found that they increased the cell capacity by 41% and the overall battery capacity by 30%.
The operating environment, manufacturing variability, and use can cause different degradation mechanisms to dominate capacity loss inside valve regulated lead-acid (VRLA) batteries. If an aging mechanism for each cell can be identified in real-time, cell usage can be adjusted by the battery management system to optimize the performance and service life of the energy storage system. In this paper, the cell voltage and pressure of new and dead VRLA batteries are monitored during testing to determine the cause of death of the cells. The new cells have fairly uniform performance with limited signs of degradation while the cells in the dead battery have widely ranging performance, especially at the end of discharge and charge. Based on the measurement data, it appears that one cell died from sulfation and another three died of dehydration. The battery capacity is mainly dictated by the sulfated cell. A desulfation charging control scheme with pressure feedback is designed to break up hard sulfate and recover capacity while minimizing water loss by using low current charging.—Shi et al.
Even better results might have occurred if sulfation were the only aging mechanism at play, but the researchers found other factors reduced capacity, as well, such as water loss.
Other mechanisms that can damage lead-acid batteries include positive electrode corrosion, irreversible hard sulfation, positive electrode softening or shedding, electrolyte stratification, internal short-circuiting and mechanical damage.
The researchers are now developing alternative models to replace the electroimpedance spectroscopy model that would allow charging right up to, but not past, sulfation in batteries where sulfation is not yet present, hoping to prevent it from occurring in the first place.
Penn State and Norfolk Southern, which operates 21,000 route-miles in 22 states, began developing locomotive No. 999 in 2008 to evaluate the application of battery technologies for railroad motive power, with particular interest in energy savings and emissions reduction.
The US Department of Energy funded this most recent study.
Ying Shi, Christopher A. Ferone, Christopher D. Rahn (2013) Identification and remediation of sulfation in lead-acid batteries using cell voltage and pressure sensing, Journal of Power Sources, Volume 221, Pages 177-185, doi: 10.1016/j.jpowsour.2012.08.013