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SAE Awards NREL Engineer for Oral Presentation Excellence

August 20, 2012

Jeremy Neubauer, a senior energy storage engineer in the National Renewable Energy Laboratory's Center for Transportation Technologies and Systems (CTTS), recently received an Excellence in Oral Presentation Award from the Society of Automotive Engineers (SAE). Neubauer was recognized for his delivery of a technical paper—co-authored by Kandler Smith, Matthew Earleywine, Eric Wood, and Ahmad Pesaran of the CTTS Energy Storage Team—at the 2012 SAE World Congress.

The paper—titled Comparison of Plug-in Hybrid Electric Vehicle (PHEV) Battery Life across Geographies and Drive Cycles—details NREL’s analysis of the impact of temperature, state-of-charge, and charge/discharge cycling conditions on PHEV battery life. Neubauer credits his co-authors for much of the analysis described in the paper, which suggests several promising strategies for reducing PHEV battery degradation.

Modeling and Analysis
The Energy Storage Team applied a battery-life model of the graphite/nickel-cobalt-aluminum lithium-ion chemistry to investigate degradation over time for various geographic environments. A vehicle thermal model was applied to realistically capture how battery temperature responded to the environment. Real-world cycling scenarios using battery charge/discharge profiles generated from PHEV simulations across 782 single-day driving cycles (taken from a Texas travel survey) were employed to calculate battery heat generation.

In his presentation, Neubauer described the team’s modeling and analysis results and also provided a comparison of active and passive battery thermal management systems.

Strategies for Reducing Battery Degradation
In seeking ways to extend battery life, the study investigated the effects of different charge strategies—including the timing, frequency, and limits of charge events.

“One strategy for reducing degradation is to only charge the battery up to 80% or 90% of its maximum capacity, but this requires that the user either sacrifices some all-electric range or pays more up front for the extra margin capacity,” Neubauer said. “However, reducing the maximum state-of-charge also decreases sensitivity to charge timing, cycle frequency, and temperature, which can make such strategies a positive net benefit overall.”

The comparison of thermal management approaches—pitting an aggressive, active refrigerant-based cooling system versus a completely passive cooling system—produced interesting results. The team found that active cooling systems can be necessary to keep PHEV batteries operating within safe limits when driving aggressively in high-temperature environments (such as hot summer days in Phoenix). In general, however, the impact of active cooling on long-term battery degradation is small.

“If an active cooling system is only functional when the vehicle is in a key-on state, it does little to counteract the effects of the environment on battery temperature when parked—which can be a major driver of battery degradation,” he added. “The hot days with long stints of aggressive driving that necessitate the active cooling system from a safe operating limit perspective are fairly infrequent, on the other hand, and thus their overall effect on degradation can be small.”

Future Work
Future work will investigate driving and charging patterns in more detail to identify designs and controls that extend the life and reduce the cost of battery systems for electric-drive vehicles.