Temperature has major effect on battery degradation: CMU study
A team led by Dr Jeremy Michalek at Carnegie Mellon University has examined plug-in hybrid electric (PHEV) battery life under various conditions. Their findings, published in the Journal of Power Sources, revealed that aggressive driving, especially in warmer conditions with "no thermal management", can reduce battery life by up to two-thirds. The research also found batteries operating in milder climates last 73–94% longer, while air cooling can increase battery life by a factor ranging between 1.5 and 6. In addition, the use of a "blended gas/electric-operation control strategy" can quadruple battery life.
In their paper, published in the Journal of Power Sources , they reported that in the absence of thermal management, aggressive driving can cut battery life by two-thirds; a blended gas/electric-operation control strategy can quadruple battery life relative to an all-electric control strategy; larger battery packs can extend life by an order of magnitude relative to small packs used for all-electric operation; and batteries last 73–94% longer in mild-weather San Francisco than in hot Phoenix.

A team at Carnegie Mellon University (CMU) led by Dr. Jeremy Michalek has investigated the implications of regional and drive cycle variations on the degradation of a plug-in hybrid electric (PHEV) battery. Modeling a PHEV with an air-cooled battery pack comprising cylindrical LiFePO 4 /graphite cells, they simulated the effect of thermal management, driving conditions, regional climate, and vehicle system design on battery life.

Air cooling can increase battery life by a factor of 1.5–6, depending on regional climate and driving patterns, they found. End of life criteria has a substantial effect on battery life estimates.



Effect of air cooling on capacity fade and battery life when annual miles driven are 14,700 miles. The battery industry traditionally defines end-of-life (EOL) as the point where the battery’s energy storage capacity drops by 20% of its initial value—indicated by the solid horizontal black line on the first two charts. (a) Capacity Fade in Phoenix, comparison of air cooling vs no cooling for two drive cycles. (b) Capacity Fade in Phoenix and San Francisco, using GPS data. The comparison of air-cooling vs. no cooling is provided for two cities (c) Improvement in battery life by air-cooling for different cases simulated. Credit: ACS, Yuksel et al. Click to enlarge.

Apart from the specific type and design of the battery, the conditions and stress factors during storage and use also affect how quickly the battery will degrade. There are various factors that affect battery life such as time, charge/discharge rate, temperature, and depth of discharge (DOD)/state of charge (SOC). The degree to which each of these factors affects degradation patterns depends on the chemistry and design.

… In this study, we aim to assess the regional and drive cycle implications of degradation of a PHEV battery. For this purpose we construct a comprehensive and modular simulation model to address three main questions: 1) How much improvement in PHEV battery life can be obtained with passive air-cooling? 2) How does this improvement vary across different regions and different driving and usage profiles? 3) What is the sensitivity of the results to the model parameters and assumptions? —Yuksel et al.

The CMU team created usage scenarios for one year of daily driving, charging and rest conditions and recorded the battery usage history. The researchers used this battery usage history to estimate the degradation over consecutive years, assuming that every year the same usage profile repeats itself.

For the vehicle, they used specifications similar to a Toyota Prius with a 5 kWh Hymotion ANR26650 LiFePO 4 /Graphite pack comprising cylindrical cells manufactured by A123 systems. This allowed them to draw on prior work and to use an air-cooled system with well-established parameters. Based on those assumptions, they developed a comprehensive simulation model to estimate battery temperature, current and state of charge profiles under the usage scenarios.

To estimate daily travel behavior of the vehicle, the team used GPS sample data from the Atlanta Regional Commission (ARC) Regional Travel Survey with GPS Sub-Sample, available at the Transportation Secure Data Center (TSDC) of the National Renewable Energy Lab- oratory (NREL).

To test the sensitivity of the results to drive cycle, they used two standard EPA fuel economy test cycles: the Urban Dynamometer Driving Schedule (UDDS), represents city driving conditions; US06 is an aggressive (high acceleration) driving schedule.

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