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Battery end-of-life considerations for plug-in hybrid electric vehicles




Wood, Eric, author
Bradley, Thomas H., advisor
Marchese, Anthony J., committee member
Young, Peter M., committee member

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Plug-in hybrid electric vehicles (PHEVs) represent an advanced vehicle technology with the potential to displace petroleum consumption with energy generated on the US electric grid. While many benefits have been associated with the increased electrification of the US vehicle fleet, concerns over battery lifetime and replacement costs remain an obstacle to widespread PHEV adoption. In order to accurately determine the lifecycle cost of PHEVs, assessment studies must make use of informed assumptions regarding battery degradation and replacement. These assumptions should approach end-of-life (EOL) metrics not only in terms of pack level degradation but also loss of vehicle efficiency and performance in order to accurately represent consumer incentive for battery replacement. Battery degradation calculations should also remain sensitive to the range of ambient conditions and usage scenarios likely to be encountered in the US market. Degradation resulting from a single duty cycle has the potential to misrepresent battery life distributions for the US fleet. In this study, the sensitivity of PHEV lifecycle cost to the battery replacement assumption is explored to underscore the need for an improved understanding of PHEV battery EOL conditions. PHEV specific battery test results are presented to evaluate the ability of industry standard life test procedures to predict battery degradation in PHEVs. These test results are used as inputs to a vehicle simulation program to understand changes in efficiency and performance with respect to battery degradation using a light commercial vehicle simulated as a blended-mode capable, parallel PHEV20. A predictive battery degradation model based on empirical data is used to explore sensitivity of battery wear to various parameters including design variables, ambient conditions, and usage scenarios. A distribution of expected wear rates for a light-duty, midsize passenger vehicle modeled as a series PHEV35 is presented to highlight the uncertainty associated with battery life subject to US ambient conditions and driving distributions. The results of this study show that active management of PHEV battery degradation by the vehicle control system can improve PHEV performance and fuel consumption relative to a more passive baseline. Simulation of the PHEV20 throughout its battery lifetime shows that battery replacement will be neither economically incentivized nor necessary to maintain performance. The spectrum of climate and usage conditions PHEVs are expected to face in the US market suggest that the assumption of a single average ambient condition for battery wear calculations may not be representative of observed behavior in the fleet. These results have important implications for techno-economic evaluations of PHEVs which have treated battery replacement and its costs with inconsistency.


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battery degradation


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