In regard to the increasing market launch of plug-in hybrid electric vehicles (PHEVs), understanding battery pack performance under electric vehicle (EV) operating conditions is essential. As lifetime still remains an issue for battery packs, it is a necessity to monitor the battery pack's state-of-health (SOH) on-board. Standard laboratory performance tests for health evaluation do not apply since operation interruptions and additional testing equipment are out of the question during ordinary EV usage. We suggest a novel methodology of performance estimation from real-life battery data. On the basis of battery pack data collected during PHEV operation, a support vector machine model capturing battery behavior characteristics is constructed. By virtually testing this battery model, access to standard performance evaluation figures can be gained. The SOH indicator "10 s discharge resistance" as known from hybrid pulse power characterization (HPPC) tests is chosen to exemplify how performance can be followed over a year. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.047211jes] All rights reserved.