Li-ion is currently the most commonly used battery chemistry in portable applications. Accurate state-of-charge (SOC) and remaining run-time indication for portable devices is important for user convenience and to prolong the lifetime of batteries. The actual SOC algorithms, which the main companies use in practice, make use of the so-called electromotive force (emf). In these SOC systems it is assumed that the emf of an Li-ion battery only depends on aging to a limited extent. In this paper, novel emf measurement and modeling efforts are presented as a function of battery aging. As will be shown, a better understanding of this dependence is useful for improving SOC accuracy. (c) 2007 The Electrochemical Society.