Determining state-of-charge (SoC) in a battery has been an important subject for the industry for decades. Despite significant efforts in the past focusing on methodologies to accurately estimate SoC in a battery, the fundamental understanding of the SoC issue has not been clear, at least in the industry where testing, control, and operation are concerned. Recently, we have been working on developing reliable techniques to identify capacity loss mechanism in rechargeable lithium batteries and to quantify contributions to capacity loss from different origins. That prompted us to re-visit the SoC issue. Strictly speaking, SoC is a static thermodynamic property of battery chemistry, which should be determined at equilibrium. On the other hand, cell capacity is a quantity of practical interest often determined by kinetics; thus, it is rate dependent. We conducted a few experiments to illustrate the accurate estimate of SoC through proper measurements. We also explained the proper correlation between SoC and rate capacity. A better understanding of the charge and discharge behavior in a battery under different rates in relation to the SoC is therefore derived. (c) 2007 Elsevier B.V. All rights reserved.