This paper presents a new technique to determine the online capacity of a lithium ion battery for Electric Vehicle (EV) applications. A mixed calendar and cycling profile is defined and applied to a high power-density 16Ah pouch type battery. Whilst the capacity loss due to cycling is irreversible, the apparent capacity loss due relaxation/self-discharge is found to be pseudo-reversible. The characteristic voltage drop during relaxation is studied in detail, and shown to exhibit behavior which can be modelled using a dynamic time constant approach. Applying this technique, an excellent fit is obtained between the actual and model predicted relaxation curves. The results are of significant practical importance, particularly in the field of lifetime prediction of Li-Ion batteries. (C) 2017 The Electrochemical Society. All rights reserved.