A model is developed to understand the discharge characteristics of lithium-silver vanadium oxide (Li/SVO) cells that are used to power implantable defibrillators. A distinctive characteristic of Li/SVO cells is the tendency for the internal resistance to increase relatively rapidly in the middle discharge. If the internal resistance continues to build, the rate capability of the cell will be compromised. It is assumed that chemical species form during a portion of the discharge by chemical reaction of a reduced phase. These species lead to the increased resistance in the cathode and at the anode. The model accounts for the greater resistance increase that occurs at slower discharge rates. It is anticipated that this model could be used to help quantitatively explain the behavior of different SVO materials, cathode formulations and electrode configurations in Li/SVO cells. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.049112jes] All rights reserved.