A large number of commercial batteries employ the HNO2-HMO2 (or Ni(OH)(2)-NiOOH) reaction in their positive electrodes. Such batteries often exhibit a so-called "memory effect" in which the available capacity at useful voltages noticeably decreases if they are used under conditions in which they are repeatedly only partially discharged before being recharged. This paper presents a rational and consistent explanation of this phenomenon based upon the thermodynamics of ternary systems. (c) 2006 Elsevier B.V. All rights reserved.