To improve a nickel/metal hydride (Ni/MH) battery's rapid charge/discharge performance and enlarge it, a precise understanding of the thermal behavior is required. This report examines numerically and experimentally the thermal behavior of the Ni/MH cell during rapid charge and discharge cycles by considering entropy changes for electrochemical reactions, the exothermic reaction for hydrogen adsorption as MH, the exothermic heat from the side reaction, the heat generation by overpotential, and the heat transfer to ambient air. The overpotential resistance and the current efficiency, the ratio of main reaction current to charge current, have been measured during rapid charge and discharge cycles with constant current. Our proposed model works well to estimate the cell temperatures during charge and discharge cycles not only at smaller currents than the rated one, but also at rapid charge (∼ 2C) and discharge (∼ 3C) currents. © 2005 The Electrochemical Society. [DOI: 10.1149/ 1.1914744] All rights reserved.