It has been established experimentally that during thermal runaway a great amount of hydrogen is given off from nickel-cadmium accumulator KSX-25. Through the thermal decomposition of nickel-cadmium accumulator electrodes, it has been shown that hydrogen had been present in the electrodes before the onset of thermal runaway. Through an analysis of the energy balance of thermal runaway, it has been shown that it is not an external source of current that causes thermal runaway, but rather a powerful internal exothermic reaction. (C) 2007 Elsevier B.V. All rights reserved.