In this study, it has been established experimentally that a probability of a thermal runaway in the nickel cadmium accumulators KSX-25 grows with an increase of a constant-voltage charge as well as of an environmental temperature and an accumulator time in service. It was shown that the thermal runaway is initiated not because of an external current supply source but instead it is a result of a powerful internal exothermic reaction. A possible thermal runaway mechanism is proposed corresponding with all the received experimental results. (C) The Author(s) 2014. Published by ECS. All rights reserved.