A device triggers thermal runaway of an electrochemical accumulator, including a flexible package that contains at least once electrochemical cell. A part of the current collectors that form the output terminals of the accumulator pass through the flexible package. The device includes two electrical current conducting elements, each respectively in electrical contact with the exterior of a part of a metal layer of the flexible package or of the metal housing, and an electrical power supply independent from the accumulator. The electrical power supply is designed to drive a current, referred to as a heating current, between the two elements so as to heat up the flexible package or the housing of the accumulator by Joule effect.