A method for the diagnosis of thermal runaway in an electric battery comprising at least one stack of electrochemical cells connected to one another in series; the diagnosis method comprises the steps of: measuring a first electric voltage (V cell-i) at the ends of each electrochemical cell; measuring a second electric voltage (V stack) at the ends of the entire stack of electrochemical cells; and diagnosing a thermal runaway of an electrochemical cell, if the first electric voltage (V cell-i) of the electrochemical cell is smaller than a first threshold value; the diagnosis method is characterized in that it comprises the further steps of: calculating a difference between the second electric voltage (V stack) and the sum of all first electric voltages (V cell-i); diagnosing a thermal runaway of an electrochemical cell, only if the difference between the second electric voltage (V stack) and the sum of all first electric voltages (V cell-i) is smaller than a second threshold value; measuring all second electric voltages (V stack) at the ends of each stack of the plurality of stacks of electrochemical cells, identical to one another, making up the electric battery; determining, among all second electric voltages (V stack) at the ends of each stack of the plurality of stacks of electrochemical cells, a maximum second electric voltage (V stackmax) and a minimum second electric voltage (V stackmin); calculating a difference between the maximum second electric voltage (V stackmax) and the minimum second electric voltage (V stackmin); diagnosing a thermal runaway of an electrochemical cell, only if the difference between the maximum second electric voltage (V stackmax) and the minimum second electric voltage (V stackmin) is greater than a third threshold value. measuring a first electric voltage (V cell-i) at the ends of each electrochemical cell; measuring a second electric voltage (V stack) at the ends of the entire stack of electrochemical cells; and diagnosing a thermal runaway of an electrochemical cell, if the first electric voltage (V cell-i) of the electrochemical cell is smaller than a first threshold value; the diagnosis method is characterized in that it comprises the further steps of: calculating a difference between the second electric voltage (V stack) and the sum of all first electric voltages (V cell-i); diagnosing a thermal runaway of an electrochemical cell, only if the difference between the second electric voltage (V stack) and the sum of all first electric voltages (V cell-i) is smaller than a second threshold value; measuring all second electric voltages (V stack) at the ends of each stack of the plurality of stacks of electrochemical cells, identical to one another, making up the electric battery; determining, among all second electric voltages (V stack) at the ends of each stack of the plurality of stacks of electrochemical cells, a maximum second electric voltage (V stackmax) and a minimum second electric voltage (V stackmin); calculating a difference between the maximum second electric voltage (V stackmax) and the minimum second electric voltage (V stackmin); diagnosing a thermal runaway of an electrochemical cell, only if the difference between the maximum second electric voltage (V stackmax) and the minimum second electric voltage (V stackmin) is greater than a third threshold value. calculating a difference between the second electric voltage (V stack) and the sum of all first electric voltages (V cell-i); diagnosing a thermal runaway of an electrochemical cell, only if the difference between the second electric voltage (V stack) and the sum of all first electric voltages (V cell-i) is smaller than a second threshold value; measuring all second electric voltages (V stack) at the ends of each stack of the plurality of stacks of electrochemical cells, identical to one another, making up the electric battery; determining, among all second electric voltages (V stack) at the ends of each stack of the plurality of stacks of electrochemical cells, a maximum second electric voltage (V stackmax) and a minimum second electric voltage (V stackmin); calculating a difference between the maximum second electric voltage (V stackmax) and the minimum second electric voltage (V stackmin); diagnosing a thermal runaway of an electrochemical cell, only if the difference between the maximum second electric voltage (V stackmax) and the minimum second electric voltage (V stackmin) is greater than a third threshold value.