A device for determining a thermal runaway condition in a nickel-cadmium tery. A current sensor develops a DC voltage proportional to battery current and a converter converts the voltage to a pulse train whose frequency is directly proportional to battery current. The pulse train frequency is monitored for fixed periods of time and changes of frequencies are used to determine the slope of the charge current. A maximum allowable slope is specified and fed into a binary up/down counter which also counts the pulse train frequency. When the difference between two pulse train frequency readings is greater than the maximum allowable, a possible thermal runaway condition is indicated and after a predetermined number of consecutive indications, an indicator is energized to provide a warning.