Fuel cell stack coolant conductivity sensor using differential voltage measurements

This invention relates, in general, to a fuel cell system and, more particularly, to the detection of leakage current in the coolant of a fuel cell system. A method and apparatus for detecting leakage current in the coolant of a fuel cell stack. The invention eliminates the use of a conductivity sensor by using a voltmeter to measure the voltage across the coolant. If the coolant voltage is at or below a predetermined level, an excessive level of conductivity in the coolant exists, and the presence of leakage current is reported. The invention can also compare the coolant voltage to two different predetermined voltage levels, the second one higher than the first. A coolant voltage at or below the lower voltage level could signal a shut down of the stack, while a coolant voltage at or below the higher voltage level could signal an alarm or another diagnostic. The invention can optionally use a second voltmeter to measure the voltage across the stack and calculate the resistivity and conductivity of the coolant.