Estimating coolant conductivity in a multi-voltage fuel cell system

A method of monitoring isolation resistance in a vehicular propulsion system, comprising:

• circulating a coolant in a coolant system fluidly coupled to a fuel cell stack that forms at least a portion of the vehicular propulsion system wherein the coolant provides thermal management within the fuel cell stack and the coolant provides electrical isolation between the fuel cell stack and a vehicle chassis;
• closing a at least one contactor in an electrical system comprising a stack voltage and a battery voltage;
• measuring a first isolation value, a second isolation value, the stack voltage, and the battery voltage;
• opening the at least one contactor in the electrical system;
• measuring a first negative isolation value;
• calculating a stack isolation resistance using the first isolation value, the second isolation value, the first negative isolation value, the stack voltage, and the battery voltage;
• calculating a coolant conductivity value;
• and
• indicating the coolant in the coolant system needs to be replaced with the coolant conductivity value crosses a threshold value.

A coolant conductivity estimation system for a fuel cell, comprising:

• a plurality of isolation sensors comprising:
• a first isolation sensor electrically coupled to a positive stack isolation resistance measurement location and used to measure a first isolation value,
• a second isolation sensor electrically coupled to a positive battery isolation resistance measurement location and used to measure a second isolation value,
• a plurality of negative sensors are electrically coupled to a negative stack isolation resistance measurement location, and a negative battery isolation resistance measurement location and used to measure a first negative isolation value,
• a stack sensor used to measure a stack voltage, and
• a battery sensor used to measure a battery voltage;
• and
• a battery monitor controller electrically coupled to the plurality of isolation sensors and comprising a processor and a non transitory computer-readable medium and executing an isolation algorithm, the isolation algorithm when executed causes the battery monitor controller to perform the steps of:
• closing at least one contactor in an electrical system comprising the stack voltage and the battery voltage,
• measuring the first isolation value, the second isolation value, the stack voltage, and the battery voltage,
• opening the at least one contactor in the electrical system,
• measuring the first negative isolation value,
• calculating a stack isolation resistance using the first isolation value, the second isolation value, the first negative isolation value, the stack voltage, and the battery voltage,
• calculating a coolant conductivity value, and
• providing indicia that a coolant needs to be replaced when the coolant conductivity value crosses a threshold value.