Bus leakage resistance estimation for electrical isolation testing and diagnostics

A method of detecting bus isolation for an electrified vehicle, the vehicle having a DC power source connectable to a positive bus and a negative bus and a chassis ground distributed within the vehicle, the method comprising the steps of:

• connecting a first fixed resistance between the positive bus and chassis ground;
• sensing a resulting first current flowing through a negative bus leakage resistance and a balanced leakage resistance;
• connecting a second fixed resistance between the negative bus and chassis ground;
• sensing a resulting second current flowing through a positive bus leakage resistance and a balanced leakage resistance;
• estimating the positive and negative bus leakage resistances in response to respective ratios of the first and second currents;
• comparing an isolation value to a threshold, wherein the isolation value is responsive to a voltage of the DC power source and a smaller one of the positive and negative bus leakage resistances;
• and
• signaling an atypical isolation when the isolation value is less than the threshold.

An electrified vehicle comprising:

• a positive bus connectable to a positive output of a DC power source;
• a negative bus connectable to a negative output of the DC power source;
• a chassis ground distributed within the vehicle;
• a first detector circuit comprising a first limiting resistance and a first sense resistance selectably connected between the positive bus and the chassis ground in order to sense a resulting first current flowing through a negative bus leakage resistance and a balanced leakage resistance;
• a second detector circuit comprising a second limiting resistance and a second sense resistance selectably connected between the negative bus and the chassis ground in order to sense a resulting second current flowing through a positive bus leakage resistance and the balanced leakage resistance;
• and
• a control circuit identifying the positive and negative bus leakage resistances in response to respective ratios of the first and second currents, wherein the control circuit identifies a balanced resistance as a higher one of the positive and negative bus leakage resistances, and wherein the control circuit monitors changes over time of the balanced resistance to identify a potential malfunction of the vehicle;
• wherein the DC power source is a fuel cell with a deionizer, and wherein the monitoring of changes in the balanced resistance is comprised of comparing the balanced resistance with a threshold to detect a malfunction of the deionizer.