Method and system for detecting contactor weld

1. A vehicle comprising:

a traction battery;

an electric machine;

a pair of contactors electrically between the traction battery and electric machine; and

a controller programmed to sequentially generate a first command to open one of the contactors and a second command to open the other of the contactors, and to inhibit a next start of the vehicle based on (i) voltage or current sensed before and after the first command is generated indicating that a first resistance between a bus electrically connected with the electric machine and a chassis of the vehicle increases after the first command is generated and (ii) a duration of continuous reduction in voltage across the other of the contactors following the second command exceeds a predetermined threshold.

2. The vehicle of claim 1 , wherein the controller is further programmed to permit a next start of the vehicle based on (i) the voltage or current sensed before and after the first command is generated indicating that the first resistance increases after the first command is generated and (ii) the duration of continuous reduction in voltage across the other of the contactors following the second command does not exceed the predetermined threshold.

3. The vehicle of claim 2 , wherein the controller is further programmed to generate output indicating the contactors are open.

4. The vehicle of claim 1 , wherein the controller is further configured to generate output indicating the other of the contactors is welded closed.

5. The vehicle of claim 1 , wherein the controller is further programmed to inhibit a next start of the vehicle based on the voltage or current sensed before and after the first command is generated indicating that the first resistance does not increase after the first command is generated, and a second resistance between the bus and chassis increases after the second command is generated.

6. The vehicle of claim 5 , wherein the controller is further programmed to generate output indicating the one of the contactors is welded closed.

7. The vehicle of claim 1 , wherein the controller is further programmed to inhibit a next start of the vehicle based on the voltage or current sensed before and after the first command is generated indicating that the first resistance does not increase after the first command is generated, and a second resistance between the bus and chassis does not increase after the second command is generated.

8. The vehicle of claim 7 , wherein the controller is further programmed to generate output indicating the other of the contactors is welded closed.

9. A method comprising:

generating a first command to open a first contactor and a second command to open a second contactor; and

inhibiting a next start of a vehicle responsive to voltage or current sensed before and after the generating the first command indicating that a first resistance between an electrical bus and a chassis of the vehicle does not increase after the first command is generated, and a second resistance between the electrical bus and chassis increases after the second command is generated.

10. The method of claim 9 further comprising generating output indicating the first contactor is welded closed.

11. The method of claim 9 further comprising inhibiting a next start of the vehicle responsive to the voltage or current sensed before and after the generating the first command indicating that the first resistance does not increase after the first command is generated, and a second resistance between the electrical bus and chassis does not increase after the second command is generated.

12. The method of claim 11 further comprising generating output indicating the first and second contactors are welded closed.

13. The method of claim 9 further comprising inhibiting a next start of the vehicle responsive to the voltage or current sensed before and after the generating the first command indicating that the first resistance increases after the first command is generated, and a duration of continuous reduction in voltage across the second contactor following the second command exceeds a predetermined threshold.

14. The method of claim 13 further comprising generating output indicating the second contactor is welded close.

15. The method of claim 9 further comprising permitting a next start of the vehicle responsive to the voltage or current sensed before and after the generating the first command indicating that the first resistance increases after the first command is generated, and a duration of continuous reduction in voltage across the second contactor following the second command does not exceed a predetermined threshold.

16. The method of claim 15 further comprising generating output indicating the first and second contactors are open.

17. A power system comprising:

a battery; and

a controller programmed to

inhibit charge of the battery according to voltage or current values sensed before and after contactors electrically connected to the battery are commanded to open and indicating that a leakage resistance associated with one of the contactors does not increase after the one of contactors is commanded to open, and a leakage resistance associated with another of the contactors increases after the another of the contactors is commanded to open, and a duration of a continuous voltage drop across another of the contactors after the another of the contactors is commanded to open exceeds a threshold.

18. The power system of claim 17 , wherein the controller is further programmed to inhibit charge of the battery according to the voltage or current values sensed before and after the contactors are commanded to open and indicating that the leakage resistance associated with the one of the contactors does not increase after the one of contactors is commanded to open, and a leakage resistance associated with another of the contactors does not increase after the another of the contactors is commanded to open.