I had to weld nuts to the outside of rounded-out allen type bolts when removing 15-year-old rear brake calipers and carriers this weekend. As I was laying under the car welding (probably running about 55 amps, MIG) with the my ground somewhere on the chassis I thought to myself "I should surely disconnect the battery." But I was wedged under a car with a welding helmet on and was already halfway done so I didn't bother. Frankly I've done it before and never caused any damage that I know of.
Anyway, you can see where this is going. Two days later, I'm driving, enjoying my new brakes when my brake pad wear sensor comes on intermittently. I assumed, of course, that I must not have plugged the sensor's connector back in completely. After a short stop the pad wear light was off and on came my battery light. After coming home and using a multimeter it's obvious that the alternator is inop.
Any chance this is a coincidence? Do you think I've messed up the ECU or perhaps the wear sensor light was a function of low voltage?