I had a student that kept missing classes due to dead batteries in the car (a mid 2000's ford focus) his uncle gave him to drive. They put 2 new alternators and a battery in it, and still had the same problems. He was at the mercy of his "mechanic" uncle to get the car fixed, so I took a look at it with him. There were lots of "rigged" items that jumped out as soon as he lifted the hood indicating his uncle's talents. Wirenuts under the hood of a car is never a good sign! Anyway, I quickly discovered resistance between the engine block and negative terminal of the battery and told him he had a cable issue of some sort. He took the car back to his uncle who said it was definitely not that, and made the guy buy a used OEM alternator at the junkyard to try. Still had same problem so his uncle gave up. He could make it to school if he charged it overnight, and the weather was clear. If he needed the wipers and headlights he was screwed...

Student brought it back to me to look at, and I noticed a spark at the back of the engine when he went to start it. Sure enough there was a burned spot where a couple of the accessory brackets bolted together - and the ground cable was bolted to the bracket that then bolted to another bolted to the head. Looked like a royal pain to get the brackets out, and seeing as how it was his uncle's car who knew it all I quickly worked out a solution. Did I mention this student is in a welding program I teach at a community college? Had him reach back in there with a 6010 electrode and nuke the place where the 2 brackets mated up. Instantly he had perfect charge from the alternator and no more dead batteries. I hope his uncle doesn't need to take those brackets apart....