The related post has the background info. TLDR version - my 16 year old daughter was gifted a car by her cousin. As she was driving it, the lights slowly dimmed and then it died. I could jump start it but as soon as I took the jumper cables off, it died. I took the old battery out and charged it, and it started fine and drove home. I assumed it had to be an alternator going bad.

So I bought a new alternator and bolted that on. It still wasn't charging. Because the battery was older, I replaced that as well on the weird, off, chance that might be part of the issue. Nope. Still not charging. I pulled the cables off and cleaned and checked the cables. They're not new, but they look in OK shape. There are no obvious breaks in any of the insulation where the cables might be grounding out.

I started testing things with a voltmeter. I had 11.8 volts at the battery. I started the car up and checked the voltage at the back of the alternator. 11 volts. It should be closer to 14, right? So I'm thinking the voltage I'm reading at the alternator is the battery back charging to the alternator. I pulled the new alternator out again and checked it over. Everything looks fine visually. I reinstalled it, but didnt connect the main wire coming off the back. Then I started the car up and tried to check the voltage coming out of the alternator. Zero.

At this point I'm thinking the brand new alternator that they sold me is bad. Is there anything else that would cause me to read zero volts coming out of the alternator? IT would be really bizarre to have an alternator go bad, then have the new replacement be bad as well.