Cascadia alternator - what does this little wire do?

I want to follow up to this thread. Thank you Rocketyman and HoneyBadger67. I had no idea that terminal spray even existed.

I actually fixed the problem back in June, but one of my batteries had a cross threaded stud, so I had to get around to fixing that.

That wire is a sense wire. The normal alternator voltage is 14.2V and without that wire, the alternator was putting out 14.7 - 14.8V. Although the battery connections looked OK, I measured the voltage at the batteries and found out that the battery was only getting 12.85V without that sense wire!! That is why the previous owner disconnected the sense wire. With the wire connected, the batteries were only getting 12.35V, which is too low to actually charge the batteries.

I ended up using a wire brush on my drill and cleaned all of the connectors. I also knocked out the studs at the end of each cable and installed real battery terminals. They cost $2. I installed copper washers over the battery studs and finally replaced the junction box. It costs $110 Canadian ($80 US) but comes with the mega fuses. There are 2 unused fuses in that box so next year, I want to wire my APU and power inverter through the junction box. The whole thing is sprayed with terminal protector spray. That is great stuff and I had no idea that it even existed.

Attached are some pictures of the result.