Battery (post-screw) loose under fuse box. How to fix?

That post is part of the main fusible link. I have a few of them on the shelf that I can get a good look at once I get home. Initial gut feeling is that something broke and you may need to install a new fusible link. I can provide better info after inspection, but wedging it in a position where it's making good contact and it can't move may at least provide you some temporary reliability.

Without actually dissecting one, I believe that stud should have a head at the bottom inside the plastic, fixed in position to allow tightening the nut. If you look at the stud, it should have two busbars that lay on top of it and the 12v battery cable terminal. Once the nut is tightened, it clamps the 12v cable terminal tightly to the busbars. One busbar provides power to the passenger side of the fusible link assembly and the other busbar provides power to the driverside. If the nut is loose, obviously the connection is going to be poor.

If you try to tighten the nut, does the entire stud turn?
Is the nut cross threaded, preventing it from being fully tightened?
Can you remove the nut to inspect the threads?
If the threads on the stud are damaged, it will limit how far down the nut can travel.
If this is the case, and I find it to be very likely, you can work around it by putting some flat washers with the correct inner diameter over the stud to take up gap and then install the nut (or a new nut if needed).

 

Your best bet is to answer the questions and follow the directions above. Otherwise its a new fusible link, which is a bit of a job since its wired and cabled in. Not just plugin.

 

Here's a close-up photo. The top one is the driverside bar, the bottom one is the passenger side bar. The stud has a beefy square base that is set into the housing. There is nothing visible from underneath, so exactly how its attached to the plastic housing isn't visible, but it is definitely strong. The stud has no play at all, so if yours is loose, something is broken. The important part is you have to get the 12v cable firmly clamped to the 2 fusible link bars. No ifs, ands or buts, because 100% of the power provided by the 12v battery passes through that connection.

 

There are several areas within the fusible link that blow just like a fuse. For most its obvious. This one takes a discriminating eye so I added hints.