Fuse 15 Under Hood?

I was changing my battery and realized this fuse isn't fully connected. It's next to the fuse box and looks like it runs through the firewall into the car. It's in a little plastic black case that opens up like an old school lighter. Does anyone know what it's for? I'd lost a picture but I'm new to these forums and now sure if that's a function because I can't figure out how.

Are you talking about the fuse that's on the post for the positive side of the battery. If so I believe that's the fuse for the alternator. Someone correct me if I'm wrong