Sparhawk1995

I just started having this exact same problem on a 2017 Journey. I have put 2 brand new duralast batteries (I had them checked at the store before I left) AND a new alternator in the vehicle. Cable coming up from the battery disconnected from fuse block reads 12.4V.. as soon as I connect to the fuse block, voltage reading drops to 3V. Any help would be appreciated. I am at wits end trying to figure this problem out.