1684 is the code what does it mean when it say battery been unhooked in the last 50 starts.

The message is there to allow the inspectors that check for emissions compliance to know whether or not you (or someone else) has disconnected the battery in any attempt to erase or clear codes in the last 50 start-stop cycles. In jurisdictions that only use the on board computer to verify emissions compliance, they will often automatically fail the vehicle for emissions on this message and you must return after the message clear to prove the vehicle is fault-free.