I had designed my own arduino nano shield for a 2WD line follower robot.
I connected battery to Vin of arduino nano and the 5v from my external 7805 to arduino nano.
Don't! OR power the Arduino with 5V onto the 5V-pin OR power it with >7V on Vin. NOT both. Your voltage regulator and the onboard regulator will try to fight to keep it at their slightly different understanding of 5V. Extra note, on a Nano it's fine to power the Arduino via 5V and connect USB at the same time. On a Uno or Mega it's not!