Problem with Arduino relay module and serial monitor when I attach a 4-cell AA battery pack

I made a plant watering system controlled by an Arduino. When I attach the 4-cell AA battery pack the serial monitor prints this:

Soil Moisture Level: 0
1
Soil Moistu⸮⸮+⸮j⸮⸮⸮⸮⸮ɕ Level: 0
button unpushed
Soil Moisture Level: 0
0
Soil Moisture Level: 0
0
Soil Moisture Level: 0
0

Also, the relay starts to switch randomly and then often stops working until I restart the system.