Describe the wiring of your Arduino, I guess there has the problem to be located. Usually the serial monitor should work even if it's powered externally. You cannot expect the serial connection to work with the PI if it won't work with your laptop.
Describe the wiring of your Arduino, I guess there has the problem to be located. Usually the serial monitor should work even if it's powered externally. You cannot expect the serial connection to work with the PI if it won't work with your laptop.