How to get a serial output without using USB power?

I would like to look at serial outputs while my project is powered by its own standalone power supply. I need to do this to check how my project behaves when using non-USB power.

Is there an easy way to do this?

So far I tried using an unprogrammed nano as a serial interface, by connecting ground, RX and TX to my project nano. It works, and I get a serial output from the unprogrammed nano, but there appears to be a tendency for the project to be powered through the ground, TX and RX connection when the project power supply is switched off, and I'm sure I heard that this is not healthy for the serial chip.

Is there another way that will not lead to back-powering, and possible damage to the second unprogrammed nano that I am using purely as a serial interface?

You could just use a USB-TTL cable in place of the "bridge" nano.

I don't know about the risk of damaging the nano.