Why does this UART Daisy Chain keep frying?
My guess is the Nano's voltage regulator overheats as you are feeding it 12V. It has to drop that down to 5V and it does it by wasting the energy as heat. Get yourself a 12-&gt;5V buck converter power supply board and use this to generate the 5V from your batteries. Your Nanos will thank you.
The power supplies are commonly called a LM2596 adjustable buck. stroomvoorziening