Program running on RPi 3b+ Very Strange Behavior - Out of Bound Index

[MadMacks]

At 63 years of age I remember starting my "coding life" in college with Basic, Logo, COBOL, and Pascal...done a lot of other languages since then of course, but I'm liking Python as I dig in a bit. Now that I've retired and live off-grid I have the time (especially in winter when I get snowed in often) to rekindle my interest in coding, so for me that's a good thing...

Regarding your earlier comment about checking the result on the statements...

result = bms.writeCharacteristic(0x15,b'\xdd\xa5\x03\x00\xff\xfd\x77',False)
bms.waitForNotifications(5)

I think you're spot on. I'm betting the random issue on occasion is the BMS device being slow to provide a response, and the code doesn't seem to care, it just moves on after 5 seconds. That would result in a "short list" and cause the out of bounds reference error. I'll check the method and see if it provides a usable result code that I can test, and test it. And/or I'll adjust the wait time from 5 seconds to 10 seconds and see if that removes the random fails in the short term. You're correct that once you go down a rat hole like "it must be something strange about the print statements fixing the issue" it can be hard to see other alternatives (wish I had a code buddy to bounce these things off of, but I guess that's what these forums are for).

Again, your thoughtful responses have been enormously helpful!