I have two of these boards - one of them will not work on battery (I even tried removing all Serial references and it still didnt work) - it does charge the battery but will not power the device.
Another board I have is working on battery without USB so the only difference is I soldered some pin headers to the one that doesn’t work.
I wish I had tried it before that but it could be related. I’m not sure if it’s a batch issue with some of them, if it’s potential heat damage (I’m not a beginner with soldering but may have to be EXTRA careful) … suffice to say, I really like this board and was trying with two before I bought a load more 