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Thanks for the idea, Andrew. I kind of did that by installing the right drivers and upgrades to Arduino on both the mac and the other windows laptop. Both other machines had to be brought up from scratch, since I hadn't done any esp32 development on them. I can now do it in my sleep. Banggood, the vendor, seems to be pretty responsive, having already asked me for more details about the failure. I'm making a youtube video about it now.
Thanks, Andrew. Yes, I did get the driver from Silicon Labs. I then tried it with both the original cable and others on another Windows laptop, then on a Mac laptop. Got the same result every time. On all of them I can see data from the watch, but can't upload anything. I put in a question to the vendor, Banggood.
If the power reads 197%, that's likely because it started as 100%, then the routine failed to clear the 3rd digit before writing the 97%.