To answer your question as simply as possible.
No - do not use the battery connection.
Use the USB (USB C power cable) or VBus Pin (Pin 14 on XIAO).
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This one is for a permanent-ish node mounted on a pole isolated from external power sources, so I am trying to find best way to use solar panel.
Yes, that would make sense. If I undestood correctly (link my output from solar charger and battery to the xiao USB input) but … that solar charger outputs 3.7V - 4.2V voltage (same as battery) I assume it is a problem if I supply that to the USB of the xiao board, that normally expects 5V no? Or maybe USB power source on these boards accept under 5V?
In this case, isn’t it better to solder my 3.7V - 4.2V output (mentioned in text scheme above) to the 2 xiao battery pins? I am thinking I can/should place a diode to prevent xiao “charging” this external power source (solar+charger+battery).
If you just use the battery input for powering the XIAO and not the USB then it should be fine to use the solar power without a diode.
Since both voltages lead into the same point (see VIN on the diagram attached) putting 3.7V on the USB C should be OK as well.
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