https://www.aliexpress.us/item/3256808606364019.html
Not to disagree with my friend…MF and with saying that dealing with battery is dangerious and dont use this with production, I will say this
It appears the “Charging” circuit of the XIAO is not that sophisticated… its basicly a 4.7v voltage regulator/gate 4.7v←>4.7v
so i would say try the unit you discribed… according to the link above, schematic, i would say
2 options power thru XIAO USB or Power thru TP USB
try hook the XIAO bottom to the Load Side, provide 18650 and power the device via TP unit
or connect 5V pin and GND of XIAO to USB Power IN on the TP
I was going to draw a schematic… but i couldnt scrape the image from the internet
I have not done any testing, so please report back your findings, but my basic opinion is the under battery pins can equally be used to power the XIAO, and this is the better way to do so, rather than using the 5V pin, or the (needs regulated) 3v3 which many people have had problems with.
also be awair without a diode, you could backfeed your USB, so do not connect to a computer or sensitive device… if using data, disconect the power at pin 5v for maximum safety for upstream devices. you dont want a high current backfeed.
I assume you are powering with a fast usb charger, do not leave the experiment unattended watch for fire or heat.