I have noticed the XIAOs can’t really charge my 18650’s fast enough (I use solar daytime that can supply ~800mA).
Since there’s no low voltage cutoff on the XIAOs you may need to either…
- use an “LV protected” cell,
- or add “LV cutoff” circuitry,
- or add code to go to “deep sleep” when the voltage gets too low (~2.5V) - and hopefully get enough charge to eventually allow repowering the device.