Thank you so much for your help. Really appreciated!
If I connect the battery to BAT+ do I need to add any protection diode or is a straight connection fine?
In my experiments I was using a Nordic PPK2 in source mode. In the initial experiment I connected it to the battery pads and then connected the device straight to the Xiao pins.
I then tried to do the same but connecting the device to the 5V pin, which is what led me to ask the initial question in this discussion.
That didn’t work so I got a breadboard out and used the PPK2 to power both the Xiao (using its 5V pin as the input) and the device.