Xiao BLE supplying 3V3 at same time as USB


I am using non-rechargeable batteries and a buck converter down to 3.3V, which I supply directly to Xiao BLE’s 3V3 pin.
Works fine, but I am worried what would happen if USB got connected now as well, with the batteries still attached?


As far as I understood the schematics, it looks like my battery current cannot flow back to a PC via USB, because of the diode next to VBUS, so that’s fine. Nothing is connected at VBAT, so also fine. No idea where VIN actually goes to?

Would I end up getting USB VBUS 5V converted down to 3.3V and my own 3.3V BOTH at Xiao BLE’s 3V3 pin?
Is there any chance of current flowing back into my non-rechargeable batteries to possibly blow them up?
Any other possible risks, or would it be fine to do it like that, if not, what would be a better idea?

Any help is greatly appreciated!

I strongly discourage you from doing this, when you connect a battery and then at the same time connect Type-C, which supplies power to the battery whether it is rechargeable or not, then this could cause the non-rechargeable battery to explode! For safety reasons, we recommend that you use a rechargeable lithium battery.

Ok, so what if I did something like this externally?

Once USB is connected, VBUS 5V will be at the Xiao BLE’s 5V pin and at the PMOS’s gate.
This will switch the external PMOS that will cut the path from my non-rechargeable batteries to the 3V3 pin and no current can flow from USB to possibly charge the batteries.

Wouldn’t that work fine and be safe?

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