When charging (picocalc switched off, usb-c cable plugged in), for one battery the light stays off and the battery does not allow switching on the device. Is it dead? For the other, the light blinks rapidly in yellow. Is it a sign that the battery is dying?
Definitely inserted the right way round ?. Maybe try in a friends charger ?.
Yeah, tried them in all possible configurations. Even flashed several stm32 firmwares to see if there was a bug with the battery charging logic…
Probably bad batteries then…