I'm confused with the wiring needed to connect the Teensy 3.6 board and a Adafruit Fona GSM800 module.
The GSM module has a lithium battery connect to its onboard LiPoly battery charging circuitry.

I connected the 5V input of the GSM board to the VIN pin on the Teensy. If I plugin a USB cable I get 5.x V from the teensy to power the Fona. The battery get's charged and that's fine.
My problem now is: if I pull the USB cable (and so the external power source) I would like to run the Teensy together with the GSM board from the lithium battery connected to the GSM board.

If someone could give me an advice for the correct wiring - this would help a lot!