I received a new Teensy 3.6 and an Adafruit PowerBoost 1000C LiPo charger board and did some experimenting today.  I'd like this project to run from a single cell LiPo, and charge when plugged into USB.  I'd also like to incorporate a soft power button that I've used on a few other projects.

To run the Teensy in this configuration, a trace on the bottom of the board that connects the USB +5v to the VIN pin must be cut.  After that, it's possible to access the USB voltage from the VUSB pin, and power the board separately from the VIN pin.  Because I'm prototyping in a breadboard, and the VUSB pin isn't breadboard friendly, I soldered a jumper onto the cut trace and secured it with tape.  This jumper gets plugged into the breadboard at the far end and can be used for a charging voltage.

The Adafruit PowerBoost board combines an MCP73871 charging IC with a tps61090 boost converter, and breaks out status pins, battery voltage, etc.  I'll use this board for prototyping and build a similar circuit into my finished design.

The soft power on feature takes advantage of the enable pin on the boost converter.  A pushbutton connected to battery voltage will momentarily turn on the boost converter and boot the Teensy.  The startup code can then pull a pin designated as power control (PCTL) HIGH, which is fed back into the enable pin.  When the button is released, the Teensy keeps itself turned on through PCTL.  With the addition of a couple of diodes, the button can also be read through and additional pin while the device is on.  In this case, the battery voltage could be as high as 4.2 or as low as 3v, so the level must be clamped to 3.3v, and an ADC pin works best.  When the Teensy wants to shut down, it sets the PCTL pin to LOW and the boost converter is disabled.