If you are using the Teensy 4.0, one possibility is to get the Adafruit Teensy 3.x feather adapter. I'm putting the Adafruit link along with a compatibility page I wrote for the old unofficial Teensy wiki site:
Note, with this adapter, you do NOT have to cut the pads underneath the Teensy. The Adafruit board will automatically charge the battery if USB power is applied, and if there is no power, it will provide power from the battery (i.e. it has diodes built-in).
On the Teensy 4.0 you can use the on/off pin to turn off the Teensy while USB is connected if you want to recharge the battery but have the Teensy be off, which is something I want to do every so often with props, where I want to turn off the lights at night, but recharge the battery.
Adafruit has some other charging boards that are similar that can be used with different pinouts:
The Tindie shop Pesky Products makes a charger board that is meant to solder directly on to the Teensy. You do have to cut the pads underneath the Teensy:
I like the Seeed lipo rider plus. It has a USB C port for charging, the JST lipo plug and a USB A port for have a short USB cable to the Teensy. Unlike some of the charging boards, this one can charge the larger lipo batteries.
There are various other similar boards that have a separate USB plug for doing the charging. You would connect the output to the VIN and ground pins (or VUSB and ground). Either you have to be careful to not connect a USB power source at the same time as the battery, or you need to have diodes if the board doesn't have its own diodes that prevent mixing two power sources:
Another possibility is not to use a lipo battery, but instead use a USB power bank, and just hook it up to the USB port. One thing to look at is a lot of power banks are made for charging phones, and a lot of times microprocessors don't use enough power, and the power bank shuts down.
