I have this charger: https://www.adafruit.com/product/2124
It works fine if I wait for the battery to get fully charged. But if the battery is low and charging, the voltage on Teensy 3.3V output drops to about 2.5V and poor thing goes into a coma.

What charger do you use which keeps 3.3V on Teensy even while charging a totally drained battery?
And I don't want to use any additional switches other than the new on/off button on Teensy 4.0
Ideally, I want to have an analog output pin on the charger which shows the battery level, so that I can show it on Teensy while the battery charges or not.
So, my Teensy should work just like my phone: it shows that the battery is low, I connect it to the USB and it keeps working with no interruption, showing that the battery is charging now.

I don't need a complete board. I'm need a chip and schematics for it, so that I can integrate it onto the new board I'm making.