I got the schema from here: https://blog.zakkemble.net/a-lithium...-load-sharing/
The battery is charging at 100ma. I checked it using the jumpers for this purpose.
The diod gets a little hot, but not too much.

The problem is that the battery has 3.8V on it, but I get only 2.9V on Teensy input and 2.8V instead of 3.3V on the power output.
Did anybody else has this working? Please share your schema.

Here is the thing. Please ignore J5. It's in case if I don't want to install the charger. When I do, I break it.
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Thanks.